CN111893431B - 20Cr2Ni4A carburizing steel with high contact fatigue resistance and preparation method thereof - Google Patents
20Cr2Ni4A carburizing steel with high contact fatigue resistance and preparation method thereof Download PDFInfo
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- CN111893431B CN111893431B CN202010823352.2A CN202010823352A CN111893431B CN 111893431 B CN111893431 B CN 111893431B CN 202010823352 A CN202010823352 A CN 202010823352A CN 111893431 B CN111893431 B CN 111893431B
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- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
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- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/18—Hardening; Quenching with or without subsequent tempering
Abstract
The invention provides 20Cr2Ni4A carburizing steel with high contact fatigue resistance and a preparation method thereof, belonging to the technical field of metal materials. The invention adopts the composite strengthening process of rare earth vacuum carburizing heat treatment and ion implantation technology, namely, rare earth elements are implanted before carburizing 20Cr2Ni4A steel, transition metal elements are implanted after carburizing, the defect density of the 20Cr2Ni4A steel is increased at a certain depth, an ideal channel can be provided for carbon atom diffusion in the carburizing process, the penetration depth and concentration of carbon atoms are improved, and the grain structure of the carburized layer is refined; but also can form a hardening ceramic phase and a nanocrystalline equal strengthening phase, improve the surface hardness, improve the mechanical property of the 20Cr2Ni4A steel and obviously improve the contact fatigue resistance of the surface.
Description
Technical Field
The invention relates to the technical field of metal materials, in particular to a 20Cr2Ni4A carburizing steel with high contact fatigue resistance and a preparation method thereof.
Background
The 20Cr2Ni4A steel is widely applied to manufacturing gears, has the characteristics of high strength, good toughness and high hardenability, and has good comprehensive performance after heat treatment. Because the heavy-duty gear bears the action of larger repeated alternating load for a long time, contact fatigue failure occurs frequently in severe service working conditions, wherein the spalling failure is the main form of hardened tooth surface contact fatigue failure, and the existence of the spalling failure can reduce the bearing capacity and the contact fatigue resistance strength of the gear.
In recent years, researchers at home and abroad have conducted many researches and studies on carburizing heat treatment of 20Cr2Ni4A steel, and have focused on improving the quality of a carburized layer so as to obtain a uniform carburized layer with high hardness and gentle hardness gradient distribution. At present, the carburizing heat treatment method comprises high-temperature carburizing, vacuum carburizing, rare earth carburizing and the like, but the problems of uneven carbon distribution of a vacuum carburized layer, insufficient depth of an effective hardened layer and the like still exist, and the contact fatigue resistance of the obtained 20Cr2Ni4A carburized steel is still to be improved.
Disclosure of Invention
The invention aims to provide 20Cr2Ni4A carburizing steel with high contact fatigue resistance and a preparation method thereof, and the invention increases the defect density of the 20Cr2Ni4A steel at a certain depth by injecting rare earth elements before carburizing the 20Cr2Ni4A steel and injecting transition metal elements after carburizing, thereby not only improving the penetration depth and concentration of carbon atoms, refining the grain structure of the carburizing layer, but also improving the surface hardness, improving the mechanical property of the 20Cr2Ni4A steel and obviously improving the contact fatigue resistance of the surface.
In order to achieve the above object, the present invention provides the following technical solutions:
the invention provides a preparation method of 20Cr2Ni4A carburizing steel with high contact fatigue resistance, which comprises the following steps:
carrying out ion implantation pretreatment on 20Cr2Ni4A steel to obtain a pretreated steel material;
carrying out vacuum carburizing heat treatment on the pretreated steel material to obtain a carburizing steel material;
performing ion implantation post-treatment on the carburized steel material to obtain 20Cr2Ni4A carburized steel with high contact fatigue resistance;
the ion implantation pretreatment adopts an implantation element which is a rare earth metal element, and the ion implantation post-treatment adopts an implantation element which is a transition metal element.
Preferably, the implantation element used in the ion implantation pretreatment is La or Y, and the implantation element used in the ion implantation post-treatment is Ti or Ti + Cr.
Preferably, the operating conditions of the ion implantation pretreatment include: the degree of vacuum was 3.5X 10 -3 Pa, the injection temperature is 15-35 ℃; the implantation energy of La was 100keV and the implantation dose of La was 2X 10 17 ions/cm 2 (ii) a The implantation energy of Y is 105keV, and the implantation dose of Y is 2X 10 17 ions/cm 2 。
Preferably, the vacuum carburizing heat treatment includes carburizing treatment, first tempering, oil quenching, and second tempering, which are performed in this order.
Preferably, the carburizing treatment is carried out at a carbon potential of 1.2%, a temperature of 920 ℃ and a vacuum degree of 1X 10 -5 The carburizing treatment is carried out under the condition of Pa, and the time of the carburizing treatment is 5h; alternate pulse injection of C during the carburization process 2 H 2 And N 2 By pulsing once C 2 H 2 And pulse once N 2 The total number of pulse cycles was 25, which was recorded as 1 pulse cycle.
Preferably, the temperature of the first tempering is 650 ℃ and the time is 3h; the temperature of the oil quenching is 820 ℃; the temperature of the second tempering is 180 ℃, and the time is 3h.
Preferably, when the implantation element of the post-ion implantation treatment is Ti + Cr, the post-ion implantation treatment includes Ti ion implantation and Cr ion implantation performed in sequence.
Preferably, when the implantation element of the post-ion implantation treatment is Ti + Cr, the operating conditions include: the degree of vacuum was 3.5X 10 -3 Pa, the injection temperature is 15-35 ℃; the implantation energy of Ti is 105keV, and the implantation dose of Ti is 2X 10 17 ions/cm 2 (ii) a The implantation energy of Cr is 105keV, and the implantation dosage of Cr is 2X 10 17 ions/cm 2 。
Preferably, when the implantation element of the post-ion implantation treatment is Ti, the operating conditions include: the degree of vacuum was 3.5X 10 -3 Pa, the injection temperature is 15-35 ℃; the implantation energy of Ti is 105keV, and the implantation dose of Ti is 2X 10 17 ions/cm 2 。
The invention provides the 20Cr2Ni4A carburizing steel with high contact fatigue resistance, which is prepared by the preparation method in the technical scheme.
The invention provides a preparation method of 20Cr2Ni4A carburizing steel with high contact fatigue resistance, which comprises the following steps: performing ion implantation pretreatment on 20Cr2Ni4A steel to obtain a pretreated steel material; carrying out vacuum carburizing heat treatment on the pretreated steel material to obtain a carburizing steel material; performing ion implantation post-treatment on the carburized steel material to obtain 20Cr2Ni4A carburized steel with high contact fatigue resistance; the ion implantation pretreatment adopts an implantation element which is a rare earth metal element, and the ion implantation post-treatment adopts an implantation element which is a transition metal element. The invention adopts the composite strengthening process of rare earth vacuum carburizing heat treatment and ion implantation technology, namely, the defect density of 20Cr2Ni4A steel is increased by injecting rare earth elements before carburizing the 20Cr2Ni4A steel and injecting transition metal elements after carburizing, thus not only providing an ideal channel for carbon atom diffusion in the carburizing process, improving the penetration depth and concentration of carbon atoms and refining the grain structure of the carburized layer; but also can form a hardening ceramic phase and a nanocrystalline equal strengthening phase, improve the surface hardness, improve the mechanical property of the 20Cr2Ni4A steel and obviously improve the contact fatigue resistance of the surface.
The results of the examples show that lanthanum and yttrium ion implantation improve the carbon diffusion coefficient by 1.12 times and 1.17 times respectively and the effective hardening depth of the carburized layer is increased by 0.08mm and 0.11mm respectively in the rare earth carburization process with carbon potential of 1.2% at 920 ℃. The rare earth ion injection pretreatment obviously improves the structure of the carburized layer, and the diameters of fine dispersed carbides in the rare earth carburized layer are smaller than 0.32mm. The surface hardness of the carburized layer after lanthanum and yttrium ion implantation is respectively improved by 3.8 percent and 6.0 percent. In the rare earth carburizing process, high-density dislocation defects and Coriolis gas clusters formed on the surface of steel by rare earth become rapid diffusion channels of carbon atoms, and the method has important effects on improving the carbon diffusion coefficient and improving the carburized layer tissue. Meanwhile, in the ion implantation transition metal composite strengthening treatment, ti implantation and Ti + Cr implantation can form a composite strengthening layer on the rare earth carburized layer, and new carbonitride phases of Ti and Cr, including TiC, tiN, tiNC, crC and CrN, exist on the surface of the composite strengthening layer. The phases have the characteristics of high hardness and high strength, and have a remarkable effect of prolonging the contact fatigue life of the 20Cr2Ni4A carburizing steel. Under the same pressing load condition, the surface hardness of the composite reinforced layer is increased, and the plastic deformation resistance is enhanced. After Ti is injected, the nano hardness of the strengthening layer is improved by 7.1 percent, and the elastic modulus is improved by 27.6 percent; after Ti + Cr is injected, the nano-hardness of the composite strengthening layer is improved by 15.1%, and the elastic modulus is improved by 28.7%. The ion implantation strengthening mechanism includes the formation of high-strength new phases at the surface and radiation damage and lattice distortion at sub-surfaces. The carburized layer treated by rare earth carburization and composite strengthening has obviously prolonged service life and obviously reduced service life dispersity.
Drawings
FIG. 1 is a flow chart of a process for preparing a 20Cr2Ni4A carburized steel having high contact fatigue resistance according to example 1 of the present invention;
FIG. 2 is an XPS spectrum of rare earth lanthanum and yttrium implanted 20Cr2Ni4A steel sample surface;
FIG. 3 is a distribution diagram of elements of a 20Cr2Ni4A steel sample after rare earth lanthanum and yttrium are implanted;
FIG. 4 is an XRD pattern of an unimplanted sample, a rare earth lanthanum implanted sample and a rare earth yttrium implanted sample;
FIG. 5 is an XRD pattern of the sample after carburization;
FIG. 6 is a surface and core metallographic structure morphology of a conventional vacuum carburized layer, rare earth lanthanum and yttrium carburized layer;
FIG. 7 is a plot of carbide diameter size distribution in the carburized layer;
FIG. 8 is a graph showing the results of Rockwell hardness measurement of the surface and the core of a carburized layer;
FIG. 9 is a microhardness profile of a carburized layer along the depth direction;
FIG. 10 is a graph of the carbon concentration profile of the carburized layer along the depth direction;
FIG. 11 is a small-angle glancing X-ray diffraction pattern of a carburized sample after implantation of Ti and Ti + Cr ions in a rare earth yttrium and nitrogen atmosphere;
FIG. 12 is an XPS spectrum of Fe, C and Cr at different depths of a rare earth carburized sample;
FIG. 13 is an XPS spectrum of Fe 2p, C1 s, cr2p, ti 2p and N1s at 10nm, 40nm, 80nm depth for a Ti ion implanted carburized sample in a nitrogen atmosphere;
FIG. 14 is an XPS spectrum of Fe 2p, C1 s, cr2p, ti 2p and N1s at 10nm, 40nm and 80nm depths of a carburized sample into which Cr ions are again injected into a sample after Ti ions are injected in a nitrogen atmosphere;
FIG. 15 is a graph of nanoindentation results under a 5mN load;
FIG. 16 is a Weibull distribution curve of contact fatigue life versus probability of failure for a rare earth carburized layer;
FIG. 17 is a scanning electron micrograph of a fatigue specimen surface of a conventional vacuum carburized layer and a rare earth lanthanum carburized layer;
FIG. 18 is a Weibull distribution of contact fatigue life versus probability of failure for a composite reinforcement layer;
FIG. 19 is a graph of surface contact fatigue failure wear marks of a rare earth yttrium carburized layer and a Ti-infused composite strengthening layer;
FIG. 20 is a cross-sectional view of fatigue failure of a rare earth yttrium carburized layer and a Ti + Cr ion implantation strengthening layer after a contact fatigue test.
Detailed Description
The invention provides a preparation method of 20Cr2Ni4A carburizing steel with high contact fatigue resistance, which comprises the following steps:
carrying out ion implantation pretreatment on 20Cr2Ni4A steel to obtain a pretreated steel material;
carrying out vacuum carburizing heat treatment on the pretreated steel material to obtain a carburizing steel material;
performing ion implantation post-treatment on the carburized steel material to obtain 20Cr2Ni4A carburized steel with high contact fatigue resistance;
the ion implantation pretreatment adopts an implantation element which is a rare earth metal element, and the ion implantation post-treatment adopts an implantation element which is a transition metal element.
The method comprises the following steps of carrying out ion implantation pretreatment on 20Cr2Ni4A steel to obtain a pretreated steel material; the implantation elements adopted in the ion implantation pretreatment are rare earth metal elements. The source of the 20Cr2Ni4A steel is not particularly limited in the present invention, and 20Cr2Ni4A steel known to those skilled in the art may be used. In the present invention, the rare earth metal element is preferably La or Y. The method preferably comprises the steps of sequentially grinding, polishing, cleaning and drying the 20Cr2Ni4A steel, and then carrying out ion implantation pretreatment; in the embodiment of the invention, the 20Cr2Ni4A steel is ground by using silicon carbide abrasive paper with the grain size of 600# to 2000# and then polished by using W3.5 diamond polishing paste, then ultrasonically cleaned for 10min by using absolute ethyl alcohol, and the 20Cr2Ni4A steel is dried by using a blower for standby after the cleaning is finished.
In the present invention, the operating conditions of the ion implantation pretreatment preferably include: the degree of vacuum was 3.5X 10 -3 Pa, the injection temperature is 15-35 ℃; the implantation energy of La was 100keV and the implantation dose of La was 2X 10 17 ions/cm 2 (ii) a The implantation energy of Y is 105keV, and the implantation dose of Y is 2X 10 17 ions/cm 2 . In the invention, the injection temperature is more preferably 20-30 ℃, and specifically, the ion injection pretreatment can be carried out at room temperature, i.e. no additional heating or cooling is needed; in the embodiment of the invention, the ion implantation pretreatment is carried out under the condition of 25 ℃.
In the invention, the equipment adopted for the ion implantation pretreatment is preferably an MEVVA (metal vapor vacuum arc ion source) IIA-H type high-current metal ion implanter researched and manufactured by the Low energy Nuclear physics research institute of Beijing university, and the main technical indexes comprise: (1) acceleration voltage: 20-80 kV; (2) ion species: various electrically conductive solid state elemental ions; (3) average ion current intensity: 5-10 mA; (4) average charge state of the ion: 1 to 3.2; (5) pulse length: 1.2ms; (6) pulse repetition frequency: 1-25 Hz; (7) cathode lifetime: is more than 8h; (8) target disk size: phi 220mm.
After the pretreated steel material is obtained, the invention carries out vacuum carburizing heat treatment on the pretreated steel material to obtain a carburizing steel material. In the present invention, the vacuum carburizing heat treatment preferably includes carburizing treatment, first tempering, oil quenching, and second tempering, which are performed in this order. In bookIn the invention, the carburizing treatment is preferably performed at a carbon potential of 1.2%, a temperature of 920 ℃ and a vacuum degree of 1X 10 -5 Pa, and the time of the carburization treatment is preferably 5h; preferably, alternating pulse injection C is performed during the carburizing treatment 2 H 2 And N 2 By pulsing once C 2 H 2 And pulse once N 2 The number of pulse cycles was recorded as 1, and 25 pulse cycles were performed in total. The invention injects C by alternate pulses 2 H 2 And N 2 And is favorable for promoting the uniform diffusion of carbon atoms. In the present invention, the temperature of the first tempering is preferably 650 ℃, and the time is preferably 3 hours; the first tempering is preferably carried out at a higher temperature in the present invention, which is advantageous in promoting carbide precipitation and residual austenite decomposition. In the present invention, the temperature of the oil quenching is preferably 820 ℃; the oil quenching is carried out under the condition, which is beneficial to improving the hardness and the toughness of the material. In the invention, the temperature of the second tempering is preferably 180 ℃, and the time is preferably 3h; and the second tempering is carried out at a lower temperature, so that the internal stress of the material can be eliminated. In the present invention, the vacuum carburizing heat treatment is preferably performed on a vacuum carburizing automatic line (EMC).
After obtaining the carburizing steel material, carrying out ion implantation post-treatment on the carburizing steel material to obtain the 20Cr2Ni4A carburizing steel with high contact fatigue resistance; the implanted elements adopted by the ion implantation post-treatment are transition metal elements. In the invention, the transition metal element is preferably Ti or Ti + Cr, namely the injection element adopted in the post-ion injection treatment can be Ti single element injection or Ti + Cr double element injection; when the implantation element of the post-ion implantation treatment is Ti + Cr, the post-ion implantation treatment preferably includes Ti ion implantation and Cr ion implantation performed in this order. In the present invention, when the implantation element of the post-ion implantation treatment is Ti + Cr, the operating conditions preferably include: the degree of vacuum was 3.5X 10 - 3 Pa, the injection temperature is 15-35 ℃; the implantation energy of Ti is 105keV, and the implantation dose of Ti is 2X 10 17 ions/cm 2 (ii) a The implantation energy of Cr is 105keV, and the implantation dosage of Cr is 2X 10 17 ions/cm 2 . In the present invention, when the implantation element of the post-ion implantation treatment is Ti, the operating conditions preferably include: the degree of vacuum was 3.5X 10 -3 Pa, the injection temperature is 15-35 ℃; the implantation energy of Ti is 105keV, and the implantation dose of Ti is 2X 10 17 ions/cm 2 . In the present invention, the implantation temperature in each ion implantation process is more preferably 20 to 30 ℃, and specifically, the ion implantation post-treatment can be performed at room temperature, i.e., no additional heating or cooling is required; in the embodiment of the invention, the ion implantation post-treatment is carried out under the condition of 25 ℃. In the present invention, the post-ion implantation treatment is preferably performed in a nitrogen atmosphere. In the present invention, the equipment used for the post-ion implantation treatment and the main technical indicators of the equipment are preferably the same as those of the equipment used for the pre-ion implantation treatment and the main technical indicators of the equipment, and are not described again.
The invention adopts the composite strengthening process of rare earth vacuum carburizing heat treatment and ion implantation technology, namely, the defect density of 20Cr2Ni4A steel is increased by injecting rare earth elements before carburizing the 20Cr2Ni4A steel and injecting transition metal elements after carburizing, thus not only providing an ideal channel for carbon atom diffusion in the carburizing process, improving the penetration depth and concentration of carbon atoms and refining the grain structure of the carburized layer; but also can form a hardening ceramic phase and a nanocrystalline equal strengthening phase, improve the surface hardness, improve the mechanical property of the 20Cr2Ni4A steel and obviously improve the contact fatigue resistance of the surface.
The invention provides the 20Cr2Ni4A carburizing steel with high contact fatigue resistance, which is prepared by the preparation method in the technical scheme. The 20Cr2Ni4A carburizing steel provided by the invention has high contact fatigue resistance, is beneficial to prolonging the service life of a workpiece under extreme working conditions and improving the reliability.
The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. It is to be understood that the described embodiments are merely exemplary of the invention, and not restrictive of the full scope of the invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
Example 1
A TXC01 direct-reading spectrometer is adopted to measure the chemical components of a 20Cr2Ni4A steel sample, the main chemical components are shown in Table 1, and the requirement of GB/T3077-1999 is met through verification.
Preparing a sample with a size required by an experiment by adopting linear cutting processing, wherein the size of the sample for observing a microstructure is 15mm multiplied by 15mm; the test specimens used for the contact fatigue test were: a ring sample having an outer diameter of phi 65mm, an inner diameter of phi 30mm and a height of 6 mm.
TABLE 1 chemical composition of 20Cr2Ni4A Steel (mass fraction,%)
As shown in fig. 1, the 20Cr2Ni4A steel sample is processed according to the process flow of ion implantation rare earth (La or Y) pretreatment → vacuum carburization heat treatment → ion implantation transition element (Ti or Ti + Cr) post-treatment, comprising the following steps:
(1) Ion (La or Y) implant pretreatment
And (3) grinding the sample by using silicon carbide abrasive paper with a 600# to 2000# granularity, then polishing by using W3.5 diamond polishing paste, then ultrasonically cleaning for 10min by using absolute ethyl alcohol, and drying the sample by using a blower after cleaning for later use.
The method adopts a MEVVA (metal vapor vacuum arc ion source) IIA-H type high-current metal ion implanter researched and manufactured by the Low-energy nuclear physics research institute of Beijing university of education to perform ion implantation pretreatment.
The main technical indexes of the MEVVAIA-H type high-current metal ion implanter include: (1) acceleration voltage: 20-80 kV; (2) ion species: various electrically conductive solid state elemental ions; (3) average ion current intensity: 5-10 mA; (4) average charge state of ions: 1 to 3.2; (5) pulse length: 1.2ms; (6) pulse repetition frequency: 1-25 Hz; (7) cathode lifetime: is more than 8h; (8) target plate size: phi 220mm.
The operating conditions of the ion implantation pretreatment are shown in table 2.
TABLE 2 operating conditions for ion (La or Y) implant pretreatment
(2) Vacuum carburization heat treatment
The whole vacuum carburization heat treatment process is finished on a vacuum carburization automatic production line (EMC), and the operating steps and parameters of the vacuum carburization heat treatment are as follows:
putting the sample obtained in the step (1) in a vacuum environment (1 multiplied by 10) with the carbon potential of 1.2% and the temperature of 920 DEG C -5 Pa) for 5h; wherein alternating pulses inject C during processing 2 H 2 And N 2 By pulsing once C 2 H 2 And pulse once N 2 Recording as 1 pulse cycle, and carrying out 25 pulse cycles in total;
after the treatment is finished, tempering at 650 ℃ for 3h, then carrying out oil quenching at 820 ℃, and finally tempering at 180 ℃ for 3h to finish vacuum carburization heat treatment.
(3) Ion (Ti or Ti + Cr) implantation post-treatment
And (3) performing ion implantation post-treatment on the sample obtained in the step (2) by referring to the method of ion implantation pre-treatment, wherein the operating conditions of the ion implantation post-treatment are shown in Table 3.
TABLE 3 operating conditions for post-ion (Ti or Ti + Cr) implant treatment (N) 2 In the atmosphere)
Characterization and Performance testing
1. Influence of ion implantation pretreatment on carburized layer structure and performance
1.1 organization structure and mechanical property of rare earth carburized layer
1.1.1 chemical composition and texture of rare earth implanted layer
The XPS spectrum of the surface of a 20Cr2Ni4A steel sample implanted with rare earth lanthanum and yttrium is shown in FIG. 2, wherein (a) in FIG. 2 is a La full spectrum, (b) is a La 3d spectrum, (c) is a Y full spectrum, and (d) is a Y3 d spectrum; wherein Ar is used in advance before XPS spectrum characterization + And removing dust on the surface of the sample by ion etching for 1 min. From (a) and (C) in fig. 2, it is clear that the XPS full spectrum contains La, Y, fe, C, O, and Si. The absorption peak of C element on the surface of the sample is measured to be 285.2eV, which is 0.4eV higher than the peak 284.8eV of the standard C element. The maps of La and Y after adjustment are shown as (b) and (d) in fig. 2, respectively, taking into account the measurement error. FIG. 2 (b) shows that three peaks of La 3d surface correspond to La 3d 3/2 (851.5 eV) and La 3d 5/2 (835.1 eV); the spin-orbit splitting value is 16.4eV, which is typical of La 2 O 3 . FIG. 2 (d) shows that the peaks at 158.0eV and 155.9eV in the Y3 dXPS spectrum correspond to Y-Y bonds, and the peak at 157.4eV corresponds to Y 2 O 3 . Thus, the rare earth lanthanum is present in the rare earth implanted layer mainly in the form of an oxide, while the rare earth yttrium is present in the rare earth implanted layer in the form of an oxide and the metal Y.
The distribution of the elements after rare earth lanthanum and yttrium implantation into a 20Cr2Ni4A steel sample is shown in FIG. 3, where (a) in FIG. 3 is the result of La implantation and (b) is the result of Y implantation. It can be seen from fig. 3 that the elemental carbon and oxygen contents of the sample surface are higher, and decrease with increasing depth, wherein the elemental lanthanum and yttrium contents tend to increase and decrease with increasing depth, and reach a minimum at a depth of about 60 nm. Thus, the implanted rare earth elements satisfied a gaussian distribution with peak concentrations of lanthanum and yttrium at depths of about 15nm and 25nm, at concentrations of 22.9wt.% and 21.2wt.%, respectively.
XRD patterns of the non-injected sample (original 20Cr2Ni4A steel sample), the rare earth lanthanum-injected sample and the rare earth yttrium-injected sample are shown in FIG. 4, wherein (a) in FIG. 4 is an XRD diffraction spectrum, (b) is a (110) peak diffraction angle, (c) is a (200) peak diffraction angle, and (d) is a (211) peak diffraction angle. As can be seen from fig. 4, the structure of the unimplanted sample is mainly ferrite of a body-centered cubic structure; due to the larger atomic radius of rare earth and the internal stress introduced by ion implantation, three diffraction peaks of alpha-Fe are shifted to the left, and the peak intensity is increased, which means that lattice distortion occurs on the surface of the implanted substrate.
The transmission electron microscope characterization results of the samples without injection, the samples after rare earth lanthanum injection and the samples after rare earth yttrium injection show that a certain amount of crystal defects such as dislocation entanglement, dislocation grids and the like exist in the 20Cr2Ni4A matrix, and a stable defect network is formed. Rare earth lanthanum and yttrium ion implantation causes lattice damage to the Body Centered Cubic (BCC) Fe matrix. The rare earth ion implanted samples formed high density dislocation entanglement compared to the unimplanted samples. Particularly rare earth yttrium ions, are implanted, the matrix is almost filled with high density dislocations, which are piled up and tangled together.
1.1.2 phase composition and texture of carburized layer
The XRD pattern of the carburized sample is shown in fig. 5, in which (a) XRD diffraction spectrum, (b) is (110) peak diffraction angle, (c) is (200) peak diffraction angle, and (d) is (211) peak diffraction angle in fig. 5. As can be seen from FIG. 5, the carburized layers of the surface layers of the three samples each consist of a BCC martensite phase and FCC retained austenite. Due to the limited spatial resolution of X-ray diffraction, lanthanum and yttrium diffraction peaks were not tested. Analysis found that the strongest diffraction peaks in the lanthanum and yttrium implanted samples were shifted to the left by 0.26 and 0.20, respectively, and the other two martensite peaks were shifted to the left, as compared to the conventional vacuum carburized layer (i.e., no rare earth element implanted). In addition, all of the crystal planes of the carburized layer after lanthanum and yttrium implantation are stronger than those of the unimplanted sample. The XRD measured the retained austenite content on the carburized layer surface as shown in table 4, and the results show that the retained austenite content in the carburized layer is slightly decreased due to the presence of rare earth.
TABLE 4 residual Austenite content of carburized case of three samples measured by X-ray diffraction
The metallographic structure and morphology of the surface and the core of the conventional vacuum carburized layer, the rare earth lanthanum and yttrium carburized layer are shown in fig. 6, (a) and (b) are the surface and the core of the conventional vacuum carburized layer, (c) and (d) are the surface and the core of the lanthanum carburized layer, and (e) and (f) are the surface and the core of the yttrium carburized layer. Fig. 6 (a) shows that the microstructure of the conventional vacuum carburized layer is composed of a matrix of martensite, and particulate carbides and residual austenite that has not been transformed into martensite are dispersed in the carburized layer. However, ultra-fine acicular martensite and fine dispersed-grain carbides are obtained in the carburized layer after ion-implantation of lanthanum and yttrium, respectively, as shown in (c) and (e) of fig. 6. As shown in (b), (d) and (f) of fig. 6, the core microstructures of the three samples are tempered martensite and a small amount of ferrite, according to the metallographic standard of GB/T25744-2010, the core structure of the conventional vacuum carburized layer reaches the standard core structure level 4, and the core structure of the carburized layer after rare earth injection meets the standard level 3 requirement. In the invention, the rare earth atoms are beneficial to the precipitation of fine carbides in austenite grains in the carburizing process, the existence of the carbides enables the martensite to be blocked and forced to be superfine, and finally, a good structure consisting of superfine martensite, a small amount of retained austenite and a large amount of fine dispersion carbides is formed on the carburized surface layer. Therefore, the structure of the carburized layer after rare earth injection is more refined.
As can be seen from scanning electron microscope characterization of carbides in the carburized layer, in the conventional vacuum carburized layer, most of the carbides are granular and unevenly distributed in martensite interstitial spaces, accompanied by large-diameter rod-like carbides. Compared with the conventional vacuum carburized layer, the carbide in the carburized layer after the rare earth is injected is more uniformly distributed. Wherein, the carbide in the carburized layer after the rare earth yttrium is injected is the finest. The carbide mainly comprises carbon, iron, chromium, nickel and manganese, and the carbide of the carburized layer after rare earth injection contains lanthanum and yttrium.
And estimating the size distribution of the carbide particles by using Nano-Measurer1.2 software and Gaussian fitting to ensure that at least 150 particles in each scanning electron microscope image are counted. The carbide particles in the sem image of fig. 6 are marked and statistics are given, and the distribution diagram of the diameter size of the carbide in the carburized layer is shown in fig. 7, where (a) in fig. 7 is a conventional vacuum carburized layer, (b) is a carburized layer after La implantation, and (c) is a carburized layer after Y implantation. The results showed that the average diameter of carbides in the conventional vacuum carburized layer was 0.35 μm, with 81.2% of the carbides having a diameter within 0.6 μm and a portion of the bulk carbides having a length greater than 5 μm. The average diameters of the carbides on the surface of the carburized layer after ion implantation of lanthanum or yttrium and vacuum carburization of 20Cr2Ni4A steel specimens were 0.25 μm and 0.17 μm, respectively. Thus, ion implantation of rare earth reduces the grain size of the carbide on the carburized layer surface. The yttrium atom radius is larger than lanthanum, so the yttrium ion implantation effect is better than the lanthanum ion implantation effect, the minimum particle size of carbide in the carburized layer is 0.06 μm, and the maximum particle size is 0.36 μm. The fact proves that the ion implantation pretreatment plays an important role in refining the structure of the carburized layer and promoting the dispersion and precipitation of fine carbides, and is beneficial to improving the surface hardness and the mechanical property of the carburized layer.
1.1.3 hardness gradient distribution of carburized layer
The fine carbides dispersed in the carburized layer play a role of strengthening a second phase, and have favorable influence on the prevention of dislocation movement, the enhancement of deformation resistance of the carburized layer and the improvement of the hardness of steel. The results of rockwell hardness measurements of the carburized layer surface and the core are shown in fig. 8. As shown in FIG. 8, the surface hardness of the conventional 920 ℃ vacuum carburized layer is 58.3HRC, after lanthanum element is injected, the surface hardness of the carburized layer can reach 60.5HRC, and after yttrium element is injected, the surface hardness of the carburized layer is improved to 61.8HRC. In contrast, the core hardness values do not vary much. The smaller the value of the error bar in the figure, the less significant the difference in the range of variation of the surface hardness. Therefore, rare earth injection can improve the surface hardness of the carburized layer, wherein the carburization effect is more remarkable after yttrium injection.
The microhardness profile of the carburized layer in the depth direction is shown in FIG. 9, in which the fluctuation value of the error bar does not exceed 25HV 1 . Experimental results show that the microhardness of the carburized layer after the rare earth ions are injected is higher than that of a conventional vacuum carburized layer under the condition of 920 ℃, and the hardness gradient does not change violently. The maximum hardness of the carburized layer after La and Y ion implantation is 805HV 1 And 822HV 1 Higher than the hardness value of the conventional vacuum carburized layer. According to the standard of GBT 9450-2005 'determination and check of the depth of the effective hardened layer of carburization quenching of steel parts',the distance from the Vickers hardness of the specified section to the surface is greater than 550HV and is the effective hardened layer thickness, and on the basis, the following can be known: the thickness of the conventional vacuum carburized layer is about 1.36mm, the carburized layer can reach 1.44mm after La injection, and the carburized layer can reach 1.47mm after Y injection (as shown by the arrows in FIG. 9). Thus, rare earth ion implantation increased the hardness of the carburized layer of 20Cr2Ni4A steel, but did not cause a significant increase in the effective hardened layer depth. And yttrium is injected to carry out vacuum carburization heat treatment to obtain a carburized layer with highest microhardness and most uniform hardness gradient distribution.
1.2 calculation of carbon diffusion coefficient
The carbon concentration profile of the carburized layer in the depth direction is shown in fig. 10. As can be seen from the graph, the carbon concentration of the surface of the conventional vacuum carburized layer is about 0.84%, and the trend of the carbon concentration change is less uniform than that of the rare earth implantation. After rare earth is implanted into ions, the carbon concentration on the surface of the carburized layer reaches over 0.9 percent, and the gradient change of the carbon concentration is more gradual. The carbon distribution measured here was approximately the same trend as the hardness gradient distribution of the three differently treated carburized specimens in subsection 1.1.3.
The vacuum carburization uses acetylene gas as a carburization source, and the acetylene gas is decomposed through a chemical reaction formula (1-1) to generate hydrogen and carbon atoms.
C 2 H 2 →2[C]+H 2 +53.5kcal (1-1)
Assuming that the diffusion coefficient of carbon is independent of the carbon concentration, the diffusion of carbon in austenite can be regarded as two-dimensional diffusion in the carburizing process, and since there is a concentration gradient in the carburizing process, the diffusion proceeds from the high concentration region to the low concentration region, and the diffusion flux is proportional to the concentration. According to Fick's second law, the carbon concentration distribution during carburization follows the formula (1-2):
in the formula: c (x, t) is the volume concentration of carbon, x is the distance from any point in the steel substrate to the surface, t is the diffusion time, and D is the carbon diffusion coefficient.
As the carburizing time is extended,carbon concentration c of steel substrate surface s From the original carbon content c 0 Gradually increases to a constant level and is in contact with the carbon potential c of the furnace gas p The balance is approached, which indicates that the surface of the steel matrix enters a carbon diffusion stage. At the beginning of the carburization process, the initial and boundary conditions of equation (1-2) are labeled c 0 = (x, 0) and c s = (0,t). Fick's second law provides a carbon diffusion concentration curve whose solution is as in equation (1-3):
in the formula: c. C x Is the concentration of carbon at a distance x from the surface,
in order to be a function of the error,
is a constant, and therefore, the relation (1-4) of the depth of the hardened layer to the diffusion time can be obtained:
wherein K is a constant.
Therefore, on the basis of the carburized effective hardened layer depth obtained from FIG. 9, under the same conditions of carburization temperature and time for the three samples, the diffusion coefficient relationships (1-5) and (1-6) for the rare earth-containing or rare earth-free carburized layer can be obtained from the equations (1-4):
in the formula D 920℃ 、
And
represents the diffusion coefficient of carbon element in the low-carbon steel after the rare earth is injected or not when the vacuum carburization heat treatment is carried out at 920 ℃.
Finally, relational expressions (1 to 7) and relational expressions (1 to 8) are obtained:
the rare earth increases the depth of an effective hardened layer by increasing the diffusion coefficient of carbon atoms, and accelerates the process of carburizing heat treatment. When the vacuum carburization temperature was 920 ℃ and the carbon potential was 1.2%, the carbon diffusion coefficients of the lanthanum-and yttrium-implanted samples were 1.12 times and 1.17 times, respectively, that of the non-implanted samples.
2. Effect of post-treatment of ion implantation on carburized layer structure and performance
2.1 organization structure of composite strengthening layer
2.1.1 phase Structure of composite Strength layer
FIG. 11 is a small angle grazing X-ray diffraction (GIXRD) pattern of a carburized sample after implantation of Ti and Ti + Cr ions in a rare earth yttrium and nitrogen atmosphere. As can be seen from the figure, the martensite peaks in the carburized layer are approximately at 44.9 ° (α ' (110)), 63.1 ° (α ' (200)) and 82.3 ° (α ' (211)) in 2 θ. After Ti and Ti + Cr ion implantation, the diffraction peak of martensite shifts to a high angle. In the carburized layer, the solid solution atomic radius (70 pm) of N atoms is smaller than that of metal atoms, such as Fe with an atomic radius of 117pm. The α '(110) peak in the Ti and Ti + Cr ion-implanted modified layer was broadened and split into two peaks, as compared to the GIXRD pattern of the rare earth yttrium carburized layer (i.e., no transition metal element implanted), and the cause of this phenomenon may be lattice expansion of α' phase and formation of nitrides and carbides. In the Ti ion implantation layerThree new diffraction peaks appear at 38.6 degrees, 44.6 degrees and 45.7 degrees, which are TiN and TiC peaks respectively and are shifted to a high-angle direction, and the phenomenon is found to be related to the change of lattice parameters after nitrogen ions penetrate into the surface of the carburized steel. Similarly, diffraction peaks at 39.6 °, 44.8 ° and 45.9 ° in the Ti + Cr ion-implanted layer correspond to TiC and Cr, respectively 7 C 3 And a CrN phase. Therefore, it can be seen from the above analysis that, due to the high energy of the transition element injection process, the injected Ti and Cr atoms enter the iron unit cell, causing lattice distortion and internal stress inside the material, and forming compounds of carbon and nitrogen to improve the surface properties of the carburized layer.
2.1.2 element distribution and valence state of composite strengthening layer
The XPS spectra of Fe, C and Cr at different depths of the rare earth carburized sample are shown in FIG. 12, in which (a) is Fe, (b) is C, and (C) is Cr in FIG. 12. As can be seen from (a) in FIG. 12, five peaks of iron were analyzed at the sample surface, of which 2p at 706.82eV and 719.87eV 3/2 And 2p 1/2 The peaks are the 2p peak of Fe, 2p at 710.50eV and 724.08eV 3/2 And 2p 1/2 Peaks are respectively Fe 2 O 3 And 2p at 713.53eV 3/2 The FeS peak corresponding to FeS is due to the presence of the impurity S element in the 20Cr2Ni4A steel. At 10nm, 40nm and 80nm, the Fe peak has a composition except Fe and Fe 2 O 3 Out of the 2p peak of FeS, 2p of FeS 3/2 The peak disappeared and Fe, roughly at 712.25eV, appeared 3 C 2p 3/2 Peak(s). In addition to this, it is clear that the intensity of the iron peak increases with increasing depth. This is because the oxygen concentration of the compound formed with iron gradually decreases as the depth increases. As can be seen from FIG. 12 (b), the main chemical bond composition of carbon is C-C bond, C-O bond and C-H-O bond, wherein Fe consisting of Fe-C bond is formed at 10nm, 40nm, 80nm 3 C, consistent with XPS analysis of Fe. As shown in FIG. 12 (c), the main chemical bond composition of Cr is a Cr-Cr bond and a Cr-O bond. 2p at the sample surface and at a depth of 10nm, approximately 576.50eV and 586.50eV 3/2 And 2p 1/2 Peaks are respectively Cr 2 O 3 2p peak of (1); 2p at 40nm and 80nm, approximately 574.60eV and 584eV 3/2 And 2p 1/2 The peaks are the 2p peak of Cr, respectively.
FIG. 13 is an XPS spectrum of Fe 2p, C1 s, cr2p, ti 2p and N1s at 10nm, 40nm and 80nm depths of a Ti ion implantation carburized sample in a nitrogen atmosphere, and in FIG. 13, (a) is Fe, (b) is C, (C) is Cr, (d) is Ti and (e) is N. Fig. 13 (a) shows that the bond type of iron is not significantly different at different depths, and Fe — S bonding is present at the surface and 10nm, and Fe — C bonding is not present at 40nm and 80nm, compared to fig. 12 (a). As the O content decreases with depth, the iron oxide peak shifts to a lower binding energy, i.e., the binding energy corresponding to Fe-O. FIG. 13 (b) shows that the bond type of C-C bond, C-O bond and C-H-O bond still exists in the surface layer of the sample after Ti ion implantation. However, cr is present at 283.05eV at 10nm depth 7 C 3 The peak of (3) and the peak of 281.82eV are TiC; cr at a depth of about 282.60eV at 40nm and 80nm 2 C 3 Peak of (2). FIG. 13 (c) shows that Cr-N bonds appear at depths of 40nm and 80nm, which corresponds to XPS spectra at 40 and 80nm for N1s in FIG. 13 (e). FIG. 13 (d) shows that the main binding bond types of Ti are Ti-N bond and Ti-O bond. Of these, 2p at the surface, 454.80eV and 460.50eV 3/2 And 2p 1/2 Peak is TiN respectively 0.96 2p peak of (3), 2p at 458.18 and 463.90eV 3/2 And 2p 1/2 Peaks are respectively TiO 2 2p peak of (1); 2p at a depth of 10nm, 455.53eV and 460.73eV 3/2 And 2p 1/2 Peaks are the 2p peak of TiN, 2p at 458.50eV and 464.17eV, respectively 3/2 And 2p 1/2 Peaks are respectively TiO 2 2p peak of (1); at a depth of 40nm, except for TiO 2 And TiN X In addition, 2p at 456.06eV and 462.83eV are found 3/2 And 2p 1/2 Peaks are respectively 2p peaks of Ti-N-C combination type; at a depth of 80nm, the Ti-N-C bond pattern disappears, and 2p at 453.97eV appears 3/2 Is the 2p peak of the Ti-Ti bond. Furthermore, the intensity of the Ti 2p peak increases with increasing implantation depth, indicating that the content of elemental Ti increases with increasing implantation depth.
Injecting Cr ions into the sample after Ti ions are injected again, injecting Cr ions into the carburized sample in the nitrogen atmosphere, and injecting the Cr ions into the surface of the carburized sample at depths of 10nm, 40nm and 80nmThe XPS spectra of Fe 2p, C1 s, cr2p, ti 2p and N1s are shown in FIG. 14, (a) is Fe, (b) is C, (C) is Cr, (d) is Ti and (e) is N. The bonds associated with the Ti 2p and Cr2p peaks are significantly different compared to fig. 13. After the re-injection of Cr, no Fe-C bond was found, and as shown in (a) of FIG. 14, the main bond type composition of Fe was Fe 2 O 3 And FeS. Next, as shown in FIG. 14 (b), cr carbide appears 10nm, 40nm and 80nm below the surface. Further, as shown in (c) of fig. 14, more Cr — Cr bonding bonds occurred, the presence of Cr nitrides was not found at the surface and at 10nm, and peaks corresponding to Cr at 574.80eV were seen at depths of 40nm and 80nm, as compared with (c) of fig. 13 2 N-related peaks. As shown in (d) of FIG. 14, no peak related to Ti-Ti bonds was found, the 2p peak of Ti disappeared, ti-N bonds replaced Ti-Ti bonds to become main bonds, and the peaks appeared at all depths belonging to TiNx and TiO 2 Characteristic peak of (2). The proportion of Cr-Cr bonds is higher and higher as the depth is increased. FIG. 14 (e) shows Cr nitride present at 10nm and at the surface 2 N, tiN nitride with Ti existing at 40nm and 80 nm.
2.1.3 Structure morphology of composite strengthening layer
The transmission electron microscope representation of the rare earth yttrium carburized layer shows that the transmission structure of the carburized layer after rare earth yttrium carburization mainly comprises lath martensite, tempered martensite, retained austenite and partial carbide. Among them, the presence of lath martensite improves the toughness and fracture toughness of the carburized layer, enhances the resistance to fatigue crack initiation and propagation on the surface, and improves the contact fatigue life of parts such as gears.
The transmission electron microscope representation of the sample after the ion implantation of Ti shows that the ion implantation of Ti leads the stress induced martensite phase transformation to occur in the whole plane of the carburized layer, thus generating highly compact twin crystal martensite, wherein the inclusion part is not completely transformed into the residual austenite of the martensite, and part of nanocrystalline phase exists; after Ti ion implantation, the carburized surface layer has a hardened ceramic phase structure, and an amorphous phase exists in the implanted surface layer.
By transmission electron microscopy characterization of the ion-implanted Ti + Cr samples, it was found that, similar to the Ti ion-implanted layer, the Ti + Cr implanted layer also had an amorphous layer, and the CrN phase was found to exist in addition to the carbide and nitride of titanium. And the nanocrystalline grains contain substantially Ti, cr, C, N, O and Fe, with relatively small amounts of Ni and Y elements contained therein. Because the Y element is injected before carburization and continuously diffuses from the surface to the interior along the cross section direction in the carburization process, the Y element is sparsely distributed on the surface.
Compared with the Ti ion implantation process, the energy of the Ti + Cr double-element implantation process is larger, and the interaction between different ions is stronger. The observed crystal defects are therefore denser and the martensitic grains are finer, which contributes to a further increase in toughness and mechanical strength. The high energy of the previous Ti ion implantation process results in more frequent interactions of the different ions, resulting in a stronger collision cascade at lower depths, resulting in a shallower amorphous layer. The second Cr ion implantation will result in more crystal defects. A large number of dislocations form and entangle with each other even across the surface of twin or lath martensite grains, disordering the atomic arrangement, resulting in the generation of more defects. In addition, more ceramic phases are formed in the Ti + Cr injection layer, and the nano CrNx phase is more beneficial to improving the surface strength and the contact fatigue resistance of the material.
2.2 Nano-mechanical Properties of the composite strengthening layer
The nano-indentation technology is used for measuring the nano-mechanical properties of two ion injection layers of Ti and Ti + Cr injected again in a nitrogen atmosphere, the pressing-in speed is 10nm/s, information such as different reinforced injection nano-hardness and elastic modulus is obtained by adopting a load of 5mN, a nano-indentation result graph under the 5mN load is shown in FIG. 15, a load-displacement curve of the carburized steel surface before and after the ion injection under the 5mN load is shown in FIG. 15, and the different stages of the curve respectively correspond to the loading, load-holding and unloading processes. As can be seen from the figure, the indenter of the nanoindenter is pressed into the surface of the carburized steel with 5mN, the maximum indentation depth of the indenter is 0.182 μm, and after unloading, the indentation still has a certain indentation depth of 0.153 μm. After Ti ions are injected into the surface of the carburizing steel, the maximum pressing depth of the pressing head is reduced to 0.135 mu m, and after unloading, the pressing depth of the indentation residue is reduced to 0.107 mu m; after Ti + Cr ions are implanted, the maximum indentation depth of the indenter is reduced to 0.129 μm, and after unloading, the indentation residual indentation depth is reduced to 0.102 μm. This indicates that the ability to resist plastic deformation of the carburized steel surface is significantly improved after the injection of transition metal ions. The nano-hardness and elastic modulus of the carburized steel surface can also be obtained through a load-displacement curve. The invention utilizes the software of the nanoindenter to calculate the nano-hardness and the elastic modulus of the surface of each sample measured under different loads, and draws the histogram of the surface hardness and the elastic modulus of the carburized steel before and after different ion implantations as shown in (b) in figure 15. As shown in fig. 15 (b), under the same press-in load condition, the resistance of the composite strengthening layer to plastic deformation is enhanced, the nano hardness of the surface layer of the rare earth yttrium carburized sample is 9.5GPa, and the elastic modulus is 214.1GPa; the maximum nanometer hardness of the strengthening layer after Ti injection is 13.01GPa, and the elastic modulus is 273.2GPa; the nano-hardness of the Ti + Cr injected composite strengthening layer is 13.98GPa, and the elastic modulus is 275.6GPa. The hardness increases by 0.68GPa and 1.83GPa, respectively, for the nano-scale compared to the unimplanted sample. The existence state of the elements of the secondary injection layer in the part 2.1.2 is combined to know that TiN, tiC hard phases and CrN and CrC ceramic phases are formed on the surface layer of the carburized layer after the Ti and the Ti + Cr are injected, so that the hardness of the injection layer is improved, and the comprehensive performance of the strengthening layer is improved.
3. Contact fatigue properties of 20Cr2Ni4A steel in different processing states
3.1 contact fatigue Life data analysis method
According to the contact fatigue test data of the strengthening layer and by combining GB 10622-89 rolling contact fatigue test method for metal materials, the rolling contact fatigue test data meets Weibull distribution functions of two parameters, so that the reliability analysis is respectively carried out on the contact fatigue life of the rare earth carburized layer and the composite strengthening layer based on the Weibull distribution functions.
Since the contact fatigue life data values show certain dispersion, the fatigue life of the part may be too short or too long due to the influence of various external factors in the test process, so that the data can be eliminated. Ten sets of effective data are obtained under the test conditions that the maximum Hertz contact stress is 1.9531GPa and the rotating speed is 2500 r/min. The contact fatigue life and failure mode of the carburized steel before and after strengthening treatment under the condition of pure rolling point contact are counted, the average life of the rare earth carburized layer and the composite strengthening layer is calculated, and the statistical result is shown in table 5.
TABLE 5 contact fatigue life of 20Cr2Ni4A carburized steel under various strengthening treatment conditions
And respectively processing the contact fatigue life data of different strengthening layers by adopting double-parameter Weibull distribution, and obtaining a P-N curve of the life distribution. The two-parameter Weibull distribution function is:
wherein P (N) is failure probability function, N is life actual value obtained by test, and N is a Beta is the slope of a two-parameter Weibull curve, namely a shape parameter, the value of beta mainly reflects the dispersity of distribution, and when the value of beta is larger, the Weibull dispersity is smaller.
According to the two parameters N of (3-2) and (3-3) α And beta values were evaluated and the results are shown in Table 6.
As can be seen from Table 6, the shape parameters β of the conventional vacuum carburized layer, rare earth La/Y carburized layer, and Ti/Ti + Cr injection-strengthened layer were 6.09, 7.76, 6.52, 8.71, and 8.93, respectively, and the characteristic lifetime N was found to be a In the order of 5.59X 10 6 、7.69×10 6 、7.98×10 6 、8.86×10 6 And 9.50X 10 6 The times of the week. Carburizing with rare earthAnd the ion implantation composite strengthening obviously improves the contact fatigue resistance of the 20Cr2Ni4A steel.
TABLE 6 beta and N of 20Cr2Ni4A carburized steel under different strengthening treatments a Estimated value
3.2 Effect of ion implantation pretreatment on contact fatigue Properties of 20Cr2Ni4A Steel
3.2.1 contact fatigue Weibull distribution Curve
The contact fatigue life Weibull distribution curve (P-N curve) of the contact fatigue life and the failure probability of the rare earth carburized layer is shown in figure 16, under the same external load condition, the contact fatigue failure probability of the carburized layer at any cycle is clear at a glance, the Weibull distribution better represents the contact fatigue life evolution rule of the carburized layer before and after the rare earth carburization, and the rare earth carburized layer can bear more load cycles under the same failure probability. In addition, the dispersion degree of the contact fatigue life of the rare earth carburized sample is obviously lower than that of the conventional vacuum carburized sample, which shows that the sensitivity of the contact fatigue life to the stress is reduced after the rare earth carburization.
3.2.2 contact fatigue failure characterization
Fig. 17 is a scanning electron microscope image of the fatigue sample surface of the conventional vacuum carburized layer and the rare earth lanthanum carburized layer, and (a) in fig. 17 is the conventional vacuum carburized layer and (b) is the rare earth lanthanum carburized layer. The graph shows that the surface of the sample after failure shows obvious fatigue phenomenon, stripping pits appear, the sample shows an irregular shape, the bottom surfaces of the two stripping pits are uneven, and sharp edges exist; after rare earth carburization, the diameter size of the spalling pit is reduced. The carburized steel surface material is peeled off and rolled to form dents, and the sample which is not carburized by rare earth is easy to fatigue, so that the fatigue life is shorter. After the abrasive grains on the carburized layer surface are stripped under continuous shearing action, obvious surface attached cracks are accompanied, and the cracks are a source for further forming near-surface layer stripping failure.
3.3 Effect of post-treatment by ion Implantation on contact fatigue Properties of 20Cr2Ni4A Steel
3.3.1 contact fatigue Weibull distribution Curve
The distribution curve of the contact fatigue life Weibull of the contact fatigue life and the failure probability of the composite reinforced layer is shown in figure 18. The contact fatigue life corresponding to different injection states, namely the Ti and Ti + Cr composite strengthening layers under different failure probability conditions under the same external load can be predicted through the graph. As can be seen from the figure, the lifetime of the Ti + Cr ion implantation composite strengthening layer is longer than that of the Ti ion implantation layer, and the sensitivity of the contact fatigue lifetime of the Ti + Cr ion implantation composite strengthening layer to the stress is reduced.
3.3.2 contact fatigue failure characterization
The surface contact fatigue failure wear scar of the rare earth yttrium carburized layer and the Ti injection composite strengthening layer is shown in FIG. 19, wherein (a) in FIG. 19 is the rare earth yttrium carburized layer, and (b) is the Ti ion injection composite strengthening layer; the surface of the wear-resistant layer is provided with a plurality of pits, the surface area of a single pit is small, the depth of the single pit is shallow, and the pits are distributed in the range of the width of a contact grinding trace. It can be seen from (a) in fig. 19 that the rare earth yttrium carburized layer fatigue sample surface had pitting pits and deeper spalling pits in the bearing ball wear traces, indicating that the test piece had failed by fatigue. It can be seen from (b) in fig. 19 that only the grinding marks generated by the ball-milling of the bearing exist on the surface of the sample after Ti implantation, the grinding mark area is smooth, shallow pear furrows and a few pitting pits exist, and therefore it can be concluded that the Ti ion implantation composite strengthening layer does not have fatigue failure.
FIG. 20 is a cross-sectional view showing fatigue failure of a rare earth yttrium carburized layer and a Ti + Cr ion implantation strengthening layer after contact fatigue test, wherein (a) in FIG. 20 is a rare earth yttrium carburized layer, (b) is an enlarged view of the position of the spallation pit in (a), (c) is a Ti + Cr ion implantation strengthening layer, and (d) is an enlarged view of the position of the spallation pit in (c). As is apparent from fig. 20 (a), peeling pits and long cracks are observed, and these cracks gradually extend upward to the surface and extend in the rolling direction, and when the cracks reach the surface of the reinforcing layer, the material is peeled off to form peeling pits. From FIG. 20 (b), it was found that secondary cracks propagating deep into the reinforcing layer were present at the bottom of the spalling pit, and the cracks propagated in a dendritic form and finally formedDeeper flaking and damage. While only a slight pitting was observed in the cross-sectional morphology of the Ti + Cr ion-implanted strengthening layer ((c) in fig. 20), as shown in (d) in fig. 20, a fatigue crack initiated downward was also observed at the enlarged pitting, but no connection between cracks was observed. As can be known from the above analysis, at the initial stage of the contact fatigue test, the rare earth yttrium carburized layer and the Ti + Cr ion implantation strengthening layer directly contact the bearing ball of the grinding part and the microprotrusions or the hard phase on the surface of the strengthening layer, the microprotrusions are subjected to micro-fracture and the hard phase is peeled off under the action of shear stress, pitting corrosion occurs on the surface of the strengthening layer, and the formed abrasive dust forms a three-body wear mode between the counter grinding ball and the strengthening layer, so that the pitting corrosion process is accelerated. Meanwhile, under the action of long-time contact cyclic load, large stress concentration is formed around the microscopic defects in the strengthening layer, so that the initiation and the expansion of the microscopic cracks are promoted, and the microscopic cracks are peeled off under the action of shearing force, and finally, the peeling failure is caused. Therefore, it can be seen that the fatigue failure of the rare earth yttrium carburized layer is the result of the combined action of pitting and spallation, and the Ti + Cr ion implantation strengthening layer is operated at 1.125X 10 7 Failure occurred after cycles, and both pitting and cracking were in the initial formation stage. From the tendency of fatigue propagation, significant tearing and wave motion occurred in the fatigue pits, indicating that the matrix spalling at the fatigue point occurred in the form of cohesive tears, which extended forward in the rolling direction. The Ti + Cr ion injection samples form different strengthening regions under different thicknesses, and certain compressive stress is generated on a steel matrix, so that the propagation of cracks is inhibited, and the contact fatigue failure process is delayed. The expression is as follows: on one hand, the surface fatigue crack initiation points are reduced; on the other hand, the propagation of subsurface fatigue cracks is suppressed. The overall effect is to improve the contact fatigue resistance of the case hardening steel, but the propagation pattern of the fatigue failure is not changed, as shown by the crack propagation angle in fig. 20 (d).
The foregoing is only a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, various modifications and decorations can be made without departing from the principle of the present invention, and these modifications and decorations should also be regarded as the protection scope of the present invention.
Claims (7)
1. A preparation method of 20Cr2Ni4A carburizing steel with high contact fatigue resistance comprises the following steps:
performing ion implantation pretreatment on 20Cr2Ni4A steel to obtain a pretreated steel material;
carrying out vacuum carburizing heat treatment on the pretreated steel material to obtain a carburized steel material;
performing ion implantation post-treatment on the carburized steel material to obtain 20Cr2Ni4A carburized steel with high contact fatigue resistance;
wherein, the injection elements adopted in the ion injection pretreatment are rare earth metal elements, and the injection elements adopted in the ion injection post-treatment are transition metal elements Ti + Cr; the ion implantation post-treatment comprises Ti ion implantation and Cr ion implantation which are sequentially carried out; the operating conditions include: the degree of vacuum was 3.5X 10 -3 Pa, the injection temperature is 15-35 ℃; the implantation energy of Ti is 105keV, and the implantation dose of Ti is 2X 10 17 ions/cm 2 (ii) a The implantation energy of Cr is 105keV, and the implantation dosage of Cr is 2X 10 17 ions/cm 2 。
2. The method according to claim 1, wherein the ion implantation pretreatment uses an implantation element of La or Y.
3. The method of claim 2, wherein the ion implantation pretreatment conditions comprise: the degree of vacuum was 3.5X 10 -3 Pa, the injection temperature is 15-35 ℃; the implantation energy of La was 100keV, and the implantation dose of La was 2X 10 17 ions/cm 2 (ii) a The implantation energy of Y is 105keV, and the implantation dose of Y is 2X 10 17 ions/cm 2 。
4. The production method according to claim 1, wherein the vacuum carburizing heat treatment includes a carburizing treatment, a first tempering, an oil quenching, and a second tempering, which are performed in this order.
5. Root of herbaceous plantThe process according to claim 4, wherein the carburizing treatment is carried out at a carbon potential of 1.2%, a temperature of 920 ℃ and a vacuum degree of 1X 10 -5 Pa, and the carburizing treatment time is 5h; alternate pulse injection of C during the carburization process 2 H 2 And N 2 By pulsing once C 2 H 2 And pulse once N 2 The total number of pulse cycles was 25, which was recorded as 1 pulse cycle.
6. The method according to claim 4 or 5, wherein the first tempering is performed at a temperature of 650 ℃ for a time of 3 hours; the temperature of the oil quenching is 820 ℃; the temperature of the second tempering is 180 ℃, and the time is 3h.
7. 20Cr2Ni4A carburized steel with high contact fatigue resistance, prepared by the preparation method of any one of claims 1 to 6.
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