CN111843087A

CN111843087A - Vacuum brazing process method for inhibiting high-temperature nitrogen precipitation in high-nitrogen steel

Info

Publication number: CN111843087A
Application number: CN202010647937.3A
Authority: CN
Inventors: 何鹏; 张墅野; 王星星
Original assignee: Harbin Institute of Technology
Current assignee: Harbin Institute of Technology
Priority date: 2020-07-07
Filing date: 2020-07-07
Publication date: 2020-10-30

Abstract

A vacuum brazing process method for inhibiting high-temperature nitrogen precipitation in high-nitrogen steel. The invention belongs to the field of high-nitrogen austenitic stainless steel brazing. The invention aims to solve the technical problems that nitrogen elements which are supersaturated and dissolved in austenite are precipitated in a nitrogen form due to the melting of the base material near a joint in the traditional fusion welding, so that air holes are formed in a welding seam, and the corrosion resistance and other comprehensive properties of the joint are reduced. The method comprises the following steps: firstly, cutting a test piece; secondly, surface pretreatment; thirdly, preparing a brazing filler metal; fourthly, brazing: and (3) assembling the test piece to be welded and the brazing filler metal paste, preserving heat for 8-12 min at 400-500 ℃, preserving heat for 8-12 min at 740-760 ℃, and then carrying out vacuum brazing at the welding temperature of 850-1050 ℃ for 5-20 min. The invention performs brazing process exploration and performance analysis on the high-nitrogen austenitic stainless steel, thereby avoiding the problem of precipitation of nitrogen element during fusion welding. The interface appearance of the joint brazed under vacuum at the brazing temperature is good. The joint has higher shearing strength, and the highest strength can reach 228.6 MPa.

Description

Vacuum brazing process method for inhibiting high-temperature nitrogen precipitation in high-nitrogen steel

Technical Field

The invention belongs to the field of high-nitrogen austenitic stainless steel brazing; in particular to a vacuum brazing process method for inhibiting high-temperature nitrogen precipitation in high-nitrogen steel.

Background

With respect to the definition of high nitrogen steel, it is currently widely believed that steel having a nitrogen content (mass%) of more than 0.4% in an austenitic matrix or more than 0.08% in a ferritic matrix is called high nitrogen steel, and due to the shortage of nickel resources in the period of shivering, many scholars propose to austenitize the structure with nitrogen element instead of nickel element, nitrogen element is more easily dissolved in solid solution than carbon element in austenite, and precipitation of carbide can be reduced, and the strength and corrosion resistance of steel can be improved. With the development of high nitrogen steel, some outstanding advantages of the steel grade, such as high strength, good toughness, good process performance and excellent corrosion resistance, are found. By replacing nickel element with nitrogen element, the steel grade has good economical efficiency and improved biocompatibility. Therefore, the high-nitrogen steel is widely applied to the fields of electric power, ships, ocean engineering, military equipment, medical instruments and the like at present.

In application, high nitrogen steel is mainly used as a structural member, and is required to have high bearing capacity and high impact resistance in electric power, ships and military equipment. Therefore, in these fields, a fusion welding method such as laser welding or telephone welding is often used as a weak link in the welded joint. In the field of medical instruments and the like, the bearing capacity and impact resistance of high-nitrogen steel as medical austenitic stainless steel are not the first criteria, and the corrosion resistance is rather the key point of the requirements in this respect.

The high nitrogen steel studied by the inventor as austenitic stainless steel has better weldability, and the traditional fusion welding method can carry out welding. However, nitrogen in high-nitrogen steel is supersaturated and is produced under nitrogen-rich pressurized condition during smelting, and the supersaturated nitrogen element is precipitated under ordinary fusion welding conditions. If supersaturated nitrogen is precipitated, the performance of the joint may be somewhat degraded. Therefore, the high nitrogen steel still has certain problems in fusion welding: (1) nitrogen is separated out from the melted joint, so that the nitrogen content of the joint is reduced, and the performance of the joint is reduced; (2) the welding process affects the heat affected zone, resulting in the formation of second phase precipitates in the zone that are detrimental to joint performance; (3) nitrogen is precipitated from the molten parent metal in the form of nitrogen to form nitrogen pores, or nitride precipitates are precipitated in the weld, further degrading the joint performance.

For the welding of high nitrogen stainless steel, at present, the welding method is mainly fusion welding method, such as shielded metal arc welding, TIG welding, laser welding, electron beam welding, etc., and friction stir welding is in a preliminary study state. How to avoid and control the absorption and escape behavior of nitrogen for the fusion welding method is an important research topic for the welding of high nitrogen steel.

Disclosure of Invention

The invention provides a vacuum brazing process method for inhibiting high-temperature nitrogen precipitation in high-nitrogen steel, aiming at solving the technical problems that nitrogen elements which are supersaturated and dissolved in austenite is precipitated in a nitrogen form due to the melting of the vicinity of a base metal joint in the traditional fusion welding, so that air holes are formed in a welding seam, and the corrosion resistance and other comprehensive properties of the joint are reduced.

The vacuum brazing process method for inhibiting the precipitation of high-temperature nitrogen in the high-nitrogen steel is carried out according to the following steps:

firstly, cutting a test piece: cutting the high-nitrogen steel in a wire cut electrical discharge machining mode to obtain a test piece with a required specification;

secondly, surface pretreatment: cleaning the surface of the test piece to remove surface impurities and oxide films;

thirdly, solder preparation: mixing AgCuNi brazing filler metal with ethyl cellulose to obtain brazing filler metal paste;

fourthly, brazing: and (3) assembling the test piece to be welded and the brazing filler metal paste, preserving heat for 8-12 min at 400-500 ℃, preserving heat for 8-12 min at 740-760 ℃, and then carrying out vacuum brazing at the welding temperature of 850-1050 ℃ for 5-20 min.

Further limiting, in the step one, the high-nitrogen steel is austenitic stainless steel with 0.5-1% of nitrogen element by mass percent.

And further limiting, and cleaning the surface of the test piece by adopting a mode of firstly polishing with sand paper and then carrying out ultrasonic cleaning in the step two.

Further limiting, the process of cleaning the surface of the test piece specifically comprises: and sequentially polishing the surface of a test piece to be welded of the high-nitrogen steel by using sand paper of 400#, 800# and 1000#, and then putting the polished test piece into an acetone solution for ultrasonic cleaning.

In the third step, the ratio of the mass of the AgCuNi brazing filler metal to the volume of the ethyl cellulose is (1.5-2.5) g: 1 mL.

In the third step, the ratio of the mass of the AgCuNi brazing filler metal to the volume of the ethyl cellulose is 2 g: 1 mL.

Further limiting, in the third step, the mass percentage of the nickel element in the AgCuNi brazing filler metal is 0.75%.

Further limiting, the AgCuNi brazing filler metal in the third step comprises the following elements in percentage by mass: ag: 71.5%, Cu: 27.75%, Ni: 0.75 percent.

Further limiting, in the fourth step, the temperature is firstly preserved for 10min at 450 ℃, and then preserved for 10min at 750 ℃.

Further limiting, in the fourth step, the welding temperature is 950 ℃, and the heat preservation time is 10 min.

And further limiting, and performing vacuum brazing in a vacuum diffusion welding furnace in the fourth step, wherein the heating mode is vacuum radiation heating.

Compared with the prior art, the invention has the following remarkable effects:

1) the invention performs brazing process exploration and performance analysis on the high-nitrogen austenitic stainless steel, thereby avoiding the problem of precipitation of nitrogen element during fusion welding. The interface appearance of the vacuum brazing joint at the brazing temperature of the invention is good, and the element diffusion area at the high-nitrogen steel base material becomes thicker along with the temperature rise. Along with the increase of the brazing heat preservation time, the thickness of an element diffusion area at the side interface of the base metal is increased, and a brazing filler metal layer is an Ag-Cu eutectic structure.

2) The brazing process parameters have a great influence on the mechanical properties of the joint. The joint has good appearance and high shearing strength at the brazing temperature of the invention, and the highest strength can reach 228.6 MPa.

Drawings

FIG. 1 is a photograph of a microstructure of a welded joint according to an embodiment;

FIG. 2 is a graph of the results of scanning of the elements of FIG. 1;

FIG. 3 is a photograph of a high magnification microstructure of a welded weld joint according to an embodiment;

FIG. 4 is a photograph of the tissue morphology of a welded joint of embodiment three;

FIG. 5 is a photograph of the tissue morphology of a welded joint of the embodiment four welded;

FIG. 6 is a photograph of the tissue morphology of a welded joint of an embodiment five welds;

FIG. 7 is a photograph of the tissue topography of a welded joint according to an embodiment;

FIG. 8 is a photograph of the tissue morphology of an embodiment hexa-welded weld joint;

FIG. 9 is a shear stress plot of a weld joint at different soak times.

Detailed Description

The first embodiment is as follows: the vacuum brazing process method for inhibiting high-temperature nitrogen precipitation in high-nitrogen steel is carried out according to the following steps:

firstly, cutting a test piece: cutting the high-nitrogen steel in a wire electrical discharge machining mode to obtain a test piece with the specification of 10mm multiplied by 20mm multiplied by 4 mm; the high-nitrogen steel is austenitic stainless steel with 0.5 percent of nitrogen element by mass;

secondly, surface pretreatment: sequentially polishing the surface of a test piece to be welded of the high-nitrogen steel by using sand paper of 400#, 800#, and 1000#, and then putting the polished test piece into an acetone solution for ultrasonic cleaning to remove surface impurities and an oxide film;

thirdly, solder preparation: mixing 2g of AgCuNi brazing filler metal with 1mL of ethyl cellulose to obtain brazing filler metal paste;

fourthly, brazing: and (3) assembling the test piece to be welded and the brazing filler metal paste, preserving heat at 450 ℃ for 10min, preserving heat at 750 ℃ for 10min, and then carrying out vacuum brazing by using an M60 type vacuum diffusion welding furnace, wherein the welding temperature is 850 ℃ and the heat preservation time is 20 min.

The second embodiment is as follows: the first difference between the present embodiment and the specific embodiment is: in the fourth step, the welding temperature is 950 ℃. Other steps and parameters are the same as those in the first embodiment.

The third concrete implementation mode: the first difference between the present embodiment and the specific embodiment is: and in the fourth step, the heat preservation time is 10 min. Other steps and parameters are the same as those in the first embodiment.

The fourth concrete implementation mode: the third difference between the present embodiment and the specific embodiment is that: in the fourth step, the welding temperature is 950 ℃. Other steps and parameters are the same as those in the third embodiment.

The fifth concrete implementation mode: the third difference between the present embodiment and the specific embodiment is that: in the fourth step, the welding temperature is 1050 ℃. Other steps and parameters are the same as those in the third embodiment.

The sixth specific implementation mode: the first difference between the present embodiment and the specific embodiment is: and in the fourth step, the heat preservation time is 5 min. Other steps and parameters are the same as those in the first embodiment.

The seventh embodiment: the first difference between the present embodiment and the specific embodiment is: in the first step, the high-nitrogen steel is austenitic stainless steel with 1% of nitrogen element by mass. Other steps and parameters are the same as those in the first embodiment.

TABLE 1 high nitrogen steel chemistry

And (3) detection test:

(one) Joint interface tissue analysis

1. The welded joint and the base metal after the heat preservation treatment according to the first embodiment are observed by a metallographic microscope and a scanning electron microscope to obtain a microstructure photograph of the joint as shown in fig. 1, and as can be seen from fig. 1, the joint has a good interface structure, and no welding defects such as holes and cracks exist at the interface. The joint is mainly divided into an Ag-Cu brazing filler metal layer and a diffusion area at the interface of the brazing filler metal and the base metal, wherein the brazing filler metal layer is mainly divided into a part A of a light color area and a part B of a dark color area according to the contrast difference, and the diffusion area is mainly a partial area generated by diffusion of the high-nitrogen steel base metal and the brazing filler metal in the base metal.

2. In order to qualitatively analyze the composition of each phase at the joint, surface scan analysis of several elements was performed, and the results of the main element scan are shown in fig. 2. The scanning result of the main elements of the interface shows that the main elements of the brazing filler metal area are Ag, Cu and Ni, wherein the part A of the light-colored area in the figure 1 is mostly Ag element and a small amount of Cu and Ni elements; in fig. 1, the portion B in the dark region is mostly Cu, and a small amount of Ag or Ni. The high nitrogen steel base metal side is mainly composed of high nitrogen steel elements such as Fe, Cr, Mn and the like.

3. In order to further determine the diffusion of the reaction products and elements between the brazing filler metal layer and the base metal during the brazing process, the interface between the base metal of the welded high nitrogen steel of the welded joint according to the embodiment and the brazing filler metal is observed in a high magnification microstructure to obtain a high magnification microstructure photograph as shown in fig. 3, and the spectral EDS analysis is performed on each point of the microstructure as shown in the figure to obtain the chemical compositions of each point as shown in table 2.

Table 2 energy spectrum analysis results (at.%) for each point in fig. 3

As is clear from table 2, points A, B in fig. 3 are a Cu-rich phase and an Ag-rich phase, respectively, and point a is mainly Cu and contains a small amount of Ag, Fe, Mn, and the like, indicating that a small amount of Fe and alloying elements in the matrix diffuse into the brazing filler metal, and since a small amount of alloying elements can be dissolved in Cu, point a in the dark region is considered to be a Cu-based solid solution containing a small amount of Ag, Fe, Mn, and is denoted as Cu [ s, s ]. The main element of the B point is Ag element and has a small amount of Cu, Mn and other elements, and the B point in the light color area is an Ag-based solid solution containing a small amount of Cu and Mn and is marked as Ag [ s, s ]. On the other hand, the region A shown in FIG. 3 is the main structure in the middle of the Ag-Cu solder layer, and it is found from the silver-copper phase diagram that the solder component used in the present test is at the silver-copper eutectic point, and therefore this region is considered to be the Ag-Cu eutectic region. The Mn element in the brazing filler metal layer is increased to a certain extent compared with other elements in the base metal, mainly because the Mn element in the base metal can be dissolved in the AgCuNi brazing filler metal, and meanwhile, the dissolution of the Mn element can improve the wettability and the fluidity of the AgCuNi brazing filler metal, and is beneficial to further wetting and spreading of the brazing filler metal. The main element of the C point is Fe element, and a small amount of Cr, Mn alloy elements and N elements exist, wherein the atomic percentage of the nitrogen element is 4.71%, the weight percentage is 1.21%, and the proportion of the whole elements is close to the components of the high-nitrogen steel, so that the area is mainly the high-nitrogen steel base material. Since the main element at point D is Ag, and there are Fe, Cr, Mn, and the like in a proportion close to the base material composition, it is considered that this region is a region where the brazing material is diffused in the high nitrogen steel base material. The E point is the interface of the brazing filler metal layer and the base metal, the element content is relatively complex, and a large amount of N elements and Ni elements are enriched besides Ag and Cu elements and high-nitrogen steel elements. The Ni element of the brazing filler metal containing Ni can form a transition layer at the interface of the brazing filler metal and the base metal in the brazing process, so that the brazing filler metal and the base metal are better combined, and the corrosion resistance can be effectively improved. In contrast, it is presumed that, in the brazing heat-insulating process, a large amount of N elements have a higher diffusion ability of the N elements in the high-nitrogen steel base material, tend to diffuse toward the grain boundary and the interface, and aggregate at the interface, and then precipitate as nitrides with the elements such as Fe, Cr, and Mn at the interface.

4. Observing the welded joint structure morphology after the third to the fifth welding of the specific embodiment to obtain a joint structure morphology chart shown in fig. 4 to 6, and as can be seen from fig. 4 to 6, when the heat preservation time is kept unchanged, the interface structures of the brazed joint obtained at 850 ℃, 950 ℃ and 1050 ℃ are better, the thickness of the brazing filler metal intermediate layer is gradually narrowed along with the increase of the brazing temperature, and meanwhile, the thickness of the transition layer at the interface of the brazing filler metal and the base metal is increased. Wherein, the joint interface structure obtained by heat preservation at 850 ℃ for 10min is shown in figure 4, which is similar to the joint interface structure shown in figure 1 and mainly comprises a brazing filler metal layer, a base metal diffusion region and a transition layer at the interface of the brazing filler metal base metal, but compared with figure 1, the Ag-based solid solution region in the middle region of the welding seam is reduced, and most regions are Ag-Cu eutectic structures. FIG. 5 shows a joint interface structure obtained by heat-insulating at 950 ℃ for 10min, in which the brazing material interlayer region is narrower than the joint interface structure of FIG. 4, but this region has both an Ag-based solid solution structure and an Ag-Cu eutectic structure, and the element diffusion region of the base material is enlarged, and the transition layer between the brazing material and the base material is also enlarged. And similarly, the thickness of a brazing filler metal layer of the joint interface tissue obtained by heat preservation at 1050 ℃ for 10min is further reduced, and a dark transition layer with certain thickness and obvious color appears at the interface of the brazing filler metal and the base metal.

5. Observing the structure and the appearance of the welded joint welded in the first, third and sixth embodiments to obtain the structure and the appearance diagrams of the joint shown in the figures 7-8, wherein the joint interface structures obtained in the three heat preservation times of 5min, 10min and 20min are better when the brazing heat preservation temperature is fixed to 850 ℃, the thickness of the brazing filler metal layer is increased to a certain extent along with the extension of the heat preservation time, and the thickness of the brazing filler metal diffusion area on the base metal side is increased to a certain extent. FIG. 8 shows that in the embodiment, the joint interface structure is obtained when the six-time heat preservation is 5min, all brazing filler metal layers in the welding seam are Ag-Cu eutectic structures, and because the heat preservation time is short and the element diffusion time is too short, the brazing filler metal hardly diffuses in the base metal compared with the joint structures with other parameters. FIG. 4 shows the interface structure of the joint obtained by the third embodiment with 10min of heat preservation time, and it can be seen that the solder layers in the weld are all uniform Ag-Cu eutectic structures, the width of the weld is wide, the joint interface is well bonded, and the base metal side has an element diffusion region with a certain thickness. FIG. 7 shows the joint interface structure obtained by maintaining the temperature for 20min according to the embodiment, in which the solder layer in the weld zone is an Ag-Cu eutectic structure and Ag-based solid solutions distributed therein are generated, because Mn element is dissolved in the solder, so that the Ag-Cu eutectic composition shifts, and a certain amount of Ag-based solid solutions are generated during cooling, and then the Ag-Cu eutectic structure is generated. And the element diffusion area of the brazing filler metal in the base material is increased to a certain extent along with the extension of the heat preservation time.

(II) detection of mechanical Properties

1. The shear stress of the welded joints welded in the second to fourth embodiments was examined, and it was found that the shear stress of the welded joint welded in the second embodiment was 228.6MPa, the shear stress of the welded joint welded in the third embodiment was 186.5MPa, and the shear stress of the welded joint welded in the fourth embodiment was 212.4 MPa.

2. The shear stress of the welded joints according to the first, third and sixth embodiments was measured, and the results shown in fig. 9 were obtained, and it can be seen from fig. 9 that the shear strength was 146.8MPa at the lowest shear strength at 5min, increased with the increase in the holding time, and reached 198.2MPa at 20 min.

Claims

1. A vacuum brazing process method for inhibiting high-temperature nitrogen precipitation in high-nitrogen steel is characterized by comprising the following steps:

2. The vacuum brazing process method for inhibiting the precipitation of high-temperature nitrogen in the high-nitrogen steel according to claim 1, wherein in the first step, the high-nitrogen steel is austenitic stainless steel with 0.5-1% of nitrogen element by mass.

3. The vacuum brazing process method for inhibiting the precipitation of high-temperature nitrogen in the high-nitrogen steel according to claim 1, wherein in the second step, the surface of the test piece is cleaned in a mode of firstly sanding by using sand paper and then ultrasonically cleaning.

4. The vacuum brazing process method for inhibiting the precipitation of high-temperature nitrogen in the high-nitrogen steel according to claim 3, wherein the process of cleaning the surface of the test piece specifically comprises the following steps: and sequentially polishing the surface of a test piece to be welded of the high-nitrogen steel by using sand paper of 400#, 800# and 1000#, and then putting the polished test piece into an acetone solution for ultrasonic cleaning.

5. The vacuum brazing process method for inhibiting high-temperature nitrogen precipitation in high-nitrogen steel according to claim 1, wherein the ratio of the mass of the AgCuNi brazing filler metal to the volume of ethyl cellulose in the third step is (1.5-2.5) g: 1 mL.

6. The vacuum brazing process method for inhibiting the precipitation of high-temperature nitrogen in the high-nitrogen steel according to claim 1, wherein the mass percentage of the nickel element in the AgCuNi brazing filler metal in the third step is 0.75%.

7. The vacuum brazing process method for inhibiting high-temperature nitrogen precipitation in high-nitrogen steel according to claim 1, wherein elements and mass percentages thereof in the AgCuNi brazing filler metal in the third step are as follows: ag: 71.5%, Cu: 27.75%, Ni: 0.75 percent.

8. The vacuum brazing process method for inhibiting the precipitation of high-temperature nitrogen in the high-nitrogen steel according to claim 1, wherein in the fourth step, the temperature is kept at 450 ℃ for 10min, and then at 750 ℃ for 10 min.

9. The vacuum brazing process method for inhibiting the high-temperature nitrogen precipitation in the high-nitrogen steel according to claim 1, wherein the welding temperature in the fourth step is 950 ℃, and the heat preservation time is 10 min.

10. The vacuum brazing process method for inhibiting the precipitation of high-temperature nitrogen in the high-nitrogen steel according to claim 1, wherein the vacuum brazing is performed in a vacuum diffusion welding furnace in the fourth step, and the heating mode is vacuum radiation heating.