CA2939134A1 - Shot material and shot peening method - Google Patents

Abstract

A method of shot peening a workpiece comprising projecting metal alloy particles at a workpiece wherein said metal alloy particles comprises Fe in combination with B, C, Cr and Nb, wherein the Fe is present at a level of greater than 50.0 atomic percent. The metal alloy particles have a Vickers Hardness (HV) of at least 1150 and an elastic modulus of greater than 200 GPa.

Description

Shot Material and Shot Peening Method Cross Reference To Related Applications This application claims the benefit of U.S. Provisional Application Serial No.
61/940,140 filed February 14, 2014.
Field of the Invention The present invention relates to a shot material for shot peening and a shot peening method and a treated article obtained by the process.
Background Shot peening is a mechanical surface treatment having the objective of enhancing the resistance of mostly metallic components or workpieces that are subjected to cyclic loadings, wear, and corrosion and other life time reducing influences. During the shot peening process, the abrasive medium, also called shot medium, is propelled onto the surface of a workpiece.
The impact of the medium dimples the surface of the workpiece. The restoring force below the dimple in the workpiece results in a hemisphere of material that is relatively highly stressed in compression and a compressive residual stress field develops over the peened surfaces as the dimples overlap during the peening process. Such generated compressive stresses can provide a wide variety of benefits to a workpiece. For example, the useful life time of a workpiece may be increased under cyclical loads or stresses caused by corrosion from stress cracking, friction, cavitation, galling, erosion and wear, as well as combinations of these kinds of stresses. It is commonly assumed that these benefits are caused by the presence of the compressive stresses on the surface of the workpiece, since such stresses reduce the tendency of surface crack formation and or crack propagation.
Shot-peening media that have been reported include (1) carbides, carbide composites, or cermets (composite material composed of ceramic (cer) and metallic (met) materials); (2) ceramics such as zirconia that are commercially available for example as ceramic beads or ceramic shot such as Microblast B-120 or Zirblast B-30 or B-400 from Saint Gobain.

Metallic based peening media are reported in U.S. Patent No. 6,658,907. An iron-based amorphous spherical particle is employed as the peening material that is said to preferably have an iron content of 45 to 55 wt. %, having a Vicker hardness (HV) in the range of 900-1100 and a Young's modulus of 200,000 MPa or less.
U.S. Patent Application Publication No. 2011/0265535 discloses an iron-based shot peening material which comprises in mass % 5 to 8% of B, 0.05-1% of C, 0 to 25% of Cr, balance of Fe and inevitable impurities, wherein B and C are contained in a total amount of 8.5% or less. B contents of less than 5% was described as providing insufficient hardness.
W02009/133920A1 discloses an iron-based shot material made of B (5-8 mass%), Al (10 mass% or less, preferably 0.5-10 mass%), Cr (0-25 mass%, preferably 1-25 mass%) and the remainder is Fe and unavoidable impurities. The HV reportedly ranged from 1150-1300.
W02012/128357 discloses a shot peening material containing, in mass %, 2-8% of B
and at least one element selected from Ti, Cr, Mo, W, Ni, Al and C in the amount fulfilling the formula: 0 ( Ti% / 10) + (Cr% / 25) + (Mo% / 10) + (W% / 6) + (Ni% / 10) +
(Al% /
10) + (C% / 1) 1. 00, and the remainder made up by Fe and unavoidable impurities and having a particle diameter of 75 p m or less.
Summary A method of shot peening a workpiece comprising projecting metal alloy particles at a workpiece wherein said metal alloy particles comprises Fe in combination with B, C, Cr and Nb, wherein the Fe is present at a level of greater than 50.0 atomic percent wherein said metal alloy has a Vickers Hardness (HV) of at least 1150 and an elastic modulus of greater than 200 GPa. The present invention also includes a workpiece that is treated by the above referenced method.
The present invention also relates to the shot peening material itself comprising metal alloy particles containing Fe in combination with B, C, Cr and Nb, wherein the Fe is present at a level of greater than 50.0 atomic percent wherein said metal alloy has a Vickers Hardness (HV) of at least 1150 and an elastic modulus of greater than 200 GPa.

2

3 Brief Description of the Drawings The detailed description below may be better understood with reference to the accompanying figures which are provided for illustrative purposes and are not to be considered as limiting any aspect of the invention.
FIG. 1 is a schematic illustration of the air-blasting cabinet for shot-peening.
FIG. 2 is a plot of comparative peening intensity saturation curves.
FIG. 3 is a plot of comparative residual stress fields for the identified workpiece.
FIG. 4 is a plot of comparative workpiece hardness values.
FIG. 4A is plot of residual stress fields for the identified workpiece.
FIG. 4B is a plot of comparative workpiece hardness values.
FIG. 4C is a comparative plot of peening intensity saturation curves.
FIG. 4D is a plot of residual stress fields for the identified workpiece.
FIG. 4E is a comparative plot of comparative workpiece hardness values.
FIG. 4F is a comparative plot of peening intensity saturation curves.
FIG. 5 is a comparative plot of peening intensity saturation curves.
FIG. 6 is a plot of comparative peening intensity saturation curves.
FIG. 7 is a plot of comparative workpiece hardness values at near peening intensity saturation.

FIG. 8 is a plot of comparative mass loss for the indicated particles.
FIG. 9 is a plot of comparative mass loss for the particles of the present invention at 1.0 hours and 6.0 hours versus Saint Gobain B120 particles.
FIG. 10 is a comparative plot of mass loss.
FIG. 11 is a comparative plot of peening intensity saturation curves.
FIG. 12 is a comparative plot of peening intensity saturation curves.
FIG. 13 is a plot of comparative workpiece hardness values at near peening intensity saturation.
Detailed Description As noted above, the present invention relates to a shot peening media that provides relatively high hardness and durability. The alloy is generally understood as Fe based and includes B, C, Cr and Nb. Optional elements include Mn, Si and V. Reference to Fe based may be understood as the feature where the majority of the alloy composition comprises iron (e.g., > 50.0 atomic percent Fe). In addition, the alloy is one that preferably includes a-Fe (ferrite) and/or 7-Fe (austenite). The alloy also preferably includes one or more of the following: (1) complex borides (e.g. KB, M2B and M3B where M is a transition metal); (2) complex carbides (e.g. MX, M2C, M3C, and M23C6 wherein M is a transition metal); (3) borocarbides (material containing both boride and carbide atoms).
Preferably the alloy composition is comprised of the concentrations identified in Table 1 below:
Table 1 ¨ Preferred Alloy Composition Units Fe B C Cr Nb Mn Si Atomic 59.0-64.0 17.5-18.8 4.4-5.1 12.7-13.1 1.4-1.7 0-0.5 0-1.3 %
Weight 74.2-77.6 4.1-4.5 1.2-1.4 14.3-15.1 2.9-3.5 0-0.6 0-0.8 %

4 In addition to the above, the alloys herein may also preferably have the following composition, in atomic percent: Fe (58.0-65.0); B (14.0-19.0); C (4.0-5.5); Cr (7.0-13.5); Mn (0-1.5);
Nb (1.4-3.5); Si (0-1.5) and V (0-6.0).
The alloys herein are preferably prepared as particles by atomization methods.
Exemplary atomization procedures include gas atomization, centrifugal atomization or water atomization. The particles may then be sized using various techniques such as screening, classification and air classification. Preferably, the particles are such that 95% of the particles have a size range (largest linear dimension through the particle) in the range of 40 p.m to 250 p.m.
Accordingly, about 5% of the particles may fall outside this range and indicate a particle size distribution in the range of 0.1 ¨ 39.9 p.m. More preferably, the D10 value in microns (percent of the population having particle sized below this number) is 50.0 p.m. The preferred D50 value (median) is 80 p.m and the preferred D90 value (90 percent of the distribution below this number) is 150 p.m.
In addition, the D10 value in microns may fall in the range of 50-100, the D50 value may fall in the range of 100-150 and the D90 value may fall in the range of 150-200.
In connection with the particles noted above, preferably, the particles have a spherical geometry. This may be understood as a shape having a set of points that are all the same distance r from a given point in three-dimensional space. The value of r is the radius.
Relatively less spherical configurations that are also contemplated include relatively more angular shapes that are referred to as grits.
The alloys are such that the particles indicate a HV value of at least about 1150.
Preferably, the HV value may be in the range of 1150-1400. More preferably, the HV value is 1250 +/- 75. Accordingly, the HV value may preferably be in the range of 1175 to 1400.
In addition, the particles are such that they have an elastic modulus of greater than 200 GPa, more preferably in the range of greater than 200 GPa up to 350 GPa.
Another feature of the particles herein is their associated durability. In that context the durability herein was characterized by projecting the particles towards a workpiece under the following range of conditions: projection pressure of 0.13 MPa to 0.82 MPa, a peening velocity of 80 m/s to 350 m/s and an Almen A intensity of 2-12 mils. The distance to the workpiece is in the range of 76 ¨ 153 mm. The workpiece and media are then preconditioned for a period of 15 minutes prior to testing. The workpiece is a 6.35 mm thick steel alloy containing about 13% manganese having a hardness of 697 Vickers. The metal particles after

5 such projection on the workpiece for a period of 18 hours is such that, for a 100 gram portion of the particles, the weight fraction of particles of less than 75 p m that are present is less than or equal to 7.0%.
The particles herein may be employed in two different types of equipment devices that are used to impart kinetic energy into the particles, such as air nozzle-type systems or centrifugally wheel based system. The examples listed herein were conducted by an air nozzle system (FIG. 1). More specifically, a Kelco air blasting cabinet was employed with the following parameters: nozzle type: venturi with an exit diameter of 7.9375 mm; distance to workpiece of 101.6 mm; mass flow rate of 2.63 kg/minute. However it is contemplated that all of the disclosed benefits would be observed for wheel based systems. In addition, the particles herein, while applied in the examples to workpieces having the indicated characteristics, would be applicable to any workpiece where the advantages of treating with impacting particles would be of benefit.
Working examples of the present invention, in comparison to various shot-peening media, are supplied below. However, it may now be appreciated herein that the relatively high hardness and durable shot-peening media herein will be applicable for processes other than shot peening, including, but not limited to de-sanding, de-scaling, and etching- prior-to-coating of any workpiece as well as for water jet or other cutting and sawing applications.
Example 1 & 1A
A metallic abrasive media of the invention (Example 1) was adopted as a peening media and tested as such. The spherical iron-based particle is comprised of 12.7 at. %
chromium, 18.8 at. % boron, 1.5 at. % niobium, 4.6 at. % carbon, 0.3 at. %
manganese, 0.8 at. % silicon, and the remainder (61.3 at. %) iron, having a specific gravity of 7.36 g/cm3, a hardness of 1284 Vickers. The particles had a particle size distribution of D10 equaling 52 micrometers, D50 equaling 83 micrometers, and D90 equaling 142 micrometers.
Other metallic abrasive media of the invention were also prepared.
Specifically, Example 1A is comprised of 7.77 at. % chromium, 14.73 at. % boron, 2.68 at. %
Nb, 5.45 at.
% C, 1.17 at. % Si, 4.68 at. % V and 62.45 at. % iron. Example 1B has the same alloy composition as Example 1.

6 For comparison, two commercial peening media were initially employed: (1) Sinto Microshot SBM-100C, having as specific gravity of 7.6 g/cm3, a hardness of 729 Vickers and a particle size distribution of D10 equaling 100 micrometers, D50 of 132 micrometers, and D90 equaling 183 micrometers; (2) Sinto AMO Beads AM-100, having as specific gravity of

7.4 g/cm3, a Vickers hardness of 788 and a particle size distribution of D10 equaling 64 micrometers, D50 of 92 micrometers, and D90 equaling 134 micrometers.
The above referenced shot media particle hardness testing was performed using a Tukon 2500 Knoop/Vickers Automated Hardness Tester (2000x optics) with a Vickers diamond indenter. Particle specimens were prepared by add-mixing them with epoxy resin, curing, and then polishing the mix to expose particle cross sections. A test load of 100 grams was used, and the average of twelve indents reported, as shown below in Table 2:
Table 2 Shot Media HV
Example 1 1284 Example lA 1236 Example 1B 1176 Sinto Micro shot SBM-100C 729 Sinto AMO BEADTM AM-100 788 Saint Gobain MicroblastTM 692 Particle size distribution was determined using a Microtrac S3500 laser diffraction analyzer. The distributions are noted below in Table 3:
Table 3 Shot Media D10( m) D50( m) D90( m) Example 1 52 83 142 Example lA 77 118 184 Example 1B 93 129 178 Sinto Micro shot 100 132 183 Sinto AMO 68 92 134 Saint Gobain 79 105 148 MicroblastTM B120 The elastic modulus of the particles of Example 1 was next determined using a nano instrumented indentation tester (IIT). Particle specimens were prepared by mixing the particles with epoxy resin, curing, and then polishing the mix to expose particle cross sections. Four particles on each sample were selected for testing. The position of the indentations within the sample was placed so as to avoid being too close to the edge of a particle, but were not otherwise particularly selected with any other criterion. Each test was done with 20 load increments, to a maximum of 20 mN, and then 20 unloading decrements using a diamond Berkovich (triangular pyramid) indenter. The indentation data was analyzed using the conventional "Oliver and Pharr" technique. The data was corrected for initial penetration of the indenter, compliance of the load frame, and area function of the indenter used. The instrument was calibrated with instrumentation for load and displacement traceable to national standards of these quantities. Example 1 was determined to have an elastic modulus (assuming a Poisson's ratio of 0.3) of 246 GPa 44.58.
In general, the workpieces that may be employed herein preferably include metal type workpieces that have a HV value of 500-1000. Such workpieces may be in a variety of geometrical forms, including but not limiting to metallic sheet, coils, springs, metal forging or tubes. Accordingly, it is contemplated herein that the shot peening method utilizing the particles identified herein may be applied to any workpiece where shot peening is utilized for any purpose, including enhancement of general wear characteristics.
For evaluation purposes, SAE 1070 steel Almen A (76 mm x 19 mm x 1.295 0.025 mm thick) strips and Almen N (76 mm x 18 mm x 0.785 0.025 mm thick) strips with hardness 472 Vickers and a surface roughness average (Ra) of 0.106 micrometers were selected as workpieces. Almen strips are used in the art to quantify the intensity of a shot peening process. Compressive stress induced by the peening operation causes the strip to deform into an arch, of which the point of maximum curvature is measured using a gage specifically designed for the measurement. Arch heights from successive Almen strips produced under set peening parameters are plotted as a function of time to determine the peening intensity saturation. Peening intensity saturation is defined as the first point beyond which the arc height increases by 10 percent or less when the peening time is doubled.

8 Peening intensity saturation was therefore determined by measuring Almen strip arch heights after peening increments of 5, 10, 20 and 40 seconds. Measurements were taken using a calibrated Electronics Inc. Advanced Almen Gage. Peening Intensity Saturation was calculated using Saturation Curve Solver software, Release 9. Post peening maximum subsurface residual compressive stress was determined by X-ray Diffraction (XRD) by profiling at 12.7, 25.4, 50.8, 101.6, and 127.0 micrometers. XRD peak width was converted to hardness for each profile.
Surface roughness measurements were taken using a Mitutoyo SJ-210 instrument.
The workpiece post peening residual stress field was determined by X-ray Diffraction (XRD) in accordance with SAE HS-784/2003 by profiling at 12.7, 25.4, 50.8, 101.6, and 127.0 micrometers. In measuring residual stress using X-ray diffraction (XRD), the strain in the crystal lattice is measured and the associated residual stress is determined from the elastic constants assuming a linear elastic distortion of the appropriate crystal lattice plane. XRD
was performed using a TEC1630 with chromium radiation, a peak of 155 20, and a 4mm diameter round collimator. A parabolic fit to the diffraction peak was used, with the k alpha 2 component of the peak subtracted out using SARATec software. Corrections to account for layer removal and exponential penetration of the beam (stress gradient effects) were also applied. Hardness was determined by peak width calibrated by microhardness measurements at 12.7 and 127 micrometers depth.
Shot peening was carried out using Kelco air blasting equipment using the following parameters: pressure of the projection of 0.55 MPa; a venturi nozzle with an exit diameter of 7.9375 mm; a distance to the workpiece of 101.6 mm; and a mass flow rate of 2.63 kg/minute. See again, FIG. 1.
The results of the peening test are summarized in Table 4 below, comparative peening intensity saturation curves are shown in FIG. 2, comparative residual stress fields are shown in FIG. 3 and corresponding comparative hardness values in the workpiece are shown in FIG. 4.

9 Peening Maximum Intensityresidual Saturation Peening Surface Surface (Almen Acompressive Time Time Roughness Hardness Arch stress of Height) workpiece (mils) (sec) (sec) (Ra/mm) (HV) (MPa) Example 1 5.82 5.72 5 2.372 525 2.657 526 697 2.332 528 702 40 2.234 543 Example lA 6.14 16.39 20 1.549 581 Example 1B 8.56 14.48 20 3.261 586 Sinto SBM-100C 4.95 9.02 5 1.188 514

10 1.666 514 20 1.66 524 40 1.516 524 Sinto AM-100 3.5 5.19 5 2.168 526 10 2.058 532 20 2.019 538 40 1.980 532 From Table 4 it can be seen that while the surface roughness of the Sinto SBM-5 is lower than that of the Example 1, the maximum residual compressive stress is also lower with nearly equivalent surface hardness. This is also reflected in the comparison of the peening intensity at saturation (Almen intensity (arch height)) versus the peening saturation time. Further, the longer saturation time of 9.02 seconds for Sinto SBM-100C
versus that of Example 1 (5.82 seconds) and the lower maximum residual compressive stress of the 10 commercial Sinto SBM-100C near the peening intensity saturation time (635 MPa at 10 seconds) versus that of Example 1 (726 MPa at 5 seconds) infers that a lower projection pressure could be used with the alloys disclosed herein for an equivalent maximum residual stress to that of Sinto SBM-100C thereby reducing cost. The same conclusion can be drawn when comparing maximum compressive stress of the commercial Sinto AM-100 near the peening saturation time (655 MPa at 5 seconds) versus that of Inventive Example 1 (726 MPa at 5 seconds). This is also reflected in FIG. 3, a plot of the XRD through thickness residual stress field profile at near peening intensity saturation. FIG. 4 is a plot of the through thickness hardness profile at near peening intensity, and provides evidence that Example 1 work hardens the surface in the same manner as Sinto SBM-100C and Sinto AM-100.
From Table 4 it should again be noted that when comparing to the commercially available Sinto product, the comparisons are considered most relevant at that point where Almen saturation has been achieved. Accordingly, it can be seen that the surface roughness of Example 1A at 20 seconds peeing time is lower than that of the Sinto SBM-100C at 10 seconds peening time, a higher maximum residual compressive stress is achieved for Example 1A versus that of the Sinto SBM-100C. Further, since the peening intensity is higher for Example 1A than that of the Sinto SBM-100C, as shown in Table 4, a deeper residual compressive stress is implied. This is confirmed upon review of the XRD profile residual stress field, FIG. 4A, and hardness plots, FIG. 4B at near saturation for both the Example 1A and the Sinto SBM-100C. In addition, the peening intensity saturation curve for Example 1A and the Sinton SBM-100C is provided in FIG. 4C.
From Table 4 it can further be seen that the surface roughness of Sinto SBM-100 after 10 seconds peeing time (after Almen saturation has again been achieved) is lower than that of Example 1B after 20 seconds (after Almen saturation has been achieved).
However, a near equivalent maximum residual compressive stress is achieved for Example 1B
versus that of the Sinto SBM-100C. Further, since the peening intensity is higher for Example 1B than that of the Sinto SBM-100C with near equivalent maximum residual compress stress, as shown in Table 4, a deeper residual compressive stress is implied. This is confirmed upon review of the XRD profile residual stress field, FIG. 4D, and hardness plots, FIG. 4E at near saturation for both the Example 1B and the Sinto SBM-100C. In addition, the peening intensity saturation curve for Example 1B and the Sinton SBM-100C is provided in FIG. 4F.

11 Example 2 In Example 2, the particles of Example 1 and Example 1B were tested for comparison with a commercial zirconia ceramic peening media, Saint Gobain Microblast B120, having a specific gravity of 3.8 g/cm3, a hardness of 692 Vickers and a particle size distribution of D10 equaling 79 micrometers, D50 equaling 105 micrometers, and D90 equaling micrometers was used. SAE 1070 steel Almen N (76 mm x 18 mm x 0.785 0.025 mm thick) strips with a hardness of 472 Vickers and a surface roughness average (Ra) of 0.106 micrometers were selected as the workpiece. Shot peening was carried out using Kelco air blasting equipment using the following parameters: pressure of the projection of 0.28 MPa; a venturi nozzle with an exit diameter of 7.9375 mm; a distance to the workpiece of 101.6 mm;
and a mass flow rate of 2.63 kg/minute. Peening intensity saturation was determined by measuring Almen strip arch heights after peening increments of 5, 10, 20 and 40 seconds.
Measurements were taken using a calibrated Electronics Inc. Advanced Almen Gage.
Peening Intensity Saturation was calculated using Saturation Curve Solver software, Release 9. Post peening maximum subsurface residual compressive stress was determined by X-ray Diffraction (XRD) by profiling at 12.7, 25.4, 50.8, 101.6, and 127.0 micrometers. XRD peak width was converted to hardness for each profile. The results of the peening test are summarized in Table 5. For Example 1, comparative peening intensity saturation curves are shown in FIG. 5, comparative residual stress fields are shown in FIG. 6 and corresponding comparative hardnesses are shown in FIG. 7. For Example 1B, comparative peening saturation curves are shown in FIG. 11, comparative residual stress fields are shown in FIG.

12 and corresponding comparative hardne s se s are shown in FIG. 13.

Table 5 Peening Maximum Intensityresidual Saturation Peening Surface Surface (Almen Acompressive Time Time Roughness Hardness Arch stress of Height) workpiece (mils) (sec.) (sec.) (Ra/um) (HV) (MPa) Example 1 3.14 6.17 5 1.524 509 669 1.42 515 643 1.485 516 587 40 1.679 527 534 Example 1B 7.03 16.35 20 1.341 540 671 Saint Gobain B120 3.09 13.42 5 0.931 500 679 10 0.872 497 701 20 0.903 505 612 40 0.894 513 555 From Table 5 it can be seen that while the surface roughness of the Saint Gobain 5 B120 is lower than that of the Example 1, near equivalent surface hardness and maximum residual compressive stress is achieved for Example 1 versus that of the Saint Gobain B120.
Further, since the peening intensity is higher for Example 1 than that of the Saint Gobain B120 with near equivalent maximum residual compress stress, as shown in Table 5, a deeper residual compressive stress is implied. This is confirmed upon review of the XRD profile 10 residual stress field, FIG. 6, and hardness plots, FIG. 7 at saturation for both the Example 1 and the Saint Gobain B120.
From Table 5, it can also be seen that while the surface roughness of Saint Gobain B120 is lower than that of the Example 1B, near equivalent maximum residual compressive stress is achieved for Example 1B versus that of the Saint Gobain B120.
Further, since the 15 peening intensity is higher for Example 1B than that of the Saint Gobain B120 with near equivalent maximum residual compress stress, as shown in Table 5, a deeper residual compressive stress is implied. This is confirmed upon review of the XRD
profile residual stress field, FIG. 12, and hardness plots, FIG. 13 at saturation for both the Example 1B and the Saint Gobain B120.

13 Example 3 In Example 3, the durability of the Example 1 and 1B, sieved to remove fines less than 75 micrometers, was tested in comparison to the durability of Sinto Microshot SBM-100, Sinto AMO Bead AM-100 and Saint Gobain Microblast B120. Shot peening was carried out using Kelco air blasting equipment using the following parameters:
pressure of the projection: 0.55 MPa; a venturi nozzle with an exit diameter of 7.9375 mm;
a distance to the workpiece of 101.6 mm; and a mass flow rate of 2.63 kg/minute. To test peening media durability, preferably, one employs a 6.35 mm thick Hadfield Manganese workpiece peened over a period of time using pre-conditioned media. A Hadfield Manganese workpiece is made by alloying steel containing 0.8-1.25 wt. % carbon with 11-15 wt. %
manganese. The workpiece has an ultimate tensile strength of 120,000 psi ¨ 140,000 psi, a yield strength of 65,000 psi ¨ 85,000 psi. The workpiece will also preferably have an initial HB
hardness value of 180-245 to a HB hardness value of >500 in the work hardened state.
Pre-conditioning is performed by peening the workpiece with each media for 15 minutes prior testing the media. Durability was evaluated by weighing the media below 75 micrometers at peening increments of 6, 12, 18, and 24 hours. This was done by removing a 100 g sample of shot at the designated time interval, sieving the 100 g mass, and weighing the fraction of particles less than 75 micrometers in diameter to identify the weight percent of fractured material. The results of this testing are shown in Table 6.

Mass Time Loss (hrs.) (%) Inventive Example 1 1 0.30 6 2.00 12 5.00 18 6.20 Inventive Example 1B 1 0.7 6 3.1 12 3.8 18 5.2

14 Sinto AM-100 6 17.70 12 27.30 18 32.60 Sinto SBM-100C 6 8.00 12 36.00 18 57.00 Saint Gobain B120 1 20.00 6 98.00 From Table 6 it is observed that Example 1 and 1B has superior durability in comparison to all of the commercial samples and in particular in comparison to the Saint Gobain Microblast B120. Mass loss for Example 1 and 1B compared to Sinto AM-100 and Sinto SBM-100C are depicted graphically in FIG. 8. Mass loss for Example 1 and (compared to Saint Gobain Microblast B120) is depicted graphically in FIG. 9.
Accordingly, it may be appreciated that the data above identifies that the shot peening metal alloy particles herein may be more broadly understood and defined as particles which, when projected at a pressure of 0.55 MPa, to a preconditioned steel workpiece (i.e. a workpiece that is peened prior to durability testing), indicate a mass loss (fraction of particles less than 75 pm), after a time period of 18 hours, of less than or equal to 20.0%, or <19.0%, or < 18.0%, or < 17.0%, or < 16.0 %, or < 15.0%, or < 14.0%, or < 13.0%, or <
12.0 %, or < 11.0%, or <10.0%, or < 9.0%, or < 8.0%, or < 7.0%. In addition, the particles are such that under the above identified testing conditions they indicate a mass loss (fraction of particles less than 75 p m) after a time period of up to 12 hours, of <5.0%.
Furthermore, the particles are such that under the above identified testing conditions they indicate a mass loss (fraction of particles less than 75 p m) after a time period of up to 6 hours of < 4.0 %. Finally, the particles are such that under the above identified testing conditions they indicate a mass loss (fraction of particles less than 75 p m) of less <1.0 %.
As can be seen from the above, the present invention provides an improvement in the use metallic particles that are iron based and combine relatively high hardness and durability when used as a metal abrasive, as, e.g., in shot-peening processes. The benefits include but are not limited to improvements in the properties that may be realized in an impacted workpiece as well as increased longevity in the metal particles employed due to the particles strength and durability as noted herein.
Example 4 In Example 4, the durability of the Example 1, sieved to remove fines less than 75 micrometers, was tested in comparison to the durability of Saint Gobain Microblast B120 at a lower projection pressure than that of Example 3. Shot peening was carried out using Kelco air blasting equipment using the following parameters: pressure of the projection: 0.28 MPa; a venturi nozzle with an exit diameter of 7.9375 mm; a distance to the workpiece of 101.6 mm; and a mass flow rate of 2.63 kg/minute. To test peening media durability a 6.35 mm thick Hadfield Manganese workpiece (described above) peened over a period of time using pre-conditioned media. Pre-conditioning was performed by peening the workpiece with each media for 15 minutes prior testing the media. Durability was evaluated by weighing the media below 75 micrometers at peening increments of 1, 3, and 6 hours. This was done by removing a 100 g sample of shot at the designated time interval, sieving the 100 g mass, and weighing the fraction of particles less than 75 micrometers in diameter to identify the weight percent of fractured material. Accordingly, it may now be appreciated that one characteristic of the shot peening media herein is that the metal alloy particles, when projected at a pressure of 0.28 MPa at a Hadfeld Manganese workpiece, Mass Time Loss (hrs.) (%) Inventive Example 6 1.0 Saint Gobain B120 1 3.6 3 7.8 6 17.3 From Table 7 it is observed that Example 1 has superior durability in comparison to the Saint Gobain Microblast B120. Mass loss for Example 1 according to the above, as compared to Saint Gobain Microblast B120, is depicted graphically in FIG. 10.

Accordingly, it may be appreciated that the data above identifies that the shot peening metal alloy particles herein may be more broadly understood and defined as particles which, when projected at a pressure of 0.28 MPa, to a preconditioned steel workpiece (i.e. a workpiece that is peened prior to durability testing), indicate a mass loss (fraction of particles less than 75 pm), after a time period of 6 hours, of less than or equal to

15.0% or lower, such as <14.0%, or < 13.0%, or < 12.0%, or < 11.0 %, or <10.0%, or < 9.0%, or <
8.0%, or <
7.0 %, or 6.0%, or < 5.0%, or < 4.0%, or < 3.0%, or < 2.0%, or < 1.0%. In addition, the particles are such that under the above identified testing conditions they indicate a mass loss (fraction of particles less than 75 p m) after a time period of up to 3 hours, of <5.0%, or <
4.0%, or < 3.0%, or < 2.0%, or < 1.0 %. Furthermore, the particles are such that under the above identified testing conditions they indicate a mass loss (fraction of particles less than 75 p m) after a time period of up to 1 hour of < 3.0 %, or < 2.0%, or < 1.0%.
Applications for the shot peening particles herein include, but are not limited to, gear parts, cams and camshafts, clutch springs, coil springs, connecting rods, crankshafts, gearwheels, leaf and suspension springs, threads, rock drills, and turbine blades. One particularly useful application has been determined to include engine valve springs, which operate to maintain the engine valve springs closed against their seating until a cam opens such valve to release pressure. Such engine valve springs particularly include engine valve springs that have a relatively high hardness, such as a chromium-silicon type valve spring alloy, which has a hardness (ASTM A877) of HRC 48-55.

Claims (21)

Claims:

1. A method of shot peening a workpiece comprising projecting metal alloy particles at a workpiece wherein said metal alloy particles comprises Fe in combination with B, C, Cr and Nb, wherein the Fe is present at a level of greater than 50.0 atomic percent wherein said metal alloy has a Vickers Hardness (HV) of at least 1150 and an elastic modulus of greater than 200 GPa.

2. The method of claim 1 wherein said metal alloy has a HV of 1150-1400.

3. The method of claim 1 wherein said metal alloy has an elastic modulus in the range of greater than 200 GPa to 350 GPa.

4. The method of claim 1 wherein said metal alloy comprises:
Fe: 59.0 ¨ 64.0 at. %
B: 17.5 ¨ 18.8 at. %
C: 4.4 ¨ 5.1 at. %
Cr: 12.7-13.1 at. %
Nb: 1.4-1.7 at. %.

5. The method of claim 1 wherein 95% of the particles have a size in the range of 40 µm to 250 µm.

6. The method of claim 1 wherein the particles have the following particle size distribution:
D10: 50 µm D50: 80 µm D90: 150 µm.

7. The method of claim 1 wherein the weight fraction of particles of less than 75 microns is less than or equal to 7.0%, after projection of said particles onto said workpiece.

8. The method of claim 1 wherein the shot is projected at a metal substance having a HV in the range of 500 to 1000.

9. The method of claim 1 wherein said alloy includes .alpha.-Fe and/or .gamma.-Fe and one or more of the following:
(1) complex borides;
(2) complex carbides; or (3) borocarbides.

10. The method of claim 1 wherein said alloy comprises:
Fe: 58.0 ¨ 65.0 at. %
B: 14.0 ¨ 19.0 at. %
C: 4.4 ¨ 5.5 at. %
Cr: 7.0-13.5 at. %
Nb: 1.4-3.5 at. %.

11. The method of claim 1 wherein the particles have the following particle size distribution:
D10: 50 - 100 µm D50: 100-150 µm D90: 150 - 200 µm

12. The method of claim 1 wherein said particles, when projected at a pressure of 0.55 MPa, to a preconditioned steel workpiece, indicate a mass loss associated with the fraction of particles less than 75.0 µm, after a time period of 18.0 hours, of less than or equal to 20.0 %.

13. The method of claim 1 wherein said particles, when projected at a pressure of 0.28 MPa, to a preconditioned steel workpiece, indicate a mass loss associated with the fraction of particles of less than 75.0 µm, after a time period of 6 hours, of less than or equal to 15.0%.

14. The method of claim 1 wherein said workpiece comprises one of a gear part, cam, camshaft, clutch spring, coil spring, connecting rod, crankshaft, gearwheel, leaf spring, suspension spring, threads, rock drill, turbine blades or engine valve spring.

15. Shot peening material comprising metal alloy particles containing Fe in combination with B, C, Cr and Nb, wherein the Fe is present at a level of greater than 50.0 atomic percent wherein said metal alloy has a Vickers Hardness (HV) of at least 1150 and an elastic modulus of greater than 200 GPa.

16. Shot peening material of claim 15 wherein said metal alloy has a HV of 1400.

17. Shot peening material of claim 15 wherein said metal alloy has an elastic modulus in the range of greater than 200 GPa to 350 GPa.

18. Shot peening material of claim 15 wherein said metal alloy comprises:
Fe: 59.0 ¨ 64.0 at. %
B: 17.5 ¨ 18.8 at. %
C: 4.4 ¨ 5.1 at. %
Cr: 12.7-13.1 at. %
Nb: 1.4-1.7 at. %.

19. Shot peening material of claim 15 wherein 95% of the particles have a size in the range of 40 µm to 250 µm.

20. The shot peening material of claim 15 wherein said alloy comprises:
Fe: 58.0 ¨ 65.0 at. %
B: 14.0 ¨ 19.0 at. %
C: 4.4 ¨ 5.5 at. %

Cr: 7.0-13.5 at. %
Nb: 1.4-3.5 at. %.

21. The shot peening material of claim 15 wherein the particles have the following particle size distribution:
D10: 50 - 100 µm D50: 100-150 µm D90: 150 - 200 µm.