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.

WO2009/133920A1 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.

WO2012/128357 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 μιη 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. 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 γ-Fe (austenite). The alloy also preferably includes one or more of the following: (1) complex borides (e.g. M]B, M ₂ B and M ₃ B where M is a transition metal); (2) complex carbides (e.g. M]C, M ₂ C, M ₃ C, and M2 ₃ C6 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

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 μιη to 250 μιη. 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 μιη. More preferably, the D10 value in microns (percent of the population having particle sized below this number) is 50.0 μιη. The preferred D50 value (median) is 80 μιη and the preferred D90 value (90 percent of the distribution below this number) is 150 μιη.

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 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 μιη 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/cm ³ , 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 IB has the same alloy composition as Example 1. For comparison, two commercial peening media were initially employed: (1) Sinto Microshot SBM-100C, having as specific gravity of 7.6 g/cm ³ , 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/cm ³ , 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

Particle size distribution was determined using a Microtrac S3500 laser diffraction analyzer. The distributions are noted below in Table 3:

Table 3

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. 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° 2Θ, 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. TABLE 4

Peening Maximum Intensity residual

Saturation Peening Surface Surface

(Almen A compressive

Time Time Roughness Hardness

Arch stress of Height) workpiece

(mils) (sec) (sec) (Ra/μηι) (HV) (MPa)

Example 1 5.82 5.72 5 2.372 525 726

10 2.657 526 697

20 2.332 528 702

40 2.234 543 685

Example 1A 6.14 16.39 20 1.549 581 982 Example IB 8.56 14.48 20 3.261 586 686

Sinto SBM-100C 4.95 9.02 5 1.188 514 705

10 1.666 514 635

20 1.66 524 684

40 1.516 524 684

Sinto AM-100 3.5 5.19 5 2.168 526 655

10 2.058 532 662

20 2.019 538 643

40 1.980 532 632

From Table 4 it can be seen that while the surface roughness of the Sinto SBM-100C 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 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-IOOC 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-IOOC at 10 seconds peening time, a higher maximum residual compressive stress is achieved for Example 1A versus that of the Sinto SBM-IOOC. Further, since the peening intensity is higher for Example 1A than that of the Sinto SBM-IOOC, 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-IOOC. In addition, the peening intensity saturation curve for Example 1A and the Sinton SBM-IOOC 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 IB after 20 seconds (after Almen saturation has been achieved). However, a near equivalent maximum residual compressive stress is achieved for Example IB versus that of the Sinto SBM-IOOC. Further, since the peening intensity is higher for Example IB than that of the Sinto SBM-IOOC 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 IB and the Sinto SBM-IOOC. In addition, the peening intensity saturation curve for Example IB and the Sinton SBM-IOOC is provided in FIG. 4F. Example 2

In Example 2, the particles of Example 1 and Example IB were tested for comparison with a commercial zirconia ceramic peening media, Saint Gobain Microblast® B120, having a specific gravity of 3.8 g/cm ³ , a hardness of 692 Vickers and a particle size distribution of D10 equaling 79 micrometers, D50 equaling 105 micrometers, and D90 equaling 148 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 IB, comparative peening saturation curves are shown in FIG. 11, comparative residual stress fields are shown in FIG. 12 and corresponding comparative hardnesses are shown in FIG. 13.

Table 5

Peening Maximum Intensity residual

Saturation Peening Surface Surface

(Almen A compressive

Time Time Roughness Hardness

Arch stress of Height) workpiece

(mils) (sec.) (sec.) (Ra/μηι) (HV) (MPa)

Example 1 3.14 6.17 5 1.524 509 669

10 1.42 515 643

20 1.485 516 587

40 1.679 527 534

Example IB 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 profile0 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 IB, near equivalent maximum residual compressive stress is achieved for Example IB versus that of the Saint Gobain B120. Further, since the5 peening intensity is higher for Example IB 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 IB and the Saint Gobain B 120. Example 3

In Example 3, the durability of the Example 1 and IB, 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.

TABLE 6

Mass

Time

Loss

(hrs.) ( )

Inventive Example

1 1 0.30

6 2.00

12 5.00

18 6.20

Inventive Example

IB 1 0.7

6 3.1

12 3.8

18 5.2 Sinto AM- 100

Sinto SBM-lOOC

Saint Gobain B120

From Table 6 it is observed that Example 1 and IB 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 IB compared to Sinto AM- 100 and Sinto SBM-100C are depicted graphically in FIG. 8. Mass loss for Example 1 and IB (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 μιη), 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 μιη) 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 μιη) 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 μιη) 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,

TABLE 7

Mass

Time

Loss

(hrs.) ( )

Inventive Example

1 1 0

3 0

6 1.0

Saint Gobain B 120 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 μιη), 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 μιη) 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 μιη) 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.