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Shot peening is widely used for automobile parts such as springs and gears to improve fatigue strength. In general, high hardness media is required for shot peening to targets with high hardness. Recently, shot peening to various tool steels such as cold and hot tool steels and high speed tool steel has been investigated and is used in some applications, and improvements in fatigue strength and die life by the increase of compressive residual stress and hardness at their shot peened surface have been reported.In this study, the properties of cold work tool steel shot peened by Fe-8%Cr-6.5%B (mass%) gas atomized powder (FeCrB powder) with 1200 HV-class Vickers hardness and long life as shot peening media were examined. Quenched and tempered JIS-SKD11 followed by shot peening by FeCrB and high speed steel powder sieved between 45 and 125 mm was used.Vickers hardness and compressive residual stress of the shot peened JIS-SKD11 surface by FeCrB powder were higher than these by high speed steel powder. As a result, JIS-SKD11 shot peened by FeCrB powder showed excellent fatigue property, Charpy impact value and wear resistance property.

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Shot peening is a surface treatment effective to obtain compressive residual stress on shot peened surface and to improve fatigue strength, and it is applied to automobile parts such as springs and gears. Based on recent trend for further reduction of automobile parts weight, higher hardness of automobile parts is required through various treatments such as carburizing.In this study, the effects of the amount of B addition on properties of gas atomized Fe 8Cr B(mass) powders for shot peening were examined. Including Fe 8Cr 6.5B as base composition, alloys with the amount of B addition from 4 to 9.5 mass were gas atomized and examined. The amount of Fe 2 B phase and Vickers hardness increased with the increase of the amount of B addition, and high hardness of 1200HV was obtained in alloys with more than 6.5 mass B addition. Density decreased with the increase of the amount of B addition, and 7.14 Mg/m 3 was obtained in the alloy with 9.5 mass B addition. No crack initiated near indentation by 1000 g load in the alloys with the amount of B addition from 5.5 to 6.5 mass.

In this study, for the purpose of developing the shot peening process obtaining both improving strength and coating with Cu, the behavior of peened surface modification by using hard media containing soft Cu phase was examined. Shot peening was carried out by Cu phase dispersed FeCrBCu media with high hardness of 1220 HV in Vickers hardness. Media particle diameter was under 45 mm. Annealed JIS SUJ2 steel was used as workpiece.On the peened surface, compressive residual stress was 440 MPa and Vickers hardness increased to 350 HV. By EDX analysis, 29 mass Cu was detected on the peened surface. Though this value was beyond the solid solution limit in aFe, no peak of the new phase by Cu adhesion was observed in XRD pattern. Furthermore, according to TEM observation near the peened surface, the nano crystalline region of aFe was observed and Cu was detected at this region by EDX analysis.Logarithmic strain near the surface caused by shot peening in this work was estimated at 4.7 calculated from the crystal grain diameter and thickness before and after shot peening, respectively. Therefore, owing to the severe plastic deformation condition of this shot peening, it seems the crystal grain refinement to nano size and super saturated solid solution of Cu in aFe were observed.

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