Microstructure of Gear Steels Treated by Laser Shock-Mechanical Shot Peening by X-Ray Diffraction Line Profile Analysis Method

ZHU Pengfei; ZHU Xiang; YAN Yixiong; GE Yibo; LI Songbai; CHEN Zhi

doi:10.16490/j.cnki.issn.1001-3660.2026.01.007

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Surface Technology ›› 2026, Vol. 55 ›› Issue (1) : 81-95. DOI: 10.16490/j.cnki.issn.1001-3660.2026.01.007

Surface and Interface Strengthening Technology

Microstructure of Gear Steels Treated by Laser Shock-Mechanical Shot Peening by X-Ray Diffraction Line Profile Analysis Method

ZHU Pengfei¹, ZHU Xiang^2,3, YAN Yixiong⁴, GE Yibo^5,*, LI Songbai^2,3, CHEN Zhi^2,3

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Abstract

Heavy-duty gears are subject to alternating loads, high torque as well as harsh service conditions, and bending fatigue is the main failure mode, seriously affecting the comprehensive performance of gears. Conventional shot peening (CSP) is a common method for gear surface strengthening. It introduces high magnitude compressive residual stress, but the induced surface roughness is relatively large, which restricts the manufacturing of high-performance gears. Laser shock peening (LSP) can introduce a deeper compressive stress layer, but the compressive stress level is lower than that of CSP. Fine particle peening (FPP) can improve the surface roughness and enlarge the compressive stress level on the exterior surface of gears, but the compressive stress layer is shallow. Therefore, the existing strengthening methods cannot meet the current technical performance requirements of heavy-duty gears. To address this issue, laser shock-mechanical shot peening compound strengthening method is proposed in this work and the induced microstructural features affecting the bending life of gears are experimentally investigated.
Carburized and ground gear steels made of 20CrMnTi are used in this work, and the dimension of each sample is 30 mm× 50 mm×5 mm. Three kinds of strengthening methods, in terms of CSP (media diameter of 0.8 mm, intensity of 0.52 mmA, coverage of 100%), FPP (media diameter of 0.05 mm, intensity of 0.14 mmN, coverage of 100%), and LSP (pulse energy of 4 J, spot diameter of 2 mm, overlap of 50%, pulse width of 20 ns), are combined to form a laser shock-mechanical shot peening compound strengthening process. Since LSP has strict requirements on the surface roughness of workpieces, this process is arranged before CSP. FPP is often used as the final strengthening process to improve the roughness of workpieces, so it is arranged after LSP and CSP. Considering the processing efficiency, CSP, FPP, and LSP are used for treating gear steels only once. This work aims to reveal the microstructure evolution of gear steels treated by laser shock-mechanical shot peening compound strengthening process. X-ray diffraction line profile analysis (XRDLPA) method is applied to comparatively study the effect of different strengthening processes (carburizing and grinding, conventional shot peening, laser shock peening + conventional shot peening, laser shock peening + conventional shot peening + fine particle peening) on the microstructure (dislocation density, crystallite size, micro-strain and phase fraction) of the gear steels. A three-point bending fatigue test is designed to reveal the effect of the compound strengthening method on the fatigue life of gear steels.
The results show that it is feasible to conduct a quantitative investigation on the microstructural characteristics of gear steels subject to laser shock-mechanical shot peening compound treatment with XRDLPA method. Laser shock-mechanical shot peening compound strengthening process can promote dislocation multiplication, crystallite refinement and phase transformation, thereby improving the surface microstructure of gear steels on multiple scales. The compound method can effectively enhance the bending fatigue life of gear steels. Specifically, with the application of the compound treatment, the dislocation density for martensite increases from 2.73×10¹⁶ to 3.09×10¹⁶ m^-2 in the form of screw-edge-mixed type. The fraction of screw dislocation decreases due to the occurrence of cross-slip, whereas the fraction of edge dislocation increases. The formation of dislocation dipoles or dislocation cells in the surface layer can be further promoted by the compound strengthening process, and the dislocation arrangement parameter M decreases from 7.3 to 5.8. With the adoption of the compound treatment, the average crystallite size on the surface of gear steels decreases from 57.1 to 36.7 nm, and the uniformity distribution of the crystallite size is improved. Because of dislocation multiplication and division mechanism, nanostructured layers are generated on the surface of gear steels. Due to the different degrees of atomic slip, the (200) crystallographic plane exhibits the smallest crystallite size and highest micro-strain, whereas the (222) plane shows the smallest micro-strain and the largest crystallite size. With the employment of the compound treatment, the martensite fraction of gear steels increases from 63.4% to 84% resulting from martensite transformation. With the use of the compound treatment, the bending fatigue life of gear steels increases by 33.6 times (under the stress amplitude of 540 MPa).

Key words

laser shock peening / mechanical shot peening / gear steel / compound strengthening / XRD line profile analysis / microstructure

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ZHU Pengfei, ZHU Xiang, YAN Yixiong, GE Yibo, LI Songbai, CHEN Zhi. Microstructure of Gear Steels Treated by Laser Shock-Mechanical Shot Peening by X-Ray Diffraction Line Profile Analysis Method[J]. Surface Technology. 2026, 55(1): 81-95

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Funding

National Natural Science Foundation of China (52405609, 52305081); The Natural Foundation of Hunan Province, China (2023JJ40049); The Key Research and Development Program of Hunan Province, China (2025JK2012)