Micro-structure and surface layer properties of 18CrNiMo7-6 steel after multistep shot peening

• New multistep SP technologies were carried out on the 18CrNiMo7-6 steel.
• The optimal peening intensity is 0.50 + 0.30 + 0.15 mmA for 18CrNiMo7-6 steel.
• CRS and micro-hardness of 18CrNiMo7-6 steel are improved after multistep SP.
• The domain size of 18CrNiMo7-6 steel is refined after multistep SP.

The effects of multistep shot peening on the structure and mechanical properties of 18CrNiMo7-6 steel have been investigated using X-ray diffraction line profile analysis. X-ray diffraction data reveals the phase transformation from austenite to martensitic phase after SP treatments. The results show that multistep shot peening can more significantly improve the mechanical properties. The optimal peening intensity of 0.50 + 0.30 + 0.15 mmA is found for 18CrNiMo7-6 steel. With the optimized shot peening treatment, the maximum compressive residual stress (located at the depth of 20 μm for martensite) and top-surface compressive residual stress of martensite attain the values of 1463 and 1256 MPa, respectively; While those of austenite are 1039 and 766 MPa, respectively. The domain size of 18CrNiMo7-6 steel decreases to 11.1 nm on the top surface after shot peening with the intensity of 0.50 + 0.30 + 0.15 mmA, and the micro-hardness on the top surface attains the peak value of 1175 HV.