The effect of shot peening on the fatigue life of laser-hardened 17-4PH steel

Because 17-4PH stainless steel has a good combination of mechanical properties and corrosion resistance, it can be suitable for various applications, such as replacing more expensive titanium alloys and polymer materials in steam turbines, marine engineering, chemical and electric power, etc. . These materials must be in service for a long time during the entire life span of the component. With the improvement of the technical requirements for the life span and safety of steam turbines, the quality of turbine blades, especially the final blades, becomes more and more important. Therefore, it is necessary to improve surface quality, such as surface hardness and wear resistance.

In order to improve the surface hardness for many years, laser hardening has been used as a promising method in the manufacture of turbine blades. In order to prevent the occurrence and growth of micro-cracks, shot peening is a cold-working mechanical surface treatment process that produces compressive residual stress on the surface. A series of results brought by shot peening, such as the improvement of surface roughness, surface hardness and surface residual stress, have an important impact on the fatigue life of machine parts.

Researchers have carried out research on the surface roughness, hardness, residual stress and fatigue life of the laser hardened 17-4PH steel used for the final stage of turbine blades before and after shot peening. The purpose of this work is to improve the fatigue life of laser-hardened 17-4PH parts and obtain a suitable compressive residual stress distribution, proper hardness and roughness distribution. The fatigue test of Matrix17-4PH, laser hardening 17-4PH and laser hardening + shot peening 17-4PH parts was carried out using a rotating bending fatigue testing machine.

Table 1 Chemical composition of laser hardened samples (wt.%)

CSiMnCrNiCuNb+TaPSAlTiNFe

≤0.055≤0.1≤0.515.00 -16.003.80

-4.503.00

-3.700.15

-0.35≤0.035≤0.030≤0.050≤0.05≤0.05 margin

The 17-4PH martensitic stainless steel was used for the test, and the chemical composition is shown in Table 1. Processed from flat material into a geometric body of 75mm×60mm×10mm. Then the sample is austenitized by keeping it at 1035°C for 1 hour, quenched by water to 25°C, tempered and cooled in a vacuum furnace at 815°C for 0.5 hours, and then tempered and cooled in a vacuum furnace at 570°C for 3 hours . After the heat treatment, a continuous wave mode CO2 laser is used for laser hardening. The size and feed speed of the laser beam are 6mm×6mm and 2mm/s, respectively. After the first shot peening, when the roughness of the sample is relatively high, using small diameter metal shots as the second shot peening medium can reduce the roughness. After shot peening, the hardness of the three typical areas close to the surface is significantly increased, because these areas exhibit high compressive residual stress, refined grain size, and increased micro-strain and dislocation density. In the shot peening affected zone, the initial residual stress distribution does not affect the final residual stress distribution, and the final residual stress distribution mainly depends on the material hardness and shot peening density. With the increase of shot peening density, not only the depth of compressive residual stress and the maximum value of residual stress increase, but also the shape of residual stress distribution changes in the three typical regions. However, when the saturation value is reached, the surface compressive residual stress remains nearly constant. Through shot peening, the fatigue life of laser hardening 17-4PH can be significantly improved. Under the load condition of 492MPa, with the shot peening density increasing from f=0.2mmA+0.1mmA to f=0.5mmA+0.1mmA, laser hardening + shot peening The fatigue life of the treated 17-4PH is increased by 30%. The results show that shot peening is an effective means to improve the fatigue life of laser hardened 17-4PH for the final stage of turbine blades.