Abstract
The present study proposes a novel method, i.e., combined severe shot peening (SP) and reversion annealing treatment, to grain-refine the surface layers of 304L austenitic stainless steel. Steel specimens were shot-peened at 0.7 MPa for 30 min, introducing 40% vol. α′ martensite, and then were annealed at 700 or 800 °C for different durations (30 s). As annealing reversed α′ martensite to austenite, the obtained surface layers consist of fully austenitic ultrafine grains. The smallest grain size obtained is about 500 nm at the top surface. SP elevates the microhardness to more than 500 HV. Although the grain-refined surface layers produced by the combined method are not as hard as that treated by only SP, they are harder (e.g., the specimen annealed at 700 °C for 30 s using a heating rate of 50 °C/s exhibited a peak microhardness of 400 HV) than the untreated surface layer (225 HV) due to grain refinement. Moreover, due to the absence of α′ martensite, they have higher corrosion resistance in H2SO4 solution than that treated by only SP.
Highlights
Surface grain refinements based on surface severe plastic deformation (SPD) treatments such as severe shot peening (SP) [1,2], ultrasonic impact/peening treatment (UIT/UPT) [3,4] and surface mechanical attrition treatment (SMAT) [5,6] have been applied widely to austenitic stainless steels (ASSs), in order to harden their surface layers and enhance their wear and fatigue resistance.significant strain-induced α0 martensite transformation and high density of microstructure defects such as dislocations may accompany with the grain-refinement process in the surface layers, degrading substantially the corrosion/stress-corrosion and hydrogen embrittlement (HE) resistance of the ASSs
Results of 60 μm was revealed in the SPed specimen, Figure 1a
Annealing and recrystallization, as the grain boundaries were partly corroded during etching in Figure 1b–f verify the occurrence of grain refinement in surface layers by annealing and Beraha’s reagent, probably because of slight sensitization
Summary
Surface grain refinements based on surface severe plastic deformation (SPD) treatments such as severe shot peening (SP) [1,2], ultrasonic impact/peening treatment (UIT/UPT) [3,4] and surface mechanical attrition treatment (SMAT) [5,6] have been applied widely to austenitic stainless steels (ASSs), in order to harden their surface layers and enhance their wear and fatigue resistance.significant strain-induced α0 martensite transformation and high density of microstructure defects such as dislocations may accompany with the grain-refinement process in the surface layers, degrading substantially the corrosion/stress-corrosion and hydrogen embrittlement (HE) resistance of the ASSs. Surface grain refinements based on surface severe plastic deformation (SPD) treatments such as severe shot peening (SP) [1,2], ultrasonic impact/peening treatment (UIT/UPT) [3,4] and surface mechanical attrition treatment (SMAT) [5,6] have been applied widely to austenitic stainless steels (ASSs), in order to harden their surface layers and enhance their wear and fatigue resistance. Lu et al [8] showed that when SP pressure is less than 0.4 MPa the grain-refinement effect plays a main role, the stress-corrosion sensitivity of 304 steel decreases with increasing SP pressure. Coatings 2020, 10, 470 effect plays a main role, stress-corrosion sensitivity increases with increasing SP pressure. Balusamy et al [9] found a deleterious effect of SMAT on corrosion resistance of 304 steel in 0.6 M
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