Microstructural characterization of surface morphology on metastable austenitic stainless steel AISI 347 after different surface modification methods
Three surface modification methods were used on metastable austenitic stainless steel AISI 347, namely (i) cryogenic turning process, (ii) ultrasonic surface modification and (iii) gas dynamic cold spraying to manufacture a martensitic surface layer on the austenitic substrate material. Previous studies have shown that such surfaces improve significantly the fatigue behavior and wear resistance.
Systematic material characterizations at the surface and in the near surface regime up to ~300 µm distance from surface were carried out, including nondestructive testing techniques like magnetic induction measurement, X-ray diffraction analysis as well as destructive testing methods, e.g. metallographic investigation, hardness test, scanning electron microscope study and focused ion beam investigation. Therefore, the relationship between manufacturing processes / parameters and surface morphology was developed.
After cryogenic turning with a feed rate f = 0.15 mm/rev, the maximum volume fraction of 18 % alpha’-martensite was detected on the surface and decreased in the subsurface. By increasing the feed rate to 0.35 mm/rev, the martensite fraction on the surface was lower (5 %) due to higher working temperature, the maximum value with 25% alpha’-martensite was determined at 50 µm from the surface and decreased in the next 100 µm. Epsilon-martensite was identified in subsurface with both turning parameters, showing a similar distribution trend but with maximum value at different depth. Microhardness profile across the gradient direction correlated well with the alpha’-martensite distribution. High tensile residual stresses (800 MPa) were determined on the surface regardless of working parameters, which decreased quickly and turned into compressive residual stress from 40 µm till 200 µm with a maximum value of – 200 MPa. After ultrasonic surface modification, the maximum alpha’-martensite volume fraction of 26 % on the surface was identified, meanwhile a compressive residual stress (- 104 MPa) was determined on the surface.
The authors gratefully acknowledge financial support from Deutsche Forschungsgemeinschaft (DFG) within the Collaborative Research Center (SFB) 926, Project-No. 172116086 as well as the technical support for surface treatment from Institute for Manufacturing Technology and Production Systems (FBK) and Institute of Particle Process Engineering (MVT) at University of Kaiserslautern.