Research On Improvement Of 40Cr Steel Properties By Ultrasonic Surface Rolling Processing

Ultrasonic surface rolling processing (USRP) is put forward, in order to improvethe general properties of surfaces. Testing device includes the improved digitalultrasonic power supply (with more accuracy and stable frequency tracing andamplitude controlling systems) and USRP operator, in which, the processing tip isdesigned as a rolling ball, so that the service life can be greatly prolonged, whichmakes USRP more practical and economical.Take as-supplied, quenched and tempered 40Cr steel as testing material. Analysismethods, including TEM, SEM, XRD, measurements of micro-hardness and surfaceroughness and so on, are used to discuss the micro structure and influence ofprocessing parameters on mechanical properties of USRP surfaces. Contrast tests ofthermal stability, corrosion and wear resistance are carried out to investigate theservice performance of USRP specimens.Micro-structure observation and test results show that: USRP surface is of gradeddistribution; surface enhancement and grain refinement are the results of plasticdeformation accumulation. As for the quenched and tempered 40Cr specimen, theprecipitated and dispersion-distributed carbide strengthens the surfaces and anchorsthe dislocations. Nanocrystallization can be realized by USRP, with grain size of 3~7nm at the top surface and the nano-structured layer is about 100 micrometers thick.Diffraction patterns indicate that: solution of cementite takes place in a certain rangefrom top surface. Micro strain decreases by dislocation movements and grainboundary formation, so micro strain is not the biggest at the top surface.USRP parameters have influence on surface hardening, surface roughness andresidual stress to some degree. Appropriate USRP treatment can reduce and eveneliminate the surface defects. However, repeated plastic deformation may damage thesurface, and micro-crack can be induced when processing energy exceeds the tensilestrength.Contrast tests show that: USRP can make the breakdown potential and corrosionpotential of 40Cr specimens move positively, which indicates the improvement ofcorrosion resistance, and the bigger the parameters are, the better the corrosion resistance will be. After annealing 2h at 400℃, surface grains grow up to some degree,and the surface micro strain decreases. Under different friction conditions, mechanismof wear surfaces varies. Generally speaking, USRP improves the wear resistance ofsurfaces, especially under lubricating, the increase is obvious.