Effects of ultrasonic impact treatment combined with the electric discharge surface alloying by molybdenum on the surface related properties of low-carbon steel G21Mn5

Abstract To enhance the fatigue, corrosion and wear behaviors of low-carbon casting steel 20GL (G21Mn5 in EN classification) it was subjected to the effects of severe plastic deformation induced by ultrasonic impact treatment (UIT) and the electric discharge surface alloying (EDSA) with molybdenum. The structural formations in the sub-surface layers were characterized by means of light microscopy, X-ray diffraction analysis and transmission electron microscopy (TEM). Several intermetallic phases were observed in the EDSA-induced layer. TEM studies of transition area between the EDSA layer and steel substrate (at a depth of 15–30 μm) demonstrate that the combined EDSA + UIT process induces the formation of a dislocation-cell structure in the alloyed ferrite grains. Both the cell boundaries and separate dislocations are often blocked by fine intermetallic and/or carbide particles. SEM analysis of fatigue fracture surfaces reveals the subsurface crack nucleation in the UIT-processed specimens instead of superficial crack initiation observed in the pristine and EDSA-processed ones. This provides enhanced fatigue strength and prolonged lifetime of the steel specimens after UIT and EDSA + UIT processes. High molybdenum content supports the improved corrosion resistance and fine intermetallic precipitates in the Fe-Mo protective layer provide the increased hardness and wear resistance of the EDSA + UIT-treated specimens. Thus, the specimens underwent combined EDSA + UIT treatment were shown to exhibit simultaneously increased fatigue strength (by ~ 5%) in low cycle fatigue regime, anti-corrosion properties (by ~ 85%) and wear resistance (by ~ 43%) in comparison with those of the pristine steel G21Mn5.