Abstract
Effects of shot-peening were investigated on the resistance to softening during tempering at 573K and the contact fatigue strength of JIS SCr420H steels carburized conventionally and carbo-nitrided to a surface nitrogen content of about 0.70mass% N. Experimental methods involved in the investigation include hardness measurement, X-ray diffraction analysis and roller-pitting fatigue test.
The resistance to softening during tempering in the carbo-nitrided layer is found to be higher than in the carburuized layer, which is due to the precipitation of γ'-Fe4N. It is further improved by shot-peening, which effect is larger after carbo-nitriding than after carburizing. The reason seems to be that recovery of dislocations introduced by shot-peening would be delayed by solute nitrogen and precipitation of γ'- Fe4N. Contact fatigue strength in terms of pitting durability and wear resistance is estimated from a roller-pitting test under a Hertz stress of 3.68 GPa added slip (-40%) under an assumption that the temperature of contact surfce rose to nearly 573K. Shot-peening is shown to improve the pitting life of the carbo-nitrided specimen by as much as 25 times, compared with only by 2times for the carburized specimen. Pitting life can be explained from the differences in the initial surface hardness and the resistance to softening. The pitting life was also found to correlate positively with the surface hardness of the specimen tempered at 573K.
