EFFECT OF COLD TREATMENT OR DEEP CRYOGENIC TREATMENT ON SELECTED PROPERTIES OF CASE-CARBURIZED 18CrNiMo7-6 STEEL

摘要

A comparative study on the influence of cold treatment (CT) and deep cryogenic treatment (DCT) on the properties of case-carburized steel was made. Specimens made of 18CrNiMo7-6 steel underwent carburizing with two different surface carbon concentrations. After thermo-chemical treatment samples were subjected to cold treatment, performed at -80°C, or deep cryogenic treatment, at -180°C, and tempered. Basic material properties, such as surface hardness, cross-section hardness profile, value and state of internal stress and retained austenite content, were measured and evaluated. Charpy u-notch impact test was employed to determine steel toughness. The effect of CT and DCT on the wear resistance of specimens was assessed by means of three rollers-cone method. Microstructures of specimens were analyzed by means of light microscopy. Cold treatment and deep cryogenic treatment allowed to continue martensitic transformation, however a value of decrease of residual austenite content was similar regardless of the employed temperature of treatment. Austenite transformation was accompanied by an increase of hardness, measured at surface or cross-sections of specimens. Reduction of wear of specimens was also observed, but more beneficial effect was achieved for the CT. Sub-zero treatment processes, irrespective of the applied temperature, caused the decrease in impact strength of steel. Measurements of residual stresses showed the occurrence of compressive stresses in all specimens. After DCT and tempering diminishing of value of stress was observed, probably due to effect of precipitation of finely dispersed carbides.