Abstract
Various thermal mechanical treatments are applied to produce differentinternal micro-structures in order to study the mechanism which mightinfluence the mechanical properties of steel. SK 3 was chosen in thisexperiment. Improvements of both tensile strength and ductility was attributed tothe presence of subgrains with sharp boundaries (about 1-2 u in size),alignment of cementite and effects due to strain-bardenning. Fatigue S-N curves were determined for all samples concerned. For bothair cooling and water quenching processes after up quench, the fatiguelife descends in the order of: lamellae pearlite with TMT, lamellaepearlite without TMT, with spheroidite going through TMT the least. Thisindicated that the morphology of carbide is one of the important factorsinfluencing fatigue resistance. As for the TMT treated samples, thewater quenched ones always have better fatigue resistance than aircooled ones due to the small amount of martensite formed in the pearlitematrix. Subgrains formed during the TMT process was also suggested to behelpful in suppressing fatigue crack initiation since the long rangedislocation movement and slip band formation were always prohibited bysubgrain boundaries. The feature of fracture surfaces were not greatly influenced by aircooled compared with water quenching process.