Investigation On Formation Of Delayed Cracks Of Wear-resistant Steel

To the problem of delayed cracking of the high strength low alloy wear resistantsteel plate NM360after cutting, the fracture, surface cracks and microstructure wereinvestigated by optical microscopy, electron scanning microscopy and energyspectroscopy, and the residual stress was tested by X ray. Micro alloying element Nb,Vand Ti in steel carbon nitrides precipitation mechanism in the process of austenizationwere studied application of thermodynamics model of precipitation. The wear resistantsteel tempering temperature and tempering time of heat preservation was optimized.The main results are as follows:(1)The nitride TiN is considerer as the dominant precipiate in high temperatureregion, which is the main factors to prevent the austenitic grain growth. V-richcomplex carbide is the main precipitate in low temperature region.(2)The fracture source was in the center of the thickness of the steel plate andthere are several center segregation in the mid-depth of the plate, the crackingassociated with obvious center segregation defects.(3)There are a large number of granular inclusions in the crack samples; thecutting edge heat affected zone thick grain structure reduced the plastic substrate.(4)organization was not uniform in the surface,1/4depth and mid-depth of thesteel, changed form lath martensite to martensite+granular bainite+ferrite, steelinadequate quenching.(5)The steel showed the characteristics of the typical stress crack, there aredifferent degree of resiual stree from edge to center, the impact of various residualstresses generating during heat treatment and the flame cutting on the defective areas isthe main reason for the formation of the delayed crack.(6)Through the tempering process optimization,the tempering temperature is450℃, tempering time of2.5H for the comprehensive performance of wear resistantsteel is excellent.