Plastic Behavior Of NM300TP Tempering Process And Its Effect On Residual Stress

High residual stress levels and severe distortion during preparation and processing are common problems faced by Low-alloy wear-resistant dual phase steels.In practical production,tempering heat treatment is usually used to reduce the quenching residual stress,but due to the relaxation mechanism of residual stress during tempering of lowalloy wear-resistant dual phase steels is unclear,the residual stress of the material is often adjusted qualitatively by raising the temperature and extending the time in the actual production process based on experience,which easily leads to the reduction of mechanical properties of wear-resistant steels during tempering.Therefore,it is a key problem to master the relaxation mechanism of residual stresses in the tempering process of wearresistant dual phase steels and optimize the tempering process so that the wear-resistant steels have good wear resistance and toughness while fully relaxing the quenched residual stresses.In this paper,the plastic behavior of the tempering process of NM300 TP and its influence on the residual stress are investigated by combining residual stress characterization and microstructure analysis,which provides a theoretical basis and process reference for the development of wear-resistant steels with low stress,high hardness and high toughness.The main research contents are as follows:(1)Tempering transformation and stress relaxation of NM300 TP.The tempering microstructure transformation includes: carbon segregation in martensite(below 75 ℃),ε-carbide precipitation in martensite(200～250 ℃),and θ-carbide in martensite and ferrite(350 ℃);influenced by the above microstructure transformation process,the residual stresses relaxation in NM300 TP tempering process also appear in the carbon segregation,ε-carbide precipitation,and θ-carbide precipitation stages.(2)The evolution mechanism of NM300 TP residual stresses during tempering: below tempering at 75 °C,the stress relaxation is controlled by the short-range diffusion of carbon atoms in martensite,and the short-range diffusion of C causes vacancy movement,which induces macro residual stress relaxation;tempering at 200 °C is controlled by the precipitation of ε-carbides in martensite,and the volume contraction of martensite caused by the precipitation of ε-carbides induces M/F interphase stress relaxation;tempering at350 °C is controlled by the synergistic ferrite/martensite precipitation of θ-carbide,the collaborative precipitation of cementite significantly reduces the stress level of NM300 TP.(3)Tempering time can affect the relaxation process of residual stress by changing the effective carbon concentration in the microstructure.Extending the tempering time can change some of the "ineffective carbon" into "effective carbon",which in turn changes the precipitation plastic behavior and the residual stress relaxation pattern.After extending the tempering time from 30 min to 45 min,part of the "invalid carbon" dissolves,resulting in overlapping precipitation of ε-carbides and θ-carbides in the microstructure,and the double plastic superposition fully relaxes the residual stress,reducing the elastic strain energy in the sample from the original 271.62 MPa·mm to76.09 MPa·mm,reducing by 72%.