| The yield strength of 5NiCrMo welded structural steel is not less than 785Mpa,this steel not only has high strength,but also has good toughness,especially demands lower toughness.5NiCrMo steel is traditionally produced by the process of quenching and tempering,but it has disadvantages of poor tempering stability,high yield ratio and so on.In order to solve these shortages and ulteriorly improve low temperature toughness,the heat treatment process of intercritical quenching and tempering(QLT)and quenching and tempering(QT)were carried out in 5NiCrMo steel.This paper systematically studied effects of different process parameters on properties and microstructures in different heat treatments.Based on it,the structure evolution process and the mechanism of improving toughness in QLT process was disscussed.The process parameters,such as intercritical quenching temperature and tempering temperature have a remarkable effect on properties.The results showed that strength trended to be decreasing then to be increasing,the yield ratio were improved and low temperature toughness decreased along with intercritical quenching temperature increasing,the obdurability maching was in optimal condition around 690℃for opimum intercritical quenching temperature.QLT-treated 5NiCrMo steel improved the tempering stability and decreased the yield ratio,especially increased the low temperature toughness.With more tempering time,the low temperature toughness under -84℃fristly trended to be increasing,then to be decreasing.The process parameters,such as intercritical quenching temperature and tempering temperature also have a remarkable effect on reversed austenite.The results showed that reversed austenitic content fristly trended to be increasing,then to be decreasing along with intercritical quenching temperature increasing.That was to say,there was a peak value of reversed austenitic content around 690℃.With higher tempering temperatures,the content of reverted austenite became more.it was under 2%in QT process,but 8.17%in QLT process.With more tempering time,Reversed austenitic content trended to be increasing,then to be decreasing in QLT process. Microstructures in QLT and QT process were observed by Optics Microscope (OM),Scanning Electron Microscope(SEM)and Transmission Electron Microscope (TEM),the structure evolution process for QLT treatment was thoroughly analyzed.As a result,tempered martensite and a little carbide were gained in QT process.The mixed microstructure of tempered secondary martensite+ferrite+a little carbide,besides granular or brillet and island or strip reverted austenite precipitated along austenitic grain boundaries and martensitic boundaries were obtained in QLT process.Reverted austenite all kept K-S relation with the matrix.With intercritical quenching temperature increasing.there existed a big difference in shape,distribution,mass number and component of secondary martensite.It caused the fluctuation of component during intercritical quenching,in the subsequent tempering process,because alloying agents were concentrated at the boundaries of secondary martensite,the temperature of these areas exceeded the transformation point,therefore It formed into reversed austenite in 5NiCrMo steel.The mechanism of improving toughness in QLT process was studied.The results showed that QLT process effectively improved low temperature toughness because of structure refining and distribution improvement of a little harmful impurity elemnts along boundaries,as well as precipitation of reversed austenite.Reversed austenite was rich in carbon,nickel and manganese,in which nickel content reached 9.47%and manganese was 1.05%.There was no other than steady reversed austenite that could improve low temperature toughness.The more reversed anstenitic content,the better low temperature toughness. |
