Effects Of Step-by-step Tempering Process On Microstructure And Properties Of High-strength Hull Steel

In this paper,the heat treatment methods of solid solution + tempering(QT),solid solution + short-time high temperature tempering + long-time low temperature tempering(QTT)are used to study two typical tempering treatments for 9NiCrMoVCu1.5 steel and7Ni2 Cr Mo V3 Cu.5 steel.Influence rule of microstructure and properties of 1000 MPa highstrength hull steel.Research indicates:Under the QT process,with the increase of tempering temperature,the two test steels have similar strength variation laws.During the tempering process,the softening of the matrix and the precipitation strengthening of the second phase jointly determine the properties of copper-containing steels.Among them,when tempered below 425°C,the copper-containing steels of the two strength grades are in an under-tempered state,and precipitation strengthening dominates;when tempered above 425°C,the steel is in an over-tempered state,and the martensitic matrix Softening dominates.Tempering temperature and tempering time have a significant effect on the precipitates of Cu particles and carbide particles in the steel.The precipitates of Cu particles in the under-tempered state are mainly fine particles smaller than 5 nm.The precipitation size of Cu particles ranges from 10 to 30 nm,and after coarsening to a certain size,they form rod-like precipitation.Under the QTT process,the change of the secondary tempering temperature has relatively little effect on the strength of the two test steels,but it still shows the change rule under the QT process.increased,the strength of the test steel decreased slightly;the extension of the secondary tempering time improved the low temperature toughness of the test steel,and the main mechanism was that the brittle phase in the test steel was continuously dissolved and transformed into stable M2 C carbides;Under the tempering time(1h),the toughness of the test steel does not change significantly.This is because if one tempering is too sufficient,the structure of the material and the precipitation of the second phase will be sufficient,and the properties tend to be solidified.The QTT heat treatment process enables the two test steels not only to obtain a high strength of 1000 MPa but also to have good low temperature toughness.Compared with the QT process,the Akv at-40°C is increased by about 150 J.This is mainly related to the dissolution and transformation of brittle phase cementite into M2 C during the second longterm tempering after a short-term high-temperature tempering treatment.Short-term hightemperature tempering makes fine and uniformly dispersed Cu precipitation nucleation sites.During the subsequent long-term low-temperature tempering process,Ni continuously diffuses and enriches near the Cu precipitation,forming obvious Cu and Ni synergistic precipitation.The size of Cu precipitation is reduced,so that the high-strength hull steel obtains an excellent match of strength and toughness.Comparing the properties of the two systems of high-strength hull steels,it is found that the 9NiCrMoVCu1.5 steel is mainly composed of Cu-rich precipitation,while the7Ni2 Cr Mo V3 Cu.5 steel has more intense synergistic precipitation.On the one hand,the higher Cu content increases the number of Cu-rich precipitates.At the same time,the precipitation of M2 C polytype carbides further improves the strength of the steel,and the strength of the latter is about 100 MPa higher than that of the former.