Q & P Process To Obtain A New Type Of Martensitic Steel Microstructure And Performance

Conventional heat treatment of martensite steels is quenching and tempering process, it has been applied to steels to meet good combinations of strength and toughness from the martensitic structure.For improving the mechanical property,a new process concept "Quenching and Partitioning Process"(Q&P)has been proposed by USA in 2003,and the results showed the potential to creat a new kind of steel microstructure with controlled amounts of retained austenite to improve the mechanical property.In this paper,several steels with different carbon content were subjected to Q&P process, and the effect of different quenching temperature(QT)on mechanical properties and microstructure were studied.The microstructure and mechanical properties of steels by Q&P process compared with conventional technology was illustrated.The Q&P process can improve the toughness of the steels greatly with the little loss of strength,so it can reach the better combination of strength and toughness of the steel;With the increase of the quenching temperature in Q&P process,the volume fraction of martensite reduces during the first quench step,the strength of the steels with different carbon content drops,and the impact toughness has a peak point,but the plasticity and hardness changes slightly;Comparing to conventional technology,the loss of strength of Q&P process reduced with the decrease of the carbon contents;For 21Si2Ni3 steel,the Q&P process can improve the toughness more greatly,and the influence tendency reduces with the increase of the carbon contents.The aspect of the microstructure,deviation from the predicted results is then indicative of incomplete partitioning,for example,loss of carbon atoms to competing processes, mainly thought to be carbide precipitation;The Q&P process can get more residual austenite than conventional technology,and the residual austenite between martensite laths distributed uniformly;The Q&P process has different microstructure features,The martensite is randomly in conventional process,but in the Q&P process,the martensite laths and the blocks which are made up of parallel laths having the same orientation is clear. With the increase of the quenching temperature,martensite laths have a more clear tendency to align themselves parallel to one another,and the continuous residual austenite between martensite laths also becomes discontinuous.The difference in morphology feature is the important reason for the change in mechanical properties.