Effect Of Tempering And Warm Deformation On Tempering Brittleness Of Martensite

In this dissertation,two types of high-hardenability medium carbon Cr-Ni quenched and tempered steels with different phosphorus contents(named 66 P and 2P)were tempered and warm deformed at 450-650 °C.The effects of phosphorus content,tempering temperature and deformation temperature on temper embrittlement of martensite were studied.The mechanical properties of the test steel after tempering and warm deformation were tested,such as hardness,tensile strength and impact at the room temperature.The microstructure and fracture morphology of the sample were analyzed by metallographic microscope,scanning electron microscope and transmission electron microscope.The grain shape and size of the sample were analyzed by electron backscattered diffraction test.The results show that both medium carbon quenched and tempered steels obtained lath martensite structure after quenching.After the tempered lath martensite structure was tempered at different temperatures,medium-temperature tempered troostite and high-temperature tempered sorbite were obtained.Compared with tempering,after warm deformation,the ferrite laths of the two test steels had a certain direction in the spatial distribution,the structure was finer,and the ferrite laths were finer.With the increase of tempering and deformation temperature,the amount of carbide precipitation gradually increased and gradually spheroidized.Phosphorus content had basically no effect on the structural evolution of the test steel.After warm deformation,the strength and hardness of the two test steels have been improved.With the increase in tempering and deformation temperatures,the impact energy of the two test steels has also been increased to varying degrees;the precipitation of carbides inhibits the growth of grains,and carbides act as a hardened phase,which strengthens and hardens.In the temperature range of 450-650 °C,the impact energy of 66 P test steel after tempering was 14-41 J.After warm deformation,the impact energy was significantly increased to 135-220 J,which was 5-9 times higher than that without tempering.The high content of phosphorus element segregates towards the grain boundary during tempering,resulting in embrittlement of the material;after warm deformation,the existence of phosphorus element led to delamination and toughening,which eliminated the temper brittleness of high P steel.