Study On Strengthening-Toughening Mechanism And Heat Treatment Of Nitrogen-Alloyed Hot-Working Die Steel

At present,the application of nitrogen in stainless steel and the mechanism of micro action have been widely reported,but the application and mechanism of nitrogen in die steel are rarely studied.Based on the mechanism of nitrogen,the research group put forward the idea of adding nitrogen to Cr-Mo-V die steel to improve its hardness and ensure good toughness.In this paper,the rationality of the micro-nitrogen alloying idea of die steel was verified by the microstructure test and process optimization test of nitrogen-containing Dievar and 8Cr3 hot work die steel.The dissolution and precipitation behavior and mechanism of nitrogen in Cr-Mo-V hot work die steel were systematically studied by means of dilatometer,scanning electron microscopy(SEM),transmission electron microscopy(TEM),X-ray diffraction(XRD)and three dimensional atomic probe(3DAP).The main conclusions are as follows:(1)The addition of trace nitrogen can significantly increase the precipitation amount and dissolution temperature of MC type carbide in Dievar steel,while the addition of trace nitrogen to 8Cr3 steel will generate nitride CrN and increase the dissolution temperature of M7C3 type carbides.In Dievar and 8Cr3 steels,nitrogen can increase the quantity and stability of quenched undissolved carbide,raise the temperature of grain coarsening,play a significant role in grain refinement,but have little effect on the content of retained austenite in the tempered structure of the two steels.The hardness and toughness of Dievar hot work die steel can be improved by trace nitrogen under conventional heat treatment,but its thermal stability is deteriorated.However,the addition of trace nitrogen to 8Cr3 steel will cause great damage to toughness due to the excessive amount and size of undissolved carbides.Although the hardness can be improved,the toughness is deteriorated seriously.(2)Compared with the steel without nitrogen,Dievar steel with nitrogen alloying can obtain satisfactory properties in a range of quenching temperature(1030??1100?).The higher the quenching temperature,the stronger the thermal stability of the structure.When the quenching temperature is between 1060? and 1080?,the hardness and thermal stability can be significantly improved without toughness loss,compared to that of nitrogen-free steel.(3)The spheroidizing structure of Dievar steel can be significantly refined by a small amount of nitrogen.The undissolved V(C,N)in the nitrogen-containing steel can be used as the nucleation core of Cr23C6 carbide and promote the refinement of Cr23C6 carbide.The amount and stability of undissolved V(C,N)carbides and the ability of refining the original austenite structure are increased by a small amount of nitrogen.The ability of refining the original austenite structure after adding nitrogen is 5.8 times higher than that of nitrogen-free steel.In the tempering process,the essential factor that determines whether the thermal stability of nitrogen-containing steel is better than that of nitrogen-free steel is the vanadium and nitrogen in the nano carbide.When the quenching temperature is 1060?,the coarsening ability of tempered carbide of nitrogen-alloyed steel is obviously weakened,and the coherent relationship with matrix is better.In the tempering structure of the two steels at 600?(4h),the metastable M3C carbide is the main one.The N and Mo elements in the nitrogen containing steel are higher than that of nitrogen-ree steel,while the V element is lower than that of nitrogen-free steel.When the tempering time is extended to 8h,M3C will change to M(C,N)and M23C6 carbides.(4)MC,M23C6,M6C and other carbides in hot work die steel can be doped with nitrogen atoms.The doping of nitrogen strengthens the covalent bond strength between atoms and makes the precipitates more stable.The effect of Trace Nitrogen energy on the thermal stability of MC,M3C and M7C3 carbides in die steel is significant.The stability of nitrogen to undissolved carbide is not only related to the absolute content of nitrogen,but also to the ratio of nitrogen to carbon.(5)In the quenching state,most of the nitrogen exists in the form of V(C,N).During the tempering process,the nitrogen and carbon are dissolved into the matrix again,and the replacement reaction takes place.The replacement direction is along(200)M(C,N)//(110)a.Fine grain strengthening and precipitation strengthening are the main strengthening methods of die steel.The addition of trace nitrogen mainly improves the performance of die steel by strengthening the level of fine grain strengthening and precipitation strengthening.Trace nitrogen can make full use of the vanadium element in Cr-Mo-V die steel to produce fine,dispersive and high roundness undissolved carbides which is a good way of alloying.At the same time,the tempering stability can be improved by raising the quenching temperature properly and ensuring the alloy element to be fully solid soluted.