Study On The Microstructures And Properties Of N-alloyed Low Carbon Martensitic Stainless Steel

Low carbon martensitic stainless steels have been used widely because of its excellent properties such as hardenbility, comprehensive mechanical properties both at ambient temperature and at ccryogenic temperature, corrosion fatiguare strength and fracture toughness, and good processing properties such as casting, forging and welding. As the most advanced breakthrough in low carbon martensitic stainless steel field, one Nb-V-N multiple microalloying concept was proposed and industilized, which not only inhibit 8 ferrite formation without increasing nickel content but also increase the strength greatly. As one patented techonology, the principles and chemistry scope are to be clairified.As preliminary research on the N-alloying theory and techonology of low carbon martensitic stainless steel, a series of testing steels with different nitrogen contents were fabricated. The effect of nitrogen addition on the phase equilibrium and martensitic phase transformation temperature were examined through thermodynamical calculation and experimental testing. By comparing the phase constitute, microstructures and mechanical properties of different steels under various heat-treatments, the effects of nitrogen on the microstructureevolution and properties were investigated. The following conclusions were drawn:1. The thermodynamic calculation on phase equilibrium diagram shows that, N-addition can raise the austentite region by inhibiting 8 ferrite formation effectively at elevated temperature and inhibit the formation of intermetallicχphase andσphase, while N-alloying promote prepcipitattion of Cr2N in steel.2. N decrease the martensitic phase transformation start tempertture Ms of low carbon martensitic stainless steel accordingly increase the amount of retained austenite at ambient temperature. When 1.0%N is doped, the Ms is decreased about 430℃, the same effect as that of carbon. 3. Upon tempering at intermedium temperature reversed austenite will be formed inside the martensitic lath. The volume fraction of reversed austenite increases with the temperature until it reach a maximum at certain temperature, beyond which the volum fraction of reversed austenite decrease and is replaced by a new striped microstructure. Acicular Cr2N phase precipitate in N-containing steels during tempering, the higher content of nitrogen is doped, the less volume fraction of reversed austenite is formed.4. As the tempering temperature increases, the strength decreases firstly until it reach a minimum and increses again, while the elongation and impact toughness increases firstly till a maximum then decrease again. As the nitrogen content increases, the strength of N-containing low carbon martensitic stainless steel increases, while ductility and toughness decrease.