Metallurgical Fundamental And Properties Of High Nitrogen Austenitic Stainless Steels

A great attention has been paid to the development of high nitrogen austenitic stainless steels, because their excellent mechanical and corrosion resistance properties. Research on the metallurgical fundamental and properties of high nitrogen austenitic stainless steels can improve the deveoplentment the steels in a large scale in China.This study is a part of key Program, "Mechanism of nitrogen in high nitrogen steels and its effect on properties" financed by National Natural Science Foundation of China and Shanghai Bao Steel Corporation. The nitrogen solubility models in different phases have been built by analyzing the metallurgical fundamental of high nitrogen austenitic stainless steels. The high nitrogen austenitic stainless steels has been manufactured by vacuum induction furnace and electro-slag remelting furnace under nitrogen atmosphere, and the mechanical and corrosion resistance properties of high nitrogen austenitic stainless steels were researched.A new thermodynamic calculation model for the calculation of nitrogen solubility in molten stainless steel have been established by introducing a new term for the effect of nitrogen partial pressure on the nitrogen activity coefficientδNp. When pN2/pθ≥1.0;δNp=0.06, and when pN2/pθ<1.0,δNp=0. The calculation results are in good agreement with the measured values. The calculation results show the nitrogen pressure can greatly improve the nitrogen solubility in molten stainless steel. Under a certain nitrogen pressure, the effect of temperature on nitrogen solubility in molten steel depends on activity coefficient of nitrogen which is determined by the composition of alloy. There is a negative effect of temperature on the nitrogen solubility when fN, 1873<-0.057 i.e., A>0, and a positive effect of temperature on the nitrogen solubility when fN, 1873>-0.057, i.e., A<0.Based on the previous literature reported, a thermodynamic calculation model has been developed to predict the nitrogen solubility in 8 ferrite phase and y austenitic phase of stainless steels. The model can well predict the nitrogen solubility in two phases. When the nitrogen pressure is increased, the nitrogen solubility of alloy inδferrite phase and y austenitic phase could be improved and the 8 ferrite zone becomes narrow and even disappeared. So nitrogen precipitation can be suppressed during the solidification of high nitrogen austenitic stainless steels. Thus it is a valid method that increasing appropriately austenitic formation elements can avoid nitrogen porosity formation during the solidification of high nitrogen stainless steels. By analyzing the micro-segregation models, a conclusion can be obtained that high nitrogen pressure and rapid directional solidification are beneficial for manufacturing high nitrogen steels to prevent nitrogen porosity formation.The high nitrogen austenitic stainless steel ingots with theoretical nitrogen content can be obtained by properly controlling the melting and casting temperature and by adding the nitrided alloys in vacuum induction furnace. The high nitrogen austenitic stainless steels with sound and compact macrostructure can be obtained after electro-slag remelting, in which the maximum nitrogen content is 0.96%. The yield of nitrogen increases with the decrease of melt rate in the ESR process. Due to electroslag remelting under nitrogen atmosphere and the aluminum as deoxidizer is continuously added into the slag, the loss of manganese decreases obviously.The cold rolled A1 high nitrogen austenitic stainless steel has recrystallized completely within 40s to 60min annealing time at 1100℃. The grain size becomes from 8.3μm to 49.5μm when prolong annealing time from 40s to 60min. When decreasing the grain size, the tensile strength, yield strength and HV increase, but the elongation decreases. The relation between the grain size and the tensile strength, yield strength and HV meet the Hall-Petch equation, as shown Rm=895.7+17.2·d-1/2, Rp0.2=507.6+11.7·d-1/2, H=253.5+6.7·d-1/2, respectively. The relation between the grain size and elongation also meets the similar Hall-Petch relationship as shown A=73.4-2.6·d-1/2.A decrease of tensile strength and yield strength of Al steel is found for the steel by increasing test temperature, in the temperature range from-196℃to 500℃. The ductility is improved with increasing the test temperature in the range of -196 to 300℃. Above 300℃, the elongation of the steel decreases. The Al steel exhibits high strength and good ductility with about 1000MPa tensile strength and 60% elongation at RT. The tensile deformation of A1 and A3 steels during uniform plastic deformation are well described by the Ludwigson equation. The n1 values of the two steels investigated decreases when the crosshead speed is increased, which implies the strain hardening of high nitrogen steel is more active under low crosshead speed. With increasing the nitrogen content in high nitrogen austenitic stainless steel, the strength factor K1, work hardening coefficient n1, transient stressεL and exp(K2) increase, but the -n2 decreases.The A1, A2 and A3 high nitrogen austenitic stainless steels investigated all show the ductile to brittle transition behavior. The ductile to brittle temperatures of A1, A2 and A3 are-85℃,-90℃and -95℃respectively. The change of fracture patterns of high nitrogen austenitic stainless steels is dimple→shallow dimple→mixture of quasi-cleavage facet and dimple→cleavage facet. The fracture modes model of A2 steel at -196℃is transgranular cleave fracture.The 316L stainless steel has more excellent uniform corrosion resistance property than high nitrogen austenitic stainless steels in 40% H2SO4 at 60℃. The uniform corrosion resistance property of A2 steel is well than A1 and A3. The high nitrogen stainless steels are not suitably used in H2SO4 solution.The high nitrogen austenitic stainless steels with the pitting corrosion potential above 1 V have more excellent pitting corrosion resistance than 316L stainless steel. With increasing the nitrogen content in steels, the pitting corrosion potential, polarization resistance Rp and critical pitting corrosion temperature increase. So nitrogen in steel improves greatly the pitting corrosion resistance. With increasing the NaCl solution concentration and decreasing the PH of the NaCI solution, the pitting corrosion potential Eb10 of A2 steel decreases a little. Thus the A2 high nitrogen austenitic stainless steel exhibits good pitting corrosion resistance in strong acid and high concentration chloride ion.The A1, A2 and A3 high nitrogen austenitic stainless steels exhibit excellent crevice corrosion resistance. When increasing the nitrogen content in steels, the crevice corrosion resistance ability can be improved.The 2mol/LH2SO4+1mol/LNaCl+0.01mol/LKSCN of EPR solution can be used to check the intergranular corrosion sensibility of Fe-Cr-Mn-Mo-N series high nitrogen austenitic stainless steels, but is not suitable for 316L stainless steel. With decreasing the scan rate, the Ir/Ia and Qr/Qa all increase. The suitable scan rate is chosen as 1.6667mV/s. When the A2 steel is heat treated within 650℃to 950℃for 2h and air cooling, the amount of precipitation arrives at the maximum at 850℃. When increasing the temperature from 650℃to 850℃, the amount of precipitation increases. Above 850℃, the amount of precipitation decreases. The EPR experiment results show that the Ir/Ia and Qr/Qa increase with increasing the temperature to 850℃, decrease above 850℃. The amount of precipitation in grain boundary is well consistent with the EPR experiments results.The amount of precipitation of A3 steel heated at 700℃for 2h followed by air cooling is most, and its intergranular corrosion sensibility is the maximum. The intergranular corrosion sensibility of A2 steel is the minimum.