Behavior Of Nitrogen In High Nitrogen Steel Preparation Processes

The steels contain more than 0.08 % (mass percent) nitrogen in a ferritic matrix or more than 0.4 %(mass percent)nitrogen with an austenitic matrix were defined as high nitrogen steels. High nitrogen steels have many advantages, i.e., high strength and toughness, good creep resistance and corrosion resistance, and so on . However, the nitrogen solubility in molten steels are very low under atmosphere pressure, so it is difficult to fabricate than common ones. Development of a cost low and effective technique to make high nitrogen steels has being payed more and more attention.For the purpose of developing new methods of manufacturing high nitrogen steels and promoting research and its application, the kinetics of nitrogen dissolution in Fe-Cr-Mn alloys under 0.1 MPa nitrogen, the kinetics of solid state nitriding of Fe-Cr-Mn stainless steels under 0.1 MPa nitrogen, the solubility and kinetics of nitrogen dissolves in molten stainless steels under pressurized induction melting under nitrogen atmosphere, the releasing of nitrogen from high nitrogen steels and influence factors during re-heating and remelting processes of those steels and solidifying processes had been investigated. The definition and classification, deveopment history, production technique, existing steels grades and their structure and properties, important properties and application of high nitrogen steels were reviewed in charpter 1. The behavior of nitrogen during high nitrogen steels manufacturing and welding processes was also reviewed.The behavior of nitrogen dissolves in Fe-Cr-Mn (Ni) alloys has been reported in charpter 2. At 1873 K , the solubility of nitrogen had been measured in liquid Fe14Cr8Mn, Fel6Cr7Mn2Mo, Fe18Cr8Ni2Mo under 0.1 MPa nitrogen, the results are 0.242%, 0.299% and 0.226%, respectively. Nitrogen dissolution in liquid alloy followed Sievert' s law. The. mass transfer coefficients in these three liquid alloys were measured at 1873 K without stirring. Their values were 5.72×10^-3 m/min, 6.13×10^-3 m/min and 7.68×10^-3 m/min, respectively.Solid state nitriding of Fe-Cr-Mn stainless steel in 0.1 MPa nitrogen had been investigated and reported in charpter 3. The results showed that the average nitrogen content in those solid state steel samples were proportional to square root of nitriding time. The solid state nitriding time needed were influenced by chemical composition of the steels. At 1473 K, when the solid state nitriding reaction arrived equilibrium, the saturated nitrogen content in Fe-Cr-Mn stainless steels can be described as w[N]_eq=0.066w[Cr] + 0.029w[Mn] — 0.608. It was confirmed that the average nitrogen content had exceeded 0.5% (mass percent) could be obtained after solid state nitriding for 12.5 h under 0.1 MPa nitrogen atmosphere at 1473 K, with the mass percent of w[Cr]+w[Mn]≥20%. The average nitrogen mass percent would decrease when the average oxygen mass percent increase in those steels. Solid state nitriding of experimental was difficult to be continued if the average oxygen mass percent exceeds or equals to 0.04 % (mass percent), but the nitrogen absorption rate could be markedly improved by adding 0.5% (volume percent) H₂ gas into nitrogen. The dissolved nitrogen could not release from Fel5Cr-0.293N steel and Fel8Cr8Mn-0.672N steel under 0.1 MPa argon until the temperature reached 1373 K. When the temperature was higher than 1373 K, the dissolved nitrogen would release and the rate of nitrogen release was influenced by chemical composition of those steels.High nitrogen Fe-Cr-Mn (Ni) stainless steels melted by pressurized induction furnace hadbeen investigated in charpter 4. The solubility of nitrogen had been measured in liquid Fel2Cr, Fel9Crl5Mn, Fe20Cr8Mn and Fel7Cr5Mn5Ni under 1.0 MPa nitrogen at 1913 K, the results were 0.391%, 1.120% , 0.899% and 0.692%, respectively. It was found that the nitrogen dissolution in liquid stainless steels deviated from Sievert' s law in present work. It was also confirmed that the mass transfer coefficient of nitrogen in these four liquid steels were influenced by the content of surface active element, oxygen and sulphur, the nitrogen dissolving rate and equilibrium nitrogen content would markedly decrease as the oxygen content increase in liquid steels. At 1913 K, the mass transfer coefficient in Fe20Cr8Mn liquid stainless steel was measured in present work under 1.0 MPa nitrogen. The value was 1.38 X 10 2mmin^' while the oxygen content beloweed 350 X \0A%, and the reaction order of nitrogen absorption was still first order.Thermal analysis (TG&DTA/DSC) and nitrogen releasing kinetics of high nitrogen steels had been investigated in charpter 5. The starting temperature of nitrogen releases from high nitrogen steels during heating process under 0.1 MPa argon atmosphere, The temperature was proportional to w[Mn] and could be described as T (°C ) = 1027.1 + 12.89n'[Mn]. The nitrogenda -—releasing kinetics of high nitrogen steels could be described as — = Ae RT(\-a) which adtwais nitrogen releasing fraction. The phase changes temperature, nitrogen releasing beginning temperatures and the activation energies of nitrogen release for four high nitrogen steels during heating process under 0.1 MPa argon atmosphere were obtained.The behavior of nitrogen in high nitrogen steel during remelting and solidifying had been investigated in charpter 6. The results showed that the nitrogen content in welding line would decrease as nitrogen partial pressure decreasing. The residual nitrogen content in Fel7Cr5Mn5Ni could keep at 0.6% (mass percent) if the nitrogen partial pressure was higher than 0.2 MPa. The nitrogen content in those steels during remelting processes did not follow Sieverf s law. It could be predicted by equation w[N]measured= "0.254 +1.239{ w[N]eq + 2X (1.46xlO"1 + 7.12X 10" 3w[Cr]-7.25 X 10"4w[Cr]2+3.38 X 10⁵w[Cr]3-8.08 X 10⁷w[Cr]4)}. Although small size high nitrogen steel parts could be welded successfully and the nitrogen loss at welding line could be restrained effectively under pressurized high frequency induction welding process, the problems, i.e., blow hole in welding line would still appear . It was also confirmed that the nitrogen absorption and desorption rate constant k was influenced by the content of surface active elements, oxygen and sulphur.The main conclusions had been summarized in charpter 7.