Composition Design And Properties Of Fe-Cr-Mn Stainless Steel For Solid Solution Nitriding

Solid solution nitriding is different from traditional nitriding and high nitrogen steel smelting.And the solid solution high nitrogen concentration nitriding layer with a thickness of more than 1 mm can be easily obtained,which provides a new idea for the industrial production of high nitrogen steel.On the basis of existing stainless steel,domestic and foreign scholars have done a lot of related researches.Many achievements have been made in the aspects of composition characteristics,nitriding process,layer performance and reaction diffusion mechanism.But so far,there is no report on developing special steel for solid solution nitriding with high nitrogen solubility and high diffusion based on thermodynamic calculation.Based on schaeffler diagram and diffusion theory,this paper designs fe-cr-mn series steels for nitriding by means of orthogonal test and coefficient of economy and practicality.A special steel for solid solution nitriding was designed and smelted,and the interaction between nitrogen and alloy elements,the reaction diffusion kinetics of nitriding process and the basic properties of nitriding layer as structural materials were studied.The results show that:Nine kinds of nitriding steels were produced by medium frequency induction furnace.The ingot has good appearance and relatively few casting defects.There are no microstructure defects such as microcracks,inclusions,pores and so on which can affect nitriding effect obviously.The cast microstructures are mainly?-phase and?-phase.CrFe and other intermetallic compounds appear with the increase of alloy content.The content of intermetallic compounds is relatively high in steel 5#to 9#with higher contents of Cr and Mn,but the intermetallic compounds in steel are basically dissolved after solid solution treatment,and?-single phase or?-?double phase are obtained.The above-mentioned Fe-Cr-Mn series steels were nitrided by solid solution nitriding under normal pressure for 24 hours,and the nitrided layers,above 0.6 mm,were obtained.Among them,the thickness of the nitriding layer of 4#steel was as high as 1.27 mm.The order of the effect of alloying elements on the thickness of nitriding layer is R_Cr>R_Mn>R_Al>R_N.The optimized composition is:Cr18Mn21N0.2AlMo2.5Nb0.15?Here in after referred to as Cr18Mn21 steel?.After nitrided by solid solution nitriding under normal pressure for 24 hours,layer thickness is up to 1.4 mm,the corrosion potential of nitrided parts in 3.5%NaCl solution was increased and the corrosion current was decreased.The impedance spectrum showed that the impedance arc radius of Cr18Mn21 steel was increased from 4000 ohm to 16000ohm.Research on its abrasion resistance shows that,the wear loss of nitriding layer of Cr18Mn21 steel with different nitriding time decreased obviously with the increase of nitriding time,and the wear resistance of the layer transformed into complete austenite with the increase of nitrogen content was obviously higher than that of the substrate.The wear surface of the layer was relatively smooth.There is no obvious fatigue shedding.Both proves that the microstructure of the infiltrated layer has good fatigue wear resistance.The kinetics of solid solution nitriding was studied and results showed that the activation energy of nitrogen diffusion in Cr18Mn21 steel was 99.67 kJ/mol by solid solution nitriding under normal pressure for 24 hours,which was significantly lower than that of stainless steel?133.1 kJ/mol?in the present solid solution nitriding study.And its nitriding efficiency is significantly higher than 304 stainless steel and other commonly used solid solution nitriding steel.