Research on Microstructure and Properties of Cr-Mn-N Austenitic Stainless Steel for Non-Magnetic Drilling Collar Application

The effects of heat treatment and Nitrogen content on recrystallization process,precipitation behavior and mechanical properties of Cr-Mn-N austenitic stainless steel for non-magnetic drilling collar(NMDC) application were studied.Stabilizing element Nb was added in to avoid the formation of Cr2N precipitates.Microstructure observation results show that recrystallization process of specimen with N content of 0.62wt% can be completed after annealing treatment at 950 ℃ for 5 min.Annealing time of longer than 5 min will lead to the growth of equiaxed grains.A great deal of nanometer grade dispersed spot-like Nb nitride is discovered in the substrate.Meanwhile,Nb-Cr-Fe intermetallic compound precipitates with average particle size of 1-2 μm are also observed,which can be thoroughly solution treated after heating to 1050 ℃ for 5min.The formation of this type of precipitates can occur during heating within temperature range 600-900 ℃.The yield strength and tensile strength of specimen with N content of 0.78wt% are approximately 880 MPa and 1100 MPa,respectively,which are both about 100 MPa higher than that of specimen with N content of 0.62wt%.This results indicate that the N content increment from 0.62wt% to 0.78wt% induces the obvious strengthen effect.The problem of serious room temperature ductility deterioration induced by Nb-Cr-Fe type precipitates can be resolved by fully solution treatment.