Fe–Cr–Mn–C–N Steels for Down-Hole Application in Sour Environments: Statistical Analysis of Mechanical Properties

Two types of Fe–Cr–Mn–C–N steels (CN66, CN71) were developed and investigated for down-hole applications. Results of the experiments were used to identify statistically significant metallurgical factors and to predict the mechanical properties of CN66 and CN71. The three factors involved in the study were cooling rate (air or water), carbon and nitrogen content (C+N, 0.66 or 0.71 in wt. %), and heat-treatment holding time (2 or 4 h). The experiments were designed with a three-factor full-factorial design with two replicates for each condition. Cooling rate and C+N content (composition) were significant variables that affected the mechanical properties of CN66 and CN71, and the most favorable mechanical properties of the developed steels were obtained for a fast cooling rate and a high C+N content. The microstructures of the CN66 and CN71 were investigated to understand these strengthening effects, and it was found that the formation of precipitates was controlled by the cooling rate and that fast cooling suppressed precipitate formation. In addition, CN66 showed that a larger amount of ferrite phase in the material than that of CN71. The mechanical properties of CN66 and CN71 were compared to Fe–Cr–Ni based non-magnetic austenitic steel, 316L stainless steel, to evaluate the application possibilities of these alloys for down-hole applications. The superior mechanical properties of CN66 and CN71 show their potential to replace 316L stainless steel in many applications.