Study on Alloying of 2Cr13 Martensitic Stainless Steel for Oil Country Tube Goods Use
|Keywords||Fe-0.2%C-13%Cr martensitic stainless steels alloying phase equilibrium carbide oil country tube goods (OCTG)|
Fe-0.2%C-13%Cr martensitic stainless steel has been made of high corrosion-resistant oil country tube goods (OCTG) graded at 80Ksi and 95Ksi, however, new 110Ksi-graded OCTG containing 13%Cr with higher strength and corrosion resistant is demanded. The upgraded properties maybe achieved through approperate alloying based on the original chemistry or heat-treatment optimization. In present research, equilibrium phase diagrams based on Fe-13%Cr-C system calculation and carbides characterization using laser scaning confocal microscopy(LSCM), scaning electronic microscopy(SEM) and transmission electronic microscopy(TEM) were conducted to investigate the effects of alloying elements, including Mo, Ni, V and N, on the phase equilibrium and carbide precipitation of Fe-0.2%C-13%Cr martensitic stainless steel, the effect of heat-treatment parameters modification on microstructure and properties were discussed as well. The following conclusions were drawn:1. The main effects of Mo on phase equilibrium of Fe-13%Cr-C system including decreaseing the (δ+γ)/γcritical point and narrowingγphase region through promotingδ/αferrite andσphase formation and enlarge the single-phase regions of them. Once the Mo content exceed 2.0wt%, x-phase will be formed in Fe-0.2%C-13%Cr at lower temperature. Ni addition enlarges theγphase region, inhibits the ferrite formation, but influences slightly on formations ofσand M23C6. The V addition promotes ferrite formation and precipitation of carbide inγphase, narrows Y phase region, but has no effect onσphase. N doption increase the (δ+γ)/γcritical temperature greatly through inhibitδ/αferrite, affectes slightly on other critical point within the adding scope of present research.2. The tempering reaction, espically the intra-lath cabides precipitation, is hingdered by Mo addition. In the Mo-bearing steel tempered at higher temperature, the carbide precipitates in larger volume fraction and better homogeniouty. The carbide precipitation will be accelerated by Ni addition, growing along certain preferred orientations. The V addition promote carbide precipitation and inhibit it growing, making it distribute more uniformly in matrix. The N content increment promotes carbide precipitating more homogeniously, in smaller particle size and larger volume fraction.3. The mechanical properties of Fe-0.2%C-13%Cr are affected slightly when 0.5% to 1.0%Mo is dopped, and the high temperature tempering brittleness caused by other alloying elements can be reduced by Mo doping. When 1.0%Ni plus 0.5Mo is doped together an optimized compresensive mechanical property is achieved, if even more Ni is added, the toughness decrease greatly because of large amount of coarse particle formed in preferred orientations. The icreased strength together with deteriorated toughness is achieved when 1.0%V is added for the over-precipitate of carbide. Small amount of N addition is benefit for the strength improment.4. The microstructure evolution of Fe-0.2%C-13%Cr stainless steel is sensitive to tempering temperature, and the carbide growth speed at fixed temperature is dominateded by diffusion of metal elements. The diffusion abilities of C and N are also affected by other alloying elements, for example, the stonge carbo-nitride forming elements such as V and Mo will inhibit the diffusion of C and N, hingdering nucleation and growth of carbide; while the non-carbide forming element Ni promote diffusion of C, accordingly promote the nucleation and growth of carbide. Decreasing the austenization temperature of V-bearing steel, not only refine the pre-austenite grain size, but decrease the dissolved content of V in austenite, accordingly abate the inhabitation effect of V on carbide growth.