| Super low carbon martensitic stainless steels are widely used for hydraulic turbines, valve bodies, pump bowls, compressor cones, impellers, and high-pressure pipes in power generation, offshore oil and gas and petrochemical industries, because of their high strength, excellent plasticity and toughness, good corrosion resistance and weldability.In this paper, the main objective of the present work is to add Nb-V-N micro-alloying in order to improve strength and avoid forming δ-ferrite. Based on13Cr5Ni2Mo super low carbon martensitic stainless steel, we design three different components stainless steels.After normalization and various tempering heat-treatment conditions, the microstructure was observed by laser scanning con-focal microscope (LSCM) and scanning electron microscopy (SEM). The phase constitute was analyzed using X-ray diffraction (XRD) spectrum. Conventional mechanical properties are also performed. Based on the requirement of corrosion potential, cyclic anodic polarization test was conducted on electrochemical specimen with three parallels in a3.5%NaCl by using CS300electrochemical testing system.These experiments reveal the relationship between microstructure and properties, the effect of tempering temperatures and time on evolution of morphology and properties. Compared with pure nitrogen13Cr5Ni2Mo and different content of niobium of Nb-V-N micro-alloying13Cr5Ni2Mo, the influence of micro-alloying on microstructure transformation, mechanical properties and pitting corrosion properties are illustrated. The principal results are as follows:1. Three kinds testing steels normalized at1050℃for0.5h have typical lath martensitic structure, retained austenite and carbonitride, while followed by tempering at550℃,575℃,600℃,625℃,650℃and700℃for2.Oh respectively, the morphology consists of tempered martensite, reversed austenite and carbonitride. At lower tempering temperature, giving priority to film-like reversed austenite forms at martensite interface or original austenite grain boundary. The volume fraction of reversed austenite increases to a maximum value tempered at650℃, and the evolution of morphology of reversed austenite is that thin-film reversed austenite decompounds and coarsens into granular, and continues to grow, then synthesizes with adjacent granular into lamellar. The transformation of the reversed austenite to martensite results in some decrease in the volume fraction after tempering at650℃.2. Due to the increase tempering temperature, yield strength decreases gradually but tempered higher than600℃it sharply drops, the amplitude of yield strength is250to300MPa. Tensile strength declines to minimum value tempered at650℃, then slightly rises, the variation range is about150-200MPa. Impact absorbing energy climbs up to maximum value at650℃, but it appears abnormal brittleness at550℃. The mechanical properties changes of Nb-V-N micro-alloying13Cr5Ni2Mo martensitic stainless steels show further enhance the strength in the form of expense of some plasticity and toughness.3. After normalizing the pitting potential is the highest value among all heat treatment processes, tempering at550℃the Eb rapidly reduces by50mV, the maximum value appears at575℃,40mV higher than550℃. From575℃to700℃, pitting potential change of testing steels with tempering temperatures shows monotonous decrease.4. From the further experiment of13Cr5Ni2Mo-0.03Nb0.12V0.05N, the formation process of reversed austenite is controlled by diffusion which decides by aging time. In the wake of aging time tempered at600℃, the nucleation and growing to saturation of reversed austenite needs4.Oh, and the reversed austenite stable volume fraction value is about5%. Because of low content of reversed austenite, the combinations of strength and toughness have little effect except for initial tempering aging time. Aging at650℃, more than2.Oh displays an unchanged value of volume content of reversed austenite, about10%. From the obtained mechanical properties at a series of aging time, before2.Oh the properties happen dramatically change due to the remarkable increase of reversed austenite. As rising tempered temperature to700℃, the formation stationary volume value is shorted at1.Oh, and properties are also basically stable. |
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