|
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 its high strength, excellent plasticity and toughness, good corrosion resistance and weldability. Since the application environment has been very harsh, the requirement on toughness, strength and resistance to seawater corrosion becomes more and more stringent. Regarding to this background, the present work is focused on reasonable heat treatment system and micro-alloying of martensitic stainless steel to improve its comprehensive mechanical properties and corrosion resistance, and to prolong the service life of steel. Based on00Cr13Ni5Mo2super low carbon martensitic stainless steel, we smelted two different components stainless steels, one is Nb Micro-alloyed, the other not. The effect of tempering temperature and Nb Micro-alloying on solid phase transformation and microstructures achieved by normalizing and tempering was investigated using dilatometer, laser scanning con-focal microscope (LSCM), scanning electron microscopy (SEM), transmission electron microscope (TEM), high-resolution transmission electron microscopy(HRTEM), X-ray diffraction (XRD), and its consequence on mechanical properties and pitting corrosion resistance properties were also examined by Mechanics performance test and cyclic anodic polarization test respectively. The principal results are as follows:1. Two testing steels normalized at1050℃for0.5h have typical lath martensitic structure and a small amount of retained austenite, while followed by tempering at550℃,575℃,600℃,625℃,650℃and700℃for2h respectively, the morphology consists of tempered martensite, reversed austenite and carbonitride. At lower tempering temperature, film-like reversed austenite forms at lath boundarues or prior austenite grain boundaries. With tempering temperature increasing, the volume fraction of reversed austenite increases. Only more sparse dislocation network can be observed in the reversed austenite, and EDS analysis revealed Ni enrichment in the reversed austenite, which verified a’→γ transformation was controlled by the diffusion mechanism. The volume fraction of reversed austenite increases with prolongation of holding time, and also can increase with heating temperature.2. Nb Micro-alloying postpons the transformation of reversed austenite,00Cr13Ni5Mo2steel obtained the most volume fraction of reversed austenite after tempered at625℃, secondary quenched martensite appeared after tempered at650℃;00Crl3Ni5Mo2Nb steel obtained the most volume fraction of reversed austenite after tempered at650, some reversed austenite would transform to secondary martensite during cooling after higher temperature tempering.3. For the two experimental steels, the yield strength and tensile strength decrease with the tempering temperature increasing, and the elongation and impact toughness rise with the tempering temperature increasing.00Cr13Ni5Mo2Nb steel showed higher yield strength, tensile strength, lower impact toughness and elongation than00Crl3Ni5Mo2steel.4. After normalizing the pitting potential value is the highest among all heat treatment processes for the two experimental steels. With the tempering temperature increasing, the pitting potential of two experimental steels showed the trend of decreasing gradually.00Cr13Ni5Mo2Nb steel showed better resistance to pitting corrosion than00Crl3Ni5Mo2steel, and the precipitation of austenite promoted the degradation of pitting resistance.
|