Effect Of Niobium And Heat Treatments On Microstructure And Mechanical Properties Of Cryogenic Steels

The cracking problems of carbon steels and low alloy steels at low temperature conditions are relevant to strength and low temperature toughness of steels, whereas the properties of the materials depend on the composition, microstructure and quality of liquid steel, and therefore all countries in the world develop high strength and high toughness cryogenic steels by optimizing the composition and microstructure. In this paper,0～0.08% weight percentage of niobium was added into LCC steel, the main alloying elements were optimized, and quenching+quenching and tempering, annealing+quenching and tempering, and normalizing+quenching and tempering three heat treatment processes were designed to develop high strength and toughness cryogenic steels. Microstructure observasion, scanning electron microscopy, spectrum analysis, X-ray diffraction analysis, low-temperature impact toughness test, and tensile strength test were done to research the effect of niobium and heat treatments on microstructure and mechanical properties of cryogenic steels.The chemical composition of the cryogenic steels were, carbon 0.17%～0.19%, silicon 0.5%～0.6%, manganese 1.1%～1.2%, phosphatides≤0.02%, sulphur≤0.02%, chromium 0.5%～0.6%, nickel 0.98%～1.00%, molybdenum 0.5%～0.6%, niobium 0.03%～0.05%.The as-cast microstructure of the cryogenic steels were pearlite+blocky ferrite. With the increase of the content of niobium in the samples, the interlamellar spacing of pearlite decreased gradually, and the microstructure was refined. The microstructure was tempered sorbite after three heat treatment processes. The microstructure was mainly homogeneous and fine polygon ferrite after quenching+ quenching and tempering, mostly lath ferrite and little polygon ferrite after annealing +quenching and tempering, and polygon ferrite and little lath ferrite and the length of lath was short after normalizing+quenching and tempering.The impact works of the cryogenic steels after quenching+quenching and tempering changed from increase to decrease with the increase of the content of niobium in the samples, and decreased faster at-40℃.When the content of niobium was 0.039%, the impact works peaked, respectively,45.7J at-20℃,29.9J at -40℃.The impact fracture morphology was brittle cleavage and little quasi-cleavage, composed of cleavage plane, tearing ridge and dimple with small size. When the content exceeded 0.039%, river pattern existed on the cleaved surfaces.The change of the impact works after three heat treatments was similar. The low-temperature impact toughness of the steels containing the same amount of Nb was best after quenching+quenching and tempering and better after normalizing+ quenching and tempering. The impact works peaked at 0.039% niobium, respectively, 45.7J,42.3J,44.4J at -20℃, and 29.9J,27.6J,28.4J at -40℃. The impact works and impact fracture morphology showed that the microstructure and mechanical properties of the cryogenic steels containing 0.039% niobium could be met after three heat treatment processes. The process of normalizing+quenching and tempering could be used in practice.The tensile strength, yield strength and percentage elongation after fracture of the cryogenic steels after normalizing+quenching and tempering first increased then decreased with the increase of the content of niobium. The variation between tensile strength and yield strength was very similar. When the content of Nb was 0.057%, tensile strength and yield strength had the maximum value 111 OMPa and 990MPa. When the content of Nb was 0.039%, percentage elongation after fracture was 17.0%. Macroscopic fracture morphology of tensile specimens was typical cup cone fracture, micro-fracture morphology was equiaxed dimples. With the increase of the content of niobium, the amount of coarsed individual dimples first decreased and then increased.When the cryogenic steels contained 0.039% niobium after normalizing+ quenching and tempering, the steels had good comprehensive performance. The impact works were 44.4J at -20℃, and 28.4J at -40℃, and tensile strength, yield strength and percentage elongation after fracture were 995MPa,885MPa,17.0%. The steels might be widely applied to the valve parts of petroleum and natural gas transmission pipeline system, with good development prospects.