The Influence Of Ni Content And Heat Treatment On Microstruture And Mechanical Properties Of 300M Steel

In the aviation and aerospace fields, 300M steel, as a typical representative of low-alloy high strength steel, due to its low content of alloying elements, low cost, simple production technology, are widely used, such as the manufacture of aircraft beams, landing gear, engine shaft, high strength bolts, solid rocket motor case and chemical high-pressure containers. Based on the 300M steel, in order to improve its toughness, we adjust Ni content of 300M steel, study the effect of Ni content to continuous cooling transformation and microstructure evolution of microstructure, the impact of changes of mechanical properties. The results obtained are as follows:With the Ni content increases from 2% to 5%, Ms temperature of 300M steel reduces from the 270℃to 220℃; Ni element promotes martensitic transformation, delays bainite transformation, with the Ni content increases, during continuous cooling transformation, bainitic transformation zone of 300M steel shifts to the right gradually; when the increase of Ni content is 2%, the critical cooling rate of bainitic transformation is about 0.28℃/s, when the Ni content increases to 4%, the critical cooling rate of bainitic transformation is about 0.05℃/s, while the Ni content increases to 5%, the bainite transformation has not occurred even the cooling rate is 0.03℃/s. With Ni content increases, the volume fraction of retained austenite of 300M steel after quenching gradually increases, which reduces the strength while increases the impact toughness of test steel.After tempered at the range from 150℃to 600℃, the tensile strength of 3% Ni test steel decreases, the yield strength and hardness increases firstly and then decreases, at 250℃³00℃reaches a peak, and when the tempering temperature is higher than 400℃, the decreased rate of tensile strength and yield strength increase; there is a second class of temper brittleness of 3% Ni steel, with the tempering temperature increase the impact toughness reaches the maximum value at 250℃ 300℃,and reaches the minimum value at 450℃.During the process of tempering ,martensite and retained austenite of test steel are decomposed, when the tempering temperature below (150℃ 350℃), a large number of tinyε-carbide are precipitated in the martensitic body , then strength and toughness of test steel achieves the best match. With further increase of the tempering temperature, the decomposition of martensite and retained austenite gets faster,ε-carbide gradually transforms into M₃C, when the tempering temperature increases to 600℃, the retained austenite volume fraction reaches the minimum value,ε-carbide transforms into M₃C completely. Keywords 300M steel; Ni content; microstructure; continuous cooling...