Studies On The Q-P And Q-P-T Heat Treatment Processes Of 300M Steel

Aircraft landing gear is one of the four key components (engines, wings, fuselage, landing gear). Aircraft landing gear is the taking off and landing system, is a key forced component of the aircraft, bearing large load and strong impact, directly related to the safety of the aircraft and the crew, so it plays a very important effect on the performance and security of the aircraft. Therefore, as the material of the aircraft landing gear and the related important parts,300M steel must be higher strength and better toughness.This study focused on 300M steel as experimental materials. We perform Q-P and Q-P-T processes on 300M steel by the crucible type salt bath furnaces. Experimental methods such as optical microscopy (OM), scanning electron microscope(SEM), transmission electron microscope (TEM), electronic universal test machine and X-ray diffraction (XRD) were used to study microstructure and mechanical properties, study effect of the austenitizing temperature, austenitizing time, quenching temperature, partitioning temperature, partitioning time, tempering temperature and tempering time of the Q-P and Q-P-T processes on the microstructure of 300M steel processed, as the content, morphology and distribution of retained austenite, precipitation and martensite in the steels, as well as the influence of the microstructure on the mechanical properties, and the variations of mechanical properties were analyzed. The main results are as follows:1. The mechanical properties of the samples processed by one step Q-P process are much better than the mechanical properties of the samples processed by the Q-T process. The best process of one step Q-P process:870(lh)-250-250(2h). With this process, about elongation of 14.0% and tensile strength of 2055MPa are obtained, and the strength-ductility product is up to 28770 MPa%. The content of retained austenite is about 4.82%. The microstructure of retained austenite are film-form and spot which locate between martensite laths and in martensite laths respectively; They improve the toughness of the steel, A certain amount of ε-carbide locate on some part of the base of martensite, they strengthen the steel by dispersion strengthen.2. The tensile strength of the samples processed by two step Q-P processes are lower than the the tensile strength of samples processed by one step Q-P processes, but the elongation and strength-ductility product make a substantial increase. After processes by the process of 870(lh)-220-350(lh), about elongation of 24.5% and tensile strength of 1542MPa are obtained, and the strength-ductility product is up to 37779 MPa%. The microstructure of retained austenite are strip which locate between martensite laths and in martensite laths respectively, the content is 5.81%. The best elongation is the samples partitioning at 350℃, the elongation of the sample partitioning at 400℃ is the lowest. The reason is the retained austenite is decomposing at the high temperature.3. The mechanical property of the samples processed by Q-P-T process are low than the mechanical property of the samples processed by Q-P process. The mechanical property of the samples tempered at high temperature is the lowest.4. A lot of ε-carbide is found in the sample processes by Q-P-T process with a temper temperature of 250℃, but the strength of 300M steel is not improved. The reason is the content of carbon in the martensite is decreased, so the strength of martensite is decreased. In addition, the carbide precipitate from the austenite, the content of carbon is decreased, the stability of the retained austenite is decreased. When the sample is extruded, the strengthen of phase change of retained austenite is decreased. Therefore, phase transformation strengthen and the precipitation strengthen are the main strengthen modes of the 300M steel processed by Q-P-T process with a low temperature.5. If 300M steel processed by Q-P-T process with a temper temperature of 500℃, a lot of precipitates of Cr is precipitated in the base of martensite and original austenite. The decomposition of the retained austenite is the main reason of the decrease of the strength and toughness.