In this study, the effect of baking heat treatment on fatigue strength and fatigue life was evaluated by performing baking heat treatment after shot peening treatment on 4340M steel for landing gear. An ultrasonic fatigue test was performed to obtain the S–N curve, and the fatigue strength and fatigue life were compared. The micro hardness of shot peening showed a maximum at a hardened depth of about 50 μm and was almost uniform when it arrived at the hardened depth of about 400 μm. The overall average tensile strength after the baking heat treatment was lowered by about 80–111 MPa, but the yield strength was improved by about 206–262 MPa. The five cases of specimens showed similar fatigue strength and fatigue life in high cycle fatigue (HCF) regime. However, the fatigue limit of the baking heat treated specimens showed an increasing tendency rather than that of shot peening specimens when the fatigue life was extended to the very high cycle fatigue (VHCF) regime. The effect of baking heat treatment was identified from improved fatigue limit when baking heat was used to treat the specimen treated by shot peening containing inclusions. The optimum temperature range for the better baking heat treatment effect could be constrained not to exceed maximum 246 °C.

In this study, shot peening (SP), quenching and tempering (QT) heat treatment, baking heat treatment and Cr-plating technologies were used to enhance the hardness, tribological properties and corrosion resistance of AISI 4340M steel. The purpose of this study is to develop repair process technology for an overhaul of landing gear applied to the MRO process and establish a quality assurance system. The effects of SP, QT heat treatment, Cr-plating, stripping after Cr-plating and Cr-plating after stripping and re-SP on the tribological properties and corrosion resistance of AISI 4340M steel were investigated, and the obtained results were compared with the base AISI 4340M steel. One of the reasons for stripping after Cr-plating is to find out how many times stripping can be done after Cr-plating. Moreover, it is important to investigate the effect of re-SP after Cr-plating on the tribological properties and corrosion resistance. The tribological properties of the specimens were investigated using a ball-on-disk tribometer at room temperature against AISI 52100 steel for 60 min under dry conditions. The corrosion resistance was investigated using a potentiodynamic polarization test in NaCl 3.5% solution. The results showed that the application of SP, QT heat treatment, Cr-plating, stripping after Cr-plating and Cr-plating after stripping and re-SP had a significant effect on the tribological properties and corrosion resistance of AISI 4340M steel. The effects of SP and Cr-plating post-treatment technologies on the wear and corrosion enhancement and mechanisms were discussed based on the microstructural and surface morphology of worn and corroded surfaces.

The materials and parts used in severe environments require high toughness and strength. This study examined the transition characteristics using Charpy impact tests after modification by a surface treatment of 4340M steel. Seven different test specimens were manufactured according to the surface treatment, and Charpy impact tests were conducted over temperatures of −100 °C, 50 °C, 0 °C, 50 °C, and 100 °C. Despite changing the conditions in the test specimens, the ductile-to-brittle transition temperature (DBTT) according to the change in absorbed energy was unclear because of the high strength and high hardness caused by the surface treatment, but it generally formed over the low-temperature range of −50 to−75 °C. The percent of brittle fracture did not decrease below 50%, even at 100 °C. High strength and hardness were maintained when surface modification of 4340M steel by shot peening was performed, even in the low-temperature range, and the deterioration of toughness was not significant. Hence, the uses and application temperature range of the material can be expanded. Thus, increasing the impact resistance against material destruction will allow the this material by shot peening to be applied to parts and equipment used in severe environments.