Effect Of Cu Content On Microstructure And Mechanical Properties Of AMS 6308 Steel

AMS 6308 steel is a kind of heat resistant gear steel invented by Carpenter Technology Corporation, whose commercial brand is Pyrowear alloy 53. Based on its deep hardenability, high temperature performance, and excellent toughness and fatigue resistance, AMS 6308 steel finds application in helicopter and aerospace drivetrain components (gears, shafts, etc.) that must operate at elevated service temperatures. At present, the chemical composition and heat treatment process of AMS 6308 steel have been studied. Meanwhile, the strengthening mechanism of M6C phase in AMS 6308 steel has also been studied. While the precipitation and secondary hardening mechanism of Cu and the effect of Cu on thermoplasticity of AMS 6308 steel have not been studied. Therefore, systematical study on the influence of Cu content on microstructure and mechanical properties of AMS 6308 steel has great significance for the engineering application of AMS 6308 steel.Based on four kinds of AMS 6308 steels contained none,0.98%,2.10% and 3.02% Cu content, the effect of Cu content on microstructure and mechanical properties of AMS 6308 steel after quenching and tempering was investigated by means of mechanical properties tests, OM, SEM, TEM and X-ray diffraction. The precipitation and secondary hardening mechanism of Cu, the effect of Cu on thermoplasticity of AMS 6308 steel were also studied. The main results obtained are as follows:At the same quenching temperature, with Cu content increasing, the strength and hardness increase gradually. The toughness of steel without Cu decreases over the entire quenching temperature range. With elevating temperature, the toughness of Cu-containing steels reduces when quenched below 980℃, while quenching above 980℃, the toughness improves gradually.Compared with the steel without Cu, the microstructure of Cu-containing steels is smaller after quenching at high temperature. At the same quenching temperature, the content of retained austenite increases with an increase in Cu content, which results in the increase of toughness of Cu-containing steels.Under the same tempeing temperature, the number of ε-Cu phases increase with an increase in Cu content, which leads to the increase of strength and hardness and the decrease of plasticity and toughness. Meanwhile, the size of ε-Cu phases gradually coarsens and its precipitation strengthening is weakened, the martensite lath recovers and recrystallizes gradually with prolonging tempering time, which contributes to the decrease of hardness.In the condition of under aging, fine ε-Cu phases precipitate largely along dislocations and in ferrite, which results in the increase of strength and hardness. The strength and hardness reach to peak due to a significant increase in the number of ε-Cu phases. In the condition of over aging, the number of ε-Cu phases decreases, the size of ε-Cu phases obviously coarsens and changes from spherical shape to bar-like shape with tempering temperature increasing, thus its precipitation strengthening is weakened.Meanwhile, the size of M6C carbides increases and the martensite lath recovers and recrystallizes gradually. All of these results in rapid decrease of the strength and hardness and gradual increase of the toughness.With the increase of Cu content, the thermoplasticity of AMS 6308 steel decreases, the precipitation temperature of Cu-rich phase increases rapidly and its content also increases. When there is segregation of Cu, a large number of Cu-rich phases precipitate during heating temperature. Some of Cu-rich phases become liquid film at heating temperature because the melting point of Cu-rich phase is low, which makes the binding force of grain boundary weaken with the infiltrating effort of Cu-rich liquid film. Then thermoplasticity of steel becomes worse and forging crack generates and expands in the effort of forging stress.With the increase of Mo content, the precipitation temperature and content of Laves phase increase. When there is segregation of Mo, a large number of Laves phases precipitate during heating temperature, which results in the decrease of thermoplasticity. This is an important cause producing forging crack.