Effect of ageing heat treatment process on the microstructure and mechanical properties for high hardness 3J40 alloy

Alloy 3J40 is a nickel-based alloy with high hardness and high elasticity. Alloy 3J40 was widely used as bears and conical pivots in many fields, such as, navigation, aviation and antiaircraft gun due to its excellent properties of elasticity, strength, hardness, corrosion resistance, nonmagnetic characteristics and so on. However, after aging treatment, the hardness of 3J40 alloy was uneven, there were large differences in hardness between the air-cooled and the furnace-cooling alloys, and the properties were unstable. Previous researches basely focused on the distribution, size, shape and other aspects of inclusions, to improve the plasticity and thermal processing properties of the alloy. Studies about properties and microstructure of the alloy devoted to the studies of the traditional Ni3Al-γ-phase. Up to now no public research report had been found about the α-Cr phase strengthening mechanism of the 3J40 alloy. In this paper, the effect of α-Cr phase on the strengthen mechanism in 3J40 alloy was investigated. By thermodynamic phase equilibrium calculation, the possible phases, which may precipitate after heat treatment, have been predicted. Relationships between the mechanical properties and the ageing heat treatment parameters, such as, ageing temperature, ageing time and cooling rate, had been studied. Microstructure of the alloy under different heat-treatment process has been analyzed, especially cared about the morphology, particles size and quantity of the α-Cr phase. And the strengthening mechanism was also investigated. The results showed that the microstructure of the aged 3J40 alloy were composed of γ-based phase, precipitated γ’ phase, (Ni3Al) phase, and α-Cr phase in granular and laminar. And the main contribution for the strengthen mechanism may be associated with the α-Cr phase laminar.