Powder metallurgy materials have inherent porosity that affects heat treatment, particularly the relationship between porosity and changes in heat treatment, while the addition of alloy elements also has a certain impact on heat treatment in order to improve density and grain size.
During heat treatment of powder metallurgy materials, rapid cooling is used to inhibit austenite diffusion and promote the formation of martensite. However, the presence of porosity significantly affects the material's heat dissipation capabilities.
Permeability decreases with the increase of porosity. On the other hand, porosity affects the density of the material, which is related to the surface hardness and quenching depth effects of the material after heat treatment, reducing surface hardness. Moreover, the existence of porosity precludes the use of saltwater as a medium during quenching to prevent corrosion caused by residual salt. Therefore, general heat treatment is performed in a vacuum or gaseous medium.
The heat treatment effect of powder metallurgy materials is related to their density, permeability, thermal conductivity, and resistance. Porosity is the main cause of these factors. When porosity exceeds 8%, gas will permeate quickly through the pores, increasing the depth of carburization during carbonitriding, reducing the effectiveness of surface hardening. If the carburizing gas permeates too quickly, soft spots will be produced during quenching, reducing surface hardness and making the material brittle and deformable.
Copper and nickel are common alloy elements that affect heat treatment effectiveness depending on their composition and type. The hardening depth of heat treatment gradually increases with increasing copper and carbon content, but decreases after reaching a certain content. Nickel alloys have greater stiffness than copper alloys, but uneven nickel content leads to uneven austenite structure.
Although high-temperature sintering can achieve good alloying effects and promote densification, different sintering temperatures, especially lower temperatures, can lead to decreased sensitivity to heat treatment (reduced alloy in the solid solution) and reduced mechanical properties. Therefore, high-temperature sintering with sufficient reducing atmosphere can obtain better heat treatment effects in powder metallurgy.
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