How Different Heat Treatment Processes (Annealing, Solution Treatment, Aging) Regulate the Hardness of Nickel-based Alloys
1. Annealing: Softening the Alloy to Reduce Hardness
Basic Principle
Effect on Hardness
For cold - worked nickel - based alloys (e.g., cold - rolled Inconel 625 sheets), annealing eliminates work hardening by erasing dislocation accumulations, resulting in a significant decrease in hardness and an increase in ductility.
For cast nickel - based alloys, annealing reduces the hardness caused by dendritic segregation and residual stresses, making the alloy easier to machine.
Example: Cold - worked Inconel 718 has a hardness of approximately 320 HB. After annealing at 980∘C for 1 hour followed by furnace cooling, its hardness drops to around 190 HB.
2. Solution Treatment: Homogenizing the Matrix to Set a Hardness Baseline
Basic Principle
Effect on Hardness
Compared with the as - cast or cold - worked state, solution treatment reduces hardness by dissolving coarse, brittle secondary phases and homogenizing the matrix. For example, as - cast Inconel 718 has a hardness of ~250 HB, and after solution treatment at 1065∘C for 1 hour and water quenching, the hardness decreases to ~210 HB.
Compared with the annealed state, solution - treated alloys have slightly higher hardness because the supersaturated solid solution has a higher degree of lattice distortion than the equilibrium matrix formed by slow cooling during annealing.
Solution treatment alone cannot maximize the hardness of precipitation - hardenable nickel - based alloys; it only prepares the microstructure for subsequent strengthening.
3. Aging Treatment: Precipitating Strengthening Phases to Maximize Hardness
Basic Principle
Effect on Hardness
For solution - treated Inconel 718, after double - stage aging (720∘C for 8 hours, furnace cooling to 620∘C for 8 hours, then air cooling), its hardness increases sharply from ~210 HB to 360–400 HB.
The hardness change during aging follows a typical trend: it increases rapidly in the early stage (as precipitates nucleate and grow to a critical size), reaches a peak when the precipitates are fine and uniformly distributed, and then decreases (overaging) as the precipitates coarsen and lose their dislocation - pinning ability.
For non - precipitation - hardenable nickel - based alloys (e.g., Inconel 600, a solid - solution strengthened alloy), aging has little effect on hardness because there are no strengthening phases to precipitate; its hardness mainly depends on solution treatment and cold working.
4. Synergistic Effect of Combined Processes
Solution treatment + Aging: The most common combination for precipitation - hardenable nickel - based alloys, which can achieve the highest hardness and strength while maintaining acceptable toughness.
Annealing + Cold Working + Solution Treatment: Used to adjust the grain size of the alloy; the hardness can be controlled by adjusting the degree of cold working before solution treatment.
