Solution Treatment

Solution Treatment: A Heat Treatment Process

Heat Treatment Services for Stainless Steels and Metal Alloys

Solution treatment refers to a heat treatment process in which the alloy is heated to a high temperature single-phase region to maintain the temperature, so that the excess phase is fully dissolved in the solid solution, and then rapidly cooled to obtain a supersaturated solid solution.

Because its operation process is similar to quenching, it is also called "solution hardening". It is suitable for alloys with solid solution as the matrix and the solubility changes greatly with the temperature change. Generally, it is a preliminary heat treatment, and its function is to prepare the best conditions for the subsequent heat treatment.

Solution treatment heats the alloy to a specific temperature, maintains that temperature long enough to bring one or more components into solid solution, and then cools rapidly to maintain the properties of the solution. Annealing is a commonly used solution treatment, sometimes referred to as solution annealing.

Solution annealing is also a common heat treatment process for many different metal families. Solution annealing may be required for stainless steels, aluminum alloys, nickel-based superalloys, titanium alloys, and some copper-based alloys.

The purpose of solution annealing is to dissolve any precipitates present in the material and to transform the material into a single phase structure at the solution annealing temperature. At the end of the solution annealing process, the material is rapidly quenched to room temperature to avoid any precipitation during cooling through the lower temperature range. Single-phase solution annealed materials will be in a soft state after processing.

Specifically, while common alloys including iron, nickel, cobalt, and aluminum are usually solution treated in some way prior to fabrication, other materials such as stainless steel require solution treatment in a vacuum furnace because they remove atmospheric gases , thereby preventing oxidation. After annealing and quenching, precipitation hardening is a common next step in the heat treatment process. The single-phase microstructure created during the solution annealing process is required prior to age hardening so that only the precipitates formed during the age hardening process will be present in the final product. The composition, size and amount of these precipitates formed during aging will determine the hardness, strength and mechanical properties of the final product after aging. To meet all these requirements, proper solution treatment of the structure prior to aging is critical.

Advantages of solution heat treatment

This process is carried out to increase the tensile strength of non-ferrous alloys, turn them into solid solution, and control the rate of structural change back to their mechanical mixture. In this heat treatment, the alloy is heated to a specific temperature below the material's melting point and then quenched or rapidly cooled. The solid solution obtained during heating remains stable due to the cooling process. At this point, aging and precipitation hardening (processes that help the material in the alloy change or come out of solid solution) are carried out to ensure that the alloy does not change its original structure over time. Aging gives the final grain structure in the metal greater tensile strength than its original state. In conclusion, this hardened form preserves the metallic memory and spring properties of the material without the risk of surface contamination of the product.

Example:

Heat Treatment of Stainless Steel Castings

Use solution treatment to soften stainless steel castings. Generally, stainless steel castings are heated to about 950-1150 degree and kept for a period of time, the purpose is to fully and uniformly dissolve carbides and various alloy elements in austenite, and then quickly quench and cool. Alloying elements such as carbon are too late to precipitate, and a pure austenite structure is obtained.

Advantages of solution treatment of stainless steel castings

1. Make the structure and composition of stainless steel castings uniform, which is especially important for raw materials, because the pouring temperature and cooling rate are different, resulting in inconsistent microstructure. As the temperature increases, the atomic activity intensifies, the σ phase dissolves, and the chemical composition tends to be uniform. A uniform single-phase structure is obtained after rapid cooling.

2. Eliminate hardening and facilitate subsequent processing. Through solution treatment, the distorted lattice is recovered, the elongated and broken grains are recrystallized, and the internal stress is relieved. The tensile strength of stainless steel castings decreases and the elongation increases.

3. Restore the inherent corrosion resistance of stainless steel castings. The corrosion resistance of stainless steel after investment casting decreases. After solution treatment, the corrosion resistance of stainless steel castings returned to the best state.

For stainless steel castings, the main factors of solution treatment are: temperature, holding time and cooling rate.