Solution Treatment of Stainless Steel
To understand why stainless steel requires solution treatment, we first need to clarify the purpose of solution treatment, its functions, which materials require it, and in which industries it is commonly applied.
First, let’s discuss the process. Solution treatment is performed to dissolve carbides, γ’ phases, and other precipitates within the matrix, obtaining a uniform supersaturated solid solution. This facilitates the re-precipitation of finely dispersed and evenly distributed carbides and γ’ strengthening phases during subsequent aging. At the same time, it relieves stresses induced by cold or hot working and enables recrystallization of the alloy. Secondly, solution treatment is carried out to achieve an appropriate grain size, ensuring the alloy’s high-temperature creep resistance. The solution treatment temperature typically ranges from approximately 980°C to 1250°C, selected based on the precipitation and dissolution behavior of phases in each specific alloy as well as service requirements, so as to ensure the necessary precipitation conditions for the primary strengthening phases and a suitable grain size.
- Commonly Treated Materials
Common materials include SUS304, SUS303, SUS316, 1Cr18Ni9, 0Cr19Ni9, and other austenitic stainless steels. In addition to austenitic stainless steels, duplex stainless steels and precipitation-hardening stainless steels—such as 630, 17-4, and others—also require solution treatment. Their purposes are largely similar.
- Purpose of Treatment
The austenitic stainless steel is heated to approximately 1100°C to completely or substantially dissolve the carbide phases, allowing carbon to dissolve into the austenite. It is then rapidly cooled to room temperature, bringing the carbon to a supersaturated state (where carbon is already stabilized and lacks the capacity and opportunity to form chromium-rich carbides with chromium). This heat treatment method is known as solution heat treatment.
The rapid cooling in solution heat treatment resembles quenching in ordinary steel, but this “quenching” is different from that of ordinary steel—the former is a softening treatment, while the latter is hardening (forming martensite). Moreover, the heating temperatures for quenching vary depending on the desired hardness but do not reach 1100°C.
- Application Fields
With rapid industrial development, the application of solution treatment heat treatment processes has become increasingly widespread. The most common application areas include stainless steel deep-drawn and stamped parts. Typical products include vacuum flasks, dinner plates, bellows, and more. It is also used in the medical industry. Of course, applications in aerospace, optoelectronics, and marine industries are numerous and not exhaustively listed here.
- What Is the Difference Between Solution Treatment and Quenching?
The differences between solution treatment and quenching can be distinguished from three aspects: material, process, and purpose.
Different Materials:
Solution treatment is generally applied to austenitic stainless steels such as 304, 316, and 630 (precipitation-hardening stainless steel), while quenching is mainly applied to stainless irons such as 410, 420 (2Cr13), and 430 (3Cr13)—martensitic and ferritic stainless steels.
Different Processes:
This is relatively straightforward to understand. Solution treatment refers to a heat treatment process in which the alloy is heated to a high-temperature single-phase region, held isothermally to fully dissolve excess phases into the solid solution, and then rapidly cooled to obtain a supersaturated solid solution.
Quenching is a heat treatment process in which steel is heated above the critical temperature, held for a certain period, and then cooled at a rate greater than the critical cooling rate to obtain a predominantly martensitic non-equilibrium microstructure.
Different Purposes:
As mentioned earlier, solution treatment is performed to modify the matrix structure of austenitic stainless steel, whereas quenching is a process used to increase hardness and strength.
