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Solid Solution Treatment of Stainless Steel Castings
Solution heat treatment is generally used for stainless steel castings. Its main purpose is to dissolve carbides or other precipitated phases in the stainless steel in solid solution so as to obtain a supersaturated single-phase structure.The cooling medium for solution heat treatment can be water, oil or air. Among them, water is the most commonly used cooling medium. Air cooling is only suitable for thin-walled steel castings.
The application range of solution heat treatment generally includes: austenitic stainless heat-resistant steel castings, austenitic manganese steel castings and precipitation hardening heat-resistant steel castings. The choice of solution temperature depends on the grade and metallographic structure of stainless steel. The solid solution temperature of austenitic manganese steel castings is generally 1000 ℃ ~ 1100 ℃. The solid solution temperature of austenitic nickel-chromium stainless steel castings is generally 1000 ℃ ~ 1250 ℃. If the carbon content in the steel casting is higher, the more insoluble alloying elements are, the higher the solution temperature should be.
In order to reduce the temperature difference between the surface of the steel casting and the center part during the heating process, the heating method for the solution treatment of austenitic stainless steel castings should be preheated at a low temperature first, and then quickly heated to the solution temperature. The holding time of the solution heat treatment depends on the thickness of the steel casting, and it is generally calculated as 1 hour for every 25mm wall thickness. It is enough as long as it is ensured that all sections of the casting are fully heated.
For precipitation hardening steel castings containing copper, the hardness of the steel castings is increased due to the precipitation of hard copper-rich phases in the as-cast state during the cooling process. In order to soften the structure and improve the processing performance, such steel castings require solution heat treatment. The solid solution temperature of precipitation hardening steel castings containing copper is generally 900 ℃ ~ 950 ℃. After cooling, a supersaturated single-phase structure with a copper mass fraction of 1.0% ~ 1.5% can be obtained.
