Common Heat Treatment Process for Steel Castings (Part 2)

Apr. 29, 2020

4. Tempering: Tempering is to heat the quenched or normalized cast steel to Ac, a selected temperature below, after holding for a certain time, and then cool at a suitable rate to make the unstable structure obtained after quenching or normalizing. A heat treatment process that transforms into a stable structure, eliminates quenching (or normalizing) stress and improves the ductility and toughness of pump body casting. The process of quenching and high-temperature tempering is usually called quenching and tempering. Quenched steel castings must be tempered in time, and normalized castings are only tempered when necessary. The properties of steel after tempering depend on the tempering temperature, time, and a number of times. With the increase of tempering temperature and time, in addition to eliminating the quenching stress of steel castings, the unstable quenched martensite is also transformed into tempered martensite, tonite, or sorbate, which makes the casting The strength and hardness of steel decreases, while the plasticity increases significantly. For some medium alloy cast steels containing alloy elements that form strong carbides (such as chromium, molybdenum, vanadium, and tungsten), the phenomenon of increased hardness and decreased toughness when tempered at 400 to 500 ° C is called secondary hardening, That is, the hardness of the cast steel in the tempered state reaches a maximum. Generally, medium-alloy cast steels with secondary hardening properties need to be subjected to multiple (1 to 3) tempering treatments.

5. Solution treatment: The solution treatment is a heat treatment process that heats the casting to an appropriate temperature and keeps it so that the excess phase is fully dissolved, and then quickly cooled to obtain a supersaturated solid solution. The main purpose of the solution treatment is to dissolve carbides or other precipitated phases in the solid solution to obtain a supersaturated single-phase structure. Generally, austenitic stainless heat-resistant steel, austenitic manganese steel, and precipitation-hardening stainless heat-resistant steel castings need to be subjected to solution treatment. The choice of solution temperature depends on the chemical composition and metallographic diagram of the steel. Austenitic manganese steel castings are generally 1000 to 1100 ° C; austenitic nickel-chromium stainless steel castings are 1000 to 1250 ° C. The higher the carbon content in the cast steel and the more refractory alloy elements, the higher the solution temperature should be. The precipitation-hardening cast steel containing copper, due to the as-cast hard copper-rich phase, precipitates during cooling, which causes the hardness of the cast steel to increase. In order to soften the structure and improve the processing performance, the high manganese steel casting needs to be subjected to solution treatment. Its solid solution temperature is 900-950°C. After rapid cooling, a supersaturated single-phase structure with a copper mass fraction of 1.0% to 1.5% can be obtained.

High Manganese Steel Casting

High Manganese Steel Casting

6. Precipitation hardening treatment (aging treatment): After the solution is treated or quenched, the casting is held at a suitable temperature at or above room temperature to form a solute atom segregation region and/or a dispersed strengthening phase in a supersaturated solid solution The process of hardening a metal is called precipitation hardening (or aging). Performed at temperatures above room temperature is called artificial aging. Its essence is: at higher temperatures, carbides, nitrides, intermetallic compounds, and other unstable mesophases are precipitated from the supersaturated solid solution and dispersed in the matrix, so that the comprehensive mechanical properties and Increased hardness. The temperature of the aging treatment directly affects the final performance of the steel casting.

7. The purpose of stress relief treatment is to eliminate casting stress, quenching stress, and stress formed by machining, and to stabilize the size. Generally, it is heated to Ac, and the temperature is kept below 100 ~ 200 ℃ for a certain time, and it is slowly cooled with the furnace. The structure of the metal casting has not changed. Carbon steel, low alloy steel or high alloy steel castings can be processed.

8. Dehydrogenation treatment. The purpose is to remove hydrogen and improve the plasticity of cast steel. It is heated to 170-200 ° C or 280-320 ° C, and it is treated by holding it for a long time. No organizational change. Mainly used for low alloy steel castings that are prone to hydrogen embrittlement.