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Carbon steel castings
Changes in mechanical properties: Tempering can eliminate quenching internal stress, reduce hardness and strength, and improve toughness and plasticity. For example, high carbon steel castings have high hardness and high brittleness after quenching. After low temperature tempering, they can greatly improve toughness while maintaining a certain hardness, so that they can meet the use requirements of tools and other tools with high hardness and toughness requirements.
Organizational transformation: The organization of carbon steel castings after quenching is mainly martensite and residual austenite. During tempering, martensite will decompose to form tempered martensite and other organizations, making the organizational structure of the casting more stable and improving dimensional accuracy.
Alloy steel castings
Changes in mechanical properties: Different alloying elements have different effects on the tempering process. Elements such as chromium and molybdenum can improve the tempering stability of steel, so that alloy steel castings can still maintain high hardness and strength when tempered at higher temperatures. Nickel elements can improve the toughness of steel, and the toughness of alloy steel is further improved after tempering.
Organizational transformation: Alloy elements will change the formation and aggregation rate of carbides. For example, alloy steel castings containing vanadium, titanium and other elements will form fine, dispersed carbides during the tempering process, resulting in secondary hardening, which significantly improves the hardness and wear resistance of the steel.
Stainless steel castings
Mechanical properties change: Tempering can eliminate some processing stresses and improve the mechanical properties of stainless steel castings. For austenitic stainless steel, tempering generally has little effect on strength, but can improve its plasticity and toughness to a certain extent. For martensitic stainless steel, tempering can significantly reduce its hardness and strength and improve toughness.
Corrosion resistance change: A reasonable tempering process can improve the corrosion resistance of stainless steel castings. For precipitation hardening stainless steel, specific tempering ensures uniform strengthening phase precipitation, improving strength and corrosion resistance. Improper tempering may cause chromium carbide precipitation at grain boundaries. This reduces local chromium content and lowers corrosion resistance.
Heat-resistant steel castings
Mechanical properties change: Tempering helps to adjust the strength and toughness of heat-resistant steel castings to meet high-temperature use requirements. Generally speaking, the strength will be reduced after tempering, but the toughness will be improved, so that the casting has better fatigue resistance and creep resistance at high temperature.
Organization stability: When heat-resistant steel is used for a long time at high temperature, organizational stability is crucial. Tempering can promote organizational homogenization, eliminate organizational defects generated during the casting process, and improve organizational stability, thereby ensuring the long-term stable operation of heat-resistant steel castings in high temperature environments.
Summary of knowledge of tempering process principles at different stages
Tempering is a heat treatment process in which the quenched metal workpiece is heated to a temperature range below the critical temperature, and then cooled to room temperature after a certain period of heat preservation. It can usually be divided into the following stages, each of which has its own unique mechanism:
Low-temperature tempering stage (150℃-250℃)
Organization transformation: Martensite begins to decompose, and supersaturated carbon atoms precipitate in the form of extremely fine carbides to form tempered martensite.
Performance changes: The hardness is slightly reduced in this stage, but it still maintains a high hardness and wear resistance. At the same time, the internal stress is partially eliminated and the toughness is improved. Mainly used for workpieces such as knives, gauges, and molds made of high carbon steel and alloy steel that require high hardness and wear resistance.
Medium temperature tempering stage (350℃-500℃)
Organization transformation: Retained austenite begins to decompose and transforms into lower bainite or tempered martensite. At the same time, martensite continues to decompose, and carbides continue to precipitate and grow together.
Performance changes: The workpiece’s hardness decreases, strength slightly drops, but toughness and plasticity improve. Internal stress is eliminated. This suits spring processing, ensuring good elastic limit, yield strength, and necessary toughness.
High temperature tempering stage (500℃-650℃)
Organization transformation: Carbides continue to gather and grow to form coarse granular carbides, and ferrite undergoes recrystallization, transforming from needle-shaped or lath-shaped to equiaxed. Finally, a tempered troostite structure is formed.
Performance changes: The workpiece becomes softer and tougher, with improved plasticity and comprehensive mechanical properties. Quenching and high-temperature tempering is called quenching and tempering. This process is widely used in shafts, gears, and connecting rods.
Higher temperature tempering stage (>650℃)
Organization transformation: carbides further aggregate and grow, ferrite grains also grow significantly, and the organization gradually tends to a stable equilibrium state.
Performance change: The strength and hardness of the workpiece continue to decrease, and the toughness and plasticity do not change much. At this time, it is mainly to eliminate stress or obtain stable organization and size.