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What are the main types of heat treatment processes?
What are the main types of heat treatment processes?

09

2022

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09

What are the main types of heat treatment processes?

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(1) Quenching. For hypoeutectoid steels with a carbon content of less than 0.8%—which include low- and medium-carbon steels— the quenching heating temperature is 30 to 50°C above the A3 line. At this temperature, the steel is held for a specific period to ensure that its microstructure completely transforms into austenite. Subsequently, the steel is rapidly cooled in water or oil, preventing the austenite from decomposing into pearlite and ferrite and instead causing it to form martensite. This process is called quenching. However, ordinary low-carbon steels with a carbon content below 0.25% are not easily quenched into martensite due to their low carbon content. Martensite is a solid solution in which carbon is dissolved in body-centered cubic iron lattice. Under a microscope, martensite appears as a white, needle-like structure. Its
　　(1) Quenching. For hypoeutectoid steels with a carbon content below 0.8%—which include low- and medium-carbon steels— the quenching heating temperature is 30 to 50°C above the A3 line. At this temperature, the steel is held for a certain period of time to ensure that its microstructure completely transforms into austenite. Then, the steel is rapidly cooled in water or oil, preventing the austenite from decomposing into pearlite and ferrite and instead causing it to form martensite. This process is called quenching. However, ordinary low-carbon steels with a carbon content below 0.25% are not easily quenched into martensite due to their low carbon content.
　　Martensite is a solid solution in which carbon is dissolved in body-centered cubic iron. Under a microscope, martensite appears as a white, needle-like microstructure. It exhibits high hardness, great brittleness, and very high strength, but low ductility and toughness.
　　When welding carbon steel and certain low-alloy structural steels, quenching may occur in the near-weld zone, resulting in the formation of a high-hardness martensitic zone. To avoid the formation of martensitic structures during welding and to prevent cold cracks, the following measures are often taken when welding steels that are prone to quenching:
　　① Preheat to slow down the cooling rate of the weld and the heat-affected zone, which helps prevent the formation of a quenched microstructure.
　　② Using a larger welding current and a lower welding speed can also slow down the cooling rate of the joint, helping to prevent the formation of quenched microstructures and cold cracks.
　　(2) Tempering. Tempering after quenching can partially restore the toughness of steel. The tempering temperature should be below the A-line (723℃). Depending on the tempering temperature, tempering is classified into high-temperature tempering (400–650℃), medium-temperature tempering (250–400℃), and low-temperature tempering (150–250℃). High-temperature tempering can completely relieve internal stresses in the steel, reduce its strength and hardness, and enhance its ductility and toughness. In contrast, low-temperature tempering results in only a slight reduction, or even no reduction, in the steel’s hardness, while its toughness is somewhat improved. The purpose of medium-temperature tempering is to relieve internal stresses and give the steel a higher elastic limit.
　　If a welded component is heated to 650℃ solely for the purpose of eliminating residual welding stresses, this process can also be referred to as stress-relief tempering.
　　(3) Tempering. For certain alloy steels and their welded structures, high-temperature tempering is performed immediately after quenching. This continuous heat treatment process is known as tempering. Tempering enables steel to maintain high impact toughness while achieving high strength—a result that cannot be attained through other heat treatment methods.
　　(4) Annealing. Steel is heated to a temperature 30–50°C above the specified value, held at this temperature for a certain period, and then cooled slowly and uniformly either to room temperature or to a temperature below a specific threshold. After holding at that lower temperature for a prescribed duration, the steel is allowed to cool in air. This entire process is referred to as annealing. Annealing reduces hardness, making the steel easier to machine; it also refines the grain structure of the steel and relieves internal stresses. Stress-relief annealing for welded structures is a type of low-temperature annealing whose heating temperature is similar to that used in high-temperature tempering, hence it is also called stress-relief tempering. The typical annealing temperature ranges from 600°C to 650°C, and the holding time is calculated at 4–5 minutes per millimeter of thickness (but should not be less than one hour). Finally, the steel is cooled either in air or in a furnace.
　　(5) Normalizing. Steel is heated to a temperature 30–50°C above the critical point, held at that temperature for a specified period, and then cooled in air. This process is called normalizing. Because cooling in air is relatively rapid, the microstructure obtained after normalizing is finer than that obtained after annealing. Consequently, for the same steel grade, the strength and hardness achieved after normalizing are higher than those achieved after annealing.

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