Thread Content
I. What is steel annealing? What are the types of annealing and their uses? Answer: Annealing of steel: Annealing is a heat treatment process in which steel is heated to a temperature above or below its critical point AC1, held at that temperature for a certain period of time, and then cooled slowly in the furnace to obtain a structure that is close to an equilibrium state. Types of annealing: Based on the heating temperature, it can be divided into annealing above or below the critical temperature AC1; the former includes full annealing, partial annealing, spheroidizing annealing, and homogenization annealing, while the latter includes recrystallization annealing and stress-relief annealing. Based on the cooling method, it can be divided into isothermal annealing and continuous cooling annealing. Uses of annealing: 1. Full annealing: Full annealing is a heat treatment process in which the steel is heated to 20–30°C above AC3, held at that temperature for a sufficient length of time to ensure complete austenitization of the structure, after which it is cooled slowly within the furnace to achieve a structure that is close to an equilibrium state. It is mainly applied to hypoeutectoid steels, with the purpose of refining grains, eliminating internal stresses and work hardening, improving plasticity and toughness, homogenizing the chemical composition and microstructure of the steel, enhancing its machinability, and removing defects such as Widmanstätten structure and banded structure in medium-carbon structural steels. 2. Partial annealing: Partial annealing is a heat treatment process in which the steel is heated to a temperature between AC1–AC3 (for hypoeutectoid steel) or AC1–ACcm (for hyper-eutectoid steel); after holding at that temperature for a certain period of time, it is cooled slowly within the furnace to obtain a microstructure that is close to an equilibrium state. For hypoeutectoid steel, if the original microstructure distribution of the steel is appropriate, incomplete annealing can be used in place of full annealing to eliminate internal stresses and reduce hardness. For hypereutectoid steels, incomplete annealing is primarily aimed at obtaining a spheroidal pearlite structure, in order to eliminate internal stresses, reduce hardness, and improve machinability. 3. Spheroidizing annealing: Spheroidizing annealing is a heat treatment process that spheroids the carbides in steel to obtain granular pearlite. It is mainly used for eutectoid steel, hypereutectoid steel, and alloy tool steel. Its purpose is to reduce hardness, improve machinability, homogenize the microstructure, and prepare it for quenching. 4. Homogenization annealing: Also known as diffusion annealing, it is a heat treatment process in which steel ingots, castings, or forged billets are heated to a temperature slightly below the solidus line and held at that temperature for an extended period of time, before being cooled slowly to room temperature. Its purpose is to eliminate dendritic and regional segregation that occurs during the solidification of ingots or castings, thereby homogenizing the composition and microstructure. 5. Recrystallization annealing: A heat treatment process in which the metal that has been cold-deformed is heated above its recrystallization temperature for an appropriate period of time, and then slowly cooled to room temperature. Its purpose is to transform the deformed grains back into uniform equiaxed grains, while eliminating work hardening and residual internal stresses, thereby restoring the microstructure and properties of the steel to their state before cold deformation. 6. Stress-relief annealing: A heat treatment process in which cold-deformed metal is heated to a temperature below the recrystallization temperature, held at that temperature for a certain period of time, and then slowly cooled to room temperature. Its main purpose is to eliminate residual internal stresses (primarily type 1 internal stresses) in castings, forged and rolled parts, welded components, and machined parts, in order to improve dimensional stability and reduce the tendency for deformation and cracking. II. What is normalizing of steel? What’s the purpose? What are the applications? Answer: Normalizing of steel: Normalizing is a heat treatment process in which steel is heated to an appropriate temperature above AC3 or Accm, held at that temperature for a sufficient time to achieve complete austenitization, and then cooled at a relatively fast rate (by air cooling, wind cooling, or spray cooling) to obtain a pearlitic structure. The essence of the normalizing process is complete austenitization followed by a pseudo-eutectoid transformation. Purpose: To refine the grain structure, uniformize the composition and microstructure, eliminate internal stresses, adjust hardness, and remove defects such as Widmanstatten structure, banded structure, and networked carbides, in order to achieve an appropriate microstructural state for the final heat treatment. Applications: 1. Improve the machinability of low-carbon steel. 2. Eliminate the hot-working defects of medium carbon steel (Weyl structure, banded structure, coarse grains). 3. Eliminate the networked carbides in hypereutectoid steel to facilitate spheroidizing annealing and prepare the microstructure for quenching. 4. As the final heat treatment, to improve the mechanical properties of ordinary structural components. III. To improve the strength of hypoeutectoid steel in production, a common method is to increase the content of pearlite in such steel. What heat treatment process should be employed? Answer: The normalizing process should be used. Reason: In hypoeutectoid steel, pro-eutectoid ferrite precipitates from the supercooled austenite during cooling. The slower the cooling rate, the greater the amount of pro-eutectoid ferrite, which results in a reduced amount of pearlite and thus a decrease in the hardness and strength of the hypoeutectoid steel. The essence of the normalizing process is complete austenitization followed by a pseudo-eutectoid transformation; by increasing the cooling rate, it is possible to reduce the amount of pro-eutectoid ferrite, thereby transforming steel with sub-eutectoid composition into an eutectoid structure – that is, increasing the amount of pearlite – and thus enhancing the strength and hardness of sub-eutectoid steel. IV. What is the purpose of quenching? How many types of quenching methods are there? Compare the advantages and disadvantages of several quenching methods? Answer: The purpose of quenching is to obtain as much martensite as possible, which can significantly increase the strength, hardness, and wear resistance of steel. When combined with various tempering processes, it enables the steel to possess high strength and hardness while also maintaining good ductility and toughness. The heat treatment process in which steel is heated to a temperature above its critical points AC3 or AC1, held at that temperature for an appropriate period of time, and then cooled at a rate faster than the critical cooling speed to obtain martensite (or lower bainite) is called quenching. Quenching methods: Based on the cooling method, they can be classified into single-fluid quenching, double-fluid quenching, staged quenching, and isothermal quenching. Comparison of advantages and disadvantages: Advantages and disadvantages of quenching methods – Single-fluid quenching: Simple to operate and widely used; 1. Suitable only for small parts with simple shapes; 2. High quenching stress; 3. Difficult to select a cooling medium with appropriate cooling capacity and properties. Double-fluid quenching: 1. Reduces structural stress and lowers the tendency for deformation and cracking in parts; 2. Suitable for larger parts; however, operation is difficult to control and requires extensive experience and skilled techniques. Staged quenching: 1. Reduces thermal stress and structural stress, thereby lowering the tendency for deformation and cracking in parts; 2. Relatively easy to control; suitable only for smaller parts. Isothermal quenching: 1. Reduces thermal stress and structural stress, significantly lowering the tendency for deformation and cracking in parts; 2. Suitable for processing parts with complex shapes and precise size requirements; suitable only for smaller parts. V. Discuss the principles for selecting the heating temperature for quenching hypoeutectoid and hyper-eutectoid steels. Why cannot the quenching heating temperature of hypereutectoid steel exceed the Accm line? Answer: Principle for selecting the quenching heating temperature: It is based on obtaining uniform and fine austenite grains, in order to achieve a fine martensite structure. Hypoeutectoid steel is usually heated to 30–50°C above AC3, while hypereutectoid steel is heated to 30–50°C above AC1. 1. In the case of hypereutectoid steel, when quenching is carried out at a temperature higher than the Accm line, all carbides dissolve into austenite, increasing its carbon content and thereby reducing the Ms and Mf temperatures of the steel. This leads to an increase in the amount of residual austenite after quenching, which in turn reduces the hardness and wear resistance of the steel. 2. If the quenching temperature for hypereutectoid steel is too high, the austenite grains become coarse, and the high carbon content results in the formation of coarse needle-like martensite with microscopic cracks after quenching, further reducing the plasticity and toughness of the steel. 3. High-temperature quenching generates large thermal stresses, leading to severe oxidation and decarburization, which also increases the tendency for the steel parts to deform or crack. VI. What are the hardenability and hardening capacity of steel? What are the factors that affect the hardenability, strength after quenching, and depth of the hardened layer in steel? Answer: Hardenability: The hardenability of steel refers to the ability of steel that has been austenitized to obtain martensite during quenching. It reflects the stability of the supercooled austenite and is related to the critical cooling rate of the steel. Its size is indicated by the depth of the hardened layer and the hardness distribution obtained when steel is quenched under certain conditions. Hardenability: The hardenability of steel refers to the ability of steel that has been austenitized to become hardened during quenching; it depends primarily on the carbon content in the martensite, and is expressed by the highest hardness that can be achieved in the quenched martensite. Depth of the hardened layer: The depth of the hardened layer refers to the depth from the semi-martensitic region to the surface of the workpiece, as measured when the steel is quenched under specific conditions. It is related to the hardenability of steel, the shape and size of the workpiece, and the cooling capacity of the quenching medium.