Six heat treatment questions
What is annealing of steel? What are the types and uses of annealing?
Annealing of steel: Annealing is a heat treatment process that heats steel to a temperature above or below the critical point AC1, and then slowly cools it with the furnace after holding it for a certain period of time to obtain a nearly equilibrium structure.
Annealing types: According to the heating temperature, it can be divided into annealing above or below the critical temperature AC1. The former includes complete annealing, incomplete annealing, spheroidizing annealing, and homogenizing annealing, and the latter includes recrystallization annealing and stress relief annealing, according to the cooling method It can be divided into isothermal annealing and continuous cooling annealing.
- 1. Complete annealing: Complete annealing is a heat treatment process in which steel is heated to 20-30°C above AC3 and kept for a long enough time, so that the structure is completely austenitized and then slowly cooled with the furnace to obtain a nearly equilibrium structure. It is mainly used in hypoeutectoid steels, and its purpose is to refine grains, eliminate internal stress and work hardening, improve plastic toughness, uniform the chemical composition and structure of steel, improve the cutting performance of steel, and eliminate the problem of medium carbon structural steel. Defects such as Widmanstatten organization and ribbon organization.
- 2. Incomplete annealing: Incomplete annealing is to heat the steel to between AC1-AC3 (hypoeutectoid steel) or AC1-ACcm (hypereutectoid steel), and then slowly cool with the furnace after holding it for a certain period of time to obtain a nearly equilibrium state The heat treatment process of the organization. For hypoeutectoid steel, if the original structure of the steel is properly distributed, incomplete annealing can be used instead of complete annealing to eliminate internal stress and reduce hardness. For hypereutectoid steel, incomplete annealing is mainly to obtain spherical pearlite structure to eliminate internal stress, reduce hardness, and improve cutting performance.
- 3. Spheroidizing annealing: Spheroidizing annealing is a heat treatment process to spheroidize carbides in steel to obtain granular pearlite. Mainly used for eutectoid steel, hypereutectoid steel and alloy tool steel. Its purpose is to reduce the hardness, improve the cutting performance, uniform structure, and prepare the structure for quenching.
- 4. Homogenization annealing: also known as diffusion annealing, it is a heat treatment process in which steel ingots, castings or forging billets are heated to a temperature slightly below the solidus for a long time, and then slowly cooled to room temperature. Its purpose is to eliminate dendritic segregation and regional segregation produced during solidification of ingots or castings, and to homogenize the composition and structure.
- 5. Recrystallization annealing: a heat treatment process in which the cold deformed metal is heated to above the recrystallization temperature for an appropriate time, and then slowly cooled to room temperature. The purpose is to transform the deformed grains into uniform equiaxed grains, eliminate work hardening and residual internal stress, and restore the structure and properties of the steel to the state before cold deformation.
- 6. Stress relief annealing: a heat treatment process in which the cold deformed metal is heated to a temperature below the recrystallization temperature, kept for a period of time and then slowly cooled to room temperature. Its main purpose is to eliminate residual internal stress (mainly the first type of internal stress) in castings, forgings, welds and machined workpieces, so as to improve dimensional stability and reduce the tendency of workpieces to deform and crack.
What is normalizing of steel? What is its purpose? What are its applications?
- Normalizing of steel: Normalizing is to heat the steel to an appropriate temperature above AC3 or Accm, hold it for an appropriate time, and then complete austenitization, and then cool it at a faster rate (air cooling, air cooling or spray) to obtain a pearlite structure Heat treatment process. The essence of the normalizing process is complete austenitization plus pseudo-eutectoid transformation.
- Purpose: Refine grain, uniform composition and structure, eliminate internal stress, adjust hardness, eliminate defects such as Widmanstatten structure, band structure, network carbide, etc., and provide a suitable structure for the final heat treatment.
- 1. Improve the cutting performance of low carbon steel.
- 2. Eliminate the hot working defects of medium carbon steel (Widmanstatten structure, band structure, coarse grains).
- 3. Eliminate the network carbides of hypereutectoid steel, facilitate spheroidizing annealing, and prepare the structure for quenching.
- 4. As the final heat treatment, improve the mechanical properties of ordinary structural parts.
In order to improve the strength of hypoeutectoid steel in production, the commonly used method is to increase the content of pearlite in hypoeutectoid steel. What heat treatment process should be used?
The normalizing process should be used.
Reason: Undercooled austenite of hypoeutectoid steel will precipitate pro-eutectoid ferrite during the cooling process. The slower the cooling rate, the more the content of pro-eutectoid ferrite, which leads to the decrease and decrease of the content of pearlite The hardness and strength of hypoeutectoid steel. The essence of the normalizing process is complete austenitization plus pseudo-eutectoid transformation. The content of pro-eutectoid ferrite can be reduced by increasing the cooling rate, and the hypoeutectoid steel can be transformed into a eutectoid structure, that is, increase Increase the content of pearlite, which can improve the strength and hardness of hypoeutectoid steel.
What is the purpose of quenching? There are several quenching methods? Compare the advantages and disadvantages of several quenching methods?
The purpose of quenching: to obtain as much martensite as possible, which can significantly improve the strength, hardness, and wear resistance of the steel. Cooperating with various tempering processes can make the steel have high strength and high hardness and good plastic toughness.
The heat treatment process of heating steel to a certain temperature above the critical point AC3 or AC1, holding it for a suitable time and cooling at a cooling rate greater than the critical cooling rate to obtain martensite (or lower bainite) is called quenching.
Quenching method: According to the cooling method, it can be divided into: single liquid quenching method, double liquid quenching method, graded quenching method, and austempering method
Comparison of advantages and disadvantages:
|Single liquid quenching method||Simple operation and wide application||
1. Only suitable for small size and simple shape workpieces.
2.The quenching stress is large.
3. It is not easy to choose a cooling medium with more suitable cooling capacity and cooling characteristics.
1.Reduce tissue stress and reduce the tendency of workpiece deformation and cracking.
2.Suitable for larger size workpieces.
|The operation is not easy to control, and requires rich experience and skilled technology.|
1.Reduce thermal stress and organizational stress, reduce the tendency of workpiece deformation and cracking.
2. The operation is relatively easy to control.
|Only suitable for smaller size workpieces.|
1.Reduce thermal stress and organizational stress, and significantly reduce the deformation and cracking tendency of the workpiece.
2. It is suitable for processing workpieces with complex shapes and precise dimensions.
|Only suitable for smaller size workpieces.|
Try to describe the selection principle of the quenching heating temperature of hypoeutectoid steel and hypereutectoid steel. Why can’t the quenching heating temperature of hypereutectoid steel exceed the Accm line?
The principle of quenching heating temperature selection: the principle of obtaining uniform and fine austenite grains in order to obtain a fine martensite structure. Hypoeutectoid steel is usually heated to 30-50°C above AC3, and hypereutectoid steel is heated to 30-50°C above AC1.
- 1. The quenching heating temperature of hypereutectoid steel exceeds the Accm line, and all carbides are dissolved in austenite, which increases the carbon content of austenite, reduces the Ms and Mf points of the steel, and increases the amount of retained austenite after quenching , Will reduce the hardness and wear resistance of steel
- 2.The quenching temperature of hypereutectoid steel is too high, the austenite grain is coarsened, and the carbon content is high. After quenching, it is easy to obtain coarse needle-shaped martensite with microcracks, which reduces the plastic toughness of the steel.
- 3. During high temperature quenching, the quenching thermal stress is large, and the oxidation decarburization is serious, which also increases the tendency of steel parts to deform and crack.
What is the hardenability and hardenability of steel? Factors that affect the hardenability, hardenability and depth of the hardenable layer of steel?
- Hardenability: The hardenability of steel refers to the ability of austenitized steel to obtain martensite during quenching. It reflects the stability of undercooled austenite and is related to the critical cooling rate of steel. Its size is expressed by the depth and hardness distribution of the hardened layer obtained by quenching the steel under certain conditions.
- Hardenability: The hardenability of steel refers to the ability of austenitized steel to harden during quenching. It mainly depends on the carbon content in the martensite and is expressed by the highest possible hardness of the quenched martensite.
- Depth of hardened layer: The depth of hardened layer refers to the depth from the half martensite zone to the surface of the workpiece measured when the steel is quenched under specific conditions. It is related to the hardenability of the steel, the shape and size of the workpiece, and the cooling capacity of the quenching medium.
Source: China Pipe Fittings Manufacturer – Yaang Pipe Industry Co., Limited (www.ugsteelmill.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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