Annealing, normalizing and quenching of steel pipes
What is annealing of steel pipe? What are the types and uses of annealing?
Annealing of steel pipe: annealing is a heat treatment process that heats the steel pipe to the temperature above or below the critical point AC1, and cools slowly with the furnace after holding for a certain time to obtain the structure close to the equilibrium state.
Annealing type: 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, homogenizing annealing, and the latter includes recrystallization annealing and stress relief annealing. According to the cooling mode, it can be divided into isothermal annealing and continuous cooling annealing.
- 1. Complete annealing: complete annealing is a heat treatment process in which the steel pipe is heated to 20-30 ℃ above AC3 for a long enough time, and the structure is fully austenitized and then cooled slowly with the furnace to obtain the structure close to the equilibrium state. It is mainly used in hypoeutectoid steel. Its purpose is to refine grain, eliminate internal stress and work hardening, improve plastic toughness, even chemical composition and structure of steel, improve cutting performance of steel, and eliminate defects such as widmanstatten structure and banded structure in medium carbon structural steel.
- 2. Incomplete annealing: incomplete annealing is a heat treatment process in which the steel pipe is heated to ac1-ac3 (hypoeutectoid steel) or ac1-accm (hypereutectoid steel) and cooled slowly with the furnace after holding for a certain period of time to obtain the structure close to the equilibrium state. For hypoeutectoid steel, if the original structure distribution of steel is suitable, incomplete annealing can be used instead of complete annealing to eliminate internal stress and reduce hardness. For hypereutectoid steel, the main purpose of incomplete annealing is to obtain spherical pearlite structure, to eliminate internal stress, reduce hardness and improve machining performance.
- 3. Spheroidizing annealing: spheroidizing annealing is a heat treatment process to spheroidize carbide in steel pipe and obtain granular pearlite. It is mainly used for eutectoid steel, hypereutectoid steel and alloy tool steel. Its purpose is to reduce the hardness, improve the cutting performance, even the structure and prepare for quenching. Normal spheroidizing annealing process is adopted for general bearing steel pipe
- 4. Homogenization annealing: also known as diffusion annealing, it is a heat treatment process that heats ingots, castings or forging billets to a temperature slightly lower than the solid phase line for a long time, and then slowly cools them to room temperature. Its purpose is to eliminate the dendrite segregation and regional segregation in the solidification process of ingot or casting, so as to homogenize the composition and structure.
- 5. Recrystallization annealing: a heat treatment process in which the cold deformed metal is heated above the recrystallization temperature for a proper time, and then cooled slowly to room temperature. Its 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-formed steel tube metal is heated to a certain temperature below the recrystallization temperature, kept warm for a period of time, and then slowly cooled to room temperature. Its main purpose is to eliminate the residual internal stress (mainly the first kind of internal stress) in steel pipes, castings, forged and rolled parts, weldments and machined workpieces, so as to improve dimensional stability and reduce the tendency of deformation and cracking of workpieces.
What is steel pipe normalizing? What is the purpose? What are the applications?
Normalizing of the steel pipe: normalizing is to heat the steel pipe to the appropriate temperature above AC3 or Accm and to heat it for the proper time to complete austenitizing. Then, the heat treatment process of pearlite like tissue is obtained by cooling at high speed (air cooling, air cooling or spray). The essence of normalizing process is complete austenitizing and pseudo eutectoid transformation.
Objective: to refine grain, uniform composition and structure, eliminate internal stress, adjust hardness, eliminate defects such as widmanstatten structure, banded structure and reticular carbide, and provide suitable microstructure for final heat treatment.
- 1. Improve the cutting performance of low carbon steel pipe.
- 2. Eliminate the hot working defects (widmanstatten structure, banded structure, coarse grain) of medium carbon steel pipe.
- 3. The net carbide of hypereutectoid steel pipe is eliminated, which is convenient for spheroidizing annealing of steel pipe and preparing for quenching of steel pipe.
- 4. As the final heat treatment, it can improve the mechanical properties of common steel tube structure.
In order to improve the strength of hypoeutectoid steel, the common method is to increase the content of pearlite in hypoeutectoid steel. What heat treatment process should be used?
The normalizing process should be used.
Cause: the undercooled austenite of hypoeutectoid steel will precipitate the proeutectoid ferrite during the cooling process. The slower the cooling speed is, the more the content of proeutectoid ferrite is, resulting in the less pearlite content, and reducing the hardness and strength of hypoeutectoid steel. The essence of normalizing process is complete austenization plus pseudo eutectoid transformation. By increasing the cooling rate, the content of proeutectoid ferrite can be reduced, and the hypoeutectoid steel can be transformed into eutectoid structure, that is, the content of pearlite is increased, so the strength and hardness of hypoeutectoid steel can be improved.
What is the purpose of quenching? How many quenching methods are there? Compare the advantages and disadvantages of several quenching methods?
The purpose of steel tube quenching is to obtain as many martensites as possible, which can significantly improve the strength, hardness and wear resistance of steel tube. In combination with various tempering processes, the steel can have high strength, high hardness and good plastic toughness.
When the steel is heated to a certain temperature above the critical point AC3 or AC1 and cooled at a cooling rate higher than the critical cooling rate after holding for a proper time, the heat treatment process for obtaining 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 isothermal quenching method
Comparison of advantages and disadvantages:
|Single fluid quenching||Simple operation and wide application||
1. Only for small size and simple workpiece.
2. Great quenching stress.
3. It is not easy to choose cooling capacity and cooling characteristics of suitable cooling medium.
|Double liquid quenching method||
1 Reduce tissue stress and reduce the tendency of workpiece deformation and cracking.
2. Suitable for workpiece with large dimension.
|Operation is not easy to control, and requires rich experience and skilled technology.|
|Step quenching method||
1. Reduce thermal stress and tissue stress, reduce the deformation and cracking tendency of workpiece
.2. Operation is relatively easy to control.
|Only for smaller workpiece.|
1. Reduce thermal stress and tissue stress, significantly reduce the deformation and cracking tendency of workpieces.
2. It is suitable for dealing with workpieces with complex shape and precise dimension.
|Only for smaller workpiece.|
The selection principle of quenching temperature for hypoeutectoid steel and hypereutectoid steel is discussed. Why can’t the quenching temperature of hypereutectoid steel exceed the ACCM line?
Selection principle of quenching and heating temperature:
In order to obtain fine martensite structure, the principle is to get even and fine austenite grains. Hypoeutectoid steel is usually heated to 30-50 ℃ above AC3 and hypereutectoid steel to 30-50 ℃ above AC1.
- 1. If the quenching temperature of hypereutectoid steel exceeds the ACCM line, all carbides will dissolve into austenite, which will increase the carbon content of austenite, reduce the MS and MF points of steel, increase the amount of retained austenite after quenching, and reduce the hardness and wear resistance of steel
- 2. Hypereutectoid steel has high quenching temperature, coarsening austenite grain and high carbon content. After quenching, it is easy to obtain coarse acicular martensite with microcracks, which reduces the plastic toughness of steel
- 3. During high temperature quenching, the heat stress of quenching is large and decarburization is serious, which also increases the tendency of deformation and cracking of steel parts.
What is hardenability and hardenability of steel? Factors affecting hardenability, hardenability and hardenability depth of steel?
Hardenability: hardenability of steel refers to the ability of austenitized steel to obtain martensite during quenching, which reflects the stability of undercooled austenite and is related to the critical cooling rate of steel. Its size is expressed by the depth of harden layer and the distribution of hardness obtained by quenching steel under certain conditions.
Hardenability: the hardenability of steel refers to the ability of austenitized steel to harden during quenching, which mainly depends on the carbon content of martensite, expressed by the highest possible hardness of quenched martensite.
Depth of harden layer: the depth of harden layer refers to the depth from semi martensitic zone to workpiece surface measured during quenching of steel under specific conditions. It is related to hardenability of steel, shape and size of workpiece and cooling capacity of quenching medium.
Source: China Steel Pipe Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.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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