Three questions about aging deformation
What is aging deformation?
Aging deformation is the general term of deformation (dimensional change and deformation) of steel parts placed at room temperature, which is also called long-term aging change. Deformation is not allowed in such precision measuring tools as micrometers and micrometers.
The manifestation of aging deformation of quenched parts is initial expansion and then contraction. And this expansion The change of shrinkage is significantly different with the carbon content of steel.
Types of aging deformation
Deformation aging can be divided into low-temperature Deformation Aging and high-temperature deformation aging.
- (1) Low temperature deformation aging. After quenching, the material is deformed at room temperature and then aged. Due to the cold deformation before aging, a large number of dislocations are introduced into the alloy. After aging treatment, the matrix recovers to form subgrain structure and obtain substructure strengthening; However, the desolvation process of supersaturated solid solution becomes complex due to cold deformation, which is related to the composition, quenching, deformation and aging of desolvation phase. Generally speaking, there are a large number of dislocations with disordered orientation in the alloy structure, which are evenly distributed in the crystal and near the grain boundary. The existence of these dislocations is conducive to the diffusion of solute atoms, the increase of the number of GP regions, the uniform distribution of desolved phases, the improvement of the strength of materials, and the elimination of precipitate free zones at grain boundaries. If a pre aging treatment is added between quenching and cold deformation, the desolvation process after cold deformation can be accelerated. Sometimes, in order to improve the thermal stability of materials, cold deformation can also be changed to warm deformation. Although low temperature deformation aging can obtain high tensile strength and yield strength, the plasticity of the material is reduced. It can also cause the decrease of creep resistance and anisotropy 862 for some aluminum alloys.
- (2) High temperature deformation aging. Direct quenching and aging after thermal deformation. This process not only improves the strength of the material due to the substructure caused by quenching and the uniform distribution of desolvent phase during subsequent aging, but also makes the material have better plasticity and structural stability due to grain fragmentation, grain boundary bending and desolvent point pinning of sub grain boundary Among the above two kinds of Deformation Aging, low-temperature deformation aging is widely used. High temperature deformation aging is only better used in Al Zn Mg alloys. However, the two kinds of deformation aging must meet the following basic requirements, that is, the matrix cannot recrystallize after quenching, and a high concentration of supersaturated solid solution should be obtained after quenching. For alloys with narrow quenching temperature range and not easy to harden through, the use of deformation aging method is limited.
What are the causes of aging deformation?
The reason of aging deformation is the existence of unstable structure (retained austenite and martensite) and residual stress during quenching, and the culprit is retained austenite. In order to achieve a certain degree of hardness by quenching, it is impossible to avoid the existence of retained austenite. Therefore, quenching hardened parts have aging deformation, which is an inevitable phenomenon.
The change of retained austenite at room temperature is slow and endlessly delayed According to the report of British g.t. brown, in Cranbrook Kent ruins The steel sheet (composed of W1 tool steel, 1.2% C, 0.2% Si, 0.03% s.0.025, p0.025) excavated on the is something from A.D. 100. It is a martensitic structure formed by water quenching and has quenching cracks. It is said that 15 ~ 20% retained austenite still exists in the hardened part. After 1860 years, retained austenite still exists, which shows how slow its transformation is!
How to prevent aging deformation
The reason of aging deformation is the expansion of quenching retained austenite during martensitization and the decomposition and contraction of martensite. Therefore, in order to prevent aging deformation, unstable retained austenite should be eliminated to stabilize it. In the method of stabilizing retained austenite, the cold treatment method should be considered first. The residual austenite is martensitized by cold treatment, and then it is better to temper at the temperature of 200 ~ 250℃ to stabilize the martensite.
Ordinary cold treatment (about 80 ℃) is enough for cold treatment, but if ultra cold treatment (- 196 ℃), it can not only prevent aging deformation. It can also effectively improve the wear resistance of parts. After cold treatment, of course, tempering treatment should be carried out.
When tempering above 300 ℃, the retained austenite will be decomposed, and martensite will also become troostite and sorbite, so there is no need to worry about aging deformation. Tempering at a temperature of about 200 ℃ can not only wear the parts, but also prevent deformation. For example, in order to prevent the aging deformation of block gauge and metal measuring tools, it is absolutely necessary to carry out the combined operation of cold treatment and tempering treatment. In addition, such tempering is effective by repeating 2 – 3 times. In any case, cold treatment plus tempering (twice) is an effective and appropriate method to prevent aging deformation.
Source: China Pipe Sleeve 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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