First, the quenching strain has dimensional change and shape change.
Quenching strain (distortion) has two types: dimensional change and shape change (deformation). The so-called dimensional change is the dimensional change caused by expansion or contraction caused by phase change during quenching, mainly referring to similar deformations such as elongation, shortening, thickening, and thinning. Deformation is mainly caused by sagging caused by the dead weight of the parts and shape distortion caused by stress, such as non-similar deformations such as warping, bending, and twisting. Of course, if the size changes, its shape also changes, so it is common to be confused about whether it is a dimensional change or a deformation, and dimensional changes and deformation often overlap. It is most appropriate to express it figuratively using quenching strain. The definition of quenching strain in metallurgy is the state where the sum of stresses generated by parts after heat treatment tends to zero.
The appearance of quenching strain involves three stages:
① Heating (based on the elimination of internal stress);
② Insulation (drooping due to self-weight, i.e. drooping bending);
③ Cooling (based on uneven cooling and phase transformation). These three stages overlap and eventually lead to the quenching strain of the parts.
Second, 6 questions about dimensional changes.
1. What is the cause of dimensional change: Usually dimensional change is caused by organizational change, i.e. expansion and contraction caused by phase transformation. Expansion occurs when quenching generates martensite, while contraction occurs when residual austenite is generated, and the amount of contraction is proportional to the amount of residual austenite. When tempering, it is generally contraction, and alloy steel that has been tempered and hardened multiple times is expansion. In addition, when cold treatment is performed, the martensite of the residual austenite expands, causing dimensional changes. The specific volume of these organizations increases with the increase in carbon content. The more carbon content, the greater the dimensional change.
2. Material and dimensional changes: The dimensional change (quenching strain) caused by quenching varies with the material of the steel. Measuring tool steels SKS3 and SKS31 (W-Cr-Mn tool steel) are steels with small quenching deformation, and can even be called steels that do not produce quenching strain. Secondly, the plastic flow line of the steel has a great influence on the dimensional change during quenching. The dimensional change along the direction of the plastic flow line, that is, in the longitudinal direction, is large; the dimensional change in the direction perpendicular to the longitudinal direction, that is, in the transverse direction, is small. Therefore, when taking materials, it is necessary to pay attention to the consistency of the plastic flow line direction. In addition, the linear segregation of carbides also affects the dimensional change.
3. Quenching and dimensional change:
(1) Dimensional change caused only by structural changes: When steel parts are quenched, various structural changes occur. These structural changes cause dimensional changes. When the austenite structure is transformed into the martensite structure (complete quenching), the dimensional change (expansion) of the part is the largest; when the austenite structure is transformed into the bainite structure, the dimensional change is about 1/3 of the above; when it is transformed into the pearlite structure (annealing), it is about 1/4 of the above. In addition, the expansion caused by martensiticization increases with the increase in the carbon content of the steel.
(2) Effect of retained austenite: Due to the effect of quenching, even if a small amount of austenite remains, the dimensional change caused by expansion will be reduced accordingly. Therefore, the presence of a small amount of retained austenite leads to a reduction in dimensional change. However, the presence of retained austenite will reduce the quenching hardness and cause aging deformation when left at room temperature.
(3) Effect of undissolved carbides: During quenching heating, the fewer carbides are dissolved in austenite; in other words, the more retained carbides, the smaller the dimensional change. Changes in the morphology and type of the retained carbides themselves do not cause volume changes, so they have nothing to do with dimensional changes.
(4) Effect of cold treatment: During cold treatment, the amount of retained austenite decreases, and the amount of martensite increases, so an expansive dimensional change occurs.
4. Tempering and dimensional change:
(1) Decomposition of martensite: The decomposition of martensite caused by tempering is the cause of shrinkage dimensional change. The amount of dimensional change varies with the carbon content of martensite. The higher the carbon content of martensite, the greater the dimensional change. However, if the state before quenching is taken as the benchmark, the comprehensive dimensional change after quenching and tempering is still expansionary in the end.
(2) The influence of undissolved carbides: If there are undissolved carbides, the carbon content of austenite decreases, and the carbides themselves do not affect the dimensional change, so the dimensional change in the first stage of tempering (tempering below 200) is contractionary.
(3) The influence of retained austenite: If there is retained austenite, the dimensional change caused by tempering is small; when the tempering temperature is above 200℃, the retained austenite transforms into bainite, causing expansionary dimensional change. Therefore, in the early stage of tempering (below 200℃), the retained austenite shrinks the size. Above this temperature, increasing the tempering temperature, the decomposition of the retained austenite will cause expansionary dimensional change.
5. Dimensional change of alloy steel:
The carbides in alloy steel often dissolve special elements, but their specific volume can be said to be almost unchanged. Therefore, the method for treating alloy steel is the same as the above method. It’s just that the amount of retained austenite varies according to the type and amount of alloying elements: the amount of carbides also varies. So dimensional changes must be considered.
6. How to reduce dimensional changes:
Dimensional changes are caused by changes in the structure after quenching or tempering. Therefore, it is impossible to eliminate dimensional changes. It can only be reduced by heat treatment methods:
(1) Expansion is caused by martensite: Contraction is caused by retained austenite, so the amount of martensite and the carbon content dissolved in martensite should be reduced, and the amount of retained austenite should be increased. However, it must be noted that the increase of retained austenite will cause aging deformation.
(2) Increase the amount of undissolved carbides (residual carbides).
(3) Use other structures other than martensite to harden the steel, among which bainite is the best. Steel with 50% bainite plus 50% martensite structure is both hard and has small dimensional changes, so it is easy to control the size.
(4) Tempering should be performed.
Post time: Nov-04-2024