What is the cause of quenching distortion? What are the factors that affect distortion?

The workpiece is prone to some distortion during the quenching and cooling process. What are the causes of these distortions? What are the factors that affect quenching distortion? Zhengzhou Gou's takes you to find out.

Causes of quenching distortion:

1. There is residual stress inside the parts before heat treatment

2. Heating temperature is high and uneven

3. The chemical composition of the steel is uneven, the hardenability is low, and the carbides are unevenly distributed.

4. Partial removal before quenching

5. Insufficient tempering, and the residual stress caused by straightening and pickling has not been eliminated.

6. The workpiece is improperly placed during tempering

Factors affecting quenching distortion:

(1) Influence of heat treatment process

Increasing the austenitization temperature, thermal stress and structural stress will increase accordingly, and have a decisive influence on the proportion of martensite, retained austenite, carbide and other phases in the quenched structure, and the specific volume of each phase Different from each other, the micro-deformation quenching of tool steel controls distortion by adjusting the austenitization temperature.

The cooling method also has a relatively large impact on quenching distortion. Reducing the cooling rate above the martensite point can reduce distortion caused by thermal stress. Reducing the cooling rate below the martensite point can reduce distortion caused by tissue stress. Graded quenching and isothermal quenching not only reduce both thermal stress and tissue stress, but also reduce tissue stress more significantly. Graded quenching and isothermal quenching are often effective methods to reduce quenching distortion. During isothermal quenching, a lower bainite structure with a specific volume smaller than martensite can be obtained, which can significantly reduce quenching distortion.

(2) Composition and original structure of steel

The thermal conductivity, hardenability, Ms point temperature of steel, the volume increment of martensitic transformation and thermoelastoplastic mechanical behavior are all related to the austenite chemical composition and austenitization temperature, which affect the thermal stress of steel. and tissue stresses and the deformations they cause have a significant impact.

The original structure of steel before quenching also has a certain influence on the quenching distortion of steel. The quenching distortion of the original structure of spherical pearlite or quenched and tempered structure is smaller than that of workpieces whose original structure is lamellar pearlite.

The segregation of carbides in tool and die steel makes the quenching distortion directional, and the expansion along the direction of the banded carbides is greater than the direction perpendicular to the banded direction.

(3) The comprehensive influence of various factors before and after quenching

In addition to the factors mentioned above, mechanical processing, plastic forming, welding and straightening before quenching will cause residual internal stress in the workpiece. If there is no stress relief treatment, distortion will be caused by the relaxation of residual stress during quenching and heating. In addition, significant distortion will also be caused by improper placement of the workpiece or poor fixture and the self-weight of the steel during quenching and heating.

(4) Size and shape of parts

The dimensional changes of the parts directly affect the quenching depth and the quenching stress distribution. As the wall thickness of the workpiece increases, the thermal stress type distortion tends to increase. When the wall thickness decreases, the tissue stress type distortion tends to increase.

The shape of steel parts has a greater impact on quenching distortion, but the shapes of workpieces vary so much that it is difficult to summarize a general rule. Roughly speaking, when the cross-section of the steel part is symmetrical and the wall thickness is relatively uniform everywhere, the cooling of each part is relatively uniform and the distortion is relatively regular. For example, if the shape of the workpiece is complex, the wall thickness varies greatly, and the shape is asymmetrical, the various parts of the steel part will be distorted. If the parts are cooled unevenly and the thickness of the hardened layer is different, serious distortion will occur.

Parts with asymmetrical sides bulge toward the fast-cooling surface under the action of thermal stress, and toward the slow-cooling direction under the action of tissue stress.

In actual production, regardless of the steel type, when asymmetric parts are fully hardened, if salt water is used for quenching, most of the bulges will be on the side that cools quickly, and if nitrate salt graded quenching is used, most of the bulges will be on the slow cooling side.

Uneven wall thickness of the workpiece will also increase the tendency of distortion. The cavity in the thin-walled part will tend to expand, while the cavity in the thicker wall will tend to shrink. If the mold cavity has no through-holes, the inner cavity will cool slowly and the bottom surface of the mold will bulge under the action of thermal stress.