What is a Cleft Palate?

Internal cracking is one of the internal defects of steel. It is manifested in different forms of internal cracks caused by shrinkage, non-metallic inclusions, and air bubbles on the acid-soaked low magnification sample. It is an intergranular with little or no inclusions in the crack. Cracking occurs mostly in high-carbon steels and high-alloy steels. The internal crack concentrated at the center of the ingot is called the axial intergranular crack. Because it does not contact the air, the inner wall is not oxidized and there is no segregation inclusion. Large compression is used during hot working. Than possible welding. Axial intergranular cracks are thermal cracks caused by tensile stress at the center caused by volume shrinkage during solidification of the ingot.

The main cause of thermal stress internal cracking is the residual stress during quenching of the workpiece. Defects in the material are the auxiliary causes of cracking. In order to prevent the occurrence of internal cracks and fractures, we should mainly consider two aspects: reducing the tensile stress of the heart and increasing the breaking resistance of the material.

1. Properly reducing the quenching temperature can reduce thermal stress and is beneficial to preventing internal cracking. Because the higher the quenching temperature, the greater the thermal stress. At the same time, too high heating temperature will coarsen the austenite grains and reduce the material strength. Appropriate pre-cooling before quenching after the workpiece is out of the furnace is also beneficial to reduce thermal stress and prevent internal cracking.

2. When quenching, pay attention to the uniform cooling of the workpiece.

3. Timely tempering is an effective measure to prevent internal cracking. Residual stress intensifies after quenching in a timely manner, and with time, the residual austenite in the surface layer continues to decompose, further intensifying the internal tensile stress. Therefore, large forgings, especially alloy steel forgings, must be tempered in time after quenching.

4. It is very important to control the heating rate. Rapid heating will increase the internal and external temperature difference and increase thermal stress. Especially for large forgings with high carbon and alloying elements, the temperature rise rate in the low temperature stage must be strictly controlled. Generally speaking, the heating rate of large forgings in the low temperature stage should be controlled at 30 ~ 70 / h. When tempering, due to the large residual stress of the workpiece, pay more attention when heating.

5. Reducing defects in the material and increasing the strength of the material itself are also important to prevent quench cracking. In order to increase the strength of the material itself, for workpieces with coarse grains, a normalization can be performed first to refine the grains and increase the strength.

6. When large workpieces with internal holes are quenched, simultaneous cooling of the internal holes is beneficial to prevent quench cracking. In the quenched and tempered heat treatment 640mm × 1100mm (with 380mm blind hole) 40Cr steel fork head, the blind hole faces downwards during lifting, and the inner wall of the blind hole cannot be sufficiently cooled, which causes cracks at the bottom of the blind hole for all 4 workpieces. Later, the blind holes were hoisted upwards, and the cooling conditions in the holes were improved. All 6 pieces processed in this batch were qualified. ^[2]