CrWMn steel mold exploded and failed, and the medium frequency induction heating furnace easily solved it

The size of a certain cold work mold is 120mmx210mmx250mm, and the workpiece material is CrWMn steel. The technical requirement is that the hardness after heat treatment is 55-58HRC. During production, it was found that explosion failure occurred during wire cutting processing after the mold was heat treated. After analyzing the causes of the defects, it was found that the use of medium frequency induction heating furnace for heat treatment can effectively avoid the occurrence of such defects.

Macroscopic inspection of the fractured mold revealed that the fracture was roughly perpendicular to the axis, the cross section was flat, smooth, slightly arc-shaped, and showed brittle fracture characteristics. The upper edge of the fracture is the crack source area, with radial patterns next to it, the middle is the crack expansion area, and the final fracture area is characterized by rapid tearing. By testing the hardness of various parts of the CrWMn steel mold, it can be seen that the hardness of the mold is obviously low. The design requires the mold hardness to be 55-58HRC, but the actual workpiece hardness is only 33-42HRC, and the hardness distribution is very uneven, so the strength performance of the mold is greatly reduced. 

The metallographic observation of the explosion mold sample found that the metallographic structure of different parts is greatly different, and the distribution is obviously uneven. The surface structure is tempered sorbite + tempered troostite. The structure of the remaining parts is tempered trostite structure + carbide and pearlite-like structure with different spacing between lamellae. It shows the morphology of pearlite and carbide obvious accumulation areas respectively, and the carbide also has intergranular distribution characteristics. The metallographic structure shows that the mold is not completely quenched and the composition is seriously uneven.

Scanning electron microscopy of the fracture morphology shows that the fractures are dominated by brittle fracture characteristics such as cleavage, quasi-cleavage and intergranular cracks. Only a small amount of dimple morphology is found at some grain boundaries, so the fracture is a typical brittle fracture.

The mold inspection results are closely related to the mold heat treatment process and wire cutting processing. The size of the mold is large, and the cooling rates of various parts of the workpiece vary greatly during quenching and cooling. The surface layer cools quickly, thereby obtaining a full martensite structure, while the internal cooling is slow, and pearlite transformation occurs. While the workpiece is continuously cooled, the pearlite transformation temperature is different in different parts, resulting in different structures. Therefore, the spacing between lamellae in the structure is different, and there is carbonization. substance precipitates. Analysis believes that the uneven metallographic structure of the mold and the local carbide segregation phenomenon are caused by the serious unevenness of the original structure: at the same time, uneven composition is also an important reason for the uneven hardness of the mold. The entire bursting mold was not quenched, and the quenched layer was thin. After the workpiece is quenched, its maximum tensile stress is located inside the mold, and the uneven structure of the mold after heat treatment increases the residual stress of the workpiece. Non-metallic inclusion defects in the mold cause stress concentration. When the stress exceeds the material strength limit, crack initiation sources appear in the weak areas of the mold. Once a crack occurs, it expands rapidly under stress. Due to poor strength, poor toughness and many defects of the failed mold, the crack propagates in a brittle manner. The workpiece is composed of carbides distributed along the grain, and its strength is low, which promotes crack propagation along the grain. In addition, the mold is not tempered enough and the quenching residual stress cannot be completely eliminated. Therefore, during the wire cutting process of the mold, cracks develop rapidly again under the action of external force, causing the workpiece to explode and destroy.

In order to prevent the CrWMn steel mold from bursting, failing and damaging, the following process improvement measures are proposed:

(1) Strictly inspect and control the chemical composition and original organizational technical requirements of the mold blank to prevent non-metallic inclusions, segregation and other defective blanks from flowing into the mold processing process.

(2) Use a medium frequency induction heating furnace to carry out a new heat treatment process, and strictly control and operate the process to completely quench the mold, reduce the quenching residual stress of the workpiece, and make the quenching structure and performance of the mold meet the technical requirements.

(3) The mold should be fully tempered to eliminate quenching residual stress and prevent mold cracking and failure due to excessive residual stress due to insufficient tempering.

(4) The tempering temperature of the original mold is too high, and the hardness of the workpiece is much lower and uneven. This is one of the important reasons for the low strength of the workpiece and cracks and fractures. Therefore, when using a medium frequency induction heating furnace for tempering, the tempering temperature should be adjusted so that the hardness of the mold after tempering meets the technical requirements and prevents the serious lack of strength of the workpiece after tempering.

Isn’t it great to use a medium frequency induction heating furnace for heat treatment of CrWMn steel molds? It not only avoids the bursting defect, but also increases its service life and meets the working performance requirements. The most important thing is that even the appearance is much more beautiful. Therefore, it gained recognition as soon as it entered the market.