With the new medium frequency induction heating equipment, you no longer have to worry about cracking defects in DIN 100MnCrW4 mold steel.

The size of a cold work mold is 23mmx310mmx330mm, and the workpiece material is DIN 100MnCrW4 steel [the main chemical composition (mass fraction) is: 0.88%C, 1.10%Mn, 0.65%Cr, 0.54%W, 0.07%V]. The heat treatment process is: heating at 820℃xlh, quenching and cooling to 50℃, and then tempering twice at 240℃×2.5h. During production, it was discovered that the corners of the mold cracked after grinding, causing the mold to fail and be scrapped.

Macroscopic inspection revealed that there were three chipping and cracks at the corners of the mold. The first crack was along the transverse direction of the workpiece, and the cracked piece collapsed and fell. The size of the cracked piece was 155mmx13mmx5mm. The second crack completely split along the length of the mold at the thinnest part of the hole wall, and was located at the large circular hole in the middle of the left side of the first cracked block. The third crack is located on the lower side of the mold, 10mm away from the lower left corner of the workpiece, showing two fine cracks extending from the edge to the thickness direction and perpendicular to the plane of the workpiece. The mold hardness is 58HLC. The surface inspection of the mold revealed that the edges, corners and openings of the workpiece were rough in machining, and the intersection surface between the large hole and the mold had no arc transition and was in the shape of a right-angled step. Moreover, the wall thickness of the workpiece in this part was the thinnest (about 6mm). It is a typical stress concentration area. Observing the fracture morphology under low magnification, it can be seen that the cracked fracture is silver-grey. After microscopic observation with a scanning electron microscope, it was found that the fracture surface had a typical rock sugar-like morphology and was an intergranular brittle fracture. Energy spectrum analysis shows that there are a large number of carbides at the fracture point of the fracture mold. Microscopic examination and analysis of the fractured mold showed that the crack propagation showed elliptical bending characteristics, the crack front edge was small, and no passivation was found. There are carbides that have not fallen off at the local cracks, and microscopic holes are formed near the carbides. The presence of unresolved carbides was found in the crack area. The microstructure here is tempered martensite + granular carbides + a small amount of massive carbides + a small amount of retained austenite. The carbide band segregation in the mold material is obvious, and the band segregation is level 3. There are semi-closed or closed secondary network carbides in the crack area locally, and elongated plastic inclusions are found in the carbide segregation zone, which is gray, and the network carbides are grade 3-4.

From the metallographic analysis of the mold crack inspection, it can be seen that the crack is caused by quenching cracking, not formed before heat treatment. Microstructural analysis shows that there are secondary network carbides and band segregation in the cracked mold, as well as local segregated carbide liquid precipitation, which is the main reason for the quenching cracking of the mold. Compared with the matrix, the carbides present in the mold are hard and brittle, and stress concentration is easily generated at the phase interface, becoming a source of microcracks. Under the action of internal stress, cracks tend to propagate along the carbide and eventually lead to quenching cracking of the workpiece. On the other hand, processing defects such as rough machining marks and unrounded transitions on the edges, corners and processing holes of the mold, as well as unreasonable structural design, make these parts prone to stress concentration and micro-cracks, which promotes mold cracks. Secondary causes of cracking.

Based on the above analysis, the process improvement measures to prevent mold quenching cracking failure are proposed as follows:

(1) The mold blank should be forged before heat treatment to eliminate band-like segregation of the mold blank and uniformize the carbide structure distribution to reduce the hidden dangers and dangers of stress concentration and cracking.

(2) After machining, new medium-frequency induction heating equipment should be used for stress relief annealing to eliminate the stress on the workpiece and reduce the tendency of quenching and cracking of the workpiece.

(3) Improve mold design and processing, improve mold processing accuracy and fillet transitions at the intersection of openings and mold surfaces to reduce stress concentration and micro-cracks.

It has been proved by many practices that after heat treatment using the above improvement measures, DIN 100MnCrW4 mold steel has no cracking defects, and the service life and workpiece performance have been greatly improved, meeting the work requirements. What makes everyone even more happy is that it can carry out large-scale batch production and can greatly improve production efficiency.