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Process of preventing deformation and cracking of mold by vacuum heat treatment

Because of the large variety specifications, complex shape and low surface roughness value, the mold is difficult to manufacture. The deformation of the mold after vacuum heat treatment will seriously affect the quality and service life of the mold, once the cracking in the vacuum heat treatment will cause the scrap of the mold, therefore, to reduce and prevent the mold vacuum heat treatment deformation and avoid its cracking is an important research topic for the majority of mold vacuum heat treatment workers.

1 Select materials correctly
The deformation and cracking of vacuum heat treatment are closely related to the steel used and its quality, so it should be based on the performance requirements of the mold. Comprehensive consideration of mold accuracy, structure and size, as well as the nature of the processing object, quantity and processing methods and other factors reasonable selection. If the general mold has no deformation and accuracy requirements, carbon tool steel can be used from the perspective of cost reduction. For easily deformed and cracked parts, alloy tool steel with higher strength and slower critical quenching cooling rate can be selected. When the mold deformation made of carbon steel does not meet the requirements, use 9Mn2V steel or CrWMn steel and other alloy steel, although the material cost is slightly higher, but to solve the problem of deformation and cracking, in general, it is still cost-effective.
At the same time of correct selection of materials, it is necessary to strengthen the inspection and management of raw materials to prevent mold vacuum heat treatment cracking due to raw material defects.

2 Reasonable vacuum heat treatment technical conditions
Reasonable formulation of technical conditions (including hardness requirements) is an important way to prevent quenching deformation and cracking. Local hardening or surface hardening can meet the requirements of use, try not to quench as a whole. For the overall quenching mold, the local requirements can be relaxed, and try not to force consistency. For molds with high cost or complex structure, when vacuum heat treatment is difficult to meet the technical requirements, the technical conditions should be changed, and the requirements that have little impact on the service life should be appropriately relaxed, so as to avoid being scrapped due to multiple repairs.
For the selected steel grades, the highest hardness that can be achieved cannot be used as the technical conditions specified in the design. Because the highest hardness is often measured with a small sample of limited size, the hardness that can be achieved with the actual size of the larger mold is very different. Because the pursuit of the highest hardness often needs to increase the quenching cooling speed, thereby increasing the quenching deformation and cracking tendency, so the use of higher hardness as a technical condition, even the smaller size of the mold will bring certain difficulties to the vacuum heat treatment operation. In short, the designer should reasonably formulate practical technical conditions according to the performance and the selected steel grade. In addition, when placing hardness requirements on the selected steel grade, the hardness range that produces temper brittleness should also be avoided.

3 Adopt reasonable vacuum heat treatment process
In order to reduce and prevent the quenching deformation of the workpiece, in addition to reasonable design of the workpiece, material selection, the development of vacuum heat treatment technical requirements, and the correct hot processing of the workpiece blank (casting, forging, welding) and pre-vacuum heat treatment, it is more important to pay attention to the following issues in vacuum heat treatment:
(1) Reasonable choice of heating temperature
Under the premise of ensuring hardening, generally a lower quenching temperature should be selected as far as possible. However, for some high-carbon alloy steel molds (such as CrWMn, Cr12Mo steel), the Ms point can be reduced by appropriately increasing the quenching temperature, and the residual austenite volume can be increased to control the quenching deformation. In addition, for high carbon steel molds with large thickness, the quenching temperature can also be appropriately increased to prevent quenching cracks. For molds that are easy to deform and crack, stress relief annealing should also be carried out before vacuum quenching.
(2) Reasonable heating
Uniform heating should be done as far as possible to reduce the thermal stress during heating. For high-alloy steel molds with large sections, complex shapes and high deformation requirements, they should generally be preheated or limited in heating speed.
(3) The correct choice of cooling method and cooling medium
As far as possible, pre-cooling quenching, fractional quenching and fractional cooling methods are selected. Precooling quenching has a good effect on reducing deformation of slender or thin molds, and can play a role in reducing deformation of molds with different thickness to a certain extent. For the mold with complex shape and wide cross-section difference, it is better to use fractional quenching. Such as high speed steel using 580-620℃ quenching, basically avoid quenching deformation and cracking.
(4) Correctly master the operation method of vacuum quenching
The correct choice of the workpiece into the medium to ensure that the mold gets the most uniform cooling and enters the cooling medium along the direction of the least resistance, and the slowest cooling surface moves towards the liquid. When the mold is cooled below Ms point, it should stop moving. For example, if the thickness of the mold is not uniform, the thick part should be quenched first; For the workpiece with large cross-section changes, the deformation of vacuum heat treatment can be reduced by adding process holes, reserving reinforcing ribs, and filling asbestos in the holes. For the workpiece with concave and convex surface or through hole, the concave surface and hole should be quenched upward in order to discharge the bubble in the through hole.

Vacuum heat treatment is one of the indispensable processing technologies in the mold manufacturing process, which has a great impact on the quality and cost of the mold, and is one of the important measures to improve the service life of the mold. Deformation and cracking are the two major problems in the mold vacuum heat treatment, and the causes are complex, but as long as the rule is mastered and the right medicine is used, the deformation of the mold can be reduced, and the cracking can be controlled.