Vacuum Heat Treating of Tool Steels

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Vacuum heat treatment refers to the process of placing mold parts in vacuum heat treatment equipment for heating, heat preservation and cooling. It is one of the current advanced mold heat treatment technologies. As the mold quality requirements are getting higher and higher, general heat treatment is difficult to meet the technical requirements. After the mold is vacuum heat treated, the surface will not be oxidized and decarburized, the quenching distortion will be small, the surface hardness will be uniform, the performance will be improved, and the service life of the mold will generally increase. , Generally, it can be increased by more than 30%. In addition, vacuum heat treatment can reduce the machining allowance (grinding or polishing) by 1/3~1/2, thereby improving production efficiency and reducing mold manufacturing costs. Vacuum heat treatment is especially suitable for high-precision molds that require stable dimensions and performance. Mold vacuum heat treatment technologies include: vacuum quenching, vacuum tempering, vacuum carburizing, vacuum nitriding, etc. Figure 1 is the vacuum heat treatment equipment for the mold, and Figure 2 is the photo of the mold.

Figure 1

Figure 2

1.Vacuum heat treatment process parameters

(1) Vacuum degree

The degree of vacuum directly affects the roughness of the mold surface, thereby affecting the surface quality performance. In order to prevent the volatilization of alloy elements on the surface of the mold, a reasonable degree of vacuum should be selected.

(2) Preheating temperature

When the heating temperature of vacuum heat treatment is 1000~1100℃, preheat once at 800~850℃; when the heating temperature exceeds 1200℃, the mold with simple shape can be preheated at 850℃ once, and the mold with large or complex shape can be preheated once at 850℃. It should be preheated twice at 500~600℃ and 800~850℃.

(3) insulation

1) Heating temperature. The heating temperature of vacuum quenching generally takes the lower limit of salt bath furnace and air furnace. The heating temperature of vacuum tempering, vacuum annealing, vacuum solution treatment and vacuum aging is generally the same as that of conventional heat treatment.

2) Holding time. Usually, the vacuum heating time is 6 times that of the salt bath furnace and 2 times that of the air furnace. The empirical formula is T=KB+T, where t is the heating and holding time (min), K is the holding time coefficient (min/mm), and B Is the effective thickness of the mold (mm), T is the time margin (or fixed time) (min).

2.Vacuum quenching of mold

(1) Preheating

Low alloy steel (40Cr, 60Si2Mn, etc.), medium alloy steel (CrWMn, 9CrSi, 5CrNiMo, etc.) can choose two-stage heating (such as 650 °C preheating → 850  C vacuum quenching heating); high alloy steel (H13, Cr12MoV, etc.) Choose three-stage heating (such as 650°C preheating → 850°C preheating → 1030°C vacuum quenching heating).

(2) Selection of heating and holding time

The holding time should not only ensure that a certain amount of carbides are fully dissolved, increase the alloy content in the austenite, and provide a guarantee for the obvious hardness recovery during tempering at the secondary hardening peak temperature, but also prevent overheating and affect the quality of the mold.

(3) Cooling method

The vacuum quenching of die steel can adopt oil quenching, gas quenching, water quenching, nitrate quenching, etc. All alloy die steels can be subjected to vacuum oil quenching to obtain a bright surface and reasonable performance. Compared with air cooling and quenching, it is easy to obtain high toughness and strength due to the fast oil cooling speed. Air cooling quenching can obtain smaller quenching distortion.

The vacuum quenching cooling method should be selected according to the shape, material, size and technical requirements of the mold.

(4) Vacuum tempering

Advantages of using vacuum furnace (such as SIMUWU RVT series vacuum tempering furnace) for tempering: accurate and uniform temperature control; ensuring no oxidation during heating and holding stages; slow cooling with the furnace, or quick cooling with air. The cooling process can be filled with high-purity N₂ or a mixture of high-purity N₂ and other reducing gases (such as H₂) to ensure no oxidation and no coloring during cooling.

The heating rate of vacuum tempering is 0.8min/mm, and the core is kept at least 2h after reaching the temperature. The tempering temperature depends on the hardness requirement. The first and second vacuum tempering are mandatory, and the third vacuum tempering can also be omitted depending on the technical requirements and final hardness.

3.Vacuum annealing of the mold

Vacuum annealing of the mold (module) is easy to achieve heat treatment without oxidation and decarburization, which is conducive to improving the surface quality and production efficiency of the mold, shortening the process cycle, and the surface of the mold can be bright and the microstructure is uniform.

(1) Ordinary vacuum annealing process

Fig. 3 is H13 (4Cr5MoSiV1) steel module ordinary vacuum annealing process. Mold annealing adopts vacuum furnace (such as RVA series vacuum annealing furnace of SIMUWU company), and slowly heats the module to 870℃ at a speed of 60℃/h. The holding time (2~4h) depends on the effective size of the module. Keep 0.8min/mm. The pressure in the heat preservation stage is controlled at 0.1~10Pa. When cooling, the furnace can be cooled in a vacuum state. When the temperature is lower than 500°C, it can be filled with 1×105Pa high-purity N₂ or a mixture of high-purity N₂ and other reducing gases (such as H₂) for cooling to ensure The surface of the module is free from oxidation and coloring. After annealing, the hardness of the module is less than 235HBW, and the structure is pearlite + uniformly distributed granular carbide.

Fig. 3

(2) H13 die steel isothermal annealing process

The vacuum furnace pressure is 0.1~10Pa, slowly rise to 875~890°C at ≤200°C/h and keep for 2~4h, then quickly cool to 710~740°C and keep for 3~4h, and cool to below 100°C with high-purity nitrogen to take out the furnace.

(3) Cr12MoV die steel isothermal annealing process

The vacuum furnace pressure is 0.1~10Pa, slowly rise to 830~870°C at ≤200°C/h and keep for 2~4h, then quickly cool to 720~740°C and keep for 3~4h, cool to below 100°C with high-purity N₂ and leave the furnace.

4.Vacuum carburizing of the mold

Vacuum carburizing is to heat the mold to an austenitized state in a vacuum furnace, carburize in a carburizing atmosphere, and then diffuse and quench. Because the mold is heated in a vacuum state, the surface of the mold is very smooth, which is suitable for carburizing treatment of molds with high surface quality requirements.

5.Vacuum carbonitriding of the mold

Vacuum carbonitriding is a chemical heat treatment process in which the surface of the mold is heated under negative pressure in a vacuum furnace and infiltrated with carbon and nitrogen elements at the same time. Compared with single carburizing, the surface hardness of the mold is higher and the wear resistance is better. , but also has a certain corrosion resistance and higher fatigue strength. Due to the low co-infiltration temperature and direct quenching after infiltration, the austenite grains are finer than single carburizing, so the core toughness of the mold is improved, the co-infiltration layer structure has no grain boundary oxidation phenomenon, and the mold performance is significantly improved.

6.Vacuum furnace equipment selection:The vacuum heat treatment furnace produced by SIMUWU is a high-quality product for vacuum heat treatment process. Good temperature control accuracy and temperature control uniformity ensure the effective vacuum heat treatment process The equipment is easy to manage and maintain, greatly reducing maintenance costs. SIMUWU company focuses on the manufacture of vacuum furnaces, has more than ten years of relevant experience, and has a good reputation in the field of vacuum furnace manufacturing. The product line includes vacuum air quenching furnace, vacuum oil quenching furnace, vacuum brazing furnace, etc., which are widely sold in developed and developing countries.

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