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Vacuum Heat Treatment Process of H13 Forging Die Steel

H13 steel is an extremely typical hot work die steel with strong toughness, strong thermal fatigue resistance and strong thermal crack resistance. It is a material with high thermal strength and obvious mold recovery effect after the action of cold and heat. It is suitable for pressing molds, extrusion molds, heat treatment cutting molds and hot forging molds. The level of heat treatment directly determines the service life of the mold. At present, more advanced heat treatment methods are used at home and abroad to improve the performance of the mold and extend the service life of the mold. Vacuum heat treatment is one of the more advanced methods in H13 mold heat treatment. The development of vacuum heat treatment technology has improved the heat treatment of H13 mold steel to a new level.

1. Preparation For the Vacuum Heat Treatment of H13 Die Steel Before Loading the Furnace

Before installing the furnace, it is required to clean the iron filings on the surface of the mold, the residual coolant during machining, and the entire mold, and then install it in a special vacuum furnace pan after drying. When drying, you should pay attention to clean the surface of the mold and the moisture in the cavity to prevent residual debris or moisture from polluting the vacuum quenching oil.

2. Mold Assembly

The mold should be placed horizontally on the special tray of the vacuum furnace with the mold face up. There should be a fixed gap between the molds. It is strictly forbidden to overlap. Uneven hardness affects the service life of the mold. When dealing with small parts such as punches or ejector rods, in order to increase production efficiency, special tooling can be made. Two reels are installed at a time. Support rods are used between the two reels to leave enough space.

3. Selection of Quenching Medium

The H13 die steel core for forging requires a hardness above HRC43, and the effective thickness of the core is generally above 80MM. The workpiece is required to be hardened. Oil is selected as the cooling medium. When selecting the oil product model, the cooling curve of the oil product should be considered, and the workpiece should be considered. Material and shape factors, the workpiece that is prone to stress concentration is easy to crack when it is in a medium with a faster cooling rate.

4. Quenching and Tempering

In order to ensure the uniform temperature of the core and surface of the mold, a three-stage stepped heating is required before heating to the austenitizing temperature, that is, heating at 500 degrees per hour to 650 degrees, and holding time for T1, heating at 300 degrees per hour to 850 degrees. Holding time T2, heating at 200 degrees per hour to 1000~1040 degrees austenitizing, holding T3 time, T1, T2, T3 are the holding time of each section, which can be based on T1=30+( 2.0- 1.5 )D, T2=30+( 1.5-1.0 )D, T3=20+( 0.5-0.25 )D, D represents the maximum thickness of the mold to calculate to ensure that the core of the mold is burnt through. When actually preparing the process, the holding time and heating speed should also refer to the shape of the parts and the stacking method during loading.

Fig 1 Process curve

When the mold is quenched, the cooling chamber needs to be filled with nitrogen and the pressure is set to above 0.025MPa to prevent the quenching oil from overflowing after the cooling chamber pressure is too low after the high temperature mold is filled with oil. Moreover, a stirrer should be installed at the bottom of the oil tank to ensure that the oil is fully flowing when the workpiece is quenched, which is conducive to the cooling of the workpiece and the uniform transformation of the structure. The cooling time of the mold entering the oil can be determined according to the cooling curve of the oil quality, and it is generally calculated at 3 to 5 minutes per centimeter of effective thickness.

Since H13 die steel has high hardness and greater stress after quenching, it should be tempered immediately after quenching and cooling to prevent die cracking. Before tempering, the quenched mold must be cleaned in a washing machine, and the impurities and moisture on the mold surface must be blown away with compressed air. According to the typical tempering curve of H13 steel, during the tempering process, as the tempering temperature increases, the hardness value gradually decreases, with a valley near 4009C, and then the tempering temperature increases, and the hardness gradually rises to around 5309. The maximum hardness is the phenomenon of secondary hardening. When the tempering temperature exceeds 6209C, the hardness begins to decrease rapidly. Tempering adopts a special vacuum tempering furnace, and the tempering holding time can be calculated as 5 minutes per millimeter of effective thickness. Since the retained austenite of the mold after quenching will be transformed into martensite after the first tempering, internal stress will be generated, so it needs two times or more tempering. Generally choose 550-600 degree tempering according to the hardness requirements. During the second tempering, the temperature is adjusted according to the actual hardness detected after the first tempering. Generally, the surface hardness of the mold can reach HRC43-47 after tempering at 570 degrees for three times.

After processing the H13 die steel according to the above heat treatment process, wire-cut the remaining sample with an effective thickness of 75MM. The hardness of the cut surface is tested. The hardness difference between the surface and the core is within 2HRC. The internal structure is tempered martensite and alloy. Carbide has reached the technical requirements. Through large-scale data statistics, the actual mold life is about 1.2 times longer than that of ordinary heat treatment, and individual batches are increased by 1.5 times or even higher. However, the factors that affect the life of hot forging dies include many factors such as die design, processing, heat treatment, and operating conditions. It is necessary to take measures from the above aspects at the same time to significantly improve the life of the mold.

5. Other Matters Needing Attention

When the vacuum furnace is not used for a long time, keep it in a vacuum state and keep the furnace dry. If it is exposed to air for a long time, it needs to be baked before reuse;

Each time the vacuum furnace is used, the oil stirring equipment should be turned on at the same time as the pre-vacuum is used to degas the quenching oil to ensure sufficient cooling speed when the oil is cooled;

The vacuum quenching oil should be preheated before the die steel is quenched. Generally, the preheating temperature is 50~70 degrees to ensure the best quenching oil fluidity;

When the equipment conditions are insufficient, the ordinary box furnace trolley furnace can also be used for tempering after vacuum quenching, but it will cause oxidation on the surface of the mold;

For H13 material mold vacuum heat treatment, factors such as different vacuum heat treatment equipment, different quenching oil models, mold shape and even ambient temperature in actual production will affect the setting of process parameters.

6. Conclusion

In summary, by analyzing the vacuum heat treatment process of the forging die made of H13 material, and continuously improving the vacuum heat treatment process parameters, the purpose of smooth surface, uniform hardness, and increased die life in actual production can be achieved. Since vacuum heat treatment equipment is time-consuming to vacuum, and the current production efficiency is still low, in the case of high requirements for molds such as rapid automatic forging production lines, the use of vacuum heat treatment can obtain higher economic value.