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Influence of Vacuum Heat Treatment on High Speed Steel Tools

1 Vacuum heat treatment process of high-speed steel tools

When performing vacuum heat treatment on high-speed steel tools, first check whether the process card is correct, and then check the appearance of the tool. The specific operation procedure is: cleaning→loading the card→putting into the furnace→preheating→heating→ vacuum gas quenching→out of the furnace→loading into the tempering fixture→vacuum tempering.

1.1 Vacuum quenching temperature

Vacuum heating has two notable features: one is heating in an extremely thin atmosphere, which can avoid defects such as decarburization of the workpiece; The power is directly proportional.

It can be seen that in the vacuum state, especially in the low temperature stage, the temperature rises slowly, so the temperature difference between the surface and the core of the workpiece is small, the thermal stress generated is small, and the distortion of the workpiece is small.

The vacuum quenching heating temperature of high-speed steel is only about 30°C lower than the melting point, and the dissolution and grain growth of carbides are relatively fast, requiring precise temperature control. Generally, the heating temperature of vacuum quenching for high-speed steel workpieces is 10-20 °C lower than that of salt bath quenching.

1.2 Holding time of high-speed steel tools

When determining the holding time of high-speed steel workpieces heated by intense fire, in addition to considering the effective thickness of the workpieces, the amount of furnace loading should also be considered, that is, the amount of furnace loading is large, and the holding time should be extended appropriately. Usually, the actual charging capacity is only 35%~45% of the rated charging capacity.

1.3 Vacuum quenching and cooling

Proper pre-cooling before quenching can reduce workpiece distortion. It is generally believed that for workpieces with an effective thickness of 20-60 mm, the pre-cooling time is 0.5-2.5 min. In order to prevent carbide precipitation from affecting vacuum quenching hardness due to too slow cooling, the gas quenching rate of high-speed steel tools at 1000-800 °C should be controlled at ≥7 °C/s.

1.4 Vacuum tempering after vacuum quenching

Vacuum quenched workpieces should also be vacuum tempered so that they do not oxidize, decarburize, corrode, have a bright surface, and have no pollution, especially for precision tools that are not finished after heat treatment, which is particularly important.

Some people also think that salt bath tempering is better than vacuum tempering, and the process is (550 ~ 560 ℃) x1 h x3 times. If the workpiece load is large, the tempering time should be extended to 1.5~2.0 h.

2 Application examples of high-speed steel tools

2.1 Vacuum heat treatment of gear hob

Vacuum quenching: 800 ℃ x 20 min for one preheating, vacuum degree 0.133 Pa; 1 000 C x 20 min for second preheating, vacuum degree 133Pa (filled with nitrogen); 1220 ℃ × 25 min (vacuum degree of 133 Pa, filled with nitrogen gas) After heating, it is gas-quenched at a pressure of 5×105Pa.

Vacuum tempering: 550 ℃ x 2 h x 3 times. As a result, the hardness of the gear hob is 64 ~ 65 HRC.

High-pressure gas quenching is the mainstream of vacuum heat treatment for high-speed steel tools. It should be further promoted and applied, and different process parameters should be used for different tools, such as gas quenching pressure, furnace loading and placement methods, etc., to ensure the quenching and tempering hardness. ≥64 HRC. High-speed steel tools such as rolled twist drills, inlaid blades of large saw blades, and broaches are not suitable for vacuum quenching. The cooling rate of vacuum quenching should be increased as much as possible, so that the cooling rate in the range of 1000 ~ 800 ℃ is ≥ 7 ℃ / s, to prevent the precipitation of carbides, and to make the tool hardness ≥ 65 HRC after quenching and tempering.