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Vacuum Heat Treatment of High Speed Straight Shank Twist Drill

High-speed straight shank twist drills are the most widely used hole processing tools. The market demand is large, and the batch output often reaches tens of thousands to hundreds of thousands. Usually, hot and cold processing processes such as hot rolling, hot extrusion, milling or grinding are used. Production. Straight shank twist drills require high hardness, high red hardness, high wear resistance and certain toughness. The heat treatment technical conditions are: the hardness of the blade is 63-66HRC, the shank is 30-45HRC, and the quenching and tempering structure is normal. Most of them use salt bath heat treatment, and only use vacuum heat treatment for a small amount of fully ground small straight shank drills or milling drills (diameter ≤ 3mm) with high precision requirements. Although the salt bath heat treatment has the advantages of fast heating speed, small distortion, high productivity, and the shank and the blade can be hardened at one time, the barium chloride used in the high temperature salt bath furnace is toxic, and the generated furnace gas, slag, and cleaning water will be harmful. cause pollution to the environment. Therefore, the high-temperature barium chloride salt bath furnace is a heat treatment equipment that has been temporarily eliminated in my country.

Vacuum heat treatment is recognized as a clean heat treatment technology with high efficiency, energy saving and no pollution. It has the advantages of no oxidation, no decarburization, small distortion and good surface quality of the workpiece, and also has the effect of degassing and degreasing, so it has become a hot spot in the development of domestic heat treatment technology. In order to reduce the pollution sources of heat treatment, domestic manufacturers began to gradually reduce and eliminate barium chloride salt bath furnaces, and transitioned to vacuum heat treatment, and finally achieved the purpose of replacing salt bath heat treatment with vacuum heat treatment.

The product to be processed is a straight shank rolled twist drill with a diameter of 2.5mm×57mm. It is a semi-finished product with a grinding head cut after rolling and a rough outer circle. The material is W6Mo5Cr4V2 high-speed steel, and the surface is oxidized blue.

Vacuum heat treatment process

(1) Clamping method and furnace capacity

The straight-shank rolled twist drill is cleaned and loaded into a stainless steel round sleeve with a size of φ25mm×65mm. The blade is uniformly upward. The sleeve is placed vertically in a rectangular material box. In the material basket, the loading capacity is about 150 kg.

(2) Preheating

After the workpiece is put into the furnace, it starts to vacuumize and heat up, and the heating rate is about 10℃/min. When the temperature of the workpiece is less than or equal to 170°C, the vacuum degree must be pumped to less than or equal to 10 Pa; when the temperature of the workpiece is greater than 170°C, nitrogen is charged to 0.15MPa, and the convection heating stage is entered.

The preheating is divided into two stages. The temperature of the first stage is 850 °C, and the temperature is kept for 45 minutes. Then nitrogen is introduced, and the vacuum degree is reduced to 150-200 Pa and the temperature is continued. hot.

(3) Austenitizing

The austenitizing temperature is 1220～1225℃, the temperature is kept for 50min, and the vacuum degree is kept in the partial pressure state.

(4) Quenching

Gas quenching was performed with nitrogen at a pressure of 0.8 MPa. When the workpiece is cooled to below 500°C, it is automatically switched to 0.2MPa air pressure for slow cooling, and it is cooled to below 65°C for release.

The vacuum heat treatment process of the straight shank rolling twist drill is shown in the figure:

(5) Tempering

Using steam furnace tempering, tempering temperature 550 ~ 560 ℃, each 3h, a total of 3 times.

Decarburization during vacuum heat treatment:

Oxidation and decarburization sometimes occur in vacuum heat treatment, which in turn causes a reduction in the life of the finished product. This kind of phenomenon generally does not occur in a vacuum atmosphere. The main reason for this problem is that the straight shank twist drill is oxidized during rolling at 1100-1150 °C, and there is a blue oxide film on the surface of the drill bit, which is heated during vacuum heat treatment. Decomposition produces an oxidizing atmosphere, resulting in oxidative decarburization or carbon depletion during quench heating.

There are two ways to deal with this problem:

(1) The heating and decomposition of Fe3Oa oxide film in the vacuum furnace is related to the degree of vacuum, that is, the pressure, and also to the heating temperature. The higher the vacuum degree, the easier it is to decompose when the temperature is higher than 570°C, and the higher the temperature, the more complete the decomposition. Therefore, a preheating of 600-650 °C can be added, and a furnace pressure of ≤10 Pa can be maintained to promote the decomposition of the oxide film and remove residual oxygen as much as possible.

(2) The rolling drill is pickled before quenching to eliminate the surface oxide film.

The products processed by the above procedures can show good processing performance after vacuum heat treatment, and have high life and no deformation, and are high-quality tool products.

Vacuum heat treatment equipment: VOGQ series vacuum heat treatment furnaces produced by SIMUWU are high-quality products for vacuum heat treatment of tooling and molds. Good temperature control accuracy and temperature control uniformity ensure the effective progress of the vacuum heat treatment process. SIMUWU specializes in 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.