Process analysis of heat treatment of 120 steel twist drill by medium frequency induction heating machine

The twist drill is a tool that drills the round hole of the workpiece through its rotating cutting relative to the fixed axis. It is named because the chip flute is helical and shaped like a twist. In the process of cutting and drilling, the twist drill must bear a lot of friction. In order to improve its wear resistance, we usually use a medium frequency induction heating machine for heating and heat treatment. The effect is very good and meets its working requirements.

Cr4W2MoV (codename 120, hereinafter referred to as 120 steel) is a medium-chromium cold-working mold steel independently developed by my country. Since its promotion and use at the National Mold Conference in 1966, more than 40 years of market testing has proven that the eutectic carbides of this steel are fine and uniform. , has high hardenability and hardenability, good mechanical properties, and also has high wear resistance and dimensional stability. It is used to replace Cr12 steel in manufacturing silicon steel sheet blanking dies for motors and electrical appliances, which can increase the service life by 1 -3 times, and can also be used to manufacture spring steel plate dies with a thickness of 1.5-6.0mm, as well as cold heading dies and cold extrusion dies. Among the few cold work die steel families, 120 steel occupies an important position, and it is also an ideal material for manufacturing ordinary machine twist drills. 120 steel has been on the market for many years and has a good reputation. The following is an introduction to its frequency heat treatment process.

1) The main chemical composition of 120 steel. Since its inception, 120 steel has entered the GB 1299-1977, GB 1299-1985, and GB/T 1299-2000 "Alloy Tool Steel" standards. Although the standard number has changed three times, its main components have never changed. See the table below for details.  

The 120 steel used to make drill bits has a slight change in composition, that is, w(C) is reduced from 1.12%-1.25% to about 1%; w(Si) is increased from 0.40%-0.70% to 1.0%-1.30 %; The amount of W and Cr has been fine-tuned, but it still conforms to the steel numbering rule. Strictly speaking, when the W(Si) in the steel exceeds 0.40%, it should be marked out, and it should be Cr4W2MoVSi exactly.

2) Annealing process. In order to make the rolled twist drill less cracked or not cracked, the hardness should be checked after the steel wire is broken. If it exceeds 255HBW, a medium frequency heating furnace must be used for complete annealing heat treatment at 840-860℃x4h; if it is lower than 255HBW, only 650-680℃x4h stress relief is enough.

3)Quenching and tempering process

1. Preheat. Use medium frequency induction heating machine for preheating heat treatment, the preheating temperature is 850-870°C, and the preheating time is twice the heating time. According to regulations, twist drills require handle hardness ≥240HV (22.5HRC), which means they can be hardened as a whole. Based on our many years of production experience and feedback from market information, it is more practical to control the hardness of the handle of the twist drill at 30-45HRC. If the hardness is too high, it is easy to make the drill sleeve rough, and it is difficult to roll characters, mark the trademark and specifications on the shank; if the hardness is too low, the shank is easy to be strained or even deformed during drilling, so it is necessary to control the hardness of the shank. The more effective measure is to master the preheating temperature and immersion depth. If the preheating temperature exceeds 870°C or is lower than 850°C, the hardness of the handle may be out of control.

2. Heating. The heating temperature is 1160-1185°C, and the heating coefficient is 5-10s/mm longer than that of high-speed steel, in order to fully dissolve the limited alloy elements.

Because the mass fraction of alloy elements contained in 120 steel is low, only M2 and 9341 steel alloy mass fraction is 47%. In order to fully exert the effect of each alloy element, the quenching heating temperature should be selected higher, and the grain size control is determined according to the specifications of the drill bit. d≤5mm is grade 8.5-9; d>5mm is grade 8-8.5. Due to the high heating temperature, there is about 30% retained austenite in the quenched structure, and the hardness after quenching is only 57-59HRC.

3. Cool. It is cooled in the same way as conventional high-speed steel twist drills, that is, neutral salt is used as the coolant, and the classification temperature is 580-620°C. Over-temperature classification for too long may cause the hardness to decrease.

4. Tempering. After the quenched workpiece is cooled to room temperature and cleaned, a medium frequency induction heating machine is used for tempering heat treatment at 540-550℃x1hx3 times. The secondary hardening peak of 120 steel is at 545-540°C, so the tempering temperature should not exceed 550°C.

After many manufacturers adopt the above process, the twist drills produced are of good quality and meet their working requirements. Here, the editor would like to remind everyone that when performing the operation process, we must be serious and careful to avoid unnecessary defects and affect the quality of the workpiece.