Heating, cooling and tempering of straight shank twist drill

1. Straight handle long twist drill

Technical requirements: Made of general high-speed steel such as M2, the overall twist drill has a hardness of ≥64HRC at 4/5 of the groove length from the drill tip, and a hardness of 30-45HRC at 25.4mm from the shank end to the blade; there are strict requirements on straightness and radial runout. Requirements, overheating is not allowed.

The difficulty in heat treatment of long twist drills with straight handles is bending and straightening. Deformation is inevitable. How to straighten it effectively after deformation is the key to the problem.

The straight handle long twist drill has a longer blade, such as the 6mm specification, the total length is 139mm, and the blade length is 91mm-10mm, the total length is 184mm, and the blade length is 121mm. The blade takes up about 2/3 of the total length. The heat treatment process is introduced as follows.

1) Preheat. Choose a suitable double-layer plum blossom clamp (also known as a spiral clamp), with a preheating temperature of 850 -860°C, leaving only 20 -30mm on the handle end, and the rest is immersed in the salt bath, and the operation must be smooth.

2) Heating. Straight shank long twist drills are generally made of M2 steel, the heating temperature is 1215-1225℃, and the heating length is the entire blade length. Lift the handle while heating.

3) Cool down. Grading isotherm, that is, after grading at 580-62℃, immediately transfer to 240-28℃ nitrate salt isotherm 1-1. Sh. The metallographic grade is controlled at 10-10.5. After isothermal cooling to room temperature, clean and straighten.

4) Tempering. Clamp the straightened drill bit on the tempering fixture and temper it 550 - 560% x lh x 4 times. After tempering, cool it to room temperature and check the bends one by one. If there is not much bending, you can calibrate it by "borrowing the handle" or counterattacking. Blade technique and straightening. If it cannot be straightened, it should be straightened and re-quenched after annealing. The re-quenching temperature should be appropriately lower.

2. Small twist drill with straight handle

Straight-shank small twist drills refer to small drill bits with a diameter of ≤3mm. They are generally made of M2 steel or 4341 steel, and are integrally quenched. The metallographic grade is 10.5-12. The hardness requirement is 62-65.SHRC, and the straightness is ≤0.06mm. Small twist drills with straight handles are generally light rods before quenching. There are two quenching methods: vacuum quenching and salt bath quenching.

The outer diameter of the sleeve is about 50mm, dozens of through holes are drilled around it, and the burrs are removed. The length of the sleeve is slightly longer than the drill bit by 4-5mrn. The amount of hanging equipment should be moderate. The preheating is generally divided into two stages. The first stage is baked in the furnace at 450-500℃ for 0.5-1h. The second stage is preheated at 860~88℃x8rnin and shakes up and down in the furnace; 1215- Heating at 1220℃ for 4 minutes, shaking up and down to ensure even heating, grading at 580 - 620℃ x 4min, immediately pour it onto the iron plate after grading, cool and clean, and temper at 550 -560℃ x lh x 3 times.