Description of annealing heat treatment of large diameter involute broaches

Large-diameter involute broaches are essential metal broaching tools for manufacturing large-scale process equipment and machinery such as automobiles, tractors, airplanes, and tanks. They are expensive and difficult to manufacture. Technician Gao introduced the annealing heat treatment process.

1) Basic parameters of broach. Among the involute broaches of a certain tool factory, there is a set of three-branch specifications that are quite representative.

① Specifications. 4238mm x 1 550mm; 238mm x 1 500mm; 238mm x 850mm, one each.

②Material. The cutting edge of the broach is made of M2 steel; the handle is made of CrWMn.

③ Shape and hardness requirements. The front handle of the broach is 45 -52HRC; the blade of the broach is 63.5 -66HRC; the rear handle of the broach is 45 -52HRC. The shape is shown in Figure 2-50.

④ Radial runout ≤0.35mm.

⑤ Features of the involute drawing blade edge. The structural characteristics of this broach are: the broach blade is hollow barrel-shaped, and the front and rear handles and the blade are plug-in connected. This design structure of the broach greatly reduces the manufacturing difficulty and cost of the broach itself while achieving the usability performance.

2) Determination of broach heat treatment process. Problems arising during heat treatment cannot be ignored. Since the diameter of the broach blade is too large, if it is bent out of tolerance, it will easily break during straightening. Due to the original structure of large-diameter materials and poor carbide distribution, cracking and uneven hardness can easily occur.

After careful research and testing, in view of the problems that are prone to occur with this broach, a heat treatment process curve for the broach blade as shown in Figure 2-51 was specially formulated.

①In order to reduce heat treatment deformation, multiple preheatings are used during the heating process of the salt bath.

②In order to ensure the hardness and reduce the heating time, the medium and upper limit quenching heating temperature is adopted.

③In the quenching cooling process, isothermal cooling is not used, but multiple classifications are used, which is beneficial to thermal alignment.

④The method of applying pressure and heat to straighten during the first tempering and cooling process has achieved good results.

3) Key points for the implementation of heat treatment process.

① Alignment process. Because the diameter of the broach is too large, its temperature is still very high even after being isothermized for 240 - 26U starting temperature (about 200℃), but at 350℃, taking advantage of the good plasticity of retained austenite, pressurize and straighten until the broach cools to about 70°C and stop pressurizing and straightening.

② Tempering process. When the broach is cooled to about 40°C, put it into a furnace at 240-260°C and heat it for 30 minutes to slow the heating. Then raise the temperature to 560°C and keep it warm for 90 minutes for tempering. After taking it out of the oven, calibrate it under pressure and heat, repeat this 4 times. The straightening effect is better after the first tempering.

There are also some domestic units that use low and high temperatures combined with tempering, which has good results. That is, after the broach is quenched, it is cooled to about 50°C, and then immediately enters 350-380°C x 1.5h nitrate salt tempering. The first tempering is the same as the first time.

There are also processes such as the secondary bainite method that are also beneficial in reducing broach cracking.

③ Aging treatment. In order to avoid scrapping the broach due to grinding cracks caused by grinding stress, it is recommended to adopt the twice aging method. That is, the first aging is after conventional heat treatment, and the broach is placed in the furnace for aging at 240 -260℃ x 12h; the first aging is after rough grinding, and the aging process is the same.

4 Conclusion. The large-diameter involute broach processed according to the above process will not crack and has a hardness of 63.5 to 66HRC. Users reported that the comprehensive mechanical properties of the broach were good and no early failure occurred.