With the rapid development of socialist modernization, the use of large boredrilling is becoming more and more extensive, such as the construction of special water conservancy and hydropower projects, pile foundation, machine and well engineering, national defense construction, mines and oil mining, and so on. The tooth insert bit realizes the low broken rock layer through pneumatic or hydraulic impactor, and adopts the way of positive circulation or reverse circulation slag discharge, which is widely recognized by the market because of its high efficiency and low cost.

The diameter of a large-diameter drill bit is generally more than 300 mm, more of which is

500,800,1000 mm, or even more than 1500-3000. The depth of the large-diameter drilling is different depending on the application, and there is a great difference in the rock formation, and most of the requirements are very strict. The large-diameter deep-hole deep-hole drill bit is very expensive, so it can prevent the early failure, and the service life of the drill bit is the primary quality control target for the production of the drill bit, and the material selection and heat treatment process are especially important. In this paper, the properties of steel for different heat treatment are compared, and the difference of the microstructure after different heat treatment is used, and the optimal material selection and heat treatment process are finally determined.

1. The material performance of deep hole large diameter DTH drill bit requires, that the poor working environment puts forward very high requirements for hardness, strength, toughness, impact resistance, wear resistance and fatigue resistance of drill head body. Considering the performance requirements of each part, it can be considered that the selection of drill head body material should meet the following requirements:

1.The surface hardness is required to enhance the wear resistance and the core toughness is better to enhance the impact resistance. In order to avoid brittle fracture and fatigue fracture, the material should have good fracture toughness, fatigue strength and fracture strength.

2.with high hardenability. In order to meet the different requirements of the performance of each part of the bit body, it is beneficial to improve the indicating yield strength, improve the fastening force of the tooth hole to the alloy tooth, and also improve the yield strength and toughness of the center part.

3.In the encounter of downhole Yanze gas-liquid can have a certain corrosion resistance.

4.The structure and hardness of the material are high at a certain high temperature, and the deformation and the like are not easy to be softened and the like.

Good process properties, such as good forging properties, heat treatment properties and so on.

5.The metallurgical quality is high, the microstructure uniformity is good, the purity is high. 

Ordinary drill bits of steel Composition and performance comparison:

At present, the main steel used for drill body in China is 42CrMo, 23CrNi3Mo, FF710, XGQ25, etc.

Table 1. Comparison of main components of common DTH drill bit：

Table 2. Comparison of mechanical properties of steel used in common DTH drill bit after heat treatment

As can be seen from Table 1 above, the most important alloy elements of steel for brazing tools include Cr,Ni,Mo,, in which chromium can increase the hardenability and secondary hardening of steel, improve the hardness and wear resistance of carbon steel without making the steel brittle, chromium can improve the strength and hardness of carbon steel in rolling state, and reduce the elongation and section shrinkage. While nickel improves the strength of steel, the damage to toughness, plasticity and other technological properties of steel is less than that of other alloy elements. Mo can improve hardenability and thermal strength in steel, prevent tempering brittleness and increase corrosion resistance in steel.