1. Selection of drill bit body material
The harsh working environment places high demands on the hardness, strength, toughness, impact resistance, wear resistance, fatigue resistance, etc. of the DTH drill bit body. Considering the performance requirements of each part, it can be considered that the overall material selection of the drill bit body needs to meet the following requirements:
(1) High surface hardness to enhance wear resistance, and good core toughness to enhance impact resistance.
To avoid brittle fracture and fatigue fracture, the material needs to have good fracture toughness, fatigue strength and fracture strength;
(2) High hardenability.
To meet the different performance requirements of each part of the body, it is beneficial to improve the surface yield strength, improve the fixing force of the tooth hole to the alloy tooth, and also improve the yield strength and toughness of the core;
(3) It can resist certain corrosion when encountering gas and liquid in the underground rock formation;
(4) At a certain high temperature, the material hardness is stable and not easy to soften;
(5) Good process performance, such as good forging performance, small residual stress after heat treatment, etc.;
(6) High metallurgical quality, good structural uniformity, less impurities, especially low phosphorus content, and uniform carbide.
Therefore, in order to meet the above requirements, with good processing performance and low material cost, 42CrM0 is more suitable.
2. DTH Drill Bit impact end face
The failure form of the DTH Bit impact end face is mainly impact fatigue. The following methods can be considered to improve its service life:
(1) Reduce surface roughness and improve contact accuracy to reduce local stress concentration and stress peak during impact;
(2) Reasonably match the hardness relationship between the piston and the impact end face. Priority should be given to improving the service life of the piston to change the current situation where the piston needs to be replaced when the drill bit is replaced;
3. Spline
(1) Processing is strictly carried out in accordance with design requirements to ensure the proper clearance and uniformity of the spline;
(2) Surface carburizing and quenching are carried out to increase the surface hardness and achieve resistance to plastic deformation and impact fatigue;
(3) Reduce surface roughness and improve contact accuracy;
(4) Increasing the cross-sectional area of the spline can increase the strength of the down the hole drill bit.
4. Selection of drill bit alloy column
In order to improve the service life of the drill bit, the cemented carbide grade should be selected according to the following three requirements:
(1) According to the drilling technical parameters. For high-frequency and low-impact drilling, alloys with high molybdenum content are often used, while for low-frequency and high-impact drilling, toughness is required, and alloys with high bonding molybdenum content are often used;
(2) According to the lithology of different rock formations. For general rock formations, cemented carbide with low cobalt content is selected to enhance wear resistance and impact effect. For hard rock formations, cemented carbide with high cobalt content is selected to improve impact toughness and avoid brittle fracture;
(3) Selection according to the working characteristics of alloy columns at different positions. Due to the different forces on the edge alloy column and the middle alloy column, the edge alloy column has a lower cobalt content to enhance wear resistance; the middle edge alloy column has a higher cobalt content to improve impact resistance.
The combination of alloy columns of different grades can reduce the relative consumption of cemented carbide while meeting the use requirements, thereby reducing costs. Compared with the performance of ordinary cemented carbide, ultrafine-grained cemented carbide and nanocomposites not only have high hardness and good wear resistance, but also have higher strength and toughness.
