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Material Selection and Heat Treatment of Big Hole DTH Hammer
Sep 03, 2018

Material Selection and Heat Treatment of Big Hole DTH Hammer


The use of large-diameter drilling is becoming more and more extensive, such as the use of the group to discuss the construction of special water conservancy and hydropower projects, construction pile foundations, machine well engineering, national defense construction, mining and oil extraction. The insert bit achieves low-break rock formation by pneumatic or hydraulic impactor, adopts positive or reverse circulation slagging method, and is widely recognized by the market for its high efficiency and low cost.

Large diameter drills are generally larger than 300mm in diameter, more often 500, 800, 1000mm, and some even reach 1500~3000. The depth of the large-diameter borehole varies depending on the application, and the rock formations encountered are also very different. In most cases, the requirements are very strict. The large-diameter deep-hole DTH drill bit itself is very expensive, so preventing early failure and ensuring the service life of the drill bit is the primary quality control target for the production of such drill bit, and its material selection and heat treatment process are particularly important. In this paper, the performance characteristics of different steels for brazing tools and the differences in microstructure after heat treatment are studied. The optimal material selection and heat treatment process are finally determined.



1. Material performance requirements for deep hole large diameter DTH drill bits

The harsh working environment puts high demands on the hardness, strength, toughness, impact resistance, wear resistance and fatigue resistance of the bit body. Considering the performance requirements of each part, it can be considered that the material selection of the bit body should meet the following requirements. Several aspects require:

1 The surface hardness is required to be high 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 needs to have good fracture toughness, fatigue strength and fracture strength.

2 has high hardenability. To meet the different performance requirements of the various parts of the bit body, it is beneficial to improve the yield strength, improve the fastening force of the perforations to the alloy teeth, and improve the yield strength and toughness of the core.

3 can have certain corrosion resistance when encountering downhole Yanke gas.

4 Under certain high temperature, the material structure and hardness are high, and it is not easy to soften and deform.

5 good process performance, such as good forging performance, heat treatment performance.

6 high metallurgical quality, good organization uniformity, high purity.



2. Comparison of composition and properties of commonly used steel for brazing tools

At present, the commonly used bit body steels in China are: 42CrMo, 23CrNi3Mo, FF710, XGQ25, etc.

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It can be seen from the above Table 1 that the most important alloying elements of the steel for the brazing tool include Cr, Ni and Mo, wherein the chromium can increase the hardenability of the steel and have the effect of secondary hardening, which can improve the hardness and wear resistance of the carbon steel. Without making the steel brittle, chromium can increase the strength and hardness of the carbon steel in the rolled state, reduce the elongation and reduction of the area; the strength of the steel increases the toughness, plasticity and other properties of the steel. The damage is less affected by other alloying elements; molybdenum can improve hardenability and heat strength in steel, prevent temper brittleness, and increase corrosion resistance in the quality;

Silicon can increase the elastic limit, yield strength and yield ratio of steel, as well as fatigue strength and fatigue ratio, improve the oxidation resistance of steel at high temperature, and greatly improve the tempering stability of steel; manganese can improve steel Wear resistance and hardenability, which combined with silicon can enhance the elastic limit of steel.

It can be seen from Table 2 that XGQ25 with Si, Mn, Cr, Ni, Mo as the main alloying elements has higher yield and tensile strength, and has high hardenability, high temperature resistance and stability. Sex, while its toughness and plasticity is relatively poor. Therefore, XGQ25 steel with high strength, high elasticity and high stability is a steel type that meets the needs of large-diameter deep-hole drills, and has high strength, high wear resistance, high stability and high temperature resistance. potential. Our company uses a large amount of this type of steel on large-diameter deep-hole DTH drill bits, which has been highly recognized by customers and the market.


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Figure 1. Large diameter deep hole DTH drill bit produced by our company


 


3. Heat treatment of XGQ25 large diameter deep hole DTH drill bit

A heat treatment process route for a large-diameter deep hole down-the-hole drill bit: after being heated at 900 ° C for a sufficient time, the furnace is cooled and normalized, and after being cooled, it is mechanically processed after tempering at 680 ° C. Then, it was strongly infiltrated in a 1.2 C% carburizing atmosphere at 920 ° C, then diffused in a carbon atmosphere of 0.8 C%, and then cooled to 840 ° C for a period of time and then quenched by oil. After high temperature tempering, secondary quenching was carried out at 820 °C. The following figure shows the heat treatment process roadmap:

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After the heat treatment process, the wire cutting technique is used for sampling, and the pattern is coarsely ground by an automatic grinder, and then polished by a No. 8 metallographic paper, and placed on a flannel for polishing. The corrosion was carried out using a 4% nitric acid solution and observed under a 100X and 400X electron optical microscope.

The surface structure is hidden needle martensite + a small amount of retained austenite, the transition layer is martensite and bainite mixed structure, the matrix is bainite structure, and the carburized layer has a depth of about 1.5 mm. The structure has the characteristics of high hardness and wear resistance, high strength and high toughness of the core, and the fine structure of the structure due to the secondary quenching, so that the bit body can obtain high strength, high fatigue and high wear resistance.

Hardness test results after heat treatment of the product:

The surface hardness reaches 59~60HRC, the core hardness is 42~43HRC, and the effective hardened layer depth reaches 1.5mm.



Figure 1 Metallographic microstructure after heat treatment


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4 Conclusion

The deep hole large-diameter down-the-hole drill bit is most suitable for XGQ25. After carburizing and secondary quenching, a fine and uniform mixture of bainite and martensite can be obtained, so that the performance potential of the material can be fully obtained. It can meet the requirements of the use of such drills, and achieve high strength, high wear resistance, high fatigue and high stability.


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