Threaded extension rods are generally used in conjunction with high-power rock drills, which have a high impact power and a much higher impact frequency than small rock drills. Threaded extension rods are used for rod drilling in medium and deep holes, and the depth of the holes is more than 10 meters or even tens of meters. Due to the high impact stress and frequency of threaded extension rods during rock drilling, and the long rod, the stress and working conditions are even worse.

The quality of drill rods depends on many factors such as steel type,  heat treatment process, structural design of drill rods, and transition between different sections. Among the many factors mentioned above, the heat treatment of drill rods is the main reason for the low service life or early failure of drill rods. Correctly selecting the heat treatment process of drill rods can effectively improve the service life of threaded extension rods.

Failure of Threaded Extension Rods and Requirements for Steel Grades

Most of the failures of threaded extension rods are in the threaded part and transition groove area, and the failure form is fatigue fracture. Fatigue fracture is a form of damage, 2/3 of the threads and transition groove fracture and 1/3 of the rod fracture. The external fatigue fractures of threaded extension rods accounted for the majority, while the internal fatigue fractures accounted for a small number. The failure mechanism of the threaded extension rod is mainly the fatigue source formed by fretting wear and impact wear, which develops into fatigue cracks, and the fatigue crack propagation leads to the final fracture of the drill rod. Therefore, the threaded extension rod should have the following properties:

① High wear resistance and high toughness

② High fatigue strength and sufficient rigidity and elasticity

③Low notch sensitivity and low fatigue crack growth rate

④It must have certain high-temperature hardness and high-temperature softening resistance

⑤ It must have high anti-corrosion performance and the ability to resist corrosion fatigue.

Selection of Steel Grades for Threaded Extension Rods

20~25CrNi3Mo steel and 24~27SiMnNi2CrMo steel are more suitable for threaded extension rods. 20~25CrNi3Mo steel is recognized at home and abroad as the most suitable steel for threaded extension rods. It has high strength and toughness, low-notch sensitivity, and a low fatigue growth rate. 24~27SiMnNi2CrMo steel can also obtain a drill rod service life slightly lower than 20~25CrNi3Mo steel after certain heat treatment and anti-corrosion treatment. While 35SiMnMoV, 32Si2Mn2MoV, and other Si-Mn-Mo steels have higher strength and certain toughness, they are far behind Cr-Ni-Mo steels.

Selection of Heat Treatment Process for Threaded Extension Rod

Comprehensive domestic and foreign decades of research and production practice of threaded extension rods, the best production process route of the threaded extension rod is low-carbon high-strength alloy hollow steel, after forming and processing for overall carburizing treatment, controlled cooling in a special cooling tank, both ends re-quenched by medium-frequency heating and low-temperature tempering, and then the inner and outer surfaces are cleaned, followed by effective anti-corrosion treatment.

1. Carburizing process

The overall carburizing treatment of threaded extension rods requires a well-deep gas carburizing furnace. Drip injection gas carburizing adopts many infusion liquid formulas, generally methanol + acetone; kerosene + air has the lowest cost, but the carbon content on the surface is high, and carbon black is prone to appear. It is ideal to use methanol + acetyl acetate as the drip solution formulation and it is easy to control the carbon potential of the carburizing process.

The control of carbon content on the surface of the carburized layer is very important for threaded extension rods. The control of carbon content on the surface of the carburized layer is often controlled by the carbon potential in the furnace. The level of carbon potential is related to the selected infusion solution formula and also to the carburizing temperature. The carburizing temperature is high, the carbon potential is high, and the carburizing speed is fast; the temperature is low, the carbon potential is low, and the carburizing speed is slightly lower.

The application of computer control technology in the carburizing heat treatment process has solved a series of carburizing heat treatment quality control problems. It can be controlled by a microcomputer to ensure the high quality of carburizing drill rods.

2. Cooling after carburizing

The main purpose of controlled cooling after carburizing is to ensure that the drill rod obtains a relatively uniform hardness and prevent excessive oxidation or decarburization of the drill rod surface during the cooling process. The special carburizing furnace for foreign threaded extension rods is equipped with a specially designed control cooling device, which can control and adjust different cooling speeds to ensure the cooling requirements of different diameters and meet the specified hardness requirements and organizational requirements.

The long drill rod will have a large bending after cooling by the overall carburizing, which adds a lot of difficulties to drill rod straightening, and after cooling to ensure good rigidity and elasticity of drill rod, the required hardness is around HRC40, and the bent HRC40 drill rod is difficult to straighten, which increases the workload of straightening. High volume production should be equipped with high efficiency straightening machine.

3. Heat treatment after carburizing

After the overall carburizing of the drill rod, to improve the wear resistance and fatigue strength of the threaded parts at both ends, quenching and low-temperature tempering treatment should be carried out. It is best to use intermediate frequency induction heating for quenching heating. The heating length is slightly longer than the transition groove (or handle groove). After quenching, the surface hardness should be ≥ HRC58; for tempering, it is recommended to use a low-temperature salt bath furnace. After quenching and tempering, the drill rod is subjected to final fine straightening, surface cleaning, and anti-corrosion treatment.