Differences in Performance of Different Threaded Extension Rod Materials

Table of Contents

The drill rod is the component that connects the drill bit and shank adapter in the rock drilling system. It transmits the driving force of the rock drill to the drill bit through the threaded connection to achieve the purpose of rock excavation. The threaded extension rod is the most varied, comprehensively specified, widely used, and economically beneficial product among top hammer rock drilling tools. Its frequency of use ranks among the best. With the continuous development of science and technology, the materials of threaded extension rods are becoming increasingly diversified. However, there are significant differences in performance between threaded extension rods made of different materials. This article will explore the performance differences of different threaded extension rod materials to help you choose the threaded extension rod that best suits your work needs.

The importance of threaded extension rod

threaded extension rod

The threaded extension rod is a hollow circular or hollow hexagonal rod with connecting threads at both ends or one end of the rod. It is the main component of the threaded connection supporting the drilling tool. It has the advantages of a wide application range, fast drilling speed, low impact energy transmission loss, high rock drilling efficiency, easy-to-realize mechanized drilling, and improved rock drilling conditions.

Requirements for steel types of threaded extension rods

The failure mechanism of threaded extension rods is mainly fatigue sources caused by fretting wear and impact wear, which develop into fatigue cracks. The expansion of fatigue cracks leads to the final fracture of the drill rod. Therefore, the threaded extension rod should have the following properties:

  • High wear resistance and high toughness;
  • High fatigue strength and sufficient rigidity and elasticity;
  • Low notch sensitivity and low fatigue crack growth rate;
  • It must have certain high-temperature hardness and resistance to high-temperature softening;
  • It must have high anti-corrosion performance and the ability to resist corrosion fatigue.

Threaded extension rod material

threaded extension rod

The primary material for producing threaded extension rods is hollow steel, which is available in carbon hollow steel and alloy hollow steel.

Carbon hollow steel

Carbon steel is an iron-carbon alloy with a carbon content ranging from 0.0218% to 2.11%. It typically also contains small amounts of silicon, manganese, sulfur, and phosphorus. Generally, higher carbon content in carbon steel increases its hardness and strength but decreases its plasticity. Its history dates back to the 14th century BC when people began smelting steel with higher carbon content using iron ore. Due to its remarkable strength and hardness, it has found widespread use in construction, manufacturing, and transportation.

Alloy hollow steel

Alloy steel is an iron-carbon alloy composed of ordinary carbon steel by adding an appropriate amount of one or more alloying elements. By adding different alloying elements and using appropriate processing techniques, this material can achieve high strength, toughness, wear resistance, corrosion resistance, low-temperature resistance, high-temperature resistance, and non-magnetic properties. Threaded extension rods made of alloy steel combine the strength of carbon steel with additional alloying elements to improve its hardness, wear resistance, and corrosion resistance. They are suitable for more complex geological conditions and long-term high-load working environments, making them ideal for challenging drilling operations.

Selection of steel grades for threaded extension rods

The material used in rock drill rods is alloy steel, but there are also many types of alloy steel, including 23CrNi3Mo, 55SiMnMo, 40SiMnCrNiMo, 35SiMnMoV, etc. The more suitable steel for making threaded extension rods are 20~25CrNi3Mo steel and 24~27SiMnNi2CrMo steel.

20~25CrNi3Mo steel is recognized at home and abroad as the steel of choice for threaded extension rods. It has high strength and toughness, low-notch sensitivity, and a low fatigue expansion rate. After being carburized and air-cooled, it has sufficient rigidity and elasticity. After carburizing, quenching, and low-temperature tempering, it has high wear resistance and high fatigue strength. After effective anti-corrosion treatment, it can achieve high corrosion fatigue resistance. The problem of the steel is that the carburizing treatment of high-temperature softening and high-temperature hardness is not good, and carburizing quenching increases the notch sensitivity of the steel.

If 24~27SiMnNi2CrMo steel undergoes the above heat treatment and anti-corrosion treatment, it can also obtain a drill rod service life that is slightly lower than that of 20~25CrNi3Mo steel. As for Si-Mn-Mo series steels such as 35SiMnMoV and 32Si2Mn2MoV, although they have higher strength and a certain toughness, they are far behind Cr-Ni-Mo series steels. The Si-Mn-Mo steel can increase wear resistance after carburizing and quenching, but the notch sensitivity increases, and the toughness decreases rapidly. The brittle fracture of the threaded extension rod significantly increases, leading to a considerable reduction in the overall service life of the drill rod.

High-strength alloy steel with a carbon content of 0.35~0.45% was used as a large-diameter threaded extension rod in foreign countries in the 1970s and 1980s. It uses a medium frequency (8000-10000Hz) surface quenching process after quenching and tempering. This approach aims to achieve a balance of toughness and surface wear resistance. However, compared to the low-carbon high-strength alloy steel with the carburizing and quenching process, there is still a significant gap in product service life. By the 1990s, this steel and process route with the foreign threaded extension rods have rarely seen.

How to choose a threaded extension rod?

In addition to material considerations, the selection of threaded extension rods must account for several other factors to ensure they meet the specific application requirements.

Identify application requirements

  • Understand the type of work the threaded extension rod will be used for, such as open pit mining, underground mining, or tunnel excavation.
  • Determine the required impact energy and rotational torque.

Dimensions and specifications

  • Select the appropriate diameter and length of the threaded extension rod according to the working conditions and equipment requirements.
  • Ensure that the selected drill rod matches the specifications of the rock drill and other connecting parts.

Material selection

  • Choose materials with high wear resistance, high toughness, and fatigue resistance, such as 23CrNi3Mo.
  • Consider the corrosive nature of the working environment and select materials with appropriate anti-corrosion capabilities.

Thread types and specifications

  • Choose a left-hand or right-hand thread type according to actual needs.
  • Select the appropriate thread specification according to the device interface.

Connection type and quality

  • Ensure a good match between the threaded extension rod and the coupling sleeve, prioritizing matching products provided by the same manufacturer.
  • Check the thread wear and avoid using a severely worn drill rod.

Bending and straightening

Inspect the bending condition of the drill rod. If significant bending is observed, consider whether to straighten or replace it.

Working parameter adjustment

According to the rock hardness, drill bit condition, and other factors, adjust the working parameters in time, such as impact pressure and propulsion force.

Brand and quality

  • Choose threaded extension rods from well-known brands. Usually, the quality of these brands is more reliable.
  • If necessary, you can seek professional advice or conduct sample testing.

Cost considerations

On the premise of meeting application requirements, consider cost factors and choose products with higher price ratios.

After-sales service

Understand the supplier’s after-sales service policies, such as return and exchange policies and technical support.


There are significant differences in performance between threaded extension rods made of different materials. By comprehensively considering factors such as the operating environment, workload, and frequency of use, you can select the threaded extension rod that best suits your needs, improving work efficiency and operating safety.

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