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2026
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09
Production and Quality of Hollow Steel for Heavy-Duty Drill Rods
H22 has declined compared with previous years, while the output of heavy-duty drill rods has increased year on year. The current domestic production volume stands at approximately 250,000 pieces, and market demand is projected to exceed 300,000 pieces in the coming years
I. Preface
With advances in rock drilling and excavation technologies, rising labor costs, and growing emphasis on work safety, the rock drill tool market has undergone gradual changes in recent years. The consumption of tapered drill rods represented by H22 has declined compared with previous years, while the output of heavy-duty drill rods has increased year on year. The current domestic production volume stands at approximately 250,000 pieces, and market demand is projected to exceed 300,000 pieces in the coming years.
Heavy-duty drill rods must withstand high-frequency impact energy of 750–780 J per blow delivered by hydraulic rock drills at a rate of 3,600 blows per minute. The drill rod transmits working energy to the drill bit via stress waves propagating at roughly 5,500 m/s to break rock. During drilling operations, the drill rod also bears a rotary torque of 600–1,000 N·m. Its outer surface undergoes severe friction against rock, and the inner bore is subject to corrosion by flushing water-air mixture at 0.8–1.0 MPa. Consequently, heavy-duty drill rods operate under extremely harsh service conditions, imposing stringent requirements on their performance and quality. Among all factors, the quality of hollow steel is of paramount importance.
II. Application Status of Heavy-Duty Drill Rods
Heavy-duty drill rods currently in service on the market mainly include the T51, T45 and H35 series. Domestic-made drill rods capture over 70% of the Chinese market share and are gradually replacing imported products. Nevertheless, a quality gap still exists compared with renowned brands such as Sandvik, primarily in drilling footage life, operational stability and failure modes.
Failures of products from brands like Sandvik are dominated by thread wear, featuring long and stable drilling footage life. In contrast, domestic products mostly fail by fracture, with highly fluctuating drilling footage life. Fractures occur on the rod body, external threads and internal threads, most of which are fatigue fractures.
III. Manufacturing Processes for Hollow Steel
Two manufacturing methods are available for hollow steel used for heavy-duty drill rods, with the respective process flows listed below:
Drilling and Rolling Process for Hollow Steel:
Electric arc furnace (or converter) → LF refining furnace → VD (or RH) vacuum degassing → continuous casting of large bloom → blooming mill breakdown → mechanical drilling → mandrel insertion → semi-continuous rolling into hollow steel
Hot Piercing & Hot Rolling Process for Hollow Steel:
Electric arc furnace (or converter) → LF refining furnace → VD (or RH) vacuum degassing → continuous casting of large bloom → rolling into round bar → hot piercing and first reducing → second reducing → hot rolling into hollow steel
From the perspective of hollow steel forming and metal flow lines, the drilling plus mandrel rolling method is more rational. In terms of yield rate, production cost and flexibility of production scheduling, the hot piercing and hot rolling process presents distinct advantages.
IV. Quality Comparison of Hollow Steel
1. Geometry and Dimensions
Hollow steel produced by drilling and mandrel rolling features flattened core holes, which cause uneven stress distribution on drill rods and tend to trigger premature product failure. Sharp corners on the inner bore induce stress concentration and initiate microcracks, resulting in component failure.
Hollow steel made via hot piercing and hot rolling has spiral lines on both inner and outer surfaces accompanied by microcracks, which may also lead to premature failure.
V. Steel Quality
23CrNi3Mo is the primary steel grade adopted for heavy-duty drill rods. Its chemical composition and major technical indicators are broadly comparable with overseas grades (Sandvik). However, gaps remain in steel cleanliness and homogeneity, mainly manifested in:
Contents of sulfur (S) and aluminium (Al, including Al₁, Alt, Als)
Gaseous contents [N], [H] in steel
Morphology of inclusions
Insufficient stability of macrostructure
VI. Conclusion
Hollow steel manufactured by both processes can meet the basic production requirements for heavy-duty drill rods. With future technological breakthroughs by rock drill tool manufacturers in production processes, thread machining, heat treatment and other areas, thread quality will be substantially improved, which will raise the bar for hollow steel quality. It is expected that hollow steel producers will carry out R&D and process optimization covering steel metallurgical quality, dimensional and geometric precision, as well as inner and outer surface quality, so as to achieve notable improvements in hollow steel quality.
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