How Hollow Drill Steel Is Made for Rock Drilling Tools

Hollow drill steel is the starting material for many hydraulic drill rods used in mining, tunneling, quarrying, and construction drilling. Although it resembles a steel tube, it must meet far stricter requirements than ordinary tubing.

A hydraulic drill rod must transfer high-frequency impact energy and rotation from the rock drill to the bit. At the same time, its internal flushing hole carries air or water to the drilling face, helping remove rock cuttings. This combination of impact loading, rotation, abrasive wear, and internal fluid flow makes material quality essential.

If hollow drill steel has poor straightness, excessive inclusions, weak mechanical properties, or inconsistent dimensions, the final drill rod may suffer from fatigue failure, thread damage, poor energy transfer, or excessive drill-hole deviation. For this reason, hollow drill steel production is a controlled metallurgical and rolling process that begins long before the rod is machined.

1. Raw Material Control Starts with Scrap Selection

The quality of hollow drill steel begins with raw-material selection. Steelmakers carefully control the scrap used in the melting process. Scrap must be sorted by type and composition, and materials containing excessive heavy metals or harmful residual elements should be removed.

This is important because residual elements can affect toughness, fatigue performance, weldability, heat-treatment response, and long-term service life. Even small amounts of undesirable elements may create problems when a drill rod is exposed to repeated impact loading.

For premium hollow drill steel, steelmakers also control non-metallic inclusions and gas content during production. Inclusions are small particles trapped within the steel matrix. If they are too large or too numerous, they can become stress concentration points and initiate cracks during drilling.

The source article gives reference values used for Swedish hollow drill steel, including an oxide-inclusion rating below 0.5 and gas content below 0.056%. These values should be understood as technical references rather than universal specifications. Each producer must establish its own material standards according to the steel grade, intended application, customer requirements, and applicable quality system.

2. Steelmaking and Refining

After scrap selection, the material enters the steelmaking process. Electric arc furnace melting is commonly used for alloy steel production. The molten steel is then refined to improve chemistry control and reduce impurities.

Secondary refining may include ladle-furnace treatment and vacuum degassing. Ladle refining allows the producer to adjust alloy composition, temperature, and cleanliness. Vacuum degassing helps remove dissolved gases such as hydrogen and nitrogen, which can reduce steel quality if present at excessive levels.

Careful control during melting and refining supports a more uniform steel composition. This is essential because hollow drill steel must respond consistently during later stages such as forging, rolling, machining, carburizing, induction hardening, and shot peening.

After refining, the steel is continuously cast into square billets. Continuous casting must be controlled to reduce segregation, internal defects, and surface imperfections. The billet becomes the starting form for the hollow-steel rolling process.

3. Billet Preparation and Piercing

Before rolling, continuously cast square billets are cleaned to remove surface defects. A clean billet surface helps prevent defects from being rolled into the finished material.

The billet is then processed through a series of forming stages. The general sequence can include rough rolling, billet opening, and conversion into a round billet. The outside surface may be peeled or conditioned before the center hole is created.

A central hole is drilled into the round billet, and a high-strength alloy-steel mandrel is inserted. The mandrel plays a critical role because it defines the internal flushing hole during subsequent rolling.

The accuracy of this step is important. The hole must remain centered and consistent, while the steel surrounding it must maintain sufficient wall thickness and structural integrity. An off-center hole can create uneven wall thickness, imbalance during rotation, and reduced fatigue performance in the final drill rod.

4. Specialized Hollow Steel Rolling

After mandrel insertion, the billet is reheated and rolled in a dedicated hollow-steel rolling mill. This rolling process reduces the outside diameter, forms the required profile, and shapes the internal flushing hole around the mandrel.

Hollow drill steel may be produced in different external profiles according to the drill rod design. Common forms include round, hexagonal, and other specialized geometries. The final profile must meet dimensional tolerances that support later machining and connection design.

The steel is then cooled in a controlled manner, often using a step-type cooling bed. Cooling rate can influence microstructure, residual stress, straightness, and mechanical properties. Controlled cooling is therefore part of the overall quality system, not simply a waiting stage.

After cooling, hydraulic equipment pulls the mandrel from the steel. The product then moves to online inspection, cutting to length, and precision finishing.

5. Inspection and Dimensional Requirements

Finished hollow drill steel is inspected before it is supplied to a drill rod manufacturer. Important checks may include:

• Outside diameter and profile dimensions

• Internal flushing-hole diameter and concentricity

• Straightness

• Surface quality

• Wall thickness consistency

• Mechanical properties

• Decarburization on external and internal surfaces

• Inclusion level and material cleanliness

Decarburization deserves special attention. It occurs when carbon is lost from the steel surface during heating. Excessive decarburization can reduce surface hardness and weaken the material’s ability to resist wear after heat treatment.

For drill rod applications, both the outside surface and the internal flushing-hole wall should meet controlled decarburization requirements. The inner wall is important because it is exposed to high-velocity flushing media and abrasive drilling cuttings during service.

6. Corrosion Protection and Delivery

After finishing, hollow drill steel can be coated with a corrosion-prevention material. The flushing hole may also receive a protective plastic plug to prevent contamination and moisture entry during transport and storage.

These precautions help preserve material quality before the drill rod manufacturing stage. Corrosion, dirt, or moisture inside the flushing hole can create processing problems later and affect the final product’s reliability.

The hollow steel is then delivered to drill rod manufacturers, where it undergoes further straightening, thread production, friction welding or end forming where required, machining, heat treatment, shot peening, final inspection, and packaging.

Why Hollow Drill Steel Quality Matters in the Field

High-quality hollow drill steel supports the production of drill rods with better fatigue strength, wear resistance, and dimensional accuracy. This can help contractors drill straighter holes, maintain stable penetration rates, and reduce downtime caused by rod failures.

The final performance of a hydraulic drill rod depends on the entire manufacturing chain. Good machining and heat treatment cannot fully compensate for poor steel cleanliness, weak mechanical properties, or inconsistent geometry in the starting material.

For this reason, reliable rock drilling tool manufacturers treat hollow drill steel as a strategic material rather than a commodity. By controlling metallurgy, rolling, inspection, and finishing, they create a stronger foundation for dependable drilling performance.