Fabrication of Sanbar® 61 hollow drill steel
Sanbar® 61 requires rapid heating to the forging temperature and, above all, the soaking time at full temperature should be as short as possible. This will minimize grain growth and decarburization, both of which drastically impair the fatigue strength.
The design of the shank and the forging of the collar are very important to the properties and performance of the drill rod. Abrupt changes in cross-section and forging defects cause stress concentrations that can severely diminish the performance of the rod and give rise to fracture. It is therefore vital that the radius between the collar and the rod is generous and defects such as laps, folds and cracks are to be avoided.
Heat the rod end locally to the correct forging temperature. Forge within the temperature range specified for Sanbar® 61, and terminate at the lower end of the temperature range. This will restrict grain growth. Forge the collar using a mandrel in the flushing hole, so that the flushing tube will not close up during forging.
Shank end hardening
The shank must be hardened after forging and machining to obtain the strength necessary for robust service. Sanbar® 61 has been developed primarily for surface induction hardening. A normalizing process is recommended in order to eliminate prior transition zones and create optimum mechanical properties.This process will decrease the grain size and result in a microstructure with high strength and good toughness. Normalize within the temperature range specified for Sanbar® 61. Recommended hardness range: 38–44 HRC. The heated zone should overlap the previously heated zone by about 25 mm.
After the above treatment, surface induction harden the shank by heating and quenching according to the recommendations above. Temper for 0.5 hour in the range of 150–250°C (300-480°F) to obtain a hardness of 55–60 HRC.
The minimum hardness value in the transition zone resulting from the brazing operation should be as high as possible. It is very important to avoid interaction between transitions zones and geometrical notches.
Shot peening of an adequate intensity and coverage is recommended. It improves fatigue strength due to:
- Introduced compressive stresses
- Increased hardness
- Smoother surface defects
Corrosion of a material subjected to fluctuating loads accelerates the fatigue process. In underground applications particularly, products should be protected to avoid premature fatigue breakages starting from the hole surface.