Fabrication of Sanbar® 20 hollow drill steel
Sanbar® 20 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 avoided.
Heat the rod end locally to the correct forging temperature. Forge within the temperature range specified for Sanbar® 20, 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. Local hardening always gives rise to a soft zone at the transition between the heated and the unheated part of the rod. This results in a lower hardness in this zone, which becomes the weakest part of the rod. Hardness in the transition zone should, therefore, be maintained at as high a level as possible. The use of induction heating to produce a high transition zone hardness level is recommended.
The heated zone should overlap the previously heated zone by about 25 mm (0.984 in.). It is also important to ensure that this zone does not coincide with the part of the rod where there are maximum stresses. Experience has shown that the soft zone should be as close as possible to the collar.
Due to a brittle microstructure after hardening, it is vital to temper the component as soon as possible according to the recommendations above. Sanbar® 20 is not suitable for case hardening.
The resulting transition zone due to local heating should be treated as above. It is very important to avoid interaction between transition 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.