Fabrication of Sanbar® 64 hollow drill steel
Sanbar® 64 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. Heat the rod end locally to the correct forging temperature. Forge within the temperature range specified for Sanbar® 64.
Overall gas carburizing gives Sanbar® 64 excellent fatigue strength and wear resistant properties. It is very important to control the atmosphere in the furnace by having a stable gas flow and an even temperature. The recommended surface carbon content is 0.5 to 0.7%. To increase the toughness, tempering needs to be carried out immediately after carburizing, according to the recommendations above.
The required case depth must be a function of the thread dimension but is in the area of 0.6 to 1.2 mm (0.020 to 0.057 in.). It is essential to ensure that rods are carburized internally in order to obtain maximum service life. The absence of internal carburizing can be extremely detrimental, especially in corrosive environments. In some applications, e.g. surface drilling in poor conditions, surface induction hardening, carried out according to the recommendations above, can be an alternative to overall carburizing.
The required case depth must be a function of the thread dimension but is in the area of 1.5 to 3 mm (0.06 to 0.12 in.).
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.