Fabrication of Sanbar® 23 hollow drill steel

Forging

Sanbar® 23 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.

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Heat the rod end locally to the correct forging temperature. Forge within the temperature range specified for Sanbar® 23.

Heat treatment

Surface induction hardening is recommended for Sanbar® 23. Hardening and tempering should be carried out according to the recommendations above. 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.).

Overall gas carburizing is an alternative method for providing Sanbar® 23 with good 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 0.4 to 0.6%.

Due to the high chromium content in Sanbar® 23, carbides can be precipitated in the case hardened layer at relatively low surface carbon contents and should be avoided. 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 the rods are carburized internally in order to obtain maximum service life. The absence of internal carburizing can be extremely detrimental, especially in corrosive environments.

Sanbar® 23 is very well suited to surface induction hardening. 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

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 protection

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.