Son Won Chol, an institute head at the Faculty of Materials Science and Technology, who successfully developed a new method of hardfacing a worn drill bit by using a build-up electrode with coatings such as nitrided ferrochromium and WC, has been working hard to make further achievements.
In the past, build-up electrodes with Fe-Cr-V-Mo-Ti, Fe-Cr-Mn-V-Ti and Fe-Cr-W-V were used to repair a drill bit for drilling rocks of low strength (f<5).
In general, a hard alloy with high bending strength is used to drill hard rock thanks to its high stability on impact load and that with low bending strength is used for soft rock.
What he used to develop build-up material of the drill bit with high bending strength and toughness was Fe-Cr-Mn-V-Ti-WC system alloy.
He added WC and nitrided high carbon Fe-Cr in coating flux to form a large amount of carbonitride in weld metal for improving the bending strength and toughness.
Based on the effect of the content of ferro-alloys on the hardness and bending strength of weld metal, it was found that the reasonable contents of ferro-alloys to ensure HRC 62 and bending strength of 480MPa were nitride ferrochromium of 37%, ferromanganese of 8%, ferrotitanium of 8%, ferrovanadium of 8% and tungsten carbide of 12%.
Then, considering the effect of weld layers on the microstructure and properties of weld metal and the geometric dimension of the drill bit, he selected two weld layers and heat input 14.20kJ/cm. Finally, he could ensure hardness of weld metal HRC64.
This method can be used to deposit a worn drill bit at coal mines and mines where the strength of rock is medium (f=5~8).
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