S-1050-HDD Grinding burn in drill string

Grinding burn occurs in hardenable (or carburised) steels as a result of high heat input, e.g. during machining or even during operation if the cooling system fails. A detailed explanation can be found at [link] If the austenitising temperature is exceeded, new hardened zones of un-tempered martensite form, which etch lightly or not at all. In the present case, involving drill pipe made of S-1050-HDD (a special steel for drill pipe similar to 1.7720 | 34CrMo4), the heat input is attributable to friction between the steel and the rock. Newly hardened zones with significantly increased hardness formed, from which cracks originated under the high operational load.

Hardness of the grinding burn zone: 645 HV0.5
Hardness of the quenched and tempered base structure: 373 HV0.5