Fracture of drilling head pipe connection due to quenching of heat affected zone (HAZ)

B. Zorc, M. Bizjak, B. Kosec, A. Nagode
2015 Metalurgija  
The drilling head pipe connection is made of quenched and tempered steel 34CrNiMo6. The steel characteristic is high hardenability. Due to unsuitable and untested welding technology the heat affected zone (HAZ) cooling rate after welding surpassed the upper critical cooling rate. During horizontal drilling of the rock, the resulting brittle martensite could not bear the dynamic loads which led to a brittle fracture of the pipe connection.
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