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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.doaj:78dd3df13179483c80d7708b94a172af fatcat:cw6efegynzh4dk3u2rosxskm4a