Root Cause Analysis of Low Impact Toughness of Cast Steel Yokes Used in Railway Freight Cars

The yoke is a safety-critical component of the coupler assembly of railway freight cars and is responsible for absorption of impact loads during service. The present study investigates the root cause of low impact toughness of a particular batch of yoke castings produced in Texmaco Steel Foundry. Examination of microstructure and fracture surface of specimens extracted from the castings reveals the presence of a substantial number of macro- and microporosities. The fracture morphology confirms that the presence of such large number of porosities increases stress concentration and lowers the load-bearing area, resulting in brittle failure and low impact toughness. EDX analysis indicates that these porosities were caused by entrapment of gases during solidification. Subsequent examination indicates that the porosities were caused due to gases generated by combustion of resins/binders used in the sand system or improper venting during the pouring operation.