Analaysis of failures encountered during rolling of rings of maraging steel

Abstract

Large-sized rings of maraging steel X 2 NiCoMo 18 8 5 (Werkstoff Nr. 1.6359) find application in aerospace. The rings are required to demonstrate high strength and fracture toughness in both radial and axial directions. The processing is accordingly carried out under controlled conditions to realize a microstructure capable of attaining the specified strength and toughness after heat treatment. The rings rolled during initial trials failed to attain the required fracture toughness. Precipitation of carbonitrides at prior austenitic grain (PAG) boundaries in the temperature range 820 -980 o C contributed to the failure. Efforts were made to do final rolling through this temperature range, to prevent a microstructure comprising of PAG boundaries decorated by carbonitrides from evolving. To this end relatively low preheating temperatures (1100 -1150 o C) were adopted. Failures were encountered in realizing the desired final size and shape of the rings. The mill tripped before the ring rolling could be completed. Failure was analyzed and it was concluded that the problem was due to the high flow stress levels encountered during ring rolling and the relatively low radial loads the mill was capable of delivering.Keywords