The Causes of Local Hardness Increasing in Powerplant Rotors and its Modification by Tempering (TECHNICAL NOTE)

Local hardness increasing on the surface of the power plant rotor may result in severe damages. One major solution for rapid repairing of the rotors is the adaptive tempering heat treatment. For this purpose, after identification of the damaged rotor, initial investigations and tests (such as visual inspection, chemical analysis, nondestructive hardness measurements and replica or on-site metallographic tests) were performed on the rotor journal. The results showed that, the occurrence of local phase transformation (i.e. tempered bainite to austenite and finally martensite) is the main factor. Finally, based on the aforementioned results, tempering heat treatment method was selected as a modifying solution for the following purposes: 1. Decreasing hardness in embrittled places (i.e. 400-690 HV) and, 2. invariable or admissible decrease of hardness in undamaged areas (250-300 HV). Experiments showed that by choosing the 680˚C/4hr as an optimum condition for tempering heat treatment, the two mentioned objectives can be met.