Published on 22 January 2013
Welding low alloy steels needs careful control of pre-weld and post-weld heat treatment, as shown by the procedure recommended when TWI was consulted on repair of equipment for rock crushing.
The equipment, containing a cast alloy steel gear wheel having a diameter of 1m and spokes 75mm square section, failed by fatigue from defects in one of the spokes. Examination showed that the defects were cracks adjacent to a repair weld which may have been carried out in the foundry or by a previous owner. The gear wheel was subjected to heavy loads in service and it was considered that any welded repair would require post weld heat treatment to relieve residual stresses and to temper the heat affected zone.
The composition of the gear was 0.42C, 0.30Si, 1.60Mn, 0.54Mo which has a high susceptibility to hydrogen induced heat affected zone cracking and requires stringent precautions when welding. The welding procedure below, recommended and supervised by a TWI welding engineer, resulted in a successful repair and avoided the lengthy shutdown which would have been necessary to obtain a new gear wheel.
Preheat the cracked region to 100◦C
- Remove the crack to half section thickness by air arc gouging and grinding.
- Check surface of groove for freedom from defects by magnetic particle inspection (MPI).
- Preheat gear wheel to 225◦C in furnace.
- Remove gear wheel from furnace and maintain preheat in region of groove by oxyfuel gas flame.
- Weld with AWS E9018-DI (1.5Mn 0,4Mo) basic covered electrodes dried at 400◦C for one hour.
- Turn gear wheel over and remove the remainder of the crack.
- Inspect groove and replace gear wheel in furnace to reheat to 225◦C.
- Remove from furnace and complete welding.
- Replace gear wheel in furnace and cool slowly to 130◦C to allow complete transformation of HAZ to martensite.
- Raise temperature to 550◦C and soak for 4 hours.
- Cool gear wheel slowly in furnace to 300◦C and then cool in air.
- Grind weld flush with surface of spoke.
- Forty-eight hours after welding, during which time and hydrogen cracking would occur, inspect weld by MPI and ultrasonic testing. The acceptance standard required is that specified by BS 4870.
During the repair, run on and run off tabs were used so that most weld stops and starts were outside the repair area. Sometimes it is not possible to control the heat treatment as precisely as described here and it is necessary to follow alternative procedures including use of austenitic stainless steel electrodes.
For information about TWI's capabilities please contact us.