Published on 21 January 2013
TWI was involved in the repair of a fatigue crack at the top inside corner of a cast steel rolling mill housing. The composition of the casting adjacent to the crack was 0.27C, 0.37Si, 1.03Mn giving a carbon equivalent of 0.47. The section thickness at the top corner varied from 50 to 100mm and according to BS 5135 a preheat of 125◦C would have been necessary to avoid hydrogen induced cracking of the HAZ when welding with basic coated ferritic electrodes. For various reasons the preheat had to be restricted to 50◦C and therefore it was decided to use austenitic stainless steel electrodes.
Because of limited experience of using these electrodes in the overhead position under conditions of high restraint, procedure testing was carried out on a welded T joint with a 75mm leg length fillet weld to represent the profile of the mill housing. A 400mm long groove was made in the T joint by air arc gouging to represent the area to be welded when the crack was removed from the mill housing. The weld metal was completely removed from the gouged area so that the tests were made on to parent plate of similar composition to that of the mill housing.
The procedure tests were carried out at TWI over a period of two weeks before it was convenient to interrupt production. During this period the crack depth was monitored periodically by ultrasonic inspection.
Welding procedure tests were carried out in turn with the following electrodes after preheating locally at 50◦C:
1. BS 2926 19.9.3 dried at 400◦C for 1 hour
2. BS 2926 29.9 dried at 400◦C for 1 hour
3. AWS A5.11 Class E Ni Cr Fe 3 dried at 250◦C for 1 hour
Electrode 1 gave HAZ cracking at the uppermost weld toe when the groove was half filled. The groove was gouged to its original dimensions and the test repeated with electrode 2.
Cracking occurred again and subsequent micro examination showed that the crack had initiated in the fusion zone where the structure was martensitic. This can occur at the junction of a stainless steel weld and mild steel parent metal where a thin layer of alloy steel is formed. Here the high restraint and high shrinkage stress of stainless weld metal in addition to the low heat input of the overhead weld caused the cracking of the fusion zone. One solution to the cracking problem would have been to butter the surface of the groove with stainless steel weld metal of the 19.9.3 or 29.9 type and fill the buttered groove with lower strength 18.10 type weld metal but the access was too restricted for buttering.
The high nickel electrode 3 was tested and the softer weld metal with its lower coefficient of expansion and freedom from a crack susceptible fusion zone produced a sound joint without cracking.
The procedure was then applied successfully to the repair of the cracked mill housing.
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