TWI Bulletin, March 1985
by Neville Gregory and S Leake
Neville Gregory, CEng, FIM, FWeldI is Head of the Advisory Section in the Production Systems Department at Abington, and S Leake, AWeldI, is Manager of Arnway Ltd.
The Welding Institute is often called upon to assist in formulating procedures for weld repair. Recently advice was sought on the repair of a large extrusion press, to be undertaken by Arnway Ltd, a Member company of the Institute. Details of the repair procedure provide an interesting account of some of the precautions necessary for successful weld repair.
Repair of engineering plant by welding is a vital industrial practice which produces economic benefits by prolonging the life of expensive components. Service failures caused by fatigue; stress corrosion cracking etc, can often be repaired by welding, which saves both on high capital cost and delay in delivery of new parts. Further incentive for repair is provided by keeping shut down periods as short as possible with minimum loss of production.
Welding repair encompasses a wide range of manual skill and high technology from relatively simple but highly skilled gas or MIG welding repair of car bodies to fully automatic repair of components in nuclear reactors. Between these extremes lies a vast range of engineering components requiring periodic repair including press frames, mill housings, extrusion presses, and steel making plant.
Because of the wide range and varying degrees of complexity of both ferrous and non-ferrous components requiring repair, companies tend to specialise in certain types of repair. Some have particular expertise in on-site repairs and others restrict their activities to components that can be transported to a workshop with facilities for machining and heat treatment. A limited number of companies offer both workshop and site repairs.
A particularly interesting repair, carried out by Arnway Ltd, was made on a 2750t capacity extrusion press at High Duty Alloy Extrusions Ltd, Workington. This repair is typical of a number of similar jobs carried out by Arnway, and photographs taken during the work show some of the difficulties that have to be overcome to accomplish a successful repair.
The cracked extrusion press
The press to be repaired is used for high speed extrusion of aluminium sections, and consists of a main cast steel cylinder 3000mm long x 1219mm diameter x 150mm wall containing oil under pressure which operates the extrusion ram. Welded to the outside of the cylinder are housings for three steel columns the primary purpose of which, in conjunction with the die platens, is to restrain the extrusion die under full ram power. This imposes high stresses on the attachment welds on the cylinder. A cross section of the main cylinder is shown in Fig.1, and Fig.2 shows part of the cylinder and the rear frame plate after removal of the columns and the side ram.
After the press had operated for about 10 years a serious loss of oil was noted, which indicated a through wall crack in the main cylinder. The press was stripped down for inspection and indeed there was a through wall crack in the cylinder which had initiated at the toe of a T butt weld ( Fig.3). The position of the through wall crack is shown in Fig.1 and the appearance of the crack inside the cylinder in Fig.4. This crack ran into the end of the T butt weld where it met the rear frame plate. The crack then ran into the rear frame for a length of 50mm. The lower fillet weld between the side ram cover plate and the cylinder was cracked for its whole length along the centreline of the weld.
The cracks described above were the most serious out of a total of seven detected and were typical of fatigue cracks. A repair procedure was agreed between Arnway and The Welding Institute. The 150mm thick plate to be welded had a carbon equivalent of 0.4%. According to BS 5135, a weld in this type and thickness of plate made with basic coated electrodes dried at high temperature would require the plate to be preheated to 50°C to avoid hydrogen induced heat affected zone (HAZ) cracking.
However, as noted in BS 5135, welds made under high restraint or in thick sections may require more stringent welding procedures. It is the standard practice of Arnway to preheat to 150°C because of the extremely high restraint in stiffened structures manufactured from thick plate.
Preheat is applied and maintained throughout the repair procedure by electric resistance heating mats, with close attention given to minimising effects of distortion.
Repair procedure
Metal removal
One of the difficulties in repairing extrusion presses is that no distortion of the hydraulic cylinder can be tolerated because on re-assembly a close fitting brass bush is refitted inside the cylinder.
All defects were removed by air arc gouging to form suitable groove shapes for deposition of weld metal with minimum defects.
To provide a general preheat for air arc gouging and welding as well as for post weld heat treatment, the inside of the cylinder was heated by electric resistance heating mats placed at intervals round the inside surface. These heaters were supported by an angle iron framework and lagged with insulating blankets. The length of cylinder heated included the whole area containing cracks.
Thermocouples were attached to the main cylinder and temperature was thermostatically controlled. The complete cylinder circumference adjacent to the cracks was preheated locally to 120-150°C. This precaution was taken to avoid any further cracking or possible extension of the cracks already present during air arc gouging. Hydrogen induced heat affected zone (HAZ) cracking could conceivably occur when air arc gouging because of the formation of crack susceptible HAZ microstructures and hydrogen arising from condensed moisture on the plate surface. The minimum amount of metal possible is removed for economic reasons, but it is necessary to provide a groove wide enough for access and manipulation of the electrode or welding gun. Grooves may have to be widened if welding is carried out in the overhead position, or if the surface of the groove is to be buttered with a layer of weld metal.
In the fillet weld that contained a longitudinal centreline fatigue crack, removal of the crack showed that gaps between the plates had caused a reduction in throat thickness. The groove was therefore made deep enough to enable a full throat thickness repair weld to be made.
To obtain sufficient access to some of the welds, both for crack removal and for welding, it was necessary to cut away part of the rear frame plate adjacent to the lower hole shown in Fig.2, so that the welder could crawl in ( Fig.5).
Repair of the end of the main through wall crack where it ran into the rear frame was effected using a V preparation formed from outside with a copper backing bar clamped into position ( Fig.6). It can be seen in Fig.6 that this groove extended across the submerged arc weld between the rear frame plate and the main cylinder. A view of the end of this groove before the crack had been completely removed is shown in Fig.7. The dark area at the right is porosity in the original submerged arc weld.
Following metal removal by air arc gouging, the temperature was allowed to fall to 50°C and the surfaces of all grooves were finished by grinding. Magnetic particle inspection was used to check that all cracks had been completely removed.
Welding procedure
The agreed welding procedure was as follows:
- Preheat to 150°C using electric resistance heating mats;
- Repair cavities by welding with basic coated electrodes baked at 300°C for 1hr and stored at 100°C minimum before use;
- After completion of welding, lower preheat to a temperature at which grinding of the welds can be carried out and grind the toes of all welds to give a smooth profile;
- Carry out MPI and ultrasonic examination to ensure soundness of repair welds;
- Heat to 580-620°C for at least six hours for stress relief. Heating and cooling rates should not exceed 60°C per hr;
- After the cylinder has cooled a final NDT examination should be carried out.
The repair was carried out as detailed above and it will be understood that welding for extended periods either inside the main cylinder or inside the housing ( Fig.5) with the parent metal preheated to 150°C is extremely uncomfortable for the welders.
However, in time experience is gained in welding in difficult situations and sound welds can be deposited, leading to successful repairs. Figure 8 shows the weld inside the cylinder filled to two thirds depth and Fig.9 shows a very awkward overhead weld being made with restricted access.
Final inspection indicated that the repair welds were free from unacceptable defects and complied with the acceptance standards of codes typified by BS 4870 Approval testing of welding procedures and BS 4871 Approval testing of welders working to approved welding procedures.
Ultrasonic and magnetic particle examination were carried out by Non-Destructive Testers Ltd following repair and prior to stress relief. It is expected that the extrusion press will have its life extended by many years.
Acknowledgement
Acknowledgement is made to High Duty Alloy Extrusions Ltd for their kind permission to publish details of this repair.
