Electrogas welding of C-Mn steel

TWI Bulletin, February 1986

by Bob Graham

Bob Graham, BSc, MSc, is a Research Engineer in the Automation Section of the Production Systems Department.

Process developments in electrogas welding now permit use of lower energy inputs, with consequent improvement in weldment toughness. These developments, which are outlined in this article, enable the process to be applied for vertical welding in a larger number of areas, particularly in shipbuilding.

Electrogas welding (EGW) has a large number of potential applications for welding vertical seams of storage tanks, for ship hull construction and for similar vertical welding applications in the field. It is widely used for these applications in Europe ^[1,2] and Japan ^[3,4] and in the USA for storage tank fabrication. In the UK it is not used in shipbuilding and only infrequently for oil storage tank fabrication.

The process is not widely used in the UK largely because the high energy (typically 15-25 kJ/mm) used means that low toughness weldments result. Developments, particularly in Japan, have occurred to improve the toughness of the weldments and so increase utilisation of the process. These developments have focused on optimising process parameters to reduce energy input, and on development of new steels more resistant to high energy inputs. ^[5]

EGW is commonly used for vertical up butt welding of plates typically 25-30mm thickness. An arc is established between a continuous wire electrode and a weldpool retained by water cooled copper shoes and protected by shielding gas (Fig.1).

Fig.1. Portable EGW equipment:
1 - control box;
2 - track;
3 - torch;
4 - water cooled copper slides;
5 - motor for vertical travel;
6 - completed welds

The process may be carried out using solid or flux-cored wires (either self-shielded or gas-shielded). Traditional equipment is often large and cumbersome incorporating a rising platform for the operator, but smaller equipment has now been developed. In all systems the weld head rises on a chain or track attached to the plate, travel speed being set in response to metal deposition rate.

A power source capacity of 750A, 50V at 100% duty cycle is required. Either a DC constant voltage (flat characteristic) or constant current (drooping characteristic) power source may be used. Welding is carried out with the electrode at positive potential and the method of welding control is dependent on the power source type, with the travel speed controlled to maintain constant arc length and uniform voltage. If a drooping characteristic power source is used, travel speed is adjusted around a nominal voltage setting, so that for example, if voltage falls below the nominal value, travel speed is increased. If a flat characteristic power source is used, travel speed is adjusted around a preset current value. Typical electrogas welding conditions are given in the Table.

Process developments

Single pass welding

Process developments for single pass EGW have been aimed at reducing energy input by using a decreased preparation size compared with the square edge preparation and 13mm gap traditionally used with 25mm plate, and by improving process efficiency. Early developments used a narrow gap preparation (7mm gap) and low current (280A), so that for welding a 25mm thick plate, energy input was reduced to 5 kJ/mm, see Table. ^[6] In practice, however, higher deposition rates are required, so higher travel speeds and currents are employed.

Welding conditions for EGW process developments

	Plate thickness mm	Preparation, dimensions mm	Wire diameter, mm	Welding current, A	Arc voltage, V	Stickout, mm	Weld speed, m/hr	Oscillation, mm/min	Energy input, kJ/mm	Observations
Traditional	25		2.4	450	37	30	3.6	-	16.6
Narrow gap	25		2.3	280	30	50	6	-	5	Low current
40°V preparation	22		3	750	50	-	10	-	13.5
25°V preparation	25		1.6	500	50	50	10	-	9	50/mm stickout
20°V preparation	30		1.6	500	48	50	9.3	450	9.3	50mm/stickout
Oscillated	30		1.2	300	34	50	3.3	450	11.1
Multipass 2 passes	38		-	470	33		6	-	9.3
4 passes	38		-	420	34		11	-	4.7
3 passes	45		-	460 550 530	32 34 33		7.9 7.6 7.7	-	6.7 8.8 8.2
Metal powder additions 2 passes	38		2.4	650		40	20	-	5.0	+50% powder

Conventionally, currents of up to 750A are used and travel speeds of 10 m/hr may be achieved. A 40°V bevel preparation is used to reduce the volume of metal required in the square edge preparation (13mm gap), whilst allowing for easier fit-up than narrow gap preparations and the rationalisation of preparations for site erection. Energy inputs of 13-15 kJ/mm are achieved by use of this technique.

For more substantial improvements in toughness, energy input must be further reduced. This has been achieved by use of a long electrical stickout high current density technique, where a 1.6mm diameter flux cored wire is fed at 500A and 50mm stickout, see Table. ^[3] This technique has been used with a 25°V bevel preparation to achieve a travel speed of 10m/hr and a reduction in energy input to 9 kJ/mm for 25mm thick plate.

The large stickout employed in this technique causes the wire to be resistively heated and this results in an increased deposition rate. A special flux cored wire has also been developed, which has a larger amount of flux and iron powder than conventional cored electrogas wires, so that the resistivity of the wire is approximately five times that of solid wires.

Multipass welding

The high current density technique has been applied to weld up to 30mm thick plate at an energy input of 9.3 kJ/mm by oscillation of the electrode. However, at greater plate thicknesses, multipass electrogas or oscillated MIG welding techniques have to be adopted if energy input is to be maintained at or below these levels.

Multipass electrogas welding is often employed for welding the lower courses of storage tanks, where double bevelled preparations are generally used. ^[7] The welding conditions employed are similar to those used in the single pass welds ( Table). However, a shaped copper shoe is required to block off the reverse side of the preparation in a two pass weld ( Fig.2). Although only two passes are generally used, three or four pass techniques have also been tried.

Fig.2. A two pass EGW weld, showing use of a shaped copper shoe

A very high deposition rate can be obtained using EGW with metal powder additions. ^[8] Equipment has been developed for this technique, which may be applied for single or multipass welding. A water cooled nozzle fabricated from U-shaped laminations is used ( Fig.3), where the metal powder is blown down independent copper tubes by argon gas on to the wire and into the weldpool. The laminations screen the powder from magnetic effects and ensure uniform feeding.

The process may be carried out using solid or flux-cored wires (either self-shielded or gas-shielded). Traditional equipment is often large and cumbersome incorporating a rising platform for the operator, but smaller equipment has now been developed. In all systems the weld head rises on a chain or track attached to the plate, travel speed being set in response to metal deposition rate.

A power source capacity of 750A, 50V at 100% duty cycle is required. Either a DC constant voltage (flat characteristic) or constant current (drooping characteristic) power source may be used. Welding is carried out with the electrode at positive potential and the method of welding control is dependent on the power source type, with the travel speed controlled to maintain constant arc length and uniform voltage. If a drooping characteristic power source is used, travel speed is adjusted around a nominal voltage setting, so that for example, if voltage falls below the nominal value, travel speed is increased. If a flat characteristic power source is used, travel speed is adjusted around a preset current value. Typical electrogas welding conditions are given in the Table.

Process developments

Single pass welding

Process developments for single pass EGW have been aimed at reducing energy input by using a decreased preparation size compared with the square edge preparation and 13mm gap traditionally used with 25mm plate, and by improving process efficiency. Early developments used a narrow gap preparation (7mm gap) and low current (280A), so that for welding a 25mm thick plate, energy input was reduced to 5 kJ/mm, see Table. ^[6] In practice, however, higher deposition rates are required, so higher travel speeds and currents are employed.

Fig.3. Nozzle developed for use with EGW with metal powder additions: ^[8]

1 - magnetic screening;

2 - water cooling;

3 - wire electrode;

4 - metal powder supply;

5 - arc;

6 - copper shoe (water cooled);

7 - shielding gas;

8 - molten pool;

9 - weld metal.

High welding speeds and low energy inputs are achieved with this technique; for example, a 38mm plate with a double V bevel preparation can be welded in two passes at 20 m/hr travel speed and 5 kJ/mm energy input per pass ( Table). The high speeds mean that the process may be difficult to control on site and this, coupled with the complexity of the equipment for site operation, means that no application has been found for the technique.

Mechanical properties

The low toughness of electrogas welds has meant that in shipbuilding applications the process has generally only been used where the impact toughness requirements for both the HAZ and weld metal are not more stringent than Lloyd'sRegister Grade 1 or 2 Classification ( i.e. Charpy impact toughness of 34J at 20°*C and 0°*C respectively). ^[9] Use of lower energy inputs, however, means that Grade 3 requirements (34J at -20°C) can be more readily achieved.

Weld metal toughness can be improved by use of alloyed consumables, so that even at high energy inputs these toughness requirements can be achieved. However, HAZ toughness depends directly on steel type and energy input: the effectof decreasing energy input on HAZ toughness is illustrated, for a C-Mn steel, by the increase in Charpy impact toughness from 40J to 80J at -20°C for electrogas welds produced at 16.6 kJ/ mm and 5 kJ/mm respectively. ^[6] Weld metal Charpy impact toughness of 42J at -20°C was obtained using an S3 1%Ni wire, welded at 17.5 kJ/mm and further improvements in toughness can be expected for lower energy inputs. ^[10]

CTOD (crack tip opening displacement) data are limited for electrogas welds. CTOD values, for welds produced by the high current density technique at an energy input of 10 kJ/mm, of δ _c = 0.35mm for an unalloyed weld metal and δ _c = 0.4mm for the HAZ (fusion boundary) at 0°C, were obtained on a C-Mn steel plate of 32mm thickness. ^[3] The same study revealed that toughness fell below a value of δ _c = 0.1mm at temperatures of -60°C for the weld metal and -30°C for the HAZ (fusion boundary).

Further data are required on welds produced by the high current density technique, particularly on the Nb treated steels, to determine the procedures to be followed if Grade 3 classification is to be achieved.

Summary

Process developments of EGW which permit a high deposition rate and a reduced energy input are outlined. These developments include use of narrow gaps, of high current density techniques where 1.6mm diameter wire is fed at 500A at50mm stickout, and of metal powder addition techniques. The reduced energy input requirement of these process variants means that improved weldment toughness results, so that, for example, in shipbuilding, Lloyd's Register Grade 3Classification should be achieved.

References