Cleaning

Cleaning may be necessary before welding and during welding (interpass) and is usually essential after welding in order to ensure maximum corrosion resistance.

Pre-weld cleaning involves dressing the cut edge and removing all contaminants such as oil, paint, grease, crayon marks, adhesive tapes, etc. The area on both sides of weld should be cleaned before welding by brushing with a clean stainless steel brush and wiped with a solvent moistened cloth. All moisture must be removed and if a flame is used care must be taken to see that any water (a product of combustion) does not remain on the surface or in the weld preparation. Liquid petroleum gas particularly creates a large amount of water when burnt.

Each welding run must be thoroughly cleaned to remove slag and spatter before proceeding with the next run. The cleaning method used (chipping, brushing, grinding) will depend on the welding process, bead shape, etc. but care should be taken to see that the weld area is not contaminated in the process. Any cleaning equipment should be suitable for stainless steel and kept for that purpose.

During welding, a gas purge on the reverse side may be advantageous.

After welding, weld spatter, flux, scale, arc strikes and the overall heat discolouration should be removed. This can involve grinding and polishing, blasting and brushing with a stainless steel wire brush, or use of a descaling solution or paste. The preferred procedure is usually dictated by end use.

Grinding and dressing is to be carried out with iron-free brushes,abrasives, etc. and should not be so heavy as to discolour and overheat the metal. Rubber and resin bonded wheels are satisfactory. Wheels should be dressed regularly to prevent them becoming loaded thereby producing objectionable scratches. In any blasting process steel shot shall not be used.

Welding Procedure

Efficient arc striking is essential in welding all types of stainless steels as indiscriminate arc striking tends to scar or burn the surface of the steel thus providing areas for premature chemical attack. Care and skill are required to obtain satisfactory weld starts and restarts on continuous seams. As both burn-through on light section material and cold lack of penetration starts on heavier material can easily result from imperfect technique, the suggested procedure is to strike the electrode in the joint approximately 8-10mm forward of the actual start point. With the arc established and electrode correctly angled it can then be rapidly taken to the start point for complete fusion of the previous weld and/or the joint root of a new weld.

Efficient tack welding on austenitic steels is essential in controlling distortion. The length of tack welds varies from 12-40mm depending on sheet or plate thickness. Generally tack welds are more closely spaced than when welding mild steel of similar dimensions.

The use of a long arc or excessive current is capable of causing losses of manganese and chromium which can impair the corrosion resistance of the weld joint. Arc length should therefore be kept as short as possible consistent with satisfactory welding performance, ie. minimum spatter, complete fusion, acceptable bead shape.

Welding should be carried out at the lowest current consistent with good fusion at the selected welding speed to minimise heat input and control distortion. Higher welding speeds can be an advantage.

Stringer beads are recommended in preference to weaving in order to keep the heat input to an acceptable level. Where weaving is necessary both the weave width and side dwell time should be kept to a minimum. Interpass temperatures generally should not exceed 150ºC except in the case of martensitic alloys.

Breaking the arc in an abrupt manner can result in slag inclusions and shrinkage cracks. Craters should be filled by using a circular motion at the end of the weld followed by a gradual lengthening of the arc to the point of extinguishing it.

Welding conditions and welding technique should be such as to produce a smooth weld surface which requires minimum dressing.

Fillet weld beads shall be of full throat thickness and correct contour, consistent with good operation. Concave fillet welds are to be avoided.

The root side of the weld must be protected against oxidation especially in gas-shielded arc welding. Protection with shielding gas is commonly applied. Back-gouging (grinding) of the root and welding from the reverse side of the joint can also be used when the design so permits.