Welds should be qualified and should be made only by welders, welding operators, and tackers qualified as required in AWS D1.1 for buildings and AWS D1.5 for bridges. Welding should not be permitted under any of the following conditions:
When the ambient temperature is below 0F When surfaces are wet or exposed to rain, snow, or high wind.
When welders are exposed to inclement conditions.
Surfaces and edges to be welded should be free from fins, tears, cracks, and other defects.
Also, surfaces at and near welds should be free from loose scale, slag, rust, grease, moisture, and other material that may prevent proper welding. AWS specifications, however, permit mill scale that withstands vigorous wire brushing, a light film of drying oil, or antispatter compound to remain. But the specifications require all mill scale to be removed from surfaces on which flange-to-web welds are to be made by submerged-arc welding or shielded metalarc welding with low-hydrogen electrodes.
Parts to be fillet-welded should be in close contact. The gap between parts should not exceed 3â„16 in. If it is 1â„16 in or more, fillet-weld size should be increased by the amount of separation. The separation between faying surfaces for plug and slot welds, and for butt joints landing on a backing, should not exceed 1â„16 in. Parts to be joined at butt joints should be carefully aligned. Where the parts are effectively restrained against bending due to eccentricity in alignment, an offset not exceeding 10% of the thickness of the thinner part
joined, but in no case more than 1â„8 in, is permitted as a departure from theoretical alignment.
When correcting misalignment in such cases, the parts should not be drawn in to a greater slope than 1â„2 in in 12 in.
For permissible welding positions, see Art. 5.18. Work should be positioned for flat welding, whenever practicable.
In general, welding procedures and sequences should avoid needless distortion and should minimize shrinkage stresses. As welding progresses, welds should be deposited so as to balance the applied heat. Welding of a member should progress from points where parts are relatively fixed in position toward points where parts have greater relative freedom of movement.
Where it is impossible to avoid high residual stresses in the closing welds of a rigid assembly, these welds should be made in compression elements. Joints expected to have significant shrinkage should be welded before joints expected to have lesser shrinkage, and restraint should be kept to a minimum. If severe external restraint against shrinkage is present, welding should be carried continuously to completion or to a point that will ensure freedom from cracking before the joint is allowed to cool below the minimum specified preheat and interpass temperature.
In shop fabrication of cover-plated beams and built-up members, each component requiring splices should be spliced before it is welded to other parts of the member. Up to three subsections may be spliced to form a long girder or girder section.
With too rapid cooling, cracks might form in a weld. Possible causes are shrinkage of weld and heat-affected zone, austenite-martensite transformation, and entrapped hydrogen.
Preheating the base metal can eliminate the first two causes. Preheating reduces the temperature gradient between weld and adjacent base metal, thus decreasing the cooling rate and resulting stresses. Also, if hydrogen is present, preheating allows more time for this gas to escape. Use of low-hydrogen electrodes, with suitable moisture control, also is advantageous in controlling hydrogen content.
High cooling rates occur at arc strikes that do not deposit weld metal. Hence arc strikes outside the area of permanent welds should be avoided. Cracks or blemishes resulting from arc strikes should be ground to a smooth contour and checked for soundness.
To avoid cracks and for other reasons, standard specifications require that under certain conditions, before a weld is made the base metal must be preheated. Tables 5.14 and 5.15 list typical preheat and interpass temperatures. The tables recognize that as plate thickness, carbon content, or alloy content increases, higher preheats are necessary to lower cooling rates and to avoid microcracks or brittle heat-affected zones.
Preheating should bring to the specified preheat temperature the surface of the base metal within a distance equal to the thickness of the part being welded, but not less than 3 in, of the point of welding. This temperature should be maintained as a minimum interpass temperature while welding progresses.
Preheat and interpass temperatures should be sufficient to prevent crack formation. Temperatures above the minimums in Tables 5.14 and 5.15 may be required for highly restrained
welds.
For A514, A517, and A852 steels, the maximum preheat and interpass temperature should not exceed 400F for thicknesses up to 11â„2 in, inclusive, and 450F for greater thicknesses.
Heat input during the welding of these quenched and tempered steels should not exceed the steel producers recommendation. Use of stringer beads to avoid overheating is advisable.
Peening sometimes is used on intermediate weld layers for control of shrinkage stresses in thick welds to prevent cracking. It should be done with a round-nose tool and light blows from a power hammer after the weld has cooled to a temperature warm to the hand. The root or surface layer of the weld or the base metal at the edges of the weld should not be peened. Care should be taken to prevent scaling or flaking of weld and base metal from overpeening.
When required by plans and specifications, welded assemblies should be stress-relieved by heat treating. (See AWS D1.1 and D1.5 for temperatures and holding times required.)
Finish machining should be done after stress relieving.
Tack and other temporary welds are subject to the same quality requirements as final welds. For tack welds, however, preheat is not mandatory for single-pass welds that areremelted and incorporated into continuous submerged-arc welds. Also, defects such as undercut, unfilled craters, and porosity need not be removed before final submerged-arc welding.
Welds not incorporated into final welds should be removed after they have served their purpose, and the surface should be made flush with the original surface.
Before a weld is made over previously deposited weld metal, all slag should be removed, and the weld and adjacent material should be brushed clean.
Groove welds should be terminated at the ends of a joint in a manner that will ensure sound welds. Where possible, this should be done with the aid of weld tabs or runoff plates.
AWS D1.5 requires removal of weld tabs after completion of the weld in bridge construction.
AWS D1.1 does not require removal of weld tabs for statically loaded structures but does require it for dynamically loaded structures. The ends of the welds then should be made smooth and flush with the edges of the abutting parts.
After welds have been completed, slag should be removed from them. The metal should not be painted until all welded joints have been completed, inspected, and accepted. Before paint is applied, spatter, rust, loose scale, oil, and dirt should be removed.
AWS D1.1 and D1.5 present details of techniques acceptable for welding buildings and bridges, respectively. These techniques include handling of electrodes and fluxes and maximum welding currents
