Where cracking occurs in or adjacent to welded joints, a satisfactory repair may be made by welding. It is important that the cracked material is gouged or machined away sufficiently to permit a full penetration repair weld to be made, with no traces of crack left behind and no new significant defects introduced.
The weld on weld is not a bad thing. Think of a multiple pass weld. That is nothing more than one weld bead deposited on another.
Running over the top of an existing weld will add little, if any strength to the joint as you're having to penetrate the new weld material, not the parent metal.
Cracks caused by spreading the bead too thin can be remedied by using enough filler and making sure your pieces fit together correctly. Besides base and filler materials, often adjusting your travel speed and voltage settings will allow enough filler material to withstand the internal stresses as the metal shrinks.
Another effective way the cracks can be fixed without welding is brazing. Unlike the traditional welding technique used to repair metal items, brazing uses minimal heat to fix your cast iron pieces. This technique is commonly used to correct cracks on cast iron items.
Cold metal stitching is used to repair cracked metal that cannot otherwise be mended via traditional methods, such as welding. The process of cold metal stitching implements specialized inserts that join the broken metal again across any cracks.
Cracks not only reduce the strength of the weld through the reduction in the cross section thickness but also can readily propagate through stress concentration at the tip, especially under impact loading or during service at low temperature.
Welded joints are normally stronger than bolted joints, in great part because their material does not have the perforations needed for bolted joints. The manufacturing process is the determining factor when it comes to joint strength: bolted joints offer simplicity, but welded joints provide higher strength.
The author notes that four of the most difficult metals to weld are aluminum, cast iron, brass, and stainless steel.
The overhead position is the most challenging welding position, as gravity can cause the weld metal and slag to fall out of the joint.
Materials such as mild, low carbon and carbon-manganese steels, are considered to be very tolerant to heat input and may accept at least two or more re-weld operations. In the case of low alloy steels, re-welds will be governed by the heat-treated condition in which they were supplied.
So if the item was welded originally, it should be weldable again for repair, right? Yes, but only if you know the materials and their conditions and whether they still are exactly as they were at fabrication time—no heat treatment or other surface conditioning has been introduced.
Welding joins metals by melting and fusing them together, typically with the addition of a welding filler metal. The joints produced are strong – usually as strong as the metals joined, or even stronger. To fuse the metals, you apply a concentrated heat directly to the joint area.
Welding without filler material
Generally, the gap must be no more than 10% of the thickness of the thinnest component. This can be relaxed for materials thicker than 1 mm. On the other hand, the gap may need to be 5% or less for materials less than 0.2 mm thick.
Depends on a lot of things including thickness of base metals, job requirements and operator skills. Generally you want tight fit up, but 1/4 inch is common on inspected structural jobs. Sometimes a filler plate is added and another weld made. Sometimes an engineers approval is required.
2 Root openings (gap) between parts to be fillet welded shall not exceed 3/16 in., except in cases involving plates 3 in. or greater in thickness.
Yes, you can use adhesives instead of welding to join metal to metal. Adhesives can provide as strong a joint as welding. Although there are, of course, caveats. If you require a reliable and durable bond (of course you do), you will need to consider the joint design.
To stop the crack from growing you need to do what's called “stop drilling”. This means you drill a tiny hole – about 1/8” diameter – just past the ends of the crack. This hole will relieve the stresses placed on the metal and keep the crack from expanding.
If the weld pool does not have enough volume after cooling to overcome shrinkage stresses, a crater crack will form, often near the end of a weld, in a high-stress, low-strength area (see Figure 1). It's the weld's way of relieving stress.
Autogenous welding is a form of welding in which the filler material is either supplied by melting the base material or is of identical composition. The weld may be formed entirely by melting parts of the base metal, and no additional filler rod is used.
There are three common types of hot cracking: segregation, bead shape and crater cracking. Segregation cracking is a type of centerline cracking, which (as its name implies) occurs through the center of the weld. It results when low melting point elements are concentrated in the center of the weld after it hardens.