Localised heat caused by welding causes the 'hot' metal to expand and contract at a different rate vs the 'cold' metal, which can cause further cracking to the casting itself, and/or cracking of the new weld metal as it solidifies, or cracking later when in service.
Preheating Instructions By preheating the iron to 500-1200 degrees F, you'll substantially reduce the internal stresses in the iron. This will help prevent the part from cracking around the weld in the heat-affected zone. If you have an infrared thermometer, then by all means use it.
Tips for welding cast iron
By heating and cooling your workpiece evenly, it is less likely to gain stresses and crack. Avoid long localized exposures to high heat. Move the workpiece around and make small welds at a time. Cool your workpiece slowly, in an oven.
Preheat and Post-Weld Heat Treatment
Slower cooling helps minimize the formation of residual stresses, which are a common cause of welding cracks. Preheating is particularly critical for thicker materials, such as high-carbon steels, and those prone to cracking.
Some causes are: inadequate preheat, incorrect bead profile, excessive bead width., incorrect Filler electrode choice, exposure to water causing hydrogen embrittlement, incorrect rod storage, contaminated shielding gas, incorrect procedure including failure to perform peening operations if required.
The major cause of a crack is when internal stresses exceed the strength of the weld metal, the base metal, or both. And once a focal point for these stresses—that is, a stress riser—develops and accumulates, a crack can propagate.
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.
Popping: If your welding sounds more like popcorn than bacon, with lots of snapping and popping, this may signify that your wire speed is too fast. “The wire is hitting the surface and not melting into the metal fully, creating a weak and dangerous weld.”
There are two compelling reasons for this. Firstly, it just looks better. And secondly – and this is the real clincher – properly cleaning welds prevents corrosion, i.e., the long-term degradation of the stainless steel.
Defects which are welded over, and not melted out, can suffer locally intensified strain age embrittlement by static or dynamic strain ageing at the region of concentrated strain at the flaw tip, leaving a planar defect with its tip in a region of low toughness (Dawes M G).
The key reason why welding cast iron can be problematic is the high carbon content. During the welding process, this carbon migrates into the weld metal and/or the heat affected zone adjacent to the weld metal, causing elevated hardness/brittleness. This is how Cast Iron gets its reputation for post weld cracking.
Becuase cast iron is comparitively brittle and prone to cracking (compared to steel), the control of heat is critical to achieving a successful weld.
White cast iron which is very hard and contains iron carbides, is normally considered to be unweldable.
Welding Techniques with Preheat
Preheating the cast iron part before welding will slow the cooling rate of the weld, and the region surround the weld. It is always preferred to heat the entire casting, if possible. Typical preheat temperatures are 500-1200 degrees F.
Granted, only a very high temperature could physically damage this durable tool. Lodge, a popular brand that makes cast iron products, says its pans can safely heat up to 650 degrees Fahrenheit, while some Staub-brand cast iron skills can handle up to 900 degrees Fahrenheit.
JB Weld is basically an epoxy glue that does an exceptionally good job of bonding to cast iron and steel along with several other materials.
We know many people want to wear their wedding or engagement rings all the time, but traditional metal rings make this very difficult in some settings -- including those like welding, where metal might not only be bulky, but also could risk issues with temperature or even electrical currents if they're worn improperly.
Finally, although it does vary depending on the situation and the type of material you're working with, pulling will usually create a deeper penetrating weld than pushing. While pulling might penetrate deeper, in most situations, pushing creates a flatter weld that covers more surface area.
Personal welding safety begins with never wearing anything synthetic that will catch sparks and either ignite or melt. Cotton clothing is far more flame resistant to sparks than a synthetic shirt, but keep in mind that you may ruin a cotton shirt while welding, especially if you're stick welding.
Techniques like back stepping can help prevent cold cracking. To perform this technique, the welding operator welds in one direction for a short length, returns to prior to the beginning of the last weld and repeats the weld pass, stopping at the start of the first weld.
Expansion and contraction of weldments is a major cause of cracking. As a component is being welded, the heat will cause a weld joint to expand. As it cools, the filler material will solidify and contract or shrink, creating internal stresses.
A chipping hammer is used after each pass to break up the slag which forms over the weld as it cools. After it is broken up with the hammer whatever remains on the work is brushed off with a wire brush. These steps are necessary to form a clean surface for the next pass.
However, cast iron is also one of the most difficult materials to weld, as it is prone to cracking due to its high carbon content, low ductility, and thermal expansion and contraction.
Another common issue with welding is the creation of brittle welds that do not hold up. Bare electrodes or electrodes of the incorrect size can lead to brittle welds. In order to produce ductile welds, be sure to use shielded arc electrodes, avoid using excessive current, and pass over the weld several times.