Welding cast iron is highly possible these days but a little complex due to the carbon content present. The carbon content is about 2 -4 % which is way more than the content present in the steel. This way the carbon can migrate to the weld and later leads to post-weld cracking. Cast Iron is one of the most challenging materials to weld, a wrong welding move can ruin the whole of it. Cast Iron is made from a combination of different portions of elements i.e. manganese, silicon, chromium, nickel, copper, and molybdenum. There are some impurities involved as well making it difficult to weld without cracking.
All cast iron categories are considered weldable although it is difficult compared to carbon steel welding. Effective in-house cast iron welding can save both time and money. So, how to weld cast iron, well, if critical parts are involved, it is better to seek the workmanship of an experienced welding facility. Here, we present you some guidelines to perform a successful cast iron welding, but first it is better to know the applications of cast iron welding.
Applications of Weld Cast Iron
Several applications of cast iron welding are as follows…
- Cast Iron Pipes
- Agricultural Equipment
- Automobile engine heads
- Engine blocks and manifolds
- Water preps
- On-machine tools
Benefits of Weld Cast Iron
When you weld cast iron, it has numerous benefits as weld metal. Some of the benefits are as follows….
- Greater fluid than steel.
- Low-cost material
- Sand casting shaping
- Lower melting point than steel
- Humidification capacity
- Robustness and rigidity
Why Welding Cast Iron is Difficult?
Cast iron welding is difficult because of the composition of cast iron. The success ratio of properly weld cast iron is 50% which is improving with the latest technology advancements. There are chances that weld cast iron can have cracks and damages once you are done with it. It is difficult to evaluate the exact strength of the material you are dealing with. It is also difficult to determine the heat a cast iron piece can withstand. If you use the correct welding techniques, you can avoid weld cracks. This involves careful heating and cooling, good welding rods, and slow cooling.
How to Weld Cast Iron in Steps
A number of steps are involved to successfully weld cast iron. These steps are explained as follow….
1. Alloy Identification
When the cast iron is rapidly heated or cooled, it could crack due to thermal stresses. So, it is required to know which type of cast iron you are working with. Here are some cast iron alloys briefly explained as follows…
- Grey Cast Iron
Grey cast iron is the most common type of cast iron alloy. It is made of iron, carbon, manganese, and silicon with 2.5 – 4 % of carbon. The carbon precipitates into graphite flakes in the manufacturing process into either a ferrite or pearlite crystalline structure. These graphite flakes have the capacity to dissolve during the welding process embrittling the weld material.
- White Cast Iron
White cast iron is graphite-free and has carbon in combined form as metal carbides. These metal carbides make the microstructure brittle. White cast iron is generally known as unweldable material.
- Malleable Iron
Malleable iron is heat-treated white iron having a lower carbon content in it. It generally has a mixture of ferrite or pearlite structure depending on how it is heat-treated. It has nodules of graphite making it more ductile comparing to the standard white iron.
- Ductile Iron
Ductile iron is quite similar to grey iron with low impurity levels. Ductile iron contains the graphite as spheroids in its matrix, unlike the grey iron that has graphite flakes. The rest of the matrix is mostly pearlite having a ferrite region surrounding the graphite spheroids.
2. Cast Cleaning
It is pertinent to clean the cast iron before welding. Cleaning the cast includes removing all the surface materials like paint, grease, and oils around the welding area. You can use grinding for removing the casting skin. You also need to wipe the cleaned surface with mineral spirits to remove the residual surface graphite prior to the welding. The moisture can be easily removed by slowly preheating the weld area for a short time. In this way, the trapped moisture can be easily eliminated.
3. Appropriate Pre-heat Temperature Selection
The heating and cooling control are necessary to avoid any stress cracking in cast iron. The main reason for heat and temperature control is thermal expansion. The stress builds when the heat is localized in a small heat-affected zone. To avoid such a scenario, localized heating is used that causes restricted expansion by having a cooler metal around it. The resulting stress depends on the thermal gradient between the heat-affected zone and the casting body. Preheating the cast iron before welding slows the cooling rate of the weld and the surrounding area. Normally, the pre-heat temperatures are between 100°C to 400°C depending on the type of cast iron.
4. Choosing the Appropriate Welding Technique
Any arc welding process can be used like flux-cored arc welding, metal inert gas welding and tungsten arc welding, etc. but it is better to use a welding technique that facilitates slow heating and cooling. Here are some welding techniques briefly explained as follows….
- Manual Metal Arc/Stick Welding
Commonly known as Manual Metal Arc/Stick Welding, it is considered the best overall process for cast iron welding if you use the correct welding rods. The choice of electrode depends on the application, color match, and the amount of post-weld machining. Iron-based and nickel-based electrodes are preferred for MMA welding. The iron-based electrode is commonly used in minor repairs of casting when color matching is required. It will produce weld metal with high carbon martensite. Nickel alloy electrodes are more suited for ductile weld metal. This electrode reduces the pre-heating and cracking by providing a lower strength weld metal.
- TIG Welding
TIG welding provides a clean cast iron weld but it is not preferred because of the highly localized heating characteristics. The skill of the welder determines the quality of the finished weld mostly in the case of TIG welding.
- MIG Welding
MIG welding is carried out with a nickel consumable with 80% argon and 20% CO2 mix. You can use the brazing wire in this welding but it isn’t recommended as the wire is significantly weaker than the casting.
- Braze Welding
Braze welding has a minimal impact on the base metal so it can be used to weld cast iron. A filler rod is required to carry out this process. The surface cleaning is important before beginning the braze welding A flux is used to prevent oxides forming by wetting and cleaning the surface allowing the filler to flow over the base metal.
- Oxy-Acetylene Welding
Oxyacetylene welding uses an electrode and an oxy-acetylene torch to generate heat. Large localized heating is formed in this process due to low heat intensity and slow heating. Slow heating is beneficial in preventing the formation of high carbon martensite. You need to preheat the metal to a higher temperature like up to 600°C to make the welding feasible. A neutral flame is used in this welding process.
When the weld cools and contracts, the residual stress starts building that can lead to cracking. By the application of compressive stress, the chances of cracking can be reduced. You can apply the compressive stress by peening which deforms the weld bead. Peening can only be used with relatively ductile weld metal that is on welds produced with nickel consumables. Peening is applied using a ball-peen hammer to deliver moderate strikes.
6. Post Weld Heating
Cracking can also happen when the weld is rapidly cooled in the case of weld cast iron. So, you need to use the insulation materials to slow down the cooling process or by periodic application of heat. It can also be done by placing the workpiece in an insulation blanket and placing it to dry sand. You can even put it over a wood fire oven and allow the metal to cool down as the fire dies.
Cast iron welding is a complex and tricky task. You need to have a significant amount of preparation to weld cast iron. However, it is a metal that is commonly welded these days using the proper techniques. Make sure to follow the correct techniques to avoid cracking. Don’t go for any shortcuts or you will end up with an internally weak or cracked cast iron weld.
Luis Ferency with experience of 35+ years is a semi-retired welder. Though his passion for welding and learning keeps him going further. He loves sharing his knowledge on his expertise in SMAW, GMAW, GTAW, and FTAW. Hence, this helped the new welders to seek leverage knowledge and improve their welding skills.