Yes, cast iron and steel may be welded together. However, it’s quite a complicated task, and most welders seem to avoid it. But, with a unique approach and deep understanding of both, it’s doable.
One of the essential aspects of welding cast iron to steel is using a low-carbon steel rod, which can help prevent cracking. A welding rod the same size as the base metal can also ensure a strong bond between the two metals. With the proper tools, it’s possible to weld most types of cast iron to steel.
I’ll go through the nitty-gritty of welding cast iron to steel. I’m not saying that my ways are the greatest; they’re simply what has worked for myself and others.
Distinctive Properties of Cast Iron and Steel
1: Carbon Composition
Both steel and cast iron contains different amount of carbon content which significantly affect their strength, hardness, and ductility.
The carbon content in steel ranges from 0.2% to 2.1%, affecting its hardness and strength. Steel with a low carbon content, such as mild steel, is soft and easy to shape, commonly used in construction, automobile manufacturing, and general fabrication. On the other hand, steel with high carbon content is much harder and more durable in cutting tools, knives, and machinery that require high strength and wear resistance.
In contrast, cast iron has a much higher carbon content than steel, typically ranging from 2% to 4%, making it more brittle than steel and more resistant to wear and tear. It is commonly used in cookware, pipes, and machinery parts that require high wear resistance and low friction.
From the above information, it is concluded that the more carbon in a metal, the harder it is to weld.
There are several basic types of cast iron, which are differentiated by their composition and properties. The most common types of cast iron include:
- Gray iron: The most commonly used cast iron accounts for about 80% of all castings and is characterized by its gray color and high carbon content, which gives it excellent wear resistance and machinability.
- White iron: It has high carbon content and is known for its hardness and resistance to wear. It is primarily used in applications where abrasion resistance is critical, such as in mining and oil drilling equipment.
- Ductile iron: Also known as nodular or spheroidal graphite iron, ductile iron has a unique microstructure that gives it higher tensile strength, toughness, and ductility than gray iron. It is often used in automotive and heavy equipment applications.
- Malleable iron: This type of cast iron is heat-treated to produce a ductile and malleable material that is easily machinable and can be forged into various shapes. It is commonly used in pipe fittings, electrical boxes, and other applications requiring high strength and durability.
- Compacted graphite iron (CGI): This type of cast iron has a graphite structure that is intermediate between gray and ductile iron, giving it a unique combination of strength, ductility, and thermal conductivity. It is used primarily in high-performance engine blocks and other applications requiring high strength and thermal stability.
2: Effect of Thermal Expansion
When cast iron and steel are exposed to high heat, they undergo thermal expansion, which increases their volume.
Cast iron has a coefficient of thermal expansion of 6.5 to 9.0 µm/m-K, causing it to expand by 6.5 to 9.0 micrometers per meter of length for every degree celsius increase in temperature. This expansion can affect tight tolerances in applications during heating and cooling cycles.
On the other hand, steel has a coefficient of thermal expansion ranging from 10.8 to 13.1 µm/m-K and can cause issues in some applications. Additionally, steel experiences a phase change at specific temperatures, such as when austenitic steel becomes ferritic upon cooling below its transformation temperature, leading to a significant volume decrease.
Welderd must consider the thermal expansion and phase changes of cast iron and steel when designing parts for high-temperature applications to ensure that the final product will meet the required dimensional specifications.
3: The Difference in Melting Points
Both cast iron and steel have a difference in melting points. Cast iron has a lower melting temperature (2200 °F) than steel(2800°F). Because of the lower melting temperature, you must modify your welder’s settings correctly and be cautious while working with the thinner cast iron parts.
4: Cast Iron Has Impurities
Cast iron naturally contains traces of impurities such as sulfur, phosphorus, dirt, and oil residues making it more prone to porosity and cracking. On the other side, steel contains nominal impurities.
8 Easy Steps to Weld Cast Iron to Steel
1: Preheat the Cast Iron and Steel
Preheating the metals is important to prevent thermal shock and reduce the likelihood of cracking during welding. Cast iron has a higher thermal expansion coefficient than steel, so it expands and contracts more during heating and cooling.
Preheating the cast iron at a temperature of around 1200°F to 1400°F helps to minimize this difference in expansion and contraction while preheating steel at a temperature of 500°F to 800°F helps to make it more receptive to the welding process.
2: Choose the Right Electrode
Electrodes containing ferronickel are best suited for welding cast iron to steel as they help minimize the difference in thermal expansion between the two metals and improve the weld’s strength and durability.
3: Amperage Range Setting
Welding cast iron to steel requires a welder with a wide amperage range, as the amperage required for welding cast iron is higher than for steel. The welder should be able to produce a high amperage range to handle the cast iron and a lower range to handle the steel.
4: Clean the Metals
Cleaning the surface of both metals before welding is important to remove any impurities or debris that may interfere with the welding process. Use a wire brush or grinder to clean the surface of both metals, and make sure the area to be welded is free of oil, grease, or rust.
5: Use a Low-Heat Input
Cast iron is prone to cracking, so using a low-heat input welding technique is important. Tack welding is recommended to prevent cast iron from warping, and the welding process should be done in short segments to avoid overheating the metal.
6: Monitor the Temperature
Continuously monitor the temperature of the metals while welding, and ensure they stay within the preheated range. If the temperature drops too low, pause the welding process and preheat the metal again to prevent cracking.
7: Allow the Weld to Cool
After welding, let the metals cool down slowly to room temperature before handling. This will help to prevent cracking and warping in the weld. If the weld is cooled too quickly, it can become brittle and prone to failure.
8: Post-weld Treatment
Post-weld treatment may be necessary to relieve any residual stresses in the weld. This can be done by heating the weld to a specific temperature and holding it for a set period. The temperature and duration of post-weld treatment depend on the specific metals being welded and the welding technique used.
Welding Rods For Welding Cast Iron to Steel
Selecting the right welding rods is very important for successful welds. Below are some welding rods recommended by industry experts.
55% Nickel Rods
These are less costly than 99% rods, machinable, and commonly used for thick section repairs. Because the expansion coefficient is more minor, these rods create fewer fusion line fractures than 99% of rods. For welding cast iron to steel, these ferrous-nickel rods are excellent.
99% Nickel Rods
These electrodes are more costly than other alternatives but deliver the most effective outcomes. 99% of nickel rods provide machineable welds and are best used on low or medium-phosphorous castings. The weld deposit created by these pure nickel rods is soft and pliable.
Steel rods are the most affordable of the three and are ideal for minor repairs and fills. Steel electrodes create harsh welds that are difficult to process and require further grinding. Despite these disadvantages, steel rods give better color matching and can withstand less-than-perfect castings than nickel rods.
Best Methods for Welding Cast Iron to Steel
You can use a TIG welder to weld cast iron to steel using a nickel rod such as ERNiFe-CL.Remember to clean both the base and filler materials before starting the weld. Don’t use mild steel rods like ER70S-6, as they are prone to cracking. Overall, it is a good option for thinner cast iron and steel sections.
Using the short circuit method, a MIG welder can weld cast iron to steel. But nickel wire and additional gas incur some extra cost.
Stick welding, also known as shielded metal arc welding (SMAW), is a popular method for welding cast iron to steel and works best when the cast iron is preheated to reduce the risk of cracking.
Welding cast iron to steel is a complex procedure that needs to be performed under certain working parameters. So I don’t recommend using it for joining critical metals if you aren’t an experienced welder.
However, if you have decided to do so, the most preferred filler metal is ferronickel .Furthermore, preheating the metal being welded increases the success rate, and make sure not to cool down the metal immediately using external sources. Instead, let it cool down slowly on its own.
Braze welding is another great option to consider to join metals together. However, it does not offer the same strength as welding.
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