When it comes to metalworking, welding is a commonly used technique to join different pieces of metal together. However, welding stainless steel to carbon steel is a particularly difficult task that should be avoided whenever possible. There are several reasons why welding dissimilar metals, such as stainless steel and carbon steel, can lead to problems and potentially compromise the strength and longevity of the finished product.
Difficulty
Combining dissimilar metals adds extra challenges to the welding process. Stainless steel and carbon steel have different properties, including different thermal expansion rates, melting points, and chemical compositions. Welding these two materials together creates a "dissimilar metal weld," which can cause problems such as cracking, distortion, and brittleness.
Welding stainless steel to carbon steel requires precise control of the temperature and heat input to ensure the proper fusion of the two metals. This can be difficult to achieve due to the different properties of each metal. Carbon steel has a lower melting point and is more electrically conductive than stainless steel. Welding stainless steel with resistance welding, for example, heats up the metal much faster than carbon steel. Waiting for the carbon steel to reach weld temperature can cause the stainless steel to overheat and become riddled with hot cracks. Using filler-based welding or preheating the plain carbon steel can mitigate this issue, but these methods aren't foolproof.
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Hot Cracking of the Stainless Steel
Hot cracking is a common issue when welding stainless steel to carbon steel. This occurs because stainless steel is more electrically-resistant than carbon steel, so it heats up faster. Waiting for the carbon steel to reach welding temperature can cause the stainless steel to overheat and become riddled with hot cracks. This problem is particularly prevalent when resistance welding stainless steel to carbon steel. Filler-based welding or preheating plain steel/mild steel can help alleviate this problem, but these methods aren't always sufficient.
Contamination
Welding stainless steel to carbon steel can also result in contamination of the stainless steel with carbon steel particles, which can lead to rust and other forms of corrosion over time.
Thermal Expansion in High-Temperature Service Conditions
This difference in expansion rates between the two metals can cause extra fatigue to the welded joint, reducing the structural integrity and useful life. In high-temperature service conditions, such as in power plants or chemical processing plants, this problem can be particularly acute. The difference in expansion rates between stainless steel and carbon steel can cause the welded joint to become fatigued, leading to structural failure over time.
Increased Bimetallic Corrosion
Stainless steel is resistant to corrosion, whereas carbon steel is not. When these two metals are welded together, the carbon steel can act as a cathode and the stainless steel as an anode in the presence of an electrolyte such as water, leading to corrosion of the stainless steel.
Stainless steel is generally used for its strong corrosion resistance. An uncovered weld of plain carbon steel and stainless steel that is exposed to extremely corrosive conditions, such as immersion in saltwater, could cause corrosion. This is because the intermingling of plain carbon steel particles with the stainless alloy compromises the protective oxide layer of the stainless, allowing rust to form. This type of corrosion is known as bimetallic corrosion and can severely compromise the integrity of the welded joint.
Reduced Weld Strength
Joining dissimilar metals can lead to weaker welds, even with filler-based welding methods. The differences in weld temperatures and operational tolerances alone can easily compromise the strength of the welded joint. Over time, this can lead to failure of the welded joint, which can be dangerous in high-stress applications.
Conclusion
Welding dissimilar metals together are difficult to do right and often produces inferior results compared to using metal alloys that are similar or the same. When it comes to welding stainless steel to carbon steel, there are several reasons why you should avoid it whenever possible. The difficulty of achieving a good weld, the risk of hot cracking, thermal expansion, increased bimetallic corrosion, and reduced weld strength all make it a risky proposition.
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