As welders and fabricators across India, we frequently work with various materials. Mild steel (MS) is ubiquitous, known for its workability and ease of welding. However, low-alloy steels are increasingly common in applications demanding higher strength, toughness, or corrosion resistance. While both are iron-based, their differing compositions demand distinct welding approaches. Ignoring these differences can lead to critical weld defects, compromising safety and project timelines.
Understanding Mild Steel Welding: The Basics
Mild steel, typically with carbon content up to 0.25%, is the backbone of general fabrication. It offers excellent ductility, formability, and good weldability without significant preheating or post-weld heat treatment (PWHT) in most cases. Its low carbon and alloy content minimise hardenability, reducing the risk of brittle heat-affected zones (HAZ). For everyday jobs, electrodes like E6013 (our SUNLIGHT-13 is a popular choice across Kota and beyond) or E7018 are standard, delivering robust, straightforward welds.
The Low-Alloy Steel Challenge: More Than Just Strength
Low-alloy steels incorporate specific alloying elements like chromium (Cr), molybdenum (Mo), nickel (Ni), manganese (Mn), and vanadium (V) in controlled amounts (usually less than 8% total). These additions significantly boost mechanical properties such as tensile strength, yield strength, and toughness, especially at elevated or cryogenic temperatures. Common examples include high-strength low-alloy (HSLA) steels or chrome-moly steels.
However, these benefits come with a catch for welding. The increased hardenability due to alloying elements makes low-alloy steels more susceptible to cold cracking (hydrogen-induced cracking) in the HAZ and weld metal. This occurs when hydrogen gets trapped, combined with high residual stresses and a brittle microstructure. This susceptibility mandates stricter control over the welding process.
Key Differences in Welding Procedure and Material
- Preheating: For low-alloy steels, preheating is often mandatory to slow the cooling rate of the weld and HAZ, reducing thermal stresses and preventing hydrogen entrapment. Mild steel typically needs minimal or no preheat.
- Electrode Selection: Low-alloy steels demand low-hydrogen electrodes (e.g., E7018, E8018-B2, E9018-B3), meticulously baked and stored to IS 814:2004 standards, to minimise hydrogen input. For mild steel, electrodes like E6013 or E7018 (our WELDARC and HITKARI GOLD brands offer a full range) are common.
- Interpass Temperature Control: Maintaining a specific interpass temperature range is critical for low-alloy steels to prevent excessively rapid cooling between weld passes, which can lead to cracking.
- Post-Weld Heat Treatment (PWHT): Many low-alloy steel fabrications require PWHT to relieve residual stresses and refine the microstructure, enhancing toughness and preventing delayed cracking. This is rarely necessary for mild steel.
- Shielding Gas & Flux: Specific shielding gases and flux compositions might be required for low-alloy steels to achieve desired weld metal properties and protect against atmospheric contamination.
Understanding the carbon equivalent (CE) of the steel is vital. Higher CE values indicate increased hardenability and a greater risk of cracking, thereby requiring more stringent preheating and low-hydrogen practices. Always consult the material specifications and welding procedure specifications (WPS) for low-alloy steels.
“Never assume. Always check your material and pick the right rod. It saves time and money in the long run.”
In conclusion, while mild steel welding is forgiving, low-alloy steels demand precision and adherence to specific procedures to ensure weld integrity. Mastering these differences is not just good practice; it’s essential for safety and quality in critical applications. Trust Prakash Electrodes for a comprehensive range of quality electrodes, from our popular SUNLIGHT-13 for general purpose work to specialized low-hydrogen options like WELDARC E7018 for challenging low-alloy steel applications. We're here to support Indian welders with electrodes that meet stringent demands.