Alloy 20 Welding – A Masterclass In Preventing Corrosion
Alloy 20 is a nickel-chromium-molybdenum stainless steel designed specifically for exceptional resistance to sulfuric acid and other harsh chemicals.
Successful alloy 20 welding requires strict heat control and the use of matching ER320LR filler metal to prevent hot cracking and preserve corrosion resistance.
If you have ever worked on industrial fluid systems or high-end chemical processing equipment, you know that standard stainless steel just doesn’t cut it. You need a material that can survive the nastiest environments without turning into a pile of rust.
I promise that by the end of this guide, you will understand the specific metallurgical challenges of working with this “super-austenitic” material. We are going to walk through the exact techniques required to keep your joints strong and corrosion-resistant.
Whether you are a garage tinkerer building a custom chemical storage unit or a professional welder leveling up your skills, mastering this process is a serious feather in your cap. Let’s dive into the details of getting these welds right the first time.
Understanding the Metallurgy of Alloy 20
Alloy 20 is a unique beast because it bridges the gap between stainless steel and high-nickel alloys. It contains roughly 33% nickel, 20% chromium, and significant additions of molybdenum and copper.
This specific chemistry is what gives the metal its legendary resistance to sulfuric acid. However, that same chemistry makes the metal susceptible to hot cracking if you treat it like standard 304 or 316 stainless.
When you heat this material, the cooling phase is critical. If your heat input is too high, you risk segregating elements that hold the grain structure together. This leads to micro-fissures that can fail under pressure.
The Fundamentals of Alloy 20 Welding
When you approach alloy 20 welding, your mindset must shift from “getting penetration” to “managing heat.” This material is sensitive to thermal buildup, which can quickly degrade its chemical properties.
You should always use the Gas Tungsten Arc Welding (GTAW) or TIG process for the best control. TIG allows you to precisely manage the heat input and the size of your weld puddle, which is essential for this alloy.
Never use standard 308 or 309 filler rods. You must use ER320LR filler metal. The “LR” stands for Low Residuals, which is a vital distinction for preventing cracks in the weld bead as it cools.
Essential Preparation and Cleaning Techniques
Cleanliness is non-negotiable when working with high-nickel alloys. Any contamination from grease, oil, or even shop dust can introduce impurities that ruin the corrosion resistance of your final joint.
Start by degreasing the base metal with a solvent like acetone. Once the surface is clean, use a dedicated stainless steel wire brush to remove any oxide layers.
Do not use a brush that has touched carbon steel. Even a microscopic amount of iron contamination can cause galvanic corrosion, which defeats the entire purpose of using a high-performance material like Alloy 20.
Managing Heat Input to Prevent Cracking
The biggest mistake DIYers make is running their machine too hot. While you want good fusion, you must keep your interpass temperature below 300 degrees Fahrenheit.
Use a temperature stick or a digital infrared thermometer between every pass. If the metal gets too hot, the copper and molybdenum can react in ways that weaken the joint structure.
If you are working on thicker plates, use a stringer bead technique rather than a weave. Weaving puts too much heat into the base metal, which increases the likelihood of cracking.
Key Settings for Your TIG Setup
- Shielding Gas: Use 100% pure Argon. Avoid gas mixes with CO2 or Oxygen as they will cause excessive oxidation.
- Tungsten: Use 2% Lanthanated tungsten. It holds a point well and provides a stable arc at lower amperages.
- Pulse Settings: If your machine supports it, use a pulse frequency of 1–2 pulses per second. This helps keep the average heat input low while ensuring proper fusion.
Post-Weld Cleaning and Passivation
Once the weld is finished, your work isn’t quite done. The heat-affected zone (HAZ) will likely have some discoloration, known as “heat tint.”
This tint is essentially chromium oxide that has been depleted from the surface. Without the chromium, that area is no longer corrosion-resistant.
You must remove this tint using a stainless steel pickling paste or a mechanical grinder followed by a flap disc. If you leave the dark heat tint, your expensive alloy will rust faster than standard mild steel.
Frequently Asked Questions About Alloy 20 Welding
Can I weld Alloy 20 to 316 stainless steel?
Yes, you can. However, you must use the correct filler metal. ER320LR is usually the standard recommendation to ensure the transition zone retains sufficient corrosion resistance.
Why does my weld keep cracking as it cools?
This is almost always due to excessive heat input or improper filler metal selection. Slow down your travel speed, lower your amperage, and ensure you are using ER320LR filler wire specifically.
Do I need to back-purge when welding this material?
If you are welding pipe or tubing, back-purging with Argon is highly recommended. It prevents “sugaring” on the inside of the root pass, which would otherwise become a focal point for corrosion.
Is Alloy 20 magnetic?
No, like most high-nickel austenitic alloys, Alloy 20 is non-magnetic. If you find your piece is strongly magnetic, you might be working with a different grade of steel.
Final Thoughts on Mastering Specialized Alloys
Working with high-performance metals like Alloy 20 is a major step up in your workshop journey. It forces you to pay attention to the science of what is happening under the arc, rather than just burning wire.
Always remember: keep your heat low, keep your metal clean, and always use the correct filler material. If you follow these steps, you will produce joints that are as durable as they are clean.
Don’t be afraid to practice on scrap pieces before starting your main project. The more you understand how the material reacts to your specific machine, the better your final results will be. Now, get out there, stay safe, and enjoy the process of perfecting your craft.
