Stainless steel is a popular material for many applications due to its corrosion resistance, durability, and aesthetic appeal. However, not all stainless steel alloys are created equal when it comes to weldability. Choosing the right grade of stainless steel for welding is important to ensure proper bonding and minimize defects. This article will examine the different welding grades of stainless steel, how they are classified, and provide recommendations on matching the correct grade to your project’s requirements.
Introduction to Stainless Steel Grades
Stainless steel is categorized into different grades based on its metallurgical structure and alloy elements. The most common grading system for stainless steels is the American Iron and Steel Institute (AISI) 300 Series numbering system. The main alloy grades are:
- 200 Series – Austenitic chromium-nickel-manganese alloys
- 300 Series – Austenitic chromium-nickel alloys
- 400 Series – Ferritic and martensitic chromium alloys
- 500 Series – Heat-resisting chromium alloys
The specific alloying elements and percentages vary within each series, producing stainless steels with different properties. When selecting a grade for welding, key factors to consider are:
- Carbon content – Higher carbon content reduces weldability
- Nickel content – Nickel improves weldability and corrosion resistance
- Chromium content – Increased chromium boosts corrosion resistance
- Ferrite/austenite structure – Austenitic grades are more weldable than ferritic
Matching the grade to the intended service environment and requirements will optimize weld quality and performance.
Common Welding Grades of Stainless Steel
Here are some of the most popular stainless steel grades used for welding applications:
Austenitic Grades
- 304 – Excellent weldability and corrosion resistance. Most versatile and widely used austenitic stainless steel.
- 316 – Added molybdenum for increased corrosion resistance to chlorides and sulfates.
- 308 – Higher carbon content improves strength and heat resistance.
- 321 – Addition of titanium prevents carbide precipitation during welding.
Ferritic Grades
- 409 – Good weldability and corrosion resistance. Used for automotive exhaust systems.
- 430 – General purpose grade, often used decoratively.
- 446 – Extra corrosion resistance with addition of titanium and niobium.
Martensitic Grades
- 410 – Basic general purpose martensitic grade.
- 416 – Addition of sulfur increases machinability.
- 420 – Higher carbon content which provides strength.
- 440A – Very high strength and moderate corrosion resistance.
Duplex Grades
- 2205 – Good weldability duplex grade with high corrosion resistance.
- 2507 – Excellent strength and resistance to stress corrosion cracking.
- 2304 – Duplex grade with added nitrogen for strength.
These are some of the most common welding grade choices, but many other specialty grades exist as well.
Welding Behavior of Stainless Steel Grades
Understanding how each stainless steel grade welds is important for success. Here are some key welding behaviors of different grades:
| Grade | Welding Behavior |
|---|---|
| Austenitic (300 Series) | Excellent weldability, low risk of defects. Can be prone to distortion due to high thermal expansion. |
| Ferritic (400 Series) | Good weldability, less ductile than austenitic grades. Prone to cracking in fully ferritic grades. |
| Martensitic (400 Series) | Weldable but high risk of cracking and brittle welds. Often requires pre/post heating. |
| Duplex | Moderate weldability, susceptible to precipitation of intermetallic phases. |
The austenitic 300 series grades offer the best overall weldability of stainless steels. Their higher nickel content and fully austenitic structure prevents cracking and improves ductility. Austenitic stainless can be readily welded using any process. On the other end of the spectrum, martensitic grades are prone to cracking and can only be welded using proper precautions.
Welding Processes for Stainless Steel
All standard welding processes can be used to weld stainless steel:
- Shielded Metal Arc Welding (SMAW) – Manual stick welding
- Gas Metal Arc Welding (GMAW) – MIG welding
- Flux Cored Arc Welding (FCAW) – Flux core welding
- Gas Tungsten Arc Welding (GTAW) – TIG welding
- Plasma Arc Welding (PAW)
- Resistance Welding – Spot, seam, projection and upset welding
- Laser Beam Welding (LBW)
For most applications, GTAW and GMAW are recommended for stainless steels. The low heat input of GTAW minimizes distortion and the high deposition rate of GMAW improves productivity. FCAW is also widely used for its ease of operation and penetration capabilities.
Matching Filler Metals
Choosing the appropriate filler metal is also essential in stainless steel welding. The filler metal should match the base metal grade to ensure proper corrosion resistance, strength and weldability. Some common filler metal matching guidelines include:
| Base Metal Grade | Filler Metal |
|---|---|
| 304/304L | ER308, ER308L, ER347 |
| 316/316L | ER316, ER316L |
| 410 | ER410 |
| 2205 Duplex | ER2209 |
Having the proper filler metal is critical to achieve sound welds and avoid potential issues with corrosion resistance or mechanical properties. Consult welding codes and manufacturer recommendations for selecting filler metals.
Recommended Welding Grades for Common Applications
Selecting the optimal stainless steel grade and welding process depends on the intended service conditions and performance requirements. Here are suggested grades for various applications:
| Application | Recommended Grades |
|---|---|
| Chemical/Petrochemical | 316L, 317L, 904L, 254smo, 654smo |
| Food/Beverage Processing | 304, 304L, 316, 316L |
| Pharmaceutical | 316L, 316Lvp, 904L, 654smo |
| Marine | 316, 317LMN, 904L, 254smo |
| Nuclear | 304/304L, 316/316L, 309/309S |
| Automotive Exhausts | 409, 439, 441 |
| Aerospace | 301, 304, 321, 347 |
Consulting corrosion data and industry standards will help narrow down the best grade choice for your specific service environment and operating conditions.
Best Practices for Welding Stainless Steel
To achieve high quality welds when working with stainless steel, follow these general best practice guidelines:
- Select the optimal grade for the application and operating environment
- Use GTAW or GMAW processes for most stainless steel welds
- Choose filler metals that match the base metal alloy
- Clean the joint thoroughly prior to welding to remove contaminants
- Use the correct shielding gas (usually argon or argon/helium mix)
- Employ proper joint design and weld preparation techniques
- Follow recommended weld parameters for the material thickness and joint design
- Use interpass temperature control for heat-sensitive grades like austenitics and duplex
- Perform post-weld heat treatment when required for martensitics and precipitation-hardening grades
Proper training, welding procedure development, and qualification testing is always advised when welding stainless steels, especially for critical applications.
Conclusion
When selecting a grade of stainless steel for welding, critical factors to consider are weldability, corrosion resistance, heat resistance, and required mechanical properties. The austenitic 300 series grades offer the best weldability. Matching filler metals to the base alloy is vital to achieve sound welds. Following proper welding procedures and best practices for joint preparation, process parameters, interpass temperature control, and post-weld heat treatment will ensure high quality welds for your stainless steel components.
