Welding Knowledge - Part 2: Introduction to Stainless Steels

Understanding Stainless Steels

Contrary to popular belief, stainless steel is not entirely impervious to staining or corrosion. In certain environments, such as those exposed to saltwater, discolouration and corrosion can occur. Even in domestic settings, stainless steel can develop dark spots, particularly from the mild salt solutions used in automatic dishwashers.

The global importance of stainless steel is underscored by its extensive use, with over 50 million tonnes consumed annually as of 2019. Renowned for its corrosion resistance, stainless steel is also valued for its strength and resistance to oxidation at elevated temperatures. The key alloying elements typically include chromium, nickel, manganese, carbon, and molybdenum, each contributing to the metal’s unique properties. Depending on the proportions of these elements, stainless steels can be classified into different structures: austenitic, ferritic, and martensitic.

For standardization, references to steel grades in this guide will follow the SAE system, which was consolidated by the Society of Automotive Engineers (SAE) in 1995 after initially being developed alongside the American Iron and Steel Institute (AISI).

The varying alloy compositions of stainless steels necessitate different welding approaches. While many fabrication challenges have been addressed, selecting the right filler metal and employing proper welding techniques remain crucial for achieving high-quality joints.

Stainless steels are used in a wide range of industries, including domestic appliances, architecture, transportation, chemicals, pharmaceuticals, oil and gas, medical devices, food and drink production, and the manufacture of fasteners and wire.

Types of Stainless Steels and Their Welding Considerations

Austenitic Stainless Steels

Austenitic stainless steels are categorized under the SAE 200 and SAE 300 series. They contain 16% to 30% chromium and 2% to 20% nickel. Molybdenum and other metals may also be added to enhance corrosion resistance and mechanical strength. These steels account for approximately 75% of the global stainless-steel market.

Welding Characteristics

• Austenitic steels can be welded using all principal arc welding processes (GTAW, GMAW, SAW).
• Their single-phase structure prevents hardening during cooling, eliminating the need for post-weld heat treatment and resulting in tough joints.
• Some compositions are susceptible to ‘sensitisation,’ where chromium carbides form at grain boundaries during cooling, leading to localised corrosion. This can be mitigated by adding small amounts of titanium or niobium.

Filler Metals

• Most austenitic steels have matching filler metals, though some exceptions exist. For example, type 304 stainless steel often uses type 308 filler, and type 321 stainless steel may be welded with type 347 filler.

Ferritic Stainless Steels

Ferritic stainless steels fall under the SAE 400 series, containing 10.5% to 27% chromium, with small amounts of molybdenum, aluminium, and titanium. These steels are less expensive than austenitic grades due to their lower nickel content and are magnetic. Typical ferritic alloys include SAE 409, 430, 434, and 446, comprising about 20% of the world’s stainless-steel market.

Welding Characteristics

• Ferritic steels offer better mechanical properties than austenitic steels, though they are generally less corrosion resistant.
• Many ferritic grades are weldable but require careful handling to prevent issues such as sensitization in the heat-affected zone and hot cracking in the weld metal, especially in thicker sections.

Filler Metals

• Austenitic filler metals are commonly used, especially the low-carbon SAE 309 alloy. Their higher chromium content helps maintain strength and corrosion resistance.

Martensitic Stainless Steels

Martensitic stainless steels, such as SAE 410, 420, 422, and 431, contain up to 18% chromium, carbon, and manganese. Combining these elements with heat treatment results in a martensitic structure, offering superior mechanical properties, creep strength, and resistance to erosion and corrosion.

Welding Characteristics

• Fusion welding of martensitic steels can be challenging. Pre- and post-weld heating are often required to avoid cracking and to achieve a tough, ductile joint.

Filler Metals

• Filler metals with compositions like the base alloys are suitable, with some containing nickel to maintain low weld ferrite content, thus preserving mechanical strength.

Duplex Stainless Steels

Duplex stainless steels were developed to combine the corrosion resistance of austenitic alloys with the strength of ferritic steels. Although these materials have developed since the 1980s, they require specialized production techniques, limiting their manufacture to specific operations. Many duplex alloys are not internationally standardised but are known by their commercial names.

Characteristics

• Duplex steels offer twice the design strength of austenitic and ferritic stainless steels.
• They exhibit excellent corrosion resistance, good toughness down to -80ºC, and are weldable in thick sections.
• These alloys are widely used in offshore oil and gas and petrochemical sectors.

Welding Considerations

• Welding duplex steels requires careful control of thermal cycles to maintain the balance of microstructures and to prevent cracking.
• Filler metals often contain more nickel than the base alloy to ensure an adequate balance of austenite in the weld, with up to 10% nickel in super duplex materials.

General Welding Precautions for Stainless Steels

Dissimilar Metals

When welding dissimilar metals, selecting a filler metal that accommodates differences in thermal expansion and maintains mechanical properties is essential. Sometimes, nickel-based fillers like Inconel 625 are used when matching expansion coefficients is critical.

Purging

Protecting the weld from oxidation is crucial, especially when welding stainless steel. Loss of chromium due to oxidation can severely reduce corrosion resistance. While the arc shield protects the upper bead, special attention must be given to safeguarding the weld root with an inert gas shield.

References

• British Stainless-Steel Association. BSSA
• AISI Steel Grades, SAE and Werkstoff Numbers - Steel Express