Welding details of stainless steel pipes

Stainless steel can be categorized into four types based on its metallographic structure at room temperature: austenitic, martensitic, ferritic, and duplex stainless steel.

When low-carbon steel is heated to 1550°F, its structure transforms from ferrite to austenite. Upon cooling, it reverts to ferrite. Austenite is non-magnetic and has lower strength but higher toughness than ferrite.

If the chromium (Cr) content exceeds 16%, the structure remains ferritic across all temperatures—hence, it is called ferritic stainless steel. If Cr > 17% and nickel (Ni) > 7%, the austenitic phase is retained across a wide temperature range—this is known as austenitic stainless steel.

Austenitic stainless steels are often referred to as Cr-Ni types, while ferritic and martensitic steels are referred to as Cr types. This article will briefly introduce welding details of stainless steel pipes.

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Alloying Elements and Welding Behavior

Stainless steels and filler metals contain:

Austenite-forming elements: Ni, C, Mn, N

Ferrite-forming elements: Cr, Si, Mo, Nb

By adjusting the content of these elements, ferrite levels in welds can be controlled.

Austenitic stainless steel pipes are the most commonly welded type, accounting for about 80% of all stainless steel pipe welds. They are easier to weld and do not typically require preheating or post-weld heat treatment.

Selecting Welding Consumables

Same Base Metal: Use matching consumables (e.g., 316 base material → 316 filler).

Dissimilar Metals: Match the filler to the more highly alloyed base metal (e.g., welding 304 to 316 → use 316 filler).

Note: Not all grades have matching welding rods. For example, 304 pipes are welded with 308 or 308L filler metals. The "L" indicates low carbon (<0.03%) and helps prevent intergranular corrosion.

Si-containing filler metals (e.g., 308Si) are beneficial in GMAW for improving wetting and deposition rate. For carbide precipitation resistance, use 347 filler metals containing Nb.

Welding Stainless Steel to Carbon Steel

When joining stainless steel to carbon steel, use fillers with higher alloy content to counter dilution. Common choices:

309L: Suitable for joining 304/316 to carbon steel

312: Used when higher Cr content is required

Note: Austenitic stainless steel has a thermal expansion rate ~50% higher than carbon steel. This mismatch can cause internal stress and cracking, requiring proper filler selection and welding procedures.

Pre-Weld Cleaning

Use chloride-free solvents to remove oil and dust.

Avoid carbon steel contamination; store and handle stainless steel separately.

Clean grooves with stainless steel-only tools (e.g., grinding wheels and brushes).

Joint cleanliness is crucial due to greater electrode compensation difficulty.

Post-Weld Cleaning & Corrosion Resistance

Stainless steel's corrosion resistance comes from its Cr-oxide protective layer. Welding can disrupt this layer and introduce iron oxides or carbides, leading to rust—especially at the heat-affected zone (HAZ).

Post-weld cleaning options:

Pickling

Polishing

Grinding

Brushing (using stainless steel-dedicated tools)

Why Weld Metal May Be Magnetic

Fully austenitic welds are non-magnetic, but welding fillers often include ferrite-forming elements to:

Refine grain size

Reduce cracking risk

This results in slight magnetism, which is normal. Ferrite levels vary depending on the service application:

LNG pipelines: 308 filler with 3–6 FN (Ferrite Number)

Standard 308: FN ~8

Welding Duplex Stainless Steel Pipes

Duplex stainless steels contain ~50% ferrite and ~50% austenite, combining high strength, corrosion resistance, and toughness. The most common grade is 2205 (22% Cr, 5% Ni, 3% Mo, 0.15% N).

To maintain balance during welding:

Use fillers with 2–4% higher Ni content

Control cooling rate to avoid excessive ferrite or intermetallic phases

Example: Flux-cored wire for 2205 may contain ~8.85% Ni.

Welding Parameter Adjustments

Welders often adjust voltage, current, arc length, inductance, or pulse width due to:

Variations in filler composition

Differences in diameter, cleanliness, or shape of the wire

These factors significantly affect weld wetting, slag removal, and arc behavior.

Controlling Carbide Precipitation

At 800–1600°F, carbon diffuses to grain boundaries and binds with Cr to form chromium carbides, reducing corrosion resistance and causing intergranular corrosion.

To prevent this:

Use low-carbon fillers (<0.04% C)

Add Nb or Ti, which preferentially bind with C (e.g., 347 fillers)

Welding Material Selection Preparation

Before selecting welding materials, gather:

Service environment details: temperature, corrosive media, expected lifespan

Mechanical requirements: strength, ductility, toughness, fatigue resistance