1. Introduction to Stainless Steel

Stainless steel contains over 12% chromium, which forms a protective oxide layer. This makes it rust-resistant. Nickel and molybdenum are often added to improve corrosion resistance. Stainless steel includes both rust-resistant and acid-resistant types.

It is classified by structure into:

Austenitic stainless steel

Ferritic stainless steel

Martensitic stainless steel

Duplex (austenitic-ferritic) stainless steel

2. Welding Characteristics by Type

2.1 Austenitic Stainless Steel

This is the most common type, usually Cr-Ni based.

Welding features:

Hot Cracking: Caused by low thermal conductivity and impurities like sulfur and phosphorus. Reduced by limiting impurities and adding 4–12% ferrite.

Intergranular Corrosion: Due to chromium depletion. Prevented by using low-carbon or stabilized welding materials.

Stress Corrosion Cracking: Often due to residual stress. Reduced by proper joint design and stress relief.

Sigma (σ) Phase Embrittlement: Forms at 800–900°C with high Cr and Mo. Controlled by limiting delta ferrite to 3–10%.

2.2 Ferritic Stainless Steel

Includes Cr12 to Cr30 grades. Ordinary grades have high C and N, limiting weldability. Ultra-pure grades have low impurities, improving corrosion resistance.

Welding features:

Grain growth at >1000°C weakens toughness.

High brittleness due to Cr, C, N, and O.

Embrittlement occurs after heating at 400–600°C or 550–820°C (σ phase).

2.3 Martensitic Stainless Steel

Includes Cr13, low-carbon, and super martensitic types. Alloying elements like Mo, Ni, and V improve heat resistance.

Welding features:

Prone to cold cracking due to hard martensite formation.

Preheating and post-weld heat treatment are needed.

Austenitic fillers may be used to reduce cracking risk.

3. Welding Material Selection

3.1 Austenitic Stainless Steel

Use fillers matching base metal.

A small amount of ferrite (4–12%) improves crack resistance.

In special cases (e.g., urea plants), ferrite must be avoided.

3.2 Ferritic Stainless Steel

Matching, austenitic, or Ni-based fillers can be used.

Austenitic fillers reduce cracking and avoid heat treatment.

Use Types 309, 310, or 308 depending on Cr content.

3.3 Martensitic Stainless Steel

Matching fillers preferred.

Austenitic fillers reduce cold cracking but may reduce strength.

Preheating (if >3 mm thick) and post-weld treatment are recommended.

4. Key Welding Guidelines

4.1 Austenitic Stainless Steel

Use energy-focused methods to reduce stress.

Lower welding current avoids electrode overheating.

Prefer small-diameter electrodes and multi-pass welding.

Keep interpass temperature below 150°C.

4.2 Ferritic Stainless Steel

Use low-energy welding to control grain growth.

Preheat to 100–150°C to reduce cracking.

Use interpass temperatures <150°C.

Post-weld anneal at 750–800°C to restore corrosion resistance.

Form for Welding Planning