8.1 Stainless & Heat-Resistant Steels

8.1 Stainless & Heat-Resistant Steels

Quick Answer: Stainless steels resist corrosion mainly because a thin chromium-rich passive film forms when Cr is high enough (commonly about 10.5–12 % and above in teaching summaries). For welding inspection, classify them by microstructure—austenitic, ferritic, martensitic, duplex—link them to ISO/TR 15608 groups (notably groups 7, 8, and 10), and watch sensitization, ferrite in austenitic deposits, contamination, heat tint, and correct consumables. Heat-resistant grades add elevated-temperature strength and oxidation resistance on top of (or instead of) wet-corrosion priorities.

Modules WT2.12–2.13 expect IWI-S candidates to recognise stainless families, name typical weldability issues, and know what the inspector must verify on the shop floor. This section builds on carbon-equivalent and grouping ideas from Chapter 5 without re-teaching Fe–C basics.

Why “Stainless” Is a Family, Not One Metal

“Stainless” is a commercial label for Fe–Cr alloys (often with Ni, Mo, N, Ti, Nb, and other additions) designed so that a passive film reforms after mild damage in a suitable environment. Welding disrupts that film through heat, oxidation, dilution, residual stress, and surface contamination. A joint can be dimensionally perfect yet fail early in service if the metallurgical or surface condition is wrong.

For the inspector, three questions dominate:

1. What family and group is on the certificate and WPS?
2. What consumable, shielding, and thermal cycle were qualified?
3. What surface and cleanliness condition will the client accept after welding?

Four Microstructure Families

Austenitic stainless steels

Austenitic grades (classic 300-series teaching examples such as 304/304L and 316/316L) are face-centred cubic (FCC) at room temperature when composition is balanced with nickel and other austenite formers. Key inspector-facing properties:

• Generally excellent toughness down to low temperatures compared with many ferritic steels
• Non-magnetic or weakly magnetic in wrought form (weld metal may be slightly magnetic if δ-ferrite is present)
• Good weldability with proper filler, gas, and cleanliness—but sensitive to hot cracking if solidification chemistry is unfavourable, and to sensitization if held in the critical temperature range
• High thermal expansion and lower thermal conductivity than carbon steel → more distortion risk and different heat-flow behaviour

Ferritic stainless steels

Ferritic grades are body-centred cubic (BCC), typically higher chromium with little or no nickel. Teaching points:

• Often magnetic
• Can suffer grain growth and toughness loss in the HAZ if heat input is high
• Limited hardenability compared with martensitic grades, but still need procedure discipline
• Filler selection and joint design matter; some applications use matching or austenitic fillers per procedure—never invent a filler on site without engineering approval

Martensitic stainless steels

Martensitic grades (for example 410-family teaching references) can harden on cooling. Inspector implications:

• Hydrogen cold cracking risk rises relative to austenitic grades—low-hydrogen practice, preheat/interpass, and PWHT may appear on the WPS
• Hardness control and post-weld heat treatment must match the qualified procedure
• Mixing martensitic base metal with generic “stainless TIG rod from the rack” is a classic nonconformance

Duplex (and related dual-phase) stainless steels

Duplex grades combine roughly balanced austenite + ferrite in the solution-annealed base metal. Welding can destroy that balance:

• Too much ferrite after welding → toughness and corrosion concerns
• Too much austenite or unfavourable precipitation → other property losses
• Heat input, interpass temperature, and filler nitrogen/alloying are procedure-critical
• Super-duplex and lean-duplex variants raise the bar further—treat them as specialist materials, not “just stainless”

ISO/TR 15608 Groups — Inspector Mapping

ISO/TR 15608 material groups allow a WPQR to cover a range of similar steels. Stainless and related alloys commonly appear in groups such as:

• Group 7 — ferritic, martensitic, and precipitation-hardening stainless steels (family of Cr steels with limited austenite formers; exact sub-grouping depends on composition tables in the technical report)
• Group 8 — austenitic stainless steels
• Group 10 — duplex stainless steels (austenitic–ferritic)

Exact sub-group numbers and composition limits live in ISO/TR 15608 itself; IWI-S training expects you to read the group on the WPS/WPQR, confirm production material certificates sit inside the qualified range (or that a documented engineering concession exists), and refuse silent substitutions of “another stainless.”

Heat-resistant and high-temperature alloys may sit in stainless groups or in other alloy groups depending on chemistry—always follow the certificate and grouping table, not marketing names alone.

Sensitization and Weld Decay

Sensitization is chromium carbide precipitation at grain boundaries when susceptible austenitic stainless spends enough time in a critical temperature band often summarised as about 450–850 °C (exact bounds vary with grade and time). Chromium is depleted next to the carbides, so the passive film is locally weak. In corrosive service this can produce intergranular corrosion, historically called weld decay when the HAZ is attacked parallel to the fusion line.

Inspector-relevant controls (as specified on procedures and material selections):

• Low-carbon (L) grades (e.g. 304L, 316L teaching labels) reduce carbon available for Cr carbide formation
• Stabilised grades (Ti or Nb additions in classic 321/347 teaching labels) tie carbon as TiC/NbC instead of Cr carbide
• Controlled heat input and interpass limit time in the danger band for multi-pass work
• Solution annealing after fabrication can restore condition when the design and code allow (rarely a free field choice)

Do not confuse sensitization with general “heat tint colour.” Heat tint is surface oxidation; sensitization is a metallurgical grain-boundary condition. Both can harm corrosion resistance; they are not the same mechanism.

Ferrite in Austenitic Weld Metals

Many austenitic stainless weld metals are designed to solidify with a small amount of δ-ferrite (often discussed in the few-percent range using ferrite number, FN). A little ferrite helps disrupt continuous liquid films and reduces solidification cracking risk. Fully austenitic solidification modes may be specified for certain corrosion or cryogenic applications but need tighter composition and procedure control.

Inspector actions:

• Confirm filler designation matches WPS (e.g. 308L vs 316L vs specialised types)
• Do not “upgrade” filler chemistry without written change control
• Understand that slight magnetism of austenitic weld beads can be normal when ferrite is present—it is not automatic proof of wrong metal, nor proof that everything is correct

Heat-Resistant Grades — Awareness Level

Heat-resistant steels and alloys are selected for creep strength, oxidation resistance, and microstructural stability at elevated temperature (furnaces, reformers, power and process plant). Teaching focus for IWI-S:

• Chemistry and heat treatment differ from wet-corrosion stainless practice
• Consumables must match elevated-temperature requirements, not only ambient corrosion labels
• PWHT, solution treatment, or stabilising treatments may be mandatory
• Service failures can be time-dependent (creep, oxidation, embrittlement)—procedure fidelity matters more than visual cosmetics alone

You are not expected to design high-temperature alloys, but you are expected to stop unapproved substitutions and to verify that heat treatment charts and consumable certificates align with the WPS.

Inspector Issues: Contamination, Heat Tint, Consumables

Iron and carbon contamination

Grinding discs, wire brushes, clamps, and benches previously used on carbon steel can embed iron particles in stainless surfaces. Those particles rust and pit the passive film. Best practice on stainless work includes dedicated stainless tooling, clean storage, and protection of finished surfaces.

Heat tint and oxidation

Weld faces and especially roots without backing gas can form coloured oxide films and, in severe cases, heavily oxidised “sugaring.” Chromium can be depleted under thick oxide. Many specifications limit residual heat tint by colour chart, pickling/passivation, or mechanical cleaning to a defined standard. The inspector verifies that the specified cleaning or passivation was done—not that the weld “looks shiny enough” by personal preference.

Shielding and backing gases

Austenitic and duplex stainless root welds commonly require inert backing (purge) with argon or approved mixtures until the root is cool enough. Loss of purge, wrong gas, or excessive oxygen residual is a process nonconformance even if the external bead looks acceptable.

Correct consumables and storage

• Matching or over-alloyed fillers only as qualified
• Low-hydrogen handling where martensitic or hardenable stainless requires it
• Segregated storage so carbon-steel electrodes never enter a stainless joint
• Traceability from electrode/wire batch to the weld record when the quality system demands it

Thermal management

Interpass temperature limits protect duplex phase balance and reduce sensitization risk. Heat input ranges on the WPS are not optional suggestions. High heat input plus slow cooling is a red flag on multi-pass austenitic or duplex work when the procedure restricts it.

Practical Shop Scenarios

1. Wrong brush: A welder cleans a 316L seam with a carbon-steel wire brush from the black-steel bench. Visual weld quality is good. Inspector action: raise NCR risk for contamination; require approved cleaning and possible surface remediation per specification.
2. No purge: Pipe root of austenitic stainless welded without gas backing. Face looks acceptable; root is black and rough. This is a classic reject condition under many process-plant specs regardless of RT acceptance of fusion.
3. Duplex heat input: Multi-pass duplex weld with uncontrolled interpass above WPS maximum. Even without cracks, metallurgical properties and corrosion resistance may be out of qualification range—stop and reassess.

Link Forward

Corrosion forms that exploit weld and HAZ weaknesses (pitting, crevice, stress corrosion cracking) appear in Section 8.4. Nickel alloys, cast irons, and dissimilar joints that often meet stainless in the field appear in Section 8.3. Always tie shop findings back to the WPS, material group, and client surface-finish requirements rather than informal “stainless practice.”