Welding Guidelines for Creep-Resistant Steels: Ensuring Quality and Durability
July 12, 2023
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Welding Guidelines for Creep-Resistant Steels: Ensuring Quality and Durability

Creep-resistant steels are essential materials in power generation, oil and gas processing, and petrochemical refining — any application where metals must perform reliably under sustained elevated temperatures for years or decades. Unlike standard structural steels, which are designed primarily for ambient-temperature mechanical performance, creep-resistant steels are engineered to resist the slow, time-dependent deformation that occurs when metals are loaded at high temperatures over extended periods.

Welding these materials correctly is one of the most demanding challenges in industrial fabrication. The same alloying elements that give creep-resistant steels their high-temperature strength also make them susceptible to specific and serious cracking mechanisms during and after welding. This guide covers the metallurgical background, the two primary cracking risks, welding procedure requirements, and ESAB's filler metal recommendations for the most commonly encountered creep-resistant steel grades.

What Are Creep-Resistant Steels?

Creep is the permanent, time-dependent deformation of a material under sustained load at elevated temperature — typically above approximately 30–40% of the material's melting point. For steels used in boilers, pressure vessels, and pipework operating at 450–650°C, creep resistance is a fundamental design requirement.

Creep-resistant steels are ferritic steels strengthened by the controlled addition of alloying elements that form stable carbides and nitrides at grain boundaries, resisting the dislocation movement that causes creep. The principal alloying elements and their roles are:

  • Chromium (Cr) — improves oxidation resistance and high-temperature corrosion resistance, particularly against sulphur-containing hydrocarbons in refinery service. Also contributes to carbide formation and solid solution strengthening
  • Molybdenum (Mo) — the primary solid solution strengthener for creep resistance; also improves resistance to hydrogen attack (Nelson curves) in high-pressure hydrogen service
  • Vanadium (V) — forms fine vanadium carbides and nitrides that are highly effective at pinning grain boundaries and dislocations, significantly improving creep strength in advanced grades such as P91
  • Niobium (Nb) — similar role to vanadium; used in combination with V in advanced creep-resistant grades to refine grain structure and improve long-term creep strength

Common creep-resistant steel grades encountered in welding include:

  • 0.5Mo — used for moderate-temperature service up to approximately 500°C
  • 1.25Cr-0.5Mo (P11/T11) — widely used in boiler and pressure vessel construction
  • 2.25Cr-1Mo (P22/T22) — the standard for high-pressure, high-temperature service in power generation and refinery applications
  • 5Cr-0.5Mo (P5) — used where higher oxidation and corrosion resistance is required
  • 9Cr-1Mo (P9) and 9Cr-1Mo-V (P91) — advanced grades for the most demanding power generation service; P91 in particular is widely specified for modern power plant construction

The Two Primary Welding Challenges

Cold cracking (hydrogen-induced cracking)

Cold cracking — also known as hydrogen-induced cracking (HIC) or delayed cracking — typically occurs within 24–48 hours of welding, predominantly in the heat-affected zone (HAZ). It is caused by the combination of three factors, all of which must be present simultaneously:

  • A susceptible microstructure — martensitic microstructure in the HAZ is highly susceptible to HIC. All of the CrMo grades form martensite in the HAZ on cooling from welding temperatures, making microstructure control critical
  • Residual tensile stress — always present in restrained weld joints; managed through joint design and post-weld heat treatment
  • Hydrogen — introduced from moisture in the flux, electrode coating, base material surface, or shielding gas. The hydrogen diffuses to regions of high triaxial stress and causes cracking

Eliminating or minimising any one of these three factors reduces the risk of cold cracking. In practice, this means preheating (to slow cooling and reduce martensite formation), using low-hydrogen consumables, and maintaining hydrogen control throughout the welding operation. For guidance on hydrogen and moisture control in flux-dependent processes, see our article on managing welding fluxes and WPS and PQR procedure management.

Reheat cracking (stress-relief cracking)

Reheat cracking — also known as stress-relief cracking or SR cracking — occurs in the HAZ during post-weld heat treatment (PWHT) or elevated temperature service, typically in the temperature range of 450–700°C. It is caused by the precipitation of carbides within grains during reheating, which strengthens the grain interiors but leaves grain boundaries relatively weak. The residual stresses relaxing during PWHT are then concentrated at grain boundaries, causing intergranular cracking.

The risk is highest in steels containing vanadium, niobium, and titanium — the same elements that provide the best creep resistance. This creates a fundamental trade-off in advanced grades like P91 and P92 that must be managed through careful procedure development.

Mitigation measures include:

  • Avoiding holding times in the 450–700°C temperature range — heat through this range as quickly as practicable
  • Controlling residual elements: phosphorus (P), sulphur (S), tin (Sn), arsenic (As), and antimony (Sb) all increase susceptibility to reheat cracking. Specify low-residual filler metals and verify base material chemistry before welding
  • Controlling heat input during welding to minimise grain coarsening in the HAZ
  • Following the specified PWHT procedure precisely — incorrect temperature, heating rate, or hold time can initiate reheat cracking rather than preventing it

Welding Procedure Requirements

Preheat and interpass temperature

Preheating is essential for virtually all creep-resistant alloy grades. It slows the cooling rate through the martensite transformation range, reduces the hardness of the HAZ martensite, and allows hydrogen to diffuse out of the weld zone before cracking can occur.

Recommended preheat temperatures depend on the specific grade, section thickness, restraint level, and hydrogen potential of the consumables being used. The IIW carbon equivalent (CE) method used for structural steels is not applicable to CrMo alloys. Specific guidance must come from the applicable material specification, the consumable manufacturer's data, or the welding procedure specification. Typical ranges are:

  • 0.5Mo: 100–150°C minimum
  • 1.25Cr-0.5Mo: 150–200°C minimum
  • 2.25Cr-1Mo: 200–250°C minimum
  • 5Cr-0.5Mo: 200–250°C minimum
  • 9Cr-1Mo / P91: 200–300°C minimum; maximum interpass temperature typically 300–350°C

Always consult the WPS or the material specification — preheat requirements for P91 in particular are strictly controlled and must not be estimated.

Post-weld heat treatment (PWHT)

PWHT is mandatory for all CrMo and advanced CrMoV grades in structural and pressure-containing applications. Its purposes are to temper the martensitic HAZ (reducing hardness and improving toughness), relieve residual welding stresses, and restore some of the creep strength properties in the weld metal that are lost during welding.

PWHT temperatures and hold times are grade-specific and code-dependent. For P22 (2.25Cr-1Mo), typical PWHT is 690–750°C for a minimum of 1 hour per 25 mm of thickness. For P91 (9Cr-1Mo-V), PWHT must be performed at 730–800°C with strict hold time requirements — and the cooling rate from PWHT must be controlled to avoid re-entering the martensite transformation range too quickly.

Hydrogen control

All consumables used for CrMo and advanced alloy welding should be specified to the lowest available hydrogen designation — H5 or better where possible. ESAB's VacPac™ electrode packaging hermetically seals low-alloy stick electrodes under vacuum, guaranteeing factory-fresh moisture content and eliminating the need for rebaking when the seal is intact. For flux-dependent processes, see our flux baking guide.

ESAB Filler Metals for Creep-Resistant Steels

ESAB offers a comprehensive range of filler metals covering all common creep-resistant steel grades across stick (SMAW), TIG (GTAW), MIG (GMAW), and submerged arc (SAW) processes.

Stick electrodes (SMAW) — low-alloy CrMo range

  • OK 76.18 — basic DC low-hydrogen electrode for 1Cr-0.5Mo creep-resistant steels; stable arc, minimum spatter, resistant to cracking and porosity. Packaged in VacPac™ for guaranteed low hydrogen
  • OK 76.16 — basic DC low-hydrogen electrode for 1.25Cr-0.5Mo steels (P11/T11); suitable for boilers, pressure vessels, and power generation piping
  • OK 76.96 — 9Cr-1Mo electrode for welding creep-resistant steels of the P9 type; especially suitable for pipe welding; requires preheat and interpass temperature of 150–269°C

TIG rods (GTAW) — for root passes and precision work

  • OK Tigrod 13.09 — Mn-Mo (0.5% Mo) TIG rod for 0.5Mo creep-resistant steels in pressure vessels and boilers; service temperatures up to 500°C
  • OK Tigrod 13.08 — Mn-Mo (1.5% Mn, 0.4% Mo) TIG rod for creep-resistant steels in pressure vessels and boilers; also suitable for low-alloy high-tensile strength steels; service up to 500°C
  • OK Tigrod 13.22 — 2.5Cr-1Mo TIG rod for P22/T22 grade creep-resistant steels in pipes, pressure vessels, and boilers; service temperatures up to 600°C; AWS A5.28 ER80S-B2
  • OK Tigrod 13.32 — 5Cr-0.5Mo TIG rod for P5/T5 grade steels and high-strength steels with minimum yield strength below 730 MPa; AWS A5.9 ER502

MIG wires (GMAW) — for production and fill passes

  • OK AristoRod 13.08 — Mn-Mo alloyed solid MIG wire for creep-resistant steels in pressure vessels and boilers; service temperatures up to 500°C; designed for ESAB's non-copper-coated wire technology for consistent feeding performance

Full CrMo range

ESAB's complete range of CrMo filler metals — including B2, B3, and B9 (P91) grades across SMAW, GTAW, GMAW, and SAW processes — is detailed in our comprehensive filler metals range. For the full technical data including preheat requirements, PWHT guidance, and mechanical properties by grade, download the ESAB Welding Guide: Creep Resistant Steels from your local ESAB representative or contact our applications team.

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