7.3 Creep-Resistant & Cryogenic Steels

7.3 Creep-Resistant & Cryogenic Steels

Quick Answer: Creep-resistant low-alloy steels (ISO/TR 15608 groups 4–6, teaching examples: Cr–Mo and related grades) serve elevated temperature under stress; welding needs hydrogen control, correct fillers, and often PWHT. Cryogenic / low-temperature steels (group 9 nickel steels, e.g. 9% Ni families) need proven toughness at design temperature. Inspectors verify certificates, impact data, WPS group coverage, and thermal controls—not just that the metal “looks like steel.”

Modules WT2.10–2.11 move beyond everyday C–Mn fabrication into steels chosen for extreme temperature service. The metallurgy goals reverse at the two ends of the temperature scale: creep steels resist slow deformation at high temperature; cryogenic steels resist brittle fracture at very low temperature. Both demand disciplined welding procedures and document-driven inspection.

Creep — What Inspectors Need to Understand

Creep is time-dependent plastic deformation under load at elevated temperature. In high-temperature plant (power, process, refining), components may operate for years under stress where ordinary room-temperature yield strength is not the only design limit. Damage mechanisms can include microvoid formation, grain-boundary cavitation, and eventual cracking—often interacting with welds, which are microstructural discontinuities.

Why welding metallurgy matters for creep service:

• Weld metal and HAZ have different creep strength and ductility than parent plate/pipe.
• Residual stresses from welding can drive or accelerate damage if not relieved when required.
• Incorrect PWHT (too low, too short, wrong cool) can leave hard, brittle, or unstable microstructures.
• Wrong filler chemistry can under-match creep strength or create weak fusion zones.

IWI-S does not design creep life, but must recognise that elevated-temperature Cr–Mo fabrication is a controlled metallurgical process, not casual structural welding.

ISO/TR 15608 Groups 4–6 (Creep-Resistant Families)

Teaching overview (always confirm exact boundaries in the current ISO/TR 15608 text):

Shop names inspectors meet include grades in the ½Cr–½Mo, 1¼Cr–½Mo, 2¼Cr–1Mo, 9Cr–1Mo (and modified) families under various EN/ASTM product standards. Exact grade designation on the certificate and WPS is authoritative—group number alone is not a purchase order.

Weldability themes for groups 4–6

1. Hydrogen cold cracking sensitivity — alloy content raises hardenability; thick-wall pipe and restrained joints need low-hydrogen consumables, baking discipline, and documented preheat/interpass.
2. Matching or deliberate under/over-matching fillers — filler selection is engineering-controlled for elevated-temperature strength and toughness; do not substitute carbon-steel electrodes.
3. PWHT / tempering — many codes require PWHT of Cr–Mo welds above thickness or service thresholds to temper martensite and relieve stress.
4. Hardness limits — post-weld (and post-PWHT) hardness surveys are common acceptance tools; high hardness can signal inadequate tempering or wrong thermal cycle.
5. Interpass and heat input — both min (hydrogen/microstructure) and max (property control) may appear on the WPS.
6. Cleanliness — contamination and copper, sulphur, or moisture issues remain relevant; joint prep standards are strict on critical piping.

Inspector checkpoints — creep steels

• Certificate grade, product standard, heat treatment condition (e.g. annealed, normalised and tempered as applicable), and chemistry match the material list and WPS group/subgroup.
• Consumable classification matches the WPS (including any special high-temperature designations).
• Preheat and interpass records exist for production welds that require them.
• PWHT charts show soak temperature/time and rates inside the qualified range; thermocouple placement suitable for wall thickness and geometry.
• Any required hardness testing after PWHT is performed and recorded.
• Dissimilar joints (Cr–Mo to C–Mn, or different Cr levels) have a specific WPS—do not improvise buttering or butter-and-PWHT sequences without procedure coverage.

Exam tip: Groups 4–6 ≈ creep Cr–Mo thinking. Group 1–3 structural practice does not automatically cover them.

Group 9 — Cryogenic / Low-Temperature Nickel Steels

ISO/TR 15608 group 9 covers nickel alloy steels used especially for low-temperature toughness—the teaching example many programmes use is the ~9% nickel steel family for cryogenic tanks and piping (LNG and related services), with related lower-Ni grades for intermediate low-temperature service depending on subgroup and product standard.

Why nickel steels for cold service?

At cryogenic temperatures, ordinary C–Mn steels can lose toughness and fail by brittle cleavage if a crack-like defect is present. Nickel additions (with appropriate processing) improve low-temperature toughness so the material can absorb energy and resist brittle fracture at the design minimum temperature.

Service examples (illustrative):

• LNG storage and process equipment
• Ethylene and other low-temperature hydrocarbon facilities
• Special low-temperature pressure equipment specified by the designer

Weldability and fabrication themes for group 9

Do not assume “more preheat is always better” from carbon-steel HICC habits without reading the WPS—cryogenic nickel steels have their own qualified thermal envelopes.

Why Toughness Testing and Procedure Control Matter

Toughness testing

Charpy V-notch impact tests (and other fracture-mechanics or drop-weight tests when specified) demonstrate that base metal, weld metal, and HAZ meet energy or lateral-expansion criteria at a test temperature linked to design minimum metal temperature. For cryogenic and low-temperature service, this is central—not optional paperwork.

For elevated-temperature creep steels, toughness still matters for startup, shutdown, and ambient handling, and codes may require impacts after PWHT. Creep rupture testing is a design/material qualification topic; production inspection usually sees procedure qualification mechanicals, hardness, and NDT rather than long-term creep tests on every joint.

Procedure control

A WPQR freezes a combination of:

• Parent material group/grade
• Process and consumable
• Thickness
• Preheat/interpass/PWHT
• Heat input window
• Joint design essentials

Production that drifts outside that envelope can produce weld metal or HAZ that no longer meet the impact or hardness results that justified the procedure. That is why IWI-S treats essential variables as safety controls, not bureaucracy.

Consequences of poor control:

• Creep end: hard HAZ, high residual stress, weak matching → early damage risk in hot service
• Cryogenic end: low-toughness weld/HAZ → brittle fracture risk under cold stress and defects
• Both ends: hydrogen cracks, repair loops, and schedule/cost disasters when NDT finds late defects

Material Certificates and Impact Requirements — Inspector Awareness

What to read on the certificate

1. Grade and product standard (e.g. specific Cr–Mo pipe/plate standard; 9% Ni plate standard).
2. Heat number and marking for full traceability into cut pieces and weld maps.
3. Chemical analysis — Cr, Mo, V, Ni, C, etc., consistent with the ordered grade.
4. Delivery / heat-treatment condition — normalised and tempered, quenched and tempered, etc., as required.
5. Tensile properties — yield, tensile, elongation within standard limits.
6. Impact test results — energy, temperature, specimen location/orientation when the standard or order requires them.
7. Any supplementary requirements — hardness, ultrasonic plate testing, Z-direction properties, etc.

Impact requirements in the inspection flow

• Confirm the design / code impact temperature for the component.
• Confirm the certificate impacts (parent metal) satisfy the purchase order and product standard.
• Confirm the WPS/WPQR qualified weld metal and HAZ impacts where required for the welding process and consumable.
• Hold production if material arrives without required impact data or with tests at the wrong temperature.
• After repairs or PWHT deviations, determine whether re-testing or engineering disposition is required—do not silently accept.

Traceability traps

• Mixed storage of Cr–Mo and C–Mn pipe with similar OD markings
• Certificate for heat A attached to plate from heat B
• Using a group 1 carbon-steel WPS on group 5 Cr–Mo “because thickness matches”
• Cryogenic Ni steel welded with leftover C–Mn electrodes “for tacking only” without an approved tack procedure
• Ignoring that impact tests were performed at −20 °C when the design needs −196 °C class verification (example of temperature mismatch thinking)

Connecting Chapters 5–7 for the Exam

Exam Focus for IWI-S

Expect WT2.10–2.11 style items that ask you to:

• Assign creep Cr–Mo thinking to groups 4–6 and cryogenic Ni steels to group 9 (overview level)
• Explain creep vs brittle fracture at low temperature as different service risks
• List welding controls for Cr–Mo: low hydrogen, matching filler, preheat, PWHT, hardness
• State why impact testing and procedure control matter for cryogenic fabrication
• Describe certificate checks: grade, chemistry, condition, impact temperature and energy
• Refuse informal cross-use of a carbon-steel WPS on creep or cryogenic materials

Exam tip: Hot service → creep steels (4–6) + PWHT/hardness culture. Cold service → toughness steels (group 9) + impact culture. Both require certificate truth and WPS discipline; neither is “just another mild steel.”