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This case study documents the supply of Inconel 625 (UNS N06625) prefabricated pipe spools for a 2.4 MTPA liquefied natural gas (LNG) liquefaction facility on the east coast of Peninsular Malaysia. The project required 127 pipe spools totaling 38 tonnes of Inconel 625 material, serving the cryogenic pre-treatment and acid gas removal sections where operating temperatures reach −162°C (LNG service) and H₂S concentrations exceed 15 mol% (sour service).
This article describes the material selection rationale, technical specification compliance, fabrication challenges and solutions, NDE and testing protocols, and the lessons learned from a project that delivered on time despite supply chain disruptions and strict quality requirements. It serves as a practical reference for procurement engineers, project managers, and material engineers working on similar LNG or offshore projects.
All 127 pipe spools were delivered within the 18-week delivery schedule with zero defects found during site erection. The project confirmed that Inconel 625 is the correct material choice for combined cryogenic + sour service in LNG pre-treatment, and that rigorous pre-fabrication inspection is the most cost-effective quality strategy.
The Malaysia LNG (MLNG) supply chain expansion project required the fabrication and supply of prefabricated pipe spools for three critical process areas: the feed gas pre-treatment section, the acid gas removal unit (AGRU), and the natural gas liquids (NGL) extraction section. All three sections operate under conditions that are outside the safe envelope of standard stainless steel and carbon steel materials.
Parameter | Feed Gas Pre-Treatment | AGRU Section | NGL Extraction | Total Project |
Number of spools | 52 | 41 | 34 | 127 |
Material grade | Inconel 625 (N06625) | Inconel 625 (N06625) | Inconel 625 (N06625) | All Inconel 625 |
Total weight (tonnes) | 14.8 | 12.3 | 10.9 | 38.0 |
Size range (NPS) | 1/2"–8" | 1/2"–6" | 1/2"–4" | 1/2"–8" |
Wall thickness range (mm) | SCH 10S–XXS | SCH 10S–XXS | SCH 10S–XXS | SCH 10S–XXS |
Operating temperature | −45°C to +80°C | −25°C to +180°C | −162°C to −140°C | −162°C to +180°C |
H₂S concentration | 15–22 mol% | 5–12 mol% | < 1 mol% | Various |
CO₂ concentration | 8–15 mol% | 3–8 mol% | < 2 mol% | Various |
Design pressure (barg) | 40–80 | 25–60 | 40–100 | 25–100 |
Corrosion mode | Sour + cryogenic | Sour (warm) | Cryogenic + CO₂ | Mixed |
This project combined two of the most challenging service conditions in oil and gas: cryogenic temperatures (as low as −162°C in the NGL section) and sour gas with high H₂S concentrations (up to 22 mol% in the feed gas pre-treatment). Neither condition alone is unusual, but their combination created design and fabrication challenges that required careful material selection and a rigorous fabrication sequence.
The simultaneous presence of cryogenic temperatures (−162°C), high H₂S (up to 22 mol%), and high pressure (up to 100 barg) across three process sections required Inconel 625 as the only technically viable material for all 127 pipe spools. No stainless steel or carbon steel grade could meet all three service requirements simultaneously.
Inconel 625 Was Selected Because No Stainless Steel Can Survive Combined Cryogenic + Sour Service
Material selection for this project was governed by two simultaneous constraints: NACE MR0175 / ISO 15156 compliance for sour service (H₂S resistance) and adequate toughness at cryogenic temperatures (−162°C) for the NGL extraction section. The initial material study evaluated four candidate alloys before selecting Inconel 625.
Alloy | Grade / UNS | NACE MR0175 Sour Service? | Min Temp (°C) | H₂S Max (mol%) | Cryogenic Toughness (−162°C) | Outcome |
Carbon Steel (A106 Gr B) | — | No | −29 | < 50 ppm H₂S | Brittle fracture risk | ✗ Eliminated |
304L Stainless | S30403 | No (no Cr > 10.5%) | −196 | < 50 ppm H₂S | Good (FCC structure) | ✗ Eliminated — sour service fails |
316L Stainless | S31603 | No (no Mo > 2% in sour) | −196 | < 50 ppm H₂S | Good (FCC structure) | ✗ Eliminated — sour service fails |
Super Duplex 2507 | S32750 | Conditional | −46 | < 10 mol% H₂S | Good above −50°C | ⚠ Borderline — fails NGL section |
Inconel 625 | N06625 | Yes — fully compliant | −196 | No limit per ISO 15156 | Excellent (γ" strengthened) | ✓ SELECTED |
Inconel 825 | N08825 | Yes — conditional | −196 | < 15 mol% (varies) | Good at −162°C | ⚠ Borderline — H₂S > 15 mol% fails |
Property | Value | Why It Matters for This Project | ASTM Spec | Acceptance Criteria |
Ni (Nickel) | ≥ 58.0% | Matrix element — sour gas resistance, cryogenic toughness | ASTM B444 Gr 1 | ≥ 58.0% |
Cr (Chromium) | 20.0–23.0% | Oxidation resistance, passive film stability in sour gas | ASTM B444 Gr 1 | 20.0–23.0% |
Mo (Molybdenum) | 8.0–10.0% | Chloride pitting resistance, sour gas resistance | ASTM B444 Gr 1 | 8.0–10.0% |
Nb + Ta (Columbium) | 3.15–4.15% | Precipitates as γ" phase — strength and toughness | ASTM B444 Gr 1 | 3.15–4.15% |
Fe (Iron) | ≤ 5.0% | Cost reduction; keeps austenitic structure stable | ASTM B444 Gr 1 | ≤ 5.0% |
C (Carbon) | ≤ 0.10% | Low carbon prevents sensitization during welding | ASTM B444 Gr 1 | ≤ 0.10% |
UTS | ≥ 690 MPa (solution annealed) | High strength at cryogenic temperatures | ASTM B444 | ≥ 690 MPa |
YS (0.2% offset) | ≥ 276 MPa | Adequate for pressure design at cryogenic temps | ASTM B444 | ≥ 276 MPa |
Elongation | ≥ 30% | Ductility needed for fabrication and impact loading | ASTM B444 | ≥ 30% |
Charpy Impact (CVN) | ≥ 54 J at −162°C | Mandatory for cryogenic LNG service | ASTM E23 | ≥ 54 J avg, ≥ 40 J individual |
Inconel 625 was the only technically compliant material for all three service sections. Its ≥ 58% nickel content provides inherent resistance to H₂S stress cracking. Its austenitic (γ" strengthened) microstructure provides superior toughness at −162°C. Super Duplex 2507 was disqualified because its maximum service temperature of −46°C cannot meet the NGL extraction section requirement of −162°C.
The LNG industry has zero tolerance for material substitution or weld defects in sour service piping. This project applied 100% NDE (Non-Destructive Examination) coverage — meaning every weld joint, every spool, was examined — regardless of whether it was a 1/2" or 8" NPS pipe. The quality plan specified five independent verification layers.
Inspection and Test Plan (ITP) Summary
Inspection Point | Activity | Standard | Acceptance Criterion | Coverage | Responsible Party | Document |
MIL (Material Incoming) — PMI | Positive Material Identification on all pipe and fittings | ASTM E1621 (XRF) | Ni ≥ 58%, Cr 20–23%, Mo 8–10%, match UNS N06625 | 100% of all material | Supplier QC + EPC QA | PMI Report |
MIL (Material Incoming) — MTR | Review material test reports for each heat | ASTM B444, EN 10204 3.1 | Chemical, tensile, hardness, CVN at −196°C | 100% of all material | EPC QA | MTR Review Form |
PW (Post Weld) — RT | Radiographic examination of all butt weld joints | ASME V Art. 2; B31.3 Table 341.3.2 | No cracks; no incomplete fusion; max 2 × 1.5 mm porosity per film | 100% of all weld joints | Third-party (SGS) | RT Films + Report |
PW (Post Weld) — Hardness | Vickers or Rockwell hardness survey on weld + HAZ | ASTM E10/E18; NACE MR0175 | Weld + HAZ ≤ 35 HRC; Base metal ≤ 40 HRC | 100% of all weld joints | JN Alloys QC + EPC QA | Hardness Survey Form |
PW (Post Weld) — Visual | Visual examination of weld surface | ASME V Art. 9; AWS D1.6 | No visible cracks, undercuts, porosity, excess reinforcement | 100% of all weld joints | JN Alloys QC | VT Report |
PD (Post Dispatch) — Final Inspection | dimensional check, documentation review, preservation | Project Quality Plan | Dimensions within tolerance; all docs complete; tagging correct | 100% of all 127 spools | JN Alloys QC + EPC QA | Final Inspection Report |
NDE Results Summary
NDE Type | Total Examinations | Defects Found | Defect Rate (%) | Repairs Performed | Final Result |
RT (Radiographic Testing) | 489 weld joints | 23 porosity indications; 3 lack of fusion; 2 undercut | 4.7% | 28 repairs (24 rewelds + 4 root repairs) | 100% passed after repair |
Hardness Survey (Weld + HAZ) | 489 weld joints | 1 joint exceeded 35 HRC (found in first article, resolved in WPS requal) | 0.2% | 1 WPS modification | 100% passed after requalification |
Visual Inspection (VT) | 489 weld joints | 14 surface defects (undercuts, excess reinforcement) | 2.9% | 14 repairs (grind + re-weld) | 100% passed |
PMI (Positive Material ID) | All pipe + fittings + filler wire | 0 substitutions | 0% | None required | 100% correct material throughout |
Dimensional Inspection | 127 spools | 1 spool out of tolerance (Spool A-24, reworked) | 0.8% | 1 re-dimension + re-check | 100% passed |
The overall defect rate of 4.7% (RT) was within acceptable limits for Inconel 625 fabrication. All 28 weld repairs were completed before dispatch, and zero defects were found during site erection — confirming that pre-dispatch NDE is the most cost-effective quality investment.
The total NDE and inspection cost was $68,000, which represented 1.8% of the spool contract value — a cost that prevented potentially catastrophic failures in sour cryogenic service.
The project was completed within the contractual delivery schedule of 18 weeks from purchase order to site delivery at the Malaysia LNG facility. The spools were dispatched in five batches, with the first batch (30 spools for the pre-treatment section) arriving on site at Week 8, enabling early erection to proceed.
Batch | Number of Spools | Section | Fabrication Complete (Week) | Site Delivery (Week) |
Batch 1 | 30 | Feed Gas Pre-Treatment | Week 7 | Week 8 |
Batch 2 | 22 | Feed Gas Pre-Treatment + AGRU | Week 9 | Week 10 |
Batch 3 | 35 | AGRU | Week 12 | Week 13 |
Batch 4 | 25 | NGL Extraction | Week 15 | Week 16 |
Batch 5 | 15 | NGL Extraction + Final Tie-ins | Week 17 | Week 18 |
Total | 127 spools | All sections | Week 17 | Week 18 |
Based on the experience of this project, JN Alloys has distilled five key lessons that apply to any LNG or offshore project involving Inconel 625 pipe spools in sour cryogenic service. These lessons can save procurement teams significant cost and schedule.
Lesson 1 — Qualify WPS for Hardness BEFORE Starting Production
The most expensive lesson from this project was discovering that the draft WPS produced HAZ hardness of 38 HRC — above the NACE MR0175 limit of 35 HRC — only after the first article spool was tested. This added 3 days and $8,000 to the project. Had the WPS been tested on a trial joint before production, this cost would have been avoided.
Always produce and test a trial weld joint BEFORE beginning production welding
Test for hardness (NACE MR0175 ≤ 35 HRC in HAZ) — not just tensile and impact
Include the trial joint test in the WPS qualification package sent to the EPC for approval
Lesson 2 — Order Inconel 625 Material with ≥ 6-Week Lead Time
Inconel 625 pipe to ASTM B444 Gr 1 requires mill production lead times of 4–8 weeks in normal market conditions. During the nickel price surge of 2022–2023, mill lead times extended to 12–16 weeks. This project was fortunate to have 18 weeks total, but the 5-day delay from material rejection highlighted the risk of tight material schedules.
Order Inconel 625 pipe at least 6–8 weeks before fabrication start
Verify Nb+Ta content on material certs BEFORE accepting — 3.12% is a rejection risk
Maintain a buffer of 1–2 extra random-length pipes for contingency cutting
Lesson 3 — NDE Budget of 2% Prevents 20% Site Rework Cost
The NDE and inspection budget of $68,000 (1.8% of total) found 28 weld defects before dispatch. If these defects had been found on site during erection, the cost of field weld repair — including scaffolding, hot work permits, specialist welder mobilization, schedule delay, and potential liquidated damages — would have been 10–20× higher.
Never reduce NDE coverage to save cost on Inconel 625 sour/cryogenic projects
Specify 100% RT on all butt weld joints — do not accept spot NDE coverage
Include third-party inspection (SGS, Bureau Veritas) as a non-negotiable requirement
Lesson 4 — Inconel 625 Welders Must Be Certified, Not Just Experienced
Inconel 625 has significantly different welding behavior from stainless steel. Its high niobium content makes it susceptible to HAZ cracking if heat input is not precisely controlled. Workers experienced with stainless steel GTAW must be retrained and requalified specifically for Inconel 625 before production.
Require welder qualification records (WPQ) per ASME Section IX for Inconel 625
Prioritize GTAW over SMAW for root passes — GTAW provides better control
Limit welders to 6–8 hours per shift — fatigue increases porosity and lack of fusion risk
Lesson 5 — PMI Is Mandatory, Not Optional
A single material substitution — using 316L where Inconel 625 is specified — in a sour cryogenic service application could lead to catastrophic failure, regulatory violations, and loss of life. PMI (Positive Material Identification) is the only defense against this risk. The project PMI budget was $12,000 — a trivial cost compared to the risk of using wrong material.
Specify 100% PMI on all base materials, weld filler metals, and weld deposits
Use XRF (portable spectrometer) for PMI — it can detect Ni, Cr, Mo content in seconds
Reject any material that fails PMI verification — never accept on mill cert alone
Final Conclusion: This LNG project confirmed that Inconel 625 is the technically correct and commercially viable material for combined cryogenic + sour service pipe spools. The key to project success was rigorous pre-fabrication planning (WPS qualification, material verification), comprehensive NDE (100% RT, hardness, PMI), and skilled fabrication labor.
For any future LNG project in sour cryogenic service: qualify first, verify always, and never compromise on NDE.
JN Alloys (Jinie Technology, Jiangsu Co., Ltd.) is a China-based manufacturer and supplier of stainless steel and nickel alloy products, specializing in fabricated piping packages for LNG, offshore, and petrochemical projects.
We supply Inconel 625, Hastelloy C-276, Duplex 2205, and 316L pipe spools, flanges, and fittings to projects in Southeast Asia, the Middle East, Europe, and North America.
For project inquiries: Market@jnalloy.com | +86-193-3990-0211 | www.jnalloy.com
