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API TR 938-B-2008: Guidelines for the Use of Nickel-Alloyed Steels in High-Temperature Hydrogen Service in Petroleum Refineries
Understanding the Technical Requirements and Compliance Framework for 9% Ni and 3.5% Ni Steels Under Hydrogen Attack Conditions
Scope of API TR 938-B-2008
API TR 938-B-2008, titled “Use of 9% Ni Steel, 3.5% Ni Steel, and Other Alloyed Steels in Hydrogen Service at Elevated Temperatures and Pressures in Petroleum Refineries and Petrochemical Plants,” is a technical report that consolidates literature and industry experience on the application of nickel-alloyed steels in hydrogen-rich environments at elevated conditions. The report focuses on materials containing 9% nickel (typically designed for cryogenic service) and 3.5% nickel, as well as other alloyed steels such as 1Cr-0.5Mo, used in hydrogen partial pressures above 0.1 MPa (15 psi) and temperatures exceeding 200°C (400°F).
The scope includes:
- Evaluation of hydrogen embrittlement (HE) and high-temperature hydrogen attack (HTHA) mechanisms specific to nickel-containing alloys.
- Guidance on material selection, heat treatment, welding practices, and in-service inspection to mitigate degradation.
- Operating limitations based on temperature, hydrogen partial pressure, and material condition.
- Application in pressure vessels, piping, and heat exchangers within petroleum refining and petrochemical processes.
When selecting 9% Ni steel for hydrogen service, verify that the operating temperature does not exceed 370°C (700°F) to minimize hydrogen embrittlement risk.
Technical Requirements
API TR 938-B-2008 does not prescribe mandatory specifications but compiles recommended practices and known failure thresholds. Key technical parameters include material composition, heat treatment cycles, mechanical properties, and welding controls.
| Parameter | 9% Nickel Steel | 3.5% Nickel Steel | Other Alloyed Steels (e.g., 1Cr-0.5Mo) |
|---|---|---|---|
| Typical Composition | 8.5–9.5% Ni, ≤0.13% C, ≤0.30% Si, 0.30–0.60% Mn | 3.25–3.75% Ni, ≤0.20% C, ≤0.35% Si, ≤0.70% Mn | Variable (e.g., 1% Cr, 0.5% Mo) |
| Maximum Service Temperature in H₂ | 370°C (700°F) | 260°C (500°F) | 455°C (850°F) |
| Heat Treatment | Quenched & Tempered or Double Tempered | Normalized & Tempered or Quenched & Tempered | Quenched & Tempered or Normalized |
| Post-Weld Heat Treatment (PWHT) | Required for thickness > 50 mm (2 in) | Required for thickness > 38 mm (1.5 in) | Per Code (ASME Section VIII Div. 1) |
| Primary Damage Concern | Hydrogen Embrittlement (HE) | Temper Embrittlement (TE) and HE | High-Temperature Hydrogen Attack (HTHA) & TE |
The report emphasizes that 9% Ni steel has limited application in hydrogen service above ambient temperature due to its high sensitivity to hydrogen embrittlement. 3.5% Ni steel may be used to moderate temperatures but is prone to temper embrittlement when exposed to the critical range of 370–540°C (700–1000°F) for extended periods. For each alloy, the recommended heat treatment must achieve a fine-grained, tempered microstructure to balance strength and toughness.
3.5% Ni steel can undergo temper embrittlement when exposed to temperatures in the 370–540°C (700–1000°F) range. Controlling silicon and phosphorus content reduces susceptibility.
Implementation Highlights
Successful application of the guidelines in API TR 938-B-2008 requires integrating material selection with operating envelope definition. The following steps are critical:
- Operating Limits: Use the report’s recommendations along with API RP 941 (Nelson curves) to confirm that the chosen material can withstand the design temperature and hydrogen partial pressure without degradation.
- Welding Procedure Qualification: Weld filler metals must have low hydrogen content; preheat and interpass temperatures should be controlled to avoid cold cracking. PWHT schedules should be followed as per the report’s guidance, especially for 9% Ni steel where residual stress relief is vital.
- Nondestructive Examination (NDE): Employ methods such as ultrasonic testing (UT) and magnetic particle testing (MT) to detect hydrogen-induced cracking (HIC) and stress-oriented hydrogen-induced cracking (SOHIC). Baseline and periodic inspections should be planned.
- Material Certification: Ensure that all mill certificates confirm composition limits, especially for residual elements (P, S, Sn, Sb, As) that promote temper embrittlement.
Proper post-weld heat treatment (PWHT) greatly reduces residual stresses and improves resistance to hydrogen cracking.
Compliance Notes
API TR 938-B-2008 is classified as a Technical Report, meaning it is not a mandatory standard and does not carry the same compliance weight as API Recommended Practices (e.g., API RP 571 or API RP 941). However, it is frequently invoked in refinery mechanical integrity programs, engineering specifications, and risk-based inspection (RBI) plans.
- Regulatory bodies may reference this report as an accepted engineering practice, particularly when evaluating the fitness-for-service of older equipment constructed with nickel steels.
- Adoption should be documented in the facility’s integrity management system, including material life-cycle tracking.
- Where conflicts exist with construction codes such as ASME Section VIII or ASME B31.3, the code requirements take precedence.
- Companies should maintain records of material certifications, heat treatment logs, welding procedure specifications (WPS), and inspection reports to demonstrate due diligence.
Do not assume that all nickel-alloyed steels have similar hydrogen service capabilities. Each alloy (9% Ni, 3.5% Ni, 1Cr-0.5Mo, etc.) has unique limitations and must be evaluated independently.
Frequently Asked Questions
Q: What is the main purpose of API TR 938-B-2008?
A: It provides guidance on the use of 9% Ni, 3.5% Ni, and other alloyed steels in refinery hydrogen service at elevated temperatures, addressing hydrogen embrittlement and temper embrittlement concerns.
A: It provides guidance on the use of 9% Ni, 3.5% Ni, and other alloyed steels in refinery hydrogen service at elevated temperatures, addressing hydrogen embrittlement and temper embrittlement concerns.
Q: How does API TR 938-B-2008 relate to API RP 941?
A: API RP 941 provides Nelson curves for carbon and low-alloy steels in hydrogen service; TR 938-B specifically addresses higher nickel alloys and their unique susceptibilities, supplementing the guidelines.
A: API RP 941 provides Nelson curves for carbon and low-alloy steels in hydrogen service; TR 938-B specifically addresses higher nickel alloys and their unique susceptibilities, supplementing the guidelines.
Q: Is compliance with API TR 938-B-2008 mandatory?
A: No, it is a technical report (TR), not a recommended practice (RP) or standard. However, it is often incorporated by reference into company specifications and regulatory requirements in the oil and gas industry.
A: No, it is a technical report (TR), not a recommended practice (RP) or standard. However, it is often incorporated by reference into company specifications and regulatory requirements in the oil and gas industry.
Q: What are the key heat treatment steps for 9% Ni steel in hydrogen service?
A: Typically quenched and tempered, or double tempered, to achieve the desired toughness and microstructure. PWHT is recommended for thicker sections to relieve stresses from welding.
A: Typically quenched and tempered, or double tempered, to achieve the desired toughness and microstructure. PWHT is recommended for thicker sections to relieve stresses from welding.
This article is published for informational purposes and does not replace the full text of API TR 938-B-2008. © 2026