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API TR 938-C-2015: Use of Duplex Stainless Steels in the Oil Refining Industry — A Comprehensive Technical Guide
Scope, Technical Requirements, Fabrication Guidelines, and Compliance Notes for Refinery Applications of Duplex Stainless Steels
Introduction
API Technical Report 938-C-2015 (API TR 938-C-2015) is a widely referenced publication by the American Petroleum Institute that addresses the use of duplex stainless steels in the oil refining industry. Although designated as a Technical Report—rather than a full standard—it provides comprehensive guidance on material selection, fabrication, welding, and performance of duplex and super-duplex stainless steels in refinery environments. This article examines the scope, technical requirements, implementation highlights, and compliance considerations associated with this critical industry document.
Scope of API TR 938-C-2015
API TR 938-C-2015 focuses on the application of duplex stainless steels (DSS) and super-duplex stainless steels (SDSS) in refinery service conditions where resistance to chloride-induced stress corrosion cracking (SCC) and general corrosion is essential. The report covers the following key aspects:
- Metallurgical characteristics of duplex and super-duplex grades (UNS S31803, S32205, S32750, S32760, and others).
- Corrosion resistance performance in typical refinery environments, including crude distillation units, hydroprocessing units, and cooling water systems.
- Fabrication limitations, welding procedures, and post-weld heat treatment requirements.
- Field experience and failure case studies.
The report does not provide mandatory requirements but serves as a recommended practice for engineers and materials specialists.
Tip: API TR 938-C-2015 is often used in conjunction with other API standards such as API 571 (Damage Mechanisms), API 650 (Tanks), and API 582 (Welding Guidelines).
Technical Requirements for Duplex Stainless Steels
Material Composition and Properties
Duplex stainless steels in API TR 938-C-2015 are defined by their balanced austenite-ferrite microstructure, typically 40–60% ferrite. The report lists typical composition ranges and mechanical properties for common DSS grades. Table 1 summarizes key requirements.
| UNS Number | Common Name | PREN (min.) | Yield Strength (MPa, min.) | Typical Refinery Application |
|---|---|---|---|---|
| S31803 | 2205 | 30 | 450 | Piping, pressure vessels in sour service |
| S32205 | 2205 (higher N) | 35 | 450 | Improved corrosion resistance over S31803 |
| S32750 | 2507 (Super-duplex) | 40 | 550 | High-chloride environments, seawater cooling |
| S32760 | Zeron 100 | 40 | 550 | Offshore and refinery service with high chlorides |
Table 1: Typical duplex grades referenced in API TR 938-C-2015 with minimum pitting resistance equivalent numbers (PREN) and yield strengths.
Corrosion Resistance Criteria
The report establishes that for refinery services, the material must resist pitting, crevice corrosion, and SCC. It provides critical limits for chloride concentration, temperature, and pH. For example, standard 2205 DSS is generally recommended for environments with chloride levels below 1000 ppm and temperatures up to 250°C, while super-duplex grades can handle up to 5000 ppm and 300°C under carefully controlled conditions.
Warning: API TR 938-C-2015 stresses that premature failure can occur if the ferrite content is outside the 40–60% range or if improper heat treatment introduces detrimental phases such as sigma (σ) or chi (χ).
Fabrication and Welding Considerations
Welding and Heat Input
Proper welding procedures are critical to maintain the austenite-ferrite balance and avoid intermetallic precipitation. The report recommends low heat input (typically ≤1.5 kJ/mm) and controlled interpass temperatures (≤150°C). Post-weld heat treatment (PWHT) is generally not required, but stress relief may be specified for complex geometries.
Inspection and NDE
Non-destructive examination methods such as liquid penetrant testing (PT) and radiographic testing (RT) are recommended for weldments. The report also includes acceptance criteria for ferrite measurement using magnetic methods or metallographic techniques.
Implementation Highlights in Refineries
API TR 938-C-2015 has been widely adopted by refiners for critical applications such as:
- Desalter effluent piping
- Overhead systems in crude distillation units
- Hydrogen reactor internals
- Cooling water headers and heat exchanger tubes
Success Story: Many refiners have successfully replaced carbon steel and 300-series stainless steels with duplex grades to reduce SCC failures in chloride-contaminated overhead streams, achieving service lives exceeding 15 years.
Compliance and Implementation Notes
Although API TR 938-C-2015 is not a mandatory code, it is often invoked in engineering specifications and purchase orders. Compliance with this document requires:
- Material traceability and adherence to ASTM/ASME specifications (e.g., A240, A790, A928).
- Qualified welding procedures per ASME Section IX or API 1104.
- Verification of PREN and ferrite content.
- Review of the manufacturer’s fabrication practices, especially cold forming and solution annealing.
Critical: The report explicitly warns against using duplex stainless steels in environments containing >5 ppm hydrogen sulfide in the presence of chlorides unless the material is in the solution-annealed condition and the temperature is carefully controlled.
Conclusion
API TR 938-C-2015 remains a cornerstone technical report for the selection, fabrication, and operation of duplex stainless steels in oil refineries. Its recommendations help minimize the risk of corrosion-related failures and ensure long-term equipment reliability. Engineers should integrate this guidance with current industry standards and specific process conditions. Future updates (expected beyond 2025) may incorporate recent advances in super-duplex grades and additive manufacturing.
Q: Is API TR 938-C-2015 a mandatory standard?
A: No. It is a Technical Report issued by API to provide guidance and best practices. However, it is frequently referenced in contractual specifications and can become a de facto requirement for refinery projects.
A: No. It is a Technical Report issued by API to provide guidance and best practices. However, it is frequently referenced in contractual specifications and can become a de facto requirement for refinery projects.
Q: What is the key difference between UNS S31803 and S32205?
A: Both are 2205-type duplex stainless steels, but S32205 has a higher nitrogen content (0.14–0.20% vs. 0.08–0.20%), resulting in improved pitting corrosion resistance (PREN ≥35 vs. ≥30). API TR 938-C allows both but provides application limits based on environment severity.
A: Both are 2205-type duplex stainless steels, but S32205 has a higher nitrogen content (0.14–0.20% vs. 0.08–0.20%), resulting in improved pitting corrosion resistance (PREN ≥35 vs. ≥30). API TR 938-C allows both but provides application limits based on environment severity.
Q: Can duplex stainless steels be used in hydrogen service?
A: Yes, but with caution. The report advises that duplex grades can be used in hydrogen environments up to about 250°C, provided hydrogen charging is minimized (e.g., avoidance of cathodic protection). Super-duplex grades have better resistance to hydrogen embrittlement.
A: Yes, but with caution. The report advises that duplex grades can be used in hydrogen environments up to about 250°C, provided hydrogen charging is minimized (e.g., avoidance of cathodic protection). Super-duplex grades have better resistance to hydrogen embrittlement.
Q: What ferrite content is acceptable per API TR 938-C?
A: The report states that the as-delivered material should have a ferrite content between 35% and 65% by volume. After welding, the heat-affected zone should maintain a ferrite content within 30–70% to avoid brittle phases and corrosion issues.
A: The report states that the as-delivered material should have a ferrite content between 35% and 65% by volume. After welding, the heat-affected zone should maintain a ferrite content within 30–70% to avoid brittle phases and corrosion issues.
Article written based on API TR 938-C-2015. Always refer to the latest edition of the Technical Report for official guidance. Last updated: 2026.