API TR 942-A-2014: Materials, Fabrication, and Repair Considerations for Hydrogen Service in Refinery Hydroprocessing Units

Introduction

API TR 942-A-2014, Materials, Fabrication, and Repair Considerations for Hydrogen Service in Refinery Hydroprocessing Units, is a technical report published by the American Petroleum Institute. This document is not a standard per se; rather, it captures industry knowledge and best practices for managing materials and fabrication challenges when pressure-containing equipment operates in high-pressure hydrogen service. It is intended to assist engineers, fabricators, and operators in making informed decisions to avoid hydrogen-assisted degradation mechanisms such as hydrogen blistering, hydrogen-induced cracking (HIC), stepwise cracking (SWC), and high-temperature hydrogen attack (HTHA).

This technical report supplements API 934-A/B/C standards and provides practical guidance for both new construction and in-service repair of hydroprocessing reactors, separators, heat exchangers, and associated piping. By consolidating experience from refinery operators and fabrication specialists, API TR 942-A-2014 helps reduce the risk of catastrophic failures due to hydrogen effects.

Scope and Applicability

API TR 942-A-2014 applies to carbon steel, low-alloy steel, and stainless steel components exposed to hydrogen-containing environments in hydroprocessing units at elevated temperatures and pressures. The scope includes:

• Material selection for hydrogen service, including resistance to HIC, SWC, and HTHA.
• Fabrication practices such as forming, welding, heat treatment, and non-destructive examination (NDE).
• Repair procedures for existing equipment in hydrogen service, including weld overlay, grinding, and heat treatment.
• Considerations for wet H₂S service where hydrogen charging is combined with aqueous sulfide environments.

The report covers both high-temperature hydrogen attack (HTHA) per API 941 and low-temperature hydrogen embrittlement mechanisms. It is particularly relevant for heavy-wall reactors (e.g., 2.25Cr-1Mo, 3Cr-1Mo-0.25V) and for equipment fabricated from plates, forgings, and pipes that are susceptible to hydrogen blistering.

Key Technical Considerations

Hydrogen-Assisted Degradation Mechanisms

API TR 942-A-2014 identifies five primary failure modes relevant to hydroprocessing units:

1. Hydrogen Blistering: Accumulation of molecular hydrogen at non-metallic inclusions, causing internal cavities close to the surface.
2. Hydrogen-Induced Cracking (HIC): Planar cracking parallel to the surface due to hydrogen pressure at elongated MnS inclusions.
3. Stepwise Cracking (SWC): Propagation of HIC-driven internal cracks in a stepwise manner, linking adjacent blister zones and reducing load-bearing cross-section.
4. High-Temperature Hydrogen Attack (HTHA):
   Decarburization and internal fissuring from reaction of hydrogen with carbides at elevated temperature; predicted by the Nelson curves (API 941).
5. Hydrogen Embrittlement: Loss of ductility and fracture toughness caused by dissolved atomic hydrogen, especially in high-strength steels or hard HAZ microstructures.

Material Selection

The report emphasizes that resistance to hydrogen blistering and HIC depends on steel cleanliness (low sulfur, controlled inclusion shapes) and the use of calcium treatment for sulfide shape control. It provides guidelines for specifying plate material per ASTM standards with supplementary requirements, e.g., A516 Gr. 70 with HIC testing, or A387 steel for HTHA resistance.

Fabrication Practices

To minimize hydrogen charging and subsequent damage, fabrication parameters must be tightly controlled:

• Welding: Use low-hydrogen processes (e.g., SAW, GTAW), maintain low weld metal diffusible hydrogen levels (H₅ or lower), and apply preheat/interpass temperature control to avoid martensitic HAZ microstructures.
• Post-Weld Heat Treatment (PWHT): Adequate PWHT reduces residual stresses and temper-hard HAZ zones. For hydrogen service, the report recommends holding temperatures on the higher side of the allowed range (e.g., 620–680°C for 2.25Cr-1Mo) to achieve sufficient tempering.
• Forming: Cold forming must be limited to avoid excessive work hardening; stress relief after severe forming is required.
• Non-Destructive Examination (NDE): Ultrasonic testing (UT) for lamination detection, and automated UT for HIC/SWC susceptibility in plate before fabrication.

Repair Considerations

The report dedicates significant attention to in-service repairs, which are common in hydroprocessing units due to extended operation periods. Repair methods covered include:

• Grinding and blending: For shallow blistering or HIC, as long as the remaining thickness satisfies the minimum required per API 510.
• Weld buildup: Using matching chemistry or low-alloy steel to restore thickness, followed by PWHT.
• Weld overlay: Stainless steel overlay (309/347) to provide a hydrogen permeation barrier and internal corrosion resistance.
• Buttering and temper bead: Techniques that may allow post-weld heat treatment to be avoided on tempered base metal under specific conditions, but careful verification is required.

In each case, API TR 942-A-2014 outlines pre- and post-repair NDE requirements, critical for ensuring the repair does not introduce new hydrogen damage or reduce the component’s structural integrity.

Implementation Highlights

When applying API TR 942-A-2014 in a project or unit, practitioners should consider three key implementation strategies:

1. Material Specifications: Purchase plates, forgings, and fittings with supplemental requirements for sulfur content (<0.005% S), shape control (Ca treatment), and HIC testing per NACE TM0284. The report lists acceptance criteria that go beyond typical ASTM requirements.
2. Weldability Testing: Perform gapped bead-on-plate or implant hydrogen cracking tests for new joint geometries, especially for thick-wall (>50 mm) components. Use the control of hydrogen input via low-hydrogen flux cored wires or basic electrodes.
3. PWHT Compliance: For creep-resistant Cr-Mo steels, develop a PWHT schedule that meets both hydrogen stress relief (reducing residual H₂) and mechanical property requirements. The report provides tables for minimum PWHT temperatures and hold times based on thickness.

Compliance and Relationship with Other API Standards

API TR 942-A-2014 is not a mandatory standard, but its recommendations are often referenced in purchase orders and repair contracts where hydrogen service is involved. When an end user requires compliance with API 934-C (for heavy-wall pressure vessels), the guidance in this technical report on fabrication and repair becomes de facto mandatory for achieving the performance goals of those standards.

Key compliance points:

• Verification that steelmakers provide HIC test results (NACE TM0284) for plate thicknesses over 25 mm in hydrogen service.
• Documentation of PWHT time/temperature profiles and results of hardness testing (maximum 225 HB for low-alloy steel in hydrogen service).
• Use of qualified welding procedures with proven low hydrogen levels (diffusible hydrogen ≤ 5 mL/100g).
• Periodic inspection for hydrogen blistering using UT or internal visual examination (per API 510).

API TR 942-A-2014 cross-references several related documents:

• API 934-A Materials and Fabrication Requirements for Heavy Wall Pressure Vessels in Hydrogen Service
• API 934-B Fabrication and Repair of Hydrogen Service Equipment
• API 941 Steels for Hydrogen Service at Elevated Temperatures and Pressures
• NACE MR0175/ISO 15156 Petroleum and natural gas industries — Materials for use in H₂S-containing environments in oil and gas production

Frequently Asked Questions