Dissimilar Steel Welding and Hardfacing for Hydrogenation Equipment

Literature Overview

This technical paper, published in Petroleum Chemical Equipment Technology (2019, Vol. 40, No. 3, pp. 57–62) by Chai Xiangdong from Sinopec Guangzhou Engineering Co., Ltd., addresses the critical challenges of dissimilar steel welding and hardfacing in hydrogenation equipment. Hydrogenation reactors and associated equipment in petroleum refining are exposed to extreme conditions: high temperatures, high pressures, and high hydrogen partial pressures, which create unique metallurgical challenges not encountered in conventional piping or pressure vessel applications.

Core Technical Challenges

Dissimilar Steel Welding Challenges

Hydrogenation equipment frequently requires joining different steel grades, such as:

Crown sections: Low-alloy steels (e.g., 1.25Cr-0.5Mo, 2.25Cr-1Mo) for hydrogen resistance
Body sections: Carbon steel or low-alloy steel for cost efficiency
Manway and nozzle attachments: Higher-grade alloys for local stress concentration

The dissimilar steel joints present several challenges:

Challenge	Description	Impact
Thermal expansion mismatch	Different coefficients of thermal expansion between materials	Thermal stress at interface during operation and shutdown
Mechanical property mismatch	Different yield strengths and ductilities	Stress concentration at the weaker material
Chemical composition difference	Different alloy content and carbon equivalent	Risk of cracking in the HAZ of the higher-alloy material
Hydrogen resistance variation	Different susceptibility to hydrogen damage	Potential for preferential hydrogen attack at the interface

Hydrogen Blistering in Hardfacing

The most critical issue addressed in this paper is hydrogen blistering in hardfaced layers on hydrogenation equipment. Hydrogen blistering occurs when atomic hydrogen diffuses into the metal and accumulates at inclusions, laminations, or interfaces, forming blisters that can lead to catastrophic failure. The key factors influencing hydrogen blistering in hardfaced layers are:

Interface stress: Residual stresses at the substrate-overlay interface can accelerate hydrogen diffusion and trap formation.
Interface hydrogen concentration: The hydrogen concentration at the interface is influenced by the operating conditions and the hydrogen permeability of the overlay material.
Carbon migration: During hardfacing and subsequent heat treatment, carbon can migrate from the substrate into the overlay, forming hard, brittle carbide layers at the interface that are susceptible to hydrogen cracking.

Hardfacing Method Selection

The paper provides clear guidance on hardfacing method selection for hydrogenation equipment:

Single-Layer Hardfacing

For single-layer hardfacing applications, band electrode submerged arc surfacing (BESAS) is recommended. This method offers:

Controlled heat input with uniform energy distribution across the band electrode width
Lower dilution rates compared to wire-based methods
Reduced residual stress due to the distributed heat input
Good metallurgical bonding with minimal carbon migration

Dual-Layer Hardfacing

For dual-layer hardfacing, the recommended approach is:

First layer: Band electrode submerged arc automatic surfacing (for good bonding and low dilution)
Second layer: Band electrode electroslag automatic surfacing (for uniform composition and controlled solidification)

The electroslag surfacing process for the second layer provides several advantages:

The molten slag pool provides excellent thermal insulation, resulting in very slow cooling rates
Slow cooling minimizes residual stresses and reduces the risk of hydrogen-induced cracking
The electroslag process produces overlays with uniform composition and fine, equiaxed grain structures
The high dilution rate in electroslag can be managed by selecting appropriate alloy compositions

Engineering Practice Analysis

Material Selection for Hydrogenation Service

The selection of hardfacing materials for hydrogenation equipment must consider the Nelson Curves (API 941) for hydrogen damage resistance. Materials must be selected to withstand the specific hydrogen partial pressure and temperature conditions of the service. Common hardfacing materials for hydrogenation service include:

Material Type	Typical Application	Hydrogen Resistance
2.25Cr-1Mo	General hydrogenation service	Good up to moderate H₂ partial pressures
3Cr-1Mo-0.25V	Higher hydrogen partial pressures	Excellent
5Cr-0.5Mo-0.25V	High-pressure hydrogen service	Very good
Austenitic stainless steel	Severe hydrogen environments	Good, but with thermal expansion concerns

Carbon Migration Control

Carbon migration is one of the most insidious failure mechanisms in dissimilar steel welds and hardfaced joints. During hardfacing and post-weld heat treatment, carbon diffuses from the carbon steel substrate into the lower-carbon overlay, creating:

A decarburized zone in the substrate (softened, susceptible to hydrogen attack)
A carbon-enriched zone in the overlay (hardened, brittle, susceptible to cracking)

The band electrode submerged arc method helps minimize carbon migration by providing lower heat input and more controlled thermal cycles compared to conventional wire-based methods. The electroslag process for the second layer further reduces carbon migration through its very slow cooling rates.

Key Questions and Reflections

The paper provides valuable practical guidance but raises several questions for further investigation. The long-term performance of band electrode hardfacing under cyclic thermal loading—typical of hydrogenation reactor startup and shutdown cycles—is not extensively addressed. Each thermal cycle induces stress at the dissimilar interface, and cumulative fatigue damage could eventually lead to interface cracking.

Additionally, the interaction between hydrogen blistering and other degradation mechanisms such as sulfidation, carburization, and creep is complex and multifactorial. In real service conditions, these mechanisms often act synergistically, accelerating degradation beyond what would be predicted from individual mechanism studies.

The paper also highlights the importance of process parameters in controlling hydrogen blistering susceptibility. Variables such as heat input, cooling rate, interpass temperature, and post-weld heat treatment schedule all influence the residual stress state and microstructure at the interface, which in turn affect hydrogen blistering resistance.

Study Insights and Conclusions

This paper provides essential practical guidance for engineers working on hydrogenation equipment. The recommended hardfacing approaches—band electrode submerged arc for single-layer and band electrode submerged arc combined with band electrode electroslag for dual-layer applications—are well-supported by the analysis of hydrogen blistering mechanisms. The emphasis on controlling interface stress, hydrogen concentration, and carbon migration reflects a deep understanding of the metallurgical challenges in this service. For engineers involved in the design, fabrication, or maintenance of hydrogenation equipment, this paper serves as a valuable reference for selecting appropriate welding and hardfacing methods that ensure long-term reliability under severe hydrogen service conditions.