Dissimilar Steel Welding and Overlay Welding of Hydrogenation Equipment

Literature Overview

This technical paper by Chai Xiangdong, published in 2019 in the journal Petrochemical Equipment Technology, addresses the challenges and solutions associated with dissimilar steel welding and overlay welding in hydrogenation equipment. The author is affiliated with Sinopec Guangzhou Engineering Co., Ltd., one of China's leading engineering companies for petrochemical projects. Hydrogenation reactors are among the most critical and demanding pieces of equipment in the petrochemical industry, operating at high temperatures (350 to 500 degrees Celsius), high pressures (10 to 30 MPa), and in the presence of hydrogen, which poses severe challenges to material integrity. The use of dissimilar steel weldments and overlay welds in hydrogenation reactors is essential for combining the strength and economy of carbon steel with the corrosion and hydrogen resistance of alloy steels and nickel-based alloys.

Core Technical Points

The primary challenge in hydrogenation equipment fabrication is the compatibility between the different materials used in the reactor construction. Typically, the reactor shell is fabricated from a low-alloy steel such as 1.25Cr-0.5Mo or 2.25Cr-1Mo, while the internal components, including the catalyst support plates, distribution rings, and inlet nozzles, may be made from higher-alloy steels or nickel-based alloys. The welds connecting these dissimilar materials must withstand not only the mechanical loads but also the corrosive effects of hydrogen, hydrocarbons, and other aggressive chemicals present in the hydrogenation process.

Hydrogen attack is the most critical degradation mechanism in hydrogenation equipment. Molecular hydrogen can dissolve in the steel and react with carbides to form methane, leading to internal void formation, cracking, and eventual failure. The susceptibility of steel to hydrogen attack is quantified by the Nelson Curve, which defines the maximum allowable operating temperature for a given hydrogen partial pressure. Materials above the Nelson Curve are susceptible to hydrogen attack and require special protection measures, including the use of nickel-based alloy overlays or cladding.

Material Selection and Welding Strategy

The material selection for hydrogenation equipment follows a hierarchical approach based on the severity of the service conditions:

The welding of dissimilar steel joints in hydrogenation equipment requires careful selection of filler metals to ensure compatibility between the base metals. The filler metal must have adequate strength, ductility, and resistance to hydrogen attack, while also accommodating the differences in thermal expansion between the dissimilar materials. For example, welding 2.25Cr-1Mo steel to Inconel 625 requires a filler metal that can bridge the large difference in thermal expansion coefficients (approximately 12 x 10^-6 /K for 2.25Cr-1Mo and 13 x 10^-6 /K for Inconel 625) without introducing excessive residual stresses.

The overlay welding of hydrogenation equipment internal components is typically performed using one of the following processes:

Welding Procedure Development and Quality Control

The development of welding procedures for hydrogenation equipment dissimilar steel welds and overlays requires compliance with applicable standards such as ASME VIII Div.1, ASME IX, NB/T 47014, and API 934. The welding procedure specification (WPS) must be qualified through a comprehensive testing program that includes mechanical property testing, microstructural examination, and corrosion resistance testing.

The following table summarizes the key testing requirements for dissimilar steel welds and overlays in hydrogenation equipment:

The non-destructive testing (NDT) of dissimilar steel welds and overlays is particularly challenging due to the differences in acoustic impedance between the dissimilar materials, which can lead to signal attenuation and reduced detection sensitivity. Ultrasonic testing (UT) and phased array ultrasonic testing (PAUT) require careful calibration and technique selection to ensure adequate detection of defects at the dissimilar material interface. Radiographic testing (RT) may also be affected by the differences in material density and attenuation between the dissimilar materials.

Engineering Practice Cases

A typical case study involves the fabrication of a hydrogenation reactor catalyst support plate made from 2.25Cr-1Mo steel with an Inconel 625 overlay on the catalyst-facing surface. The overlay thickness is typically 3 to 6 mm, deposited in 2 to 3 passes using a combination of SAW for the root pass and GMAW or FCAW for the subsequent passes. The welding procedure is qualified in accordance with ASME IX, and the overlay is tested for bond strength, hardness, and intergranular corrosion resistance.

The key quality control measures for this application include:

• Preheating the base material to 200 to 300 degrees Celsius to reduce residual stresses and prevent cracking.
• Controlling the interpass temperature to below 250 degrees Celsius to minimize grain growth and maintain the mechanical properties of the overlay.
• Using a low-hydrogen welding process and ensuring proper shielding gas coverage to prevent hydrogen-induced cracking.
• Performing post-weld heat treatment (PWHT) at 700 to 750 degrees Celsius for stress relief and to improve the ductility of the overlay.
• Conducting comprehensive NDT including UT, MT, and PT to ensure the absence of defects.

Study Insights and Reflections

This research underscores the critical importance of proper welding procedure development and quality control in the fabrication of hydrogenation equipment. The use of dissimilar steel welds and overlays is essential for ensuring the long-term integrity of hydrogenation reactors, but it also introduces significant technical challenges that must be carefully managed.

The key insight from this work is that the welding of dissimilar materials in hydrogenation equipment requires a holistic approach that considers not only the mechanical properties of the weld but also the metallurgical compatibility, corrosion resistance, and long-term durability under the specific service conditions. The selection of filler metals, welding processes, and welding parameters must be tailored to the specific material combination and service environment, and the welding procedure must be rigorously qualified and documented.

The economic implications of proper welding practice in hydrogenation equipment are substantial. The cost of a single hydrogenation reactor can exceed tens of millions of dollars, and the consequences of welding-related failures, including catalyst damage, unplanned shutdowns, and safety incidents, can be catastrophic. Therefore, the investment in qualified welding procedures, experienced welders, and comprehensive quality control is not only justified but essential for the safe and reliable operation of hydrogenation equipment.