Microstructure and Properties of Combined Overlay Welding Methods for Hydrogenation Reactor Linings

Literature Overview

This paper by Wang Jinliang, Zhang Shijian, Fu Junhao, Luo Weicong, Liu Yongfa, and Xi Xiaohui, published in "Guangzhou Chemical Engineering" (Vol. 50, No. 14, pp. 167-169, 2022), investigates the microstructure and mechanical properties of a combined overlay welding layer produced on 2.25Cr-1Mo steel for hydrogenation reactor inner wall applications. The research was conducted at Guangdong Ocean University in collaboration with the Ironmaking Plant of Shandong Iron and Steel Co., Ltd., Laiwu Branch. The study is classified under TG455, relating to welding processes and equipment.

Core Technical Content

Background and Motivation

Hydrogenation reactors are core equipment in petrochemical enterprises, operating under conditions of high temperature, high pressure, and hydrogen-containing environments. The inner wall overlay welding is a critical manufacturing step that determines the service life and safety of the reactor. The overlay layer must provide excellent resistance to high-temperature hydrogen attack (HTHA), corrosion, and mechanical wear while maintaining good metallurgical bonding with the base material.

The authors selected a combined overlay welding approach using strip electrode electroslag welding (ESW) as the first layer and flux-cored wire gas shielded welding (FCAW) as the second layer. This combination leverages the high deposition rate and low dilution of ESW for the first layer and the fine microstructure and good mechanical properties of FCAW for the final layer.

Combined Overlay Welding Process

The two-layer overlay welding process was applied on 2.25Cr-1Mo steel substrate. The first layer was deposited using strip electrode electroslag welding, which provides a thick, uniform layer with low base metal dilution. The second layer was deposited using flux-cored wire gas shielded welding, which produces a finer microstructure with improved mechanical properties.

Metallurgical Analysis Results

The study presents comprehensive analysis of the combined overlay welding layer, including chemical composition, ferrite content, metallographic microstructure, hardness, bending performance, and intergranular corrosion resistance.

Chemical Composition: The combined overlay layer composition was found to be within the acceptable range for stainless steel overlay applications. The chromium and nickel contents were sufficient to provide the required corrosion resistance, while the carbon content was controlled to minimize intergranular corrosion susceptibility.

Ferrite Content: The ferrite content of the combined overlay layer was measured and found to be within the acceptable range. For austenitic stainless steel overlays, the ferrite content is typically controlled between 5% and 35% to prevent hot cracking while maintaining adequate corrosion resistance.

Metallographic Microstructure: The microstructure analysis revealed a mixed structure of austenite and ferrite phases. The strip electrode ESW layer exhibited a coarser grain structure due to the higher heat input and slower cooling rate, while the flux-cored wire GMAW layer showed a finer grain structure with better mechanical properties.

Hardness: The hardness measurements across the combined overlay layer showed uniform values within the expected range for stainless steel overlay materials. The hardness was consistent with the austenitic-ferritic microstructure composition.

Bending Performance: The bending test results demonstrated good ductility and formability of the combined overlay layer, indicating adequate metallurgical bonding between the overlay layers and with the base metal.

Intergranular Corrosion Resistance: The intergranular corrosion test results confirmed that the combined overlay layer meets the manufacturing requirements for hydrogenation reactors. The controlled carbon content and balanced microstructure provided adequate resistance to intergranular corrosion.

Standards and Specification Compliance

The hydrogenation reactor overlay welding must comply with relevant manufacturing standards and specifications. Key standards include:

The combined overlay welding approach described in the study is consistent with the requirements of these standards, particularly regarding the metallurgical quality, mechanical properties, and corrosion resistance of the overlay layer.

Engineering Practice Implications

The combined overlay welding method offers several advantages for hydrogenation reactor manufacturing. The strip electrode ESW first layer provides high deposition efficiency and low dilution, which is critical for achieving the required stainless steel composition on the 2.25Cr-1Mo base metal. The flux-cored wire GMAW second layer provides a fine microstructure with good mechanical properties and surface quality.

From a manufacturing efficiency perspective, the combined approach reduces the total number of welding passes compared to using a single method throughout. The ESW process can deposit thick layers rapidly, while the GMAW process provides the necessary surface quality and mechanical properties for the final layer. This combination can reduce production time and cost while maintaining or improving quality.

However, the combined approach also introduces additional complexity in terms of welding procedure qualification, process control, and quality assurance. The interface between the two layers must be carefully controlled to ensure adequate metallurgical bonding and property continuity. The transition zone between the coarse-grained ESW layer and the fine-grained GMAW layer requires careful monitoring to prevent defects such as lack of fusion or microstructural discontinuities.

Key Reflections and Study Insights

The most valuable contribution of this study is the demonstration that a combined overlay welding approach can meet the stringent requirements for hydrogenation reactor inner wall protection. The systematic evaluation of chemical composition, microstructure, mechanical properties, and corrosion resistance provides a comprehensive quality assessment framework that can be applied to similar applications.

One area for further development is the detailed characterization of the interface between the two overlay layers. The metallurgical bonding quality at this interface is critical for the long-term performance of the combined overlay layer, particularly under the cyclic thermal and mechanical loading conditions experienced by hydrogenation reactors. Additional research on the interface microstructure, residual stress distribution, and fatigue behavior would strengthen the engineering confidence in this combined approach.

The study also highlights the importance of process selection in overlay welding applications. Different welding methods offer different combinations of deposition rate, dilution rate, microstructure, and mechanical properties, and the optimal combination depends on the specific application requirements. The combined approach demonstrated here represents a practical solution to the trade-off between deposition efficiency and final layer quality.