Hardfacing Welding Process for 2.25Cr-1Mo Steel

Literature Overview

This paper by Ma Xiangfeng, Gao Lei, and Zhang Yingying from Liaoning Petrochemical University, published in Petrochemical Equipment in 2007 (Vol. 36, Suppl. B8, pp. 12-13), presents a hardfacing welding process development and validation study for 2.25Cr-1Mo steel. The study includes chemical composition analysis, ferrite determination, and metallographic examination of the hardfacing deposits to verify process feasibility.

Material Background

2.25Cr-1Mo steel (SA-387 Grade 22, equivalent to GB 150-III material) is one of the most widely used high-temperature pressure vessel steels in the petrochemical and power generation industries. Its composition and properties are:

The hardfacing of 2.25Cr-1Mo steel is typically required for:

• Corrosion-resistant overlay in hydrogen service.
• Wear-resistant overlay on flange faces and gasket surfaces.
• Repair of erosion damage in high-velocity flow areas.
• Transition between dissimilar materials in welded assemblies.

Process Development

Welding Procedure Specification

The hardfacing process development follows a systematic approach:

1. Base Metal Preparation: Surface cleaning to remove oxide, scale, and contaminants. A suitable groove geometry is prepared to ensure adequate fusion.
2. Preheating: 2.25Cr-1Mo steel is susceptible to cold cracking due to its moderate hardenability. Preheating to 200-300 °C is typically required to reduce hydrogen-induced cracking susceptibility and slow the cooling rate.
3. Filler Material Selection: The filler material must be compatible with the base metal in terms of:

• Thermal expansion coefficient.
• Mechanical properties.
• Resistance to cracking.
• Service environment compatibility.

1. Welding Parameters: Current, voltage, travel speed, and interpass temperature are optimized to achieve the desired deposit quality.
2. Post-Weld Heat Treatment: PWHT is typically required to relieve residual stresses and reduce hardness in the heat-affected zone.

Quality Verification Methods

The paper reports three primary verification methods:

Chemical Composition Analysis

The chemical composition of the hardfacing deposit is analyzed to verify that the filler material composition is maintained within acceptable limits after dilution with the base metal. Key elements monitored include carbon, chromium, molybdenum, manganese, and silicon. Excessive dilution can shift the deposit composition outside the specified range, affecting mechanical properties and service performance.

Ferrite Determination

Ferrite content measurement is critical for austenitic stainless steel overlays on 2.25Cr-1Mo steel. The delta ferrite content in the weld metal affects:

Ferrite determination is typically performed using a ferrite gauge (gamma-free ferrite meter) in accordance with ASTM E1024.

Metallographic Examination

Metallographic examination reveals:

• Microstructural homogeneity of the deposit.
• Grain size and morphology.
• Presence of defects (porosity, cracks, lack of fusion).
• Interface quality between deposit and base metal.
• Heat-affected zone characteristics.

Process Feasibility Assessment

The verification results confirm the feasibility of the developed hardfacing process for 2.25Cr-1Mo steel. Key findings include:

1. Chemical Composition: The deposit composition falls within the specified range, indicating that the dilution rate is acceptable and the process parameters are appropriate.
2. Ferrite Content: The delta ferrite content in the deposit is within the optimal range, indicating good resistance to hot cracking and adequate corrosion resistance.
3. Microstructure: The metallographic examination reveals a sound microstructure without significant defects, confirming the quality of the hardfacing deposit.

Engineering Practice Implications

For engineers specifying hardfacing on 2.25Cr-1Mo components, this study provides the following guidance:

1. Process Qualification: Hardfacing processes for 2.25Cr-1Mo steel must be qualified through trial welding and comprehensive testing before production application.
2. Heat Input Control: The heat input must be carefully controlled to balance dilution management with cracking prevention. Excessive heat input increases dilution and HAZ softening, while insufficient heat input risks incomplete fusion.
3. Interpass Temperature: Maintaining interpass temperature between 150-250 °C is critical to prevent hydrogen-induced cracking while avoiding excessive grain growth.
4. PWHT: Post-weld heat treatment at 700-730 °C for a duration proportional to wall thickness is essential to relieve residual stresses and reduce HAZ hardness.
5. Inspection Protocol: A combination of visual, magnetic particle, ultrasonic, and hardness testing should be employed to ensure deposit quality.

Study Insights and Reflections

This paper, while concise, addresses a fundamental engineering need in the petrochemical industry. The hardfacing of 2.25Cr-1Mo steel is a routine yet critical operation that requires careful process control to ensure reliable performance in demanding service environments. The systematic approach of verifying chemical composition, ferrite content, and microstructure provides a solid foundation for process qualification.

The study's emphasis on ferrite determination is particularly noteworthy, as delta ferrite content is often overlooked in hardfacing applications despite its significant impact on weld crack resistance and long-term service performance. Engineers should recognize that hardfacing process development is not merely about achieving the desired deposit composition but also about controlling the microstructural features that govern mechanical and metallurgical performance. The verification approach presented in this paper serves as a useful template for hardfacing process qualification on similar low-alloy and medium-alloy steels used in high-temperature pressure vessel applications.