Transition Layer Weld Overlay Process Study for 14Cr1MoR Welding

Literature Overview

The paper by Zou Yuqing (2013) from Jilin Electronic Information Vocational and Technical College investigates the transition layer weld overlay process used in the welding of 14Cr1MoR steel, a chromium-molybdenum low-alloy steel widely used in pressure vessel fabrication. 14Cr1MoR is a Chinese standard designation for a steel with approximately 1.25% Cr, 0.5% Mo, and 0.3% C, equivalent to P91 or 9Cr-1Mo steels in Western standards. The steel is used in high-temperature pressure vessels and heat exchangers operating at temperatures up to 600°C, where its excellent creep strength and oxidation resistance are essential.

Technical Background

14Cr1MoR steel is a martensitic ferritic steel that is supplied in the quenched and tempered condition. Its microstructure consists of tempered martensite with fine precipitates of chromium carbides and molybdenum carbides. The steel has a high hardenability and is susceptible to cold cracking during welding if the cooling rate is not properly controlled. When welding 14Cr1MoR to dissimilar materials such as carbon steel, stainless steel, or nickel-based alloys, a transition layer is often required to:

• Reduce the dilution of alloying elements from the base metal into the weld metal.
• Prevent excessive hardening and cracking in the weld zone.
• Ensure a smooth transition in thermal expansion coefficients between the two materials.
• Provide a compatible microstructure that can accommodate the thermal stresses during service.

Transition Layer Design

The transition layer is typically deposited as a single or double pass weld between the base metal and the final weld metal. The composition of the transition layer is designed to be intermediate between the base metal and the final weld metal, gradually bridging the composition gap. For 14Cr1MoR welded to carbon steel, the transition layer might be a low-alloy steel with 0.5-1.0% Cr and 0.2-0.4% Mo, while for welding to stainless steel, the transition layer might be a duplex stainless steel or a nickel-based alloy.

Process Parameters and Welding Procedure

The paper presents a detailed study of the welding procedure for the transition layer on 14Cr1MoR. The following table summarizes the key process parameters:

The preheating temperature is critical for 14Cr1MoR because the steel has a high hardenability and is susceptible to cold cracking. A preheating temperature of 200-250°C ensures that the cooling rate is sufficiently low to avoid martensitic transformation in the HAZ. The interpass temperature must be maintained within the specified range to prevent excessive grain growth and to ensure adequate toughness.

Microstructural Analysis

The paper includes metallographic analysis of the transition layer and the HAZ. The following observations were made:

• The HAZ of 14Cr1MoR exhibits a tempered martensitic structure with fine precipitates, similar to the base metal but with slightly coarser grains due to the thermal cycle of welding.
• The transition layer weld metal shows a mixture of ferrite and austenite phases, with the austenite content depending on the composition of the transition layer material.
• The interface between the transition layer and the final weld metal is clean and free of defects, indicating good metallurgical compatibility.
• The hardness profile across the weld shows a gradual transition from the base metal hardness (250-300 HV) through the transition layer (200-250 HV) to the final weld metal (150-200 HV), indicating a smooth hardness gradient.

Defect Analysis and Quality Control

The following table summarizes the common defects encountered during transition layer welding of 14Cr1MoR and the corresponding countermeasures:

The quality control of the transition layer weld includes visual inspection, magnetic particle testing (MT), ultrasonic testing (UT), and hardness testing. The weld must be free of cracks, lack of fusion, and porosity exceeding the acceptance criteria specified in the applicable code. The hardness of the HAZ must not exceed 350 HV (or the specified limit) to ensure adequate toughness.

Post-Weld Heat Treatment

Post-weld heat treatment (PWHT) is essential for 14Cr1MoR weldments to relieve residual stresses and to temper the martensitic HAZ. The PWHT is typically performed at 720-750°C for 2-4 hours, followed by controlled cooling. The paper emphasizes that the PWHT must be performed after the entire weld sequence is complete, including the transition layer and the final weld metal, to ensure uniform stress relief. The cooling rate during PWHT must be controlled to avoid re-hardening of the HAZ.

Study Insights and Reflections

This paper provides a thorough analysis of the transition layer welding process for 14Cr1MoR, a material that is increasingly used in high-temperature pressure vessel applications. The study highlights the importance of process control in welding dissimilar materials, particularly the need for careful selection of the transition layer composition and the welding parameters. The findings are directly applicable to the fabrication of heat exchangers, superheater tubes, and other high-temperature pressure equipment where 14Cr1MoR is used in combination with other materials.

The paper also underscores the importance of post-weld heat treatment in ensuring the long-term performance of 14Cr1MoR weldments. The PWHT not only relieves residual stresses but also tempers the martensitic HAZ, improving toughness and reducing the risk of delayed cracking. The study reinforces the need for rigorous quality control, including non-destructive testing and mechanical property verification, to ensure that the weld meets the required performance criteria.