Zhuojin Pipe Fitting Co., LtdStud Submerged Arc Overlay Welding of Inconel 625 on 12Cr2Mo1 Forging
Literature Overview
Published in Chemical Engineering Equipment and Piping (2024, Vol. 61, Issue 1, pp. 24–30), this paper by Zhan Lining and colleagues from the Hebei Provincial Special Equipment Supervision and Inspection Institute and the Hebei Provincial Pressure Vessel Inspection and Evaluation Technology Innovation Center, presents a comprehensive study on the stud submerged arc overlay welding of Inconel 625 on 12Cr2Mo1 forged components. The research addresses the quality assurance of overlay welding for heat exchanger tube sheets in high-pressure gas and circulating hydrogen service. The study includes analysis of the weldability of both the base material and the overlay material, development of a welding procedure qualification (WPQ) test, and evaluation of the overlay layer's chemical composition, bending properties, hardness, microstructure, and corrosion resistance. The actual production application on a heat exchanger tube sheet, which has been in stable operation for three years without welding or corrosion defects, provides strong validation of the proposed welding procedure.
Core Technical Findings
The study provides detailed quantitative data on the overlay welding process and results:
- Welding consumables: The EQ62-50 electrode strip combined with ES200 flux was selected as the optimal consumable combination for the overlay welding of Inconel 625 on 12Cr2Mo1. This combination provides adequate alloying element transfer, good slag coverage, and stable arc characteristics.
- Welding technique: A combination of circumferential and straight-line overlay welding was employed to deposit the Inconel 625 layer on the tube sheet. This technique ensures uniform coverage and minimizes the risk of incomplete fusion or lack of penetration.
- Corrosion resistance: The as-welded overlay layer exhibited a corrosion rate of 1.50 mm/a, while the solution-treated overlay layer had a corrosion rate of 1.17 mm/a. The ratio of as-welded to solution-treated corrosion rates was 1.28, which satisfies the requirement of JB/T 4756—2006 that specifies a ratio less than or equal to 1.5.
- Mechanical properties: The overlay layer met the requirements for chemical composition, bending properties, and hardness as specified in the design documents. The microstructure was examined to ensure the absence of deleterious phases such as sigma phase or Laves phase, which can form in nickel-based alloys during welding or heat treatment.
| Parameter | Specification | Test Result |
|---|---|---|
| Consumables | EQ62-50 / ES200 | Qualification passed |
| As-welded corrosion rate | ≤ 1.5 mm/a (implied) | 1.50 mm/a |
| Solution-treated corrosion rate | ≤ 1.2 mm/a (implied) | 1.17 mm/a |
| Corrosion rate ratio | ≤ 1.5 (JB/T 4756—2006) | 1.28 |
| Bending properties | Per design specification | Qualified |
| Hardness | Per design specification | Qualified |
| Service life | Stable operation | 3 years, no defects |
Interpretation of Technical Points
The overlay welding of Inconel 625 on 12Cr2Mo1 is a challenging metallurgical problem due to the significant differences in thermal conductivity, coefficient of thermal expansion, and chemical composition between the two materials. 12Cr2Mo1 is a low-alloy steel with moderate strength and good high-temperature performance, while Inconel 625 is a nickel-chromium-molybdenum superalloy with excellent corrosion resistance and high-temperature strength. The dissimilar joint between these materials is susceptible to several failure modes, including:
- Hot cracking: The high sulfur and phosphorus content in the base metal can segregate to the grain boundaries of the weld metal during solidification, promoting hot cracking. The use of low-sulfur consumables and appropriate welding parameters can mitigate this risk.
- Cold cracking: The high carbon equivalent of 12Cr2Mo1 and the hydrogen sensitivity of the nickel-based overlay material can lead to hydrogen-induced cold cracking. Preheating, low-hydrogen consumables, and post-weld heat treatment are essential to prevent this failure mode.
- Dilution and composition control: The dilution of the Inconel 625 overlay layer by the 12Cr2Mo1 base metal can alter the chemical composition of the weld metal, potentially reducing corrosion resistance. The welding procedure must be designed to minimize dilution while ensuring adequate fusion.
- Phase stability: Nickel-based alloys are susceptible to the formation of deleterious intermetallic phases, such as sigma phase (Cr23C6) and Laves phase (NiMo), during welding or prolonged exposure to elevated temperatures. These phases can reduce ductility and corrosion resistance. The microstructural examination of the overlay layer is critical to ensure the absence of these phases.
The corrosion rate ratio of 1.28 between the as-welded and solution-treated conditions is a key performance indicator. This ratio indicates that the solution treatment effectively reduces the corrosion rate, likely by dissolving any precipitated carbides or intermetallic phases that could act as initiation sites for localized corrosion. The fact that the ratio is well below the 1.5 limit specified by JB/T 4756—2006 provides confidence in the long-term corrosion resistance of the overlay layer.