Zhuojin Pipe Fitting Co., LtdHardfacing Process Research on Chromium-Nickel Austenitic Stainless Steel Materials
Literature Overview
The study by Zhang Yixia, Gao Junsong, and Chen Juhui, published in 2010 in "Materials Development and Application" (Vol. 25, No. 4, pp. 57-60), investigates the hardfacing of 15CrMoR steel plates using different welding consumables and processes. The authors, representing China Construction Second Engineering Bureau and the 725th Research Institute of China Shipbuilding Industry Corporation, conducted systematic hardfacing trials to determine whether E347L electrodes alone could provide an acceptable overlay on 15CrMoR substrate without an intermediate transition layer. Their conclusion is definitive: omitting the E309L transition layer and hardfacing directly with E347L on 15CrMoR is unreasonable and unacceptable from a metallurgical standpoint.
Core Technical Findings
The 15CrMoR steel is a low-alloy martensitic steel commonly used in pressure vessel applications under high-temperature and high-pressure conditions. Its microstructure consists primarily of tempered martensite with carbide precipitates, giving it moderate hardness and good creep resistance. When overlaying austenitic stainless steel directly onto this substrate, several critical metallurgical challenges arise:
| Parameter | E347L Direct Hardfacing | E309L Transition + E347L Overlay |
|---|---|---|
| Dilution rate | High (excessive base metal mixing) | Controlled at first layer, low at subsequent layers |
| Fusion line microstructure | Wide martensitic zone, possible brittle phases | Narrow transition zone, controlled dilution |
| Cracking susceptibility | High due to thermal stress concentration | Significantly reduced |
| Corrosion resistance of overlay | Compromised by dilution | Maintained as designed |
The fundamental issue is that E347L, being a Ni-Cr stabilized austenitic stainless steel, is designed for weld metal applications where dilution is minimal. When used as the first layer directly on a low-alloy steel substrate, the high dilution rate introduces excessive carbon and manganese from the base metal into the weld metal. This leads to the formation of hard, brittle carbides at the fusion line and creates a zone of high residual stress due to the mismatch in thermal expansion coefficients between the ferritic substrate and austenitic overlay.
Metallurgical Analysis of the Transition Layer Requirement
The E309L transition layer serves a critical metallurgical function that cannot be bypassed. E309L is a high-nickel austenitic stainless steel specifically designed for dissimilar metal welding applications. Its high nickel content (typically 22-27%) provides several advantages:
- It dilutes the carbon content from the base metal to a level that prevents harmful carbide precipitation at the fusion line.
- Its high ductility accommodates the thermal stresses that develop during cooling, preventing cracking.
- It creates a metallurgical bridge between the ferritic substrate and the more austenitic overlay, reducing the thermal expansion coefficient gradient.
The paper's conclusion that E347L alone is unreasonable is consistent with established welding metallurgy principles. In dissimilar metal welding, the transition layer is not optional but essential when the base metal and overlay metal have significantly different compositions. The 15CrMoR steel contains approximately 0.2-0.3% carbon and 0.9-1.1% chromium, while E347L requires a minimum of 17% chromium and 9% nickel to maintain its austenitic structure and corrosion resistance.
Engineering Practice Implications
For engineers working on pressure vessel and piping applications involving dissimilar metal overlays, this paper reinforces several critical practices:
- Always use a compatible transition layer when overlaying austenitic stainless steel onto low-alloy or carbon steel substrates.
- The transition layer should be E309L or equivalent for most low-alloy steel substrates, as it provides the necessary nickel dilution capacity.
- Hardfacing process parameters must be controlled to minimize base metal dilution, particularly heat input and travel speed.
- Post-weld heat treatment may be required to relieve residual stresses in the overlay system.
The practical consequence of ignoring the transition layer requirement is that the overlay may appear acceptable during initial inspection but fail prematurely under service conditions. The wide martensitic zone at the fusion line becomes a crack initiation site, and the compromised corrosion resistance leads to intergranular attack or pitting in corrosive environments. This is particularly dangerous in pressure vessel applications where failure can have catastrophic consequences.
Study Insights and Reflections
This paper, while seemingly straightforward in its conclusion, addresses a fundamental principle in dissimilar metal welding that is sometimes overlooked in field practice. The tendency to simplify procedures by eliminating transition layers, often driven by cost or schedule pressures, can lead to serious quality issues. The 725th Research Institute's involvement in this study underscores the importance of metallurgical research in supporting engineering decisions. Engineers should always consult applicable codes and standards, such as ASME B31.3 and NB/T standards for nuclear applications, which typically mandate transition layers for dissimilar metal welds. The systematic approach taken by the authors, testing multiple configurations and documenting the metallurgical consequences, provides a model for how hardfacing procedures should be qualified in engineering practice.