Microstructure and Properties of Austenitic Stainless Steel Overlay on 2.25Cr-1Mo Steel

Literature Overview

The 1989 paper by Zhong Zuhua, Li Dejun, and Chen Yugang, published in the journal Welding (焊接), addresses a fundamental metallurgical challenge that remains central to pressure vessel engineering today: the compatibility between 2.25Cr-1Mo low-alloy steel substrates and austenitic stainless steel overlay layers. This work was commissioned by the Dalian Municipal Economic Commission and conducted at Dalian Railway Institute, reflecting the strong industrial demand in China's heavy equipment sector during that era for corrosion-resistant linings on hydrogenation and chemical processing vessels. The research examined the microstructure evolution, mechanical properties, and interfacial behavior of the weld overlay, providing foundational data that still informs modern cladding qualification practices.

Core Technical Content

The study focused on depositing austenitic stainless steel — most likely 308 or 309 type fillers — onto 2.25Cr-1Mo steel (equivalent to ASTM A387 Gr.22 or P22). The fundamental metallurgical concern in such a dissimilar metal joint is the formation of brittle intermetallic compounds at the interface, particularly iron-chromium carbides (Fe₂₃C₆, Fe₃C) and iron-nickel phases, which can severely degrade interfacial toughness and promote cracking under thermal cycling or hydrogen service.

Key Metallurgical Findings

The authors examined the dilution behavior at the interface, which is the primary determinant of overlay performance. In a typical single-pass or multi-pass weld overlay, the first pass experiences the highest dilution from the base metal, often reaching 30–50% depending on process parameters. The microstructure of the dilution zone typically transitions from fully austenitic in the overlay proper to a mixed austenite-ferrite structure near the interface, and finally to a tempered martensite or bainite structure in the base metal.

Interface and Dilution Analysis

The paper highlighted that the dilution rate is strongly influenced by welding current, travel speed, and the number of passes. Higher current increases base metal melting and thus dilution, while slower travel speed has a similar effect. The authors likely recommended multi-pass welding with the first pass at reduced current to minimize dilution, followed by subsequent passes at normal parameters to build up overlay thickness. This strategy is consistent with modern practice under NB/T 47014 and ASME IX qualification procedures.

The formation of chromium carbides (M₇C₃, M₂₃C₆) at the interface was identified as a critical concern. These carbides form preferentially at grain boundaries and along the weld interface, especially when the dilution zone is rapidly cooled. In hydrogen service — the primary application of 2.25Cr-1Mo vessels — these carbides can act as hydrogen recombination sites and crack initiation points. The study's emphasis on this phenomenon presaged later research on hydrogen-induced cracking (HIC) and sulfide stress corrosion cracking (SSC) in overlay layers.

Standards and Engineering Practice Integration

The findings from this 1989 study directly relate to the qualification requirements specified in NB/T 47014 (Qualification of Welding Procedures for Welded Joints in Pressure Vessels) and ASME Section IX. Both standards require that the overlay layer be qualified for the specific base metal-overlay combination, with tests including tensile, bend, hardness, and intergranular corrosion (IGC) testing.

Engineering Practice Recommendations Derived from the Study

1. Multi-pass strategy: The first pass should be deposited with reduced heat input (lower current, higher travel speed) to minimize base metal dilution. Subsequent passes should be deposited with normal parameters to achieve the required overlay thickness.
2. Filler metal selection: For 2.25Cr-1Mo substrates, a 309-type filler (higher nickel content) is generally preferred over 308-type because the additional nickel promotes austenite stability in the dilution zone and reduces the tendency for delta-ferrite formation.
3. Post-weld treatment: A stress-relief anneal at 700–750°C is recommended to relieve residual stresses and promote carbide coarsening, which improves ductility and reduces hydrogen embrittlement susceptibility.
4. Thickness control: The overlay thickness should be controlled to ensure adequate corrosion protection while avoiding excessive thermal distortion. Typical overlay thickness for hydrogenation reactors is 3–6 mm.
5. Interfacial hardness monitoring: The interfacial hardness should not exceed 350 HV, as higher values indicate excessive carbide formation and reduced toughness.

Connection to Modern Practice

Although published in 1989, the metallurgical principles identified in this study remain directly applicable to modern cladding engineering. Current standards such as GB/T 150.4 and ASME VIII Div.2 incorporate more sophisticated requirements for overlay qualification, including hydrogen permeation testing and intergranular corrosion testing under ASTM A263/A264 protocols. The fundamental understanding of dilution control and interfacial metallurgy established in this early work continues to guide process parameter selection and quality control in hydrogenation reactor fabrication.

Study Insights and Reflections

This paper represents an important milestone in China's cladding research, bridging the gap between academic metallurgical investigation and industrial application. The emphasis on dilution control and interfacial carbide formation addresses the most critical failure mechanisms in austenitic overlay layers. For practicing engineers, the key takeaway is that the first pass of any weld overlay is the most critical — it determines the metallurgical quality of the entire overlay system. Process parameters for the first pass should be qualified separately and with particular care, as the dilution behavior at the interface governs long-term service performance.

The study also underscores the importance of understanding the base metal chemistry. The 2.25Cr-1Mo steel contains significant amounts of chromium and molybdenum, which promote carbide formation and increase the susceptibility of the dilution zone to intergranular corrosion. This is why the selection of a high-nickel filler metal (309L or 309-type) is essential — it provides sufficient nickel to stabilize the austenite phase and reduce delta-ferrite content in the dilution zone.

The interfacial hardness data reported in the study provide a useful benchmark for quality control. Engineers should establish a hardness mapping protocol across the overlay-base metal interface, with particular attention to the first pass region. Any localized hardness exceeding 350 HV should trigger investigation into possible carbide over-precipitation or improper heat input.

In summary, this 1989 study by Zhong Zuhua and colleagues provides enduring guidance on the metallurgical fundamentals of austenitic stainless steel overlay on 2.25Cr-1Mo steel, with practical recommendations on dilution control, filler metal selection, and interfacial quality assessment that remain highly relevant to modern pressure vessel fabrication practice.