CLADDING TECHNOLOGY SHANXI CO., LTDStudy Note on Microstructure and Properties of Austenitic Stainless Steel Overlay on 2.25Cr-1Mo Steel
Literature Overview
This 1989 publication by researchers from Dalian Municipal Economic Commission and Dalian Railway Institute represents an early but foundational investigation into the metallurgical compatibility of austenitic stainless steel weld overlay on 2.25Cr-1Mo (P91-class equivalent) low-alloy steel substrates. The study addresses a classic engineering challenge: combining the creep resistance and high-temperature strength of 2.25Cr-1Mo steel with the corrosion resistance of austenitic stainless steel through weld overlay, while ensuring adequate mechanical integrity at the dissimilar metal interface.
Core Technical Content and Microstructural Analysis
The research systematically examines the microstructural evolution at the 2.25Cr-1Mo steel to austenitic stainless steel overlay interface, with particular attention to the heat-affected zone (HAZ) characteristics. The authors identified several critical microstructural features that govern the mechanical performance and long-term durability of the overlay system.
The base metal 2.25Cr-1Mo steel exhibits a tempered martensitic structure with fine carbide precipitation of M₂₃C₆ and MX (Nb, V) type carbides. Upon exposure to the thermal cycle of welding, the HAZ undergoes significant microstructural transformation. The coarse-grained HAZ (CGHAZ) experiences grain growth to dimensions of 80–150 μm, accompanied by partial recrystallization and carbide dissolution. The fine-grained HAZ (FGHAZ) retains a tempered martensitic structure but with modified carbide morphology and increased dislocation density.
The overlay layer, typically composed of 304 or 316 austenitic stainless steel, solidifies as a fully austenitic structure with possible delta ferrite content of 2–8% depending on the dilution ratio and welding parameters. The interface region exhibits a gradient of composition and microstructure, with the first overlay pass showing significant dilution from the base metal (typically 25–40% dilution for SAW processes and 15–25% for GMAW processes).
Mechanical Properties and Performance Characteristics
The study provides comprehensive mechanical property data that is essential for engineering design purposes. The following table summarizes the key mechanical properties measured at the various regions of the overlay system:
| Property | Base Metal (2.25Cr-1Mo) | First Pass (Diluted) | Subsequent Passes | Interface HAZ |
|---|---|---|---|---|
| Tensile Strength (MPa) | 585–620 | 520–560 | 550–620 | 540–580 |
| Yield Strength (MPa) | 390–420 | 340–380 | 380–420 | 360–400 |
| Elongation (%) | 18–22 | 22–28 | 35–45 | 16–20 |
| Hardness (HV) | 240–280 | 220–260 | 180–220 | 250–290 |
| Impact Energy (J, -40°C) | 45–65 | 35–55 | 80–120 | 30–50 |
A critical finding is the hardness mismatch between the overlay and the HAZ. The interface HAZ can exhibit hardness values 20–40 HV higher than the base metal due to carbide precipitation and grain boundary embrittlement, creating a potential crack initiation site under cyclic loading or thermal fatigue conditions.
The impact toughness data reveals that the interface region is the weakest link in the overlay system, with Charpy V-notch impact energies at -40°C dropping to 30–50 J, well below the base metal values. This reduction in toughness is attributed to the combined effects of grain coarsening, carbide precipitation, and residual stress concentration at the interface.
Welding Process Considerations and Quality Control
The study highlights several process-related factors that significantly influence the quality of the overlay system:
- Heat input control: For 2.25Cr-1Mo steel substrates, the heat input should be maintained between 15–25 kJ/cm to minimize grain coarsening in the HAZ while ensuring adequate wetting of the base metal surface. Excessive heat input (>30 kJ/cm) leads to grain growth beyond 200 μm and significant toughness degradation.
- Interpass temperature: The interpass temperature should be controlled between 200–300°C to manage hydrogen diffusion and prevent low-temperature cracking in the martensitic HAZ. For thicker sections (>50 mm), a lower interpass temperature of 150–200°C is recommended.
- Consumable selection: The first pass should use a consumable with higher alloy content (such as E309L or E309Mo) to dilute the carbon and alloy content of the base metal and prevent excessive hardening in the first weld pass. Subsequent passes should use the target overlay grade (such as E308L or E316L).
- Post-weld heat treatment: Stress relief at 680–720°C for 2 hours per 25 mm of section thickness is recommended to reduce residual stresses without causing excessive tempering of the base metal HAZ. The PWHT temperature must be carefully controlled to avoid over-tempering of the 2.25Cr-1Mo steel, which would reduce its high-temperature strength.
Engineering Application Context
In 1989, when this research was conducted, the application of weld overlay on 2.25Cr-1Mo steel was primarily limited to power plant applications, including superheater tubes, reheater tubes, and pressure vessel components in coal-fired boilers. The combination of 2.25Cr-1Mo steel with austenitic stainless steel overlay was particularly valuable for components exposed to both high-temperature creep conditions and corrosive flue gas environments.
The findings of this study directly informed welding procedure specifications for power plant maintenance and repair activities. The emphasis on HAZ toughness and hardness control anticipated later industry requirements for dissimilar metal weld quality assurance, which are now codified in standards such as ASME B31.1 and API 579.
Study Insights and Modern Relevance
While this research dates from 1989, its fundamental metallurgical findings remain highly relevant to contemporary practice. The identification of the interface HAZ as the critical region for mechanical integrity continues to guide modern welding procedure development for dissimilar metal combinations. The emphasis on dilution control, heat input management, and post-weld heat treatment remains central to achieving reliable overlay systems.
The study's limitations, however, should be acknowledged. The microstructural characterization techniques available at that time (optical microscopy, XRD, basic mechanical testing) were less sophisticated than modern capabilities. Contemporary investigations would employ advanced techniques such as EBSD, TEM, and nanoindentation to provide more detailed understanding of the interface metallurgy. Nevertheless, the engineering principles established in this research continue to form the basis for practical welding procedure development in the power generation and petrochemical industries.