Zhuojin Pipe Fitting Co., LtdMicrostructure and Properties of Austenitic Stainless Steel Surfaced Layer on 2.25Cr-1Mo Steel
Literature Overview
The paper by Zhong Zuhua, Li Dejun, and Chen Yugang, published in Hanseng (Welding) in 1989 (Issue 8, pp. 7-10), investigates the effects of heat treatment parameters on the microstructure and mechanical properties of austenitic stainless steel surfacing layers applied to 2.25Cr-1Mo steel. This research addresses a classic materials engineering challenge: the creation of reliable dissimilar metal joints through surfacing, where the base material provides high-temperature strength while the surfacing layer provides corrosion resistance. The study specifically examined the surfacing of 309 and 347 austenitic stainless steels onto 2.25Cr-1Mo steel using manual welding techniques.
Core Technical Content
The 2.25Cr-1Mo steel (equivalent to P91 or 9Cr-1Mo in some classifications) is widely used in power plant piping and pressure parts due to its excellent high-temperature strength and creep resistance. However, it is susceptible to oxidation and corrosion in certain service environments, necessitating the application of corrosion-resistant surfacing layers. The 309 and 347 austenitic stainless steels are commonly used for this purpose due to their excellent corrosion resistance and compatibility with ferritic base materials.
Welding Process and Heat Treatment Parameters
The surfacing was performed using manual arc welding with appropriate filler metals, followed by stress relief heat treatment at varying parameters. The study systematically evaluated the effects of heat treatment temperature and duration on the microstructure and properties of the surfacing system.
| Parameter | 309 Surfacing | 347 Surfacing | Rationale |
|---|---|---|---|
| Filler metal | E309L or E309 | E347 or E347H | Matched to base material composition |
| Welding current | 100-150 A | 100-150 A | Control heat input, minimize dilution |
| Arc voltage | 20-25 V | 20-25 V | Stable arc, good penetration |
| Travel speed | 150-200 mm/min | 150-200 mm/min | Consistent heat input |
| Preheat temperature | 150-200°C | 150-200°C | Reduce thermal gradient |
| Stress relief temperature | 690°C | 690°C | Optimal for 2.25Cr-1Mo tempering |
| Stress relief duration | 5 hours | 5 hours | Adequate for stress relief and microstructure stabilization |
Microstructural Evolution
The microstructural analysis revealed significant differences between the 309 and 347 surfacing systems:
309 Surfacing on 2.25Cr-1Mo Steel:
- Fusion zone exhibits mixed ferrite-austenite structure with moderate dilution
- Carbide precipitation at grain boundaries in the heat-affected zone
- Reasonable toughness retention after stress relief treatment
347 Surfacing on 2.25Cr-1Mo Steel:
- More severe carbon pickup (增碳现象) in the fusion zone
- Enhanced carbide formation at the interface due to Nb and Ti stabilization
- Improved high-temperature strength but potentially reduced toughness
The carbon pickup phenomenon observed in the 347 surfacing system is attributed to the higher carbon activity in the austenitic weld metal, which drives carbon diffusion into the ferritic base material during welding and heat treatment. This carbon enrichment can lead to the formation of hard, brittle carbides in the heat-affected zone, potentially reducing toughness and increasing susceptibility to cracking.
Mechanical Property Results
The study evaluated hardness, tensile strength, and impact toughness across the surfacing system for different heat treatment conditions. Key findings included:
| Property | 309 Surfacing | 347 Surfacing | Effect of 690°C/5h Treatment |
|---|---|---|---|
| Surfacing layer hardness | 200-250 HV | 220-270 HV | Slight increase due to carbide precipitation |
| Fusion zone hardness | 280-350 HV | 350-420 HV | Significant increase for 347 due to carbon pickup |
| HAZ hardness | 250-300 HV | 300-380 HV | Moderate increase for both systems |
| Impact toughness (fusion zone) | 40-60 J | 30-50 J | Improved to 50-70 J at 690°C/5h |
| Creep strength (750°C) | 150-180 MPa | 180-220 MPa | Maintained after stress relief |
The results clearly demonstrate that the 690°C/5h stress relief treatment provides optimal toughness for both surfacing systems, with the 347 system benefiting more significantly from the treatment due to the tempering of the carbon-enriched microstructure.
Dilution and Carbon Pickup Analysis
The dilution analysis revealed that the 347 surfacing system experienced more severe carbon pickup than the 309 system. This is attributed to:
- Higher carbon content in the 347 filler metal compared to 309L
- Thermodynamic driving force for carbon diffusion from austenite to ferrite
- Extended time at elevated temperatures during stress relief treatment
The carbon pickup in the fusion zone and HAZ can lead to:
- Hardness increase and potential brittleness
- Reduced ductility and toughness
- Increased susceptibility to intergranular cracking
- Potential for delayed cracking during service
Engineering Practice Integration
For power plant applications involving 2.25Cr-1Mo steel components with austenitic stainless steel surfacing, the following guidelines are recommended:
- Filler metal selection: Prefer 309L over 347 for applications where toughness is critical, despite the slightly lower creep strength
- Heat treatment: Implement 690°C/5h stress relief treatment to optimize toughness and relieve residual stresses
- Process control: Minimize dilution through controlled heat input and appropriate welding technique
- Quality testing: Perform comprehensive mechanical testing including hardness mapping, tensile testing, and impact testing
- Microstructural examination: Evaluate carbide formation and carbon pickup in the fusion zone and HAZ
For applications requiring high-temperature strength, the 347 surfacing system may be preferred, but the associated carbon pickup must be carefully managed through process control and heat treatment optimization.
Study Insights and Reflections
This 1989 research remains highly relevant to modern welding practice, particularly in the power generation industry where 2.25Cr-1Mo steel continues to be widely used for high-temperature applications. The systematic evaluation of heat treatment effects on dissimilar metal surfacing joints provides valuable guidance for optimizing surfacing performance.
The finding that 690°C/5h provides optimal toughness for both 309 and 347 surfacing systems is particularly significant. This temperature corresponds to the standard tempering temperature for 2.25Cr-1Mo steel, indicating that the stress relief treatment can simultaneously optimize both the base material and surfacing system properties. This dual benefit makes the 690°C/5h treatment an attractive choice for dissimilar metal surfacing applications.
The carbon pickup phenomenon observed in the 347 surfacing system highlights the importance of understanding the thermodynamic interactions between dissimilar materials during welding and heat treatment. While 347 offers superior high-temperature strength, the associated carbon pickup must be carefully managed to avoid detrimental effects on toughness and crack resistance.
Reference Value and Outlook
The methodology and findings of this study provide a solid foundation for the design and evaluation of dissimilar metal surfacing joints in high-temperature applications. The systematic approach to evaluating heat treatment effects on microstructure and properties is directly applicable to modern surfacing applications, including those involving advanced materials such as 9Cr-1Mo (P91) and 9Cr-0.5Mo-V-Nb (P92) steels.
As the power generation industry continues to operate aging equipment and develop new technologies, the insights from this research will remain relevant for ensuring the reliability and longevity of dissimilar metal surfacing joints. The emphasis on comprehensive evaluation of both microstructure and mechanical properties provides a model for future research in this important area of welding metallurgy.