Effect of Pre-Welded Transition Layers on Microstructure and Properties of 12Cr2Mo1R-S30408 Dissimilar Steel Joints

Literature Overview

This study, published in Materials Reports (2022, Vol. 36, No. 12, pp. 145-149) by Liu Guangyin and colleagues from Lanzhou University of Technology and Lanzhou Lanchuan Heavy Equipment Co., Ltd., addresses a persistent challenge in power generation and petrochemical equipment fabrication: the dissimilar steel welding of thick-section low-alloy heat-resistant steel (12Cr2Mo1R) to austenitic stainless steel (S30408). The research proposes pre-apply 2-3 mm thick transition overlay layers using three different consumables—E309L, ENiCrFe-3, and ENiCrMo-3—on the 12Cr2Mo1R side, followed by a hybrid welding sequence of GTAW root pass and SAW filling/capping passes. The work is supported by the National Natural Science Foundation of China (Grant No. 51775256).

Core Technical Content and Methodology

The fundamental difficulty in welding 12Cr2Mo1R to S30408 lies in the significant disparity in thermal expansion coefficients, carbon potential, and dilution behavior between the two base metals. Without an appropriate transition strategy, the weld metal is subject to carbon migration toward the austenitic side, intergranular embrittlement at the fusion line, and formation of brittle intermetallic phases. The authors adopted a three-step methodology: (1) pre-overlay of transition layers on the 12Cr2Mo1R side using SMAW; (2) root welding by GTAW and filling/capping by SAW; (3) comprehensive characterization including optical microscopy (OM), scanning electron microscopy (SEM), electron probe microanalysis (EPMA), microhardness profiling, and tensile testing.

Transition Layer Consumable Selection

The three transition layer consumables represent progressively different alloy compositions:

The selection logic follows a classic engineering principle: as the dilution ratio from the ferritic base metal increases, the transition layer must contain progressively higher Ni and alloying elements to maintain sufficient austenite stability and avoid formation of detrimental intermetallic phases at the interface.

Key Findings on Microstructure and Elemental Distribution

Interface Chemistry

EPMA analysis revealed significant concentration gradients of Cr, Ni, and Fe at the 12Cr2Mo1R/overlay interface. This is expected due to the large compositional difference between the base metals and the transition layers. The carbon enrichment phenomenon at grain boundaries near the fusion line is a critical finding. Carbon migration toward the austenitic side is thermodynamically driven by the lower carbon solubility in austenite compared to ferrite at elevated temperatures. This carbon enrichment can lead to sensitization and intergranular corrosion susceptibility in the heat-affected zone.

Carbide Precipitation Behavior

A particularly important observation is the precipitation of M₂₃C₆ carbides in the ENiCrMo-3 overlay layer. M₂₃C₆ is a chromium-rich carbide that typically forms at grain boundaries during slow cooling or in the temperature range of 500-800°C. Its presence indicates that the Mo-containing transition layer, while providing good mechanical properties, may be susceptible to sensitization if not properly heat-treated. The E309L and ENiCrFe-3 layers did not show significant M₂₃C₆ precipitation, suggesting better resistance to carbide formation under the tested conditions.

Ferrite Content Comparison

The ferrite content in the weld metal of the E309L overlay was found to be higher than in the ENiCrMo-3 and ENiCrFe-3 overlays. This is consistent with the lower Ni content of E309L, which provides less austenite stabilization. Excessive ferrite in austenitic weld metals can lead to cracking sensitivity, while insufficient ferrite increases susceptibility to solidification cracking. The optimal ferrite content for austenitic stainless steel weld metals is typically 5-15% delta ferrite, as assessed by the Ferrite Number (FN).

Mechanical Properties and Fracture Analysis

Tensile Performance

All three transition layers produced joints with ductile fracture characteristics, as evidenced by the dimpled fracture morphology. This is a positive indicator that none of the transition strategies introduced brittle intermetallic phases at the interface in sufficient quantity to cause catastrophic failure.

Hardness Distribution

The maximum hardness at the interface between the overlay layer and the base metal increased in the order E309L < ENiCrFe-3 < ENiCrMo-3, and decreased toward the weld center in all cases. The hardness gradient is attributed to:

• Higher alloy content in ENiCrMo-3 and ENiCrFe-3 providing solid solution strengthening
• Potential microstructural hardening at the fusion line due to rapid solidification
• Carbide precipitation contributing to localized hardening in Mo-containing layers

The hardness progression reflects the trade-off between strength and toughness: higher hardness at the interface correlates with lower elongation, as observed in the tensile data.

Engineering Practice Integration

Application Context

12Cr2Mo1R is a widely used material for high-pressure boiler tubes, reactor shells, and heat exchanger components operating at 450-580°C in power generation and petrochemical applications. S30408 (equivalent to 304/304L) is used for corrosion-resistant components in the same systems. Dissimilar welds between these materials are common in:

• Boiler-to-condenser connections
• Reactor internals with austenitic heat exchanger tubes
• High-pressure pipelines with stainless steel spools

Process Recommendations

Based on this study and general engineering experience, the following process parameters and considerations are recommended:

1. Pre-overlay thickness: 2-3 mm is sufficient to establish a transition zone without excessive material cost
2. Heat input control: Limit SAW heat input to 20-30 kJ/mm to minimize HAZ softening in 12Cr2Mo1R
3. Preheat temperature: 150-200°C for 12Cr2Mo1R to reduce hydrogen-induced cracking risk
4. Interpass temperature: Maintain 150-250°C to control cooling rate and avoid excessive carbide precipitation
5. Post-weld heat treatment (PWHT): 700-720°C for 2 hours per 25 mm thickness to relieve residual stresses and stabilize the microstructure

Material Selection Guidance

Key Questions and Reflections

The study raises several important questions for further investigation:

1. Long-term creep behavior: The tensile data reflect room-temperature properties, but 12Cr2Mo1R components often operate at 450-580°C. How does the transition layer perform under long-term creep conditions? Creep rupture testing at service temperatures would be essential for qualification.
2. Thermal cycling resistance: Power plant components experience frequent start-stop cycles. The fatigue crack initiation and propagation behavior at the dissimilar weld interface under thermal cycling has not been addressed.
3. Carbide precipitation kinetics: The M₂₃C₆ observed in the ENiCrMo-3 layer warrants further investigation into its precipitation kinetics as a function of PWHT temperature and duration.
4. Hydrogen-induced cracking susceptibility: The study does not address the hydrogen cracking risk in the 12Cr2Mo1R HAZ, which is a well-known concern for this material due to its martensitic-ferritic microstructure.
5. Non-destructive testing challenges: The hardness gradient at the interface may complicate UT calibration. Standard NDT procedures may need modification for these dissimilar joints.

Study Insights and Implications

This study provides valuable data on the microstructural evolution and mechanical performance of three transition layer strategies for 12Cr2Mo1R/S30408 dissimilar welds. The key insight is that the choice of transition layer consumable involves a multi-objective optimization: strength, toughness, corrosion resistance, dilution control, and long-term stability must all be balanced.

From a practical standpoint, the E309L transition layer offers the best combination of strength and ductility, making it suitable for applications where mechanical performance is paramount. The ENiCrFe-3 layer provides a good balance of properties and may be preferred where corrosion resistance is a secondary concern. The ENiCrMo-3 layer, while showing the highest hardness and potential for enhanced corrosion resistance, exhibits lower ductility and the risk of M₂₃C₆ precipitation, which may limit its applicability in high-temperature service.

The study reinforces the importance of the pre-overlay strategy in dissimilar steel welding. By establishing a controlled transition zone, the direct dilution of austenitic weld metal by the ferritic base metal is mitigated, reducing the risk of intermetallic phase formation and carbon migration. This approach is particularly valuable for thick-section components where complete control of the weld microstructure is challenging.

For engineering practice, the following recommendations emerge: (1) always use a transition layer for thick-section dissimilar welds between low-alloy steels and austenitic stainless steels; (2) select the transition layer consumable based on the dominant failure mode of the application (mechanical loading vs. corrosion); (3) implement rigorous PWHT procedures to stabilize the microstructure and relieve residual stresses; and (4) conduct comprehensive NDT including hardness mapping and metallographic examination at the fusion line.

The research contributes to the growing body of knowledge on dissimilar steel welding and provides actionable guidance for engineers designing and fabricating high-pressure equipment in power generation and petrochemical industries. Future work should focus on long-term performance under actual service conditions, including creep, fatigue, and corrosion-fatigue interactions.