Effect of Pre-Edge 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) by Liu Guangyin and colleagues from Lanzhou University of Technology and Lanzhou Lanshi Heavy Equipment Co., Ltd., addresses the persistent challenge of welding thick-section low-alloy heat-resistant steel (12Cr2Mo1R) to austenitic stainless steel (S30408). The research proposes a practical solution involving pre-edge surfacing of 2–3 mm thick transition layers using three different consumables—E309L, ENiCrFe-3, and ENiCrMo-3—followed by A-TIG root welding and submerged arc welding (SAW) for fill and cap passes. The work is funded by the National Natural Science Foundation of China (Grant 51775256) and represents a meaningful contribution to the field of dissimilar metal welding in heavy equipment manufacturing.

Core Technical Approach

The methodology follows a well-structured experimental design. The base material pairing of 12Cr2Mo1R (a 9Cr-1Mo type heat-resistant steel used extensively in high-temperature pressure vessels and piping) with S30408 (a standard austenitic stainless steel) creates a classic dissimilar joint where significant metallurgical incompatibility exists. The key innovation lies in the pre-edge surfacing strategy, which introduces a graded transition zone between the two dissimilar metals before the main butt weld is executed.

The welding sequence employed—A-TIG for root followed by SAW for fill and cap—is a well-established high-productivity combination for thick-section fabrication. The A-TIG process provides excellent penetration control and minimal dilution at the root, while SAW delivers high deposition rates and good mechanical properties in the fill passes.

Microstructural Analysis and Key Findings

The microstructural examination using optical microscopy (OM) and scanning electron microscopy (SEM), supplemented by electron probe microanalysis (EPMA), reveals several critical observations. At the 12Cr2Mo1R base metal to surfacing layer interface, a pronounced concentration gradient of Cr, Ni, and Fe elements is observed. This gradient is a direct consequence of the large difference in elemental composition between the ferritic-martensitic base metal and the austenitic or nickel-based surfacing alloy.

A particularly noteworthy finding is the carbon enrichment phenomenon at grain boundaries near the fusion line. This carbon segregation is driven by the high carbon activity of the 12Cr2Mo1R base metal and the limited carbon solubility in the austenitic or nickel-based transition layer. In the ENiCrMo-3 surfacing layer, M23C6 carbide precipitation was identified, which is a common intermetallic phase in Cr-Ni-Mo systems. The formation of M23C6 carbides is significant because they can act as stress concentrators and potential crack initiation sites under thermal cycling or mechanical loading.

The ferrite content in the weld metal follows a clear trend: E309L produces the highest ferrite content, while ENiCrMo-3 produces the lowest. This is consistent with the known metallurgical behavior—E309L, being a Cr-Ni austenitic weld metal with lower Ni content, has a higher tendency to form delta ferrite, whereas the high-Ni nickel-based alloys strongly suppress ferrite formation. The ferrite content is critical because excessive ferrite can promote sensitization and intergranular corrosion, while too little ferrite may lead to hot cracking susceptibility.

The hardness measurements show that the maximum hardness at the base metal to surfacing interface increases in the order E309L < ENiCrFe-3 < ENiCrMo-3, with hardness decreasing toward the center of the surfacing layer. This gradient is attributed to the varying dilution levels and the formation of different microstructural constituents near the fusion boundary.

Engineering Practice Implications

From an engineering perspective, the selection of transition layer consumable must be evaluated against multiple criteria. E309L offers the highest tensile strength (608.1 MPa) and elongation (21.7%), making it attractive for applications where mechanical performance is paramount. However, its higher ferrite content raises concerns about long-term corrosion resistance in aggressive environments. ENiCrFe-3 provides a balanced compromise with good mechanical properties and moderate ferrite content. ENiCrMo-3, while exhibiting the lowest tensile strength (564.5 MPa) and elongation (18.2%), offers the highest interface hardness and lowest ferrite content, which may be advantageous for corrosion resistance but at the cost of reduced ductility.

The carbon enrichment at grain boundaries near the fusion line is a concern that warrants further attention in service applications. In hydrogen-containing environments or high-temperature service, this carbon segregation could potentially promote carbide precipitation and subsequent embrittlement. The presence of M23C6 carbides in the ENiCrMo-3 layer is particularly relevant for hydrogenation reactor applications where carbide stability under hydrogen attack is a critical design consideration.

Key Questions and Reflections

Several questions arise from this study that merit further investigation. First, the long-term creep and thermal fatigue behavior of these joints under cyclic thermal loading conditions typical of power plant or petrochemical service is not addressed. Second, the corrosion resistance of the three transition layers, particularly in chloride-containing or hydrogen-rich environments, should be systematically evaluated. Third, the effect of post-weld heat treatment (PWHT) on the microstructure and residual stress distribution at the interface would provide additional insight into the practical applicability of these transition layer strategies.

The study demonstrates that the pre-edge surfacing approach is a viable and practical solution for thick-section dissimilar steel welding. The choice of consumable should be guided by the specific service conditions, with E309L being suitable for mechanically demanding applications and ENiCrMo-3 being preferred where corrosion resistance is the primary concern. The carbon enrichment phenomenon at the fusion boundary remains a fundamental metallurgical challenge that requires careful control through proper welding procedure design and heat input management.

Summary and Conclusions

This study provides valuable experimental data on the microstructural evolution and mechanical performance of 12Cr2Mo1R/S30408 dissimilar steel joints with different pre-edge transition layers. The systematic comparison of E309L, ENiCrFe-3, and ENiCrMo-3 consumables offers practical guidance for welding engineers working on thick-section dissimilar joints. The identified carbon enrichment at grain boundaries and M23C6 carbide precipitation are important metallurgical features that must be considered in service life assessment. The tensile strength and elongation results confirm that all three transition layer strategies produce joints with acceptable mechanical properties, with E309L delivering the best strength-ductility combination. The study reinforces the principle that a properly designed transition layer can effectively manage the metallurgical incompatibility between dissimilar steels, but the selection must be carefully matched to the specific service environment and loading conditions.