Microstructural Evolution of CLAM/15-15Ti Dissimilar Steel TIG Welded Joints Before and After Heat Treatment

Literature Overview

The paper by An Mingming, Jiang Zhizhong, Sun Fenglian, Wu Qingsheng, and Chen Jianwei investigates the microstructural evolution and hardness characteristics of a dissimilar steel welded joint between Chinese Low Activation Martensitic (CLAM) steel and 15-15Ti stainless steel. Published in the Journal of Harbin University of Science and Technology in 2019 (Vol. 24, No. 6, pp. 41-46), this work was supported by the Jiangsu Provincial Nuclear Equipment Materials Engineering Laboratory Open Project (Grant No. 1006-56XCA17012) and the National Natural Science Foundation of China (Grant No. 51501184). The research is significant for nuclear fusion reactor applications where dissimilar material joints are unavoidable due to the need to combine structural and functional components.

Material Background and Welding Challenges

CLAM steel is a low-activation 12Cr-1Mo martensitic steel developed for the structural components of fusion reactors, particularly for the first wall and blanket modules. Its low activation property is achieved by substituting Cr for Fe and Mo for W, reducing the production of long-lived radioactive isotopes during neutron irradiation. The 15-15Ti stainless steel is a high-chromium austenitic stainless steel containing approximately 15% Cr and 15% Ni with Ti stabilization, used for its excellent corrosion resistance and thermal stability. The combination of these two materials in a welded joint creates a significant metallurgical challenge due to the vast differences in their chemical compositions, thermal expansion coefficients, and phase transformation behaviors.

The carbon content differential between the two base metals is particularly problematic. CLAM steel typically contains 0.08-0.12% C, while 15-15Ti stainless steel contains less than 0.08% C. During welding, carbon diffusion from the martensitic side toward the austenitic side can lead to carbide precipitation in the heat-affected zone (HAZ), causing embrittlement. Additionally, the dilution of the weld metal by the dissimilar base metals creates a complex composition that is difficult to control.

Welded State Microstructure and Hardness

The as-welded weld metal consists of lenticular martensite with a small amount of delta ferrite. The hardness in the weld zone ranges from 407 HV to 463 HV. The presence of delta ferrite in the weld metal is a consequence of the high chromium content in the dilution mixture. The lenticular martensite morphology suggests that the weld metal solidified in a ferrite-austenite mode, with the austenite transforming to martensite during cooling. This microstructure provides adequate strength but may have limited toughness, particularly at low temperatures.

Hardness Distribution in the As-Welded Joint

The CLAM steel HAZ is divided into two distinct regions: the fully quenched zone and the partially quenched zone. The fully quenched zone experienced sufficient cooling rates to transform the entire microstructure to martensite, resulting in elevated hardness. The partially quenched zone, located at a greater distance from the fusion boundary, experienced cooling rates that were too slow for complete martensitic transformation, leading to a mixed microstructure with notable softening. This softening is a critical concern for the mechanical integrity of the joint, as it creates a region of reduced strength that may become a preferential site for deformation or cracking under load.

The 15-15Ti stainless steel HAZ represents the lowest hardness region in the entire joint. This is because the austenitic stainless steel does not undergo a martensitic transformation during welding cooling. Instead, the HAZ experiences grain growth and possible precipitation of carbides, which can slightly reduce hardness. The combination of a soft zone in the stainless steel HAZ and a hardened zone in the martensitic HAZ creates a significant hardness gradient across the joint, which is a well-known challenge in dissimilar steel welding.

Post-Weld Heat Treatment Effects

After post-weld heat treatment (PWHT), the microstructural and mechanical properties of the joint underwent significant changes. The weld metal microstructure transformed from as-welded lenticular martensite to tempered martensite, accompanied by an increase in the volume fraction of precipitates. The hardness decreased by approximately 20% compared to the as-welded state. This reduction in hardness is accompanied by an improvement in toughness, as the tempered martensite is less brittle than the as-quenched condition.

The heat treatment also reduced the extent of the high-hardness region in the joint. The fully quenched zone in the CLAM HAZ was tempered, reducing its hardness and improving its ductility. The softening in the partially quenched zone was partially reversed by the precipitation of carbides during the tempering process. However, the fundamental hardness gradient across the joint persisted, as the 15-15Ti stainless steel HAZ remained the softest region.

Comparison of As-Welded and Heat Treated States

Engineering Practice Implications

For nuclear fusion reactor applications, the mechanical integrity of dissimilar material joints is paramount. The hardness gradient across the CLAM/15-15Ti joint creates a risk of stress concentration at the interface between the hard and soft zones. Under cyclic loading or thermal cycling, this gradient can lead to fatigue cracking or creep deformation. The PWHT is essential for reducing the residual stresses induced by welding and for tempering the martensite in the weld and HAZ. However, the heat treatment cannot eliminate the inherent hardness mismatch between the two materials.

From a design perspective, the soft zone in the 15-15Ti HAZ should be considered in the stress analysis of the component. The joint may require a larger transition radius or a different joint configuration to reduce stress concentration. Additionally, the weld metal composition should be carefully controlled to minimize the formation of brittle phases such as sigma phase, which can form during prolonged exposure to elevated temperatures.

Study Insights and Reflections

This research highlights the fundamental challenge of joining dissimilar steels with vastly different compositions and properties. The PWHT is effective at tempering the weld metal and reducing the hardness gradient, but it cannot resolve the intrinsic material mismatch. Future work should explore the use of interlayer materials or transition welds that can gradually change the composition from one base metal to the other, thereby reducing the severity of the hardness gradient. The findings are directly applicable to the design of fusion reactor components and provide a baseline for understanding the long-term behavior of dissimilar material joints under irradiation and thermal cycling conditions.