Post-Weld Heat Treatment of CLAM Steel Joints Fabricated by TIG Welding

Literature Overview

This paper, authored by Qiao Jiansheng, Huang Yina, and Wan Faron from the School of Materials Science and Engineering at University of Science and Technology Beijing and the Beijing Key Laboratory of New Energy Materials and Technology, was published in Nuclear Science and Engineering in 2009 (Vol. 29, No. 3, pp. 239–246). Funded by the National Natural Science Foundation of China (Grant No. 50771017) and the National Basic Research Program of China (Grant No. 2007ID102), the study addresses a critical challenge in nuclear engineering: the post-weld heat treatment (PWHT) of Cast and Rolled Austenitic Ferritic-Martensitic (CLAM) steel welded joints. CLAM steel is a low-activation martensitic/ferritic steel developed for use in fusion reactor structural components, and the welding and post-weld treatment of this material are essential for ensuring the mechanical integrity of fusion reactor structural assemblies.

Background on CLAM Steel and Welding Challenges

CLAM steel is a low-activation 9Cr steel with a composition similar to the well-known 9Cr-1Mo steel family but with reduced concentrations of activation-producing elements such as Ni, Co, and W. The base metal has a tempered martensitic microstructure with a hardness of approximately 200–250 HV and good high-temperature creep resistance, making it suitable for structural applications in fusion reactor blanket and divertor systems.

The welding of CLAM steel presents several significant challenges:

1. High hardenability: Like other 9Cr steels, CLAM steel has a high hardenability due to its high carbon equivalent (CE ≈ 0.5–0.6). This means that the heat-affected zone (HAZ) and weld metal can develop very hard, brittle martensitic structures during welding, leading to poor toughness and susceptibility to hydrogen-induced cracking.
2. Tempered martensite base metal: The base metal microstructure is tempered martensite, which is sensitive to thermal cycling. During welding, the HAZ undergoes a transformation cycle that can produce untempered martensite if the cooling rate is too fast, or coarse grain growth if the peak temperature is too high.
3. Weld metal dilution: The weld metal composition is affected by dilution from the base metal, and the resulting microstructure depends on the dilution ratio, cooling rate, and post-weld treatment.
4. Residual stress: The thermal stresses generated during welding can be substantial, particularly in thick-section components, and these residual stresses can promote delayed cracking and reduce fatigue life.

The fundamental problem identified in this paper is that TIG welding of CLAM steel produces weld joints with increased strength and hardness in the weld zone but reduced toughness. This is a classic manifestation of the strength-toughness trade-off in martensitic steels, where the high hardness achieved by untempered or under-tempered martensite comes at the expense of ductility and fracture resistance.

Heat Treatment Process Investigation

The authors systematically investigated the effects of tempering temperature, tempering time, and repeated tempering on the hardness of the weld zone. The experimental approach involved welding CLAM steel specimens using TIG welding with appropriate filler metal, followed by various PWHT schedules, and then measuring the hardness distribution across the weld joint.

Tempering Temperature Effects

The tempering temperature is the most critical parameter in PWHT of martensitic steels. At temperatures below approximately 500°C, the tempering process primarily involves the decomposition of retained austenite and the precipitation of fine carbides, which reduces hardness only slightly. As the tempering temperature increases above 550°C, the tempering process becomes more effective at dissolving and coarsening carbides, leading to significant hardness reduction and toughness improvement.

Tempering Time Effects

The tempering time affects the degree of carbide coarsening and the uniformity of the tempering response. Short tempering times may not allow complete transformation of the microstructure, particularly in the weld metal where the cooling rate during welding can produce a fine, retained austenite-rich microstructure that requires longer times to fully transform.

Repeated Tempering

The authors investigated the effect of repeated tempering — specifically, tempering at 760°C for 1 hour twice in succession. Repeated tempering is a well-known technique for improving the temper stability of high-strength steels, particularly those with high alloy content. The first tempering cycle partially decomposes the martensite, and the second cycle completes the transformation and further coarsens the carbides, resulting in a more stable and uniform microstructure.

Optimal Heat Treatment Schedules

Based on the experimental results, the authors identified two optimal PWHT schedules:

Both schedules achieved the primary objective of reducing weld zone hardness to levels comparable with the base metal, thereby improving toughness and reducing the risk of hydrogen-induced cracking. The choice between the two schedules depends on practical considerations: Schedule 1 is simpler to implement (single cycle), while Schedule 2 may be preferred when furnace capacity or cycle time constraints make a 2-hour hold impractical, or when the weld cross-section is thick enough that heat penetration during a single cycle is insufficient.

Connection to Engineering Practice

The PWHT of CLAM steel weld joints has direct relevance to the fabrication of fusion reactor structural components, including blanket modules, divertor structures, and plasma-facing components. In these applications, the weld joints must withstand:

• Thermal cycling: Repeated heating and cooling during reactor operation, with temperature swings from cryogenic to several hundred degrees Celsius.
• Neutron irradiation: Exposure to high neutron flux, which causes embrittlement, swelling, and helium/hydrogen accumulation.
• Mechanical loading: Structural loads from plasma pressure, electromagnetic forces, and gravitational loads.
• Thermal stress: Thermal gradients between the plasma-facing surface and the backing structure.

The PWHT schedule must be designed to produce a weld joint with adequate toughness, creep resistance, and irradiation resistance. The hardness matching criterion — ensuring that the weld zone hardness is within ±50 HV of the base metal hardness — is a widely used acceptance criterion in nuclear and pressure vessel codes (ASME Section VIII, RCC-M, and other standards).

The TIG welding process is particularly suitable for CLAM steel because it provides excellent control over heat input, which is critical for minimizing HAZ grain growth and controlling the dilution ratio. The low heat input of TIG welding also reduces the risk of cracking during the welding process itself, which is important for high-hardenability steels.

Key Questions and Reflections

Several important considerations arise from this work:

First, the study focuses primarily on hardness as the primary quality metric. While hardness is a convenient and rapid indicator of microstructural state, it does not directly measure toughness, which is the critical property for fracture resistance. A comprehensive PWHT qualification program should include Charpy V-notch impact testing, fracture toughness testing (KIC or JIC), and possibly creep rupture testing, depending on the service conditions.

Second, the study does not address the effect of PWHT on weld residual stress. While tempering does reduce residual stress through stress relaxation, the extent of stress relief depends on the tempering temperature and time, and a dedicated stress relief cycle (typically at 550–650°C) may be required in addition to the tempering cycle for hardness matching. In practice, the PWHT schedule must be optimized to achieve both hardness matching and residual stress relief, which may require a multi-stage heat treatment.

Third, the study does not consider the effect of PWHT on the weld metal composition and microstructure in the presence of different filler metals. The dilution ratio and the resulting weld metal composition depend on the filler metal chemistry, and the PWHT response of the weld metal may differ from that of the base metal and HAZ. A comprehensive study should evaluate multiple filler metal options and their interaction with the PWHT schedule.

From a standards perspective, the PWHT of martensitic steels is governed by requirements in ASME Section III (for nuclear components), ASME Section VIII (for pressure vessels), and RCC-M (for French nuclear codes). These codes specify minimum and maximum tempering temperatures and times based on the steel grade and the thickness of the component. The schedules identified in this paper — 760°C for 2 hours or 760°C for 1 hour twice — are consistent with the general requirements of these codes for 9Cr-1Mo steels, which typically specify PWHT at 760–790°C for 1 hour per 25 mm of thickness.

Study Insights and Implications

This paper provides a clear and practical approach to solving the PWHT challenge for CLAM steel weld joints. The identification of two optimal schedules — a single 2-hour cycle and a repeated 1-hour cycle — offers flexibility for different fabrication scenarios, which is a significant practical contribution.

The choice of 760°C as the tempering temperature is well-founded. At this temperature, the tempering process is effective enough to fully decompose the martensite and coarsen carbides to a degree that matches the base metal hardness, but not so high that it risks over-tempering or grain coarsening. The 760°C temperature is also within the range where stress relaxation is significant, providing a beneficial reduction in residual stress as a secondary effect.

The investigation of repeated tempering is particularly valuable. In industrial practice, repeated tempering is sometimes used for high-strength steels where a single tempering cycle does not produce a fully stable microstructure. The finding that two 1-hour cycles at 760°C achieve the same result as a single 2-hour cycle provides an alternative approach that may be advantageous in certain production scenarios, such as when furnace capacity is limited or when the component geometry makes uniform heating during a single long cycle difficult.

For engineers working on fusion reactor component fabrication, this paper provides a practical foundation for PWHT process development. The methodology — systematic variation of tempering parameters, hardness measurement, and identification of optimal schedules — is transferable to other low-activation steels and ferritic/martensitic steels used in nuclear and energy applications. The key insight is that the PWHT schedule must be tailored to the specific steel grade, welding process, and service conditions, and that a one-size-fits-all approach is not appropriate for advanced materials.