Tempering Temperature Effects on TIG Weld Microstructure and Mechanical Properties of 9Cr0.11V Steel

Literature Overview

Che Huanwen and Yao Jun, from Baotou Railway Vocational Technical College and Huazhong University of Science and Technology respectively, published this study in Casting Technology (2017, Vol. 38, No. 10, pp. 2383-2385). The research investigates how post-weld tempering temperature influences the microstructure and mechanical properties of TIG welds in 9Cr0.11V steel. This alloy belongs to the 9Cr-1Mo class of modified creep-strength steels, widely used in supercritical and ultra-supercritical power plant boiler tubing, where it must withstand high temperatures (up to 650°C) and high pressures over extended service periods. The study examines the effect of tempering temperatures ranging from lower to higher values, with particular attention to the 850°C condition.

Core Technical Findings

The study demonstrates that tempering temperature has a profound and systematic effect on the weld microstructure and mechanical properties. As tempering temperature increases, the hardness decreases progressively from the weld surface toward the weld center, with the most significant drop occurring at 850°C. At this temperature, the tempering martensite transforms into a more uniform and coarser microstructure, resulting in substantially improved impact toughness.

The fracture surface analysis revealed that as tempering temperature increased, the number of dimples on the fracture surface increased, indicating a transition from brittle to ductile fracture mode. This is a critical finding for power plant applications, where the weld must not only resist creep deformation but also possess adequate toughness to prevent catastrophic brittle fracture under thermal cycling conditions.

Metallurgical Analysis

The 9Cr0.11V steel weld metal, deposited during TIG welding, typically solidifies as martensite due to the rapid cooling rates inherent in arc welding. This as-welded martensite is hard but brittle, with high residual stresses that can lead to hydrogen-induced cracking if not properly relieved. Tempering at elevated temperatures transforms this martensite into tempered martensite, reducing hardness while improving toughness and relieving residual stresses.

The transformation kinetics at 850°C are particularly significant. At this temperature, the carbon in solution within the martensite begins to precipitate as fine carbides (primarily M23C6 and MX-type carbides), which coarsen and spheroidize with increasing tempering temperature. The coarsening of tempered martensite at 850°C, while reducing hardness, creates a more homogeneous microstructure that is less susceptible to stress concentration and crack initiation. The uniformity improvement at this temperature is attributed to the completion of the tempering transformation, where most of the retained austenite and untempered martensite have been converted.

Engineering Practice Integration

In power plant boiler fabrication, the post-weld heat treatment (PWHT) temperature is typically specified by design codes and standards such as ASME Section VIII Division 1, NB/T 47014, and relevant Chinese national standards. For 9Cr-1Mo class steels, the standard PWHT temperature range is 760-790°C for conventional applications and 780-810°C for advanced applications. The study's finding that 850°C tempering produces the most favorable microstructure and toughness suggests that the standard PWHT range may be conservative for certain applications, though it is important to note that higher tempering temperatures can also accelerate creep degradation during long-term service.

The balance between toughness improvement and creep resistance is a central challenge in the design and fabrication of supercritical boiler tubing. Higher tempering temperatures improve toughness and reduce residual stresses but may accelerate creep cavity formation and carbide coarsening during service. Welding engineers must therefore select PWHT parameters that optimize the entire service life of the component, not merely the as-welded mechanical properties.

Study Insights and Implications

This paper provides valuable data for welding engineers and heat treatment specialists working with 9Cr-1Mo class steels in power generation applications. The clear demonstration that 850°C tempering significantly improves toughness through microstructural uniformity provides a quantitative basis for evaluating whether standard PWHT procedures should be modified for specific service conditions. The study also highlights the importance of correlating microstructural observations with mechanical property measurements, as the hardness-toughness relationship in tempered martensitic steels is not always straightforward. For future work, the authors' approach could be extended to include long-term creep testing at service temperatures to validate that the improved as-fabricated toughness translates into enhanced long-term reliability, which is ultimately the primary concern for boiler and pressure vessel applications.