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

Literature Overview

The paper by Che Huanwen and Yao Jun (2017, Foundry Technology, Vol. 38, No. 10, pp. 2383-2385), authored by researchers from Baotou Railway Vocational Technical College and Huazhong University of Science and Technology, investigates the effect of tempering temperature on the microstructure and mechanical properties of TIG-welded 9Cr0.11V steel. This is a maraging-type or precipitation-hardening alloy steel used in high-temperature applications such as power plant components, boiler tubes, and pressure vessels. The study addresses a critical practical question: how does post-weld heat treatment (PWHT) tempering temperature influence the weld metal properties, and what is the optimal tempering temperature for achieving a balance between strength, toughness, and microstructural uniformity?

Core Technical Findings

Microstructural Evolution with Tempering Temperature

The study demonstrates that at 850°C tempering, the weld metal hardness decreases significantly compared to the as-welded condition. The tempered martensite grains become larger and more uniform, and the distribution of carbides becomes more homogeneous. The impact toughness improves markedly, as evidenced by an increase in the number of dimples on the fracture surface, indicating a transition from brittle to ductile fracture mode.

Mechanical Property Comparison

The trade-off between strength and toughness is clearly demonstrated. As tempering temperature increases, hardness and tensile strength decrease while impact toughness and ductility improve. The optimal tempering temperature depends on the application requirements: for high-temperature service where creep resistance is important, a lower tempering temperature (700-750°C) may be preferred to retain higher strength; for low-temperature or cyclic loading applications where toughness is critical, a higher tempering temperature (800-850°C) is beneficial.

Process Analysis and Metallurgical Mechanisms

The 9Cr0.11V steel is a high-chromium alloy with vanadium additions for precipitation hardening and grain refinement. The welding process produces a martensitic weld metal due to the rapid cooling rate and high carbon equivalent of the alloy. The as-welded microstructure contains retained austenite, which is metastable and can transform during subsequent heating, leading to dimensional instability and potential cracking.

Tempering Mechanisms

1. Carbon precipitation: At tempering temperatures above 500°C, carbon precipitates from the martensite in the form of fine carbides (VC, Cr23C6, Cr7C3), reducing dislocation density and hardness.
2. Martensite decomposition: At temperatures above 700°C, the martensite decomposes into tempered martensite (ferrite + carbides), which is more ductile and stable.
3. Carbide coarsening and spheroidization: At temperatures above 800°C, fine carbides coarsen and spheroidize, reducing their pinning effect on dislocations and further softening the microstructure.
4. Retained austenite transformation: During tempering, retained austenite can transform to martensite or bainite, depending on the cooling rate after tempering. This transformation can introduce new residual stresses.

The study notes that at 850°C, the tempered martensite grains are larger and more uniform, which is attributed to the dissolution of fine carbides and the coalescence of austenite islands. The improved uniformity reduces local stress concentrations and promotes homogeneous deformation, which enhances toughness.

Residual Stress Considerations

Post-weld heat treatment also serves to relieve welding residual stresses. The tempering process involves heating the weld region to a temperature where creep and stress relaxation occur. For 9Cr0.11V steel, the stress relief temperature is typically in the range of 750-850°C, which coincides with the tempering temperature range. However, the cooling rate after tempering must be controlled to avoid re-introducing high residual stresses or causing martensitic transformation in the HAZ. A controlled cooling rate of 100-200°C/h from the tempering temperature is recommended.

Engineering Practice Implications

The findings of this study are directly applicable to the fabrication and repair of high-chromium alloy steel components in power generation, petrochemical, and nuclear industries. The selection of tempering temperature is a critical process decision that must be made based on the service conditions of the component.

Standard Recommendations for 9Cr Series Steels

The study also highlights the importance of monitoring the microstructure after tempering. Metallographic examination should be performed to verify that the tempered martensite is uniform and that no untempered martensite or retained austenite remains. Hardness mapping across the weld, HAZ, and base metal should be conducted to ensure that the hardness profile is acceptable and that there are no soft zones that could lead to localized deformation or cracking.

Study Insights and Reflections

This paper provides a clear and practical demonstration of the tempering temperature effect on 9Cr0.11V steel TIG welds. The systematic approach of examining microstructure, hardness, impact toughness, and fracture morphology at different tempering temperatures is methodologically sound. The identification of 850°C as a temperature that significantly improves toughness while maintaining acceptable strength is practically useful. However, the study does not address the effects of tempering on creep properties, which are critical for high-temperature applications. Additionally, the study does not examine the HAZ properties, which may be more susceptible to embrittlement than the weld metal. For engineering practice, I recommend that the tempering temperature be selected based on a comprehensive assessment of service conditions, including temperature, pressure, cyclic loading, and corrosion environment. The study also underscores the importance of PWHT as a critical process step for high-chromium alloy steels, and that the tempering temperature must be carefully controlled to achieve the desired balance of properties.