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

Literature Overview

This 2017 study by Che Huanwen and Yao Jun from Baotou Vocational and Technical College and Huazhong University of Science and Technology investigates how tempering temperature affects the microstructure and mechanical properties of TIG welds in 9Cr0.11V steel. This alloy, often designated as 9Cr-0.11V or similar, is a martensitic creep-resistant steel developed for high-temperature applications in power plant boiler tubes, supercritical steam lines, and nuclear reactor components. The study was published in a casting technology journal, reflecting the connection between solidification microstructure and subsequent heat treatment response.

Core Technical Content

The 9Cr0.11V steel has a nominal composition of 0.11%C, 9%Cr, 1.8%W, 0.2%Mo, and 0.11%V, designed to provide excellent creep strength at temperatures up to 650°C while maintaining good weldability. The TIG welding process was performed with a filler metal matched to the base composition, typically in the form of ER9Cr-0.11V wire. The as-welded microstructure consists of lenticular martensite with a high dislocation density and fine M23C6 carbide precipitates along grain boundaries.

The tempering treatment was conducted at temperatures ranging from 650°C to 800°C in increments of 50°C, with each treatment held for 2 hours in air. The microstructural evolution during tempering follows a well-defined sequence. At 650°C, the martensite begins to decompose into tempered martensite with fine carbide precipitation, and the dislocation density decreases moderately. At 700°C, significant carbide coarsening occurs, and the matrix transforms to a tempered sorbite structure. At 750°C, the microstructure approaches a tempered bainite morphology with coarse M23C6 and MX (vanadium carbide) precipitates. At 800°C, excessive grain growth and carbide coarsening lead to a soft, coarse microstructure with degraded mechanical properties.

Key Technical Parameters and Analysis

The mechanical properties show a clear trade-off between strength and toughness as tempering temperature increases. At 650°C, the weld retains high strength but exhibits limited ductility and moderate impact energy. At 700°C, a balanced combination of strength and toughness is achieved, making this temperature optimal for most engineering applications. At 750°C, strength decreases significantly but ductility and impact energy improve substantially. At 800°C, the weld becomes too soft for high-temperature service, with yield strength falling below 360 MPa.

The creep resistance of the tempered welds was also evaluated at 650°C under a stress of 100 MPa. The rupture life increased from 120 hours at 650°C tempering to 280 hours at 700°C tempering, then decreased to 180 hours at 750°C tempering. This non-monotonic behavior is attributed to the competing effects of carbide coarsening (which reduces precipitation hardening) and dislocation recovery (which improves dislocation mobility). The optimal tempering temperature of 700°C provides the best compromise between strength retention, toughness, and creep resistance.

Engineering Practice Implications

For pressure vessel and heat exchanger fabrication using 9Cr0.11V steel, the tempering temperature is a critical parameter that must be specified in the WPS and verified through heat treatment documentation. The recommended tempering temperature range of 680–720°C for 2 hours provides a reliable window for achieving acceptable mechanical properties. The post-tempering hardness should be verified to be in the range of 230–260 HV, and tensile strength should meet the minimum requirement of 540 MPa as specified in relevant standards such as NB/T 47002 or ASME II.

The study also emphasizes the importance of cooling rate control during tempering. Air cooling is preferred over furnace cooling to avoid excessive grain growth during the cooling phase. For thick-section welds above 25 mm, the tempering treatment should be followed by a low-temperature stress relief at 300°C for 2 hours to minimize residual stresses without affecting the tempered microstructure. Non-destructive testing after tempering should include ultrasonic testing (UT) to detect any temper embrittlement or microcracking that may have developed during the heat treatment.

Study Insights and Reflections

This research provides a clear and systematic understanding of how tempering temperature influences the metallurgical and mechanical outcomes of 9Cr0.11V steel TIG welds. The identification of 700°C as the optimal tempering temperature for this alloy offers a practical guideline for welding procedure development. The study also highlights the importance of matching the tempering treatment to the intended service conditions: for high-temperature creep service, a slightly lower tempering temperature (680°C) may be preferred to retain more precipitation hardening, while for room-temperature service with high toughness requirements, a higher tempering temperature (720°C) may be more appropriate. Engineers should always verify the actual microstructure and mechanical properties through metallographic examination and mechanical testing rather than relying solely on the specified tempering temperature, as variations in welding parameters, joint geometry, and cooling conditions can significantly affect the tempering response.