TWI Industrial Member Report Summary 744/2002
With the need to improve thermal efficiency, reduce costs and reduce emissions, a new generation of 9-13%Cr ferritic steels has evolved for increased service temperatures up to 620°C. These new steels also allow a significant reduction in thickness relative to conventional grades such as P91 and P22, which in turn facilitates reduced weight and fabrication costs.
Extensive research and development in Japan, Europe and the USA has led to the development of a number of candidate alloys typically containing 9-13%Cr, ~0.5%Mo and 1-2%W with additions of Ni, Nb and V. To date only two of the newly developed grades, namely NF616 and HCM12A, have achieved ASME Code Case approval (cases 2179 and 2180 respectively). Whilst welding consumables are available for these alloys there is a need to understand more fully the role of composition in the microstructural development and mechanical properties of weld metals. Furthermore, an improvement in the mechanical properties of weld deposits should be possible, through controlled compositional variations, to support on-going material developments for service temperatures as high as 650°C.
This programme of work aimed to explore the improvement in toughness that could be achieved in weld metals for the W-containing steels NF616 and HCM12A through variations in the deposit chemistry. In the light of the results obtained previously on weld metals for grade 91 and NF616 variations in Ni and Co to the baseline compositions have been explored. For the 10Cr series, an increase in Cr, taking the level to approximately the mid-range specified for HCM12A, in combination with the addition of ~1% Co has also been investigated. This report presents the results of this investigation, and provides comparison with earlier studies on weld metals for grade 91.
- Determine the effect of variations in Co, Ni and Cr on microstructural development and toughness of weld metals for selected examples of the new, W-containing 9-13%Cr steels.
- Compare the toughness of the W-containing weld deposits with data previously generated for weld metals for grade 91 (W-free).