New 9-13%Cr weld metals
With the drive for higher operating temperatures and pressures to improve the thermal efficiency of new plant, as well as the bid to extend the life of existing plant, there have been significant changes in the materials used. For operation up to 620°C, a new generation of ferritic steels with 9-13%Cr has been developed, containing additions of tungsten (1-2%) to give improved high temperature properties over the traditional modified 9Cr grades. These materials, like ASTM A213 grade 91, are predominantly martensitic, with varying amounts of retained delta-ferrite. Such materials have been the subject of extensive research programmes, for example EPRI project 1403-5019 in the USA, [1] and COST 501 and 522 [2] , and currently COST 536 in Europe. A number of these materials now have ASME code case approval, and indeed have been introduced into actual plant for full-scale trials. However, further long term trials are required and, in view of the difficulty of matching the properties of the cast and PWHT weld metal microstructure with those of the parent material, additional welding consumable development is needed before we can expect to see the widespread application of these grades.
The principal grades that have evolved to date are:
- E911- as studied in COST 501
- NF616, produced by Nippon Steel [A213 T92/A335 P92]
- HCM12A, produced by Sumitomo Metal Industries [A213 T122/A335 P122]
- TB12M, produced by Forgemasters Steel and Engineering Ltd
Chemical composition of 'new' 9-13%Cr steels
| Element | Grade 91 | E911 | NF616 | HCM12A | TB 12M | |
|---|---|---|---|---|---|---|
| C | 0.08-0.12 | 0.10-0.13 | 0.07-0.13 | 0.07-0.14 | 0.10-0.15 | |
| Mn | 0.30-0.60 | 0.30-0.60 | 0.30-0.60 |
 0.70 |
0.40-0.60 | |
| Si | 0.20-0.50 | 0.10-0.30 |
0.50 |
0.50 |
0.50 max | |
| S | 0.010 max | 0.010 max | 0.010 max |
0.010 |
0.010 max | |
| P | 0.020 max | 0.020 max |
0.020 |
0.020 |
0.020 max | |
| Cr | 8.00-9.50 | 8.50-9.50 | 8.50-9.50 | 10.00-12.50 | 11.0-11.30 | |
| Mo | 0.85-1.05 | 0.90-1.10 | 0.30-0.60 | 0.25-0.60 | 0.40-0.60 | |
| W | - | 0.90-1.10 | 1.50-2.00 | 1.50-2.50 | 1.60-1.90 | |
| Ni | 0.40 max | 0.20-0.40 |
0.40 |
0.50 |
0.70-1.0 | |
| Cu | - | - | - | 0.30-1.70 | - | |
| V | 0.18-0.25 | 0.15-0.25 | 0.15-0.25 | 0.15-0.30 | 0.15-0.25 | |
| Nb | 0.06-0.10 | 0.06-0.10 | 0.04-0.09 | 0.09-0.10 | 0.04-0.09 | |
| N | 0.030-0.070 | 0.050-0.080 | 0.030-0.070 | 0.040-0.100 | 0.04-0.09 | |
| Al | 0.04 max | - |
0.040 |
0.040 |
0.010 max | |
| B | - | - | 0.001-0.006 |
0.005 |
- | |
| Sn | - | - | - | - | 0.010 max | |
| As | - | - | - | - | 0.010 max | |
| Sb | - | - | - | - | 0.005 max | |
| Creep strength in 10 5 hours at: [3] | 600° | 94 | (115) | (115) | (115) | (150*) |
| ASME Code case 2179-3 | 650° | 50 | (65) | (60) | (60) | (80*) |
| * 10,000 hr; ( ) estimated | ||||||
These materials offer considerable advantages over conventional grade 91. The use of NF616 (ASME code case 2179-3 and now ASTM A335 P92), for example, may allow a ~35% increase in allowable stress at 600°C. This in turn permits a decrease in section thickness, and thereby a reduction in weight, as well as lower welding costs. Consequently, the through-wall temperature gradients will be lowered, giving a reduction in the thermal fatigue loading experienced.
The diagram given ( Fig.1) compares the relative wall thickness for a pipe of 290mm internal diameter for operation at 557°C, 20MPa for grade 22, grade 91 and grade 122, although the benefits of the new advanced ferritic steel are even more pronounced at higher temperatures and pressures [4] .
|
Fig.1. Variation in wall thickness with material grade (with an internal diameter of 290mm) for service at 557°C 20MPa. [4] |
On-going steel developments are now looking at non-austenitic steels that are suitable for service up to 650°C to give further improvement to the thermal efficiency of ultra supercritical power plant [5] . One of the new steels, NF12, designed for boiler application, contains ~12%Cr, ~2.5%W and ~2.5%Co, the addition of cobalt preventing the retention of delta-ferrite in the microstructure. [5,6] A rotor steel, HR1200, has also been developed, intended for use in ultra supercritical turbine rotors for service at temperatures of 620 and 650°C. This steel contains alloying additions of W, V, Nb, Co, and B, and a low N content of ~200ppm. Data generated to date on the material have indicated that it exhibits excellent creep rupture strength, corresponding to that of the precipitation hardened austenitic alloy A286, but with a more favourable (lower) coefficient of thermal expansion. A bolting and blading material has also been developed [7] TAF650, which possess extremely good high temperature properties, significantly above those of AISI 422. Further work is required to optimise these alloys and their heat treatment, and to develop the associated welding consumables.
References
| N° | Author | Title |
|---|---|---|
| 1 | Proc EPRI/National Power Conf New steels for advanced power plant up to 620°C 11 May 1995 | |
| 2 | Staubli M, Mayer K-H, Kern T U, Vanstone R W, Hanus R, Stief J and Schönfeld K-H: | 'COST 522 - Power generation into the 21 st century: Advanced steam power plant, Proc. 3 rd EPRI Int. Conf. on Advances in materials technology for fossil power plants, Swansea, Wales, April 2001 |
| 3 | Orr J, Woollard L and Everson H: | 'The development and properties of a European high strength 9CrMoNbVWN steel - E911.' Proc Int Conf on Advanced Steam Plant, London, 21-22 May 1997 |
| 4 | Yang Z, Fong M A and Gibbons T B: | 'Steels for thick section parts: comparison of economics of usage in a typical design' Proc EPRI/National Power Conf on New steels for advanced power plant up to 620°C, London, 11 May 1995 174-183 |
| 5 | Fujita T: | 'Future ferritic steels for high temperature service.' Proc EPRI/National Power Conf on New steels for advanced power plant up to 620°C, London, 11 May 1995 190-200 |
| 6 | Naoi H H et al: | 'Mechanical properties of 12Cr-W-Co ferritic steels with high temperature creep rupture strength' Proc 5th Int Conf on Materials for advanced power engineering, Liege, October 1994 |
| 7 | Fujita T: | 'Heat resistant steels for advanced power plant' Advanced materials and processes 1992 141 (4) 42-47 |
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