TWI Industrial Member Report Summary 948/2010
by D J Abson, J Rothwell
Improved thermal efficiency of power plant has been the main driver for the development of the martensitic-ferritic 9-12%Cr creep-resistant steels known as creep strength enhanced ferritic (CSEF) steels. Good progress has been made in developing such steels, which are used particularly in the forged form as tubes and pipes in boilers, but are also being considered for rotors, and their cast variants for turbine casings. The target operating temperature for these steels is 650°C, with a common target design life of 100,000hrs. Increasingly, the demand for efficiency is linked to efforts to reduce CO2 emissions, in order to meet environmental obligations and minimise any form of carbon tax that may be levied in the future.
The improvement of tensile and creep rupture strength, achieved by the development of CSEF steels is attractive to designers who want to take advantage of the higher hot tensile and creep strength to reduce pipe wall thicknesses, and thereby minimise thermal stresses for a more reliable plant. This is increasingly important for today's power plant which are subject to temperature cycles in an effort to respond to the peaks and troughs of demand and improve profitability. However, although parent alloy developments have been making good progress, weldment performance has taken a back seat. Various cracking mechanisms associated with the weldments can and do occur, and weldments are usually of greatest concern in power plant fabrications. In particular the cross-weld creep performance for CSEF steels is an issue, with the typical failure mechanism being the low strain to failure mechanism, Type IV, which is recognised as fracture occurring in the outer region of the heat affected zone (HAZ). Only recently have codes such as ASME I taken into account the major difference between parent and cross-weld creep performance.
Aside from performance of the steel and weldments themselves, productivity is very important in order to meet the heavy demand for new power plant, especially in China and India. The newer steels generally allow reduced wall thickness which in turn reduces joint thickness, weld volume and welding times. Other measures to improve productivity include, replacing manual metal arc (MMA) welding with flux-cored arc (FCA) welding and the use of higher productivity processes, such as narrow gap welding (including narrow gap tungsten inert gas (TIG) welding) and electron beam (EB) welding. New, reduced pressure electron beam (RPEB) welding technology may offer very high quality welds at substantially reduced welding times, particularly for thick section components.
In this investigation, which is TWI's contribution to the European Collaboration in Science and Technology (COST) programme, narrow gap TIG, FCA and RPEB welding were employed to produce a series of butt welds in a new European steel named FB2, a forged, boron containing steel. These welds were subjected to metallurgical examination, and to a range of mechanical tests, primarily aimed at establishing weld metal and cross-weld properties, including cross-weld creep rupture strength. The compositions of the rutile flux-cored and metal-cored wire were based on the compositions of weld metals developed in earlier TWI studies.
- Produce and evaluate cross-weld creep data for the 9%Cr steel FB2, welded by a range of high productivity processes.
- Produce and evaluate weld metal toughness data for FB2 weldments made by a range of high productivity processes and including the use of experimental consumables.