Henry Granjon Prize Competition 2008 Joint Winner — Category B: “Materials Behaviour and Weldability” Evolution of Microstructure and Mechanical Properties of the Heat-Affected Zone in 9Cr Steels

Abstract

Martensitic 9–12% chromium steels are favoured grades for thick-walled components in ultra super-critically-operated thermal power plants. Welding in all its variations is the major joining and repair technology for such components. Several years of in-service experience and long-term creep testing data of cross-welds of various martensitic steel grades have revealed the inter-critical or fine-grained region of the heat-affected zone as the weakest part of such welded constructions. Within this work, based on long-term creep tests of cross-weld specimens, the predominant failure mode of Type IV cracking is described. Reduced creep strength of cross-welds is related to an enhanced microstructural degradation in the heat-affected zone. Therefore, the formation of the heat-affected zone and resulting microstructure is investigated using advanced microscopy, physical weld simulation and X-ray diffraction. Phase transformations occurring during characteristic weld thermal cycles in the heat-affected zone are recorded by in-situ X-ray diffraction using high-energy synchrotron radiation. The evolution of microstructure in the heat-affected zone is studied by physical weld simulation and subsequent metallographic examination. Within this work, a novel 9Cr-3-W-3Co steel with balanced additions of boron and nitrogen is produced and investigated. The base material shows superior creep strength at650°C up to the current maximum testing time of 19 000 h. Additionally, the new steel has high potential to overcome Type IV failures by the suppression of grain refinement in the heat-affected zone.