Fracture Toughness Characterisation of the Martensitic Chromium Steel P91

The thermal efficiency of thermal power plants has been significantly enhanced by increasing the operation temperature and pressure up to 650°C and 300 bars, respectively. These operation conditions make high demands on the durability and fracture toughness in this temperature range. These requirements meet heat-resistant chromium steels due to its alloying elements and their heat treatment. One representative is the X10CrMoVNb9-1steel, which is specified as P91 according to the US ASME Boiler and Pressure Vessel Code. The range of loading requires the determination of fracture toughness values in the ductile range. In case cracks in a structure initiate and propagate by ductile mechanisms, it is conventional to measure the material’s resistance to crack growth, generally in terms of J, and to plot this against the crack extension, Δa, to give a J-resistance curve. The J-R curves can be constructed using either a multiple specimen or a single specimen approach. But since, in the most cases the material available for testing purposes is limited; the application of the single specimen method becomes a necessity. The techniques available for this purpose are unloading compliance, potential drop, acoustic emission, ultrasonic measurements among others. In ductile materials, first the blunting of pre-existing cracks occurs during loading followed by formation of voids ahead of the crack tip at critical strain. These voids finally coalesce with the crack tip leading to crack propagation. Hence, the ductile crack initiation cannot be defined as a point in the J-Δa curve but rather as a process which occurs over a range. The parameters JIc or J0.2BL are engineering estimates of fracture toughness defined by the intersection of the 0.2 mm offset construction line and the J-R curve. They are measured at a point near the initiation of ductile crack extension and provides a provisional JQ or J0.2BL(B) value, which when properly qualified against the criteria proposed in the standard procedures like ASTM E1820 and ISO 12135 becomes the fracture toughness parameter JIc or J0.2BL respectively. These values are taken as the initiation fracture toughness of the material. Another criterion for prediction of fracture toughness is measuring the stretch zone width (SZW). This procedure is considered to give an accurate value of initiation fracture toughness, close to the onset of crack initiation with the degree of crack tip blunting increasing proportionally to the toughness of the material. But the determination of SZW is subjective and prone to error due to different perceptions of the users. In this paper J-R curves were measured on Charpy size SE(B) and C(T) specimens of P91 steel using the single unloading compliance technique. The fracture toughness parameters were determined according to the test standards ASTM E1820 and ISO 12135 and whose evaluation differences and validity criteria were assessed. Despite the available sophisticated instrumentation used for obtaining the J-R curves and strict implementation of the recommended test procedure, a scatter in the initial portion of the curve is persistent. This creates problems in the evaluation of the measured J-R curves and in obtaining valid and unambiguous JIc/J0.2BL values according to the test standards ASTM E1820 and ISO 12135. The influence of these scatter on these fracture toughness values is discussed. Accompanying fractographic studies were performed to assess the J-R curves and to determine the SZW. In addition the test temperatures were extended towards the lower ductile-to-brittle transition range, where fracture toughness values are determined by using the Master Curve approach.