Creep of 2 1/4 Cr/Mo ferritic steel

• Main Author: Willis, M. R.
• Published: Swansea University 1991
• Subjects: 669
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.636660

The creep and fracture properties of a ferritic 2 1/4Cr1Mo steel has been determined over the stress range 160-350MPa at 783-873K using high precision, constant stress creep testing machines. With the aid of electron microscopy of replicas, energy dispersive analysis and electron diffraction techniques the changes in precipitate morphology and distribution during creep were studied and were found to be in accordance with existing literature. In all cases, normal creep curves were recorded whereby after the initial strain on loading the strain rate decreased to what appeared to be a steady state before the onset of an accelerating stage. In all cases, the tests were carried out in air to fracture. A transition in fracture mode with applied stress was apparent and where brittle, intergranular failure was observed, fracture occurred by the link-up of cavities nucleated at incoherent sulphide particles. The variation of rupture life and secondary creep rate with testing condition gave a stress exponent of n = 8-11, and an activation energy for creep of 430 kJmol<SUP>-1</SUP>. The existence of a steady state during creep was doubted and by describing the creep curves as a consequence of just two competing events, the variation of strain, ε, with time, τ, was found to be accurately described. The well behaved nature of the θ functions enabled predictions to be made not only of entire creep curves, minimum creep rates and times to rupture but also of data relevant to engineering design. Prediction from short term tests in this manner could thus obviate the need for long term testing and increase the range of understanding of not only 2 1/4 Cr1Mo steel but also a wide variety of new creep resistant materials.