Abstract

    Knowledge of the TTT or CCT diagrams of steels is an important factor in the thermo-mechanical processing of steels. Much experimental work has been undertaken to determine such diagrams. However, the combination of wide alloy specification ranges, coupled with sharp sensitivity to composition changes plus a dependency on grain size, means that it is impossible to produce enough diagrams for generalised use. To this end significant work has been undertaken over recent decades to develop models that can calculate TTT and CCT diagrams for steels. Almost without exception, these models have been shown to be limited in applicability to carbon and low alloy steels. The aim of the present work is to develop a model that can provide accurate TTT and CCT diagrams for general steels, including medium to high alloy types, tool steels, 13%Cr steels etc., for inclusion in the software programme JMatPro. This aim has been achieved and the present paper provides a background to the calculation method and present results of an extensive validation of the model against experiment.

Introduction

    As part of the development of the software programme JMatPro, phase transformation models have been included for a variety of alloy types [1,2,3,4] e.g. Al-alloys, Ni-based superalloys, Ti-alloys and for TCP phases such as σ and χ in stainless steels. JMatPro also incorporated a cap-ability to calculate transformations involving ferrite, pearlite and bainite in HSLA steels closely based on the model of Kirkaldy [2]. To bring this calculation capability in-line with JMatPro's capability for other alloys, where high alloy contents can be routinely handled, work has been under-taken to extend this model capability to steels of high alloy content.

    There is a hugely extensive published literature concerned with the transformations in steels but, of this, only a small part is given over to the calculation of TTT and CCT diagrams. The pioneering work of Kirkaldy and co-workers [5,6] showed that it was possible to calculate quite accurate TTT and CCT diagrams as well as the Jominy hardenability for low alloy steels. Later work by Bhadeshia [7,8] used a different methodology to determine start curves for ferrite and bainite transformations and tested the model against experiment. The model of Bhadeshia has been ex-tended by Lee [9,10] to cover slightly higher concentrations. However, although successful for low alloy steels these models are limited when it comes to more highly alloyed types.

    One of the drawbacks of both models has been the use of dilute solution thermodynamics in calculating transfor-mation temperatures. This can now be overcome using thermodynamic models [11] that provide high quality results for steels in general, ranging from stainless steels, to tool steels as well as the low to medium alloy range types.

    The aim of the present work is to combine the more extensive thermodynamic models with a kinetic model to see if the composition range of applicability could be extended to cover a wider range of steels, including the highly alloyed types. The model of Kirkaldy was chosen as the basis for the new calculations as there is a clearly identifiable set of input parameters that are required and which can be readily calculated. It is also has empirical parameters that can be adjusted easily and controllably.

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