The steel industry is a severely competitive place to do business. So any technique for rapidly evaluating alternative alloy compositions, estimating the properties of off-specification melts and highlighting potential ways of reducing costs could offer companies considerable advantages. A versatile new software-based approach is now available that should prove useful to producers and end-users alike who are looking to improve their materials, heat treatment regimes and other processing operations. The software enables users to predict key transformation diagrams for the materials they are making and processing.
Time-temperature transformation (TTT) and continuous cooling transformation (CCT) diagrams are acknowledged as the best reference points for steel heat treatment, alloy production and alloy design. However, a large number of practical experiments are required to generate sufficient diagrams to cover the multitude of alloy compositions and heat treatments found in industry. The measurements themselves are time-consuming,
and their interpretation is not always straightforward.
As a result of these factors, TTT and CCT diagrams are often not available for the specific compositions required, nor do they always cover the full range of cooling rates, temperatures and grain sizes that may be of interest for a specific application.
Many years ago, this problem was partially solved by the introduction of the Jominy Hardenability test, which provides indirect information about critical cooling rates and related mechanical properties via the measurement of hardness values. However, this test does not provide any basic structural information. A more comprehensive computerised approach was developed by Kirkaldy in response to the oil crisis of the 1970s.But this approach could only function through the use of many empirical correlations, largely because the complex interaction between alloying elements could not be formulated explicitly at that time.
Since then, techniques for the calculation of multi-component phase diagrams such as CALPHAD have become well established. These new techniques can handle more than a dozen components in the case of steels, and can more accurately account for various interactions
between components than the earlier methods.
This means it is possible to derive more accurate driving forces for the precipitation of individual phases or of twophase products such as pearlite. When combined with theories of nucleation and growth, it is possible to make quantitative predictions of TTT and CCT curves, derive the related Jominy Hardenability and extend the calculations to other mechanical properties. The new software tool JMatPro, which is an acronym for Java-based Materials Properties software, is a suite of computer programs that has been developed by Thermotech and Sente Software for the prediction of a broad range of materials properties. Figure 1 shows the procedure adopted for the prediction of TTT, CCT and Hardenability diagrams. The user simply inputs their choice of alloy composition, austenite grain size and austenitising temperature, together with a choice of how the answers should be displayed.
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