Modelling the hardenability and tempering of high strength steels

• Autorzy: Eliasson J. , Siwecki T. , Rodell B.

Warianty tytułu

PL

Modelowanie zdolności do hartowania i odpuszczania stali o podwyższonej wytrzymałości

• Konferencja: 14th KomPlasTech Conference, Zakopane, January 14-17, 2007
• Języki publikacji: EN

Abstrakty

EN

The aim of the present work is to develop the hardenability and tempering models for various quenching and tempering treatments of high strength/martensitic steels. The main aim for the steel industry is to reduce the number of tests (Jominy) that will be carried out during development of new hardened steel grades, thereby reducing costs in the development stage. Other important aims are tailoring of mechanical properties with respect to alloying of the steels and reduction of weight by usage of steels with higher strength. In many cases steels are over alloyed to ascertain full hardenability. This can however increase the cost of the steel with no beneficial gain in the mechanical properties. With the help of modelling hopefully over alloying can be avoided. So, the benefit with modelling is reduced costs for material development as well as production costs with improved properties of the final products. Basis for the hardenability model is divided in three sub-models: 1. Dissolution of particles during reheating. Alloying elements generally increase hardenability when in solution in the austenite. 2. Calculation of hardenability distance kinetically depending on the dissolution of particles. Equilibrium of elements in solution or in particles is determined with ThermoCalc with a database for HSLA steels. Multiplication factors for alloying elements based on Grossmans work are used for calculation of Ideal diameter for 50% martensite in the centre of a bar. For comparison with Jominy bars, the Ideal diameter is transformed to Jominy distance. Also boron is considered since many quenched steels contain this alloying element, having a large effect on hardenability. 3. Grain growth with consideration taken to particles and their dissolution. Larger grains increase hardenability. The hardenability model will be used specifically for steel containing less than 0.4% C. For low carbon steels modified relations should be used. Below 0.2% C alloying elements such as Cr and Mo contribute less to hardenability. Therefore other relations must be used to fully describe the influence of alloying elements on hardenability. Combination effects occur, not only due to presence of particles and their ability to dissolve during reheat. Also interaction effects such as those for Mo and Ni in low carbon steels. According to the literature a high Ni-content (>0.75 wt-%) increases the influence of Mo on hardenability. In general the influence of individual alloying elements are lower than for steels with higher C-contents. The tempering model is based on experimental information from the literature. In this sense it is empirical. By combining the data from hardenability calculations using the hardenability model room temperature hardness and ultimate tensile strength after tempering in the temperature range 20-700 °C are calculated. The final properties are dependent on dissolution of alloying elements already during reheat before quench. In this paper comparisons of different alloys are given for hardening and tempering. Examples of verifications to experimental data are given both for the hardenability model as well as the tempering model. The tempering model generally functions well in comparison with experimental data. The pre-history before quench is important and affects the level of hardness and strength. In current work we try to take this into consideration by first applying the hardenability model.

PL

Celem projektu jest opracowanie modeli hartowalności i odpuszczania dla różnych zabiegów obróbki cieplnej wysoko wytrzymałych stali martenzytycznych. Model hartowalności opracowany w KIMAB jest podzielony na trzy składowe: (i) Rozpuszczanie cząstek w czasie wygrzewania, (ii) Wyznaczenie obszarów zahartowanych w oparciu o kinetykę rozpuszczania cząstek i (iii) rozrost ziaren (uwzględniając wpływ rozmiaru i objętości cząstek na efekt piningu Zenera. Model hartowalności przewiduje Idealną Średnicę, odległość Jominy'ego dla 50% martenzytu oraz twardość powierzchni stali po hartowaniu. Przykłady obliczeń dla stali z SSAB Oxelósund są przedstawione w artykule i porównane z wynikami doświadczeń. W K1MAB opracowano również doświadczalny model odpuszczania, który opiera się na danych doświadczalnych zaczerpniętych z literatury. Obliczona została twardość i wytrzymałość na rozciąganie hartowanych stali, otrzymanych z Ovako Bar, po odpuszczeniu w temperaturach 150°-700°C, i wyniki porównano z pomiarami. Model odpuszczania funkcjonuje poprawnie i ogólit skano zgodność z danymi doświadczalnymi. Niemniej j historia zmian temperatury przed hartowaniem ma wpływ i powinna być uwzględniona, ponieważ rzutuje i twardość i wytrzymałość stali. Stosując razem modele hartowalności i odpuszczania można badać wpływ nierozpuszczonych cząstek na własności stali po obróbce cieplnej.

Słowa kluczowe

EN

hardenability; carbide dissolution; modelling; tempering; grain growth; microalloyed steels

• Wydawca: Wydawnictwa AGH
• Czasopismo: Computer Methods in Materials Science
• Rocznik: 2007
• Tom: Vol. 7, No. 1
• Strony: 54--60
• Opis fizyczny: Bibliogr. 9 poz., rys.

Twórcy

autor

Eliasson J.

autor

Siwecki T.

autor

Rodell B.

• Corrosion and Metals Research Institute, KIMAB Drottning Kristinas vag 48, S-114 28 Stockholm, Sweden 2 SSAB Oxelósund AB, Sweden

Bibliografia

• ASTM Standard A 255-95, 1995, Standard Test Method for End-Quench Test for Hardenability of Steel, 32-51.
• de Retana, A.F., Doane, D.V., 1971, Predicting the hardenability of carburizing steels, Metal Progress, 100, 65.
• Eliasson, J., Siwecki, T., 2004, Influence of alloying elements on mechanical properties in tempered martensitic steels, Research report of Swedish Institute for Metals Research, IM-2004-524.
• Grange, R.A., Hribal, C.R., Porter, L.F., 1977, Hardness of tempered martensite in carbon and low-alloy steels, Met. Trans. A, 8A, 1775-1785.
• Grossman, M.A., 1942, Hardenability calculated from chemical composition, Trans. AIME, 150, 227-229.
• Hillman, P., Hillert, M., 1975, On the effect of second-phase particles on grain growth, Scand. J. Metall. 4,211-219
• Rodell, B., Siwecki, T., 2000, Hardenability model for steel: Part l. Dissolution of particles, grain growth and ideal diameter, Research report of Swedish Institute for Metals Research, IM-2000-565
• Siwecki, T., Rodell, B., 2003, Modelling the effects of microalloying on the hardenability of high strength steels, Symp. on the thermodynamics, kinetics, characterization and modelling of: austenite formation and decomposition, Chicago, 227-245.
• www.thermocalc.se, ThermoCalc Software.