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Licensed Unlicensed Requires Authentication Published by De Gruyter April 23, 2021

A Model to Predict the Microstructural Constituents after Quenching and Partitioning of Martensitic Stainless Steels

Ein Modell zur Vorhersage der mikrostrukturellen Bestandteile nach dem Abschrecken und umverteilenden Anlassen von korrosionsbeständigen martensitischen Stählen
S. Kresser, R. Schneider, H. Zunko and C. Sommitsch

Abstract

The typical heat treatment of martensitic stainless steels comprises hardening and subsequent tempering. Depending on the application and size of the component, tempering is carried out either at low temperatures (< 300 °C) or at high temperatures (> 500 °C). In this paper, tempering at lower temperatures is examined.

First, the austenitizing step is considered in greater detail and an optimized formula for the calculation of the MS temperature of such steel grades is created in order to enable to be modelled. For the calculations, the austenite composition is determined at different austenitizing temperatures using thermodynamic simulation. Furthermore, the transformation of austenite into martensite during quenching is described with the help of the Koistinen-Marburger equation.

The second part deals with effects in the material at low holding temperatures. Here, the influence of different hardening temperatures and interception temperatures of the quenching procedure is investigated. There is no complete partitioning at temperatures of 300 °C. Certain tempering processes can also take place, such as the formation of transition carbides, so-called M3C carbides. A typical tempering with formation of stable Cr-rich carbides does not occur at this low temperature. Finally, the calculated results of the model correlate well with microstructural investigations (XRD, LOM). ◼

Kurzfassung

Die typische Wärmebehandlung von korrosionsbeständigen martensitischen Stählen besteht aus Härten und anschließendem Anlassen. Abhängig von der Anwendung und der Bauteilgröße wird bei niedrigen Temperaturen (< 300 °C) oder hohen Temperaturen (> 500 °C) angelassen. In diesem Artikel wird das Anlassen bei niedrigen Temperaturen untersucht.

Als erstes wird die Austenitisierung und Abkühlung genauer betrachtet, um eine optimierte MS-Formel für diese Stähle zu erstellen und somit in weiterer Folge eine Modellierung zu ermöglichen. Für die Erstellung der MS-Formel wird die Austenitzusammensetzung bei verschiedenen Härtetemperaturen benötig, diese wird mithilfe einer thermodynamischen Simulation berechnet. Des Weiteren wird die Umwandlung des Austenits in Martensit während des Abschreckens mithilfe der Koistinen-Marburger-Gleichung beschrieben.

Der zweite Teil des Artikels befasst sich mit dem Anlassen bei niedrigen Temperaturen. Hier wurde der Einfluss von verschiedenen Härtetemperaturen und Abfangtemperaturen beim Abschrecken untersucht. Es wird auch berücksichtigt, dass es zu einem teilweisen Umverteilungseffekt – vor allem des Kohlenstoffs in den Austenit – während des Haltens auf Anlasstemperatur kommt. Abschließend werden die berechneten Ergebnisse des Modells mit mikrostrukturellen Untersuchungen (XRD, LIMI) verglichen, welche eine gute Übereinstimmung aufweisen. ◼

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Published Online: 2021-04-23
Published in Print: 2021-04-30

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