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The stability of austenite is one of the most dominant factors affecting the toughness properties of high Co-Ni steels such as Aermet 100 and AF1410. Thus, the aim of this work was to get a deeper understanding on the impact of combined heat and cryogenic treatment on the stability of retained and reverted austenite. In order to characterize the evolution of the phase fraction of austenite during tempering at different temperatures and times, X-ray diffraction analyses were carried out. The stability of austenite, which was formed during tempering, was analyzed with dilatometric investigations by studying the transformation behavior of the austenite during cooling from tempering temperature down to -100°C. Additionally, transmission electron microscopy investigations were performed to characterize the chemical composition and phase distribution of austenite and martensite before and after tempering.Keywords: high Co-Ni steel, reverted austenite, retained austenite, high temperature X-ray diffraction, dilatometer investigations Stabilność austenitu jest jednym z najbardziej dominujących czynników mających wpływ na ciągliwość stali Co-Ni, takiej jak Aermet 100 i AF1410. Celem pracy było głębsze zrozumienia wpływu skojarzonego nagrzewania i obróbki kriogenicznej na stabilność austenitu szczątkowego i przemienionego. Ewolucję udziału fazy austenitycznej podczas odpuszczania w różnych temperaturach i czasach przeprowadzono stosując rentgenowskie badania dyfrakcyjne. Stabilność austenitu, który powstał podczas odpuszczania, badano metodą dylatometryczną, analizując zachowanie się austenitu podczas odpuszczania w temperaturach poniżej -100°C. Zrealizowano badania metodą mikroskopii elektronowej w celu określenia składu chemicznego i rozkładu austenitu i martenzytu przed i po odpuszczaniu.Słowa kluczowe: stal Co-Ni, austenite przemieniony, austenite szczątkowy, rentgenowska dyfraktometria wysokotemperaturowa, dylatometria toughening effect of austenite can be characterized by its stability -the resistance against transformation [5]. It is assumed that the higher the stability of austenite is, the higher is also the toughening effect of austenite. The stability of austenite is affected by its chemical composition and the size of austenite grains [6]. It is reported that austenite grains exhibit increasing resistance against transformation with decreasing size [7]. Haidemenopoulos [5] claimed that this is a result of the lower number of potential nucleation sites for phase transformation due to smaller grain sizes. In contrast to this, it is described in [8] that transformation of small austenite grains in shape memory alloys is harder because of the increasing energy barrier for transformation with decreas-

Purpose of this work is to investigate the effects of reverted and retained austenite on the fracture toughness of high Co-Ni steels. Different cryogenic and tempering treatments are applied to a high Co-Ni steel in order to alter the austenite content, its chemical composition and hence its stability against transformation to martensite. Austenite stability is analyzed by applying thermal cycles in the dilatometerand by X-ray diffraction. For each heat treatment, the fracture toughness is characterized by J-integral and fractographic crack growth resistance measurements. The results indicate that cryogenic and two-step tempering treatments lead to a higher austenite stability and impede austenite transformation during cooling after tempering. Consequently, the toughness is considerably improved.