Unlike the well‐known effect of alloy elements in promoting the ferritic or austenitic solidification of stainless and acid‐resisting chromium‐nickel steels, kinetic effects have as yet not been so widely looked into. For this reason, the impact of the solidification rate on the ratio of the amounts of ferritic and austenitic liquid solidification was investigated for the steels of grades X8CrNiTi18.10 and X8CrNiMoTi18.11. A microanalysis for the determination of the primary ferrite content of samples taken from ingots of different size and at different distances from the ingot surface for a total of 161 heats revealed the following: – Increasing solidification rate causes the primary ferrite content produced during solidification to rise for steels with peritectic solidification sequence due to the resultant approach of the distribution coefficient to unity. – Increasing solidification rate causes the austenite content to rise for steels with a primary simultaneous crystallization of austenite and ferrite due to a low total segregation in case of austenite crystallization as compared with ferrite crystallization. – The effect of an elevated solidification rate is qualitatively equivalent to a shift of the saturation lines of the three‐phase space l+δ+γ in the Fe—Ni—Cr ternary system for liquid and γ‐crystals in the direction S with the two saturation lines approaching each other. Hence, contrary to what is expected according to the equilibrium diagram of Schürmann and Brauckmann, austenitic Cr—Ni steels solidify primarily in peritectic mode and, in the area of the line of the double‐saturated liquid, through a primary simultaneous crystallization of austenite and ferrite. – The boundary composition between primary ferritic and primary austenitic crystallization changes with an increase in cooling rate by seven orders of magnitude from 1.25 to 1.70 as expressed in the ratio of the Cr—Ni equivalents according to Hammar and Svensson.

The effect of cryoforming at 77 K on the flow and work‐hardening characteristics was investigated considering the martensitic transformation behaviour in austenitic Cr Ni steels with different nickel contents. The test steels can be divided into two groups relating to the flow and work‐hardening characteristics and martensitic transformation behaviour at 77 K. The first group comprises steels with less than 16 % nickel, the second group those with more than 16 % nickel. The flow curves of the first‐group steels show two inflection points on the basis of γ → α'‐transformation. αγ'‐martensite is observed and ∊‐ and α∊'‐martensite too. The flow curves of the second‐group steels do not show any inflection points. The γ → α'‐martensitic transformation is not induced, ∊‐ and α∊'‐martensite are provable by light and scanning microscopy. The stress‐strain intervals were determined for the individual martensite transformations at 77 K in the test steels. They are dependent from the nickel content. The stress which specifies the first inflection point on the flow curve and the minimum of the work‐hardening rate, respectively, characterizes the stress for initiating the deformation‐induced αγ'‐martensite formation. Transformation of the austenite to α' martensite will end in achieving a stress of 1200 to 1400 MPa, i.e. in achieving the second inflection point of the flow curve and the maximum of the work‐hardening rate, respectively. The stress interval is not dependent from the nickel content.

On the basis of well‐known thermodynamic equations, a model is proposed which allows the computation of both the solution behaviour and the composition of the carbonitride precipitates of the microalloying elements in steel. Apart from the concentration of the microalloying elements and the carbon and nitrogen content, the following aspects were taken into account: the non‐stoichiometric composition of carbonitrides, a regular solution behaviour of the mixture of carbide with nitride, the effect of third elements on the activity of carbon and nitrogen, the possible primary binding of nitrogen by aluminium. The calculation is based on the knowledge of the solubility products of pure carbides and nitrides. Comparison with experimental results of carbonitride precipitates in microalloyed niobium and vanadium steels support the model‐based predictions concerning the temperature‐dependent composition and solubility of carbonitrides under equilibrium conditions.

Das Anliegen der Studie war es, zu ermitteln, in welcher Weise martensitische Gefüge, die in austenitischen Cr‐Ni‐Stählen durch gezielte Kryoumformung bei 77 K gebildet wurden, eine Auswirkung auf das Lochfraßverhalten haben. Die Untersuchungen wurden an einer Legierungsreihe mit Basisgehalten von 17% Chrom und abgestuften Ni‐Gehalten von 8–20% durchgeführt. Die Charakterisierung der Gefüge mittels röntgenographischer, magnetischer, metallographischer, mechanischer und chronopotentiostatischer Untersuchungen führte zu Aussagen über Zusammenhange zwischen Martensitumwandlung und Eigenschaften. Die Gegenüberstellung der Ergebnisse aus Messungen zum Verfestigungsverhalten, Fließverhalten und Korrosionsverhalten zeigte, daß Verfestigungskennwerte und Lochfraßpotentiale ähnliche Veränderungen in Abhängigkeit vom Umformgrad erfahren. Eine Verbesserung des Lochkorrosionsverhaltens tritt ein, wenn durch hinreichend große plastische Deformation vorwiegend α′‐Martensit direkt aus dem Austenit entsteht. Dagegen weisen Gefügezustände mit ϵ‐Martensit und daraus teilweise umgewandelten Anteilen aus α′‐Martensit ein ungünstigeres Lochfraßverhalten aus.

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