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The corrosion resistance of martensitic stainless steels (MSS) depends strongly on the chemical composition and the applied heat treatment. Both determines the distribution of the alloying elements in the microstructure and the resulting material properties. The addition of nitrogen is known to be beneficial for the pitting corrosion resistance of stainless steels. In case of MSS this effect is not only connected to nitrogen itself because nitrogen can be used to substitute carbon which also influences the result of the heat treatment process. This paper shows the effect of nitrogen on the corrosion resistance in relation to the hardening process of MSS. Therefore the effects of austenitization duration, austenitization temperature and cooling rate on microstructure, hardness and corrosion resistance were studied on the MSS X30CrMoN15 1 and X50CrMoV15. The effect of different cooling rates was studied in the range of > 100 K/s down to 1 K/s using the jominy end quench test. The changes in corrosion resistance were detected with electrochemical potentiodynamic reactivation (EPR) and by the determination of critical pitting potentials. Besides this experimental approach thermodynamic calculations with the software thermocalc will be presented and used for the interpretation of the effect of nitrogen on the corrosion resistance of MSS.
Das Korrosionsverhalten von kohlenstoffhaltigen martensitischen nichtrostenden Stählen variiert in Abhängigkeit der Wärmebehandlung (WB) und der damit eingestellten Mikrostruktur deutlich stärker als bei kohlenstoffarmen ferritischen und austenitischen nichtrostenden Stählen. Bei erhöhtem Kohlenstoffgehalt bestimmt die diffusionsgesteuerte Bildung und Auflösung von Chromkarbiden die Verteilung von Chrom und Kohlenstoff im Gefüge. Bisher lag der Fokus von Forschungsarbeiten zum Einfluss der WB auf dem Anlassen im allgemein bekannten Sensibilisierungsbereich dieser Werkstoffgruppe zwischen 200 °C und 700 °C und der dort auftretenden Chromverarmung. Mit der gezielten WB des X46Cr13 (1.4034) wird gezeigt, dass Temperatur und Dauer beim Austenitisieren sowie die anschließende Abkühlung beim Härten das Korrosionsverhalten schon vor dem Anlassen signifikant beeinflussen. Auf der Basis von thermodynamischen Berechnungen wurden definierte WB ausgewählt, um gezielt unterschiedliche Volumengehalte von Chromkarbiden im Gefüge und somit auch unterschiedliche Chrom bzw. Kohlenstoffgehalte im Mischkristall zu erzeugen. Anschließend wurden die resultierenden Gefügezustände hinsichtlich Chromkarbidanteil und Härte verglichen und das Korrosionsverhalten mit der elektrochemisch potentiodynamischen Reaktivierung (EPR) sowie durch einen Schnelltest mit der KorroPad-Prüfung untersucht. Dabei konnte ein direkter Zusammenhang zwischen WB, Mikrostrukturänderungen und Korrosionsverhalten festgestellt werden. Mit steigender Austenitisierungstemperatur wird der Anteil an Chromkarbiden reduziert und der Kohlenstoff- und Chromgehalt der Matrix erhöht, bis eine vollständige Auflösung der Chromkarbide gegeben und die chemische Nennzusammensetzung der Legierung im Mischkristall erreicht ist. In der direkten Folge wird die Ausbildung der für nichtrostende Stähle charakteristischen Passivschicht erleichtert und das Lochkorrosionsverhalten verbessert. Die Abkühlrate hat neben den Austenitisierungsparametern ebenfalls einen großen Einfluss auf das Korrosionsverhalten. So führt eine langsame Abkühlung an Luft zu einer Chromverarmung im Gefüge, die eine deutlich erhöhte Lochkorrosionsanfälligkeit zur Folge hat. In Abhängigkeit der WB von kohlenstoffhaltigen nichtrostenden Stählen können Mikrostruktur, Härte und Korrosionsbeständigkeit in einem weiten Bereich variieren.
The novel heat treatment concept of “Quenching and Partitioning” (Q&P) enables producing low alloyed martensitic steels with high strength and formability. Therefore austenite, retaining from a quenching temperature between Mf and Ms, is stabilised by carbon diffusion. This stabilised austenite transforms in martensite (TRIP effect) under mechanical loading. Current investigations on the Q&P heat treatment of martensitic stainless steels reveal a further enhancement of mechanical properties due to higher amounts of austenite. Thus a tensile strength of 1.800 MPa and a maximum elongation of 20 % are possible, while mechanical properties under compression are much higher due to a distinct SD effect.
The presentation aims to gain a further understanding of the Q&P heat treatment and the resulting mechanical properties for the steel X46Cr13, especially the mechanical behaviour under dynamic mechanical loading condition. Results of drop work and Split Hopkinson Pressure Bar test confirms the findings of quasi static compression test. Furthermore, Charpy impact tests show higher impact toughness compared to the common heat treatment of quenching and tempering (Q&T). Complementary investigations indicate a higher corrosion resistance of Q&P compared to Q&T.
Co-Cr-Mo-Werkstoffe sind im Bereich der Biomaterialien weit verbreitet und werden für Endoprothesen eingesetzt. Deren Lebensdauer ist jedoch durch tribologische und korrosive Dauerbelastung begrenzt. Um die Anzahl operativer Eingriffe am Patienten zu minimieren, ist es notwendig den Werkstoff hinsichtlich Verschleißbeständigkeit und Korrosionsbeständigkeit zu optimieren. Hierfür ist ein umfassendes Verständnis der legierungsabhängigen Phasenbildung und –entwicklung während Herstellung und Wärmebehandlung notwendig. An einer speziellen Co-Cr-Mo Legierung werden verschiedene Untersuchungen zur Gefügecharakterisierung durchgeführt. Dabei werden die auftretenden Phasen thermodynamisch berechnet, mittels XRD nachgewiesen, die Phasenmorphologie durch EBSD analysiert, die Elementverteilung durch EDX-Analyse ermittelt und die Ergebnisse durch Farbätzen verifiziert. Das Ziel der Untersuchungen ist es, Optimierungspotentiale bei Herstellung und Wärmebehandlung zu identifizieren.
Hardness and pitting corrosion resistance are the major quality criteria of cutlery. Both are achieved by the heat treatment (austenitization, quenching and tempering) of the normally used martensitic stainless steels. The established quality control method regarding the pitting corrosion resistance is an alternating immersion test in 1 % NaCl solution at 60 °C according to DIN EN ISO 8442. This standard test shows a high deviation, which limits any optimization of the heat treatment process. New approaches for corrosion testing of martensitic stainless-steels were developed and used in the last years to connect the weak pitting corrosion resistance of martensitic stainless-steels with the phenomenon of chromium depletion. The tempering temperatures used in the industrial heat treatment of cutlery are too low to explain the appearance of chromium depletion. For this reason, a systematic investigation of three heat treatment parameters (austenitization time, cooling speed and tempering temperature) were performed on the martensitic stainless-steels X50CrMoV15 (1.4116) to detect their contribution to chromium depletion. The electrochemical potentiodynamic reactivation (EPR), which is very sensitive to any change of the microstructure, was used to quantify the degree of chromium depletion. The KorroPad indicator-test was applied to correlate low pitting corrosion resistance to the presence of chromium depletion. The results of all investigations allow conclusions about the very small process window, which is necessary to achieve cutlery with high pitting corrosion resistance.
Nickel-free, nitrogen alloyed austenitic stainless-steels, with about 19 wt.-% Mn and 0,8 wt. % N, are an interesting alternative to classic CrNi austenitic stainless steels due to their superior mechanical properties (Rm > 900 MPa, A5 > 50 %, Av > 350 J) in the solution annealed condition. The formation of chromium-rich nitrides during suboptimal heat treatment, processing or application leads to an inhomogeneous distribution of alloying elements in the microstructure, which reduces the corrosion resistance. Consequently, an accurate knowledge of the sensitization behavior is indispensable for the use of nickel-free, high-nitrogen austenitic stainless steels. The relationship between artificial aging, phase formation and corrosion resistance was investigated on the alloys X8CrMnN18-19 (1.3815) and X8CrMnMoN18-19-2 (1.4456), both alloyed with 0,8 wt.-% Nitrogen, in the present work. The microstructural evolution was studied by LM and SEM while the corrosion resistance was characterized with the electrochemical potentiodynamic reactivation (EPR) and the KorroPad indicator-test. Both alloys showed increased corrosion susceptibility within critical aging parameters. Finally, a sensitization diagram was described successfully for both alloys showing the positive effect of molybdenum.
The copper alloyed stainless steel 1.4542 (X5CrNiCuNb16-4) is used in different areas due to its good mechanical properties and corrosion resistance. Strength and corrosion re-sistance can be adjusted by the heat treatment, which is of importance for the application of this alloy. The mechanical properties (strength and hardness) are attributed to the dispersive precipitation of the copper rich ε–Phase. The additional precipitation of chromium carbides can reduce the corrosion resistance. Different ageing states were produced to investigate the precipitation behaviour with various methods. Furthermore, the influence of cold-rolling on the precipitation behaviour was studied in comparison to a solution annealed state without deformation. The microstructure was studied by SEM and the variations of hardness and magnetic proportion were characterised. The electrochemical potentiodynamic reactivation (EPR) was used to determine the corrosion resistance and detect chromium depletion in all heat-treated states. The results show that a work hardening accelerates the precipitation rate, while ageing at 600 °C reduces the corrosion re-sistance due to chromium depletion.
The copper alloyed stainless steel 1.4542 (X5CrNiCuNb16-4) is used in different areas due to its good mechanical properties and corrosion resistance. Strength and corrosion resistance can be adjusted by the heat treatment, which is of importance for the application of this alloy. The mechanical properties (strength and hardness) are attributed to the dispersive precipitation of the copper rich ε–Phase. The additional precipitation of chromium carbides can reduce the corrosion resistance. Different ageing states were produced to investigate the precipitation behaviour with various methods. Furthermore, the influence of cold-rolling on the precipitation behaviour was studied in comparison to a solution annealed state without deformation. The microstructure was studied by SEM and the variations of hardness and magnetic proportion were characterised. The electrochemical potentiodynamic reactivation (EPR) was used to determine the corrosion resistance and detect chromium depletion in all heat-treated states. The results show that a work hardening accelerates the precipitation rate, while ageing at 600 °C reduces the corrosion resistance due to chromium depletion.
The copper alloyed stainless steel 1.4542 (X5CrNiCuNb16-4) is used in different areas due to its good mechanical properties and corrosion resistance. Strength and corrosion resistance can be adjusted by the heat treatment, which is of importance for the application of this alloy. The mechanical properties (strength and hardness) are attributed to the dispersive precipitation of the copper rich ε–Phase. The additional precipitation of chromium carbides can reduce the corrosion resistance. Different ageing states were produced to investigate the precipitation behaviour with various methods. Furthermore, the influence of cold-rolling on the precipitation behaviour was studied in comparison to a solution annealed state without deformation. The microstructure was studied by SEM and the variations of hardness and magnetic proportion were characterised. The electrochemical potentiodynamic reactivation
(EPR) was used to determine the corrosion resistance and detect chromium depletion in all heattreated states. The results show that a work hardening accelerates the precipitation rate, while ageing at 600 °C reduces the corrosion resistance due to chromium depletion.
Quenching and partitioning (Q&P) heat Treatment increases the deformability of high-strength martensitic steels. Therefore, it is necessary to have some metastable austenite in the microstructure, which transforms in martensite during plastic deformation (TRIP effect). The austenitic-martensitic microstructure is gained by an increased austenitization temperature, water quenching and additional partitioning. The partitioning enables local carbon diffusion, which stabilizes retained austenite and leads to partial reversion of martensite to austenite. The influence of partitioning time was studied for the martensitic stainless steel AISI 420 (X46Cr13, 1.4034). In line with these efforts, metallographic, XRD and EBSD measurements were performed to characterize the microstructural evolution. The mechanical properties were tested using tension and compression loading. Additional corrosion investigations showed the benefits of Q&P heat treatment compared to conventional tempering. The reversion of austenite by the partitioning treatment was verified with EBSD and XRD. Furthermore, the results of the mechanical and corrosion testing showed improved properties due to the Q&P heat treatment.
