Ronald Schnitzer

Precipitation evolution in a Ti-free and Ti-containing stainless maraging steel.

Stainless maraging steels have a Cr content higher than 12wt% and show a excellent combination of high strength and ductility, which make them attractive for use in machinery fields and aircraft applications. The massive increase of strength during ageing treatment of maraging steels is related to a precipitation sequence of various nm-scaled intermetallic phases. The peak hardness especially in Ti-containing maraging steels can be reached after short-time ageing at temperatures around 500 degrees C. However, precipitation reactions in different stainless maraging steels are not fully understood, especially the evolution from clustering over growing to coarsening. In the present work a commercial maraging steel and a Ti-containing model alloy are investigated and compared to each other. The steels were isothermally heat treated at 525 degrees C for a range of times. Special emphasis was laid on the correlation of hardness to the formation and presence of different kinds of precipitates. The isothermal aged samples were investigated by using two advanced three-dimensional energy compensated atom probes (LEAP and 3DAP) both in voltage mode and in laser mode. The atom probe data were correlated to standard hardness measurements. The results show that the partial substitution of Al by Ti results in a different precipitation behaviour. While the Ti-free maraging steel exhibit only one type of precipitate, the Ti-containing grade shows a change in the type of precipitates during ageing. However, this change leads to an accelerated coarsening and thus to a faster drop in hardness.

Herstellung lamellarer Gefügetypen in intermetallischen TiAl- Legierungen und deren Charakterisierung

Kurzfassung Im Rahmen dieser Arbeit wurden zwei lamellare Gefügetypen in γ-TiAl-Basislegierungen untersucht. Zum einen erfolgte die Herstellung der voll lamellaren Gefüge mittels einer Glühung im α-Einphasengebiet und anschließend langsamer Abkühlung. Das resultierende Gefüge besteht aus lamellaren γ-TiAl/α2-Ti3Al-Kolonien, wobei diese die ehemaligen α-Körner darstellen. Innerhalb einer Kolonie sind die γ/α2-Lamellen parallel angeordnet und weisen eine bestimmte kristallographische Orientierungsbeziehung auf. Zum anderen konnte mit Hilfe der Massivtransformation und anschließender Auslagerungsglühung eine neuartige lamellare Mikrostruktur eingestellt werden, die auf konventionellem Weg nicht realisierbar ist. Dabei scheiden sich aus dem massiv transformierten γm feine α2-Ti3Al-Lamellen aus, die in einem Winkel von 70,5° zueinander orientiert sind. Neben einer ausführlichen Beschreibung der Mikrostruktur werden die Orientierungsbeziehungen beider lamellarer Gefügetypen während der Entstehung beschrieben und die Keimbildung der γ-Lamellen in α-Körnern mittels Transmissionselektronenmikroskopie (TEM) untersucht.

High resolution imaging of martensitic all-weld metal

ABSTRACT Martensitic steel welds show promising results regarding their strength while they may tend to be brittle. As martensite is a quite complex microstructure, high resolution techniques like electron backscatter diffraction (EBSD) and atom probe tomography (APT) are valuable tools for an in-depth characterisation. In this study, the average block size and misorientation distribution of martensitic all-weld samples were evaluated with EBSD. A lower carbon content led to a smaller block size and consequently a higher toughness of the all-weld sample. Furthermore, APT revealed a concentration fluctuation of the main alloying elements and particles with a high carbon content. It is discussed how these methods can be used in the future to design the microstructure to achieve optimum properties.

Transformation of reverted austenite in a maraging steel under external loading: an in-situ X-ray diffraction study using high-energy synchrotron radiation

Abstract Reverted austenite strongly influences the mechanical properties of maraging steels. Therefore, X-ray diffraction using high-energy synchrotron radiation was applied for in-situ studying of the reverse martensitic phase transformation under external load in a PH 13-8 Mo maraging steel. The volume fraction of austenite, the domain size of the crystallites, and the lattice parameters were determined as function of strain for differently aged samples. It is shown that the reverted austenite is not mechanically stable under external load. Scanning of the fractured sample along the axial direction shows that the volume fraction of the austenite and the domain sizes strongly depend on the distance from the point of fracture initiation.

Creep investigation and simulation of CB2 joints using similar rutile CB2 flux-cored wire

The modified 9Cr-1.5Mo-1Co steel with boron addition, designated as CB2, is one of the most promising and creep-resistant cast steels for the application in thermal power plants. To produce big cast components for steam power plants, the welding processes are an integral part of the manufacturing procedure. Therefore, an appropriate filler metal with similar properties as the base material is required. So the CB2 filler metal is target for intensive researches. In this work, creep rupture tested welding joints of rutile CB2 flux-cored wires are investigated. The objective of the paper is to compare two filler metal compositions with different nickel contents after two different creep conditions. The first comparison is concerning the creep resistance, and the second comparison is concerning the simulated microstructure with its precipitates. Differences and agreements are discussed.

Influence of alloying elements on the mechanical properties of high-strength weld metal

ABSTRACT High-strength lightweight constructions are a crucial part of transportation systems and steel constructions optimised for low energy consumption. In this investigation, the aim is to understand the influence of different alloying elements on the mechanical properties of all-weld metal samples of high-strength filler metals. Metal-cored wires with adjusted chemistry were produced and the measured yield strength is compared with calculated values which were obtained by thermodynamic and kinetic simulations. By increasing the content of the matrix alloying elements, no increase in strength could be achieved, but compared to that, higher strength was obtained by the addition of Ti, Nb and Al in combination. Furthermore, the influence of different Ti, Al and N contents is presented and discussed.

Effect of tempering time on the mechanical properties of P91 flux cored wire weld metal

ASME Grade 91 is one of the main materials for heavy-wall pipes in thermal power plants. Matching flux cored wires for welding P91 have already been available for several years and become more and more popular as flux cored arc welding (FCAW) offers several technical and economic advantages. To achieve high toughness of the weld metal at ambient temperature, temperature and/or time of post-weld heat treatment (PWHT) can be increased. As the temperature has to be kept below the transformation temperature Ac1, the range of suitable temperature is very small. In this contribution, the effect of increased tempering time on the mechanical properties at ambient temperature and elevated temperatures is investigated and the influence on long-term properties is discussed. Longer PWHT increases toughness of all-weld metal. MatCalc simulations and scanning electron microscopy (SEM) investigations indicate a coarsening effect of precipitates without detrimental effect on long-term creep properties.

Microstructural Characterization of Martensitic All-Weld Metal Samples

Abstract The development of welding consumables is permanently challenged with matching the increasing strength and toughness of thermomechanically treated or quenched and tempered steels. A martensitic microstructure offers promising prospects to guarantee such requirements in the all-weld metal as well. A thorough characterization of the microstructure is indispensable to understand its effects on the mechanical properties. This paper shall summarize how light optical microscopy and electron backscatter diffraction can be used to deepen the understanding of fully martensitic weld metal. It was found that electron backscatter diffraction offers a variety of information that light optical microscopy cannot deliver, particularly regarding the prior austenite grain structure. Grain sizes of the primary dendrites, the prior austenite grains and the martensitic structure can be measured with a combination of these techniques.

Welding of S960MC with undermatching filler material

High strength structural steels are in high demand thanks to their favorable mechanical properties. They offer high strength with sufficient toughness and good forming capabilities. Applications range from shipbuilding, to offshore constructions, cranes, and pipelines. A lot of current research focuses on weldability of high strength low alloy (HSLA) steels, especially improving the toughness in the weld zone, i.e., weld metal (WM) and heat affected zone (HAZ). In the present work, four different fusion welding processes using undermatching filler metal are compared on 8-mm thick sheets of S960MC structural steel. The welding processes include electron beam, laser hybrid, plasma, and gas metal arc welding. The welded joints are characterized by means of mechanical testing, tensile, impact, and hardness testing, and microstructural investigaton, light optical, and scanning electron microscopy. Furthermore, microprobe analysis of the weld metal was used to investigate the chemical composition of the weld metal.

Behaviour of a maraging steel under quasi-static and dynamic compressive loading

The class of stainless maraging steels exhibits an excellent combination of very high strength and hardness, ductility and toughness, combined with good corrosion resistance. Due to precipitation hardening 0.2% yield stress values of up to 2.4 GPa can be achieved. In many applications like crash worthiness or ballistic protection the materials are loaded at high strain-rates. The most important characteristic of material behaviour under dynamic loading is the dynamic yield stress. In this work, compression tests have been conducted at strain-rates of the order of 5 × 10−3 s−1 up to 3 × 103 s−1 to study the materials behaviour. In order to investigate the influence of temperature, dynamic compression tests have been performed in the temperature range from −40°C to 300°C. The Johnson-Cook equation has been used to describe the materials behaviour under dynamic loading. From strain-rate jump tests the activation volume was determined in order to identify the dislocation mechanisms which control the thermally activated part of plastic deformation.