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Hot-work tool steels are used for die casting, extrusion molding and drop forging tools and thus are exposed to high temperatures and elevated stresses during application. To get insight into the materials' behavior under those conditions, short-term creep tests are performed at temperatures in the range of 550 – 590 °C and at stress levels ranging from 400 – 750 MPa. A steady-state creep range is not observed. Instead only a minimum strain rate appears. This minimum is followed by an extended tertiary creep range. Based on the observed stress exponent at 590 °C recovery-controlled dislocation creep has been identified as the dominant creep mechanism. During creep carbide coarsening, recovery and recrystallization occur, as observed by scanning electron microscopy and transmission electron microscopy.

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Induction heat treatment facilities have a wide application range for heat treatment of cylindrically shaped materials in the steel processing industry due to their reduced process-time and high throughput. The adjustment of the heat treatment process usually aims at reaching a desired hardness. However, the question arises whether the full potential of the applied material is actually exploited. Therefore, this work systematically investigates the influence of the primary microstructure, austenitisation and tempering conditions to the resulting notch impact energy and flow behaviour of a 50CrMo4 quenched and tempered steel, with normalised and soft-annealed prior microstructures. The heat treatments, performed with a laboratory induction heat treatment facility, show that low austenitising temperatures lead to a distinct yield point with reduced strain hardening, while increasing the tempering heating rate results in the precipitation of smaller carbides and a significant increase in tensile strength. Austenitising needs to be adjusted to the primary microstructure to reach an optimum solution state to exploit the hardness and notch impact energy potential.

Hot-work tool steels represent a group of steels which are used for metal forming operations at elevated temperatures, e. g. die casting, extrusion molding and drop forging. During application such working tools are exposed for a short time to both high temperatures and mechanical stresses. Short-term creep tests of two chromium rich martensitic hot-work tool steels which basically differ in their molybdenum, carbon and silicon content were conducted at temperatures in the range of 540 8C to 600 8C and at stress levels ranging from 280 MPa to 600 MPa. Both steels showed a minimum strain rate, at approximately 1/6 time to rupture, instead of a steady-state creep range. The stress exponents of both steels indicate dislocation creep as the dominant creep mechanism. Microstructural investigations showed that in both steels the secondary hardening carbides are type of MC and M 2 C. A higher content of molybdenum and carbon causes a higher volume fraction of these carbides and results in a better short-term creep behavior.Keywords: Short-term creep / Hot-work tool steel / Precipitates / Carbide evolution / Transmission electron microscopy / Warmarbeitsstähle werden häufig als Werkzeuge beim Druckgießen, Strangpressen oder Gesenkschmieden eingesetzt. Während des Einsatzes sind diese Stähle immer wieder hohen Temperaturen und Spannungen ausgesetzt. In dieser Arbeit wurden an zwei chromreichen Warmarbeitsstählen, welche sich hauptsächlich in ihrem Molybdän-, Kohlenstoff-und Siliziumgehalt voneinander unterscheiden, Kurzeitkriechversuche durchgeführt. Die Versuchstemperaturen lagen dabei zwischen 540 8C und 600 8C, währenddessen die Spannungen von 280 MPa bis 600 MPa reichten. Bei beiden Stählen trat eine minimale Kriechrate bei ungefähr einem Sechstel der Bruchzeit auf. Ein konstanter sekundärer, so genannter "Steady-State", Kriechbereich wurde bei keinem der durchgeführten Kriechversuche festgestellt. Der Spannungsexponent von beiden Stählen weist auf Versetzungskriechen als den dominierenden Kriechprozess hin. Untersuchungen der Mikrostruktur an beiden Stählen ergaben Sekundärhärtekarbide vom Typ MC und M 2 C. Ein erhöh-ter Molybdän und Kohlenstoffgehalt führt zu einem höheren Volumenanteil an diesen Karbiden, verbunden mit einem besseren Kurzzeitkriechverhalten.

In many cases a predominant failure mechanism of parts and components made of structural steels is fatigue. Such damage is followed by a ductile or brittle residual fracture. Unfortunately, we may sometimes observe a disastrous brittle damage of steels, which show plastic deformation and ductile fracture under normal loading conditions. Many reasons are responsible for such material behaviour. On the one hand, low surrounding temperatures, local stress concentrations (e.g. notches, cracks) and impact loading, favour brittle fracture. On the other hand, brittle material response or a shift of the transition temperature to higher values is a consequence of inadequate microstructure (Widmannstätten structure), blue brittleness and temper embrittlement (due to carbon, phosphorous and impurities on grain boundaries) as well as high nitrogen contents in steels with low aluminium concentration. Hydrogen or hydrogen induced stress corrosion cracking and liquid metal embrittlement lead also to disastrous failure. Zinc layers may cause brittle fracture, too. Fig. 1a gives an overview of parts of broken hangers, used to fix belt conveyors on the ceiling of a mine. They were manufactured by bending a wire (12 mm in diameter) made of mild steel of the type S235JR (AISI 570-36). To protect them against corrosion, all parts were plated with a thin zinc layer. For this hot dip galvanisation was used. The broken parts reveal plastic deformation. A more detailed investigation of the fracture surface (Fig. 1b) shows a glittering structure and a dark spot at the edge of all broken parts. The latter can be attributed to the point of crack initiation. Therefore, the cracks start from the inner side of the bended wire. This phenomenon is clearly visible in Fig. 2a, where the bending zone is depicted. After metallographic preparation the parts reveal many branched cracks starting from the edge of the hanger (Fig. 2b). All these cracks are coated with a more or less thick layer of zinc. From fractographic investigations it is visible (Fig. 3a, b) that the initial crack can be attributed to liquid metal embrittlement. A typical feature within these zones is the separation of grain boundaries. This damage is followed by cleavage fracture (Fig. 3b), caused by the initial sharp cracks and impact loading at low ambient temperature.From Fig. 2a we can point out that cracks are initiated within very small zones on the inside of the bending radius. The location of the main cracks is not exactly within the compression region during bending. Plastic bending deformations during manufacturing cause residual tensile stresses within the original compression zone, whereas the stresses within original tension zone change from tension to residual compression stresses. The localisation of the highest tensile stresses depends on the manufacturing procedure and on the used tools. Detailed information is provided by finite element calculation. Varying the geometry of the tools does not lead to the highest tensile stresses being located at the inner bendin...

Purpose: Applications for highly corrosive environments and cyclic loading are often made out of austenitic stainless steels. Corrosion fatigue and crack propagation behaviour has been studied to determine failure processes and damage mechanisms. Approach: CrNiMo stabilized austenitic stainless steel and CrMnN austenitic stainless steel in solution annealed and cold worked condition are compared. S/N curves and crack propagation rate curves are recorded in 43 wt% CaCl2solution at 120 °C, which resembles most severe potential service conditions. For comparison these experiments are also performed in inert glycerine. Additionally, the electrochemical behaviour of these materials has been studied. Findings: The CrMnN steels have excellent mechanical properties but are very susceptible to stress corrosion cracking in the test solution. The fatigue limit as well as the threshold for long crack growth are significantly reduced in corrosive environment. Moreover these steels exhibit a remarkable increase in the propagation rate, which is extremely pronounced in the near threshold region. This effect is enhanced by cold working. CrNiMo steels also show a reduction in the fatigue limit, but it is less pronounced compared to CrMnN steels. The threshold is significantly reduced in corrosive environment, but propagation rate is lower in corrosive environment compared to inert glycerine. Possible explanations of this surprising behaviour are discussed.