Papers by Author: Wolfgang Bleck

Abstract: During the hot rolling of bainitic steels, time and temperature must be controlled within narrow limits to avoid undesirable ferritic or martensitic phase fractions. In order to design a reliable process window for the production of bainitic steels, the effects of the different process parameters on the phase transformation and the final properties of a microalloyed and a non-microalloyed steel were investigated. Thermomechanical tests with the possibility of producing secondary samples were conducted to analyze the influence on the mechanical properties strength and toughness. Transmission electron microscopy (TEM) and electron probe micro analysis (EPMA) were used to investigate the origin of the differing properties. In particular, it has been found that thermomechanical rolling of the microalloyed steel leads to an improvement in strength. This is partly due to the transformation kinetics and partly to strain-induced precipitations. Further, the hardening behavior is affected by the secondary phase within the bainitic matrix configured through the cooling strategy. Coarse Martensite/Austenite (MA) structures reduce toughness, whereas finely dispersed MA islands increase the hardening potential. Furthermore, the results from the material experiments were used to develop a rate model in combination with a nucleation model to predict the kinetics of the phase transformation and the shape of the bainitic microstructure.

Abstract: The microstructures and the mechanical properties of two Fe-26Mn-xAl-1C steels with 8 and 10 % Al have been investigated at different strain rates. The results show that Fe-26Mn-10Al-1C steel possesses higher strength and at the same time higher ductility than Fe-26Mn-8Al-1C steel at both low and high strain rates. The strengths of the steels increase and ductility declines slightly with increasing strain rate. These observations can be attributed to the different strain hardening mechanisms acting at different strain rates. Planar slip occurs and microbands form duringthe steady state stage, whereas deformation twinning occurs in the final stage ofdeformation. The higher strain hardening at high strain rates are due to the strong increase in the twinning propensity. The strain hardening at high strain rates also depends on the adiabatic heating, causing a competition between softening and strain hardening.

Abstract: Al containing intermetallic phases have been evaluated in various bcc and fcc steels. Attractive application options have been derived for hot working tools steels with respect to a reduction of resource critical alloying elements and in cold formable steels by the combined density reduction and strength increase.

Abstract: Hot forming of metals is important to achieve the desired shape and properties of products. The change in process behavior and final properties of materials during processing is essential for the design of new processes and products. Finite-element-software (FE-software) is nowadays widely used in industry to design and optimize hot forming processes. These FE codes need a proper description of the material behavior being developed for specific processes.In this paper the capability of a recently developed one-point-model will be discussed to predict material behavior for the carbon steel (C-steel) family. In detail, the following topics will be discussed: (i) description of the material behavior in varying processes, (ii) definition of material properties depending on alloying content, (iii) capability to describe whole alloy families, (iv) adjusting the model parameters by a small number of experiments.

Abstract: The ‘Quenching and Partitioning’ (Q&P) concept aims to increase the strength level of conventional TRIP-assisted advanced high strength steel (AHSS) by replacing ferritic constituents by tempered martensite. The Q&P heat treatment process involves austenitization and interrupted quenching followed by carbon partitioning from martensite to austenite at elevated temperatures. The final microstructure is traditionally investigated at room temperature after metallographic preparation by microscopy and x-ray analysis with laboratory tubes. Besides other disadvantages the established characterization methods are not adequate to observe the development of the microstructure during Q&P treatment. In the present work the microstructural evolution during Q&P processing was monitored by in-situ diffraction experiments using very hard (100 keV) synchrotron x-ray radiation. Debye-Scherrer rings were recorded as a function of time and temperature during the heat treatment in a state-of-the-art dilatometer (type Bähr DIL805AD) at the Engineering Materials Science beamline HARWI-II (HZG outstation at Deutsches Elektronensynchrotron (DESY), Hamburg). The diffraction patterns contain quantitative information on the phases present in the sample (for more details cf. Abstract Carmele et al, this conference). The evolution of the austenite phase fraction during the partitioning treatment at the quench temperature (1-step Q&P) is discussed exemplarily for a Si-based TRIP steel with additions of Ni.

Abstract: Innovative steel materials of the third generation of advanced high-strength steel (AHSS) are based on complex multiphase microstructures on a submicron scale, which are adjusted in a heat treatment procedure. Established methods for microstructural characterization are usually applied after the heat treatment process (ex-situ) at room temperature and comprise amongst others X-ray analysis based on laboratory tubes with photon energies of several keV. The corresponding penetration depths are on the micron scale. Additionally, the results may be affected by the metallographic preparation process. Using very hard synchrotron X-ray radiation with photon energies of up to 100 keV, penetration depths in the millimetre range are realized and macroscopic volumes (mm³) can be investigated. Furthermore the photon flux of synchrotron sources is several orders of magnitude higher compared to laboratory tubes. Consequently in-situ measurements during a heat treatment process can be performed. Using the example of the standardized multiphase TRIP steel HCT690T, a microstructural investigation with high energy synchrotron X-ray radiation is discussed and compared to established diffraction methods using Co-and Cu-K_α-radiation. In-situ diffraction measurements during a heat treatment are exemplarily shown.

Abstract: Temperature development during plastic deformation affects the stability of retained austenite and thus the mechanical properties in transformation-induced plasticity (TRIP) steels. In this work, we used a thermo-camera to monitor the temperature development during a step-wise tensile test of an Al-containing multiphase TRIP steel. The tensile tests were performed by loading the specimen at six straining rates ranging from 5 to 30 s-1 to a stress of 700 MPa and then holding for 15 min, followed by further loading at 50 s-1 until fracture. It is found that temperature increases about 13 – 18 °C during the first loading process and drops back to room temperature with a time-constant of around 2 min. The increment of temperature increases with increasing straining rate. The temperature increases around 30 °C during the second loading process. The distribution of temperature over the specimen surface is found to be rather homogeneous along the longitudinal direction in most cases, except for the ending points of two loading processes. The measurement of temperature development is found to be consistent with previous numerical simulation on the temperature development under constant stress in TRIP steels.

Abstract: Gas tungsten arc welding was applied to join AISI 304 stainless steel and AISI 1020 carbon steel sheets with three types of consumables – AISI 308L, AISI 309L and AISI 316L stainless steel wires. Weld metals produced by all consumables exhibited the identical hardness of ca. 350 HV. This value was higher than those of stainless steel and carbon steel base metals, indicating the relatively high strength of weld metals. The corrosive behaviour of weld metals was investigated by a potentiodynamic method. Specimens were tested in 3.5 wt% NaCl solution saturated by laboratory air at 27°C. A pitting potential of weld metal produced by the AISI 309L consumable was higher than those of weld metals produced by the AISI 308L and AISI 316L consumables respectively. The chemical compositions and microstructure of weld metals were also investigated. The pitting corrosion resistance of weld metals produced by different consumables is discussed in the paper in terms of the pitting resistance equivalent number (PREN) calculated from the chemical compositions and the content of delta ferrite in the austenite matrix of the weld metals.

Abstract: The process of thixoforming incorporates a series of forming processes in the semi-solid state, which can be categorized between the conventional processes of forging and casting and combines the advantages of these processes. Thixoforming of steels in the semi-solid state, requires round, solid particles (globulites) in a liquid matrix which is deformed with low forming forces. In order to achieve laminar material flow and to produce segregation-free components, the material must fulfil diverse criteria. First, the melting interval should be as large as possible for an easy temperature regulation. Next, low solidus and liquidus temperatures are advantageous regarding tool loading. Additionally, thixoformable steels should show a melting behaviour that is finegrained and globular. Furthermore, these steels should possess low contents of intraglobular liquid phase fractions. This paper gives a survey of the current state of steel Thixoforming and deals with the development of adaptive heat treatment strategies. Regarding the structure formation and the development of suitable heat treatment strategies, the once semi-solid state yields new structures that can be applied in ways not previously possible with conventional hardening processes. New microstructures and up to date unknown better mechanical properties can be adjusted with an optimised heat treatment strategy. By this, new fields of application for thixo-materials can be entered and also advanced procedures for special applications can be established. For example the steel X210CrW12 leads to a very hard material with high wear-resistance, which can be used at higher temperatures than the conventional hardened material. In general, new generic microstructures after thixoforming results in unexpected favourable mechanical properties. Problems arise with respect to segregation and pores which resulting in inhomogeneous property distributions.

Abstract: Due to the coexistence of different micro structural components and their interactions, multiphase steels offer an excellent combination between high formability and strength. On the micro-scale, the fracture examination shows large influence of different phases and their distributions on the mechanical properties and failure mechanisms. Considering the influence of multiphase microstructure, an approach is presented using representative volume elements (RVE) in combination with continuum damage mechanics (CDM). Herein, the influence of the material properties of individual phases and the local states of stress on the material formability as well as the failure behavior can be examined. By means of the RVE-CDM approach, a precise criterion for the deformability characterization in sheet metal forming of multi phase steels is presented.