Papers by Author: Heinz Palkowski

Abstract: Three ductile irons with different aluminum-and manganese-content were subjected to two thermo-mechanical schedules. In the first schedule, a total deformation of φ_t = 0.3 is applied on the ductile irons in the austenitic region before the austempering process. In the second schedule, the materials are subjected to deformation of 0.2 in the austenitic region and deformation of 0.1 during austempering (ausforming). Mechanical deformation of austenite prior to the transformation “stage I” pronouncedly accelerated the transformation due to increasing the nucleation sites of ausferrite. This increase has its impact on enhancing the microstructural uniformity and refining the ausferrite platelets. On the other hand, the retained austenite content was not significantly affected by the applied ausforming. Remarkable increase in hardness, strength and ductility of the ausformed ductile iron due to the latter effects is observed.

Abstract: Microstructure evolution and tensile properties were studied in a bainitic pipeline steel grade by performing a number of physical simulations on samples machined out of an industrially produced transfer bar. In these simulations, the cooling interval between roughing and finishing stages (t_V) was varied from 5 s to 180 s. The austenite status after this cooling interval, regarding the prior austenite grain size and precipitates, simulates the condition of austenite before entering the finishing mill. The finishing parameters and the subsequent cooling strategy were kept unchanged throughout all the applied simulation processes. The gradual increase in t_V resulted in a gradual increase of the granular bainite phase on the expense of the aciculare ferrite. This resulted in an incremental decrease in ultimate tensile strength and yield strength with increasing t_V. However, this behavior approached a steady state condition after which the t_V has limited/insignificant effect on the ultimate-and yield strength. This saturating value of t_V is process parameter dependent.

Abstract: In the last decade, wide research was carried out on developing light-weight materials with improved mechanical properties, for instance comparable specific stiffness and strength, improved isolation, superior vibration and sound damping properties. To satisfy the increasing demands, Metal-Polymer-Metal Laminates have been numerously developed. Here, a short background of composites structures is presented.

Abstract: High-performance metal/polymer/metal hybrid sandwich composites are attractive materials for lightweight constructions in automotive, aerospace and naval engineering world-wide. Due to the excellent combination of mechanical, thermal and elastic properties and, as a result of high forming potential, they can be used in areas of high vibration, where high damping properties of the polymer are demanded and at the same time high strength and stiffness are given by the metal. Disadvantages can be given in case of mechanical or thermal joining of these polymer-based sandwiches because of the elastic behaviour as well as low melting temperature of the polymer. Local metal plate insertions in the soft core at the place of joining can be a solution for such kind of problems. But forming behaviour of sandwich materials with and without local inlays differs strongly. Sandwich composites of that type were produced by roll-bonding. Their quality and their position were controlled by Lockin thermography. The forming behaviour of sandwiches with different geometry, size, type and the position of the inlays was tested by deep drawing and bending and analysed with the help of digital photogrammetry and compared to experimentally obtained mechanical properties. As a result, the local inlays, as well as their geometry, size and type strongly influence the forming limit conditions. The differences in flow behaviour of non-reinforced and reinforced sandwich regions after deep drawing and bending will be presented, as well as the influence of the position of the inlays.

Abstract: The increased demand for advanced high-strength steels (AHSS) is driven mainly by the need of the automotive industry to reduce weight and to improve safety. Beside good ductility and high strength, those steels have a high bake hardening (BH) effect, giving additional contribution to the strength of structural parts, subjected to the paint baking process. In this paper results are presented gained from hot rolled martenstitic steels. For the simulation of changing conditions within the final hot rolling process the specimens were hot deformed using different schedules changing reductions and temperatures been selected according to the non-recrystallization temperature By controlling the deformation temperature and the amount of strain below non-recrystallization during the thermomechanically controlled process it was possible to refine the martensitic steel structure, resulting in an improved strength and BH behaviour. A wide spectrum of mechanical properties could be obtained. The best strength and BH levels were recorded where the deformation was applied below T_nRX at the highest amount of strain.

Abstract: Using thermo-mechanical simulator equipped with dilatometry system, two ductile iron alloys with different Mn-content are processed by combining both, well defined deformation process and subsequent controlled cooling in a single processing chain to control the final microstructure. Accordingly, ductile irons with four different structrues are produced namely, martensite, ausferrite, martensite+ferrite and ausferrite+ferrite. Depending on the dilatometric measurments, the ferrite formation temperature-range has been defined for both alloys. Preferential transformation of austenite to ferrite at graphite nodules during cooling is observed. It is also observed that the formation of ferrite during cooling results in both decreased martensite start of the undecomposed austenite and accelerated kinetics of ausferrite formation.

Abstract: The production process and the forming behaviour of locally reinforced steel/polymer/steel (316L/PP-PE/316L) hybrid sandwich composite materials (SMS) have been investigated. The effect of simple plate reinforcements with different size, shape and geometry on the forming limits of SMS was studied. As a local reinforcement, the simple solid steel and mesh steel plate inlays with central and edge positions were chosen instead of a polymer core as a sandwich laminate. In order to increase the adhesion properties between the metal and polymer layers, corona discharge and plasma preliminary surface treatments were applied prior to the sandwich production. Both, deep drawing and stretching cup-forming tests were performed in order to analyse the forming behaviour as well as the failure of SMS with and without local inlays subject to different tensile loadings. The influence of the local reinforcement on the bending behaviour was determined by three and four-point bending processes. Stress-strain curves and thinning behaviour of SMS with local reinforcements under the different forming loads were determined using digital image correlation via photogrammetry. The forming behaviour strongly depends on the quality, geometry and size of the local plate inlays. Owing to the different positions of reinforcement as well as to the different polymer content around of inlays, failure of SMS by bending and drawing differs. The sandwich samples with mesh reinforcement demonstrate better formability by drawing and bending than that of samples with solid plates. In order to minimise the loss in formability of sandwich samples during deep drawing, the size of the centred reinforcement has to be larger than the punch diameter.

Abstract: This research deals with processes leading to local strengthening effects in Advanced High Strength Steels (AHSS). Dual phase (DP), retained austenite (RA) - both hot and cold rolled - and complex phase (CP) steels have been investigated to examine the effect of thermal and mechanical processing parameters on local properties. For this purpose, a method has been investigated to achieve local strengthening, namely local deformation and local heat treatment. Samples were locally deformed by bending and embossing processes. A local deformation with defined pre-strains leads to enhanced hardness and strengthening. A subsequent aging treatment leads to a further increase in mechanical properties. Local heat treatment was applied using a laser and an electron beam. Following local heat treatment with selected parameters, the microstructure of the surface and the cross section as well as the mechanical properties were evaluated by light optical, scanning as well as transmission electron microscopy, hardness measurement, tensile testing and thermal modelling. It can be stated that with partial heat treatment, local high strengthening can be produced. At lower heat treating temperatures, this effect could be attributed to bake hardening. With increased heat treating temperature, the initial microstructure near the surface is affected. A model can be improved, which defines the correlation between the influencing parameters and the local properties. The influence of over-aging in locally strengthen regions has been studied. For this investigation, parameters are stable to locally adjust the strengthening effect. Partial strengthening of AHSS by local deformation or local heat treatment can open up new fields of applications for locally using the strengthening effect to only influence relevant areas of interest, thus providing the potential for saving energy and designing the component’s behaviour.

Abstract: The investigations deal with processes leading to local effects of strengthening in multiphase steels, being characterized by good formability, continuous yielding, high strength and a strong bake hardening and ageing effect. Dual phase and complex phase steels are under investigation to examine the effect of thermo-mechanical processing parameters on local ageing ability and its use for designed properties. For this purpose local heat treatment by laser are studied, as well as stability of local ageing on the adjusted strength. A remarkable increase of the hardness in the heat treated zone was observed. Stability of the local strengthening effect could be confirmed. Partial heat treatment of multiphase steels by laser can open a new field of application for the local use of the strengthening effect to influence only relevant areas, thus giving potential for energy saving.

Abstract: Dilatometric measurements were used to design the processing parameters of two types of bainitic steels. The first type is a hypoeutectoid ultra fine bainite steel, for which the dilatometer was used to locate the temperature at which cementite is completely dissolved during intercritical annealing (TC). The intercritical annealing temperatures are then selected will above TC. To obtain the martensite start temperatures (MS), the steel is quenched to the room temperature (RT) from these selected temperatures and then the bainite transformation temperatures were selected to be well above MS. The dilatometer was then used to monitor the bainite transformation kinetics from which the required time frames for cessation of the bainitic reactions were estimated. In the second type, bimodal bainite had been produced in thermo-mechanically processed TRIP-steel. A deformation dilatometer is used to perform three deformation-steps before slow cooling to form approx. 30% polygonal ferrite. The material was then rapidly cooled to the first bainite formation temperature. During this step, the dilatometer was used to monitor the bainite reaction from which the required time for 50% decomposition of austenite is estimated. The martensite start of the undecomposed austenite was located by quenching to RT. The second bainite transformation step was then performed well above the new MSII to form a second generation of finer bainite.