Papers by Author: Dirk Ponge

Abstract: We address the differences in yield stresses between hot and cold rolled medium manganese steel showing continuous yielding. Continuous yielding in both, the hot and cold rolled samples were resulting from reverted austenite islands plastically deforming first and less strain in the tempered martensite matrix. At higher global strains, strain was taken up not only by the reverted austenite, but also by tempered martensite and fresh martensite formed from the austenite through martensitic phase transformation during deformation. Strain localization was also observed in the hot rolled samples. This localization is caused by cumulative deformation of colonies of lamellar reverted austenite islands. It is interpreted in terms of the spatial alignment of austenite colonies to the loading direction in addition to the crystallographic orientation.

Abstract: Duplex stainless steels (DSSs) are based on the Fe-Cr-Ni system and formed by ferrite (30-70%) and austenite. They have high tensile strength, good toughness and weldability and excellent corrosion resistance including stress-corrosion cracking and resistance to localized corrosion. The increase of the raw materials of the last years, there has been a motivation to develop new stainless steels with lower contents of nickel and molybdenum. Lean duplex stainless steels (LDSSs) are almost Mo free and nickel content lower than 4%. The lean duplex grades are expected to substitute not only 304/316 grades but also other duplex stainless steel grades. LDSSs are used for structural applications and for the less corrosion conditions such as liquor tanks and suction rolls. The aim of the present work was to study the kinetics of recovery and recrystallization of the lean duplex stainless steel 1.4362 during annealing treatment. The material was subjected to hot rolling and cold rolling of 70%, annealing treatment for different times at temperatures from 1000 to 1100°C and subsequently water quenched. Optical microscopy and electron back scattering diffraction (EBSD) were employed to study the evolution of the microstructure during the annealing treatment. After cold rolling austenite exhibited more strain hardening than ferrite. Consequently, the driving force of the austenite for recrystallization is higher. During annealing, recovery took place in ferrite, while the austenite remained nearly unrecovered until beginning of recrystallization. The layered grain morphology produced during cold rolling remained after the annealing treatment. The volume fraction of the phases did not show significant changes with the annealing time. Nevertheless, the volume fraction of austenite decline with an increasing of annealing temperature. After 60 seconds at 1100°C, annealing primary recrystallization had progressed in both phases, which show a bamboo-structure where the grain boundaries ran perpendicular to the phase boundaries. Grain growth progressed for longer annealing times. After 600 seconds, the bamboo-structure started to change for a more globular grain structure, pearl-structure. It continued and became more pronounced at longer annealing times. At lower temperatures, the recrystallization behavior is similar; however, the structure was refined. Recovery is favored by the high stacking fault energy of ferrite and the layered grain morphology. The lean duplex stainless steel 1.4362 shows a similar recrystallization kinetics compare with standard duplex stainless steels.

Abstract: In this study the effect of thermomechanical treatment on the microstructure of austenite and martensite and the mechanical properties of a medium carbon silicon chromium spring steel with different levels of impurities is investigated. Results are presented for conventional heat treatment and for thermomechanical treatment (TMT). Compared to conventionally heat treated samples austenite deformation improves strength and ductility. Thermomechanically treated samples are not prone to embrittlement by phosphorous. TMT influences the shape and distribution of carbides within the matrix and at prior austenite grain boundaries. It is shown that utilization of TMT is beneficial for increasing the ultimate tensile strength to levels above 2200 MPa and at the same time maintaining the ductility obtained at strength levels of 1500 MPa by conventional heat treatment. The endurance limit is increased and embrittlement does not occur.

Abstract: The ferritic rolling strategy allows for the production of two different hot strip grades, a "soft" and a "hard" hot strip. The "soft" hot strip is rolled in the upper ferrite region and a sufficiently high coiling temperature ensures direct recrystallisation in the coil. The "hard" hot strip is rolled at relatively lower temperatures in the ferrite temperature region and exhibits a strained microstructure with a desirable rolling texture. Furthermore, these ferritic rolled hot strips can be used as initial strip for subsequent cold rolling. The current investigation focuses on the development of the recrystallisation texture of cold rolled and annealed ferritic rolled hot strip for different cold reductions. For this purpose "soft" and "hard" hot strips were produced on a laboratory hot rolling mill. These strips were cold rolled with a total reduction of 40 to 80% to a final thickness of 0.5mm. Subsequently the strips were subjected to simulated continuous annealing, using a salt bath furnace. The macro texture of both types of specimens was measured and correlated to the mechanical properties, including the Lankford values. A very different development of the recrystallisation texture and hence mechanical properties has been observed. However, both grades yielded improved deep-drawing properties.