Marek Smaga

Investigation and modelling of the plasticity-induced martensite formation in metastable austenites

Abstract Mechanical stress–strain hysteresis, temperature and magnetic measurements were performed for the detailed characterisation of the cyclic deformation behaviour of metastable austenitic steels with particular attention to plasticity-induced martensite formation. Specimens of AISI 304, AISI 321 and AISI 348 stainless steels were investigated under plastic strain control at ambient temperature. On the basis of comprehensive experimental data, a model was developed for the description of the plasticity-induced martensite formation in metastable austenites under cyclic loading. The process of martensite formation was described as a function of the cumulative plastic strain and the cumulative strain energy density.

Fatigue Life Calculation of Metastable Austenitic Stainless Steels on the Basis of Magnetic Measurements

Abstract Monotonic and cyclic plastic deformation of metastable austenitic steels lead to a phase transformation from paramagnetic austenite into ferromagnetic α′-martensite. The deformation-induced changes of the magnetic properties are directly related to the accumulated plastic strain and therefore to the actual fatigue state. This paper includes a detailed characterization of the deformation-induced austenite-martensite-transformation in metastable austenitic steels. The cyclic deformation behaviour was evaluated by mechanical stress-strain hysteresis and temperature measurements. With in-situ Ferritescope magnetic measurements, the development of the α′-martensite, and the change in the magnetic induction due to the Villari effect were investigated. On the basis of far-reaching cross effects of mechanical and magnetic properties, measurements of magnetic-mechanical hysteresis loops were performed. Hence a method for fatigue life calculation based on the change in magnetic properties was developed.

Influence of Surface Morphology on the Fatigue Behavior of Metastable Austenitic Steel

Using a low temperature turning process with carbon dioxide cooling in the cutting zone a variation of the morphology at the specimen surfaces of the metastable austenitic steel AISI 347 was realized. In LCF and HCF fatigue tests at ambient temperature and 300 °C the influence of the surface morphology on the cyclic deformation behavior and fatigue life was investigated by the measurement of stress-strain hysteresis. An additional magnetic measurement allows the characterization of the phase transformation from paramagnetic austenite in ferromagnetic α´-martensite during the turning processes and during cyclic loading. The surface morphology was studied in detail by SEM and x-ray investigations.

Cyclic Deformation Behavior of Austenitic Steels in the Temperature Range -60°C ≤T ≤550°C

In this investigation specimens of the austenitic steels AISI 304, AISI 321 and AISI 348 were investigated in fatigue tests in the temperature range -60°C ≤ T ≤ 550°C. A detailed microstructure-based characterization of the cyclic deformation behavior of austenitic steels was performed by means of stress-strain hysteresis, electrical resistance and magnetic measurements. Up to ambient temperature the occurring deformation induced martensite formation was measured in-situ with a ferritescope during cyclic loading. The temperature range for dynamic strain aging was reliably identified by means of a temperature increase fatigue test with one single specimen.

Influence of Cyclic Deformation Induced Phase Transformation on the Fatigue Behavior of the Austenitic Steel X6CrNiNb1810

The austenitic steel X6CrNiNb1810 (AISI 347) was investigated in isothermal total strain-controlled tests at ambient temperature and T = 300 °C in the LCF-and HCF-range. The phase transformation from paramagnetic austenite (fcc) into ferromagnetic α´-martensite ́(bcc) leads to cyclic hardening and to an increase in fatigue life. At 300 °C no α´-martensite formation was observed in the LCF-range and the cyclic deformation behavior depends basically on cyclic hardening processes due to an increase of the dislocation density, followed by cyclic saturation and softening due to changes in the dislocation structure. In the HCF-range an increase in fatigue life was observed due to ε- and α´-martensite formation. Measurements of the mechanical stress-strain-hysteresis as well as temperature and magnetic properties enable a characterization of the cyclic deformation behavior and phase transformation in detail. The changes in the physical data were interpreted via microstructural changes observed by scanning-and transmission-electron-microscopy as well as by x-ray investigations. Additionally electromagnetic acoustic transducers (EMATs) developed from the Fraunhofer Institute of Non-destructive Testing (IZFP) Saarbrücken were used for an in-situ characterization of the fatigue processes.

Influence of Morphology of Deformation Induced α´-Martensite on Stress-Strain Response in a Two Phase Austenitic-Martensitic-Steel

In this research work specimens of the metastable austenitic steels AISI 304 and AISI 347 with one phase (fully austenitic) and two phase (austenitic-α ́-martensitic) microstructure were monotonically loaded at ambient temperature. Using stress-strain and temperature measurements the deformation behavior was characterized in detail. To study the influence of morphology of deformation induced α ́-martensite on the stress-strain response a phase field model for α ́-martensite transformations was developed. With this approach it was possible to model the two phase austenite-α ́-martensite microstructure and investigate the deformation behavior on the micro level. With optical microscopy, magnetic and x-ray measurements the microstructure characterization of fully austenitic and austenitic-α ́-martensitic steels was realized.

Magneto-Mechanical Behavior of Iron during Monotonic and Cyclic Loading

In the present study, flat specimens from polycrystalline α-iron were monotonically and cyclically loaded at ambient temperature for the investigation of magneto-mechanical behavior. The magnetic flux density was measured by a Hall-sensor in in-situ and ex-situ experiments. For the characterization of the magnetic microstructure of α-iron Kerr microscopy was used. Additionally, Moessbauer spectroscopy of specimens in initial state and after failure was performed.

Fatigue Monitoring of Austenitic Steels with Electromagnetic Acoustic Transducers (EMATs)

Early detection of fatigue processes in the cyclically loaded metastable austenitic steel AISI 347 was performed by electromagnetic acoustic transducer measurements in total strain-controlled low-cycle fatigue tests at ambient and elevated temperature. The changes in the physical data were interpreted via microstructural changes observed by scanning- and transmission-electron microscopy, as well as x-ray investigations. The application of physically based measurement data, e.g., time-of-flight from electromagnetically activated ultrasonic signals in austenitic fatigue specimens and total strain, enables measurements of a new hysteresis relationship. In analogy to the common stress–strain hysteresis, this hysteresis gives information about the actual state of fatigue and shows significant changes in shape before specimen failure.

Konzept zur Oberflächenkonditionierung beim kryogenen Hartdrehen

Kurzfassung Beim Hartdrehen führt eine kryogene Prozesskühlung gegenüber der Trockenbearbeitung zu vorteilhafteren Randschichtzuständen. Im Drehprozess auftretende Störgrößen können jedoch zu Abweichungen vom angestrebten Randschichtzustand führen. Diese unerwünschten Abweichungen können das Bauteilverhalten im Einsatz negativ beeinflussen. In diesem Beitrag wird ein Konzept vorgestellt, welches es ermöglicht, die im Drehprozess auftretenden Störgrößen zu kompensieren, um somit definierte Randschichtzustände robust einzustellen.

Innovative Experimental Approaches and Physical Measurement Methods for Fatigue Monitoring and Life Assessment

The present contribution gives an overview on innovative methods to characterize cyclic deformation and lifetime behavior of metallic materials and hybrid joints based on high precision measurement of electrical resistance, temperature and magnetic properties during fatigue testing. General aim is to minimize the number of fatigue tests for reliable S-N curve calculation. Moreover, instrumented cyclic hardness tests allow short-time assessment of cyclic hardening in case of limited availability of test material. The methods are applied to a wide range of materials, from carbon steels, over cast iron and metastable austenitic steels to ultrasonically welded Al-alloy/polymer matrix composites.