Effect of Nitrogen on the Deformation Behaviour of High‐Nitrogen Austenitic Stainless Steels

The deformation behaviour of high‐nitrogen austenitic steels with the base composition of Fe‐18Cr‐10Mn containing various contents of nitrogen was investigated. Two deformation modes including deformation‐induced martensitic transformation (DIMT) and deformation twinning (DT) were observed depending on the nitrogen content. In the alloys with lower nitrogen contents, γ→?→α' martensitic transformation sequentially occurred, whereas DT acted as a main deformation mode and DIMT was suppressed in the alloys with increasing nitrogen content. Both DIMT and DT showed strong crystallographic orientation dependence. The competing mechanism between them was discussed in terms of the variation of stacking fault energy with nitrogen content.     

Spontaneous and Deformation‐Induced Martensite in Austenitic Stainless Steels with Different Stability

The fraction and microstructure of spontaneous and deformation‐induced martensite in three austenitic stainless steels with different austenite stability have been investigated. Samples were quenched in brine followed by cooling in liquid nitrogen or plastically deformed by uniaxial tensile testing at different initial temperatures. In‐situ ferritescope measurements of the martensite fraction was conducted during tensile testing and complemented with ex‐situ X‐ray diffractometry. The microstructures of quenched and deformed samples were examined using light optical microscopy and electron backscattered diffraction. It was found that annealing twins in austenite are effective nucleation sites for spontaneous α'‐martensite, while deformation‐induced α'‐martensite mainly formed within parallel shear‐bands. The α'‐martensite formed has an orientation relationship near the Kurdjumov‐Sachs (K‐S) relation with the parent austenite phase even at high plastic strains, and adjacent α'‐martensite variants were mainly twin related (<111> 60° or Σ3).     

On the Relationship between Mechanical Properties and Mechanisms of Plastic Deformation in Metastable Austenitic Steels

The requirements of the automotive industry for materials exhibiting increased structural performances are continuously increasing. These materials must remain ductile during the forming operations while also exhibiting improved strength and energy absorption capacities. New highly alloyed steel grades have been studied for a few years now, due to their exceptional mechanical properties resulting from interactions between dislocation plasticity, transformation plasticity and mechanical twinning. This study deals with the mechanical properties of steel grades presenting high manganese compositions. At room temperature, several phase transformations (γ→α', γ→? and ?→α′) were found to occur when the samples are deformed. The effect of different annealing conditions on the mechanical properties and the transformation sequence is analysed. The evolution of the work‐hardening of the samples is interpreted in connection with the kinetics of the phase transformations.     

Modelling and Closed‐loop Control of Complex Tube Forming Based on an Optimized Application of Martensite Formation in Austenitic Stainless Steels

Metastable austenitic steels undergo deformation‐induced martensitic transformation which can lead to a distinct increase of fatigue strength at an optimal volume fraction of martensite. This effect was used in the present study to define the local strength behaviour of a structural component part for the very high cycle fatigue (VHCF) regime. The investigation was on a discontinuous two‐stage forming process that consists of U‐O‐forming and rotary draw bending and results in a cross tube of a trailer coupling as exemplary dynamically loaded component. The volume fraction of martensite can be adjusted by means of plastomechanical simulation of the forming process and its parameters as part of the online process control. The formation of martensite shows a strong dependence on forming parameters (e.g. temperature and strain‐rate) and batch variations. These disturbance variables can only be taken into account by a closed‐loop control. Non‐isothermal material models were analysed according to their simulation accuracy of the martensite evolution. For the online control various hierarchical mathematical models were studied with regard to time effort and model error.     

High‐Strength Steels in Welded State for Light‐Weight Constructions under High and Variable Stress Peaks

In design codes (Eurocode, British Standard and others) for the dimensioning of welded joints, no distinction is made between low, medium and high strength steels. Because of a lack of general knowledge about the benefits of high‐strength steels and also because of missing information in design codes, in many cases design engineers still use low or medium strength steels (R_p0.2 < 400 MPa) and compensate for high loads under constant or variable amplitude loading or overloads by increasing dimensions. Given this situation, it was deemed necessary to establish criteria for the design of light‐weight welded constructions under high and variable stress peaks using new classes of high strength steels, such as S355N (normalized), S355M (thermomechanically treated), S690Q (water quenched) and S960Q (water quenched), and to perform more reliable evaluations of the fatigue performance of high strength steel structures subjected to complex loading with regard to light‐weight design and economics. For the comparison of the fatigue strengths of the investigated steels the notch factors present were taken into account. Additionally, the real damage sums were determined in order to give recommendations for the fatigue life estimation. Under constant amplitude loading, no significant difference in the bearable local stress amplitudes for the butt welds can be detected for the four investigated steels. Under variable amplitude loading, the butt welded (lower notch factor) high strength steel S960Q has advantages in the case of the normal Gaussian spectrum and in the case of overloads, especially under pulsating loading. For the transverse stiffeners (high notch factor), slight advantages for the high strength steel S960Q exist, only in the case of pulsating overloads. However, the advantages of high strength steels in case of static loading are indisputable. In most of the investigated cases, overloads lead to a benefit in fatigue life.