High-Alloy TRIP Steel Research Articles

Electron Beam Melting of various structures and their quasi-static compression behavior

The feasibility of printing structures made of high-alloy TRIP steel by Electron Beam Melting (EBM) is demonstrated in this study. Three different structure geometries are discussed. During the melting process, approximately 1.4 wt.% manganese evaporate independent of the printed structure. The structures differ in the arrangement of cell walls and the local material distribution in the in-plane array. To evaluate the strength of the structures, quasi-static compression tests were carried out in out-of-plane and in-plane direction. It is shown, that a homogenous distribution of cell walls over the structure cross section and stiffened cell walls with short distances between the cell wall nodes contribute to a high compressive strength.

Martensite formation during tensile deformation of high-alloy TRIP steel after quenching and partitioning route investigated by digital image correlation

High-alloy austenitic TRIP steel was manufactured using quenching and partitioning route (Q&P) for further enhancement in strength which leads to a microstructure consisting of thermally induced martensite, carbides and stabilized austenite. The austenite is stabilized during partitioning due to diffusion of carbon and nitrogen from supersaturated martensite into the austenite. For evaluation of deformation behavior micro tensile specimens were manufactured and deformed in situ inside scanning electron microscope. The evolution of the microstructure was characterized by X-ray diffraction before and after deformation. During in situ deformation two austenite orientations (〈001〉 and 〈101〉 regarding load axis) were investigated and occurring strain localizations were observed by means of digital image correlation. The strain localizations could be correlated with formation of ε- and twinned α’-martensite. Using high resolution EBSD (HR-EBSD) the transformation behavior during tensile deformation is discussed. Due to the inhomogeneous internal stress distribution and the increased defect density in the austenite, preferred martensite formation is to be expected despite the increased chemical stability. This observation is attributed to the so-called constraining effect.

Tensile Behavior of Cast and Electron Beam Welded Interstitially Strengthened High-Alloy TRIP Steel

Due to their enormous potential for energy absorption, high-alloy TRIP/TWIP steels are designed to fulfill contrary requirements of crash-relevant structures such as strength, toughness, and ductility and even excellent weldability. To further optimize these mechanical properties and reduce the production costs at the same time, high-alloy TRIP/TWIP steels with an increased content of carbon and nitrogen and decreased amount of nickel are produced. The mechanical properties of the base materials and the capability for the phase transformation are determined for different temperatures (−40 to +60 °C) in quasistatic tensile tests. In comparison, welding experiments with an electron beam are performed and the microstructure is examined via light optical microscopy. The welded sheets are also tested and the mechanical properties determined via micro-hardness measurements and quasistatic tensile tests. While a TRIP effect is noticeable during loading at room temperature, optimal properties of the base material can be achieved at temperatures below 0 °C. The welded samples show a ferritic–austenitic microstructure and solidified as primary ferritic with joint austenite and ferrite formation in the solidification interval.

The influence of dilution on dissimilar weld joints with high-alloy TRIP/TWIP steels

In dissimilar welded joints without filler materials, the dilution is one of the most important factor for the weldability. Due to other factors, the high quality of the welding seam and a good reproducibility of welding parameters the electron beam can be advantageously used to join challenging materials. Several dissimilar welded joints were successfully produced as a result of an optimised variation of the beam offset and the beam current to achieve suitable levels of dilution. For microstructural evaluation, the welded joints were examined by light optical and scanning electron microscopy methods, such as Electron Backscatter Diffraction (EBSD) and Wavelength-dispersive X-ray spectroscopy (WDS). To characterise the mechanical properties and the deformation behaviour of the welded material, microhardness measurements and tensile tests were carried out. For the welds of a high-alloy TRIP steel and the steel DP590, an optimised dilution lead to flawless joints with a tensile strength superior to the TRIP steel. Furthermore, welds of a zirconia-reinforced TWIP steel with an as-cast TRIP steel free of pores and cracks could be produced.

Improved mechanical properties by high-energy milling and Spark Plasma Sintering of a TRIP-matrix composite

A composite reinforced with 5 vol% MgO partially stabilized zirconia and a high-alloyed TRIP-steel matrix (16% Cr, 6% Ni, 6% Mn) was processed. The objective was to trigger a phase transformation under mechanical stresses within the steel and the ceramic. Therefore, the composite was processed by both mixing (110 r/min) and dispersing by intense milling (250 r/min) of the initial powders, and subsequent Spark Plasma Sintering. During Spark Plasma Sintering, the dwell time was varied (5 min, 10 min). The steel/MgO partially stabilized zirconia interfaces were improved through the intense milling and the 10 min dwell time. Furthermore, 50% of the MgO partially stabilized zirconia transformed to the monoclinic phase under quasi-static compressive loading (10−3 s−1) and the composite had a 1% offset compressive yield strength of 665 MPa. Independent from the processing route, 65% of the steel transformed to α′-martensite.

ИЗМЕНЕНИЕ ФАЗОВОГО СОСТАВА ВЫСОКОЛЕГИРОВАННОЙ ТРИП-СТАЛИ В ПРОЦЕССЕ СТАТИЧЕСКОГО И ЦИКЛИЧЕСКОГО ДЕФОРМИРОВАНИЯ

ИЗМЕНЕНИЕ ФАЗОВОГО СОСТАВА ВЫСОКОЛЕГИРОВАННОЙ ТРИП-СТАЛИ В ПРОЦЕССЕ СТАТИЧЕСКОГО И ЦИКЛИЧЕСКОГО ДЕФОРМИРОВАНИЯ

Identification of Defect Prone Peritectic Steel Grades by Analyzing High-Temperature Phase Transformations

Continuous casting of peritectic steels is often difficult and critical; bad surface quality, cracks, and even breakouts may occur. The initial solidification of peritectic steels within the mold leads to formation of surface depressions and uneven shell growth. As commercial steels are always multicomponent alloys, the influence also of the alloying elements besides carbon on the peritectic phase transition needs to be taken into account. Information on the solidification sequence and phase diagrams for initial solidification are lacking especially for new steel grades, like high-alloyed TRIP-steels with high Mn, Si, and particularly high Al contents. Based on a comprehensive method development, the current study shows that differential scanning calorimeter measurements allow a clear prediction if an alloy is peritectic (i.e., critical to cast). In order to confirm these results, thermo-optical analyses with a high-temperature laser-scanning-confocal-microscope are performed to observe the phase transformations in situ up to the melting point.

Ultrafine grained high-alloyed austenitic TRIP steel

High-alloyed cast TRIP steel showing pronounced martensitic phase transformation during plastic deformation like tensile, compressive or cyclic loading exhibits concurrently high strength and high ductility. Further increase in the yield strength/ultimate tensile strength can be realized by a smaller austenitic grain structure. An ultrafine grain size of the austenite can be realized among others by well situated thermo-mechanically controlled processing like combination of cold rolling and subsequent specific heat treatment. Cast plates of high-alloyed TRIP steel were cold rolled to different deformation degrees. Subsequently, heat treatment experiments at different annealing temperatures and annealing times were performed. The microstructures after heat treatment were investigated by X-ray diffraction and scanning electron microscopy. The obtained grain size of the reverted austenite was determined by EBSD measurements. Finally, tensile tests on reverted austenitic steel specimens were performed in order to determine the influence of the grain size on the mechanical properties. The results show that plates of a high-alloyed cast TRIP steel can be cold rolled up to 90% of thickness reduction leading to high amount of α′-martensite (80%). Specific heat treatment of 90% cold rolled TRIP steel results in ultrafine grained reverted austenitic microstructure with mean grain size of about 1μm. Tensile tests of heat treated steel specimens revealed an enormous increase in the yield strength up to 1000MPa. The ultrafine grained austenitic steels still show the TRIP effect at a grain size of 1μm.

Stress Induced Phase Transformations in TRIP-Steel / Mg-PSZ Composites

Composite materials and micro- and macrostructure designs have been the focus of numerous scientific studies over the past few years according to their crashworthiness [1-3]. Crashworthiness is concerned with the absorption of energy through controlled failure mechanisms and modes that enable a defined load profile during energy absorption [4]. Cellular materials, such as metal foams, are materials which display a unique combination of physical and mechanical properties, e.g. for crash box applications. The defining characteristic of metal foams is a very high porosity, typically in the range of 70 to 90 vol. %. In principle, cellular metals can be manufactured from gas, liquid or solid phases and currently the most advanced methods involve melt-metallurgical processes [5]. Several groups have produced foam structures by using hollow spheres to form the cells of the material [5, 6]. These materials exhibited plateau stresses of 5 MPa and 23 MPa respectively, with volume specific energy absorptions SEA of 2 MJ/m3 and 10 MJ/m3 respectively, up to 50 % strain [6, 7]. By combining ceramics with ductile metals, failure-tolerant metal matrix composites (MMCs) can be created. With regard to application of the MMCs as wear resistant materials in metal forming tools a prolongation of the life time and the resultant reduced equipment downtimes have been achieved by active steel infiltrating of porous zirconia structures with the aid of Ti as activator [8]. A very promising approach concerning zirconia/steel - composite materials with superior mechanical properties has been demonstrated by Guo et al. using a low-alloyed TRIP steel in combination with an Y-PSZ – ceramic [9, 10]. In a previous study honeycomb structures were formed from composites of high-alloyed austenitic stainless TRIP-steel AISI 304 with Mg-PSZ with different mixing proportions due to ceramic extrusion at room temperature and sintering at 1350 °C for 2 h in an 99.9 % Argon atmosphere [11]. One of the most promising manufacturing route to produce open cell composite foams is based on the patent of Schwartzwalder [12] by the replication method using polyurethane sponge as a template. The polymer foam is impregnated in a powder slurry (this first coating contributes as an adhesive porous layer for further coating processes), the ceramic slurry is squeezed out of the functional pores and cold spray coatings are applied in order to eliminate defects out of the squeezing process and reach the critical wall thickness for acceptable mechanical properties. In [13] the authors reported about foams with 90 Vol% high alloyed TRIP-steel and 10 Vol% Mg-PSZ. Up to 50 % compressive strain a remarkable enhancement of the SEA was observed in comparison to comparable structures with TRIP-steel only.

Microstructure Evolution and Phase Transformation in a Novel High-Alloyed TRIP Steel Observed during <i>in-Situ</i> Tensile and Cyclic Deformation

The tensile and cyclic deformation behaviour of a new metastable austenitic stainless cast steel were studied in-situ in a scanning electron microscope (SEM). The metastable steel was investigated in cast condition. The in-situ tests in the SEM show the evolution of the microstructure in dependence on the deformation degree for uniaxial deformation and the number of cycles during fatigue, respectively. The development of the microstructure (i.e. deformation bands) are correlated with the places, where the phase transformation of the metastable austenite into the hcp structure or the bcc ’- martensite (TRIP – effect; TRansformation Induced Plasticity) takes place by applying different SEM-techniques as electron backscatter diffraction (EBSD). In addition, calculations of local deformation fields developing during tensile and cyclic deformation using digital image correlation (DIC) are presented.

Simulation of martensitic transformations in TRIP-steel and Fe-based shape memory alloy

Several important effects in the mechanical behavior of materials are due to martensitic transformations. In case of TRIP-steels, the γ→ α ′ martensitic transformation during plastic deformation is responsible for high strength combined with an outstanding ductility. Another martensitic transformation ( γ→ ε) is the reason for the perfect shape recovery in Fe-based shape memory alloys (SMAs). By means of a finite element analysis the evolution of martensite both in a TRIP-steel and in an Fe-based SMA is investigated. The model is able to explain the experimentally observed dependence of the austenite start temperature in an Fe-based SMA on the extent of prior martensitic transformation. Dilatometric experiments with an high-alloyed TRIP-steel are used to validate the predictions of the TRIP-simulations.