Zn Diffusion and α-Fe(Zn) Layer Growth During Annealing of Zn-Coated B Steel

Direct hot press forming of Zn-coated 22MnB5 steels is impeded by micro-cracks that occur in the substrate due to the presence of Zn during the forming process. A study was therefore undertaken to quantify concentration of Zn across the α-Fe(Zn) coating and on grain boundaries in the α-Fe(Zn) layer and the underlying γ-Fe(Zn) substrate after isothermal annealing of Zn-coated 22MnB5 at 1173 K (900 °C) and to link the Zn distribution to the amount and type of micro-cracks observed in deformed samples. Finite difference model was developed to describe Zn diffusion and the growth of the α-Fe(Zn) layer. The penetration of Zn into the γ-Fe(Zn) substrate after 600 seconds annealing at 1173 K (900 °C) through bulk diffusion is estimated to be 3 μm, and the diffusion depth of Zn on the γ-Fe(Zn) grain boundaries is estimated to be 6 μm, which is significantly shorter than the maximum length (15 to 50 μm) of the micro-cracks formed in the severely stressed conditions, indicating that the Zn diffusion into the γ-Fe(Zn) from the α-Fe(Zn) during annealing is not correlated to the depth of micro-cracks. On the other hand, the maximum amount of Zn present in α-Fe(Zn) layer decreases with annealing time as the layer grows and Zn oxidizes, and the amount of Zn-enriched areas inside the α-Fe(Zn) layer is reduced leading to reduced length of cracking. Solid-Metal-Induced Embrittlement mechanism is proposed to explain the benefit of extended annealing on reduced depth of micro-crack penetration into the γ-Fe(Zn) substrate

Role of heating conditions on microcrack formation in zinc coated 22mnb5

Zinc coated steels for hot press forming are intended to offer active corrosion resistant ultra-high strength steel for applications in the automotive industry. During hot-press forming, zinc can infiltrate the underlying grain boundaries during the heat and forming step, leading to intergranular cracking of the base metal. In this study, samples of coated 22MnB5 steel were heated in a furnace at temperatures of 880, 900 and 920°C for 240 to 600 s and subsequently hot-formed into U-shaped profiles. Microstructure, morphology and depth of cracks were analysed using conventional optical and scanning electron microscopy, energy dispersive spectroscopy and focused ion beam imaging. The distribution of Zn in the heated coatings and underlying substrate was measured in detail by energy dispersive spectroscopy in transmission electron microscopy with samples taken out from the coating/base metal interfaces applying an in-plane focused ion beam lift-out method. In general the heating conditions do not appear to have effect on the density and spatial distribution of cracking in the top-wall and in the side-wall, but concerning the depth of penetration into the martensite/prior austenite substrate there is clear evidence that with extended heating time and temperature the depth of the penetration into the substrate and also the amount of penetrating cracks inside the top-wall is reduced. However full understanding of the role of heating condition on microcrack formation in zinc coated 22MnB5 has not been achieved at this stage yet; further work is underway and will be published later

Analysis of new Gleeble tensile specimen design for hot stamping application

Hot tensile testing is useful to understand the material behavior at elevated temperatures. Hence it is of utmost importance that the test condition is accurate enough to derive stress-strain data in fully austenitic state and to ensure homogeneous deformation throughout the gauge length of the specimen. But present limitation of standard Gleeble hot tensile sample geometry could not be used to achieve a uniform temperature distribution along the gauge section, thus creating errors of experimental data. In order to understand the effect of sample geometry on temperature gradient within the gauge section coupled electrical-thermal and thermo-mechanical finite element analysis has been carried out using Abaqus, with the use of viscoplastic damage constitutive equations presented by Li [1]. Based on the experimental study and numerical analysis, it was observed that the new sample geometry introduced by Abspoel [2], is able to achieve a better uniformity in temperature distribution along the gauge length; The temperature deviation along the gauge length within 25 ∘C during soaking and 5 ∘C after cooling and onset of deformation); also the strain deformation is found to be almost homogeneous

Analysis of new Gleeble tensile specimen design for hot stamping application

Hot tensile testing is useful to understand the material behavior at elevated temperatures. Hence it is of utmost importance that the test condition is accurate enough to derive stress-strain data in fully austenitic state and to ensure homogeneous deformation throughout the gauge length of the specimen. But present limitation of standard Gleeble hot tensile sample geometry could not be used to achieve a uniform temperature distribution along the gauge section, thus creating errors of experimental data. In order to understand the effect of sample geometry on temperature gradient within the gauge section coupled electrical-thermal and thermo-mechanical finite element analysis has been carried out using Abaqus, with the use of viscoplastic damage constitutive equations presented by Li [1]. Based on the experimental study and numerical analysis, it was observed that the new sample geometry introduced by Abspoel [2], is able to achieve a better uniformity in temperature distribution along the gauge length; The temperature deviation along the gauge length within 25 ∘C during soaking and 5 ∘C after cooling and onset of deformation); also the strain deformation is found to be almost homogeneous

Zn diffusion and α-Fe (Zn) layer growth during annealing of Zn-coated B steel

Direct hot press forming of Zn-coated 22MnB5 steels is impeded by micro-cracks that occur in the substrate due to the presence of Zn during the forming process. A study was therefore undertaken to quantify concentration of Zn across the α-Fe(Zn) coating and on grain boundaries in the α-Fe(Zn) layer and the underlying γ-Fe(Zn) substrate after isothermal annealing of Zn-coated 22MnB5 at 1173 K (900 °C) and to link the Zn distribution to the amount and type of micro-cracks observed in deformed samples. Finite difference model was developed to describe Zn diffusion and the growth of the α-Fe(Zn) layer. The penetration of Zn into the γ-Fe(Zn) substrate after 600 seconds annealing at 1173 K (900 °C) through bulk diffusion is estimated to be 3 μm, and the diffusion depth of Zn on the γ-Fe(Zn) grain boundaries is estimated to be 6 μm, which is significantly shorter than the maximum length (15 to 50 μm) of the micro-cracks formed in the severely stressed conditions, indicating that the Zn diffusion into the γ-Fe(Zn) from the α-Fe(Zn) during annealing is not correlated to the depth of micro-cracks. On the other hand, the maximum amount of Zn present in α-Fe(Zn) layer decreases with annealing time as the layer grows and Zn oxidizes, and the amount of Zn-enriched areas inside the α-Fe(Zn) layer is reduced leading to reduced length of cracking. Solid-Metal-Induced Embrittlement mechanism is proposed to explain the benefit of extended annealing on reduced depth of micro-crack penetration into the γ-Fe(Zn) substrate

Lignin-Based Additives for Improved Thermo-Oxidative Stability of Biolubricants

There is an environmental concern regarding the use of petroleum-based lubricants, which are generally toxic and nonbiodegradable. Biobased lubricants, such as vegetable oils, are the alternative: they show excellent lubricity, are readily biodegradable and nontoxic. However, a major disadvantage of using vegetable oils in lubricant applications is their lack of thermo-oxidative stability, which can be improved by antioxidant additives. Here, we propose the use of lignin-based additives in biolubricant formulations to improve this feature, based on lignin’s known antioxidant properties. To ensure a stable dispersion in vegetable oil, lignin was partially esterified. Antioxidant properties of lignin before and after palmitoylation were demonstrated in a 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay. Four different lignin-based fractions, commercial Protobind P1000 soda lignin from straw, solvolytically fractionated Protobind P1000 lignin and two lignin fractions from reductively catalyzed fractionation (RCF) of native birch wood, were tested in biolubricant formulations with castor oil as base oil. Those lignin fractions exhibited excellent performance compared to butylated hydroxytoluene (BHT), a commonly used petroleum-based antioxidant. Formulations of modified lignin in castor oil possess improved thermo-oxidative stability, as illustrated by their increased oxidation induction time. Additionally, rheological and tribological tests demonstrate similar, or in some cases improved, lubricating properties compared to castor oil. This study showcases the successful incorporation of lignin-based antioxidants in biolubricant formulations, tackling the major disadvantage of vegetable oils as environment-friendly lubricants