Hydrogen diffusivity and trapping have been studied in two advanced high strength steel grades and model samples using electrochemical permeation test. Microstructures of CP1000 and DP1000 steels consist of ferrite, martensite and a small fraction of retained austenite. In addition, bainite is present in CP1000. Model phases with predominance of a particular phase have been prepared by specific heat treatment. DP1000 has shown the lowest diffusivity among all materials, while ferritic model sample has shown the highest. Differences in hydrogen diffusion coefficient values are linked to trapping microstructural characteristics and grain size.
K E Y W O R D Shydrogen-induced cracking, hydrogen permeability, steel
Production volumes of advanced high strength steels (AHSS) are growing rapidly due to material and energy savings they provide in a number of application areas. In order to use their potential fully, it is necessary to minimize any danger of unexpected failures caused by hydrogen embrittlement. It is possible only if deeper understanding of underlying mechanisms is obtained through further research. Besides description of main grades of AHSS and mechanisms of HE, this paper reviews available tools for determination of hydrogen content and susceptibility to HE focusing on atmospheric conditions. Techniques such as slow strain rate testing, constant load testing, electrochemical permeation technique, scanning Kelvin probe and scanning Kelvin probe force microscopy have already been used to study the effect of hydrogen entered under atmospheric exposure conditions. Nanoindentation, hydrogen microprint technique, thermal desorption spectroscopy, Ag decoration or secondary ion mass spectrometry can be also conducted after atmospheric exposure.
Deep-sea manganese nodules are polymetallic oxidic ores that can be found on a seabed. Aluminothermic reduction is one of the possibilities of manganese nodules processing. This process obtains the polymetallic alloy with a high content of Mn and a varying content of Al, depending on the ratio between aluminum and nodules. The corrosion behaviors of three experimental Mn-based alloys produced by aluminothermic reduction with a content of Mn > 50 wt % were studied. The electrochemical testing in potable water and model seawater was used to explain the corrosion mechanism of Mn-based alloys. The results showed that the corrosion rate of experimental Mn-based alloy decreases with the increase in aluminum content in both potable water and model seawater. It was observed that the uniform corrosion of experimental Mn-based alloys is changed with an increase in aluminum content in alloy to localized corrosion, which was caused by microcells in an environment of model seawater. In contrast, the formation of a semi-protective layer of corrosion products was observed on the surface of Mn-based alloys with a higher content of aluminum in potable water. Moreover, the pitting corrosion of tested Mn-based alloys was observed neither in potable water nor in model seawater.
Hydrogen absorption into steel during atmospheric corrosion has been of a strong concern during last decades. It is technically important to investigate if hydrogen absorbed under atmospheric exposure conditions can significantly affect mechanical properties of steels. The present work studies changes of mechanical properties of dual phase (DP) advanced high strength steel specimens with sodium chloride deposits during corrosion in humid air using Slow Strain Rate Test (SSRT). Additional annealed specimens were used as reference in order to separate the possible effect of absorbed hydrogen from that of corrosion deterioration. Hydrogen entry was monitored in parallel experiments using hydrogen electric resistance sensor (HERS) and thermal desorption mass spectrometry (TDMS). SSRT results showed a drop in elongation and tensile strength by 42% and 6%, respectively, in 27 days of atmospheric exposure. However, this decrease cannot be attributed to the effect of absorbed hydrogen despite the increase in hydrogen content with time of exposure. Cross-cut analysis revealed considerable pitting, which was suggested to be the main reason for the degradation of mechanical properties.
The phase composition and portion of individual phases in advanced high-strength steels (AHSS) CP1000 and DP1000 was studied by complementary microscopic and diffraction techniques. CP1000 and DP1000 steel grades have a high strength-to-density ratio and they are used in many applications in the automotive industry. The microstructure of the CP1000 “complex phase” steel consists of ferrite, bainite, martensite and a small amount of retained austenite. DP1000 is a dual phase steel, which has a structure of a ferritic matrix with islands of martensite and a minor amount of retained austenite. The influence of selected etchants (Nital, LePera, Beraha I, Nital followed by metabisulfite, Nital followed by LePera, and Nital followed by Beraha I) on the microstructure image is described. X-ray diffraction, neutron diffraction and light optical, scanning and transmission electron microscopy were used in this work for advanced characterization of the microstructure and phase composition. The information provided by each technique is critically compared.
New devices for measurement of hydrogen permeability in steels using electric resistance sensors have been assessed. Diffusion coefficients of two types of advanced high strength steels obtained with the traditional electrochemical permeation technique using the Devanathan-Stachurski permeation setup and the novel techniques are compared. The EPT technique is found to be more appropriate for laboratory assessment of hydrogen permeability due to generally better control over the experiment and data evaluation. The novel setups can be a good choice for comparative hydrogen measurements in real environments, including atmospheric ones, because of simpler sample preparation and small size of hydrogen probes.
K E Y W O R D Satmospheric corrosion, hydrogen induced cracking, hydrogen permeability, steel
For the comprehensive investigation of advanced high strength steel grades, like complex and dual phase steels, Atomic Force Microscopy (AFM) and Scanning Kelvin Probe Force Microscopy (SKPFM) have proven to be useful tools, especially for analysis of hydrogen permeability of the individual steel phases. However, for these studies a preparation route, exposing the microstructure of the steel, is necessary. Various sample preparation methods were examined, focusing on electropolishing and sputtering, and the selected route was optimized to guarantee reproducibility and stability of the prepared surface. Electropolishing was shown to be highly efficient to selectively reveal the individual steel phases without introducing strong topographical features disturbing the AFM measurements. A subsequent sputtering step was introduced to improve the stability and preservability of the surface up to several months. Finally, distinction of the steel phases via AFM/SKPFM, was complemented and compared with results from EBSD and XRD.
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