Pornwasa Wongpanya

Numerical Modelling of Cold Cracking Initiation and Propagation in S 1100 QL Steel Root Welds

Although the phenomenon of hydrogen assisted cold cracking (HACC) and respective avoidance procedures have extensively been investigated in the seventies and eighties, the reasons for recent failures are still a lack of knowledge about the basic hydrogen effects on steel microstructures and, in particular, a lack of welding procedure specifications and standards accounting directly and consistently for cold cracking avoidance in modern high strength structural steels with yield strengths of up to 1 100 MPa. In previous several contributions the consequences of various heat treatment procedures targeted at HACC avoidance have been shown, as for instance their effects on stress-strain build up and on hydrogen diffusion in high strength steel welds. But, a principal interaction of three local influences on hydrogen assisted cold cracking, i.e. local microstructure; local mechanical load and local hydrogen content have not yet been studied in detail for these materials. For this, a numerical model for HACC has been developed, accounting particularly for crack-initiation and crack-propagation criteria, like the hydrogen redistribution during the process of cracking. The numerical model has been used to investigate HACC in such materials, i.e. in the weld microstructures of an S 1100 QL steel, under severe restraint and various hydrogen levels. The results were achieved by in depth thermal and structural finite element simulations combined with numerical hydrogen diffusion modelling. By such procedure, HACC in single-layer welded plates with thickness of 20.0 mm at realistic restraints has been studied. As a particular result, it turned out that the crack-initiation location is typically in the centre of the weld metal (WM), where only a single crack is initiated at hydrogen contents of up to 10.0 Nml/100 g Fe. But, it was evidently shown by such analyses that the crack-initiation location is shifted into the HAZ and that multiple cracking occurs at higher hydrogen contents of up to 15.0 Nml/100 g Fe.

Investigation of pitting corrosion of diamond-like carbon films using synchrotron-based spectromicroscopy

Corrosion behavior of diamond-like carbon (DLC) films was evaluated via potentiodynamic polarization in a 3.5 wt. % NaCl solution with pH 2 at room temperature. The polarization results elucidated that the corrosion resistance of the films was enhanced with the variation of the chemical compositions and film thicknesses. The use of the spectromicroscopy method in the investigation of the pitting corrosion was a success in this study. Formation of orbital mapping, bonding state, and composition of the DLC films at non-corrosion and corrosion areas indicate the different structures between DLC and graphite oxide. Also, the π* (C=C) and π* (C=O) bonding states were found to increase in the corrosion areas. This finding not only exhibited the increase of sp2 content but also promoted the rise of oxygen atoms in corrosion zones. Consequently, the present results indicate that the synchrotron–based spectromicroscopy plays an important role in the characterization of the corrosion on DLC films.

A Comparative Study of Wear and Oxidation Behaviors of End Mill Coated by PVD Coatings

The objective of this research is to study wear behaviors of TiN, nanolaminated AlCrN and nanocomposite TiAlSiN coated on cemented carbide end mill deposited by cathodic arc physical vapor deposition methods in comparison with uncoated end mill. Wear behaviors were investigated by nanoindentation hardness test, scratch test and cutting test. Oxidation test was also done in air at temperatures of 700°- 900°C in order to evaluate resistance of oxidation. In the nanoindentation hardness and scratch tests, nanocomposite TiAlSiN coating exhibited higher hardness than TiN and nanolaminated AlCrN coatings. The nanolaminated AlCrN coating represented the highest adhesion ability in terms of critical load and the lowest coefficient of friction in comparison with the TiAlSiN and TiN coatings, respectively. The cutting performance, represented in terms of maximum flank wear as a function of cutting length, was found to be highest in the AlCrN coating. Oxides of these coatings, i.e., TiO2 for TiN, TiO2 for TiAlSiN and Cr2O3 for AlCrN, generated at different temperatures of 700°, 800° and 900°C, respectively. From all of results, it is obvious that the AlCrN coating exhibited more excellent wear resistance and oxidation resistance than the uncoated end mill, TiN coating and TiAlSiN coating.

Increasing Tool Life by AlCrTiSiN Film

A cemented carbide end mill was coated with AlCrTiSiN film by cathodic arc physical vapor deposition methods (CAPVD). A performance of AlCrTiSiN film for increasing tool life was evaluated by scratch test, cutting test and oxidation test in comparison with uncoated end mill. From the scratch test, the AlCrTiSiN film was helpful to resist crack and delamination of coating. From the cutting test, the maximum flank wear of AlCrTiSiN film was about two times higher than that of the uncoated end mill resulting in tool life extension. From the oxidation test, the first oxide of AlCrTiSiN film generated after service life was a titanium dioxide (TiO2) at a temperature of 900oC. From all of results, it revealed that the AlCrTiSiN film significantly enhanced the tool life of the cemented carbide end mill.

Effects of the thickness on the microstructure and corrosion behavior of a TiAlN film on 4140 steel

Abstract TiAlN films with thicknesses of 250, 500 and 750 nm were deposited on 4140 steel using the reactive direct current co-unbalanced magnetron sputtering method. The effects of the film thickness on the microstructure were revealed using a field emission scanning electron microscope, an image analyzer and X-ray diffraction. The results showed that grain size apparently increased when the film was at greater thicknesses whereas porosity, lattice strain and dislocation density decreased. Meanwhile, the results of anodic polarization tests in air-saturated 3.5 wt.-% NaCl solution at pH levels of 2, 7 and 10 and at 25 °C showed lower corrosion potential in thicker film, attributed to a lower ratio of grain boundary area to individual grain area. However, a more stable passive film with higher pitting potential was formed. By analyzing using X-ray absorption spectromicroscopy, oxidation of Ti into TiO2 was found on the corroded surface of TiAlN film.

Oxidation and Adhesion of Decorative Nickel-Chromium Plating on Ferritic Stainless Steel

Decorative Ni-Cr plating on ferritic stainless steel was exposed to pure carbon dioxide (CO2) at 200 and 300oC for 180 hours under continuous and cyclic oxidation tests. After oxidation test, weight changes of samples exposed to CO2 were found to be less than that of samples exposed to air. However, weight changes were dependent on temperature and oxidation conditions. Heat-quench test was operated at temperature of 200-500oC for determining adhesion behavior of decorative Ni-Cr plating. From the heat-quench test, it revealed that adhesion ability between Ni-Cr plating layer and substrate increased with thickness of Ni pre-plating layer at temperature of 200 and 300°C. But, at temperature above 300°C, the adhesion ability increased with surface roughness of substrate before coating. In order to achieve important properties of decorative Ni-Cr plating, i.e., oxidation resistance, beautiful appearance and good adhesion, surface of substrate was necessarily prepared under medium roughness and then it was pre-plated with the thickest Ni layer before Cr coating was applied.

Corrosion Behaviors and Mechanical Properties of CrN Film

Corrosion behaviors and mechanical properties of CrN film were evaluated. The CrN film was deposited on the surface of H13 steel by the magnetron sputtering method. The corrosion behaviors of the uncoated and CrN coated samples were studied in air-saturated 3.5 wt.% NaCl solution with various solution pHs, i.e., pH 2, 7 and 10 at 27°C by the electrochemical technique. The mechanical properties of the CrN coated samples were evaluated using nanohardness, ball on disk and scratch testers. The results revealed that the CrN coated samples had higher corrosion resistance than the uncoated samples at all pHs. Smooth substrate roughness enhanced corrosion resistance and also decreased the wear rate of the CrN coated samples on H13 steel. Corroded area was evaluated by synchrotron X-ray photoemission electron spectroscopy. It revealed that corrosion resistance of the CrN film was from oxidation of Cr into Cr2O3.