J. K. Wu

The influence of microstructure on hydrogen transport in carbon steels

An electrochemical permeation technique was performed on mild steel and S45C medium carbon steel. Mild steel shows higher permeation rate and diffusivity than medium carbon steel. The highest permeability and diffusivity for heat treated medium carbon steels are observed in the spheroidized condition and least permeability and diffusivity are in the martensitic condition. Annealed and normalized structures yield intermediate values of permeation rate and diffusivity. A monolayer of AgBr emulsion was coated on the determination site of a specimen to observe hydrogen transport in carbon steel. This hydrogen microprint technique also confirms that the main diffusion path is lattice and carbide/ferrite interfaces in pearlitic and spheroidized steels, and lath interfaces in martensitic steel.

Structure, hardness and adhesion properties of CrN films deposited on nitrided and nitrocarburized SKD 61 tool steels

Abstract The structure, hardness and adhesive strength of CrN/plasma nitrided, gas nitrided and nitrocarburized and single CrN coated SKD 61 tool steel were studied. The CrN film was deposited by the cathodic arc ion plating (AIP) deposition process at low temperature (200°C). By Rockwell N-impact indentation testing and scratch testing, the surface failure mode between the CrN film and (un)pretreated substrate was used to determine the adhesion property. The CrN coated plasma and gas nitrided specimens showed better adhesion than that of the nitrocarburized specimens. The compound layer in nitrided layers was monophase ɛ-Fe2–3N which causes substrate hardening and provides excellent hardness, and adhesive strength for composite coated steels.

Hydrogen transport and degradation of a commercial duplex stainless steel

Hydrogen embrittlement of 2205 duplex stainless steel has been evaluated using electrochemical permeation measurement, hydrogen microprint technique and tensile test in this study. Due to hydrogen transport in 2205 duplex stainless steel is mainly lattice diffusion in ferritic phase, more hydrogen distribution, higher permeation rate and diffusion in ferritic phase were detected. Brittle fracture was observed in both the ferritic and austenitic phases of hydrogen precharged specimen.

Effect of hydrogen environment on the notched tensile properties of T-250 maraging steel annealed by laser treatment

In the present work, slow displacement rate tensile tests were performed to find out the influence of ageing condition and hydrogen-charging on the notched tensile strength and fracture characteristics of T-250 maraging steel aged at various conditions. The influence of embrittling species in the environment on the notched tensile strength was accessed by comparing the measured properties in air, gaseous hydrogen and H2S-saturated solution. The hydrogen diffusivity, permeation flux and apparent solubility of various specimens determined by electrochemical permeation method, were correlated well with the microstructures and mechanical property. The results indicated that the peak-aged (H900) specimen was highly sensitive to hydrogen embrittlement even in gaseous hydrogen. In contrast, the microstructures of over-aged (H1100) specimen comprising of reverted austenite and incoherent precipitates could trap large amount of hydrogen atoms, resulting in decreased hydrogen permeability and hydrogen embrittlement susceptibility. The solution-annealed specimen had the highest diffusion coefficient and the lowest quantity of trapped hydrogen among the specimens, showing high susceptibility to sulfide stress corrosion cracking. In the presence of notches, hydrogen atoms were prone to segregate and trap at grain boundaries, resulting in the formation of intergranular fracture.

A brief review note on mechanisms of hydrogen entry into metals

Metal hydrides can serve as hydrogen storage materials either through chemical reaction or by thermal decomposition. Severe embrittlement can also occur in many metals by very small amounts of hydrogen. There are numerous ways for hydrogen to enter metals. In this paper, mechanisms of hydrogen entry into metals in gas phase and electrochemical process are briefly reviewed.

Hydrogen permeation in a submerged arc weldment of TMCP steel

Abstract The effects of the heterogeneous microstructure at the base metal, the heat affected zone (HAZ) and weld metal on hydrogen permeation in thermo-mechanical controlled rolling (TMCP) steel weldments have been investigated. The base metal with equiaxed refined ferrite and scattered fine grain pearlite has the highest permeation rate and effective diffusivity. The HAZ with bainite shows the lowest values of the permeation rate and effective diffusivity. Weld metal yields a higher permeation rate coupled with an intermediate diffusivity value. The hydrogen apparent solubility is low for the base metal, intermediate for the HAZ and high for the weld metal. The mechanisms of hydrogen diffusion path and hydrogen traps are discussed and experimentally confirmed using the hydrogen microprint technique. The high diffusivity paths and the hydrogen trapping site are the grain boundary and the ferrite/carbide interfaces for both the base metal with refined ferrite and the HAZ with bainitic microstructure. The spaces among the basket-weave acicular ferrite, where the martensite and the retained austenite (M/A) constituents present, are the main hydrogen trapping sites for the weld metal.

Notch tensile properties of laser-surface-annealed 17-4 PH stainless steel in hydrogen-related environments

Abstract Slow displacement rate tensile tests were performed to determine the notched tensile strength (NTS) of 17-4 PH stainless steel with various microstructures in hydrogen-related environments. Solution-annealed (SA), peak-aged (H900), over-aged (H1025), and laser-annealed (LA) specimens were included in the study. Based on the results of NTS in air, the NTS loss in both gaseous hydrogen and H 2 S-saturated solution was used to access the detrimental effects of hydrogen in 17-4 PH steel subjected to different treatments. Electrochemical permeation tests were also employed to determine the hydrogen permeation characteristics of the 17-4 PH steel plate with various microstructures. The result indicates that all the specimens have low NTS loss in gaseous hydrogen but significantly suffer from sulfide stress corrosion cracking (SCC), especially for the soft SA specimen. It was deduced that high hydrogen diffusivity and less trapped hydrogen atoms in the SA matrix provided rapid transport of massive hydrogen atoms into highly stressed region, and deteriorated the NTS tested in the saturated sulfide solution. On the other hand, H1025 specimen consists of the blocky austenite together with Cu-rich precipitates uniformly distributed in the grain interior; dense and coarse precipitates are also observed along prior austenite grain boundaries. Hydrogen atoms tend to be trapped along grain boundaries, and lead to the formation of intergranular fracture for H1025 specimen tested in the H 2 S solution. Fine and homogeneously distributed precipitates in the H900 matrix result in uniformly trapping of hydrogen atoms, so it behaves superior properties than other specimens. The decohesion of precipitate/matrix interfaces induces quasi-cleavage fracture of the H900 specimen tested in H 2 S solution. Finally, the application of laser-annealing treatment on the H900 specimen cannot improve its resistance to sulfide SCC, because the laser-annealed zone is susceptible to hydrogen embrittlement in the H 2 S solution.

High temperature and corrosion properties of cathodic-arc-plasma-deposited CrN coatings

Abstract CrN film was deposited on a WC-30Co substrate using the cathodic arc plasma deposition process, and the effects of annealing in nitrogen atmosphere and a vacuum chamber were investigated. CrN film comprises mainly CrN and β -Cr 2 N phases. Annealed at high temperature, it is possible to decompose CrN to Cr and Cr 2 N phase. The corrosion performance and hardness of CrN coating are both excellent. Micrographs of a coating after potentiodynamic tests still show homogeneous Cr- and N-rich phases on the corroded specimen. Annealed in an N 2 atmosphere at 1000 °C, the CrN-coated specimen has the best corrosion resistance and good hardness.

Correlation between corrosion potential and polarization resistance of rebar in concrete

The relationship between corrosion potential thermodynamics and polarization resistance (electrode kinetics) was analyzed. The influence of cement replacement materials (fly ash and slag) on corrosion resistance of steel in concrete in 3.5% NaCl solution was also investigated by corrosion potential and polarization resistance measurements. Replacement of cement by 10% fly ash shows improved corrosion resistance by reinforcing steel in concrete while concrete with 15 and 30% replacement of cement with slag results in a significant reduction in corrosion resistance of reinforcing steel.

Surface modification of Ti-6Al-4V alloy by salt cyaniding and nitriding

Abstract A cyaniding process, operating in a molten cyanide salt, and a nitriding process, operating in a Tufftride salt bath, were applied to modify the near-surface microstructure of Ti-6Al-4V alloy. The surface-hardened layers have been characterized with respect to their hardness and microstructure. The corrosion and wear performance can both be improved by cyaniding and nitriding.