Applications of ultra-high strength steel sheets to automotive bodies have expanded steadily in recent years. Various methods are used to evaluate resistance to hydrogen embrittlement (HE), which is one problem of ultra-high strength steels. In this study, the critical HE conditions obtained by the SSRT, CSRT and 4-point bending test were compared by using the same materials. The materials were two ultra-high strength steel sheets with tempered martensite microstructures, one with the SCM435 composition and the other a V-added steel containing many hydrogen trapping precipitations. The specimens were charged with hydrogen by the cathodic charging method. The specimens used in the SSRT and CSRT were machined with notches on both sides of the parallel part. The values of the stress concentration factor (Kt) of the specimens were 4.26 and 1.76. A coupon-shaped specimen was used in the 4-point bending test. The critical HE conditions evaluated by the average applied stress and the average hydrogen content of the specimen were different depending on the test methods. The HE conditions were also evaluated by the local stress and the local accumulated diffusible hydrogen content at the fracture initiation point. The critical condition evaluated by the 4-point bending test was located in a higher stress and higher hydrogen content region compared with the critical conditions obtained by the CSRT and the SSRT.KEY WORDS: slow strain rate test; conventional strain rate test; 4-point bending test; local stress; local diffusible hydrogen content; delayed fracture.

No abstract

TiO 2 particle-polymer composite coatings were applied to the surface of a 5083 aluminum alloy. After using a knife to create an artificial defect, polarization resistance was monitored in artificial seawater at a temperature of 30 8C. The polarization resistance of the specimen coated with the composite polymer containing 3 vol% TiO 2 particles increased significantly over time, suggesting that the composite coating had self-healing properties. A carbon-containing 2-mm thick film was found on the coated aluminum substrate at the site of the artificial defect. The formation of the film was related to the dissolution of bisphenol A (BPA), which is a chemical precursor of the polymer coating that behaved as an inhibiting agent.

No abstract

A remarkable difference in thermal desorption spectra of hydrogen obtained from pre-strained highstrength steel specimens which were charged with hydrogen by two different methods was observed. One charging method is by immersion in NH4SCN solution and the other is by cyclic corrosion tests. In order to understand the difference, we simulated numerically thermal hydrogen spectra of the pre-strained high-strength steel. As a result, it was found that the difference of desorption spectra results from the difference of initial hydrogen states which is caused by the amount of charged hydrogen. It was also found that the desorption spectrum in the case of cyclic corrosion test is more sensitive to the initial hydrogen state than that of the immersion case because the amount of charged hydrogen in the former is not as enough as that in the latter.