SummaryThe microstructure and the local texture of a large IMI 834 forging were characterized using the Electron Back Scattered Diffraction (EBSD) technique. Crystallographic domains called macrozones and formed by a majority of primary α p grains with their c axes in nearly the same direction were found. They had a band-like structure, parallel to the axial direction of the forging. The influence of these macrozones on the cold dwell-fatigue properties was studied. Several samples were tested under cold dwell-fatigue conditions. The crack initiation and the short-distance propagation region optically matched a bright region that contained numerous quasi-cleavage facets. The analysis of the EBSD measurements showed that this bright region was enclosed within a sharp textured region with c axes at less than 30 • from the loading axis. The crystallographic features of the crack nucleation site and the crack propagation path were also analysed.

The following cases of hydrogen influence on pipeline metal were considered: gaseous hydrogen under internal pressure in notched pipes and electrochemically generated hydrogen on external pipe surface from soil aqueous environment. The burst tests of externally notched pipes under pressure of hydrogen and natural gas (methane) were carried out after the pipe has been exposed to a constant ''holding'' pressure. It has been shown that even for relatively ''soft'' test conditions (holding pressure p ¼ 20 bar and ambient temperature) the gaseous hydrogen is able to penetrate into near surface layers of metal and to change the mechanism of local fracture at notch. The sensitivity to hydrogenating of given steel in deoxygenated, near-neutral pH NS4 solution under soft cathodic polarisation was studied and the assessment local strength at notches in pipeline has been made for this conditions. Here, the relationship between hydrogen concentration and failure loading has been found. The existence of some critical hydrogen concentration, which causes the significant loss of local fracture resistance of material, was also shown.

No abstract

Protecting the modern high-strength steels against corrosion is a challenge because the coating technology must be compatible with forming and must preserve the mechanical performances. Batch galvanizing after hot stamping could provide a simple solution to this complex problem. A commercial high-strength martensitic steel containing 13 wt.% Cr, 0.35 wt.% Si, 0.3 wt.% Mn and 0.15 wt.% carbon has been galvanized with a commercial zinc alloy. Galvanizing produces a~15 μm thick coating that is bright, continuous and metallurgically bonded. The intermetallic layer is made of ς crystals, which forms an open 3-dimensional structure. Tin, nickel and aluminium are found able to moderate the Sandelin effect. Comparison with other steels galvanized the same way indicates that chromium slows down the kinetics of the metallurgical reaction. Chromium distributes both in the ς and η phases, and follows a diffusion-like profile in the coating. The nickel from the alloy concentrates in the Fe-Zn intermetallic compound. Aluminium segregates at the surface and interface. It also provides a gettering effect that fixes silicon in sub-micron particles dispersed in the ς and η phases. Tensile experiments and fatigue tests demonstrate that the mechanical performances of the martensitic steel are preserved after coating. Comparison with similar experiments performed on a TRIP800 steel indicates that using galvanized martensitic steel is best worth in static applications.

No abstract