Dynamic and post-dynamic recrystallization under hot, cold and severe plastic deformation conditions

Relation between microstructure and mechanical properties of a low-alloyed TRIP steel

Hydrogen induced cracking (HIC) of API X80 and API X100 pipeline steels have been investigated in high pH carbonate–bicarbonate environment using slow strain rate testing (SSRT) method. It has been found that while both steels are highly susceptible to HIC, and diffusible hydrogen content is higher in API X80 than in API X100, the later steel is more vulnerable than the former at high (more negative) cathodic potential. This higher susceptibility can be primarily attributed to the combined effect of (1) separation of bainitic lath boundaries due to hydrogen trapping in these locations, (2) mobile hydrogen, and (3) stress. The charging–discharging experiments followed by SSRT experiments in air suggest that, the cracks that appeared due to lath boundary separation did not cause the reduction of ductility by themselves, rather it was the diffusible hydrogen that forced these cracks to propagate and, ruptured the steel with very low percent reduction of area (%RA). Despite the fact that the mobile hydrogen content plays a key role in causing the embrittlement, the large number of cracks in API X100 steel, resulting from the bainitic lath boundary separation at high cathodic potential, superseded the effect of higher diffusible hydrogen content in API X80 steel. The general conclusion is that bainitic lath type microstructure is more vulnerable to HIC at high cathodic potential than the ferritic/granular bainitic ones. It has been also found that applying cathodic protection can lead to excessive hydrogen embrittlement in both of the abovementioned steels in high pH carbonate–bicarbonate environment and, therefore, efforts need to be invested in developing nobler (more positive corrosion potential) and better HIC resistant steels.

Effect of bainitic microstructure on the susceptibility of pipeline steels to hydrogen induced cracking

The application of the electron backscatter diffraction technique to the investigation of phase transformations is reviewed. The wide variety of results obtained using this technique is illustrated and discussed, focusing on thermodynamics and kinetics of phase transformations, solidification, solid state phase transformations, environmentally assisted reactions and thin film deposition. Emphasis is also placed on two rapidly growing developments: coupling electron backscatter diffraction with advanced experimental techniques and with more and more complex modelling of phase transformations and of resulting material properties.

Application of electron backscatter diffraction to the study of phase transformations

Quantitative analysis of variant selection in ausformed lath martensite

Austenite texture and bainite/austenite orientation relationships in TRIP steel

Solute drag effects during dynamic recrystallization were studied using four different nickel-sulphur alloys containing from 1 to 20 wt. ppm of sulphur. It was shown in an earlier paper that the steady state stress for dynamic recrystallization depends on the sulphur concentration. The aim of this paper is to discuss the effect of sulphur on the steady state grain size during dynamic recrystallization. The compression tests were carried out between 650°C and 1200°C at a constant strain rate of 10 s -1 until a strain of 1 was attained. The samples were then quenched in water within Is. The grain sizes were measured after interrupted compression tests using both optical micrography and orientation imaging microscopy (OIM) measurements. As expected, the grain sizes decrease when the solute concentration is increased. A model taking into account the reduction of the mobility of random grain boundaries due to their interaction with sulphur and the occurrence of twinning is proposed to account for the steady state grain size during dynamic recrystallization.

Ph. Bocher

Papers

Austenite texture and bainite/austenite orientation relationships in TRIP steel

Effect of solutes on the dynamic recrystallization microstructure of nickel