Ludmila Hyspecká

Intergranular corrosion of AISI 316L steel

Abstract The degree of sensitization (DOS) of austenitic stainless steel AISI 316L to intergranular corrosion (IGC) was determined by means of electrolytic etching in oxalic acid and electrochemical potentiokinetic reactivation (EPR) tests completed by metallographic inspection. An analytical electron microscope, equipped with an energy-dispersive X-ray spectrometer, was used to examine the microstructure of the steel investigated, which had been annealed for 3.60×10 4 to 1.91×10 7 s at a temperature of 650°C. The kinetics of precipitation of secondary phases (M 23 C 6 carbides, Laves phase Fe 2 Mo) were studied by means of quantitative metallography. The volume fractions of chromium-rich M 23 C 6 carbides along grain boundaries and Laves phase Fe 2 Mo in the matrix were determined. Chromium concentration profile measurements across the grain boundary, in the vicinity of M 23 C 6 carbides, have been performed using the analytical electron microscope. All results concerning microstructural changes were quantitatively related to the IGC characteristics by statistical normalization.

Examination of hydrogen interaction in carbon steel by means of quantitative microstructural and fracture descriptions

The relations between the quantitative microstructural characteristics and the resistance of carbon steels to hydrogen-induced cracking (HIC) were studied for plates used in the oil and refinery industry. The width of the pearlitic bands and the degree of banding were considered if the testing of the resistance to HIC was performed in accordance with the NACE TM 0284 standard. The role of the degree of banding was important while that of the width of pearlitic bands was negligible. Additional hydrogen embrittlement testing of tensile specimens oriented in longitudinal and through-thickness directions revealed that hydrogen strongly increased the anisotropy of mechanical properties. These changes could be correlated with the geometric characteristics of nonmetallic inclusions (MnS) and the pearlitic bands in different metallographic sections. A quantitative description of fracture surfaces has been made by means of a profilometric method.

Use of quantitative metallography in the evaluation of hydrogen action during martensitic transformations

Experiments based on image analysis method and electrochemical potentiostatic tests were performed with particular attention to the influence of hydrogen on the stabilisation of austenite of Fe–32 wt.%Ni alloy. This alloy exhibits great hydrogen absorption capacity (25 wt. ppm) and this presence modifies the martensitic transformation essentially at temperatures near MS (−40°C). In the whole range of the quenching temperatures, the difference between the volume fraction of martensite, measured on specimens with hydrogen or without hydrogen, is constant. It was found that the value of the interfacial area of the martensite plate groups SV is the best parameter describing the interfacial area between martensite and austenite. It was also found that there is a general relationship, expressed by a linear function, between this morphological parameter and an anodic current I.

Mechanical study of instability of austenitic FeNiC alloys—Effect of hydrogen

Abstract The mechanical properties of different Fe Ni C alloys, in which Ni and C contents are correlated in order to ensure roughly equal M s temperatures, are investigated considering three austenitic states: water-cooled (γ), cathodically hydrogen charged at 300°C (γ + 300°C/H 2 ), and heat-treated at 300°C (γ + 300°C) for comparison. The true stress σ versus true strain ɛ are approximated by σ = K 1 + K 2 ɛ 1/2 . Except for 0.006 wt %C, the fitting displays two or three domains of strain characterized by higher values of the slope K 2 at high deformations. For carbon content beyond ≈ 0.2 wt% C this slope increase is due to strain induced martensite. As a consequence transformation induced plasticity (TRIP) effect, confined to medium carbon contents, is observed. At the same time the stress-strain diagrams exhibit instabilities in the form of serrated yieldings. The critical stress and strain of their onset is correlated to the number of Frank-Read sources (FRS) activated by the plastic flow. In the case of higher carbon alloys and higher strains, the increased slope K 2 is thought to be due to another strengthening mechanism involving carbon atoms in the solid solution, associated with Portevin-Le Chatelier (PLC) effect. At low carbon content the effect of hydrogen in prior austenite is negligible, but at high contents the embrittlement and cracking of the strain induced martensite is immediate.

Behaviour of hydrogen in FeNiC alloys

Abstract A particular cathodic charging technique was used to evaluate the behaviour of hydrogen on Fe25.66Ni0.31C alloy. This technique is based on the electrolysis of water injected in a molten salts bath. The electrolytic charging conditions were chosen as −2.05V/Ag and 300°C. The quantities of hydrogen extracted from specimens of different diameters after electrolysis over different durations were used to calculate both the substantial surface concentration ( C 0 = 3 cm 3 H 2 /cm 3 metal) and the hydrogen concentration profiles in austenite. After quenching at −65°C, a linear relationship between the hydrogen concentration and the amount of retained austenite was determined. A critical concentration C k = 0.06 cm 3 H 2 /cm 3 metal, initiated microcracks in martensite. The behaviour of hydrogen on austenite is discussed in terms of grain boundary and dislocation trapping.