Jean-Bernard Vogt

The effects of an Fe-Zn intermetallic-containing coating on the stress corrosion cracking behavior of a hot-dip galvanized steel

This study deals with the mechanical behavior of galvanized, interstitial free (IF) steel in air and sodium chloride water. Tensile tests conducted in air at different strain rates lead in general to a ductile behavior. It can become brittle when tested in a NaCl solution at particular strain rate values. Fractographic analysis shows a brittle fracture similar to hydrogen embrittlement (HE). A detailed analysis taking into account both the individual mechanical strength of the phases on the coating as well as the electrochemical reaction between Zn and water allow the explanation of this embrittlement. Hydrogen can be produced and can penetrate the cracks formed inside the intermetallic phases of the coating during loading, thus, reaching the steel substrate. The HE of the IF steel, assisted by zinc dissolution and galvanized coating low toughness, appears to be the cause of the loss of ductility observed in the presence of specific environmental conditions, strain rates and morphologies of the coating.

Fatigue properties of high nitrogen steels

Abstract The paper reports the different effects that nitrogen produces on the fatigue properties of austenitic and duplex stainless steels. High nitrogen steels exhibit higher stress levels to strain cycling than conventional ones and a strong tendency to softening. In general, the fatigue resistance is increased with nitrogen alloying but there exists a limit upon which the effect becomes negligible. In the single phased alloys, these properties result from strong interactions between nitrogen atoms and dislocations, occurrence of planar slip and of short range order and in some cases decrease of stacking fault energy. In dual phased alloys, the role of nitrogen appears in addition with an indirect way. Being a γ stabilizer, addition of nitrogen in duplex stainless steels tends to produce more austenite than ferrite which appears beneficial for controlling the softening and for the distribution of fatigue damage. Finally, it is shown that a too large amount of nitrogen restricts the safe fields of application.

Liquid metal embrittlement of the martensitic steel 91: influence of the chemical composition of the liquid metal. Experiments and electronic structure calculations

Abstract In previous works [Scripta Mater. 43 (2000) 997; J. Nucl. Mater. 296 (2001) 256], we showed that the martensitic steel 91 is prone to liquid metal embrittlement (LME) by liquid lead provided that some metallurgical conditions are fulfilled. In this work, we report results of LME of the steel 91 in contact with Pb–Bi and other low melting temperature metals such as Sn and Hg. Our experimental results can be interpreted within the framework of the surface energy reduction models for LME. To account for the experimental observations, we performed electronic structure calculations to assess the chemical interaction between low melting temperature metal atoms and iron surfaces. Our results allow to establish a simple criterion that can give trends on the embrittlement power of a liquid metal in contact with iron.

Embrittlement of the martensitic steel 91 tested in liquid lead

Two potential problems are encountered in the case of intimate contact between liquid metals and metallic substrates: grain boundary wetting and liquid metal embrittlement (LME) which both induce a degradation of the mechanical properties. Tensile tests were carried out on a 9% Cr 1% Mo martensitic (Grade 91) steel in a liquid lead environment at temperatures ranging between 623 and 773 K. The Grade 91 steel was submitted to heat treatments in order to modify its hardness and also to produce either ferritic or martensitic grains. Smooth and notched specimens were used. We found out that by combining adapted heat treatments and the notch effect, it is possible to create conditions severe enough that lead to LME. Our experimental observations (transgranular failure) are compatible with the expectations of traditional mechanisms based on a reduction of the surface energy and/or adsorption induced chemical bond softening at the steel surface in contact with liquid lead.

Low cycle fatigue life enhancement of 316 L stainless steel by nitrogen alloying

Abstract Tests carried out at room temperature on 316 L stainless steels with different nitrogen contents show that nitrogen improves the low cycle fatigue resistance of the materials. However, saturation occurs when nitrogen content is above 0.12 weight per cent. The microstructural aspect is also studied; the deformation is more difficult and more planar when nitrogen is present. Moreover, nitrogen delays the formation of cells. A single relation, derived from the Manson-Coffin formula, describes the low cycle fatigue behaviour of these steels by taking into account plastic strain range and nitrogen content.

Role of the microstructure on fatigue properties of 475°C aged duplex stainless steels

Abstract This paper deals with the role of microstructure, nitrogen content and α/γ phases volume fraction, on fatigue properties of duplex stainless steels (DSS) aged at 475°C. The results obtained from this study show that the α/γ volume fraction plays an important role on fatigue lives. The fatigue resistance is not deteriorated in an aged DSS with 70% of austenite. The stress response to strain cycling is strongly affected by the difference of hardness between ferrite and austenite. A 475°C aged DSS containing 0.4% nitrogen and 70% of ‘hard austenite’ lead to interesting fatigue properties which do not strongly differ from that obtained in annealed DSS.

Low cycle fatigue behaviour of a precipitation hardened Cu-Ni-Si alloy

Low cycle fatigue tests were performed at room temperature to investigate the role of the microstructure of a Cu-Ni-Si alloy on the stress response to strain cycling and on the fatigue resistance. The cyclic accommodation consisted in a hardening followed by a softening. TEM analysis showed that in some grains dislocations remained isolated and confined between precipitates while in other grains dislocations piled up at δ-Ni 2 Si precipitates and then cut them. Repetitive cutting allows their dissolution and formation of precipitate-free bands where the plastic deformation is localised. The Manson-Coffin diagram exhibited two regimes according to the proportion of grains involved in the plastic deformation accommodation.

Hydrogen-induced phase transformation in a high nitrogen austenitic stainless steel

Abstract The effect of hydrogen on the stability of the austenite of the Fe19Cr19Mn0.9N was investigated. Cathodic charging at room temperature was used for the introduction of hydrogen. The investigation took into account the charging duration on the transformation of the austenite and the effect of ageing at room temperature on the transformed phases. It is shown that a transformation of the g FCC phase into the g e expended FCC phase, into the e H martensite and into the g H hydride occurs. Reversion of the hydride into the g phase was observed after 288 ks but not for the e H martensite. The consequences of the microstructural evolution on cracking are then evaluated.

Fatigue damage evaluation of a power plant component from analysis of the dislocation structures

Abstract The paper presents a method for estimating the degree of fatigue damage in an engineering ferritic–bainitic steel. A dislocation microstructure chart is first established from laboratory fatigue tests. Misorientation measurements are carried out after fatigue failure between dislocation cells produced by cyclic loading in ferrite grains and between dislocation cells in bainite originated by the heat treatment. There is shown to be a clear dependence of these values on the strain amplitude. On this basis, the analysis of the microstructure of a steel taken from a component which failed in service allowed the determination of the strain amplitude value to which the component had been subjected.

Initiation and Growth of Short Cracks during Cycling in an Aged Superduplex Stainless Steel

The kinetics of short crack growth during cycling has been studied in a superduplex stainless steel in aged condition. After few cycles, slip lines appear distributed in both phases but the preferred phase for microcrack nucleation is the ferrite. Contrary to the exponential behavior observed in the as-received material, the growth rate of microcracks in aged condition follows a rather linear law. Internal dislocation structures were studied in the near surface region; microbands that sometimes extend over several grains were found at approximately 45º of the tensile axis on ferrite grains. The origin of the microbands has been analyzed and correlated with the microcracks.