I. Serre

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.

LCF Mechanisms of the 25 Cr 7 Ni 0.25 N Duplex Stainless Steel Investigated by Atomic Force Microscopy

Abstract Surface relief evolution during the fatigue life has been studied by Atomic Force Microscopy in a duplex stainless steel fatigued at a total strain range Δεt = 1.6%. In austenite, hill-like extrusions, observed since the beginning of the fatigue life, were the basic morphology. For ferritic grains, the modification took place later and the disposition of the persistent slip markings was irregular in a given grain. Highly rugged areas were the more developed surface morphology in ferrite after 60 percent of the fatigue life. These areas seemed to be connected with the austenitic neighbour grain in terms of surface activity and disorientation, and are proposed as the possible crack nucleation sites in the ferrite, near the ferrite austenite interfaces. It is shown that the origin of crack nucleation in ferrite is strongly dependent on the relative crystallographic orientation of the austenite and its plastic activity.

Liquid metal-induced embrittlement of a Fe9Cr1Mo martensitic steel

Publisher Summary This chapter analyzes the influence of liquid metal on the mechanical behavior of Fe9Crl Mo martensitic steel. The recommended heat treatment for the T91 steel consists of an austenization at 1050°C followed by air quenching and then tempering at 750°C for 1 h. The fatigue tests are conducted in a fully push-pull mode at imposed total strain variations ranging from 0.4 to 2.4%. Tempering at 500°C increases the yield stress up to 900 MPa, but the plasticity domain remain large up to 15% at fracture after a test at 350°C in air and liquid Pb. A strong reduction in the fracture elongation for the test in liquid lead compared to that in air suggests a possible embrittlement of the steel by the liquid metal. The fractographic analysis reveals ductile fracture with characteristic dimples for specimens tested in air, and cleavage-like transgranular brittle fracture for specimens tested in the liquid metals. Close inspection of a specimens surface reveals that the machining of the notch causes the formation of a hard cold-worked coat 10 μm thick that breaks during tensile tests. In a liquid metal environment the microcracks generated in this manner have a brittle propagation, while this does not appear to be the case in air.

Plastic Deformation Quantified by Atomic Force Microscopy Measurements for Duplex Stainless Steel under Monotonic and Cyclic Loading

By atomic force microscopy, the plastic deformation marks resulting from monotonic and cyclic plastic deformation were analysed to study the plasticity in each phase of Duplex Stainless Steels. In austenite, straight slip bands were observed after monotonic loading. These straight slip bands seem to serve as fatigue extrusion nucleation sites, which are the marks of the accommodation of the cyclic plasticity by the austenite. In ferrite, after monotonic loading, slip bands, could be classified into two different groups depending on whether they result from the bulk activities of ferrite or whether their formation is assisted by the plastic deformation of austenite. It was found that the crystallographic misorientation based on a Kurdjomov-Sachs relationship is the factor controlling one or the other type. After the first 5 loading cycles, the ferrite presents only monotonic plastic marks. This suggests no direct contribution of the ferrite to the accommodation of the cyclic plasticity.

AFM Study of Microstructure Role in the Cyclic Plasticity of Martensitic Steel

The paper presents the description of FSMs (fatigue slip markings) and the evaluation of cyclic plasticity markings of a 12%Cr martensitic steel by AFM surface analyses. The microstructure of a 12%Cr martensitic steel, quenched and tempered in air, consists of prior austenite grains, packets, blocks, and laths. The low cycle fatigue (LCF) test at a total strain range Δεt=1.2% was interrupted at different life fractions for the surface relief investigation by AFM. The localization of FSMs relatively to the different microstructural interfaces of the studied steel was proposed. The principal FSMs, appeared in the first fatigue cycle, are likely to be localized at the packet and block boundaries, while the secondary one (appeared later at 44% of lifetime) are at the lath boundaries or in the laths. The height of principal and secondary FSMs increases constantly during LCF cycling.

Mechanical Resistance in Liquid Lead-Bismuth of 316L/T91 Welded by Electron Beam

The paper deals with the mechanical strength in air and in liquid lead bismuth at 300°C and 380°C of a T91/316L weld joint assembled by electron beam process. Prior to this, the individual response of each base material has been investigated using the small punch test. The T91 is tested according to several tempering conditions. 316L and T91 tempered at 750°C did not exhibit any sign of liquid metal embrittlement. However, for the T91 steel tempered at 600°C or 500°C, a ductile to brittle transition is induced by liquid lead bismuth. After tempering at 650°C and 700°C, liquid lead bismuth promotes a decrease in mechanical properties and a reduction of the ductility of materials. The load-displacement curves in air of the weld joint are similar to that of the T91 base material. The plastic deformation is mainly concentrated in the T91 part of the weld joint which promotes cracking in this material. Testing in liquid lead bismuth bath results in a reduction in ductility and the formation of brittle cracks. The T91/weld interface is found to be rather resistant as it cracks late in the test and after a large crack propagated in the T91 steel.Copyright

Fatigue Behaviour of a 9Cr1MoNbV Martensitic Steel in a Liquid Metal

Abstract The low cycle fatigue behaviour of the T91 martensitic steel is studied in the range Δεt from 0.4% to 2.4%, at 300°C, in air and in liquid Lead Bismuth Eutectic (LBE). It is shown that the cyclic stress response consists of a cyclic softening that is not modified by the environment. However, the fatigue life is reduced after fatigue in LBE as compared to air and the effect is especially marked at high strain range. Metallographic analysis of the external surfaces and of transverse cross sections of specimen show that the short crack density is very low in the specimen failed in liquid metal while it is high for tests in air. Fracture surface observations show that multiple crack initiations occurred in air. In liquid metal, the fracture surfaces were flat and contained widely spaced fatigue striations. Strain localization promoted by the liquid metal is responsible for the decrease in fatigue resistance.