S. Hereñú

Hydrogen effects on low cycle fatigue of high strength steels

Abstract Hydrogen absorption occurs during steelmaking processes and causes detriment on mechanical properties, such as plasticity, fatigue strength and tensile strength, among others. The main purpose of the present paper is to study the hydrogen effects on the low cycle fatigue behaviour of a high strength steel, resulphurised and microalloyed. Before the cyclic tests, samples are cathodically charged using a H2SO4 acid solution. In some samples, poisons are added. The flow stress evolution during cycling was studied by analysing the so called ‘back’ and ‘friction’ stresses derived from the hysteresis loops. Fatigued specimens were observed through scanning electron microscopy and transmission electron microscopy. Additionally, the metallographic technique known as ‘silver decoration’ allows evaluation of the hydrogen distribution in the structure by applying energy dispersive analysis. The higher stress levels and cyclic softening rates exhibited by hydrogen charged samples in comparison with uncharged ones are related with the friction stress behaviour. The hydrogen is found mainly associated with MnS inclusions.

Beneficial Effects Induced by High Temperature Cycling in Aged Duplex Stainless Steel

The cyclic behavior of type DIN 1.4460 duplex stainless steels in as-received and aged conditions was investigated at room temperature and at 500°C. At room temperature, the aged samples showed embrittlement effects such as loss of toughness and reduction of fatigue life. At 500°C, beneficial effects of the synergy between temperature and strain cycling were observed. It is proposed that at high temperature in the ferrite the strain cycling can decompose the chemical composition fluctuations, promoting a demodulation of the spinodal decomposition formed by aging.

Microstructural Changes in a Duplex Stainless Steel During Low Cycle Fatigue

Abstract Low cycle fatigue of SAF 2507 duplex stainless steel has been studied at different temperatures in two thermal conditions: unaged (as-received) and aged (100 hours at 475°C). The corresponding cyclic flow stress components, the friction, σF, and the back stresses, σB, have been analyzed in relation to the observed microstructure. σF attains higher values at room temperature than in the intermediate range of temperatures in both thermal conditions due to the influence of the lattice friction stress on screw dislocations in the ferrite phase. The cyclic softening observed at all temperatures in aged samples is caused by the decrease of the back stress σB. The increase of σB in unaged samples at 300°C is consistent with the high dislocation density observed in the ferrite phase ascribed as a typical manifestation of dynamic strain aging.

Cyclic deformation and microstructural behaviour of reduced activation ferritic–martensitic steels

Abstract The present paper presents results about cyclic behaviour and the evolution of the dislocation structure of reduced activation ferritic–martensitic steels and commercial martensitic steels AISI 410 and 420. The variation of the free dislocation density within subgrains and subgrain size was mainly analysed during the cyclic softening of EUROFER 97 steel. From the analysis of the flow stress components, the friction and back stresses, and the information of the evolution of the dislocation structure, it could be concluded that the softening of tempered martensitic steels at 20°C is produced by the contribution of the friction stress and aided later by the back stress.

Characterisation of inclusions in resulphurised microalloyed steel

Abstract Resulphurised steels, due to their good mechanical properties and machinability, are employed in the manufacture parts of trucks and high strength components. The microstructure control of these steels can improve their toughness without losing strength. In this sense, the chemical composition, morphology and distribution of MnS achieved with different thermomechanical processes affect the mechanical properties of the steel. In this work, the morphology and distribution of MnS inclusions in resulphurised steel 15V45 were analysed. Samples of the steel in its as cast condition and after hot rolling were examined using light and scanning electron (SEM) microscopies. Microhardness measurements and chemical composition of different MnS types were carried out to evaluate the plastic behaviour of each inclusion. The SEM images have provided information about the nucleation sites and size of the inclusions. Applying a Fe–Mn–S diagram under different thermomechanical conditions, this study could predict ductility problems and crack generation during hot rolling processes.

Hydrogen Influence on the Mechanical Behaviour of High Strength Steel

Abstract Though numerous studies have been devoted to hydrogen embrittlement in steels, up to date there is not a general agreement about the effect of hydrogen on the mechanical behaviour. The purpose of this paper is to analyze the influence of hydrogen on the mechanical response of high strength steels. Samples were cathodically charged with hydrogen, previous to low cyclic fatigue and tensile tests at room temperature. The presence of hydrogen produces softening effects on the cyclic behaviour and improvements in the fatigue life for low hydrogen contents. The stress-strain curves of tensile tests on pre-charged samples depend on the strain rate imposed. Both tensile and fatigue response could be explained by the hydrogen enhancement of dislocation mobility mechanism.