A.F. Armas

Mechanical and microstructural behaviour of isothermally and thermally fatigued ferritic/martensitic steels

Isothermal low cycle fatigue (LCF) and thermal low cycle fatigue results in the temperature range between room temperature and 823 K of the quenched and tempered reduced activation ferritic/martensitic steels F82H mod. and EUROFER 97 are reported. Under these test conditions both steels show, after the first few cycles and for both types of tests, a pronounced cyclic softening up to failure. The softening during LCF tests described by a simple empirical relationship is dependent on temperature but independent of the total strain amplitude of the tests. From the analysis of the hysteresis loops and corroborated by electron microscopy observations it can be concluded that the cyclic softening is produced by the softening observed in the internal stress as a consequence of the evolution of the microstructure. During cycling, the martensitic lath structure with high dislocation density and carbides along the lath interfaces evolves to a softer dislocation subgrain structure. This conclusion could be correlated with transmission electron microscopy observations.

Dynamic strain ageing evidences during low cycle fatigue deformation in ferritic–martensitic stainless steels

Abstract The influence of dynamic strain ageing (DSA) on the strain cyclic behaviour of ferrite–martensite stainless steels was investigated at temperatures ranging from room temperature to 823 K. For fully annealed AISI 420 initial hardening followed by a saturation stage was observed at each test temperature. This steel was found to be susceptible to DSA as evidenced by the temperature independent stress saturation observed between 523 and 723 K. Normalized and tempered MANET II and F82H mod. softens during cyclic loading at all temperatures. In this steel DSA manifestations were observed on plotting the peak tensile stress difference between hysteresis loops obtained at different strain rates. Strongly abnormal behaviour with higher peak tensile stresses corresponding to slower strain rates was observed in the temperature range between 500 and 700 K. It is proposed that DSA mechanisms caused by the drag of solution carbon atoms is responsible for this unusual behaviour.

Effect of strain rate on the cyclic hardening of Zircaloy-4 in the dynamic strain aging temperature range

Abstract Low cycle fatigue (LCF) tests were conducted in Zircaloy-4 with a total strain range of ±0.5% in the temperature range 573–873 K where dynamic strain aging (DSA) manifestations in uniaxial tensile tests were reported. A time dependent linear cyclic hardening stage is the principal feature observed in this alloy. The cyclic hardening rate exhibits a peak in this temperature range. In order to evaluate the strain rate effects on the cyclic hardening, tests at strain rates 2 × 10 −3 s −1 and 2 × 10 −4 s −1 were performed. The location of the cyclic hardening rate peak depends on the strain rate suggesting the operation of a thermally activated mechanism. Transmission electron microscopy (TEM) observations show that the linear hardening stage is characterized by a two-dimensional wall structure that evolves from the activation of two slip systems. DSA takes place during strain-controlled fatigue deformation at the same temperature range of that occurring during tensile deformation.

Thermal fatigue behavior and dislocation substructures of 316-type austenitic stainless steels

Abstract Results of a study on cyclic behavior and dislocation structures of type AISI 316L austenitic stainless steel arising from cyclic thermal stresses are reported. The test method consists in an ohmic heating device to allow a thin tubular test specimen to serve as its own heater, converting any longitudinal thermal deformation of the specimen into elastic or inelastic deformation. The effects of thermal stress cycling on the cyclic hardening and/or softening behavior and the accompanying microstructural changes in the specimen were evaluated. A cyclic hardening is observed for all temperature changes. This hardening is followed by a continous softening that occupies the major part of total fatigue life. Only for the temperature amplitude of 473–823 K an extended saturation plateau was observed. The dislocation structure formed after failure depends on the temperature amplitude. These structures will evolve from a veins and walls structure for the lowest temperature amplitude to a completely equiaxed cell structure for the highest temperature amplitude.

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.

Thermal fatigue on steels for iter and advanced fusion reactors

Abstract In first wall structures of fusion reactors like ITER high dose radiation will be an important issue. At present thermal cycling is considered, beside the radiation damage, as the most detrimental lifetime limiting phenomenon for the above structure. A test method for performing uniaxial low cycle thermal fatigue tests on metals is described. The tubular specimens are thermally cycled between a fixed low temperature (200°C) and a variable high temperature (550°C to 750°C) in air atmosphere. Total mechanical strain amplitudes between 0.4% and 1.4% are evaluated in case of AISI 316L and between 0.3% and 0.5% in case of ferritic-martensitic steel. Thermal fatigue life, of both steels examined, is found to be shorter than isothermal, strain controlled fatigue life at a temperature equal to the high temperature of thermal cycling. Crack initiation occured in most cases on nonmetallic inclusions or impurities. Dislocations are of a well defined cell configuration, occupying the largest volume fraction ...

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.

Dislocation substructures developed in martensitic steels under thermal fatigue

Abstract Thermal fatigue tests were carried out on a martensitic steel, DIN denomination W. Nr. 1.4914, commonly named MANET I. The tests were performed in air by allowing the sample to serve as its own heater and converting any longitudinal thermal deformation of the specimen into elastic or inelastic deformation. The low temperature was held constant and equal to 473 K and variable values, 823, 873, 923, 973 K for the high temperature were selected. The effects of different thermal cycling ranges on the mechanical behavior and the accompanying microstructural changes in the specimen were evaluated. A continous softening preceded by a stability period was observed in all thermal fatigue tests. Higher temperature changes produce an accelerated softening process. The original lath structure evolves to a mixed structure of expanded laths and subgrains or a fully subgrain structure depending on the temperature range.

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.