Birger Karlsson

Inhomogeneity in plastic deformation of two-phase steels

Abstract Plastic deformation in ferritic - martensitic and ferritic - pearlitic steels with 0.3 vol.% inclusion in a continuous matrix has been studied. Continuum mechanics (finite element method, FEM) has simultaneously been applied to a corresponding two-phase model. The strain field is very inhomogeneous and the inhomogeneity increases substantially with increasing yield stress ratio. Continuum mechanics successfully predicts the measured hardness variation. Fracture criteria are discussed on the basis of the results obtained.

Plastic deformation of eutectoid steel with different cementite morphologies

Abstract Pearlite and spheroidite of different coarseness have been studied in eutectoidal steels. While the yield stress in the different structures is governed by the mean dislocation slip distance in the ferrite according to the Hall — Petch relation, the work-hardening rate is determined by the cementite morphology and is independent of the interparticle distance. For spheroidite this is explained by the load transfer from ferrite to cementite. In the case of pearlite, an important additional contribution comes from plastic constraint effects in the nodules. Experimental results on workhardening behaviour are supplemented by FEM calculations on model structures.

Plastic deformation of ferrite—pearlite structures in steel

Abstract The yield and work hardening of ferrite - pearlite aggregates with continuous ferrite matrix have been studied, special attention being paid to the relative deformation of the individual constituents, which was followed by TEM and measurements of microhardness and X-ray line broadening. FEM calculations of the deformation of inclusions of similar geometry in a softer matrix have been performed for varying hardness ratio between inclusion and matrix. hard and soft pearlite inclusions plastify at different degrees of overall strain. The flow stress of the aggregate structure can be described in terms of load transfer from the matrix to the inclusions, using a mixture rule which divides both stress and strain among the constituents in the ratio of their volume fractions.

Microstructural evaluation and interpretation of the mechanically and thermally affected zone under railway wheel flats

Abstract During a wheel–rail slide, the temperature is high enough to austenitise the material close to the contact surface. The material is rapidly cooled by heat conduction into the rest of the wheel when the wheelset starts rolling again and thus martensite can form in the surface layer. In the present investigation, the mechanically and thermally affected zones (TAZ) under railway wheel flats have been studied by optical microscopy, scanning electron microscopy and hardness testing. The wheel flat samples were generated in full-scale experiments on the last axle of a train consisting of a locomotive pulling three carriages. The axle load, train speed and locking time of the last axle were varied to simulate several different situations that arise in revenue traffic. The entire population of samples has earlier been studied, but the present investigation goes more into detail on a limited number of samples. Several discoveries are made and suggestions of mechanisms are presented. Among the most important findings are stretched MnS inclusions that act as crack nucleation sites, limited grain growth indicating low austenitisation temperatures and reformation of ferrite and pearlite in the deformed surface layers.

Precipitation behaviour in heat affected zone of welded superaustenitic stainless steel

AbstractThe present work concerns the characterisation of intermetallic phases formed in the heat affected zone of a welded superaustenitic stainless steel of composition Fe–0·02C–3Mn–24Cr–7·3Mo–22No–0·5Cu–0·5N (wt-%). Grain boundary precipitates at various distances from the fusion line have been investigated regarding crystal structures, compositions, and particle morphologies. A correlation of precipitation with the temperature history recorded in the heat affected zone was also performed. Two different precipitates, σ and R, were detected in the heat affected zone. These precipitates were evenly distributed along grain boundaries and had platelike shapes with typical lengths of 200–900 nm and 30–300 nm for σ and R, respectively. Near the fusion line, coexistence of R and σ phases was observed but, at larger distances, only R phase was found, indicating a lower temperature of formation for this phase.

Crack initiation and growth during low-cycle fatigue of discontinuously reinforced metal-matrix composites

Abstract The initiation and growth of surface cracks during low-cycle fatigue of an AA6061 alloy composite containing (a) 15 vol% of SiC particles and (b) 20 vol% of Saffil short fibres were studied. For both of the composites, cracks are initiated very early in the fatigue life irrespective of the cyclic strain amplitude. A full three-dimensional FEM simulation demonstrated that the inhomogeneous distributions of stresses and strains induced by the particles and fibres are enhanced at a free surface relative to the interior of the material. This promotes surface crack initiation either by cracking of the reinforcement or near interfacial cracking in the matrix. The growth rates of the surface microcracks are an order of magnitude faster than the growth rates of long through-cracks at corresponding values of the crack growth parameter ΔJ . It was further observed that surface cracks shorter than 100 μm grow at a decelerating rate, yet did not show any threshold behaviour. Despite this early initiation of microcracks in the composites, the particulate composite notably showed a larger tolerance against plastic strain cycling than the unreinforced matrix in the lower range of cyclic plastic strain amplitudes.

The influence of grain size and fracture surface geometry on the near-threshold fatigue crack growth in ferritic steels

Fatigue crack growth in the threshold region is very much affected by the microstructure. A large portion of this microstructural influence is due to changes in the crack closure stress intensity Kcl. The present paper is concerned with the influence of the grain size on the near-threshold fatigue crack growth in ferritic steels, with special emphasis on the influence of the microstructurally induced fracture surface roughness on the crack closure stress intensity. The experiments were performed on ferritic steels with grain sizes varying between 8 and 82 μm. An increase in the nominal and effective fatigue crack growth threshold values with increasing grain size was found. Quantitative fractographic measurements showed that the fracture surface roughness rises with the grain size independently of the stress intensity level. The results also indicate that Kcl is proportional to H13, where H is the fracture surface roughness as expressed by the mean standard deviation of height. By rationalizing the experimental data, ΔKth has been quantified in terms of grain size effects and the fracture surface roughness.

High temperature low cycle fatigue properties of Ti-48Al-2W-0.5Si gamma titanium aluminide

The low cycle fatigue properties of the γ-TiAl alloy Ti-48Al-2W-0.5Si (at.%) have been examined. Seven batches of the material with slightly varying compositions were used, elucidating the large influence of the microstructure on the low cycle fatigue properties and fracture behaviour. A duplex fine grained microstructure shows superior properties in comparison to a material low in aluminium with a coarse grained nearly lamellar structure due to larger isotropic hardening and smaller Bauschinger effect, which lead to smaller inelastic strains and damage in each cycle. The fractographic study showed that failures are mostly initiated at the surface of the test specimens. Interlamellar crack initiation is predominant in the nearly lamellar structure, whereas the duplex material shows initiation mainly in large γ-grains or internally in interdendritic γ-areas. The larger scatter in fatigue life of the nearly lamellar material is explained by the anisotropic properties of the lamellar colonies in combination with the large colony size. The probability to have a large lamellar colony with the lamellas oriented perpendicular to the loading axis producing premature failure is increased as the content of this microconstituent is increased. Even if the duplex structure has the lowest fracture toughness of the two types of microstructures studied here, its grain size is smaller than the critical crack length for instant failure, leading to some stable fatigue crack growth and thus explaining the smaller scatter in fatigue lives for this microstructure.

Homogenization by two-phase diffusion

Abstract Using the finite difference method the homogenization of two-phase structures with moving interfaces has been studied. Boundary conditions involving solid/solid and solid/liquid structures representing typical technical situations have been utilized. As a special case the austenitization of normalized steels with different carbon contents has been investigated. The effects of austenitization temperature, concentration dependent diffusion coefficient and size distribution have been analyzed. It is shown that within the geometries and boundary conditions studied, the total homogenization time for solid/solid and solid/liquid structures is controlled by the final one-phase diffusion and that total homogeneity is achieved for Dt/a2 = 0.3. It is also demonstrated that for austenitization the linearly concentration dependent diffusion coefficient can be approximated by that corresponding to the mean concentration. Increasing the standard deviation of the austenite cell size distribution drastically increases the austenitization time.

Cyclic deformation and fatigue behaviour of 7Mo-0.5N superaustenitic stainless steel-slip characteristics and development of dislocation structures

Abstract The present work concerns the development of dislocation structures and surface slip markings during cyclic straining of a superaustenitic stainless steel. The composition of the tested material was Fe–25Cr–22Ni–7.6Mo–3Mn–0.46N (wt%). Two total strain amplitudes, 2.7×10 −3 and 1.0×10 −2 , were employed and specimens were investigated at specific numbers of cycles corresponding to certain stages on the cyclic hardening/softening curve. For both strain amplitudes, the developed dislocation structures are strongly planar and with increasing strain amplitude, the slip mode gradually changes from single slip to multiple slip. The short range ordering between Mo and N, as indicated by an atom probe investigation, is broken down during strain cycling leading to increased slip planarity. Early stages of cycling show dislocation multiplication. With increasing number of cycles, the dislocations are gradually grouped together in planar bands with high dislocation density, surrounded by dislocation-poor areas. The evolution of such bands is associated with decreasing effective stresses, while the internal stresses are only slightly reduced. Macroscopic slip bands, similar to PSBs, are formed upon prolonged cycling at the high amplitude. The slip markings created on the specimen surface show strong similarities with the bands of localised slip observed in the dislocation structures of the bulk.