Local approach concepts and the microstructures of steels

The paper describes cleavage fracture models that relate local fracture stresses and fracture toughness values to the sizes of brittle initiating particles. In a “quasi-homogeneous” steel, i.e. one possessing a smooth distribution of small particle sizes, a “typically coarse” particle is present in every sample tested and the failure stress or fracture toughness is single valued. When random experimental errors are included, the single valued function becomes a Gaussian distribution. Spatially heterogeneous microstructures produce quite different distributions and these are discussed with respect to extrapolations to low failure probabilities.     

Development of ultrafine microstructures and superplasticity in Hadfield manganese steels

Ultrafine grains were developed in Hadfield manganese steels through appropriate thermomechanical processing. The steels contained from 1.2 to 1.7 wt.% C and 12.3–16.3 wt.% Mn. The austenite grains. 2–8 μm in size, were stabilized against grain growth by a dispersion of fine carbides, typically less than 1 μm. The processed materials were evaluated for superplastic properties at elevated temperatures (750–900 °C). Values for the strain rate sensitivity exponent m in the expression ranged from 0.37 to 0.65. The value of m in the superplastic regime was found to depend on composition, grain size and temperature. At 23 °C, the fine-grain steels showed higher yield strengths and hardness values, but lower ductility, relative to values reported for commercially processed materials.     

EBSD study of micromechanisms involved in high deformation ability of DP steels

Notched and un-notched tensile specimens of fine grained commercial DP780 steel were deformed in uniaxial tension until fracture. Micro-texture analysis was performed by using an FE-SEM equipped with an EBSD detector and the data were analyzed to quantify orientation gradients within the microstructure of the deformed specimens in terms of Image Quality, Inverse Pole Figure and Taylor Factor map. High deformation ability of DP steels was found to be mostly due to such mechanisms as grain rotation, void creation and evolution, substructure formation within the ferrite grains and the highly plastic stretching of martensite during the deformation process. The true strain of martensite was measured up to 64% and 74% for the un-notched and notched specimens, respectively.     

Development of structural steels with re resistant microstructures

Abstract

This paper is concerned with the design and characterisation of fire resistant steels for building construction. Steel design considerations are discussed. Issues raised include controlling the grain size, properties of substitutional elements, and processing. New experimental fire resistant steels microalloyed with molybdenum and niobium, or tungsten, titanium, and boron have been made and their microstructures and tensile properties characterised. The steels possess satisfactory high temperature strength, owing partly to their relatively large grain sizes compared with conventional steels. The nature of equilibrium precipitation has been calculated using Thermo Calc. Optical microscopy, SEM, TEM, and differential scanning calorimetry have been used to determine the physical characteristics. The strengthening mechanisms observed on the experimental steels of this study could be attributed to secondary formation of fine precipitates, in line with previous observations.     

Comparison of the microstructures in continuous-cooled and quench-tempered pre-hardened mould steels

The increased demand for plastic mould steels in pre-hardened condition has drawn the attention to this specific type of steel. As a result, more investigations are performed to understand microstructure and properties. In this work, the microstructures of two pre-hardened plastic mould steels, one quench-tempered (Uddeholm Impax HH) and the other continuously cooled (Uddeholm Nimax), are studied in delivery condition by means of different microscopy techniques and are linked to their production procedure. The results show that the quench-tempered material contains large amounts of M₃C carbides formed within the martensite plates as well as at the lath- and prior austenite grain boundaries. A few coarser Cr-rich M₇C₃ carbides have also been found. In comparison, the microstructure of the continuously cooled material consists of mainly bainite with much lower density and finer cementite particles. The hardness is with ∼40 HRC more or less constant over the cross section of both materials.     

Influence of carbon on development of deformation microstructures in Hadfield steels

Abstract

The microstructural development during rolling is compared in two Hadfield steels, one having low carbon content (0·65 wt-%) and the other high content (1·35 wt-%). Differences in substructure are observed which are due not to small changes in stacking fault energy, but to carbon segregation, which occurs in the low carbon steel (through vacancy diffusion) but not in the high carbon steel. This is demonstrated using Mössbauer spectroscopy and is in agreement with systematic characterisation of microstructures by optical and transmission electron microscopy. In the low carbon steel mixed microstructures are formed which contain intrinsic stacking faults, deformation twins, and brass type shear bands. In the high carbon steel mixed substructures of dislocation tangles, deformation twins, and shear bands (both copper and brass type) are found to develop. In spite of the difference of substructure development during rolling in the two steels, the difference in stacking fault energy is measured to be small (~2 mJ m^?2, i.e. <10% of the stacking fault energy).

MST/1417     

Plastic deformation and creep damage evaluations of type 316 austenitic stainless steels by EBSD

The inspection method of plastic and/or creep deformations has been required as the quantitative damage estimation procedure for structural components especially used in electric power plants. In this study, the method using electron backscatter diffraction (EBSD) was applied to the deformation and damage evaluation of austenitic stainless steels strained by tension or compression at room temperature and also tested in creep at high temperature. It was found that the value of Grain Average Misorientation (GAM) which showed the average misorientation for the whole observed area including over several dozen grains, was a very useful parameter for quantifying the microstructural change as either the plastic or creep strain increased. The unique linear correlation was obtained between GAM and plastic strain in tension and compression. For creep damage evaluation, the difference of grain average misorientation from the value of the unstrained specimen (ΔGAM) showed an excellent correlation with the inelastic strain below strain at which the tertiary creep began.     

Comparative study on microstructures and properties of Mn-containing and Mn-free bainitic steels

ABSTRACT

The effects of Mn on the microstructure and impact-abrasion wear resistance of bainitic steel were studied. Results showed that the Mn-containing steel possessed finer microstructure and higher volume fraction of retained austenite, in comparison with the Mn-free steel. This was caused by lower transformation temperature and higher strength of undercooled austenite. The weight loss of Mn-free steel varying with the impact load was larger than that of Mn-containing steel. High strength, hardness and toughness of Mn-containing steel were conducive to improving wear resistance. More retained austenite in Mn-containing steel played an active role in work hardening and hindering crack propagation. However, the portion of retained austenite that induced martensitic transformation was the same with increasing impact-wear load.     

SEM/EBSD based in situ studies of deformation induced phase transformations in supermartensitic stainless steels

Abstract

Recent year's equipment design has enabled the combination of in situ deformation tests with near real time electron backscatter diffraction (EBSD) mapping of the microstructure evolution in the scanning electron microscope (SEM). The present work involves studies of deformation induced phase transformations in supermartensitic steel containing ～40 vol.-% retained austenite at room temperature. The martensite formation was initiated already at low strains, and increased gradually with increasing plastic strains up to ～10%. It was observed that the martensite formed homogeneously within the microstructure, independent of the crystallographic orientations of the retained austenite. But no new martensite variants, besides those already present in the as received condition, did form during deformation. At the same time, the mutual distribution of these variants remained approximately constant throughout the deformation process.     

Interface microstructures in diffusion bonding of titanium alloys to stainless and low alloy steels

Abstract

Commercial purity Ti and a Ti 6242 alloy have been diffusion bonded to an AISI 316L stainless steel and an AISI 4130 low alloy steel. The microstructures of the as processed products have been analysed using optical metallography, scanning electron microscopy (SEM), and scanning transmission electron microscopy (STEM) techniques. The interdiffusion of the different elements through the interface has been determined using energy dispersive spectroscopy microanalysis in both a SEM and a STEM. For the combinations AISI 316L–commercially pure Ti and AISI 316L–Ti 6242 several regions surrounding the original interface have been observed. Starting from the 316L side, first a α phase is observed, followed by an Fe₂ Ti intermetallic, an FeTi intermetallic, and finally an Fe₂Ti₄O oxide just before the Ti and Ti 6242. Because the diffusion ofTi in Fe is faster than the diffusion of Fe in Ti, a Kirkendall effect is produced. In the AISI 4130–Ti 6242 combination a thin layer of TiC is observed at the interface, limiting the interdiffusion of elements.

MST/1746     

Characterization and quantification methods of complex BCC matrix microstructures in advanced high strength steels

The development of comprehensive and reliable microstructure characterization tools is very necessary for the understanding and modelling of both the formation of different phases during processing and the relationship between microstructure and mechanical properties. A series of samples containing different phases with a BCC structure have been characterised using different techniques including optical microscopy (OM), field emission gun scanning electron microscopy (FEG-SEM) with secondary electron (SE) and back scattered electron (BSE) modes, electron back scattered diffraction (EBSD) and transmission electron microscopy (TEM). It is shown that it is difficult to distinguish ferrite from bainite especially granular bainitic ferrite using conventional OM and FEG-SEM (SE) techniques, whereas FEG-SEM (BSE) and EBSD are the most suitable techniques to differentiate them. A new EBSD method has been developed to dissociate and quantify ferrite, bainite and martensite in multi-phase AHSS steels. This method has been proven to be pertinent and has been validated using reference specimens.     

Classification and reconstruction of three-dimensional microstructures using support vector machines

Reconstruction of three-dimensional (3D) microstructures is posed and solved as a pattern recognition problem. A microstructure database is used within a support vector machines framework for predicting 3D reconstructions of microstructures using limited statistical information available from planar images. The 3D distributions of the grain size of the reconstructed polyhedral microstructures exhibit qualitative agreement with stereological predictions. Amenability of the approach for studying microstructure–property relationships is shown by comparing the computed properties of reconstructed microstructures with available experimental results. Combination of classification methodology and principal component analysis for effective reduced-order representation of 3D microstructures is demonstrated. The pattern recognition technique discussed uses two-dimensional microstructure signatures to generate in nearly real-time 3D realizations, thus accelerating prediction of material properties and contributing to the development of materials-by-design.     

Characterization of transformed and deformed microstructures in transformation induced plasticity steels using electron backscattering diffraction

The microstructure of transformation induced plasticity (TRIP) steels was characterized by means of electron backscattering diffraction (EBSD) technique to identify and quantify their different microstructures such as ferrite, bainite, and retained austenite. Further, the strain distribution in ferrite and retained austenite was analyzed during deformation. The TRIP steels were annealed by austempering for different durations to investigate the effect of the austempering time on the volume fraction change of the microstructural constituents. The quantitative analysis by EBSD coupled with an image contrast analysis revealed that the amount of retained austenite decreased and the amount of bainite increased with increasing austempering time. The mechanical properties of the TRIP steels were also affected by the austempering time. The maximum elongation was obtained in the sample austempered for 5 min, probably because of the good stability of retained austenite. The strain distribution in bcc and fcc phases during tensile deformation was characterized by evaluating the changes in the average local misorientation of the phases.     

A coupled EBSD/EDS method to determine the primary- and secondary-alpha textures in titanium alloys with duplex microstructures

A method for separating the textures of primary alpha (α_p) and secondary alpha (α_s) in alpha/beta titanium alloys with a duplex microstructure was developed. Utilizing electron backscatter diffraction (EBSD) and energy-dispersive spectroscopy (EDS), the approach relies on the non-uniform partitioning of alloying elements between primary alpha and regions containing secondary-alpha lamellae and residual beta matrix phase. The method was evaluated using samples of Ti–6Al–4V for which vanadium partitions strongly to secondary alpha + beta regions. The technique thus provides a useful tool for quantifying the evolution of deformation texture in the primary alpha and transformation texture in secondary alpha formed via decomposition of the beta matrix following hot working or final heat treatment.     

Improvement of toughness of low carbon steels containing nitrogen by fine microstructures

An attempt has been made to improve the toughness of low carbon steels containing total nitrogen from 58 to 150 mass ppm by hot rolling at temperatures lower than the A₁ point (control-rolling). The control-rolled steels had fine microstructures and Charpy impact values higher than 100 J at 273 K. Strain aged steels had Charpy impact values higher than those as control-rolled. The cause is discussed.     

The microstructures and tensile properties of aged Fe-xMn-8Al-0.8C low-density steels

ABSTRACT

The microstructural evolution and deformation behaviour of Fe-(13, 16, 18)Mn-8Al-0.8C (wt-%) low-density steels were investigated in the present work. During aging treatment, the increase in the volume fraction of κ’-carbides significantly improved the yield strength. The formation of κ/α lamellae increased the strain hardening rate. However, large fractions of intergranular κ-carbides deteriorated the ductility of the materials. Mn element stabilised austenite, restricted the precipitation of intergranular κ-carbides and delayed the formation of intragranular κ’-carbides, leading to the uniform distribution of the strain. Increasing Mn content also postponed the peak aging. A good balance of strength and ductility can be achieved by short-time aging treatment in the low-Mn steels or long-time aging treatment in the high-Mn steels.     

Direct TEM observation of microstructures of the austenitic/carbon steels welded joint

The successive alteration of the microstructure from the weld metal zone through weld bonds to the heat-affected zone of a Cr₁₈Ni₁₃ austenitic-0.45% C steels welded joint has been observed using transmission electron microscopy. A new type of microstructure, called pearlite-like structure, has been found and the microstructural details of the weld interface (or fusion line) have been studied. Based on the definition of various zones of the dissimilar steels welded joints under the optical microscope, the transmission electron microscopic characteristics, including microstructures and compositions, of each zone are described and discussed.     

The influence of heat treatment and resulting microstructures on the thermophysical properties of martensitic steels

This research study investigates the influence of heat treatment on the thermal conductivities of three different tool steels at room temperature. The results reveal not only that tempering plays a decisive role in their thermophysical properties, but also that the changes in thermal conductivity due to heat treatment are dependent on the degree of alloying. Isobaric heat capacities c _p are found to be less dependent on heat treatment than thermal diffusivities a. The results are discussed with respect to the resulting microstructures and, for high-tempered conditions, under consideration of Calphad calculations. The results are relevant for the thermal design of tools, in particular, for tailored tempering of tools used for press hardening.     

Computational modeling of crack propagation in real microstructures of steels and virtual testing of artificially designed materials

A computational approach to the optimization of service properties of two-phase materials (in this case, fracture resistance of tool steels) by varying their microstructure is developed. The main points of the optimization of steels are as follows: (1) numerical simulation of crack initiation and growth in real microstructures of materials with the use of the multiphase finite elements (MPFE) and the element elimination technique (EET), (2) simulation of crack growth in idealized quasi-real microstructures (net-like, band-like and random distributions of the primary carbides in the steels) and (3) the comparison of fracture resistances of different microstructures and (4) the development of recommendations to the improvement of the fracture toughness of steels. The fracture toughness and the fractal dimension of a fracture surface are determined numerically for each microstructure. It is shown that the fracture resistance of the steels with finer microstructures is sufficiently higher than that for coarse microstructures. Three main mechanisms of increasing fracture toughness of steels by varying the carbide distribution are identified: crack deflection by carbide layers perpendicular to the initial crack direction, crack growth along the network of carbides and crack branching caused by damage initiation at random sites.