Papers by Author: Harshad K.D.H. Bhadeshia

Abstract: There is a confused and contradictory literature on the role of small concentrationsof niobium on the development of the so-called local brittle zones in steels. These zones consistof a mixture of martensite and austenite and hence, their formation should be predictable usingmodern microstructure calculation methods. Following an assessment of the most relevant literature,a mathematical model is presented which enables three quantities to be calculated, the fractionof martensite, the carbon concentration of the martensite, and its ability to influence toughness.Examples are presented for particular linepipe steels, and then the generic effect of alloying elementsother than niobium, on the development of local zones.

Abstract: A Theory is Developed for Martensite Variants that have Different Start Temperatures but Existin the same Steel. Themethod Enables the Volume Fractions of each Kind Ofmartensite to be Followed Asa Function of the Steel Temperature. the Problemis Relevant to the Calculation of Detail in Transformationtexture when Phase Changes Occur under the Influence of External Stress. it should Allow for the Firsttime, the Estimation of both the Location of Crystallographic Poles on a Stereographic Projection, Andthe Diffraction Intensity Associated with that Location. it is Found that the Increment of Transformationas a Function of Undercooling is Identical for all Variants, once Simultaneous Transformation Begins.Any Variance in the Absolute Fractions is due to the Differences in the Martensite-Start Temperature.

Abstract: There is now a great deal known about the atomic mechanisms of solid–state phasetransformations, and this knowledge can be exploited to determine the distribution of crystalorientations. It is possible to estimate accurately, the crystallographic texture, transformationstrains and details of the microstructure, particularly in the context of steels. The concepts havenow been applied to design metallic alloys which compensate automatically for the residualstresses which develop in engineering components when they are cooled heterogeneously fromelevated temperatures. Such materials are now in commercial use and represent an innovationresulting directly from phase transformation theory.

Abstract: Large-diameter steel pipes are produced by induction seam-welding followed by induction-assisted heat treatment. The microstructure and distribution of crystal orientations have been studied and related to the mechanical properties of the welded regions. The welding and heat-treatment process leads to a microstructure, a simple observation of which can not explain the observed variations in toughness in the vicinity of the welding joint, because the crystallographic grain size, which represents the scale of similarly oriented adjacent grains, is much coarser than the ordinary grain size. Furthermore, heating the affected zone into the austenite phase field followed by cooling does not completely eliminate the coarse regions of similarly oriented grains. The consequences of this on mechanical properties are discussed.

Abstract: The level of residual stresses generated in fusion welds has been a major area of interest for many years. For steels, a major influence on the final state of stress is through martensitic transformation. This is because the martensitic transformation is accompanied by significant shear and volume strains. One way to mitigate the development of residual stress is by controlling the onset of the transformation such that the associated strain is able to compensate for thermal contraction all the way down to ambient temperatures. In the past it has only been possible to follow the evolution of the phase transformation during cooling of the weld metal using indirect methods such as dilatometry and differential scanning calorimetry. This paper describes the first work in which the phases present are characterized directly during the cooling of reheated weld metal at conditions typical of those encountered during welding by installing a thermomechanical simulator on a synchrotron diffraction beam line at ESRF.

Abstract: The mechanical properties of a bainitic microstructure with slender ferrite plates (20-65 nm in thickness) in a matrix of carbon-enriched retained austenite were characterized. The microstructure is generated by isothermal transformation at temperatures in the range 200-300°C. A yield strength as high as 1.5 GPa and an ultimate tensile strength between 1.77 to 2.2 GPa was achieved, depending on the transformation temperature. Furthermore, the high strength is frequently accompanied by ductility (£ 30%) and respectable levels of fracture toughness (< 45 MPa m0.5). This unusual combination of properties is attributed to the exceptionally fine scale of the carbidefree bainitic microstructure and the associated retained austenite.

Abstract: The choreography of atoms during the course of the bainite transformation has major consequences on the development of structure. In particular, the scale and extent of the structure is dependent directly on the fact that the atoms move in a disciplined fashion. This information can be exploited to develop unconventional alloys - for example, rail steels which do not rely on carbides for their properties, and the hardest ever bainite which can be manufactured in bulk form, without the need for rapid heat treatment or mechanical processing.