Recent advances in friction-stir welding : Process, weldment structure and properties

A model is reviewed, that describes the endpoint of carbon partitioning between supersaturated ferrite and retained austenite. A new process, quenching and partitioning (Q&P), has been developed recently to intentionally employ such partitioning in creating useful ferrous microstructures containing retained austenite. The process involves quenching austenite below the martensite-start temperature, followed by a partitioning treatment to enrich the remaining austenite with carbon, thereby stabilizing it to room temperature. Recent experimental studies have confirmed that Q&P provides a viable means to create microstructures containing carbon-enriched retained austenite, and attractive property combinations have been achieved in a variety of materials, while opportunities remain for further optimization. Furthermore, some implications of the partitioning model with respect to fundamentals of the bainite transformation are discussed, including the possibility of displacive growth under carbon diffusion control, with an austenite composition at the α/γ interface represented by the (adjusted) T0 composition. It is suggested that individual movements of iron atoms are likely during growth of Widmanstatten ferrite, and that there may be a need for further consideration of thermally activated iron-related processes in general.

Partitioning of carbon from supersaturated plates of ferrite, with application to steel processing and fundamentals of the bainite transformation

This article describes the development of a new multi-platform software program called JMatPro for calculating the properties and behavior of multi-component alloys. These properties are wide ranging, including thermophysical and physical properties (from room temperature to the liquid state), time-temperature-transformation/continuous-cooling transformation diagrams, stress/strain diagrams, proof and tensile stress, hardness, coarsening of γ′ and γ″, and creep. A feature of the new program is that the calculations are based on sound physical principles rather than purely statistical methods. Thus, many of the shortcomings of methods such as regression analysis can be overcome. With this program, sensitivity to microstructure can be included for many of the properties and the true inter-relationship between properties can be developed, for example in the modeling of creep and precipitation hardening.

Using JMatPro to model materials properties and behavior

Cementite dissolution in heavily cold drawn pearlitic steel wires

The morphology of continuously cooled bainite in steels is examined in this review and compared with that of isothermally transformed bainite. Some experimental observations of microstructures in continuously cooled commercial steels are also presented; these results demonstrate the presence of microstructures which are not easily defined in terms of any of the “classical” bainitic morphologies. The numerous terms created over the last 50 years to describe specific bainite morphologies have led to some confusion, and it is suggested that the commonly used terminologies do not adequately describe the full range of bainitic microstructures which are observed. A general definition for the bainite transformation is proposed. A classification system for bainitic microstructures is also developed, encompassing both isothermally transformed and continuously cooled bainites. It is suggested that primary bainite morphologies be classified as B1, B2, or B3, depending on whether the acicular ferrite is associated with (1) intralath precipitates, (2) interlath particles/films, or (3) discrete regions of retained austenite and/or secondary transformation product(e.g., martensite or pearlite), respectively. Possible ambiguities with the descriptions of autotempered martensite and Widmanstatten ferrite are also discussed, and a number of areas are identified where further work would be helpful.

Joseph Goldstein

Papers

Phase transformations in ferrous alloys