Carbon Enrichment In Austenite Research Articles

Transformation of austenite into bainitic ferrite and martensite

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Effect of bainite transformation and retained austenite on mechanical properties of austempered spheroidal graphite cast steel

Austempered ductile iron (ADI) has excellent mechanical properties, but its Young's modulus is low. Austempered spheroidal graphite cast steel (AGS) has been developed in order to obtain a new material with superior mechanical properties to ADI. Its carbon content (approximately 1.0 pct) is almost one-third that of a standard ADI; thus, the volume of graphite is also less. Young's modulus of AGS is 195 to 200 GPa and is comparable to that of steel. Austempered spheroidal graphite cast steel has an approximately 200 MPa higher tensile strength than ADI and twice the Charpy absorbed energy of ADI. The impact properties and the elongation are enhanced with increasing volume fraction of carbon-enriched retained austenite. At the austempering temperature of 650 K, the volume fraction of austenite is approximately 40 pct for 120 minutes in the 2.4 pct Si alloy, although it decreases rapidly in the 1.4 pct Si alloy. The X-ray diffraction analysis shows that appropriate quantity of silicon retards the decomposition of the carbon-enriched retained austenite. For austempering at 570 K, the amount of the carbon-enriched austenite decreases and the ferrite is supersaturated with carbon, resulting in high tensile strength but low toughness.

Austenite decomposition during continuous cooling of an HSLA-80 plate steel

Decomposition of fine-grained austenite (10-µm grain size) during continuous cooling of an HSLA-80 plate steel (containing 0.05C, 0.50Mn, 1.12Cu, 0.88Ni, 0.71Cr, and 0.20Mo) was evaluated by dilatometric measurements, light microscopy, scanning electron microscopy, transmission electron microscopy, and microhardness testing. Between 750 °C and 600 °C, austenite transforms primarily to polygonal ferrite over a wide range of cooling rates, and Widmanstatten ferrite sideplates frequently evolve from these crystals. Carbon-enriched islands of austenite transform to a complex mixture of granular ferrite, acicular ferrite, and martensite (all with some degree of retained austenite) at cooling rates greater than approximately 5 °C/s. Granular and acicular ferrite form at temperatures slightly below those at which polygonal and Widmanstatten ferrite form. At cooling rates less than approximately 5 °C/s, regions of carbon-enriched austenite transform to a complex mixture of upper bainite, lower bainite, and martensite (plus retained austenite) at temperatures which are over 100 °C lower than those at which polygonal and Widmanstatten ferrite form. Interphase precipitates of copper form only in association with polygonal and Widmanstatten ferrite. Kinetic and microstruc-tural differences between Widmanstatten ferrite, acicular ferrite, and bainite (both upper and lower) suggest different origins and/or mechanisms of formation for these morphologically similar austenite transformation products.

Quantitative Analysis of the Dilatation by Decomposition of Fe-C Austenites; Calculation of Volume Change upon Transformation

A detailed analysis has been made of the length changes due to decomposition of iron-carbon austenites into ferrite and carbon enriched austenite above the A 1 temperature, and into ferrite and cementite below the A 1 temperature, using lattice-parameter data for the phases involved. The calculations have been performed as a function of carbon content, temperature and degree of transformation. A general definition for the total degree of transformation has been proposed for the case that more than one product phase develops. This definition also holds when the product phases develop consecutively, allowing an analysis of the kinetics of the austenite decomposition. Above the A 1 temperature, the relation between the degree of transformation and the dilatation is linear within good approximation. However, below the A 1 temperature a strong deviation from such a linear relationship occurs, as the result of subsequent decomposition of austenite into ferrite and carbon enriched austenite, and decomposition of the carbon enriched austenite into ferrite and cementite.

Isothermal Transformation Products in a Cu-bearing High Strength Low Alloy Steel.

A time-temperature-transformation (TTT) diagram has been determined for a copper-bearing steel, whicn is a low alloy TMCP variant of the ASTM A710 type of structural steel. Quantitative mesurements have been supplemented by optical microscopy and transmission electron microscopy to investigate the isothermal transformation behaviour as well as the associated precipitate morphologies. It is shown that the kinetics and product phases of the polymorphic transformation and precipitation reactions are sensitive to both temperature and time. The TTT-diagram shows a prominent transformation region for bainitic structures, at temperatures intermediate between those of polygonal ferrite and martensite. In the intermediate region, the microstructures were characterised by a ferritic matrix with a lath and/or plate shaped grains containing a high dislocation density, together with a minor dispersed phase. For a short holding time (5 sec) at intermediate temperatures (580-430°C), the island phase was identified as untempered twinned and lath martensite, autotempered twinned and lath martensite, and martensite/austenite constituent, depending on the level of carbon partitioning in the remaining austenite before quenching in water. For a longer holding time, the carbon enriched austenite regions decomposed to carbide and ferrite by coupled growth. Polygonal and quasi-polygonal ferrite were formed at relatively high transformation temperatures and these microstructures contained a low dislocation density and were associated with interphase e-copper precipitates.

Crystal-structure changes during tempering of hardened high-strength cast irons with nodular graphite

1. The martensite of high-strength cast iron is decomposed by a two-phase mechanism in the first stage of tempering (up to 100°C); the second stage of martensite decomposition occurs at 100–300°C and has the same character as that of high-carbon steel. 2. The decomposition of residual austenite occurs in two stages: The austenite is decomposed with the segregation of α-phase crystals and the carbon enrichment of intact austenite segments in the first stage (100–400°C); the decomposition is completed in the second stage (400–550°C) with the segregation of carbide from the carbon-enriched residual austenite. 3. Decomposition of residual austenite during tempering begins at 100°C in silicon cast iron, and at 200°C in manganese cast iron.