Dynamic transformation of deformed austenite at temperatures above the Ae3

A mathematical model has been developed to predict the internal stresses generated in a steel ingot during thermal processing. The thermal history of the ingot has been predicted by a finite-element, heat-flow model, the subject of the first part of this two-part paper, which serves as input to the stress model. The stress model has been formulated for a two-dimensional transverse plane at mid-height of the ingot and is a transient, elasto-viscoplastic, finite-element analysis of the thermal stress field. Salient features of the model include the incorporation of time-temperature and temperature-dependent mechanical properties, and volume changes associated with nonequilibrium phase transformation. Model predictions demonstrate that the development of internal stresses in the ingot during thermal processing can be directly linked to the progress of the phase transformation front. Moreover, the low strain levels calculated indicate that metallurgical embrittlement must be very important to the formation of cracks in addition to the development of high tensile stresses.

Mathematical model of the thermal processing of steel ingots: Part II. Stress model

Effect of Mn and Si on the dynamic transformation of austenite above the Ae3 temperature

Compacted graphite iron (CGI) plays an increasingly important role in the cylinder head of diesel industry because of its good thermal and mechanical properties. In this study, tensile strength, elongation to fracture and the corresponding damaging mechanism observed by in-situ technique at different temperatures have been investigated. The experimental results of CGI show that with increasing the temperature, the tensile strength decreases slightly at first and then dramatically, and the elongation to fracture decreases initially, and then increases. The former trend is mainly controlled by the transitional deformation mechanisms from slip band stage (the dislocation movement inhibition) to boundary sliding stage (vacancy diffusion). Based on these, the corresponding quantitative relationships are firstly proposed: double-exponential and exponential relationships, respectively. In addition, the elongation to fracture is initially divided into uniform and softening elongations. Accordingly, the latter trend is controlled by the similar transitional mechanisms. The newly proposed relationships and corresponding mechanisms can provide new clues for the optimizing design of CGI and the understanding of similar phenomenon for other cast irons and even some metallic materials.

Transition of tensile strength and damaging mechanisms of compacted graphite iron with temperature

Bimetal consisting of high-carbon steel and low-carbon steel was prepared by centrifugal composite casting, and thermo mechanical treatments were carried out by hot compression tests using Gleeble 3500 thermo mechanical simulator. The characteristics of microstructure and hot deformation behaviour of the bimetal were investigated. It is concluded that a sound metallurgical bond was achieved by micro mass transfer across the interface between the constituent steels. Four different microstructural regions (including low-carbon steel region, interface region, fully pearlitic region and pro-eutectoid ferritic/pearlitic region) with different mechanical properties were found, which resulted in heterogeneity of plastic deformation within the bimetal. Regardless of strain rate, the low-carbon steel layer preferentially extruded from the bimetal when deformation temperature was below 900 °C, whereas the extrusion was not observed at 900, 1000 and 1100 °C. This indicated that the high-carbon steel/low-carbon steel bimetal can be processed like monolithic material.

Microstructure and hot deformation behaviour of high-carbon steel/low-carbon steel bimetal prepared by centrifugal composite casting

The plastic-flow behavior of ferrite + pearlite, pearlite + cementite, and austenite + cementite mixtures in plain carbon steels has been examined over the temperature range 500 to 1050 °C, strain-rate range 6 x l0−6 to 2 x l0−2 s−1, and carbon range 0.005C to 1.89C. Up to the eutectoid temperature the strength of the ferrite + pearlite mixture more than doubles as the carbon content increases from 0.005C to 0.7C, so that whereas in low-carbon steels the ferrite is weaker than the higher temperature austenite phase, in eutectoid steels the fully pearlitic structure is stronger than the fully austenitic structure. Manganese and silicon strengthen ferrite more effectively than they do austenite. A 0.17 pct phosphorus addition strengthens the ferrite + pearlite mixture across the range of microstructures from fully ferritic to fully pearlitic. Beyond the eutectoid composition, the amount of proeutectoid cementite does not significantly affect the strength of the pearlite, but above the eutectoid temperature it appreciably strengthens the austenite and cementite mixture at the strain rate of 2 X 10-2 s-1.

P. J. Wray

Papers

High temperature plastic-flow behavior of mixtures of austenite, cementite, ferrite, and pearlite in plain-carbon steels