Dynamic .GAMMA..RAR..ALPHA. Transformation during Hot Deformation in Iron-Nickel-Carbon Alloys.

Abstract

Fe-6mass%Ni-(0.0008ϳ0.29)mass%C alloys were hot-deformed in torsion at 600-720°C (above the cooling transformation start temperatures A r3 ) after austenitization. An in-situ X-ray diffraction study revealed that g→a transformation occurred during deformation in a wide range of condition, even above A 3 p (paraequilibrium g→a transformation temperature). Corresponding to this transformation, apparent decrease in deformation stress from that expected for austenite was observed. Microstructural study of the specimens quenched after the deformation showed that a large amount of fine-grained ferrite was formed due to the deformation. The analysis of deformation stress and chemical driving-force of the transformation indicated that the transformation occurred in order to reduce the total energy of deformed material since the deformation of energy of a was revealed to be considerably smaller than that of g and the amount of deformation energy saved by the transformation was shown to be much greater than the chemical energy consumed by the transformation at the tested temperatures.KEY WORDS: low carbon steels; nickel steels; hot deformation; grain refinement; thermomechanical heat treatment; strain-induced transformation. study during hot deformation has been undertaken in low carbon Fe-Ni alloys. The in-situ X-ray diffraction technique had been successfully applied to find dynamic recrystallization of aluminum. 7) The experimental results of the in-situ X-ray study have been already reported. [8][9][10] This report puts together all the results of the in-situ X-ray study done in these reports and also includes the results of deformation stress measurement and microstructural study. In the deformation stress measurement, it was aimed to determine the difference in deformation stress between g and a more accurately. The cause of DT will then be discussed on the basis of these three kinds of experimental results. Experimental MaterialsThe compositions of the alloys used in this study are shown in Table 1. Nickel was added to reduce transformation temperature so as to make high temperature X-ray studies easier. It also enhances the hardenability of the alloys. Table 2 gives the transformation temperatures of the alloys. Equilibrium transformation temperatures were calculated by using Thermo-Calc. In this alloy system transformation during cooling was known to start from their paraequilibrium transformation start temperature, A 3 p , 11) not from orthoequilibrium transformation start temperature, A o 3 . Experimental cooling transformation start temperatures, A r3 , and transformation heating finish temperatures, A c3 , and a martensite start temperature, Ms, were adopted from preceding work, 12) in which alloys of the compositions close to the present ones were used. The alloys were vacuum-melted and hot-rolled to 3-mm thick plates. They were sliced and drawn into wires of 2-mm diameter, which were cut to 42 mm in length for torsion test. Apparatus and ProceduresThe schematic setup of X-ray diffraction experiment during t...

Dynamic Transformation of an X70 Steel under Plate Rolling Conditions

“…1,2) They showed that fine grains of ferrite were produced by deforming above the Ae 3 and that the volume fraction of ferrite increased with the strain and decreased with the temperature as well as the holding time after the last pass. They later employed in situ X-ray diffraction to confirm that the ferrite was being formed during straining.…”

“…They later employed in situ X-ray diffraction to confirm that the ferrite was being formed during straining. 3) More recently Basabe et al, 4) conducted torsion tests on a 0.036% Nb microalloyed steel and reported that both the forward and the reverse transformation were retarded by the addition of niobium. In some follow-up experiments, Grewal et al 5) showed that ferrite can be formed everywhere within the austenite phase field.…”

Combined Macro–Micro Modeling for Rolling Force and Microstructure Evolution to Produce Fine Grain Hot Strip in Tandem Hot Strip Rolling

“…However, the free energy of austenite could not increase that much even if the energy of the dislocation cell was taken into consideration, and , thus, the diffusion transformation was reasonable as represented by the intragranular transformation model. As for ferrite transformation during rolling, Yada et al [19][20][21] have proposed massive transformation. By assuming pipe diffusion due to dislocation cells generated inside the grains, however, it was possible that ferrite of 1 mm or so transforms in as short a time as 0.003 s. 22) As flow stress of ferrite is less than that of austenite, rolling force would decrease.…”

Implications of total content of silicon, aluminium, chromium and formation of thin ferrite films on low ductility at high temperature in non oriented electrical steels

“…al. [38] reported that ferrite was formed during the deformation, even above Ae 3, with para-equilibrium, which was analyzed by in-situ X-ray diffraction method. Furthermore, no evidence of formation of ferrite films at austenite grain boundaries was presented in prior work for electrical steels [33] .…”