Microstructural Evolution during Spheroidization Annealing of Eutectoid Steel: Effect of Interlamellar Spacing and Cold Working (original) (raw)
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Deformation Induced Pearlite Transformation and Spheroidization: Effect of Alloying Additions
Advanced Materials Research, 2011
The aim of this research is to investigate the effect of Cr and Al (strong ferrite formers) on the strain-induced γ-to-pearlite transformation in eutectoid steels. The microstructure evolution during the hot deformation of three eutectoid steel grades was investigated using hot torsion testing. More specifically, the steels were deformed to strain levels varying from ε = 0,5 to ε = 1,5 at their specific Ar1 temperature. Hot deformation of the undercooled austenite leads to strain-induced γ-to-pearlite transformation and to the almost instantaneous spheroidization of the formed carbides. The corresponding microstructures consist of submicronic cementite particles and ferritic grains that are 1-5 μm in size. It is shown that 1,5% Cr addition and 0,5% Al addition increase the equilibrium transformation temperature but slower significantly the kinetics of the strain-induced transformation and consequently reduce the kinetics of cementite spheroidization and of ferrite recrystallization.
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Pearlitic steels containing from some 0,8 to 0,95% C belong to the group of unalloyed steels of the quality class destined for cold drawing or rolling, they find application mainly as wires used for reinforcement of tires, hoses or rope production. However this group of steels characterized low machinability, as a consequence of presence hard plates of cementite, the intervention ameliorative this property is spheroidizing annealing. The object of the tests presented in paper was wires of pearlitic steel obtained after the successive stages of cold plastic working. The aim of the tests was to show that the condition of the material and mainly degree of plastic deformation have a clearly influence on the effectiveness of pearlitic steel spheroidization.
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Tailoring the Processing Route to Optimize the Strength-Toughness Combination of Pearlitic Steel
Metallurgical and Materials Transactions A, 2022
The present study fine tunes the processing route of a eutectoid steel to shape an optimum strength–toughness combination through appropriate microstructural design. A fully lamellar coarse pearlite microstructure leads to poor strength and toughness. Hot deformation prior to the isothermal treatment breaks down the lamellar pearlite to a spheroidized structure. Moreover, reducing the hot deformation temperature not only refines the pearlite nodule size but also increases the spheroidized pearlite fraction, which thereby improves the toughness of the steel. However, no proportionate increase in yield strength was obtained due to insignificant change in the interlamellar spacing. Remarkable refinement in lamellar spacing and increase in the spheroidization amount was ensured when the hot deformation was carried out just below the eutectoid temperature, owing to the strain-induced pearlite transformation. The presence of a mixed microstructure of fine lamellar pearlite along with spheroidized pearlite constituents simultaneously improves both the yield strength and toughness of the steel. Optimum strength–toughness combination was attained when the hot deformation strain (just below the eutectoid temperature) was increased up to 45 pct. Subsequent increase in strain creates deformation bands and traces of strain-induced bainite in the microstructure, which again deteriorates the tensile elongation of the steel.
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