Kinetics of static recrystallization and strain induced martensite formation in low carbon austenitic steels using impulse excitation technique (original) (raw)
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Isij International, 2001
Using torsion tests a improved model has been constructed to predict the static recrystallization kinetics of deformed austenite in low alloy and microalloyed steels. The model quantifies the influence of the most common elements (Ci Si, Mn, Mo) in low alloy steels and the typical elements (V, Nh) in microalloyed steels, when they are in solution. Activation energy (Q) is the parameter sensitive to the content and nature of each alloying element, and an expression for Q is shown as a function of the percentage of each one. Nb is the element that contributes most to increasing the value of Q, and thus that which most delays recrystallization kinetics. C is seen to be the only alloying element that contributes to lowering the value of Q, and thus to accelerating recrystallization kinetics. Extrapolation of the expression of Q to pure iron in the austenitic phase gives a value of 148 637 J mol(-1), which is similar to other values found in the literature for the grain boundary self-dif...
ISIJ International, 1996
The newly established technique of stress relaxation has been applied to measure the kinetics of static and metadynamic recrystaliization of austenite in a low-carbon steel subsequent to compression executed at a strain rate of 0.1 or O.OI s~1 at 900'C or I OOO'C.The characteristics of static recrystallization were found to be consistent with those previously reported from double-stage deformation tests. Metadynamic recrystallization, contrary to static one, showed no dependence on strain and hardly any on temperature. but significant dependence on strain rate. TheAvrami exponents were almost identical for the two processes, about I ,5-1 .6 at 0.1 s~i, but decreased to I .0-1 .3 at 0.01 s~1. Metadynamicrecrystallization resulted in complete softening except when relaxed after compression to a strain of 0.3 or beyond at a low strain rate of O.OI s~1 , The law of mixtures approach was found to be more accurate than the uniform softening model to describe recrystallization in partially recrystallized and subsequently deformed austenite. The results confirm the feasibility of the stress relaxation technique as an efficient method for investigating recrystallization kinetics in hot-deformed austenite.
international journal of iron and steel society of iran, 2019
One of the most promising ways to produce a grain-refined microstructure in some steel materials is the thermomechanical processing route of subcritical recrystallization annealing of a cold-deformed martensite structure. In the present study, the microstructural evolutions and the associated recrystallization kinetics under various subcritical annealing heat treatment conditions are explored in an API X120 grade, advanced, High-Strength, Low-Alloy (HSLA) steel with an initial cold-deformed martensite microstructure. The steel sheet was the subject of a conventional cold rolling process for moderate true strain of 60% followed by isothermal recrystallization for different temperature-time combinations. Optical microscopy and scanning electron microscopy were used to characterize the microstructural evolutions, and the recrystallization kinetics was evaluated by hardness measurements with the aid of the Johnson-Mehl-Avrami-Kolmogorov (JMAK) relationship. The experimental results indi...
Kinetics of Austenite Recrystallization during the Annealing of Cold-rolled Fe-Mn-Al-C Steel
2020
In the current study, the recrystallization behavior of 75% cold-rolled Fe-22Mn-10Al-1.4C steel during annealing at 750, 770, 790, 810, and 830°C was studied. X-ray diffraction patterns and optical microscopy were used to characterize microstructures. The Vickers Micro-hardness test was used to characterize recrystallization kinetics during annealing. Johnson-Mehl-Avrami-Kolmogorov (JMAK) model was used to evaluate the experimental data. The as-homogenized microstructure illustrated only austenite with a high fraction of annealing twins, and austenite to martensite phase transformation was not observed after quenching at a high temperature and also until high thickness reduction. Avrami exponent was decreased from 0.76 to 0.42, with increasing the annealing temperature from 750 to 830°C. The activation energy value was determined to be ~175 kJ/mol, which was slightly higher than the diffusion activation energy of carbon in austenite.
Dynamic Recrystallization Behavior in a Low-carbon Martensite Steel by Warm Compression
ISIJ International, 2008
The dynamic recrystallization behavior during warm compression for a low carbon martensite steel was investigated to make clear the effects of initial martensite block size, compression strain and pre-tempering before compression. It is found that the average size of recrystallized ferrite grains is influenced neither by the initial martensite block size (austenitizing temperature) nor by the amount of compression strain. The pre-tempering before compression shows two competitive effects: cementite particles precipitated during pre-tempering at 600°C promotes the occurrence of dynamic recrystallization while the decrease in dislocation density during pre-tempering at a higher temperature delays the dynamic recrystallization. Dispersed cementite particles suppress ferrite grain growth. Hence, there is an optimum tempering condition before warm-compression in order to obtain fine grained microstructure.
Retransformation (α′→γ) kinetics of strain induced martensite in 304 stainless steel
Materials Science and Engineering: A, 2000
Coupons of austenitic 304 stainless steel (g) were transformed to approximately 90% martensite (a%) and 10% austenite by rolling at 77 K. Subsequently the reverse a%g transformation was instigated by heating the coupons to 680°C. The retransformation was monitored, in situ, by dilatometry and neutron Bragg edge diffraction (BED). Results from the two techniques show good agreement and suggest that the transformation kinetics are best described by two Avrami exponents, n=2.5 and n=0.2 respectively. A limited discussion of the lattice parameter evolution during the transformation is included. Possible mechanisms for growth dynamics and stress relaxation are discussed.
A Physical Analysis of the Stress Relaxation Kinetics of Deformed Austenite in C-Mn Steel
steel research international, 2006
The softening kinetics following hot deformation of austenite have been characterised using the stress relaxation technique. Samples were deformed in compression for a variety of temperatures, strains and strain rates. At low strains where recovery was the only softening mechanism, the stress relaxation kinetics have been analysed using a recovery model previously proposed in the literature, the main parameters being activation energy and activation volume. The activation energy for recovery was found to be 314 kJ/mol, whilst the activation volume was inversely proportional to the internal stress. At higher strains where austenite recrystallization occurred as well, the stress relaxation kinetics were modelled using the recovery model combined with a single grain model for recrystallization. Reasonable agreement was obtained between model and experiment for a variety of deformation conditions. Analysis of the model parameters and experimental data indicated that the nucleation density for recrystallization depended only on the applied strain for the range of deformation conditions imposed. In addition the mobility of recrystallizing boundaries was best explained by solute drag due to manganese atoms.
Metals
Interrupted and continuous hot compression tests were performed for eutectoid steel over the temperature range of 850 to 1050 °C and while using strain rates of 0.001, 0.01, 0.1, and 1 s−1.The interrupted tests were carried out to characterize the kinetics of static recrystallization(SRX) and determinate the interpass time conditions that are required for initiation and propagation of dynamic recrystallization (DRX), while considering that the material does not contain microalloying elements additions for the recrystallization delay. Continuous testing was used to investigate the evolution of the austenite grain size that results from DRX. The results indicate that carbon content accelerates the SRX rate. This effect was observed when the retardation of recrystallization due to a decrease in deformation temperature from 1050 to 850 °C was only about one order of magnitude. The expected decelerate effect on the SRX rate when the initial grain size increases from 86 to 387 µm was not ...