A dislocation-attraction model for the first stage of tempering

Speer

et al. 2009

Metall Mater Trans A

The kinetics of the isothermal transformation, which occurs during a single-step quenching and partitioning (Q and P) process, was analyzed by means of dilatometry. Experiments revealed that an isothermal transformation with very specific characteristics occurred during the hold at the quenching temperature below the martensite start temperature. Although evidence for isothermal martensite transformation has been reported in the past for Fe-C alloys and steel, it is the first time that it is recognized to take place during the Q and P processing of low-carbon ferrous alloys.

“…For Figure 9(c), the transformation rate was increased by the existence of~10 pct volume fraction of Lement et al [20] commercial steel with 0.6 to 1.4 wt pct C~1…”

Section: Discussionmentioning

confidence: 90%

Observation of an Isothermal Transformation during Quenching and Partitioning Processing

Speer

et al. 2009

Metall Mater Trans A

“…͑1a͒ had to be modified. [8][9][10][11] However, in a previous paper 16 we showed, based on Monte Carlo simulations, that for one-dimensional growth in twodimensional space, where grains grow with equal probability in two orthogonal orientations, it is more correct to modify Eq. ͑1b͒.…”

Section: ͑2͒mentioning

confidence: 96%

“…Another option is, of course, to extend the mathematical formulation of the JMAK theory, adding ͑one or more͒ new variables that provide freedom to improve the agreement in case anisotropic growth occurs. [8][9][10][11] However, the aim of the present work is to derive an analytical description that is theory based and not phenomenological, i.e., where all variables have physical meaning.…”

Section: Introductionmentioning

confidence: 99%

Extension of the Johnson-Mehl-Avrami-Kolmogorov theory incorporating anisotropic growth studied by Monte Carlo simulations

Kooi

2006

Phys. Rev. B

An analytical theory has been developed, based on Monte Carlo ͑MC͒ simulations, describing the kinetics of isothermal phase transformations proceeding by nucleation and subsequent growth for d-1 dimensional growth in d dimensional space ͑with d 2 or 3͒. This type of growth is of interest since it is generally anisotropic, leads to hard impingement, and obtains strong deviations from the traditional Johnson-Mehl-Avrami-Kolmogorov ͑JMAK͒ theory. Within the MC simulations 1D growth can occur with equal probability in two or three different nonparallel orientations in 2D space. In 3D space 2D growth can occur with equal probability in three ͑or two͒ different orthogonal orientations. The MC simulations show that initially always a regime is present where JMAK theory prevails, but that after a well-defined critical time a transition to a blocking regime occurs. Both regimes are characterized by clearly different, but nearly constant values of the Avrami exponent which depend on the dimensionality of growth and space and on the time dependence of nucleation. The dependence of the critical time and of the extended fraction within the blocking regime ͑based on the concept of the extended volume of the JMAK theory͒ on the nucleation and growth parameters has been extensively analyzed and all results of the MC simulations have been captured within the analytical theory.

“…(11) Com relação ao segundo estágio de envelhecimento, o valor de energia de ativação encontrado, 118 ± 10 kJ/mol, está de acordo com os resultados de energia de ativação obtidos por Waterschoot, Verbeken e De Cooman (12) (123,8 kJ/mol) e Cheng et al (13) (111 kJ/mol -126 kJ/mol), para o estágio de precipitação de carbonetos de transição, carboneto ε e/ou carboneto η, durante o revenimento da martensita na faixa de temperatura entre 120°C e 200°C. Em linha com esses resultados, também foi obtida por Lement e Cohen, (14) uma lei cinética, segundo o modelo de Harper, com o expoente do tempo igual a 0,5, para a formação do carboneto ε durante o revenimento da martensita, baseando-se em um modelo controlado pela difusão induzida por gradiente de concentração. Portanto, em função dos valores de expoente do tempo e de energia de ativação obtidos, pode-se associar o segundo estágio de envelhecimento do aço estudado à formação de carbonetos de transição durante o revenimento da martensita na faixa de temperatura entre 125°C e 185°C.…”

Section: Resultsunclassified

Estudo Do Envelhecimento Após Deformação Em Um Aço Dual Phase De Baixa Resistência Mecânica Laminado a Frio

Murari¹,

Melo²,

Gonzalez³

2009

TMM

ResumoA cinética de envelhecimento após deformação em aço Dual Phase laminado a frio é estudada por meio de ensaios de tração. Após a etapa de recozimento contínuo, o aço foi pré-deformado de 0,5% em tração e envelhecido no intervalo de temperatura entre 50°C e 185°C, para tempos variando entre 1 minuto e 4.915 minutos. O aço estudado apresenta dois estágios de envelhecimento, o primeiro, entre 50°C e 125°C (para tempos inferiores a 72 minutos) e o segundo entre 125°C (para tempos superiores a 72 minutos) e 185°C. As variações no valor Bake Hardening sugerem, para o primeiro estágio de envelhecimento, um processo controlado pelo ancoramento das deslocações na ferrita devido à formação de clusters e/ou carbonetos de transição, com uma energia de ativação próxima de 70 kJ/mol e obedecendo a uma lei cinética descrita pela equação de Harper com expoente do tempo igual a 0,4. No segundo estágio de envelhecimento, o fenômeno é controlado pelo revenimento da martensita. A energia de ativação correspondente a esse estágio é de cerca de 120 kJ/mol e sua cinética pode ser descrita pela mesma equação, porém com um expoente do tempo igual a 0,5. Palavras-chave: Aço bifásico; Envelhecimento após deformação; Bake hardening. STATIC STRAIN AGING STUDY IN A COLD ROLLED DUAL PHASE STEEL OF LOW MECHANICAL RESISTANCE AbstractThe kinetics of static strain aging in cold rolled Dual Phase steel is studied by means of tensile tests. After the continuous annealing step, the specimens were pre-strained with a tensile strain of 0.5% and then aged in the temperature range of 50°C to 185°C and time intervals ranging from 1 minute to 4,915 minutes. The steel studied showe two strain aging stages: the first one between 50°C and 125°C (for times shorter than 72 minutes) and the second one between 125°C (for times longer than 72 minutes) and 185°C. The changes in the Bake Hardening values suggest, for the first stage, a process controlled by the locking of the dislocations in the ferrite due to the formation of clusters and/ or transition carbides, with an activation energy close to 70 kJ/mol and following a kinetic law described by the Harper equation with a time exponent of 0.4. In the second stage, the phenomenon is controlled by tempering of martensite. The corresponding activation energy is approximately 120 kJ/mol and the kinetics of this stage can be described by the same equation, however with a time expoent of 0.5.