No abstract

The coupled influence of the initial metallurgical state and the heating rate to austenite on the occurrence of heterogeneous grain growth during austenitization of an ASTM A213 -T91 steel has been studied. To that aim, two-step thermal cycles were designed. In the first step, different starting metallurgical conditions were obtained by treating the as-received material at 780 C for increasing times up to 6 hours. In the second step, "in situ" austenitization was performed by heating to austenite at rates of 1, 30 and 50 ºC/s and then holding at 1050 ºC for 30'.Two types of austenite grain structures were obtained after austenitization, namely, homogeneous and heterogeneous. The homogeneous structure was characterized by a smooth size distribution of approximately equiaxed, normally grown grains. The heterogeneous structure, instead, exhibited the exaggerated growth of a few austenite grains embedded in a small to medium-sized "matrix".For the 1 ºC/s heating rate and all of the initial metallurgical states, only homogeneous grain growth was observed, whereas for the 50 ºC/s heating rate only heterogeneous grain growth was observed regardless the starting metallurgical condition. Instead, the occurrence of homogeneous or heterogeneous grain growth after heating at 30 ºC/s was observed to be a function of the time of previous tempering. Some explanations of the phenomenon are advanced taking into account the precipitation state of second phases.

No abstract

Heterogeneous grain growth during austenitization in ASTM A213-T91 steel has been studied using a Gleeble 3500 thermomechanical simulator. Starting from a uniform, fine austenite grain size distribution after 1 min austenite holding time, a heterogeneous austenite grain size distribution was observed after 15 min austenite holding time at 1 060°C and 1 080°C. The state of precipitation of second phase particles was studied in detail as a function of austenitization time. The particle size distribution changed towards a bimodal feature as a function of time whereas the count frequency of the coarse precipitates increased and the count frequency of fine precipitates diminished with time. V-rich precipitates were dissolved during austenitization and only Nb-rich particles remained after 15 min austenitizing time. The experimental observations on the evolution of the austenite grain size distribution are rationalized on the basis of the Gladman's approach to abnormal grain growth.

Optimización del proceso de extracción del mucílago de Linum usitatissimum utilizando un diseño secuencial Optimization of the extraction process of Linum usitatissimum mucilage using a sequential design