Avaliação dilatométrica da reversão das martensitas induzidas por deformação em um aço inoxidável austenítico do tipo ABNT 304

Abstract

RESUMOAços inoxidáveis austeníticos da classe ABNT 304, quando deformados, sofrem transformações de fase, gerando as fases martensíticas epsilon, de estrutura hexagonal compacta, e alfa-linha, tetragonal de corpo centrado. As propriedades mecânicas, assim como a conformabilidade desse aço, dependem da morfologia, distribuição e fração volumétrica das martensitas. Ensaios de tração foram realizados nas temperaturas de -50ºC a 20ºC, com deformação verdadeira de 0,03 a 0,12, com o intuito de obter diferentes quantidades de martensitas. Determinou-se a fração volumétrica de martensita alfa-linha através de um ferritoscópio, indicando a diminuição da martensita alfa-linha, com a elevação da temperatura de deformação. Ensaios dilatométricos foram conduzidos na faixa de temperaturas de 50 a 1000ºC a 1ºC/s e mostraram duas transformações de fase, nas faixas de temperaturas de 50 a 200ºC e 500 a 800ºC, que foram relacionadas às transformações reversas epsilon→gama e alfa-linha→gama. A quantidade transformada de epsilon cresce até um máximo, enquanto que a quantidade transformada de alfa-linha aumenta, continuamente, com a deformação. A formação de epsilon precede o aparecimento de alfa-linha e diminui sua quantidade à custa do aumento de martensita alfa-linha.Palavras-chaves: martensita induzida por deformação, aço inoxidável austenítico, dilatometria. Dilatometric evaluation of strain-induced martensite reversion in type AISI 304 austenitic stainless steel ABSTRACTAustenitic stainless steels can form strain-induced martensites when deformed. The mechanical properties, as the formability, depend on morphology, distribution and volumetric fractions of phases generated, epsilon-martensite, with a close-packed hexagonal structure, and alpha-prime martensite, which possesses a body-centered cubic structure. Tensile tests were carried out in temperatures in the range of -50 to 20ºC with true strains ranging from 0.03 to 0.12, in order to obtain different volumetric fractions of epsilon and alpha-prime martensites. The amount of alpha-prime martensite was measured by using a ferritoscope indicating that the alpha-prime martensite fraction increases with strain for a constant temperature and decreases with the temperature for a constant strain. The epsilon and alpha-prime martensites reversion was observed in the temperature range of 50 to 200ºC and 500 to 800ºC, respectively, by dilatometer tests. The epsilon martensite volumetric fraction first rises with the strain and, at higher deformations, drops from a maximum value, which depends on the deformation temperature. The epsilon martensite occurs before alphaprime martensite and its amount decreases with the increase on the volumetric fraction of alpha-prime martensite.

The Investigation of Strain-Induced Martensite Reverse Transformation in AISI 304 Austenitic Stainless Steel

“…It is a typical TRIP (Transformation-Induced Plasticity) effect, where the creation of the hard martensitic phase causes the increase of the work hardening rate at a higher strain and enhanced ductility. [2,3] However, at temperatures lower than 253 K (À20°C), the SIM formation rate is high enough to cause substantial reduction of ductile austenitic regions of materials at the early stages of deformation, thus an elongation decrease is observed. Similarly, hardness change exhibits a characteristic point at 263 K (À10°C) when a change in the curve slope is observed.…”

“…This phenomenon corresponds to the SIM formation. [2,3] A clear influence of deformation temperature on elongation as well as UTS (ultimate tensile strength) is observed. For comparison purposes, data of hardness, elongation, UTS with respect to deformation temperature are presented in Figure 2.…”

“…The maximum volume fraction of e martensite is strongly dependent on the deformation temperature and usually takes place at strains lower than 0.15. [2,3] As reported by Tavares et al, [4] it is possible to obtain a different transformation succession, i.e., c fi a¢ fi e when applying high pressure, for instance by HPT (High Pressure Torsion) processing at the highest possible pressure and deformation conditions. The influence of BCC martensite and/or its formation on the materials properties such as mechanical, [5,6] corrosion resistance, [7,8] formability, [9][10][11][12] hydrogen cracking susceptibility, [13][14][15] and fracture propagation [16] have been investigated.…”

“…Both aspects are well-known features of metastable stainless steel materials related to the kinetics of c fi e and e fi a¢ transformations. [3] It is assumed that the maximum of the reaction rate (e and a¢ phase formation, respectively) occurs at the derivative extrema. [3] As the temperature drops, both extrema are shifted toward lower strains ( Figure 3) and this phenomenon follows a linear relationship with respect to deformation temperature.…”

“…Previous dilatometric studies revealed e fi c reverse transformation between RT and 473 K (200°C) and a¢ fi c reverse transformation in the range of 773 K to 1073 K (500°C to 800°C). [3,20] Both (e fi c, a¢ fi c) transformations are shifted to higher temperatures with the increase of heating rate (at the heating rate higher than 0.1 K/s). [21] Tomimura et al described two types of possible reverse transformation (a¢ fi c) mechanisms: 1st diffusive by nucleation and growth of new austenite grains and 2nd diffusionless shear reversion.…”