Cleavage micromechanisms on microalloyed steels. Evolution with temperature of some critical parameters Alberto Echeverria a, * , J.M. Rodriguez-Ibabe b a LORTEK, Centro de Investigaci on en Tecnolog  ıas de Uni on, B o La Granja s/n, 20240 Ordizia (Glpuzkoa), Spain b CEIT, Materials Department, P Manuel de Lardizabal 15, 20018 San Sebastian, Spain Received 8 October 2002; received in revised form 23 July 2003; accepted 1 September 2003 Abstract Brittle fracture behaviour evolution with temperature is studied in order to identify features that progressively may prevent cleavage fracture and lead to ductile fracture. Some key parameters (r  F , fracture nuclei size, position of fracture origin) are sys- tematically analysed based on 4PB tests carried out with steels containing a wide distribution of coarse TiN particles. Some definite trends are envisaged. Ó 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Toughness; Bainitic steels; Brittle fracture; Micromechanics 1. Introduction It is well known that steel breaks in a brittle way by cleavage at low temperatures. With increasing tempera- ture the fracture behaviour evolves towards ductile tougher fracture, an intermediate transition temperature interval existing where both micromechanisms are active. The ductile–brittle transition does not occur sharply, but rather in a smooth way (sharpness depends on mi- crostructure and C content). This transition is intimately related to the temperature sensitivity of the yield stress of iron, as it is the case for all bcc transition metals. Generally speaking, ductile behaviour is expected at high temperatures because plastic flow occurs before cracking and brittle behaviour at low temperatures where the inverse situation holds. Considering the brittle temperature range, aspects such as, location of fracture origins in a specimen, size of the microstructural features that trigger fracture and cleavage stress change at a microscopic level in a subtle but progressive way with increasing temperature. The micromechanical changes taking place in the brittle re- gion before the lower ductile–brittle transition is reached are not clear yet. Wallin et al. [1] have proposed an in- crease in the effective surface energy for fracture with temperature while Gibson et al. [2] alternatively consider a decrease in the eligibility of particles for fracturing in order to provide an improved fit between experimental results and statistical models. Other experimental evi- dences show that as the temperature increases fracture initiates increasingly closer to the position of the peak stress in sharp cracked specimens [3]. These changes are driven by the physics underneath brittle cleavage fracture; thus a study of how these fea- tures evolve with temperature can prove suitable to dee- pen into the micromechanism governing brittle fracture. The present work is based on 4PB tests carried out with microalloyed steels at )196 °C and )100 °C and focuses in the above mentioned fracture behaviour evolution with temperature before ductile fracture be- comes operative. 2. Materials and methods Two Ti–V microalloyed steels were investigated cor- responding to two levels of C, 0.21 and 0.37 wt% (re- ferred as C21 and C37 respectively––see chemical composition in Table 1). Steel C37 is a resulphurized forging steel with enhanced machinability. Steel C21 has been isothermally transformed at 375 °C in a salt bath after an austenitization period of 30 min at 1050 °C, * Corresponding author. Tel.: +34-943-886593; fax: +34-943- 882303. E-mail address: aetxeberria@lortek.es (A. Echeverria). 1359-6462/$ - see front matter Ó 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2003.09.003 Scripta Materialia 50 (2004) 307–312 www.actamat-journals.com