Materials Science and Engineering A284 (2000) 195 – 201 Effect of prestrain on fracture toughness of HSLA steels S. Sivaprasad a , S. Tarafder a, *, V.R. Ranganath a , K.K. Ray b a Fatigue and Fracture Group, National Metallurgical Laboratory, Jamshedpur 831 007, India b Department of Metallurgical Engineering, Indian Institute of Technology, Kharagpur 721 302 India Received 23 August 1999; received in revised form 10 January 2000 Abstract The effect of tensile prestraining on the ductile fracture behaviour of two varieties of Cu-strengthened HSLA steels has been studied. It was observed that for both the HSLA steels the fracture toughness, J C , remained invarient up to 2% prestrain, beyond which it decreased deleteriously. The nature of variation of fracture toughness with prestrain has been explained in terms of variation of the tensile flow properties of the steels with prestrain. A model correlating fracture toughness and the tensile flow properties has been proposed and it is demonstrated that the predictions of the model matched closely with the experimental observations. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Prestrain; Fracture toughness; Tensile flow www.elsevier.com/locate/msea 1. Introduction Low carbon, Cu-strengthened HSLA steels were de- veloped for naval structural applications demanding high strength and toughness together with enhanced weldability [1,2]. The improved properties of such steels are obtained through reduction of carbon content, and increased hardenability and additional precipitation strengthening via quenching and aging treatments. To fabricate naval structures, rolled and heat treated plates of steels are cold deformed into shape and welded together. As these steels are produced in the quenched and tempered condition they can neither be hot worked, for structural fabrication, nor be heat-treated to relieve the stresses developed during cold forming. As a consequence, the material enters into service in the cold deformed condition. Cold deformation is essen- tially a prestraining process that alters the dislocation substructure of a material, and hence the manner in which it may deform subsequently. As the mechanics of fracture is intimately connected with dislocation dy- namics, it is expected that cold deformation will have a profound influence on the fracture behaviour of the material. The effect of prestrain on fracture toughness was studied by Amouzouvi and Bassim [3–5] in annealed 4340 steel. It has been reported that fracture toughness increased with prior deformation up to 2% prestrain, at which a four-fold increase in toughness was obtained, compared to that in the undeformed material. On im- position of prestrains 2%, the toughness decreased, falling below that of the undeformed material at 10% prestrain. Clayton and Knott [6] have reported that fracture toughness of HY-80 steel remained unaltered up to a certain small prestrain level, after which it decreased with prestrain. In contrast to the above, studies conducted on 310 stainless steel [7], quenched and tempered 4340 steel and 316 stainless steel [8], and an aluminium alloy [9] revealed that fracture toughness continuously decrease with prestrain. A brief overview of these investigations is summarized in Table 1. From the literature it appears that the existing knowledge on the effect of prestrain on fracture toughness does not portray any generalized pattern. Since the Cu-strength- ened HSLA steels are put into service in prestrained condition, it is imperative that the effect of prestrain on the fracture behaviour of these steels be examined so that predictions of the material behaviour after fabrica- tion may be based on better perspective. In the present investigation, the effect of tensile pre- straining on the fracture toughness of two varieties of * Corresponding author. Tel.: +91-657-426091; fax: +91-657- 426527. E-mail address: star@csnml.ren.nic.in (S. Tarafder) 0921-5093/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII:S0921-5093(00)00739-5