Published by Maney Publishing (c) IOM Communications Ltd Effect of test temperature on fracture toughness of modified 9Cr–1Mo steel B. Shashank Dutt*, M. Nani Babu, S. Venugopal, G. Sasikala and A. K. Bhaduri The quasi-static fracture behaviour (J–R curves) of modified 9Cr–1Mo (P91) steel was studied. The J–R curves were established at 298, 653, 823 and 893 K, and fracture toughness J 0?2 at 0?2 mm of crack extension was determined. The value of yJ 0?2 at 653 K was lower compared to that at 298 K followed by increases in J 0?2 values at 823 and 893 K. The decrease in J 0?2 at 653 K can be attributed to the influence of dynamic strain aging. At 893 K, a significantly higher (more than 200%) J 0?2 was observed, since plastic deformation of the net section, rather than crack growth, occurred in this condition. Keywords: Modified 9Cr–1Mo, Fracture toughness, Dynamic strain aging, Temperature Introduction Modified 9Cr–1Mo (P91) steel is selected as the material for steam generator components of sodium cooled fast breeder reactors. Mechanical properties of this material, including fatigue and creep, have been extensively studied. 1–4 The influence of thermomechanical treat- ments, chemical composition and heat treatments, including thermal aging, on the microstructure and the mechanical properties of this steel has also been investigated. 5–10 Fracture behaviour of this steel has been found to be sensitive to the above factors. 7–10 Studies on the quasi-static fracture toughness (J 1C is the crack extension resistance under crack tip plane strain conditions) and, specifically, the J–R curves for P91 steel however have been limited. 11,12 In a previous study in the authors’ laboratory, the influence of long term aging on J–R curves for a P91 steel plate of 12 mm thickness was examined at test temperatures of 300, 653 and 803 K using 10 mm thick compact tension (CT) specimens. 13 The validity of the J 0?2 values for qualification as a thickness independent material toughness property J Ic was examined in Ref. 13, and it was concluded that these values do not qualify as a thickness independent material property J Ic . In view of these, further investigations were carried out on 20 mm thick specimens fabricated from another plate. In plain 9Cr–1Mo steel dynamic strain aging (DSA) has been reported 14 to occur in the temperature range 523–723 K. The manifestations of DSA in tensile tests include serrations, negative strain rate sensitivity, a maximum or peak in variation of flow stress and workhardening rate with temperature and a ductility minimum. 15 Previous investigations 16–21 show varying effects of DSA on the fracture toughness of different materials. The possible effect of DSA on fracture toughness of P91 steel has been examined. Therefore, a campaign was taken up to establish J–R curves for the P91 steel at various temperatures in the range 298– 893 K, which covers the operating temperature range. Experimental Material The P91 steel plate of 24 mm thick was received after thermomechanical processing in normalised (1333 K for 60 min) and tempered (1053 K for 60 min) condition (NT). The chemical composition of the P91 steel is Fe– 0?10C–0?41Mn–0?26Si–0?33Ni–0?018P–0?002S–9?27Cr– 0?95Mo–0?21V–0?074Nb–0?013Al–0?044N (wt-%). For metallography, the specimens were polished to fine diamond finish (y0?5 mm) and etched with Vilella’s reagent (45 mL glycerolz15 mL nitric acidz30 mL hydrochloric acid). Figure 1 shows the tempered mar- tensite microstructure of the NT material. Tensile testing Tensile tests were carried out on cylindrical specimens machined from the P91 plate having 28?6 mm gauge length and 4?0 mm diameter as per ASTM E8 22 standard at 298, 653, 823 and 893 K at a constant strain rate of 1?2610 24 s 21 . The fractured specimens were observed under scanning electron microscope (SEM) at magnifications of 6100 to 6500. Fracture toughness testing Compact tension specimens of 20 mm thick were fabricated in transverse-longitudinal orientation as per ASTM E399-90 23 specification as shown in Fig. 2. The CT specimens were subjected to fatigue precracking at room temperature to obtain a sharp fatigue crack ahead of the notch using a resonant fatigue machine. The starting frequency of the load cycle was y106 Hz. The crack length at the end of precracking (initial crack length a 0 for the fracture test) was targeted between 25 and 27 mm corresponding to an a 0 /W in the range of about 0?50–0?54, where W is the width of the specimen. Materials Technology Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, India *Corresponding author, email shashank@igcar.gov.in ß 2011 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 15 June 2010; accepted 12 July 2010 DOI 10.1179/026708310X12815992418094 Materials Science and Technology 2011 VOL 27 NO 10 1527