Effect of Reheating Temperature and Cooling Treatment on the Microstructure, Texture, and Impact Transition Behavior of
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EMERGENCE of the concept of ductile-brittle transition in ferritic steel and its related research work can be directed to the premature brittle fracture of welded Liberty ships and T-2 tanks (under freezing water condition) during the second world war.[1] Since then, several research studies have been dedicated to the development of steel plates having high strength and low impact transition temperature for naval application. United states navy developed HSLA-80 and HSLA-100 grades having minimum yield strength of 552 and 690 MPa, respectively, and Charpy impact toughness greater than 81 J at 193 K (80 C).[2–4] In a similar way, Defence Metallurgical Research Laboratory (DMRL), with its industrial partners, has developed naval grade HSLA steels to satisfy the special requirements of Indian Navy.[5] The different mechanical properties required in this investigated naval grade steel are as follows: yield strength ‡390 MPa, ultimate tensile strength of 510 to 690 MPa, elongation ‡16 pct and MD. BASIRUDDIN SK, A. GHOSH, and D. CHAKRABARTI are with the Department of Metallurgical and Materials Engineering, Indian institute of Technology Kharagpur, Kharagpur 721302, India. Contact e-mail: [email protected] N. RARHI and R. BALAMURALIKRISHNAN are with the Defence Metallurgical Research Laboratory, Kanchanbagh, P.O. Hyderabad, Andhra Pradesh 500058, India. Manuscript submitted August 30, 2016 METALLURGICAL AND MATERIALS TRANSACTIONS A
Charpy V-notch impact toughness ‡78 J at 213 K (60 C).[5] In order to ensure a good weldability and bendability, the steel needs to maintain low-carbon level (15 deg misorientation).
to ‘Pmax’ are indicated on the load vs time plots in Figure 11. The temperature, at which Pgy and Pmax coincide, is considered as the general yield temperature, Tgy[31,32] and the stress acting at that temperature represents cleavage fracture stress, rf.[31,32] The Tgy and rf values determined from the load-time plots of the investigated samples, following the established procedure,[31] are listed in Table IV.
From Figures 10, 11, and Table IV, it is evident that among the investigated samples, HT1223FC showed the best impact properties and satisfactorily achieved the required impact toughness level [‡78 J at 213 K (60 C)]. The impact toughness of HT1223AC at 213 K (60 C) also just exceeded the minimum toughness requirement. No other sample could meet the minimum specified impact toughness level. HT1423FC and HT1523FC showed relatively sharp impact transition behavior with high transition temperatures such that the required impact toughness could not be achieved at 213 K (60 C), in spite of their high USE. The HT1223WQ sample showed gradual decrease in impact toughness with the decrease in temperature which offered low impact transition temperatures values. However, the minimum USE value of this sample is a matter of concern. Thus, present finding clearly highlights the beneficial effect of applying an annealing or normalizing treatments from a low austenitizing temperature of ~Ar3 + 50 K, in terms
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