Observation of serrated flow in APMT™ steel
Introduction
Traditionally, zirconium based alloys are used as fuel cladding materials in light water reactors (LWRs). Exothermic reaction with steam under accident conditions may lead to production of hydrogen with possibility of catastrophic consequences. Hence, the I2S-LWR reactor project [1] is considering an aluminum-rich (about 5 wt%) ferritic steel as an alternative accident-tolerant fuel cladding material to zirconium-based alloys. This type of material creates an aluminum-based oxide scale protecting the alloy at elevated temperatures. Kanthal APMT™ is advanced ferritic steel with a composition of Fe–21.5Cr–5.0Al–3.1Mo–0.04C–0.34Si–0.16Mn (wt%) steel, produced via a powder metallurgy process by Sandvik Inc., and is generally used for making high temperature furnace heating elements. With excellent high-temperature corrosion/oxidation resistance, this steel has the potential of being used as a fuel cladding material for nuclear applications.
In the present work, a preliminary investigation was performed on the influence of strain rate and temperature on serrated flow observed during tensile deformation of APMT™ alloy. While there are high temperature tensile data available above 800 °C for this alloy in the open literature [2], the tensile properties of this alloy in the low temperature range are not available. Given that the normal LWR core outlet temperature is around 593–693 K, it is important to study the alloy in this temperature range. However, the experimental campaign designed for the current work included a broader temperature range (298–773 K) to gain further scientific insights into the relevant phenomena.
Earlier, ferritic–martensitic steels such as Grade 9 [3], [4], [5], [6], [7] and Grade 91 [8], [9], [10], [11], [12], [13] 9Cr–1Mo steel, P9 and P91 [14], [15] and P92 [16] have been reported to exhibit dynamic strain aging (DSA) over the temperature range of 498–693 K. Occurrence of DSA in these steels has been manifested by serrated plastic flow, plateau in yield and tensile strength, negative strain rate sensitivity and ductility minima [8], [9], [10], [11], [12]. DSA in the conventional 9Cr–1Mo steel has been attributed to interactions of dislocations with carbon atoms [3], [4], [5], [6], and with nitrogen atoms in the modified 9Cr–1Mo steel [8], [9], [12]. On the other hand, Keller et al. [11] suggested that DSA occurs due to carbo-nitrides. Serrations due to DSA have also been characterized by internal friction in several materials like austenitic stainless steel [17], nickel based superalloy [18], and zirconium–hydrogen alloy [19].
Based on the nature of serrations, A-, B-, C- and D-type of serrations have been reported in earlier experiments with ferritic steels [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]. Type A serrations exhibit repeated initiation and continuous propagation of deformation bands along the specimen gage length. Sleeswyk [20] and McCormick [21] in their diffusion-controlled models of DSA attributed the occurrence of A-type serrations to diffusion of solute atoms to freely gliding dislocations. Sharp rise in load occurs when the solute locks dislocations, and the load drops when dislocations unlock from the solute atmosphere. Following A-type serrations, repeated locking and unlocking of dislocations due to highly mobile solutes lead to small B-type serrations about the mean level of loads in load–elongation curves. Type C serrations are oscillations of the load–elongation curve below the mean level of load. Type D serrations occur as plateaus due to band propagation similar to Luders band with not much work hardening or strain gradient ahead of the moving band front [22], [23].
Generally, DSA has been found to be detrimental to tensile ductility, low cycle fatigue (LCF) life and creep resistance of structural components which are causes of concern for structural applications [24]. Hence, the role of DSA occurring in a LWR-specific service temperature range (553–693 K) on the tensile behavior of APMT™ alloy is addressed in this study.
Section snippets
Methods
An APMT™ steel rod of 12 mm diameter and 1830 mm length was procured from Sandvik for this study. Round tensile specimens of 25.4 mm gauge length and 6 mm gauge diameter were subjected to tensile tests in the temperature range of 298–773 K in air using nominal strain rates of 10−4, 3×10−4, 10−3, 3×10−3, and 10−2 s−1. The tensile specimen was allowed to dwell at the test temperature for 30 min to reach temperature stabilization throughout the gage section before starting elevated temperature tests.
Results and discussion
The steel exhibited serrated flow in the temperature range of 523–673 K. Fig. 1a shows the variation in serration types at different strain rates at a temperature of 573 K while Fig. 1b shows the different types of serrations at different temperatures for a strain rate of 10−3 s−1. The curves were shifted from their respective original positions to avoid overlapping and make the serrations distinct. Lower strain rates like 3×10−4 s−1 and 10−4 s−1 exhibited the dominance of A or A+C serrations,
Conclusions
The APMT™ steel exhibited serrated flow during tensile deformation within a temperature range of 523–673 K at all strain rates. Negative strain rate sensitivity predicts that serrated flow is due to DSA in this temperature range caused by interactions between mobile dislocations and interstitial solutes whose mobility is affected by high concentration of substitutional solutes. Further analysis is needed to fully assess the application of APMT™ steel as a fuel cladding material for the I2S-LWR
Acknowledgment
The research was performed using funds received from the DOE Office of Nuclear Energy's Nuclear Energy University Programs (NEUP). We would also like to acknowledge Professor Bojan Petrovic and Dr. Paolo Ferroni for facilitating the work.
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