Elsevier

Materials Science and Engineering: A

Volume 733, 22 August 2018, Pages 338-349
Materials Science and Engineering: A

A study on tensile properties of Alloy 709 at various temperatures

https://doi.org/10.1016/j.msea.2018.06.089Get rights and content

Abstract

In recent years, there have been several advancements in energy production from both fossil fuels and the alternate “clean” sources such as nuclear fission. These advancements are fueled by the need for more efficient systems that will optimize the use of the depleting fossil fuel reserves and shift the focus to cleaner sources of energy. The efficiency of any power generation cycle is dependent on the ability of the structural material to withstand increased peak operating temperatures. Advanced austenitic stainless steels have been in the focus as structural material for the next generation nuclear power plants, due to their strength, corrosion resistance, weldability and the wide range of temperatures at which the austenite phase is stable. Alloy 709, a recently developed advanced austenitic stainless steel, is being investigated in this paper. In this study, tensile tests were conducted on dog-bone samples of Alloy 709 in an in-situ scanning electron microscope (SEM) loading and heating stage, equipped with electron backscatter diffraction (EBSD), at various temperatures. The in-situ experiments indicated that the material primarily accommodated deformation by slip at lower temperatures. Void formation and coalescence at grain boundaries preceded slip at higher temperatures. Although crack initiation at all elevated temperatures was intergranular, the crack propagation through the material and the final fracture was transgranular ductile. Additionally, tensile tests were conducted on larger cylindrical samples at 550, 650 and 750 °C in air. The results of tests conducted in air and in-situ were found to be in agreement, at these temperatures.

Introduction

Structural materials used in the next generation nuclear power plants with improved efficiencies need to operate at high temperatures and withstand resultant extreme conditions. Therefore, superior mechanical strength, creep resistance and corrosion resistance are some of the desired properties in a candidate structural material. Advanced austenitic stainless steels are being investigated for these applications. Austenitic stainless steels are Fe-C-Ni-Cr alloys, where the Cr is added for corrosion resistance and Ni is added to counteract the ferritic stabilizing nature of Cr and stabilize the austenite phase. Several austenitic stainless steels have been developed with variations in their chemistry and heat treatment to suit different applications. These steels, specially the advanced austenitic stainless steels rely on secondary phases or precipitates for their characteristic strength and mechanical properties. Sourmail has reviewed the common precipitates observed in creep resistant austenitic stainless steel [1]. The 316 H steel is a heat resistant, higher carbon variant of the 316 Stainless Steel (18Cr-12Ni austenitic stainless steel) and is often used to compare the performance of newer alloys. The precipitates observed in this alloy are mainly M23C6, a predominantly Cr rich carbide typically at grain boundaries, and intermetallics such as Fe2Mo, FeCrMo and σ-phase [2]. Although, fine intermetallic Laves precipitate have shown to improve creep properties in an austenitic steel [3], after long aging times these precipitates coarsen at grain boundaries and triple points [2] reducing the creep strength of the alloy. High Temperature Ultrafine Precipitation Strengthened Steels (HT-UPS) developed by ORNL outperform the 316 H steels in creep rupture life [3]. The strength of this 14Cr-16Ni austenitic stainless steel is attributed to the fine MC, M6C, M23C6 and FeTiP precipitates that nucleate on dislocations. [3] Carroll studied the fatigue properties of the HT-UPS alloy and found that oxidation was a major problem in these alloys when compared to the 316 H [4]. To improve corrosion resistance, advanced HT-UPS was developed with added Aluminum. The passive chromium oxide layer in conventional stainless steels is vulnerable in atmospheres containing water vapor [5]. Alumina forming austenitic steels, specifically the advanced HT-UPS with added Al, performed better in terms of oxidation resistance in water vapor environments, which was further enhanced with added Nb [5], [6]. HT-UPS alloys with added Al and no Ti or V also performed better in terms of creep resistance when compared to other variants [7].

Alloy 709 is a 20Cr-25Ni advanced austenitic stainless steel developed as an improvement over the existing advanced austenitic stainless steels. The high Ni content provides increased austenite stability [8]. Sourmail et al. [9] have studied the effects of high temperature on the microstructure and secondary phases in the NF709 alloy. The NF709 alloy is a proprietary alloy of Nippon Steel & Sumimoto Steel, similar in composition to the Alloy 709. The NF709 alloy reported 0.05 wt% Ti content while the Alloy 709 studied in this paper contains < 0.01 wt% Ti. The authors found coarse undissolved nitrides, carbides and carbonitrides such as M23C6 and (Nb,Ti)CN, in the NF709, after aging [9]. This precipitate evolution was also simulated by Shim et al. [10]. NF709 possesses highest creep rupture strength amongst the austenitic steels. Preliminary studies performed indicated that Alloy 709 is superior to the HT-UPS alloys in tensile strength, thermal stability, creep-fatigue, sodium compatibility and weldability [8]. The excellent creep resistance and corrosion resistance of the Alloy 709 has made it the ideal candidate for next generation nuclear power plants.

In this study, in-situ scanning electron microscope (SEM) tensile tests were conducted on Alloy 709 to establish its yield and ultimate tensile stress at various temperatures, from room temperature to 1000 °C. To characterize the behavior of the material and dominant deformation mechanisms at different temperatures and strain rates, electron backscatter diffraction (EBSD) was used to observe microstructural evolution and phase changes in the alloy.

Section snippets

Experimental setup

A 400-pound ingot of Alloy 709 was fabricated using vacuum-induction melting (VIM) and electro-slag remelting (ESR) processes. 203 mm diameter round ingot from the VIM was homogenized at 1250 °C for 4 h. Half of this ingot was hot forged to a 203 mm × 34.9 mm bar at 1100 °C. 1/3 of the hot-forged bar was rolled to 102 mm × 20.3 mm at 1100 °C. The hot- rolled bar was finally annealed at 1100 °C for 2 h, followed by water quenching. The composition of the alloy is shown in Table 1. Preliminary

Microstructural characterization

The microstructural observation showed that the alloy comprises of an austenitic matrix with equiaxed grains of an average size between 48 and 50 µm (Fig. 3(a)). Room temperature SEM observations show some large clusters of inclusions along the rolling direction plus some isolated transgranular precipitates. EDS analysis was performed at room temperature to establish the composition of the precipitates. The compositions of the matrix and different precipitates are shown in Fig. 3. Majority of

550 and 650 °C

In-situ SEM images of the sample surface provide insights into deformation regimes in the sample, the nature of crack propagation and changes in grain morphology. The behavior of the alloy at room temperature and 550 °C are closely similar. At 550 °C and 650 °C, the plastic deformation is primarily accommodated via the formation of slip bands at all strain levels (Fig. 9(a) & (b)). By increasing strains, the density of the slip bands increases, and multiple slip systems are observed in some

Conclusions

In-situ SEM tensile experiments were conducted to investigate tensile properties and deformation mechanisms of the Alloy 709 in a temperature range of room temperature to 950 °C. Following conclusions can be drawn:

  • Alloy 709 shows typical stress-strain curves of austenite stainless steels with an excellent work hardening capability up to a temperature of 650 °C and a superb ductility at all temperatures.

  • Serrated stress strain curves were observed at 550, 650 and 750 °C under the faster strain

Declaration of Interest

None.

Acknowledgement

This study is part of a project funded by the Department of Energy (DOE) nuclear energy university program (NEUP) award 2015-1877/DE-NE0008451, and Research Council of United Kingdom (RCUK) award number EP/N016351/1. The authors would also like to thank Hitachi High Technologies America for ion-milling the insitu SEM samples.

References (17)

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