Nuclear Grade Austenitic Stainless Steel Research Articles

Grain growth behaviors during solution annealing based on the heterogeneous microstructure in hot rolled nuclear grade austenitic stainless steel

Series of solution annealing experiments and quasi in-situ observation were employed to investigate the grain growth behaviors based on the heterogeneous microstructure with deformation bands and dynamic recrystallization (DRX) grains in hot rolled 316H austenitic stainless steel (ASS). It was worked out that during early stage of direct heating at high temperature of above 1000℃, the initial static recrystallized grains with different sizes would nucleate rapidly on the deformation band and DRX grains. With the extension of holding time, they gradually grew up to develop into the grains with large size differences, during which the larger grains continued to swallow adjacent smaller grains even presenting abnormal annexation or growth. However, by adopting stepped heating from low to high temperature, the grains evenly grew and presented relatively uniform distribution, which provides new idea for regulating grain size in manufacturing heavy ASSs.

Stress corrosion cracking failure of friction stir welded nuclear grade austenitic stainless steel

Stress Corrosion Cracking behavior of Austenitic Stainless Steels (SS) in a chloride environment has been investigated by several methods. In this investigation failure of emerging structural material for nuclear reactor (316LN grade austenitic stainless steel) and its Friction Stir Welded (FSW) joint due to stress corrosion cracking were analyzed in a chloride environment. FSW joint was fabricated between 316LN plates with optimized process parameters. Initially, hardness and tensile properties were analyzed across the weld zone to evaluate the mechanical properties of the welded joint. Microstructural characterization of base metal (BM) and weld zone were done by Optical Microscope (OM), and Scanning Electron Microscope (SEM). Grain boundary analysis was done by Transmission Electron Microscope (TEM). Microstructural evolution revealed that the FSW process is capable to refine and reduce grain size nearby 12 times that of the base metal by Continuous Dynamic Recrystallization mechanism (CDRX). Improved strength and hardness are also recorded at the weld zone compared to the base metal. Boiling 45 wt% MgCl2 solution at constant load condition as per ASTM standard G36 was used to assess SCC failure on base metal and FSW joint. Failure due to Chloride Stress Corrosion Cracking (CSCC) on weld samples were investigated and compared with its base metal samples at different tensile stress conditions (20%, 40%, 60%, 80%, 100% of base metal yield stress values). From the experimental result steady-state elongation rate (ISS), Transition time (tss) and time to failure (tf) were evaluated and generalized equations to predict time to failure of base metal and FSW weldment was generated. The impact of microstructural morphology on SCC failure of the FSW joint was investigated. The fracture location of the weld joint, nature of crack advancement of both base metal and FSW joint was analyzed by the SEM study. A study of fracture surface displayed brittle nature transgranular failure for base metal as well as FSW samples.

Investigations on mechanical properties and microstructural examination of activated TIG-welded nuclear grade stainless steel

Nuclear grade austenitic stainless steel 321 contains titanium for severe corrosive conditions and is used for process equipment, aircraft exhaust manifolds and boiler shells. Taguchi-based grey relational analysis technique has been considered to optimize the input parameters for welding SS321 material using activated tungsten inert gas welding process. The optimized input parameters were identified from the multiple output responses, i.e. arc length of 3 mm, welding current of 220 Amps and welding speed of 120 mm/min (A1B3C1). The analysis of the variance table was performed to calculate the importance of the input parameters in the weldment quality. Also, the influence of the heat input on the depth of penetration and bead width at different arc lengths has been studied. The mechanical integrity is evaluated in terms of tensile strength for base metal (BM) (621 MPa) and weld metal (WM) (624 MPa) specimens by conducting the uniaxial tensile test. The 180° bend test with WM sample revealed the ductility level and was free from fissures and cracks. Charpy impact test results depicted the toughness values of BM (123.17 J) and WM (113.46 J) samples. Microstructural investigations at the SS321 WM highlighted the existence of columnar and equiaxed dendrites in the fusion zone. Moreover, the formation of titanium carbide reduces the chances of weld decay. Ferrite number measurement shows that the amount of delta ferrite is higher in WM (5.9) than in BM (1.2). The existence of dimples and voids in the fractographic analysis of the failed specimens (tensile and impact) confirms the ductile type of failure.

Microstructure and mechanical properties of the 316 stainless steel nuclear grade experimental component made by wire and arc additive manufacturing

An experimental component of nuclear grade austenitic stainless steel 316 (SS316) shielding plate was manufactured by wire and arc additive manufacturing (WAAM) method. Its microstructure, tensile properties at room and high temperatures, Vickers hardness, and impact properties were analyzed. The results show that the microstructure of WAAM SS316 product is mainly composed of austenite (γ) and delta-ferrite (δ) phases. The δ exhibits a fine vermicular morphology in the austenite matrix and is distributed at the boundaries of and inside the grains. The tensile properties of WAAM SS316 product are comparable to those of wrought SS316 and exceed the corresponding requirements of the nuclear industry. WAAM SS316 product has good uniform impact toughness in all directions, and the impact energy can fully meet the technical requirements of the nuclear industry with excellent performance. The results of nondestructive testing show that the quality of the WAAM SS316 shield plate product meets the evaluation requirements of the nuclear industry.

Correlation between Tensile Deformation Behavior and Microstructural Morphology of Nuclear Grade Austenitic Stainless Steel Weld Joints using Infrared Thermography Technique

Tensile deformation behavior of nuclear grade Austenitic Stainless Steel (SS) and its weld joints fabricated by Gas Tungsten Arc Welding (GTAW) and Activated flux Gas Tungsten Arc Welding (AGTAW) processes were studied and correlated with relevant microstructural morphologies using Infrared Thermography (IRT) technique. The microstructure of base metal showed a complete austenite phase. GTAW Fusion Zone (FZ) exhibited both primary ferrite and primary austenite mode of solidification. Meantime, AGTAW FZ exhibited only primary austenite mode of solidification A strain rate of 4.4x10-4 s-1 was used during the tensile test of the base metal and weld samples. The failure locations of the base metal, GTAW and AGTAW samples were noticed at the center of the gauge portion, the base metal side away from Fusion Line (FL) and Heat Affected Zone (HAZ) respectively. Temperature variations of the base metal and weld zones were recorded in the form of thermograms using the IR camera at the different stages of the tensile deformation. During deformation study, peak temperatures of 39.2 oC, 38.8 oC and 34 oC were observed at the base metal, GTAW and AGTAW samples respectively. The lesser peak temperature of the AGTAW sample compared to the base metal and GTAW samples indicated that the AGTAW sample undergone lesser deformation. Moreover, tensile deformation behaviors of the base metal and weld samples were correlated with their microstructural morphologies using corresponding temperature curves.

A Calorimetric Study of Recovery and Recrystallization in Fe–14.4 Cr–15.4 Ni–2.4 Mo–2.36 Mn–0.25 Ti–0.04 C–0.05 P (mass %) Austenitic Stainless Steel

AbstractThe stored energy release and the kinetics of recovery and recrystallization processes in 70 and 85% coldworked nuclear grade austenitic stainless steel have been investigated by dynamic calorimetry, the results of which are supplemented by optical microscopy and microhardness measurements. Distinct thermal arrests arising from recovery and recrystallization events that occur upon slow heating are around 815 and 1070 K, respectively. The peak recovery and recrystallization temperatures are found to be sensitive to prior coldwork and heating rate. Faster rates of heating pushed the recovery stage to higher temperatures overlapping with the domain of recrystallization, which resulted in a decreased dissipation of stored energy due to recovery. As a result, faster heating rates, served to accelerate the kinetics of recrystallization owing to higher driving force. It is also found that the dissolution of fine TiC particles lead to secondary recrystallization at higher temperatures. Modeling of lattice softening kinetics yielded values of effective activation energies for recovery and recrystallization processes in the range, 198 and 336 kJ mol−1, respectively.

Microstructure and dry sliding wear resistance evaluation of plasma nitrided austenitic stainless steel type AISI 316LN against different sliders

In this work, Plasma Nitriding was carried out at a temperature of 570°C on nuclear grade austenitic stainless steel type AISI 316 LN (316LN SS) in a gas mixture of 20% N2–80% H2 to improve the surface hardness and thereby sliding wear resistance. The Plasma Nitride (PN) treated surface has been characterized by Vickers microhardness measurements, Scanning Electron Microscopic (SEM) examination, X-ray Diffraction (XRD) and sliding wear assessment. The average thickness of the PN layer was found to be 70μm. Microhardness measurements showed a significant increase in the hardness from 210 HV25g (unnitrided sample) to 1040 HV25g (Plasma Nitrided sample). The XRD reveals that PN layer consists of CrN, Fe4N and Fe3N phases along with austenite phase. The tribological parameters such as the friction coefficient and wear mechanism have been evaluated at ambient conditions for PN treated ring (PN ring) vs. ASTM A453 grade 660 pin (ASTM pin), PN ring vs. Nickel based alloy hard faced pin (Colmonoy pin), PN ring vs. 316LN SS pin and 316LN SS ring vs. 316LN SS pin. The wear tracks have been analyzed by SEM, Energy Dispersive X-ray Analysis (EDX) and Optical Profilometry. The untreated 316LN SS ring vs. 316LN SS pin produced severe wear and was characterized by a combination of delamination and adhesion wear mechanism, whereas wear mechanism of the PN rings reveals mild abrasion and a transfer layer from pin materials.

High Temperature Heat Capacity of Alloy D9 Using Drop Calorimetry Based Enthalpy Increment Measurements

Alloy D9 is a void-swelling resistant nuclear grade austenitic stainless steel (SS) based on AISI type 316-SS in which titanium constitutes an added predetermined alloying composition. In the present study, the high-temperature enthalpy values of alloy D9 with three different titanium-to-carbon mass percent ratios, namely Ti/C = 4, 6, and 8, have been measured using inverse drop calorimetry in the temperature range from 295 to 1323 K. It is found that within the level of experimental uncertainty, the enthalpy values are independent of the Ti–C mass ratio. The temperature dependence of the isobaric specific heat CP is obtained by a linear regression of the measured enthalpy data. The measured CP data for alloy D9 may be represented by the following best-fit expression: CP (J · kg −1 · K −1 ) = 431 + 17.7 × 10 −2 T + 8.72 × 10 −5 /T 2 . It is found that the measured enthalpy and specific heat values exhibit good agreement with reported data on 316 and other related austenitic stainless steels.

Study on Plastic Deformation and Hardening Mechanism of Austenitic Stainless Steels at Room and Cryogenic Temperatures

Plastic deformation and hardening mechanism of nuclear grade austenitic stainless steel, SUS316NG, at room and cryogenic temperatures were investigated using advanced indentation method and observations by transmission electron microscope (TEM). First, tensile and compressive pre-strains were applied at room temperature up to true strain values of 20%. Detailed indentation testing revealed that the strain hardening was detected when the indentation sizes were more than 50 μm, while it disappeared when the indentation size decreased down to 1 μm. This behavior was explained by the slip band size due to dislocation structure. Dual indenter method successfully predicted the stress-strain response of the pre-strained specimens. Second, Charpy impact tests were performed at room temperature and 77 K. The fracture toughness was sufficiently large at 77 K, however, ductility was slightly decreased compared with that at room temperature. Martensitic transformation was observed near the notch tip for the specimen tested at 77 K. This hardening was detectable by the indentation testing even if the indentation size was less than 1 μm.

Thermal property characterization of a titanium modified austenitic stainless steel (alloy D9)

The temperature dependence of lattice parameter and enthalpy increment of alloy D9, a titanium modified nuclear grade austenitic stainless steel were studied using high temperature X-ray diffraction and inverse drop calorimetry techniques, respectively. A smooth variation of the lattice parameter of the austenite with temperature was found. The instantaneous and mean linear thermal expansion coefficients at 1350 K were estimated to be 2.12 × 10 −5 K −1 and 1.72 × 10 −5 K −1, respectively. The measured enthalpy data were made use of in estimating heat capacity, entropy and Gibbs energy values. The estimated isobaric heat capacity C p at 298 K was found to be 406 J kg −1 K −1. An integrated theoretical analysis of the thermal expansion and enthalpy data was performed to obtain approximate values of bulk modulus as a function of temperature.

Stimulated amplification of acoustic emission signals during deformation

This study show that by injecting acoustic waves of a suitable frequency and amplitude on deforming nuclear grade austenitic stainless steel, there is an enhancement of total power of acoustic emission. The amplification of AE power is attributed to the activation of subcritical source in the material by the injected acoustic waves. This technique can be applied for stimulating the subcritical sources. Hence this technique offers a possibility for studying the dislocation dynamics during deformation and studying subcritical AE sources