Room Temperature Charpy Impact Research Articles

Effects of Short-Time Tempering on Impact Toughness, Strength, and Phase Evolution of 4340 Steel Within the Tempered Martensite Embrittlement Regime

While tempering reactions under furnace conditions have been extensively studied, short-time tempering at higher temperatures, as might apply to induction tempering, has not been thoroughly explored. The mechanical behavior and phase development of short-time (1, 10, and 100 seconds) and conventionally (3600 seconds) tempered 4340 steel are compared at an equivalent degree of tempering defined by the Hollomon–Jaffe tempering parameter. The tempering parameter accurately predicts hardness values across short-time and conventional tempering conditions, but is less able to describe phase evolution associated with short-time tempering. Room temperature Charpy impact toughness and ductile-to-brittle transition temperature systematically improve with shorter tempering times at an equivalent tempering parameter. In particular, rapid tempering significantly increases toughness within the tempered martensite embrittlement regime. Relative to conventional tempering, shorter tempering times exhibit higher retained austenite content for a given tempering parameter, although no systematic or significant difference in cementite or transition carbide content is observed between time conditions. The retained austenite decomposition behavior indicates that the relationship between the classical tempering stages is altered at short tempering times, where increased overlap and/or “re-ordering” of stage II and III tempering are observed.

Influence of filler metals and PWHT regime on the microstructure and mechanical property relationships of CSEF steels dissimilar welded joints

The nuclear grade modified 9Cr1Mo (P91) and 9Cr-0.5Mo-1.8WVNb (P92) are commonly used material for power plant components operating in the temperature range of 600–650 °C. For such steels, the creep strength at the elevated operating temperature is the prime criteria. However, microstructure stability and notch toughness also play an important role, especially for the welded joints to meet the pressure test at room temperature. The room temperature mechanical properties of the welded joint are strongly influenced by the welding process, filler composition, composition of the base plate and heat treatment performed after the welding. In present work, P91 and P92 dissimilar multi-pass welded joints have been produced using the different filler rod. The microstructure of the welded joint has been studied for the different filler composition in various heat treatment condition. The room temperature Charpy impact toughness and tensile properties for the different welded joints (different filler) have been also studied and related with the microstructure of the welded joint. From the results, it has been concluded that a higher amount of ferrite stabilizer in P92 filler promotes the formation of the δ ferrite in the weld fusion zone.

Room-Temperature Charpy Impact Property of 3D-Printed 15-5 Stainless Steel

In this study, the room-temperature Charpy impact property of 3D-printed 15-5 stainless steel was investigated by a combined experimental and finite element modeling approach. The experimentally measured impact energy is 10.85 ± 1.20 J/cm2, which is comparable to the conventionally wrought and non-heat treated 15-5 stainless steel. In parallel to the impact test experiment, a finite element model using the Johnson–Cook material model with damage parameters was developed to simulate the impact test. The simulated impact energy is 10.46 J/cm2, which is in good agreement with the experimental data. The fracture surface from the experimentally tested specimen suggests that the 3D-printed specimens undergo predominately brittle fracture.

Effect of Mo-rich Fillers in Pulsed Current Gas Tungsten Arc Welding of Inconel 718 for Improved Strength and Hot Corrosion Resistance

The formation of Laves phase in the fusion zone of Inconel 718 welds is considered to be detrimental and requires major attention, as this phase deteriorates the mechanical properties and high-temperature corrosion resistance. This present study addresses the use of current pulsing technique on the gas tungsten arc welding of 5-mm-thick Inconel 718 plates. Nb-free fillers, namely ERNiCrMo-10 and ERNiCrMo-14, are chosen for joining these plates. Microstructure studies revealed the segregation of Mo-rich phases at the inter-dendritic regions of the fusion zone of both the weldments. Further, the occurrence of detrimental Laves phase was minimized while employing Mo-rich fillers. It is inferred from the tensile studies that the fractures occurred in the parent metal for ERNiCrMo-10 filler and at the fusion zone while employing ERNiCrMo-14. Room-temperature Charpy impact studies also affirm that the impact toughness of ERNiCrMo-10 welds is significantly greater than that of ERNiCrMo-14. Hot corrosion studies were performed on the coupons by exposing them to a synthetic molten salt environment comprising Na2SO4 + 60%V2O5 at 800 °C for 50 h. Owing to the presence of higher amounts of Cr2O3 and NiO, both fusion zones exhibited better hot corrosion resistance than the parent metal. The novel outcome of the study reiterates that with the use of current pulsing and Mo-rich fillers, the detrimental phase shall be controlled.

Study of Aging-Induced Degradation of Fracture Resistance of Alloy 617 Toward High-Temperature Applications

For the Alloy 617, the effect of aging on the fracture energy degradation has been investigated after aging for different time periods at 1023 K (750 °C). A sharp reduction in impact energy (by ~55 pct vis-a-vis the as-received material) after 1000 hours of aging, as evaluated from room-temperature Charpy impact tests, has been observed. Further aging up to 10,000 hours has led to a degradation of fracture energy up to ~78 pct. Fractographic examinations using scanning electron microscopy (SEM) have revealed a change in fracture mode from fibrous-ductile for the un-aged material to intergranular mode for the aged one. The extent of intergranular fracture increases with the increasing aging time, indicating a tendency of the material to undergo grain boundary embrittlement over long-term aging. Analysis of the transmission electron microscopy (TEM) micrographs along with selected area diffraction (SAD) patterns for the samples aged at 10,000 hours revealed finely dispersed γ′ precipitates of size 30 to 40 nm, rich in Al and Ti, along with extensive precipitation of M23C6 at the grain boundaries. In addition, the presence of Ni3Si of size in the range of 110 to 120 nm also has been noticed. The extensive precipitation of M23C6 at the grain boundaries have been considered as a major reason for aging-induced embrittlement of this material.

Methodology for Estimating Thermal and Neutron Embrittlement of Cast Austenitic Stainless Steels During Service in Light Water Reactors

Cast austenitic stainless steel (CASS) materials, which have a duplex structure consisting of austenite and ferrite phases, are susceptible to thermal embrittlement during reactor service. In addition, the prolonged exposure of these materials, which are used in reactor core internals, to neutron irradiation changes their microstructure and microchemistry, and these changes degrade their fracture properties even further. This paper presents a revision of the procedure and correlations presented in NUREG/CR-4513, Rev. 1 (Aug. 1994) for predicting the change in fracture toughness and tensile properties of CASS components due to thermal aging during service in light water reactors (LWRs) at 280–330 °C (535–625 °F). The methodology is applicable to CF-3, CF-3M, CF-8, and CF-8M materials with a ferrite content of up to 40%. The fracture toughness, tensile strength, and Charpy-impact energy of aged CASS materials are estimated from known material information. Embrittlement is characterized in terms of room-temperature (RT) Charpy-impact energy. The extent or degree of thermal embrittlement at “saturation” (i.e., the minimum impact energy that can be achieved for a material after long-term aging) is determined from the chemical composition of the material. Charpy-impact energy as a function of the time and temperature of reactor service is estimated from the kinetics of thermal embrittlement, which are also determined from the chemical composition. The fracture toughness J-R curve for the aged material is then obtained by correlating RT Charpy-impact energy with fracture toughness parameters. A common “predicted lower-bound” J-R curve for CASS materials of unknown chemical composition is also defined for a given grade of material, range of ferrite content, and temperature. In addition, guidance is provided for evaluating the combined effects of thermal and neutron embrittlement of CASS materials used in the reactor core internal components. The correlations for estimating the change in tensile strength, including the Ramberg/Osgood parameters for strain hardening, are also described.

Effect of Ageing at 700 °C on Microstructure and Mechanical Properties of S31042 Heat Resistant Steel

To investigate the effect of high temperature ageing on the microstructure and mechanical properties of S31042 steel, solid solution treatment at 700 °C was carried out for various time from 10 to 6000 h. Experimental results showed that the change of mechanical properties is closely related to the amount of precipitated phases. During ageing from 10 to 300 h, precipitation in the tested steel increases rapidly, and correspondingly, the high temperature yield strength and room temperature hardness of tested steel increase rapidly. Meanwhile, the thickness of the secondary phase on grain boundaries widens sharply and the room temperature Charpy impact absorb energy decreases. Ageing beyond 300 h, the precipitation in the steel increases gradually and the precipitates coarsen to a certain extent. The high temperature yield strength of the steel keeps stable, and the room temperature Charpy impact energy and hardness decrease slowly. Ageing beyond 3000 h, the mechanical properties of the steel tend to be stable. The main precipitates are M23C6, NbCrN and NbC in the tested steel.

Effects of tungsten addition and heat treatment conditions on microstructure and mechanical properties of microalloyed forging steels

In the present work, the effects of tungsten (W) addition and heat treatment conditions on the microstructure, impact toughness and tensile properties of microalloyed forging steels were studied. Four kinds of microalloyed forging steels were produced by varying W additions (0, 0.5, 1 and 2wt%). Heat treatment was carried out at temperatures ranging from 840 to 950°C followed by air and furnace cooling. Mechanical tests were used to evaluate the room temperature Charpy impact and tensile properties. The resulting microstructures were analysed via optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM). The results show that the microstructure and mechanical properties of the microalloyed forging steels are closely related to the W content. Correct selection of heat treatment temperature and cooling method is very important to enhance the mechanical properties. The optimum heat treatment temperatures obtained in this study are 920, 900, 880 and 860°C for the steels with 0%, 0.5%, 1% and 2% W, respectively. Both the impact toughness and tensile properties of the microalloyed forging steels have been significantly enhanced after heat treatment at the optimum temperatures followed by air cooling.

Enhancing mechanical properties of a low-carbon microalloyed cast steel by controlled heat treatment

In the present work, detailed studies were made on the optimization of microstructure and mechanical properties of a low-carbon microalloyed cast steel through control of heat treatment conditions. Specimens were austenitized at temperatures ranging from 950 to 1200°C for 2h followed by different cooling methods (furnace, air and water). For analyzing the effect of holding time on mechanical properties, some cast specimens were austenitized at 1100°C for different times followed by furnace cooling. After heat treatment, mechanical tests were employed to evaluate the room temperature Charpy impact and tensile properties. The characterization of complex precipitates formed during heat treatment process was investigated by using analytical electron microscopy. The results show that dissolution of vanadium-containing precipitates plays an important role in the abnormal growth of austenite grains at 1150°C. Further growth in austenite grains at 1200°C is caused by the dissolution of Ti-containing particles and the reduction of the amount of precipitates. Correct selection of the austenitizing temperature, holding time and cooling method is very important to improve the mechanical properties of the steel. Heat treatment at 1100°C for 2h followed by furnace cooling leads to the best combination of excellent Charpy impact and tensile properties.

Thermal Stability of a Ni-Cr-W-Mo Alloy - Long-Term Exposures

HAYNES® 230® alloy, based on the Ni-Cr-W-Mo system, is extensively used in gas turbines and other applications at high temperatures. It is recognized that the microstructure of many alloys under long-term service conditions can change. Microstructural changes may affect mechanical properties which in turn may determine component design life in service. Given the importance and widespread usage of 230 alloy, a number of studies on its thermal stability have been conducted. This study will focus on the effect of long-term thermal exposures at temperatures ranging from 649 to 871°C (1200 to 1600°F) on the microstructure and properties of 230 alloy. Room and elevated temperature tensile, and room temperature Charpy impact toughness properties of thermally exposed material are determined and correlated to observed changes in the microstructure. Microstructural analyses were performed using extraction/X-ray diffraction together with optical and scanning electron microscopy. The mode of fracture was ascertained by fractography of the thermally-exposed and broken test samples. The paper presents previously unpublished results for long-term exposures with durations ranging up to 50,000 hours (~ 6 years).

Cleavage Initiation in Ti-V-N and V-N Microalloyed Forging Steels

The effects of matrix microstructure and features of non-metallic inclusion (morphology, type, volume fraction and size) on cleavage initiation in medium-carbon Ti-V-N and V-N microalloyed resulphurized forging steels have been determined by examining fracture surfaces produced in room temperature Charpy impact tests. The steels were generally Al-deoxidised but one V-N steel was Si-deoxidised. It has been found that, in the Ti-treated steel, having a ferrite-pearlite microstructure, brittle fracture initiation occurred at cracked coarse (Ti,V)(C,N) single phase or [(Ti,V)(C,N)/Al2O3/MnS] multi-phase inclusions. In the Ti-free steels, cleavage initiation was dependent on matrix microstructure and non-metallic inclusions. In the low strength Ti-free steels, with a ferrite-pearlite microstructure, the absence of a continuous grain boundary ferrite layer led to initiation from interfacing pearlite colonies. For the bainitic microstructure, cleavage initiated close to the notch, but the microstructural feature responsible could not be identified. For the ferritepearlite microstructure in the Si-deoxidised V-N steel, cleavage initiated at cracked Mn-Al-(Ca) silicate inclusions. The higher matrix strength and more continuous nature of grain boundary allotriomorphic ferrite in the V-N steel deoxidised with Al was associated with cleavage initiation from V-rich (V,Ti)(C,N)-containing inclusions. These were generally of smaller size than those in the Ti-treated steels.

Effects of high temperature excursion on mechanical and fatigue properties of nuclear pressure vessel steel during fabrication

Presented is study concerning the effects of introducing a high temperature cycle, within the range of 1200 to 1500°C, during the fabrication heat treatment on the mechanical and fatigue properties of a reactor pressure vessel (RPV) steel. The tensile properties, viz., tensile strength and reduction in area both exhibited small discontinuous decreases over the temperature range of 1200 to 1500°C while the room temperature Charpy impact energy was dramatically reduced by 50%. At room temperatures in a deoxygenated, demineralised aqueous environment (PWR water) it was observed that a high temperature cycle caused a significant increase in fatigue crack growth behaviour. However at ΔK levels above 8 MPa √m the high temperature cycled steel (termed overheated) and the as-received steel (termed normal) exhibited similar fatigue crack growth rates. The increased fatigue crack growth rates were caused by the occurrence of significant amounts intergranular failure facets of the fatigue surfaces of the overheated steel. In PWR water at a temperature of 120°C it was shown that, at initial ΔK levels the overheated pressure vessel steel exhibited fatigue crack extension characteristics which were around ten and twenty times slower (depending upon the overheating temperature) than the normal pressure vessel steel data. Such differences were explained in terms of the fractographic features. It was argued that the high temperature cycle treatment caused the absences of EAC growth in PWR water at 120°C by causing the liquation and subsequent re-precipitation of numerous, very small, sulphide inclusions at prior austenite grain boundaries. Finally, at higher ΔK levels, a high temperature cycle promoted the onset of stage III fatigue, typified by the occurrence of intergranular failure, by reducing the fracture toughness of the pressure vessel steel.