Thermal Aging Embrittlement Research Articles

The Microstructure and Mechanical Properties of a 15-6 PH Stainless Steel with Improved Thermal Aging Embrittlement Resistance.

The evolution of the microstructure and the mechanical properties of a 15-6 martensite precipitated hardened (15-6 PH) stainless steel after thermal treatment and long-term aging at 480 °C were investigated. Compared with 17-4 PH steel, the content of Cr decreased and Ni increased in the newly developed 15-6 PH steel; therefore, reversed austenite formed after thermal treatment at 620 °C of the solution-treated 15-6 PH steel. Although the reversed austenite may reduce the strength of the steel, it is very beneficial for the inhibition of the aging brittleness of the steel. During the accelerated thermal aging at 480 °C, the Cu-rich phase gradually coarsened, and its crystal structure changed, while the reversed austenite phase sightly increased and the Charpy impact energy maintained a rather high value. The increase of the reversed austenite content can offset the reduction of the strengthening effect of the Cu-rich phase and therefore maintain an excellent impact property of the material after thermal aging.

Rapid repair of thermal aging embrittlement in 17-4PH martensitic stainless steel using pulsed electric current

The 17-4PH stainless steel serves as the main steam isolation valve in a light water reactor due to excellent comprehensive mechanical properties and corrosion resistance. Nonetheless, the thermal aging resulting from prolonged service poses a threat to the safe operation of nuclear power plants. The mechanism of thermal-aging embrittlement of 17-4PH stainless steel and its repairing behavior under the action of the pulsed electric current were studied using transmission electron microscopy and 3D atom probe tomography. The ductile to brittle transition occurred following the spinodal decomposition and G phase formation during the thermal aging of 5000 h at 673 K, resulting in a 100 HV increase in hardness of the 17-4PH stainless steel, of which 94.4 HV was attributed to the spinodal decomposition. Under the action of a pulsed electric current at 733 K, the hardness and impact energy can be restored to its original state, whereas heat treatment at the same temperature is unable to restore these properties. This is because the pulsed electric current treatment decreased the thermodynamic barrier to dissolution for the spinodal decomposition and G phases. The Kolmogorov-Johnson-Mehl-Avrami model was used to calculate the kinetics of spinodal decomposition dissolution. The results suggest that the pulsed electric current could increase the diffusion pre-exponential factor of Cr element and accelerate the elimination of spinodal decomposition. This process ultimately facilitates the quick regeneration of the 17-4PH stainless steel's performance.

In Situ Characterization of 17-4PH Stainless Steel by Small-Angle Neutron Scattering.

17-4PH martensitic steel is usually used as valve stems in nuclear power plants and it suffers from thermal aging embrittlement due to long-time service in a high-temperature and high-pressure environment. Here, we characterized the evolution of microstructures at the nano-scale in 17-4PH steel by in situ small-angle neutron scattering (SANS) with a thermo-mechanically coupled loading device. The device could set different temperatures and tensile so that an in situ SANS experiment could dynamically characterize the process of nanoscale structural changes. The results showed that with increasing thermal aging time, the ε-Cu phase precipitates and grows as the temperature is 475 °C and 590 °C, and the ε-Cu phase is spherical at 475 °C but became elongated cylinders at 590 °C. Moreover, the loading stress could aid in the growth of the ε-Cu phase at 475 °C.

Mediating phase decomposition to avoid thermal aging embrittlement in a duplex stainless steel

Due to the presence of miscibility gap in the body-centered-cubic structure iron‑chromium alloy system, the ferrite will decompose into the Fe-rich α phase and Cr-rich α´ phase, resulting in the inherent “475 °C embrittlement”. In this study, a pulsed electric field is introduced to mediate the phase decomposition for avoiding aging embrittlement. Combining nanoindentation testing and atomic-scale characterization, the results show that almost no aging hardening and phase decomposition behaviors occur with the pulsed electric field intervening, and the thermodynamic analysis indicates that the pulsed electric field can shrink the spinodal domain. Further, the life evaluation shows that the equivalent aging time of the duplex stainless steel is 18.2 years without the application of the pulsed electric field, while the equivalent aging time of the duplex stainless steel is 1.2 years under the action of the pulsed electric field, which is equivalent to an extension of the service life by 17 years. This study offers an opportunity for resisting the aging embrittlement process to prolong the longevity of components. • The ferrite decomposition is suppressed by pulsed electric field during thermal aging. • Spinodal decomposition plays the major role in thermal aging embrittlement. • Pulsed electric field induces the spinodal domain to shrink for avoiding phase decomposition. • The equivalent service life of duplex stainless steel is extended by 17 years.

Effect of microstructure evolution at the interphase boundary on the thermal ageing embrittlement of CF8M cast austenitic stainless steel

Thermal ageing embrittlement of a cast austenitic stainless steel, CF8M with 25% of δ-ferrite, was evaluated using various mechanical tests such as micro-hardness, small punch, tensile, and fracture toughness (J-R) tests. To simulate long-term service at operating temperature in light water reactors (LWRs), CF8M was thermally aged at 400 °C up to 10,000 h. After thermal ageing for 5000 h, phase separation into Fe-rich and Cr-rich phases by spinodal decomposition and G-phase precipitation were observed in δ-ferrite, resulting in significant hardening in δ-ferrite. As ageing duration increased to 10,000 h, additional changes in microstructure and micro-hardness in δ-ferrite were not significant. On the other hand, fracture properties, such as fracture toughness and small punch (SP) energy showed further reduction with ageing time, which could be related to elemental segregation at the austenite/ferrite interphase boundary. After reversion heat treatment at 550 °C for 1 h, spinodal decomposition was mostly disappeared, but G-phase precipitate in δ-ferrite and elemental segregation at the austenite/ferrite interphase boundary were not recovered, which could be responsible for the incomplete recovery of mechanical properties.

Applicability of non-destructive laser ultrasound and non-linear ultrasonic technique for evaluation of thermally aged CF8 duplex stainless steel

Duplex stainless steel is a structural material that is highly used in nuclear power plants, especially in the reactor core components and pipelines. Long-term exposure of duplex stainless steel in a high-temperature working environment will cause thermal aging embrittlement. Material deterioration will affect the long-term reliability of nuclear power plants. In this work, we used two different ultrasonic techniques, namely Laser Ultrasound Technique (LUT) and Non-linear Ultrasonic Technique (NUT) for measuring the material properties. The LUT has unique advantages such of being non-contact, non-destructive, and remotely operable, while NUT has considerable measurement sensitivity to the microstructure changes of materials. Therefore, this research mainly focused on duplex stainless steel after the heat aging treatment. The changes in the material properties of the duplex stainless steel after the heat aging treatment were investigated by ultrasonic testing and traditional material testing methods. The results of this study show that the thermal aging with higher duration, the ferrite phase decomposes into Cr-rich αˊ-phase and Fe-rich α-phase. The hardness of both the austenite iron phase and the ferrite iron phase increase with the increase in heat aging time. The phase velocity of duplex stainless steel increases significantly up to a heat aging of 2400 h, and then the phase velocity decreases significantly. After thermal aging treatment at different times, the non-linear factor did not change significantly due to the hardness changes. The non-linear response caused by the heat aging treatment of duplex stainless steel is minimum.

Effect of step cooling and P-segregation to grain boundaries on the tensile and fracture toughness properties of A533B plate and A508 forging steels

Although available data do not report significant thermal ageing of reactor pressure vessel materials, this should be reconsidered in the perspective of long term operation where exposure time is significantly higher than the available reported experimental data. One of the element that is reported to promote thermal ageing degradation is phosphorus. The thermal ageing embrittlement susceptibility is assessed by submitting reactor pressure vessel steels to a step cooling heat treatment known to induce P-segregation to grain boundaries resulting in intergranular rather than transgranular cleavage fracture. Thermal ageing susceptibility was experimentally assessed on two typical reactor pressure vessel steels with P-content of 0.017% and 0.006%. The experimental data including tensile, Charpy impact and fracture toughness tests, show a very consistent picture of the various mechanical properties. Upon step cooling, intergranular fracture is promoted and P-content has a major effect on the thermal ageing susceptibility. However, quantitative assessment shows that the effects of step cooling on the mechanical properties of both steels in their as received condition are not sufficiently high to induce thermal ageing embrittlement and therefore their susceptibility to thermal ageing is low. Consequently, given the low P-content of most Belgian RPV materials, it can be concluded that they will be not prone to thermal ageing in the perspective of long term operation.

Effect of precipitates on hardening of 17-4PH martensitic stainless steel serviced at 300 °C in nuclear power plant

When 17-4PH martensitic stainless steel serviced at 300 °C for long term in nuclear power plant, the thermal aging embrittlement and hardening will be significant. It will seriously affect the safety and economic operation of nuclear power plant. In this work, Three-Dimensional Atom Probe Technology (3DAPT) was used to characterize the element distribution, and the evolution of Cu-rich precipitate and Cr-rich precipitate with service time were analyzed. The results show that with the service time increasing, the number density and size of Cu-rich precipitate and Cr-rich precipitate in the material increase obviously, which leads to the increase of critical shear stress with dislocation. The main reason to hardening is the nucleation and growth of Cu-rich precipitates and Cr-rich precipitates, which the contribution of Cu-rich precipitates fits Russell-Brown model. The contribution of Cr-rich precipitates to hardening increases with the service time, and is greater than that of Cu-rich precipitates.

Investigation on effect of thermal aging embrittlement on residual stresses of austenitic stainless steel repair welds in nuclear components subject to seismic loads

This paper presents investigation results on effect of thermal aging embrittlement on residual stresses of austenitic stainless steel repair welds in nuclear components subject to seismic loads. First, the welds on austenitic stainless steel 316L plates were manufactured consisting of the austenitic stainless steel weld metal with the δ—ferrite content sensitive to the thermal aging embrittlement. One of the welds was embrittled via conducting thermal aging acceleration test. Second, validity of finite element weld residual stress analysis was verified by performing finite element residual stress analysis and holedrilling method for the unrepaired welds. Third, effect of the thermal aging embrittlement on the residual stresses of the repair welds was evaluated by comparing the residual stresses according to whether the original welds were thermally embrittled. Last, effect of seismic loads was reviewed for the case where the repair welding was performed on the original welds being thermally embrittled. As a result, it is found that the effect of the thermal aging embrittlement on the residual stresses of the unrepaired welds is insignificant whereas tensile residual stresses of the repair welds increase in proportion to the increase in yield strength at room temperature due to the thermal aging embrittlement. In addition, the residual stresses are significantly mitigated after the seismic loads.

Non-hardening embrittlement mechanism of pressure vessel steel Ni-Cr-Mo-V welds during thermal aging

The mechanical performance of reactor pressure vessel materials is an important factor in the safety and economics of the operation of a nuclear power plant. The ductile-to-brittle transition temperature tested by Charpy impact test is the key parameter for evaluating the reactor pressure vessel embrittlement. In this article, the study of thermal aging embrittlement of temperature sets of reactor pressure vessel surveillance Ni-Cr-Mo-V steel weld metal was conducted by Charpy impact test. The thermal aging effect on the impact fracture behavior was analyzed. The impact test of the three batches of weld surveillance sample indicated that the weld metal embrittled during thermal aging. The study of impact fracture and Auger electron spectroscopy indicated that the element P segregated to the grain boundaries and lowered their cohesion strength during the long-term thermal aging. Therefore, the non-hardening embrittlement of Ni-Cr-Mo-V steel welds in a reactor pressure vessel caused by segregation of impurity elements P occurs during thermal aging.

Reconstructing the decomposed ferrite phase to achieve toughness regeneration in a duplex stainless steel

Duplex stainless steels suffer from thermal aging embrittlement that results from severe phase decomposition in ferrite phase after a long-term service at temperatures of 550–700 K, leading to the severe performance deterioration of duplex stainless steels. To ensure reliability and extend the service life of fabricated components made of duplex stainless steels, the development of techniques to efficiently and completely regenerate the deteriorated performance induced by spinodal decomposition and precipitation are extremely important. In this study, a novel pathway–an external electric field, is developed to eliminate the emerging Cr-rich (α′) phase and Fe-rich (α) phase resulting from spinodal decomposition as well as to dissolve the precipitates of G-phase in ferrite by introducing extra electrical free energy. The investigation is evidenced by microstructural and mechanical analyses using atom probe tomography, transmission electron microscopy, and nanoindentation. This high-efficiency (performance recovery above 90%), low-energy consumption, online repair at the service temperature (700 K) is considerably superior to the traditional heat treatment process, which requires off-site repair at high temperatures (> 823 K). This new concept of manipulating precipitates using electric current to reconstruct the decomposed microstructure and achieve performance regeneration is expected to further stimulate the interest of researchers to extend the service life of materials by this means.

Electrochemical investigation of in-service thermal aging in two CF8M cast stainless steels

In-service thermal aging of CF8M cast austenitic stainless steel was investigated in materials removed from the steam generator inlet and crossover elbows of the Ringhals 2 pressurized water reactor nuclear power plant unit after approximately 92kh of full operating time. The thermal aging of these materials was investigated using the double loop electrochemical potentiokinetic reactivation method, coupled with indentation hardness measurements and microstructural characterizations, to identify correlations between the electrochemical behavior and traditional methods of investigating thermal aging embrittlement effects in cast stainless steels. While this electrochemical method can be easily employed to quantify thermal aging effects in materials aged at higher temperatures (e.g. greater than 350∘C), this study highlights the difficulties encountered when electrochemically evaluating the aging of materials exposed to nuclear power plant operating conditions.

Mechanical Properties of a Thermally-aged Cast Duplex Stainless Steel by in Situ Tensile Test at the Service Temperature

Z3CN20.09M cast duplex stainless steel (CDSS) has been used for primary coolant water pipes in pressurized water reactors due to its excellent mechanical properties. Such pipes operate at an elevated service temperature (~320 °C) and experience issues of thermal aging embrittlement. In situ tensile tests were conducted to investigate the deformation mechanisms of Z3CN20.09M CDSS after long-term thermal aging at 475 °C for up to 2000 h in both optical microscope and scanning electron microscope at 320 °C. For the 320 °C tests, the tensile stress and other mechanical properties, e.g. the yield stress and the ultimate tensile strength, increase during the thermal aging process and recover to almost the same level as the unaged condition after annealing heat-treatment, which is caused by the formation and dissolution of precipitation during aging and anneal heat-treatment, respectively. For the slip mechanism, straight slip lines form first in the austenite phase. When these slip lines reach the austenite/ferrite interface, three kinds of slip systems are found in the ferrite phase. During the fracture process, the austenite phase is torn apart and the ferrite phase shows a significant elongation. The role of the ferrite phase is to hold the austenite matrix, thus increasing the tensile strength of this steel.

Effect of Precipitate on Thermal Aging Effect of 17-4PH Martensitic Stainless Steel Used as Valve Stem in Nuclear Power Plant

The valve stem used in the main steam system of nuclear power plant is usually 17-4PH martensitic stainless steel. When it served in 300°C for a long time, the thermal aging embrittlement of valve stem will be significant, with the performance of the ductile brittle transition temperature (DBTT) and the hardness increased, the upper stage energy (USE) decreased. It will increase the risk of brittle fracture of the valve stem, and seriously affect the safety and economic operation of nuclear power plant (NPP). Similar cases have occurred in foreign nuclear power plants. Therefore, it is important to study the thermal aging effect of the 17-4PH steel used as valves in nuclear power plant. In this work, the 17-4PH martensitic stainless steel samples served in nuclear power plant for many years were studied, and they exhibit obvious thermal aging embrittlement. By use of small angle neutron scattering (SANS) and three-dimensional atomic probe (3DAP), the nanosize precipitate in stainless steel is studied. The results show that the size of the larger cluster (~7nm) in stainless steel increases and the volume fraction of the cluster with size of ~1nm increases obviously after thermal aging. The larger nanosize precipitate was growing up during long service at high temperature, and precipitation of the smaller ones continuously occurred. Combing with the results of 3DAP, the nanosize clusters were formed by segregation of Ni, Mn and other elements with Cu-rich cluster, which are mainly in the form of Cu core and Ni-Mn shell.

Proposal of guidelines for structural integrity assessment on thermal aging embrittlement of cast austenitic stainless steel in BWR environment

Proposal of guidelines for structural integrity assessment on thermal aging embrittlement of cast austenitic stainless steel in BWR environment

Crack growth evaluation by XFEM for nuclear pipes considering thermal aging embrittlement effect

AbstractSince thermal aging embrittlement (TE) causes loss of fracture toughness and may lead to failure, it is one of structural integrity issues to be assessed in nuclear industry. In this study, crack growth simulation by using extended finite element method (XFEM) was carried out for CF8M base metal and ER316L weld metal, which are typical vulnerable materials. First, user subroutines were constructed to incorporate variation of mechanical properties of two materials due to the TE. In particular, with regard to ER316L, weld residual stress (WRS) distribution was considered. Second, validity of the subroutines and XFEM was confirmed by comparing with analytical and experimental data. Finally, crack growth analyses for nuclear pipes were performed by applying the verified methodology. Thereby, differences of the crack growth amounts and paths as the presence or absence of the TE and WRS were quantified and discussed.

Mechanical properties and kinetics of thermally aged Z3CN20.09M cast duplex stainless steel

Cast stainless steels used in nuclear power plants suffer from fracture toughness losses owing to thermal aging after long-term service at temperatures ranging from 280–320°C. To study the thermal aging embrittlement of Z3CN20.09M duplex stainless steel produced in China, accelerated thermal aging experiments were carried out at 350, 380, and 400°C for up to 10000 h. Microhardness and Charpy impact energies were measured at different aging times. The microhardness of ferrite increased drastically over the initial aging time of 2000 h at 380 and 400°C and then slowly reached HV0.01 560. In contrast to this observed change in microhardness, Charpy impact energies sharply decreased after initial aging and then gradually reached a minimum value. Taking the microhardness of the ferrite phase as the parameter describing the thermal kinetics of the stainless steel samples, the activation energy of thermal aging was calculated to be 51 kJ/mol. Correlations between the thermal aging parameter, P, and ferrite microhardness and between P and Charpy impact energy were also analyzed. The results showed that the activation energy calculated from the ferrite microhardness is much more reasonable than that obtained using other parameters, such as chemical composition and impact energy.

Effects of long-term thermal aging on bulk and local mechanical behavior of ferritic-austenitic duplex stainless steels

Ferritic-austenitic cast duplex stainless steels (CDSS) undergo significant thermal aging embrittlement in service in light water nuclear reactors. Cast CF–3 and CF–8 duplex stainless steels have been laboratory aged for up to 17,200 h at four different temperatures (280 °C, 320 °C, 360 °C, 400 °C) in order to characterize the evolution of microstructure and mechanical properties. Standard Charpy V-Notch (CVN) impact testing and tensile testing have been conducted to measure the progression of embrittlement of the steels. Concurrently, nanoindentation measurements were performed to measure the hardening of the individual ferrite and austenite phases. Atom probe tomography (APT) analysis of the constituent phases is used to investigate the relationship between spinodal decomposition of the ferrite and the mechanical property evolution during aging. The effects of the local microstructure and property changes in the ferrite phase on the overall bulk mechanical property degradation of these steels are discussed.

Mechanical properties and eddy current testing of thermally aged Z3CN20.09M cast duplex stainless steel

To study the thermal aging embrittlement of Z3CN20.09M duplex stainless steel produced in China, accelerated thermal aging experiments were carried out at 380 °C up to 9000 h. Microhardness measurements, Charpy impact and eddy current tests were performed on aged samples to characterize their thermal aging embrittlement. The results showed that the signal amplitude of eddy current decreased with the increase in aging time. Two quantitative correlations of the eddy current signal amplitude with both the Charpy impact energy, and the Vickers microhardness of the ferrite phase are obtained. The study showed that eddy current testing could be used to non-destructively evaluate the thermal aging embrittlement of cast duplex stainless steels.

Effect of long-term aging on the fracture toughness of primary coolant piping material Z3CN20.09M

In this study, a series of accelerated thermal aging tests were performed on primary coolant piping materials (Z3CN20.09M) at 400 °C for 0, 2000, 5000 and 18,000 h to investigate the resulting microstructure change and fracture toughness. To further assess their fracture toughness, 0.2 mm offset line method and stretch zone width method were both employed to determine the fracture toughness. The results indicated that after long-term thermal aging, round shaped particles and characteristic morphology of spinodal decomposition were found in ferrite, thus resulting in the thermal aging embrittlement of Z3CN20.09M. Subsequently, the J-R curves and J-T curves all decreased significantly with the increasing thermal aging time. Furthermore, the fracture toughness parameters J50, JSZW(2D), JSZW(3D) and dJ/dΔa were first decreased rapidly and then slow saturated. In addition, the JSZW(2D) and JSZW(3D) values were significantly lower than JQ while the reduction of the critical crack size based on JSZW(2D) and JSZW(3D) parameters was about 30% compared with JQ, and thus JSZW was relatively close to the onset of crack initiation.