Creep behaviour and long-term creep life extrapolation of alloy 617 for a very high temperature gas-cooled reactor

Creep behaviours for Alloy 617, such as creep properties, oxidation and creep fracture morphologies were investigated by a series of creep tests with different stress levels at 1223 K. Creep constants of A, n, m, and C were obtained at 1223 K for Alloy 617. To accurately predict the long-term creep life (or strength) for Alloy 617, a multi constant method with two C in the Larson-Miller (LM) parameter was newly proposed instead of the conventional one with a unique C. The conventional method did not thoroughly match with the creep rupture data, and revealed an overestimation for the prediction of the long-term creep strength. On the other hand, the multi constant method revealed a good agreement with the creep rupture data, and this method was more accurate than the conventional one. The multi constant analysis can be used to accurately predict the long-term creep rupture life of Alloy 617 above 10⁵ h for 1173 K, 1223 K and 1273 K.     

Long-term creep characterization of Gr. 91 steel by modified creep constitutive equations

This paper focuses on the long-term creep characterization of Gr. 91 steel using creep constitutive equations. The models of three such equations, a combination of power-law form and omega model (CPO), a combination of exponential form and omega model (CEO), and a combination of logarithmic form and omega model (CLO), which are described as sum decaying primary creep and accelerating tertiary creep, are proposed. A series of creep rupture data was obtained through creep tests with various applied loads at 600 °C. On the basis of the creep data, a nonlinear least-square fitting (NLSF) analysis was carried out to provide the best fit with the experimental data in optimizing the parameter constants of an individual equation. The results of the NLSF analysis showed that in the lower stress regions of 160 MPa (σ/σ_ys <0.65), the CEO model showed a match with the experimental creep data comparable to those of the CPO and CLO models; however, in the higher stress regions of 160 MPa (σ/σ_y > 0.65), the CPO model showed better agreement than the other two models. It was found that the CEO model was superior to the CPO and CLO models in the modeling of long-term creep curves. Using the CEO model, the long-term creep curves of Gr. 91 steel were numerically characterized, and its creep life was predicted accurately.     

An improved methodology for determining tensile design strengths of Alloy 617

This paper presents an improved methodology for determining high-temperature tensile design strengths of Alloy 617, which is regarded as one of main structural materials for very high temperature reactor (VHTR) system. In establishing time-independent allowable stress values, an existing ASME standard procedure is preliminarily analyzed and their limitations are pointed out. Then, an improved methodology, which has a consistent and quantifiable design margin at low and high temperatures for tensile strengths, is proposed and compared with the ASME method. To find suitable curves of temperature trend to the tensile strength data, three fitting methods are demonstrated, and a statistical technique is adopted for design use. The results will be utilized to reasonably determine the tensile design strengths of Alloy 617 for application in the VHTR system.     

Improvement of long-term creep life extrapolation using a new master curve for Grade 91 steel

This study aims to improve the long-term creep life extrapolation of Grade 91 (Gr. 91) steel using a new master curve of a hyperbolic sine (“sinh”) form. In the master curve for extrapolating creep rupture life, a controversial point of a polynomial form, which has usually been used in time–temperature parametric methods, was preliminarily indicated. The validity of the new master curve was verified. A large body of creep rupture data for Gr. 91 steel was prepared from different sources of available worldwide data at 500 °C to 700 °C for Gr. 91 steel. The material constants of the Larson–Miller, Orr–Sherby–Dorn, and Manson–Harferd parameters were obtained using the polynomial and sinh forms on the basis of the rupture data. Long-term creep rupture life was extrapolated for up to 60 years using the two forms that defined the master curves. Results of long-term creep life extrapolation showed that the master curve of the polynomial form exhibited overestimation due to the divergent curves in the low stress ranges, whereas the master curve of the sinh form revealed goodness in the low stress ranges beyond the experimental data. The proposed sinh form was superior to the polynomial form. The sinh form could be applied to extrapolate the long-term creep rupture life of other heat-resistant steels. The creep rupture stresses predicted by the sinh function were comparable to those of RCC-MRx code.     

Characterization of the Q* parameter for evaluating creep crack growth rate for type 316LN stainless steel

In this study, the Q* parameter was characterized to evaluate the Creep crack growth rate (CCGR) of type 316LN stainless steel. Creep crack growth (CCG) data were obtained by CCG tests under different applied loads at 600°C. An additional CCG test was conducted at 550°C to investigate the possible temperature dependence of the stress intensity factor. An equation using the Q* parameter for evaluating CCGR was proposed, and this parameter was characterized and compared with the typical C* fracture parameter, which is commonly used. The Q* parameter exhibited good linearity of the data, exhibiting no nonlinearity-induced dual value at the early stage. The Q* parameter was suitable for characterizing the CCGR regardless of different applied loads and types of steels. In addition, fracture microstructures near the crack revealed a typical intergranular fracture mode, and this fracture was dominantly propagated along the grain boundary. The cracks were developed by the growth and interlinking of cavities, which were attributed to the precipitates forming along the grain boundary.

     

Microstructural Investigation of Alloy 617 Creep-ruptured at High Temperature in a Helium Environment

A very high temperature reactor （VHTR） is one of the next-generation nuclear reactors chosen by the Generation IV International Forum. A Ni-base superalloy, Alloy 617, is considered as a primary candidate material for an intermediate heat exchanger （IHX） and hot gas duct （HGD） of the VHTR because of the superior creep resistance at a high temperature above 850 ~C. In this study, the microstructures of the specimens creep- ruptured at high temperatures in a helium environment were investigated. The decrease in rupture time was more pronounced with increasing creep temperature. In the specimens crept at 950 ~C, the external Cr-oxide layer of the specimens increased in thickness with increasing creep rupture time, and delaminated after a long-time creep. The high creep stress induced a deep penetration of carbide depletion along the grain boundaries at the early stage of the creep test. The creep temperature enhanced the growth rate of the decarburized zone depth clearly, but the temperature effect on the growth of the external oxide and internal oxide was not well understood as the surface reaction and creep stress affected the microstructures complexly.     

Creep-fatigue crack behaviour of a mod. 9Cr-1Mo steel structure with weldments

An assessment of a creep-fatigue crack initiation and growth for a Mod. 9Cr-1Mo steel structure with weldments has been carried out based on an extended French RCC-MR A16 procedure. The A16 defect assessment guide provides assessment procedures for a creep-fatigue crack initiation and growth for an austenitic stainless steel, but no guidelines are available yet for a Mod. 9Cr-1Mo steel. A creep-fatigue structural test was carried out for a Mod. 9Cr-1Mo specimen with two hours of a hold time at 873 K and various primary nominal stresses. σ _d approach was employed to evaluate a creep-fatigue crack initiation, and an assessment of a creep-fatigue crack growth at a defect has also been carried out. The evaluation results were compared with the observed images from the structural test.     

Comparative study on the high-temperature tensile and creep properties of Alloy 617 base and weld metals

This paper presents a comparative investigation on the high-temperature tensile and creep properties of Alloy 617 base metal (BM) and weld metal (WM) fabricated by a gas tungsten arc weld process. The WM had higher yield strength and lower ultimate tensile strength than the BM does; however, its elongation was significantly lower than that of the BM. The creep curve of the BM and WM was somewhat different from that of typical heat-resistance steel, and did not show a textbook creep. The WM exhibited a longer creep rupture life, lower creep rate, and lower rupture ductility than the BM. However, as the creep rupture time reached approximately 36,800 h, the creep life of the WM was expected to be almost similar to that of the BM; and after 36,800 h, its creep life was expected to be worse than the BM. Loner creep tests is needed to investigate the long-term creep life of the WM. The creep failure mode of the BM and WM was obviously an intergranular cracking of the cavity formation and growth mechanisms, although it was more evident in the WM. The BM had a more ductile fracture surface than the WM.     

A numerical approach to determine creep constants for Time-Temperature Parametric methods

This study dealt with how to mathematically obtain creep constants of C, Q/2.3R, T _a and t _a used in Time-Temperature Parametric (TTP) methods. A number of creep rupture data from 550 °C to 800 °C for Type 316LN stainless steel were collected through a worldwide literature survey and experimental data produced at KAERI. It was found that the creep rupture data revealed a linear relationship when the data were plotted by the semilogarithmic coordinates of log (t _r ) versus stress. On the basis of this linear relationship, the creep constants of C, Q/2.3R, T _a and t _a used in the Larson-Miller (LM), the Orr-Sherby-Dorn (OSD), and the Manson-Haferd (MH) methods were obtained by a new numerical method. This approach can be utilized as a useful method to predict the long-term creep life of Type 316 stainless steel.     

Generation of isochronous stress-strain curves with a nonlinear least square fitting method for modified 9Cr-1Mo steel

This study deals with the generation of isochronous stress-strain curves for modified 9Cr-1Mo steel. Material property data (s_y, D, and m) from tensile tests and strain-time curves from creep tests were obtained at 550 °C. On the basis of the experimental data, the creep curves were characterized by Garofalo’s creep model. The three parameters of P ₁, P ₂ and P ₃ in Garofalo’s model could be optimized properly by a nonlinear least square fitting analysis. The three parameters showed a good stress dependency with a linear relation. However, the P ₃ parameter, which represents the steady state creep rate, exhibited two-slope behavior with different stress exponents at a transient stress of about 250 MPa. The results verify that the long-term creep curves for G91 steel can be modeled by Garofalo’s model with only several short-term creep data. When the modeled creep curves are used, long-term isochronous curves of up to 10⁵ h can be successfully constructed.