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.     

Nickel-base superalloys for USC power plants

The advanced ultra-supercritical power plants of the future will utilize steam pressures and temperatures that are too high for traditional ferritic steels,thus requiring austenitic materials.Older nickel-base superalloys such as 263 and 617 were initially evaluated under the European THERMIE project beginning in the 1990s.An entirely new age-hardened alloy 740 which possesses exceptional fireside corrosion resistance and creep strength was also developed for boiler tubing capable of serving at 700C.Subsequently,interest in the USA considered other product forms such as steam header piping and steam turbine forgings for service as high as 760C.A more stable and weldable alloy version now called 740H was developed to meet these more demanding conditions.This paper summarizes the current status of work on alloys 740 and 740H.     

Creep rupture strength of tungsten-alloyed 9-12 % Cr steels for piping in power plants

The creep rupture strengths of the three tungsten-alloyed 9-12% Cr-steels E 911, NF 616 and HCM 12A are discussed. It is shown that Larson-Miller assessments may lead to an overestimation of the long-term creep rupture strength because of microstructural changes. The presented assessments indicate that all three alloys will have similar creep rupture strengths in the range of 110-120 MPa for 10⁵ h and 600°C. To obtain reliable estimates for creep rupture strength, however, further long-term tests are needed. In any case, the tungsten-alloyed steels will enable the main steam temperature to be raised by about 20°C over that possible with P 91.     

Structure and elevated temperature properties of carbon-free ferritic alloys strengthened by a laves phase

A Laves phase, Fe₂Ta, was utilized to obtain good elevated temperature properties in a carbon-free iron alloy containing 1 at. pct Ta and 7 at. pct Cr. Room temperature embrittlement resulting from the precipitation of the Laves phase at grain boundaries was overcome by spheroidizing the precipitate. This was accomplished by thermally cycling the alloys through theα →γ transformation. The short-time yield strength of the alloys decreased very slowly with increase in test temperature up to 600°C, but above this temperature, the strength decreased rapidly. Results of constant load creep and stress rupture tests conducted at several temperatures and stresses indicated that the rupture and creep strengths of spheroidized 1 Ta−7 Cr alloy were higher than those of several commercial steels containing chromium and/or molybdenum carbides but lower than those of steels containing substantial amounts of tungsten and vanadium. When molybdenum was added to the base FeTa-Cr alloy, the rupture and creep strengths were considerably increased. Formerly with Lawrence Berkeley Laboratory.     

Structure and elevated temperature properties of carbon-free ferritic alloys strengthened by a laves phase

A Laves phase, Fe₂Ta, was utilized to obtain good elevated temperature properties in a carbon-free iron alloy containing 1 at. pct Ta and 7 at. pct Cr. Room temperature embrittlement resulting from the precipitation of the Laves phase at grain boundaries was overcome by spheroidizing the precipitate. This was accomplished by thermally cycling the alloys through the α→γ transformation. The short-time yield strength of the alloys decreased very slowly with increase in test temperature up to 600°C, but above this temperature, the strength decreased rapidly. Results of constant load creep and stress rupture tests conducted at several temperatures and stresses indicated that the rupture and creep strengths of spheroidized 1 Ta-7 Cr alloy were higher than those of several commercial steels containing chromium and/or molybdenum carbides but lower than those of steels containing substantial amounts of tungsten and vanadium. When molybdenum was added to the base Fe-Ta-Cr alloy, the rupture and creep strengths were considerably increased. M. Dilip Bhandarkar, formerly with Lawrence Berkeley Laboratory.     

Studies on Creep Deformation and Rupture Behavior of 316LN SS Multi-Pass Weld Joints Fabricated with Two Different Electrode Sizes

Effect of electrode size on creep deformation and rupture behavior has been assessed by carrying out creep tests at 923 K (650 °C) over the stress range 140 to 225 MPa on 316LN stainless steel weld joints fabricated employing 2.5 and 4 mm diameter electrodes. The multi-pass welding technique not only changes the morphology of delta ferrite from vermicular to globular in the previous weld bead region near to the weld bead interface, but also subjects the region to thermo-mechanical heat treatment to generate appreciable strength gradient. Electron backscatter diffraction analysis revealed significant localized strain gradients in regions adjoining the weld pass interface for the joint fabricated with large electrode size. Larger electrode diameter joint exhibited higher creep rupture strength than the smaller diameter electrode joint. However, both the joints had lower creep rupture strength than the base metal. Failure in the joints was associated with microstructural instability in the fusion zone, and the vermicular delta ferrite zone was more prone to creep cavitation. Larger electrode diameter joint was found to be more resistant to failure caused by creep cavitation than the smaller diameter electrode joint. This has been attributed to the larger strength gradient between the beads and significant separation between the cavity prone vermicular delta ferrite zones which hindered the cavity growth. Close proximity of cavitated zones in smaller electrode joint facilitated their faster coalescence leading to more reduction in creep rupture strength. Failure location in the joints was found to depend on the electrode size and applied stress. The change in failure location has been assessed on performing finite element analysis of stress distribution across the joint on incorporating tensile and creep strengths of different constituents of joints, estimated by ball indentation and impression creep testing techniques.     

Microstructure Evolution During Short Term Creep of 9Cr–0.5Mo–1.8W Steel

P92 steel (9Cr–0.5Mo–1.8W) was subjected to a heat treatment of 1050 °C/30 min/air cooling/780 °C/120 min/air cooling followed by 1080 °C/30 min/air cooling/740 °C/60 min/air cooling to obtain tempered martensite microstructure, for better creep strength. Stress rupture tests carried at 600 °C in the range of 250–350 MPa resulted in rupture times in the range of 200–3000 h. Straight line plot of stress rupture curve indicated no major change in deformation mechanism. Coarsening of precipitates and substructure development were the main reasons for microstructure degradation, consequently leading to reduced hardness of the sample. Gauge and grip portions of the same sample were sectioned to comparatively evaluate the effects of stress and aging. Gauge portion of 3000 h sample showed considerable change in the microstructure in terms of boundary migration, while that of grip portion hardly evolved. The ruptured samples exhibited predominantly ductile fracture with elongated cavities at higher rupture times.     

Assessment of Creep Deformation,Damage, and Rupture Life of 304HCu Austenitic Stainless Steel Under Multiaxial State of Stress

The role of the multiaxial state of stress on creep deformation and rupture behavior of 304HCu austenitic stainless steel was assessed by performing creep rupture tests on both smooth and notched specimens of the steel. The multiaxial state of stress was introduced by incorporating circumferential U-notches of different root radii ranging from 0.25 to 5.00 mm on the smooth specimens of the steel. Creep tests were carried out at 973 K over the stress range of 140 to 220 MPa. In the presence of notch, the creep rupture strength of the steel was found to increase with the associated decrease in rupture ductility. Over the investigated stress range and notch sharpness, the strengthening was found to increase drastically with notch sharpness and tended toward saturation. The fractographic studies revealed the mixed mode of failure consisting of transgranular dimples and intergranular creep cavitation for shallow notches, whereas the failure was predominantly intergranular for relatively sharper notches. Detailed finite element analysis of stress distribution across the notch throat plane on creep exposure was carried out to assess the creep failure of the material in the presence of notch. The reduction in von-Mises stress across the notch throat plane, which was greater for sharper notches, increased the creep rupture strength of the material. The variation in fracture behavior of the material in the presence of notch was elucidated based on the von-Mises, maximum principal, and hydrostatic stresses. Electron backscatter diffraction analysis of creep strain distribution across the notch revealed localized creep straining at the notch root for sharper notches. A master curve for predicting creep rupture life under the multiaxial state of stress was generated considering the representative stress having contributions from both the von-Mises and principal stress components of the stress field in the notch throat plane. Rupture ductility was also predicted based on the multiaxial state of stress.     

Influence of Prior Fatigue Cycling on Creep Behavior of Reduced Activation Ferritic-Martensitic Steel

Creep tests were carried out at 823 K (550 °C) and 210 MPa on Reduced Activation Ferritic-Martensitic (RAFM) steel which was subjected to different extents of prior fatigue exposure at 823 K at a strain amplitude of ±0.6 pct to assess the effect of prior fatigue exposure on creep behavior. Extensive cyclic softening that characterized the fatigue damage was found to be immensely deleterious for creep strength of the tempered martensitic steel. Creep rupture life was reduced to 60 pct of that of the virgin steel when the steel was exposed to as low as 1 pct of fatigue life. However, creep life saturated after fatigue exposure of 40 pct. Increase in minimum creep rate and decrease in creep rupture ductility with a saturating trend were observed with prior fatigue exposures. To substantiate these findings, detailed transmission electron microscopy studies were carried out on the steel. With fatigue exposures, extensive recovery of martensitic-lath structure was distinctly observed which supported the cyclic softening behavior that was introduced due to prior fatigue. Consequently, prior fatigue exposures were considered responsible for decrease in creep ductility and associated reduction in the creep rupture strength.     

The effect of helium environment on the creep rupture properties of Inconel 617 at 1000°C

The effect of oxygen contents in helium and in vacuum on the creep rupture properties of Inconel 617 has been investigated at 1000°C under the stress of 3.5 Kg/mm². The main results are as follows. 1) The creep rupture properties in 99.9999 pct and 99.995 pct helium and in high vacuum are almost the same as those in air. 2) The oxygen in helium causes a remarkable decrease of the creep rupture time and similar effect of oxygen is also found in the controlled pressure of air. 3) The effect of oxygen on the creep rupture properties is attributable to the decarburization which tends to decrease the high temperature strength.     

Small Punch Creep Studies for Optimization of Nitrogen Content in 316LN SS for Enhanced Creep Resistance

Small punch creep (SPC) studies have been carried out to evaluate the creep properties of 316LN stainless steel (SS) at 923 K (650 °C) at various stress levels. The results have been compared with uniaxial creep rupture data obtained from conventional creep tests. The minimum deflection rate was found to obey Norton power law. SPC rupture life was correlated with uniaxial creep rupture life. The influence of nitrogen content on the creep rupture properties of 316LN SS was investigated in the range of 0.07 to 0.14 wt pct. SPC rupture life increased and the minimum deflection rate decreased with the increase in nitrogen content. The trends were found to be in agreement with the results obtained from uniaxial creep rupture tests. These studies have established that SPC is a fast and reliable technique to screen creep properties of different experimental heats of materials for optimizing the chemical composition for developing creep-resistant materials.     

Threshold Stress Creep Behavior of Alloy 617 at Intermediate Temperatures

Creep of Alloy 617, a solid solution Ni-Cr-Mo alloy, was studied in the temperature range of 1023 K to 1273 K (750 °C to 1000 °C). Typical power-law creep behavior with a stress exponent of approximately 5 is observed at temperatures from 1073 K to 1273 K (800 °C to 1000 °C). Creep at 1023 K (750 °C), however, exhibits threshold stress behavior coinciding with the temperature at which a low volume fraction of ordered coherent γ′ precipitates forms. The threshold stress is determined experimentally to be around 70 MPa at 1023 K (750 °C) and is verified to be near zero at 1173 K (900 °C)—temperatures directly correlating to the formation and dissolution of γ′ precipitates, respectively. The γ′ precipitates provide an obstacle to continued dislocation motion and result in the presence of a threshold stress. TEM analysis of specimens crept at 1023 K (750 °C) to various strains, and modeling of stresses necessary for γ′ precipitate dislocation bypass, suggests that the climb of dislocations around the γ′ precipitates is the controlling factor for continued deformation at the end of primary creep and into the tertiary creep regime. As creep deformation proceeds at an applied stress of 121 MPa and the precipitates coarsen, the stress required for Orowan bowing is reached and this mechanism becomes active. At the minimum creep rate at an applied stress of 145 MPa, the finer precipitate size results in higher Orowan bowing stresses and the creep deformation is dominated by the climb of dislocations around the γ′ precipitates.     

Stress and temperature dependence of creep in Alloy 600 in primary water

The stress and temperature dependence of creep of commercial nickel-base Alloy 600 was investigated through constant load, step-load, and step-temperature creep tests in deaerated primary water containing 40 to 60 cc/kg hydrogen. To analyze creep rates for Alloy 600 in the mill-annealed (MA) condition, effective stresses were estimated using applied stresses and instantaneous strains. The apparent activation area was determined to be 7b ² by the multiple regression analysis of creep rates. The apparent activation energy for creep has a weak stress dependence and was determined to lie between 188 and 281 kJ/mole for the effective stress range of 117 to 232 MPa. Creep rates were better correlated with effective stress than applied stress and the stress exponent of Alloy 600 MA was determined to be 2.2 at 337 °C and 5.1 at 360 °C. The magnitudes of the stress exponent, activation energy, and activation area can be interpreted to support a creep mechanism controlled by dislocation-climb and nonconservative motion of jogs in commercial Alloy 600 MA. The activation area agreed with that determined from carbon in solution, implying thermally activated dislocation glide as another possible creep mechanism.     

中国超超临界电站锅炉关键材料用钢及合金的研究现状

 综述了近几年国内用于超超临界火电机组锅炉的马氏体耐热钢、奥氏体耐热钢、镍基耐热合金的研究情况。国内学者们对马氏体和奥氏体耐热钢高温强化机理和生产工艺进行了系统研究,并实现了马氏体/铁素体耐热钢T/P91、T/P92与奥氏体耐热钢S31042、S30432、TP347HFG的国产化,对新型9%Cr马氏体耐热钢展开了研究。对镍基合金的投入和研究还不够,研究内容不够系统,但表现出较好的势头,学者们正对Inconel 740、Alloy 617及其改型合金和Alloy 263合金进行较广泛的研究。对用于超超临界火电机组关键材料的研究提出了建议,展望了国内耐热钢及合金的研究趋势。     

超超临界火电机组用关键锅炉钢性能分析

对T/P92、T/P122、S30432(Super 304H)、S31042(HR3C)、9Cr3W3Co等先进锅炉钢的性能进行了综合评述,对比分析了这几种钢管及其焊接接头的持久强度、抗腐蚀性能和常规力学性能数据,采用等温线法和Larson-Miller法外推了这些锅炉钢的10万h持久强度,探讨了这几种钢管许用应力的设定及在工程设计和应用的问题。     

Accelerated Stress Corrosion Crack Initiation of Alloys 600 and 690 in Hydrogenated Supercritical Water

The objective of this study is to determine whether stress corrosion crack initiation of Alloys 600 and 690 occurs by the same mechanism in subcritical and supercritical water. Tensile bars of Alloys 690 and 600 were strained in constant extension rate tensile experiments in hydrogenated subcritical and supercritical water from 593 K to 723 K (320 °C to 450 °C), and the crack initiation behavior was characterized by high-resolution electron microscopy. Intergranular cracking was observed across the entire temperature range, and the morphology, structure, composition, and temperature dependence of initiated cracks in Alloy 690 were consistent between hydrogenated subcritical and supercritical water. Crack initiation of Alloy 600 followed an Arrhenius relationship and did not exhibit a discontinuity or change in slope after crossing the critical temperature. The measured activation energy was 121 ± 13 kJ/mol. Stress corrosion crack initiation in Alloy 690 was fit with a single activation energy of 92 ± 12 kJ/mol across the entire temperature range. Cracks were observed to propagate along grain boundaries adjacent to chromium-depleted metal, with Cr₂O₃ observed ahead of crack tips. All measures of the SCC behavior indicate that the mechanism for stress corrosion crack initiation of Alloy 600 and Alloy 690 is consistent between hydrogenated subcritical and supercritical water.     

Effect of Constraint on Creep Behavior of 9Cr-1Mo Steel

The effect of constraint on creep rupture behavior of 9Cr-1Mo steel has been investigated. The constraint was introduced by incorporating a circumferential U-notch in a plain cylindrical creep specimen of 5 mm diameter. The degree of constraint was increased by decreasing the notch root radius from 5 to 0.25 mm. Creep tests were conducted on plain and notched specimens at stresses in the range of 110 to 210 MPa at 873 K (600 °C). The creep rupture life of the steel was found to increase under constrained conditions, which increased with the increase in degree of constraint and applied stress, and tended to saturate at a higher degree of constraint. The creep rupture ductility (pct reduction in area) of the steel was found to be lower under constrained conditions. The decrease in creep ductility was more pronounced at a higher degree of constraint and lower applied stresses. Scanning electron microscopic studies revealed a change in fracture behavior with stress and degree of constraint. The fracture surface appearance for relatively lower constrained specimens at higher stresses was predominantly transgranular dimple. Creep cavitation-induced intergranular brittle fracture near the notch root was observed for specimens having a higher degree of constraint at relatively lower stresses. The creep rupture life of the steel under constrained conditions has been predicted based on the estimation of damage evolution by continuum damage mechanics coupled with finite element analysis of the triaxial state of stress across the notch. It was found that the creep rupture life of the steel under constrained conditions was predominantly governed by the von-Mises stress and the principal stress became progressively important with increase in the degree of constraint and decrease in applied stress.     

The effect of Nb,V, N and Al on the creep rupture strength of 9–12 % Cr steel

The creep resistance of advanced chromium steels can be significantly increased due to precipitation of very small particles of vanadium nitride VN. The solubility and precipitation of VN, Nb(C,N) and AIN in austenite and ferrite was analysed using relevant solubility products. The calculated values of nitrogen in solid solution were used for assessment of creep rupture strength of chromium steel (mean considered chemical composition, mass contents in %: 0.18 C; 10.5 Cr; 1.0 Mo; 0.2 V; 0.07 Nb; 0.05 N; 0.01 Al). Increasing N mass contents from 0.03 to 0.07 % leads to increasing creep rupture strength in 100 000 h at 600°C of about 60 %. Lowering AI mass contents from 0.045 to 0.005 % produces higher creep rupture strength of about 30 %.