Creep life assessment of an overheated 9Cr-1Mo steel tube

Crude oil heater 9Cr–1Mo steel tubes from a refinery plant were studied, after 24 years of service at nominally 650 °C and 27 MPa, to predict their remanent lives. The investigation included dimensional, hardness and tensile measurements in addition to accelerated stress rupture tests between 650 °C and 700 °C and microstructural examination. Tube specimens were taken from two sections, the overheated side and the side which only saw the nominal operating temperature. The method employed involved the prediction of the increase in temperature with increasing sediment deposition during the operating life times using an FEM model. In addition the predicted temperatures are used to derive appropriate creep properties at relevant temperatures in a 3D pipe FEM creep analysis to predict the pipe deformation rate. All compare well with the actual service exposed pipe measurements and layer deposition. The overheated side revealed a small loss of creep strength in a stress rupture test. A layer of sediment (appr. 10 mm thickness) consisting basically of sintered carbon (coke) spread over the inside of the tube was acting as a thermal barrier causing the temperature to rise above 650 °C. Analysis for the overheated side predicted an upper bound temperature of ≈800 °C and a life of about 50 h suggesting that failure by creep rupture could occur rapidly in the sediment region.     

Creep degradation in welds of Mod.9Cr-1Mo steel

Creep life assessment technology has not yet been developed for Mod.9Cr-1Mo steel welded joints due to the unique degradation and failure mechanisms in the heat affected zone. Nevertheless, there is strong demand from power plant operators for the development of nondestructive damage detection and life assessment technology for weldments of this steel. In this study creep rupture testing using a large-size welded joint was conducted, and creep and damage detection tests were carried out to elucidate the degradation mechanism. Also the microstructure and hardness changes in the heat affected zones of the welds were investigated to clarify the degradation mechanisms of such welds in comparison with base metal during creep. In general, the changes in hardness distribution along the welds were difficult to correlate with the life consumption, although degradation in welds due to creep was successfully detected. Accordingly, a new approach to degradation evaluation and creep life assessment by hardness measurement method for Mod.9Cr-1Mo steel welds is proposed.     

超临界汽轮机9Cr-1Mo汽缸补焊

介绍了超临界汽轮机9Cr-1Mo高压内缸补焊试验研究,制定了合理的工艺,已应用于超临界机组汽缸焊接生产。     

Investigation of ratcheting characteristics of modified 9Cr–1Mo steel by using the Chaboche constitutive model

In this paper, an inelastic constitutive equation of the Chaboche model, which can simulate ratcheting behaviour at high temperatures, is investigated for modified 9Cr–1Mo steel. The plastic modulus, which primarily governs the calculation scheme for plasticity, is derived for the Chaboche model and implemented into a computer program with the radial return algorithm to compute the cyclic backstress components. With the cyclic test data of modified 9Cr–1Mo steel, the Chaboche three-decomposed constitutive parameters of the kinematic hardening rule and the isotropic softening rule, which controls the cyclic softening characteristic of modified 9Cr–1Mo steel, are identified by a simple method. By using the identified constitutive parameters, the ratcheting behaviour of modified 9Cr–1Mo steel is simulated to investigate the effects of mean stress levels at a certain stress amplitude and ratcheting parameter's values. In particular, the progressive deformation instability phenomena due to the cyclic softening characteristics of modified 9Cr–1Mo steel are investigated.     

Thermal cycling of transition joints between modified 9Cr–1Mo steel and Alloy 800 for steam generator application

The effect of accelerated thermal cycling on a joint between modified 9Cr–1Mo steel (Grade 91) and Alloy 800 welded with Inconel¹ 82 and 182 filler material is discussed. This is part of a trimetallic transition joint involving Grade 91–Alloy 800–316LN austenitic stainless steel for steam generator application. It has been shown that, during thermal cycling following the typical post-weld tempering treatment at 760 °C for 2 h, no carbon diffusion occurs from the ferritic steel towards the weld metal. There is, in fact, a hardness increase at the ferritic steel/weld metal interface which is probably a result of work hardening. Carbon migration sets in only after unusually long post-weld heat treatments for 20 and 50 h at 760 °C followed by thermal cycling. Significantly, even under the most severe thermal cycling test conditions imposed, no cracking or oxide notching could be detected, thus demonstrating the superior performance potential of modified 9Cr–1Mo steel as part of a trimetallic configuration.     

Determination of apparent diffusivity of hydrogen in 9Cr-1MoVNbN steel using hot extraction-PEMHS technique

In the present study, apparent diffusivity of hydrogen at different temperatures in 9Cr-1MoVNbN steel was determined using the data obtained from the measurement of hydrogen evolved from specimens charged with hydrogen. Cylindrical specimens of the 9Cr-1MoVNbN steel in normalized condition were charged with hydrogen electrochemically to saturation level. Hot extraction technique was used for collection of hydrogen evolved from the specimens at different temperatures in a chamber. Subsequently, measurement of evolved hydrogen was carried out using a Nafion based proton-exchange-membrane hydrogen sensor (PEMHS). Using these data in the diffusion equation for a finite cylinder, apparent diffusivity of hydrogen in steel was determined at temperatures in the range of 25–400 °C. Hydrogen lost from the specimens prior to start of the collection of hydrogen evolved was estimated. Apparent diffusivity data, earlier obtained, were revised by considering this lost hydrogen in the estimation. Apparent diffusivities calculated using this method agreed well with the values reported in the literature.     

A dynamic fracture-toughness-based reference temperature characterization for the weld metals of modified 9Cr–1Mo steel in two different weld positions

The ductile–brittle transition temperatures (DBTT) of modified 9Cr–1Mo steel welds from two different weld positions, namely down hand (1G) and overhead (4G), have been evaluated and compared using the ASTM E 1921-05 based reference temperature (T₀) approach, but under dynamic-loading conditions. The reference temperatures thus obtained, termed as T₀^dy to signify the dynamic condition, have been found to be higher for the 4G position than the 1G position. A scanning electron microscopic study of the fracture surfaces close to the fatigue crack front reveals that while lath boundary fracture is the dominant mechanism for brittle crack initiation in both the welds, the higher T₀^dy value is linked to the higher concentration of probable crack initiation sites in the 4G position. The experimentally obtained Weibull slope in both the welds has been found to be different (7.526 and 7.205 for the 1G and 4G positions, respectively) from the ‘fixed slope of 4’ assumption, used in ASTM E 1921-05. However, in the present instance, the ‘fixed Weibull slope of 4’ concept yields more conservative T₀^dy values compared to those obtained using the experimentally determined Weibull slope. For these welds, the RT_NDT-based ASME K_IR curve proved to be ultra-conservative compared to the realistic dynamic fracture toughness variation described by the Master Curve indexed with T₀^dy.     

Prediction of Type IV creep failure of a seam-welded mod. 9Cr-1Mo elbow based on microscopic damage simulation

Utilising the random-fracture-resistance model of grain boundaries, micro-macro combined creep damage simulation was applied to the prediction of the distribution of small defects in the FGHAZ (fine-grained heat-affected zone) of longitudinal welds in an actual-size elbow of modified 9Cr-1Mo (9Cr-1MoVNb) steel subject to internal pressure at 923 K. Based on the simulation results, a prediction scheme for the final rupture life of welds was considered using the damage mechanics concept together with effective stress. The applicability of nonlinear fracture mechanics was also discussed, assuming the initial crack length determined from the microscopic simulation results. The results thus obtained are summarized as follows: As the simulation results showed, the peaks of small defect density in the subsurface could be predicted, corresponding well with the observed results. Final failure life prediction based on the damage mechanics concept was found to be applicable, by considering both the final failure surface connecting the weakest grain boundaries and the effective stress against this surface. The fracture mechanics approach was also found applicable when assuming the initial crack length from the high peaks of the simulated small defects in the last stage of creep life.     

Creep damage evaluation of 9Cr–1Mo–V–Nb steel welded joints showing Type IV fracture

By conducting long-term creep rupture tests for 9Cr–1Mo–V–Nb (P91) steel welded joints, creep rupture properties and microstructures were examined. Creep rupture tests were conducted at three temperatures of 823, 873 and 923 K, under applied stresses of 160–230, 80–130, and 40–80 MPa, respectively. The rupture locations were found to shift from the weld metal at the higher stress condition to the fine-grained HAZ adjacent to the base metal at lower stress conditions at 873 and 923 K. The relationship between microstructural changes and crack nucleation site and propagation path was clarified. A remarkable decrease of dislocation density and growth of precipitates of M₂₃C₆ and Laves phase during creep was observed in the vicinity of the fine-grained HAZ adjacent to the base metal for the Type IV fractured welded joint specimen. The stress–strain distribution in the welded joint was investigated by the finite element method (FEM) using creep data of the simulated HAZ specimen. It was found that the observed crack initiation site and crack growth path coincided better with the distribution of the stress triaxiality factor than that of the equivalent creep strain.     

Evaluation of creep damage in heat affected zone of thick welded joint for Mod.9Cr–1Mo steel

Mod.9Cr–1Mo steel has been used for boiler components in ultra-supercritical (USC) thermal power plants. The creep strength of welded joint of this steel decreases due to the formation of Type IV cracking in heat affected zone (HAZ) at higher temperatures. The present paper aims to clarify the damage processes and mechanisms of the welded joint for Mod.9Cr–1Mo steel. Long-term creep tests of base metal, welded joint and simulated fine- grained HAZ were conducted at 550, 600 and 650 °C. Creep tests using thick plate welded joint specimen were interrupted at several time steps, and evolutions and distributions of creep damages were measured quantitatively using laser microscope. It is found that creep voids initiate at early stage of creep life (0.2 of life), the number of creep voids increases until 0.7 of life, and then voids coalesced into the macro crack at the later stage of life (0.8 of life). Creep damages concentrate mostly at a quarter depths of the plate thickness within the fine-grained HAZ of the present welded joint. The experimental creep damage distributions were compared with the computed results by using the FEM analysis. Both creep strain concentration and high stress triaxiality in fine-grained HAZ of welded joint are considered to accelerate the creep void formation and growth.