Creep Embrittlement Research Articles

Defeating creep embrittlement under high-stress levels through heterogeneous grain architecture in a L12-strengthened multicomponent alloy

L12-strengthened multicomponent alloys exhibit excellent mechanical properties over a wide temperature range. However, defeating the intermediate-temperature creep embrittlement is challenging. Herein, we demonstrated that the creep damage tolerance can be enhanced in a heterostructure consisting of high-density low-angle boundaries and disconnected high-angle grain boundaries. Our results show that the rearrangements of the preexisting dislocations coupled with the interactions between precipitates and various faults can be simultaneously activated in the heterostructured alloy, which helps to accommodate the cumulative strain for a prolonged creep life. This work provides a feasible route to optimize the creep rupture resistance under high-stress levels among precipitation-hardened polycrystalline alloy systems.

Asset management strategy for HAZ cracking caused by sigma-phase and creep embrittlement in 304H stainless steel piping

This study aims to develop an asset management strategy to address weld cracking on 304H austenitic stainless-steel piping welded with 308H filler metal. The 304H NPS 16 outlet line from the primary reformer fired heater to the primary reformer which operates in high temperature service at around 620 °C was the principal case study. A metallurgical evaluation characterized the failure as sigma-phase and creep embrittlement in the heat-affected zone; however, sigma-phase embrittlement was the more predominant damage mechanism. Sigma-phase is the most common intermetallic phase precipitating in austenitic stainless steels. The cracks due to embrittlement were exacerbated by a significant amount of sigma-phase formation, which initiated at the fusion line and propagated at the coarse-grained heat-affected zone which were interconnected with creep voids. Welding procedures were developed using two alternative filler metals, ER16-8-2 and ER19-10H, as a repair option and for new fabrication. The philosophy was to reduce the sigma-phase formation by controlling the heat input; thereby limiting the ferrite; and the probability for embrittlement. The results from the mechanical and metallurgical examination of the welded specimens proved that the ER16-8-2 filler metal possessed superior resistance to sigma-phase formation. Acoustic emission testing and a combination of field metallographic replication and ferrite testing were identified as the most effective predictive technologies to detect the incipient cracking.

High Temperature Creep and Hydrogen Embrittlement Failure of a Steam Trap Bypass Tube

A coal fired power plant with its normal operation temperature of 540oC in which its steam trap bypass tube in that power plant was totally fractured. The aim of this study is to explore the evidence related to the steam trap bypass tube failure and to determine the failure mechanism and the root cause of the failure and to give an appropriate recommendation. From the evidence with considering the contribution factors such as temperature, pressure and environment, a fault analysis was made and it can be concluded that the cause of failure to the steam trap bypass is due to multi causes which consists of creep failure and hydrogen damage.

Study of Creep Damage in a 10.86% Cr Heat Resistant Steel using Synchrotron X-Ray Microtomography

Synchrotron X-ray microtomography(SR-μCT) scans have been carried out on sample coupons extracted from the fracture specimens of a 10.86% Cr heat resistant steel exposed to crep deformation at 873K over stresses of 120, 150, and 180 MPa. The 3D cavitation characteristics in terms of void volume fraction, numbwer density and size distribution as a function of the applied stress has been determined by quantitative analysis of the reconstructed tomograohy slice datasets. The relationship between heterogenous spatial distribution of creep voids and variation in rupture life has been exploited in terms of microstructural sites during the onset of creep embrittlement.

A Damage Mechanics Model for Creep and Oxygen Embrittlement in Metals

This paper first describes a creep damage mechanics model that shows a marked performance for both steady-state creep and tertiary creep, as evidenced by comparisons with creep test results for two metallic materials. Then, using the creep damage mechanics model as a foundation, a framework is presented for treating the phenomenon of stress-accelerated grain-boundary oxygen embrittlement (SAGBO) in metals (e.g. superalloys) at elevated temperatures and under oxidizing environments. This framework establishes a combined creep and oxygen embrittlement damage model that incorporates the effect of oxygen diffusion along grain boundaries and oxidation-accelerated cavitation. The damage model has been applied to round-bar creep tests of a steel alloy that exhibits the SAGBO phenomenon. Predictions of the creep curve in air and its dependence on the bar size have been obtained based on a simplified analysis of the tests. Comparisons between model predictions and test results show that the model can capture the essential features of the SAGBO phenomenon and can adequately describe the creep and oxygen embrittlement behavior of metals such as superalloys.

The role of impurities during creep and superplasticity at very low stresses

It is well documented that impurities play an important role in the deformation and fracture of polycrystalline materials. For example, the results of a number of studies have demonstrated that the presence of a very small of amount of impurities in polycrystalline materials can explain many phenomena such as temper embrittlement in steels, creep embrittlement, and enhancement of ductility in the intermetallic compound Ni3Al. This article reviews the details of two high-temperature deformation phenomena whose characteristics are, according to very recent experimental evidence, influenced or controlled by impurities. The first phenomenon, micrograin superplasticity, deals with the ability of fine-grained materials (d<10 µm, where d is the grain size) to exhibit extensive neck-free elongations during deformation at elevated temperatures above 0.5 Tm, where Tm is the melting point. The second phenomenon, Harper-Dom creep, refers to the anomalous creep behavior of large-grained materials at very low stresses and temperatures near the melting point. It is shown that while these two phenomena are different in terms of the conditions of occurrence and the characteristics of deformation, they share three common features: (1) stresses applied to produce deformation are very small; (2) impurities control the deformation characteristics such as the shape of the creep curve, the value of the stress exponent, and the details of the substructure; and (3) boundaries play a key role during deformation.

The kinetics of creep embrittlement under long-term static loading

The formulation and solution technique for the problem of construction of evolutionary equations of creep embrittlement of metal materials are considered. Embrittlement is regarded as loss of plasticity depending on the time to fracture. Experimental data for high-temperature steels and alloys are systematized and analyzed, and, on this basis, two main classes of materials, those that are subjected to ductile fracture and those that undergo embrittlement, are singled out. The kinetics of embrittlement is considered as regards the strain of instantaneous fracture and obeys the power law. The conditions of change in the nature of fracture are formulated, which allows one to calculate the coordinates of the inflection points on delayed-fracture curves.

ステンレス鋼ＦＣＡＷ溶接金属における再熱割れに関する研究 ２ クリープ破断試験による溶接金属の高温延性の評価

To evaluate the high temperature ductility, which is considered to affect the reheat cracking sensitivity, creep rupture tests with Type 308 stainless steel weld metals made from various commercial welding materials at 823 K-1023 K have been performed. To clarify the effect of bismuth on creep embrittlement, creep rupture test with FCAW weld metal including Type 308, Type 316 and Type 347 containing various bismuth contents have been carried out at 823 K and 973 K.The experimental results indicate that with an increasing Larson-Miller parameter value, the rupture strength of the Type 308 weld metal decreased linearly with decreasing rupture ductility. Among the welding processes using commercial welding materials, the FCAW weld metal showed the poorest rupture ductility. However the bismuth-free FCAW weld metal showed a better ductility compared to the bismuth containing FCAW weld metal. It is concluded that the main factor which causes creep embrittlement in the FCAW weld metal is bismuth which is usually added to the flux for improvement of slag detachability. Also, the Type 316 and Type 347 weld metals were embrittled by bismuth as in the case of the Type 308 weld metal.The characterization of cracks in the creep rupture test specimen indicated that regardless of the bismuth content in the weld metal, the fracture mechanism can be categorized into two types, the transgranular type and the intergranular type. At the higher test temperature and/or the prolonged rupture time, the intergranular type fracture becomes dominant and the rupture ductility decreases. Bismuth is considered to severely affect the grain boundary embrittlement and enhances the degradation of rupture ductility.

Effect of Content of Impurites and PWHT Conditions on Creep Embrittlement Cracking of Cr-Mo Steels

Effect of Content of Impurites and PWHT Conditions on Creep Embrittlement Cracking of Cr-Mo Steels

Influence of prolonged aging on creep embrittlement susceptibility

The influence of prolonged aging on creep embrittlement susceptibility of 1 Cr− 1 2 Mo steel was investigated. Based on the results of notch creep tests, it has been found that the creep life (time to failure) of material subjected to prolonged aging [heat-affected zone (HAZ) material in the as-received state] is increased in the lower stress zone, and thus creep embrittlement susceptibility is improved compared with material in (simulated HAZ material) which has not been subjected to prolonged aging. While the fractures which occurred in materials that had been subjected to prolonged aging were mainly transgranular, those that occurred in material which had not been subjected to prolonged aging were mainly of an intergranular nature. Consequently, it is believed that the improvement in creep embrittlement due to prolonged aging is attributable to reduced transgranular strength and enhanced strength of the grain boundaries.

Creep Embrittlement of Structural Components in Catalytic Reformer Reactor.

During the last ten years, more than 70 incidents of creep embrittlement damage of structural components in catalytic reformer reactors which had been in long-term operation have been reported. Creep embrittlement is the loss of creep ductility in the heat-affected zone of welds or base metal. Creep void density, as related to stress concentration, revealed a critical stress intensity for a catalytic reformer reactor that was actually operated for the last 19 years. There was a threshold Larson-Miller parameter in the range of 19.0×103 to 19.5×103, above which creep embrittlement occurred. Creep crack growth tests were also performed to evaluate crack propagation and remaining life of the components. For the safety of these components, it was necessary to evaluate the creep void density because of the shortage of long-term crack propagation data.

An analysis of the creep embrittlement of Inconel alloy X-750 due to its prior exposure to reduced air pressure at 1150°C

Inconel alloy X-750 test pieces exposed to a reduced air pressure at 1150°C for various times exhibited significant embrittlement during subsequent creep tests in air at 700°C. This embrittlement was characterized by a systematic reduction in time and strain to fracture and an acceleration of the minimum creep rate with increasing time of prior exposure. Earlier work has shown that the near-surface environmental damage in the form of heavily cavitated boundaries and γ-free zones are responsible for this form of embrittlement. The purpose of this investigation is to provide a comprehensive analysis of the effect of the above two forms of damage on the time and strain to fracture and the minimum creep rate. Towards this end, a three-zone creep fracture model has been developed to explain the experimental results.

The role of reverse temper embrittlement on some low and high temperature crack extension processes in low carbon, low alloy steels: A review

The present paper attempts to review the influence or role that reverse temper embrittlement (RTE) plays on the low temperature ( <400° C ) environmentally assisted cracking (EAC) processes in aqueous environments and the high temperature ( > 400° C ) creep dominated crack growth processes prevalent in low carbon, low alloy steels. RTE is a generic problem in iron based alloys and causes a marked reduction in the cohesion strength of grain boundaries due to the segregation of impurity solute atoms. It has been shown that there is a strong possibility that RTE can promote or enhance EAC in low alloy steels (the heat affected zone in weldments being the most susceptible) subjected to aqueous environments at temperatures below about 350°C. At higher operational temperatures, viz. above 400°C, the characteristics of creep embrittlement are discussed and three microstructural features which contributed to this phenomena are highlighted. Proposals aimed at mitigating creep embrittlement processes in low alloy steels are forwarded. In certain steels it was recorded that two embrittlement processes were prevalent at operational temperatures in the range 450–550°C, viz. RTE and an irreversible creep embrittlement which was prevalent when large deformation occurred in service. Indeed both 2·25 Cr1Mo and Cr-Mo-V steels were severely temper embrittled at service temperatures of 430°C while the Cr-Mo-V steels did not exhibit evidence of RTE at higher service temperatures of around 530°C. However, the effects of RTE in promoting creep embrittlement are, as yet, unclear.

Metallographic evidence for the formation of gaseous internal oxides in nickel-base superalloys

Many nickel-base superalloys exhibit creep embrittlement after exposure to oxygen-containing environments at high temperatures. This embrittlement is characterized by a region of profuse grain boundary cavitation immediately below the surface, and four mechanisms have been proposed in the literature to account for it. Since each mechanism has microstructural consequences, these have been checked metallographically. From the results given in this paper it is concluded that, for two nickel-base superalloys, cavitation is nucleated by bubbles of a gaseous internal oxide of carbon.

On the Less Known Effects of Environment and Specimen Geometry on Creep Fracture of Nickel Alloys

Attention has been drawn to the fact that during exposure of nickel to air at elevated temperatures, oxygen diffuses through the grain boundary and reacts with carbon forming gas bubbles of carbon monoxide and/or carbon dioxide. Voids are produced due to the high pressure generated by the gas bubbles which cause creep embrittlement. Alloying nickel with 3.8 weight per cent aluminium, which is a stronger oxide forming element as compared to carbon, could not prevent diffusion of oxygen during exposure of the alloy to air at elevated temperatures. The diffusing oxygen reacts even in the Ni-Al-C system, with carbon and forms gas bubbles of CO and CO/sub 2/ which lead to creep embrittlement. The presence of 15 weight per cent chromium in nickel (Inconel 600 and Inconel alloy X-750), however, plays an interesting role on the kinetics of oxygen diffusion. At temperature 1050 degree Celsius and below, during exposure to air at atmospheric pressure, oxygen can diffuse along the grain boundary and reacts with carbon forming gas bubbles of carbon monoxide, where as at 1120 degree Centigrade and above, the formation of 'barrier' oxide scale, chromium oxide, at the surface prevents diffusion of oxygen. On the other hand, the presence of chromium, as observed in Inconel alloy X-750, does not play an effective role in preventing diffusion of oxygen along the grain boundary in poor vacuum. Circumstantial evidence suggests that diffusing oxygen reacts with carbon and forms gas bubbles of CO with consequential creep embrittlement, reduction in creep life and enhancement in creep rate. It has been shown that exposure of the alloy in poor vacuum has a much more damaging effect on the creep properties than that of an atmospheric pressure. It is, therefore, emphasized that exposure of nickel base superalloys to poor vacuum at elevated temperatures must be avoided; in particular this effect is more pronounced in the alloy with thin section size. It is also shown that even in the absence of oxygen interaction prior to creep testing, section size has considerable influence on the creep properties which is believed to be caused by oxygen interaction during creep deformation. For the same section size, the creep properties may be different in specimens of different geometrices. But for the same ratio of cross-sectional area to perimeter (A/P), rupture lifetime becomes independent of specimen geometry. Failure analysis of an aero engine blade has been presented as a case study to emphasize that while designing a component, utmost care must be excercised to avoid thin sections.

Effect of weld heat input and creep strain on the elevated temperature and crack growth properties of austenitic steels

A potential material class for use at 600°C and more, e.g. for steam turbines with improved thermal efficiency, are austenitic steels. Using these steels with welded joints, it is to be considered that, by superposition of weld residual stresses and service stresses, extensive creep strains – and in the worst case crack formation – can occur locally. To assess the influence of these effects on service behaviour, different material states of CrNi‐steels and Incoloy 800 were investigated with respect to strength, ductility and, especially, to crack and creep crack growth in the temperature range around 600°C. It is shown that creep embrittlement, not microstructural changes as effected by weld heat input, causes heat affected zone (HAZ)‐reheat cracking. Creep embrittlement can be avoided by special design and fabrication rules.

Creep Embrittlement of Low Alloy Steels for High Temperature Equipments

Creep Embrittlement of Low Alloy Steels for High Temperature Equipments

Ｃｒ‐Ｍｏ鋼のクリープぜい化とその評価方法

This investigation was carried out to clarify the cause of cracks which are often detected in the 11/4 Cr- 0.5Mo steel chemical reactor vessel in use. Furthermore, the prevention of such cracks and rapid evaluation of creep embrittlement were also examined.Firstly, appearance of cracks detected was investigated by the light microscope and SEM. The cracks were identified as the typical intergranular fracture initiated and propagated in the coarse grained HAZ adjacent to the fusion line of weldment. Grain boundary segregation of impurities such as Sn, Sb and S was revealed by Auger electron spectroscopy.Secondary, several kinds of test including creep rupture test and high temperature tention test with slow extention rate technique (SERT) were conducted with the synthetic weld HAZ specimens directly sampled from the materials in which cracks were detected. Results of the creep rupture test indicated the creep embrittlernent behavior of specimens clearly by the low creep ductility and the small notch rupture strength ratio. Creep rupture strength of notched specimen coincided with the stress applied to the part of the reactor vessel where cracks were detected. As a result, it was verified that cracks detected were due to the creep embrittlement phenomenon.Thirdly, it was confirmed that reheat treatment (900°C) was available to prevent the creep embrittlement of coarse grained weld HAZ.Finally, SERT test using notched specimen was usefull for the accelerated evaluation of creep embrittlement by taking notice of fracture mode. In addition to high temperature atmosphere, such technique is also available for reappearance of damage caused by high temperature and high pressure hydrogen surroundings.

Scanning Auger investigations of embrittlement phenomena in Cr-Ni and Cr-Mo steels

Temper embrittlement of martensitic Cr-Ni-steels and creep embrittlement of Cr-Mo steels were investigated by scanning Auger spectroscopy. The objective of this work was to determine the chemical composition of the embrittled grain boundaries as a function of thermal and mechanical treatment. It was confirmed that temper embrittlement of Cr-Ni-steel is linked to phosphorus segregation and to a significant enrichment of Cr and C at the grain boundaries. This process can be reversed if a de-embrittlement annealing is applied at temperatures above 600°C. We achieved positive effects at 625 and 650°C even with relatively short tempering times. The creep embrittlement of Cr-Mo steel in the temperature range between 300 and 600°C leads to the promotion of grain boundary cavities. The segregation of trace elements takes place heterogeneously. Phosphorous appears only at the flat grain boundary facets, while S, Sn, Sb and Cu are enriched in the cavities. These elements have an influence on the cavity nucleation and the cavity growth.

Intergranular creep embrittlement by non-soluble impurity: Pb in precipitation hardened Al–Mg–Si alloys

AbstractMinute amounts of Pb induce intergranular embrittlement of precipitation hardened Al–Mg2Si alloys in creep conditions but not in tension and toughness tests. Auger and Rutherford backscattering analyses of impact-fracture surfaces revealed the presence of submonolayer Pb coverages which were first attributed to a segregation of Pb to grain boundaries. However, in view of the contradictions which arose from the two types of results, further experiments using scanning Auger and X-ray microanalysis revealed that in fact Pb was essentially in precipitated form, its solubility being much smaller than expected, and that it spread on the surface by diffusion immediately after fracture. The preferential association between Pb globules and large AlFeSi inclusions favours this process since the latter particles initiate transgranular dimples. The surface analysis results are discussed on the basis of a semiquantitative description of the possible distribution of globules on fracture surfaces. The surface di...