Analysis of the tube-sheet cracking in slurry oil steam generators

Tube-sheet cracking is a severe problem in the oil refinery industry with the consequences of shortened service life and increased costs. In this study, the mechanisms of the tube-sheet cracking in slurry oil steam generators have been investigated using optical and scanning electron microscopy with energy dispersive spectrometry. It has been found that the cracks always occurred in the shortest tube–tube ligaments. All the cracks initiated near the expansion joints about 15 mm away from the oil-side surface where stress concentration existed. It was concluded that stress corrosion cracking occurred in the tube sheets due to the high-temperature (200–242 °C) and high-pressure (3.5 MPa) water, rather than the slurry oil containing elemental sulfur and chlorine.     

Failure analysis of heat exchanger tubes

Primary waste heat exchanger tubes of material ASTM A213 grade T11 failed after operation of only three and a half months. The heat exchanger was of the bayonet type with boiler water inside the tubes and secondary reformer outlet process gas at the shell side. The heat exchanger environment was rich in hydrogen, carbon monoxide and nitrogen. The temperature of the process gases was 960 °C and the heat exchanger was producing steam at a temperature of 306 °C and a pressure of 1500 psig. The failed, used and new heat exchanger tubes were subjected to stereo/optical microscopy, chemical analysis and hardness testing. The cause of the failure was thoroughly investigated using optical/scanning electron microscope equipped with energy dispersive spectrometer. The study revealed that the material was exposed to thermal cycling and excessive local heating. The same was also confirmed by simulated experimentation. These conditions lead to thermal fatigue of the material with consequent failure.     

Using the HELIOS facility for assessment of bundle-jacket thermal coupling in a CICC

In a Cable In Conduit Conductor (CICC) cooled by forced circulation of supercritical helium, the heat exchange in the bundle region can play a significant role for conductor safe operation, while remaining a quite uncertain parameter. Heat exchange between bundle and jacket depends on the relative contributions of convective heat transfer due to the helium flow inside the bundle and of thermal resistance due to the wrappings between the cable and the conduit.In order to qualify this thermal coupling at realistic operating conditions, a dedicated experiment on a 1.2 m sample of ITER Toroidal Field (TF) dummy conductor was designed and performed in the HELIOS test facility at CEA Grenoble. Several methods were envisaged, and the choice was made to assess bundle-jacket heat transfer coefficient by measuring the temperature of a solid copper cylinder inserted over the conductor jacket and submitted to heat deposition on its outer surface.The mock-up was manufactured and tested in spring 2015. Bundle-jacket heat transfer coefficient was found in the range 300–500 W m⁻² K⁻¹. Results analysis suggests that the order of magnitude of convective heat transfer coefficient inside bundle is closer to Colburn–Reynolds analogy than to Dittus–Boelter correlation, and that bundle-jacket thermal coupling is mainly limited by thermal resistance due to wrappings. A model based on an equivalent layer of stagnant helium between wraps and jacket was proposed and showed a good consistency with the experiment, with relevant values for the helium layer thickness.     

Effect of stress ratio on fatigue lifetime of high Nb containing TiAl alloy at elevated temperature

The effect of stress ratio (R) on fatigue lifetime of a cast Ti–45Al–8.0Nb–0.2W–0.2B–0.1Y (at.%) alloy was investigated at 750 °C. Fatigue tests with various stress ratios ranging from 0.1 to 1 were performed using a mini servo-hydraulic fatigue machine inside a chamber of scanning electron microscope (SEM). Fatigue crack initiation and propagation behavior was studied by in situ SEM observation and fatigue fracture mode was examined by fracture surface analysis. It is found that fatigue lifetime shows a reversed S-type curve with the increase of stress ratio. At R ranging from 0.1 to 0.4, creep–fatigue interaction dominates the fatigue lifetime and the fatigue lifetime reaches its minimum value at R = 0.3. At R ranging from 0.4 to 1, creep damage dominates the fatigue lifetime and the fatigue lifetime exhibits inverse proportional relation with R. Meanwhile, with the increase of stress ratio, the fatigue crack initiation sites transform from lamellar interface at R = 0.1, to lamellar interface and colony boundary at R = 0.3, and to lamellar colony boundary at R = 0.5. Accordingly, the fatigue fracture mode transforms from transgranular cracking, to transgranular and intergranular cracking, and to intergranular cracking.     

Analysis of material degradation and life assessment of 25Cr–38Ni–Mo–Ti wrought alloy steel (HPM) for cracking tubes in an ethylene plant

Radiant tubes made of wrought 25Cr–38Ni–Mo–Ti alloy steel (HPM) have been in-service for 76,500 h as cracking tubes in an ethylene plant and they are expected to provide reliable service for 100,000 h (11.4 years) or more. During service, the tube inner surfaces were operated at temperature in the range of 820–835 °C within which thermal cracking process occurred. These aged tubes were assessed to ensure continued safe operation. The assessment of material degradation was carried out using optical microscopy, scanning electron microscopy (SEM) in combination with energy dispersive X-ray (EDX) analysis, X-ray powder diffraction (XRD) analysis, Vickers microhardness measurement and stress rupture test to obtain stress–Larson–Miller parameter (LMP) curves for remaining life prediction. Results showed that microstructural degradation was observed at the inner surface of the radiant tubes marked by the damage of protective oxide film containing Cr₂O₃, Fe₂O₃ and SiO₂. Once this film was removed, carburization occurred and free C atoms involved during cracking of ethylene easily penetrated along austenitic grain boundaries. In addition, carbon diffusion into the tube metal seemed to promote precipitation of Cr₂₃C₆ at grain boundaries and within the grains resulting in a sharp increase in hardness. The outer surface of the radiant tubes, on the other hand, was exposed to higher temperature, typically 1040–1100 °C during operation and creep damage seemed to be the main cause of material degradation. Based on stress rupture test, the remaining life of the radiant tubes is expected to be 21,107 h (2.4 years) consistent with the design life. In the present investigation, factors affecting creep are discussed.     

Stress corrosion cracking of stainless steel pipes for Methyl-Methacrylate process plants

In a Methyl Methacrylate (MMA) plant, tree-like transgranular cracks were found near the weld of a pipe that had been used for transferring MMA material at 110 °C and 0.77 kg/cm². The pipe was made of ASTM A312 TP304 stainless steel.In this study, it was shown that the failure was due to the stress corrosion cracking (SCC) caused by the chloride that remained in the pipe. Corrosion pitting occurred on the inside surface of the pipe. The stress corrosion cracking started from the pits and grew out through the thickness. Concentrated chloride was found in the deposit stuck to the pipe in addition to the pre-process MMA materials. Many work-hardened grains were observed in the area of SCC, providing the evidence of high residual stress due to welding, which could serve as the driving force for the SCC. Recommendations are made for preventing further failure due to SCC in such cases.     

The crack development on the micro- and mesoscopic scale during the pyrolysis of carbon fibre reinforced plastics to carbon/carbon composites

Investigations were conducted to get an insight into the crack development during the carbonization of CFRP components and to understand the mechanisms ruling this cracking. It was found that the major crack types are micro-cracks caused by fibre–matrix-debonding, transversal cracks and partial delaminations. Cracking starts with the onset of pyrolysis as the reaction gases are captured in the still compact material leading to internal pressures and consequently to a channel network. At 490 °C (in the case of a heating rate of 10 K/min) the first micro-cracks evolve due to fibre–matrix-debonding resulting in a homogenisation of the stress distribution. Driven by the high shrinkage in this temperature regime the initiation of transversal cracking takes place soon afterwards at 515 °C resulting in a steep increase in the transversal crack density up to approximately 550 °C. Partial delaminations develop from 520 °C onwards as the crack deflection of the transversal cracks. The development of the latter interrupts the stress transfer between warp and weft fibres, so that the further cracking is mainly driven by local stresses inside the C/C segments between the transversal cracks. Thus the further cracking is dominated by fibre–matrix-debonding occurring in all sizes from the submicro- to the microscopic scale. Although the coarse crack pattern mainly develops up to approximately 570 °C, fibre–matrix-debonding at higher temperatures and especially during cool down plays the major role in terms of crack activity as monitored by acoustic emission.     

Mechanical test on the ITER TF jacket

This paper focuses on mechanical tests on the ITER TF jacket 316LN stainless steel material. During manufacture the conductor will be compacted, spooled and aged at approximately 650 °C after cable insertion. Therefore, sample jackets were prepared under compaction, stretching and annealing in order to simulate the manufacturing process of TF coils. The mechanical properties of the given material were measured at 4.2 K and 300 K. Young’s modulus, yield strength (0.2% offset) and elongation are reported. SEM images showed that the TF jacket had more pronounced ductile behavior. The elongation at failure is more than 30% at cryogenic temperature, showing that the TF jacket has high mechanical performance. It is concluded that the results complies with ITER requirements.     

Evaluation of stress corrosion cracking and hydrogen embrittlement in an API grade steel

Stress corrosion cracking (SCC) and hydrogen embrittlement (HE) of pipeline steels in contact with soil was investigated. Different soils were prepared in order to determine their physical, chemical and bacteriological characteristics. Slow strain rate testing was carried out by using aqueous extracts from soil samples and NS4 standard solution. Stress vs. strain curves of API 5L grade X46 steel were obtained at different electrode potentials (E_corr, 100 mV below E_corr and 300 mV below E_corr) with 9 × 10⁻⁶ s⁻¹ and 9 × 10⁻⁷ s⁻¹ strain rate. In addition, the hydrogen permeation tests were carried out in order to evaluate the susceptibility of hydrogen penetrates into theses steels. The results demonstrated the incidence of cracking and their dependence on the potential imposed. In that case, cracking occurred by stress corrosion cracking (SCC) and the hydrogen embrittlement (HE) had an important contribution to cracking initiation and propagation. Cracking morphology was similar to the SCC reported on field condition where transgranular cracking were detected in a pipeline collapsed by land creeping. It was important to point out that even under cathodic potentials the material showed the incidence of secondary cracking and a significant reduction of ductility.     

Behavior and failure of uniformly hydrided Zircaloy-4 fuel claddings between 25 °C and 480 °C under various stress states,including RIA loading conditions

The anisotropic plastic behavior and the fracture of as-received and hydrided Cold-Worked Stress Relieved Zircaloy-4 cladding tubes are investigated under thermal–mechanical loading conditions representative of Pellet–Clad Mechanical Interaction during Reactivity Initiated Accidents in Pressurized Water Reactors. In order to study the combined effects of temperature, hydrogen content, loading direction and stress state, Axial Tensile, Hoop Tensile, Expansion Due to Compression and hoop Plane Strain Tensile tests are performed at room temperature, 350 °C and 480 °C on the material containing various hydrogen contents up to 1200 wt. ppm (hydrides are circumferential and homogeneously distributed). These tests are combined with digital image correlation and metallographic and fractographic observations at different scales. The flow stress of the material decreases with increasing temperature. The material is either strengthened or softened by hydrogen depending on temperature and hydrogen content. Plastic anisotropy depends on temperature but not on hydrogen content. The ductility of the material decreases with increasing hydrogen content at room temperature due to damage nucleation by hydride cracking. The plastic strain that leads to hydride fracture at room temperature decreases with increasing hydrogen content. The influence of stress triaxiality on hydride cracking is negligible in the studied range. The influence of hydrogen on material ductility is negligible at 350 °C and 480 °C since hydrides do not crack at these temperatures. The ductility of the material increases with increasing temperature. The evolution of material ductility is associated with a change in both the macroscopic fracture mode of the specimens and the microscopic failure mechanisms.     

Analysis of the sealing performance and creep behavior of the inner casing of a 1000 MW supercritical steam turbine under bolt relaxation

The creep behavior and sealing performance of the inner casing of a 1000 MW supercritical steam turbine were investigated during 200,000 h of steady operation at high temperatures. The influence of the stress relaxation of bolts on creep behavior and sealing performance was specifically demonstrated. A constitutive creep model based on continuum damage mechanics and a multiaxial creep strain formula was used to describe the stress–strain behavior and calculate the multiaxial strain. Due to significant bolt relaxation in the high-temperature region, stress in the steam inlet region decreased dramatically; likewise, multiaxial creep strain decreased markedly in this region. Contact pressure significantly decreased during the first 10,000 h, especially in the regions between bolts 1 and 9, and the largest decrease in contact pressure exceeded 340 MPa. This reduced sealing performance at high temperatures. Further comparison of the contact pressure and the opening displacement at the contact surface was carried out with and without bolt relaxation. The massive difference of 153 MPa between these two cases in the primary creep phase demonstrated that bolt relaxation significantly influences sealing performance.     

A study of cooling rate of the supercooled water inside of cylindrical capsules

An experimental apparatus was developed to investigate the supercooling phenomenon of pure water inside cylindrical capsules used for cold storage process. The Phase Change Material (PCM) used was distilled water. The external coolant material was a water–alcohol mixture (50% vol.), controlled by a constant temperature bath (CTB) in four fixed values (?4 °C, ?6 °C, ?8 °C, and ?10 °C). Temperatures varying with time were measured inside and outside the capsule. Cylindrical capsules with internal diameter of 30 mm, 45 mm, and 80 mm, with 1.5 mm wall thickness were made in aluminum, bronze or acrylic materials. The Cooling Rate (CR) was investigated for different positions on the internal wall of the capsule, for different external coolant temperatures (T_c), different capsules diameters and different materials. The results showed that the cooling rate is a strong function of the angular position on the internal wall, the coolant temperature, the capsule material, and the capsule's diameter.     

Root causes of fire in a solvent pipe at a petrochemical plant

The root causes of a leak followed by a small fire in a 2 in. solvent pipe in a Petrochemical Plant are discussed. The fire occurred during blanket heating to 300 °C to dissolve polymer clogs. Fractographic and metallographic analyses showed that the failure is characterized by intergranular through the thickness propagation of longitudinal cracks initiated at the outer pipe surface. This cracking mode is called r-type cavitation. The leaking solvent self ignited, polymer deposits inside the pipe carbonized, microstructure changed. All these indicate that pipe temperature during blanket heating reached 550 °C, much above the specified 300 °C maximum. At this temperature yield strength of the pipe material got below nominal hoop stress due to normal internal pressure. The failure of the temperature control system in one of the heating blanket sets was the main event that caused the fire. However, other conditions and exceeded barriers are discussed, related with safety procedures and insufficient support of the ongoing investigation by the contractor involved in the incident. As a result, changes in the declogging and safety procedures were introduced.     

Experimental investigation of the influence of different leaking gases on the heat transfer in a HVMLI cryogenic tank after SCLIV

This paper presented an experimental investigation of the influence of different leaking gases on the heat transfer process in a high-vacuum-multilayer-insulation (HVMLI) cryogenic tank after sudden catastrophic loss of insulation vacuum (SCLIV). The experiments were conducted with the breakdown of the insulation vacuum with nitrogen, air, helium, oxygen, argon, carbon dioxide and the gas mixture of argon and carbon dioxide. The maximum value of the venting rate and heat flux could be ordered as following: CO₂ > O₂ > Ar > the gas mixture > He > Air > N₂, while the average value of the venting rate and heat flux could be ordered as following: O₂ > Ar > He > the gas mixture > CO₂ > Air > N₂. The temperature distribution indicated that phase change heat transfer happened in the insulation jacket after the five different gases including air, argon, the gas mixture of argon and carbon dioxide, oxygen and carbon dioxide were introduced into the insulation jacket.     

Field studies of steam oxidation behavior of austenitic heat-resistant steel 10Cr18Ni9Cu3NbN

The steam oxidation of austenitic steel 10Cr18Ni9Cu3NbN at about 605 °C after in service for 12,000 h and 34,696 h in a 660 MW USC power plant was investigated. Results show that a double layer structure consisting of a magnetite outer layer and Cr-rich inner layer was observed. After 12,000 h, shot blasting treatment improved the steam oxidation resistance of 10Cr18Ni9Cu3NbN steel compared with previous report. Scale oxide which was beneficial from shot blasting treatment spalled off within a short time before 34,696 h. After initial scale exfoliation, the newly formed scale oxide was more likely to be the oxide in 9–12%Cr steels, which was attributed to the low chromium under the initial oxide.     

Design,development and fabrication of indigenous 30 kA NbTi CICC for fusion relevant superconducting magnet

The fusion relevant superconducting magnet is under development in India using a cable-in-conduit-conductor (CICC) with operating current of 30 kA at 5.5 T and 4.5 K. The 30 kA NbTi based CICC is designed on the basis of desired critical design parameters as well as mechanical fabrication considerations. The 30 kA CICC has been designed having square cross-section (30 mm × 30 mm) consisting NbTi as superconducting cable, SS316LN as jacket material and SS304 foil as wrapping around the cabled strands. The design configuration of 30 kA NbTi CICC has been discussed in this paper. The NbTi base high current carrying strands have been fabricated indigenously using direct extrusion and cold drawing process. The 100 m long NbTi–Cu strands twisting, insertion of cabled strands into a circular conduit has been developed with pull through technology. The welding process qualification and effects of cold work on jacket material at room temperature have been elaborated in this paper. The manufacturing parameters and quality procedures for development of CICC have been successfully established and demonstrated with fabrication of 100 m NbTi based CICC without any technical difficulties.     

High-temperature low cycle fatigue behavior of a gray cast iron

The strain controlled low cycle fatigue properties of the studied gray cast iron for engine cylinder blocks were investigated. At the same total strain amplitude, the low cycle fatigue life of the studied material at 523 K was higher than that at 423 K. The fatigue behavior of the studied material was characterized as cyclic softening at any given total strain amplitude (0.12%–0.24%), which was attributed to fatigue crack initiation and propagation. Moreover, this material exhibited asymmetric hysteresis loops due to the presence of the graphite lamellas. Transmission electron microscopy analysis suggested that cyclic softening was also caused by the interactions of dislocations at 423 K, such as cell structure in ferrite, whereas cyclic softening was related to subgrain boundaries and dislocation climbing at 523 K. Micro-analysis of specimen fracture appearance was conducted in order to obtain the fracture characteristics and crack paths for different strain amplitudes. It showed that the higher the temperature, the rougher the crack face of the examined gray cast iron at the same total strain amplitude. Additionally, the microcracks were readily blunted during growth inside the pearlite matrix at 423 K, whereas the microcracks could easily pass through pearlite matrix along with deflection at 523 K. The results of fatigue experiments consistently showed that fatigue damage for the studied material at 423 K was lower than that at 523 K under any given total strain amplitude.     

Failure analysis of a welded impeller in coke oven gas environment

In this study, a centrifugal impeller made of FV520B martensitic precipitation hardening stainless steel failed after 12 months service in coke oven gas environment. An increase in vibration was recorded prior to the breakdown. Analysis revealed that fracture initiated at the welded joints and the main failure mechanism was sulfide stress corrosion cracking (SSC). Efforts were made to assess the cracking susceptibility of base metal and weld metal. Results indicated that welded joints experienced lower fracture toughness and higher susceptibility to SSC cracking. This paper brings out the details of investigation and suggests remedial measures to improve performance of this welded impeller under aggressive environment.     

Investigation of stress corrosion cracks in a UNS S32750 superduplex stainless steel

Duplex and superduplex stainless steels are corrosion resistant alloys with many uses in chemical and petrochemical industries. It is generally accepted that these alloys present stress corrosion resistance superior to austenitic grades, but it does not mean that they are immune to this type of failure. Under severe conditions of temperature, stress, low pH, high chloride and H₂S contents superduplex steels may fail environmentally assisted cracking (EAC). In this work, superduplex UNS S32750 steel specimens were subjected to critical environmental conditions which produced stress corrosion cracks. In the first experiment the material was tested by slow strain rate tensile tests at 80 °C in a solution with 115,000 ppm of chloride, H₂S partial pressure of 6.75 psia, and pH = 3.0. In a second experiment the material was subjected to four bend beam test in a solution similar to experiment 1, but with a H₂S partial pressure of 30.0 psia. Finally, a third test was conducted in a bend plate of superduplex steel subjected to MgCl₂ saturated solution at 154 °C. The cracks produced in the three experiments showed quite different features, which were investigated by optical and scanning electron microscopy.     

Experimental and finite element analysis of fatigue performance of copper power conductors

Fatigue performance of a 95 mm² stranded copper conductor was investigated. Individual copper wires were tested in tension–tension loading with a stress ratio R = 0.1. The specimens were taken from the core wire and from the inner and outer layer of the conductor. Due to the compacting process that was applied during manufacturing, geometrical irregularities were observed on the wires from the outer and inner layers. Finite element (FE) analyses were performed to investigate the combined effects of these irregularities and of material plasticity on the fatigue performance. The FE models were validated by convergence studies. Full cross section conductors were tested in a specially designed rig providing constant tension and variable (reversed) curvature simulating bending inside a bellmouth. In this test the fatigue failures were found to be governed by local bending effects in individual wires. The data from the single wire tests are presented on S–N format and applied in a model for prediction of fatigue strength of full section copper conductors.