High Temperature Creep Strength Research Articles

Experimental in-situ metallographic investigations of industrial hydrocarbon processed fired heater tubes

In the present investigations, the coker unit heater tube has been evaluated using in-situ metallography, after an exposure at a temperature of 615 °C for around 52 000 h. The heater tubes in petroleum industries are mostly damaged by creep failures; hence metallography assessments are conducted for safe operation of industrial units. The primary material of construction for fired heater tubes is ASTM A213 GrT9 chromium molybdenum ferritic steel and maximum primary creep strains are estimated as 11.2761 × 10−2and 11.2802 × 10−2for temperature 888 and 923 K, respectively. In-situ metallography test was conducted at finish pass surface on one butt weld joint section of two different passes (pass I and pass II) of heater tubes. Hardness is within limit and chemical composition stands confirmed within material standards. A heater tube weld joint comprising weldment metal, heat affected zone and base metal were found in good condition, and no indications of creep were observed. Tempered martensite is observed only in weld metal and heat affected zone which aids in high temperature creep strength of metal at weld joint and heat affected zone. No martensitic structure was observed on the base metal; however base metal creep strength was ascertained by fine precipitated alloy carbides within the ferritic matrix base metal structure. Recommendations were further provided to monitor the performance of heater tubes and for an early prediction of secondary creep.

High-Temperature Creep and Long-Term Strength of Metallic Materials

The article consideres the loosening of metallic materials and an irreversible change in density during high-temperature creep is considered as damage parameter. On the basis of this parameter and taking into account the law of mass conservation, interrelated equations for creep deformation and damage parameter are proposed. Approximate and exact solutions for creep deformation and continuity parameter are obtained and criteria for long-term strength are formulated. The corresponding theoretical curves are constructed. It is shown that the theoretical relationships for the damage parameter describe the experimental results on the density change during the high-temperature creep of various metallic materials well.

Research progress on high temperature materials for 700°C ultra-supercritical coal-fired units

Nowadays, material research worldwide for 700°C ultra-supercritical firepower generator units (USC-FPGUs) is focus on the evolution of microstructures and mechanical properties as well as the research progress of high-temperature materials, which are used for fabricating water-cooled wall, superheater/reheater tube, main steam pipe and high-temperature steam header. HCM12 steel and Inconel 617 alloy are suited for high-temperature segment of water-cooled wall. Inconel 740H alloy developed from Inconel 740 alloy after modification is the preferred material for superheater/reheater, main steam pipe and high-temperature steam header due to its excellent persistent creep strength and welding performance. In future research, it is necessary to further verify the high-temperature persistent strength and creep strength of the aforementioned materials through practice. Meanwhile, the optimum hot working process should be exploited to improve the hot-workability and welding performance as well as reduce the cost of materials.

Damage mechanism of nickel-based creep-resistant alloys strengthened by the Laves phase at the grain boundary

ABSTRACTThis study proposes a design guideline for polycrystal Ni-based model alloys with high ductility and 100-MPa creep rupture strength beyond 800°C and 105 h. These alloys are strengthened by both the precipitation of fine γ′ particles inside the grain and the Laves phase at the grain boundary. For investigating the damage mechanism, transformation from the non-equilibrium Laves phase to the σ phase at the grain boundary and formation of the equilibrium needle-like Laves phase inside the grain are promoted by increasing the Fe concentration. The rupture time of Fe-free alloys significantly increases because of the equilibrium Laves phase at the grain boundary owing to a suitable Mo equivalent. In particular, W addition can help achieve high-temperature creep strength. The precipitate-free zone (PFZ) is predominantly formed by prior migration at the grain boundary without precipitation. Creep rupture occurs at the precipitation/matrix interface in the PFZ. Therefore, transformation control from the Laves to the σ phase at the grain boundary suppresses creep degradation. Consequently, a Ni-based alloy with strength >100 MPa and rupture elongation >20% at 800°C and 105 h is fabricated using Larson–Miller parameter conversion, and the alloy design guideline’s validity is confirmed.

Enhanced high-temperature mechanical properties of ARAA by thermo-mechanical processing

We studied effects of thermo-mechanical processing (TMP) on microstructure and mechanical properties of the Korea RAFM steel, ARAA, as a feasibility study for enhancement of creep-resistance. Austenitized ARAA plates were subjected to three different TMP schedules: i) 20% hot-rolling at 1000 °C, ii) 20% hot-rolling at 700 °C, and iii) 20% hot-rolling at 700 °C followed by 30% cold-rolling, all of which were then subjected to tempering at 750 °C. 20% hot-rolling resulted in strengthening effects on the as-tempered ARAA in both tensile and short-term creep tests, the extents being greater for that rolled at lower temperature. However, additional cold-rolling promoted recrystallization of tempered martensite structure, leading to a significant loss of strength. Enhanced tensile and creep strength of the TMPed ARAA can be attributed to the larger number of fine MX carbides and slightly higher density of dislocations. It is concluded that ausforming, which is simple and readily applicable to industry, is a method that can effectively improve the high-temperature creep strength of ferritic-martensitic steels without a loss of impact toughness.

Thermodynamic Alloy Design of High Strength and Toughness in 300 mm Thick Pressure Vessel Wall of 1.25Cr-0.5Mo Steel

In the 21st century, there is an increasing need for high-capacity, high-efficiency, and environmentally friendly power generation systems. The environmentally friendly integrated gasification combined-cycle (IGCC) technology has received particular attention. IGCC pressure vessels require a high-temperature strength and creep strength exceeding those of existing pressure vessels because the operating temperature of the reactor is increased for improved capacity and efficiency. Therefore, high-pressure vessels with thicker walls than those in existing pressure vessels (≤200 mm) must be designed. The primary focus of this research is the development of an IGCC pressure vessel with a fully bainitic structure in the middle portion of the 300 mm thick Cr-Mo steel walls. For this purpose, the effects of the alloy content and cooling rates on the ferrite precipitation and phase transformation behaviors were investigated using JMatPro modeling and thermodynamic calculation; the results were then optimized. Candidate alloys from the simulated results were tested experimentally.

Microstructural features, mechanical properties and high temperature failures of ferritic to ferritic dissimilar welds

ABSTRACTDissimilar metal welds (DMWs) between ferritic steel grades are found extensively in the construction of thermal power plants. The potential combinations and approaches for joining dissimilar ferritic steels are nearly limitless. For DMWs, the difference in alloy composition (specifically chromium and carbide-forming elements) provides the main driving force for carbon diffusion during welding, post-weld heat treatment and long-term service at elevated temperatures. Since the high temperature creep strength of local, carbon-denuded zones can be dramatically reduced from that of the parent or filler material, the service performance of ferritic DMWs can be severely reduced. This article reviews experimental observations on microstructural evolution in dissimilar ferritic welds, activities to describe the observed phenomena by modelling and simulation and discusses the performance of these welds at high temperature. Lastly, a well-engineered approach to the design of ferritic DMWs is discussed in the context of thermal power plants which are subject to damage by creep.Abbreviations: HAZ: heat affected zone; PWHT: post weld heat treatment; GMAW: gas-metal arc welding; SMAW: shielded-metal arc welding; GTAW: gas tungsten arc welding; SAW: submerged arc welding; DMW: ferritic dissimilar metal weld; CGHAZ: coarse-grained heat affected zone; FGHAZ: fine-grained heat affected zone; CDZ: carbon-denuded zone; CEZ: carbon-enriched zone; CSEF: creep strength enhanced ferritic

Microscopic plastic response in a bulk nano-structured TiAl intermetallic compound processed by high-pressure torsion

Intermetallic compounds of titanium aluminides (Ti-Al) have attracted much attention for their excellent properties including light-weight, high specific strength and high-temperature creep strength. Nevertheless, bulk Ti-Al require suitable processing routes for improving the mechanical properties, especially plasticity and ductility. Accordingly, the present study applies a strategy of grain refinement through high-pressure torsion (HPT) processing which is an effective processing technique for difficult-to-work alloys due to the large compressive hydrostatic stresses that are developed during processing. A γ-based Ti-45Al-7.5Nb intermetallic compound was processed by HPT for 5 and 10 turns under 6.0GPa at room temperature. The duplex microstructure was successfully refined to have ultrafine laths with thicknesses of 40–100nm after 10 HPT turns. Improved hardness was recorded by both Vickers microhardness and nanoindentation measurements. The X-ray diffraction profiles confirmed the occurrence of phase transformation from γ-TiAl to α2-Ti3Al after HPT. The macroscopic plastic flow was estimated through the nanoindentation technique by applying both Berkovich and spherical indenters. The results predict an improved plastic behavior by increasing the strain rate sensitivity and a significant increase in yield strength whereas a slight decrease is measured in the strain hardening capability after HPT for 10 turns. Furthermore, this nanoindentation study demonstrates an alternative and promising technique of small-scale testing for predicting the macroscopic plastic flow behavior of ultrafine-grained materials.

Effect of Y/Ti atomic ratio on microstructure of oxide dispersion strengthened alloys

Oxide dispersion strengthened (ODS) alloys are the leading candidate materials for advanced fission and fusion reactor components because of their excellent high temperature creep strength and radiation tolerance. The promising performance of ODS alloys relies on a high-density population of nano oxides. Here, four 9Cr-ODS alloys with varying initial Y/Ti atomic ratio were processed using a combined route of mechanical alloying (MA) and hot isostatic pressing (HIP). Microstructural characterizations of the consolidated alloys were examined by advanced transmission electron microscopy (TEM). Great differences in oxide distribution behavior were observed. 9Cr-ODS alloy with a Y/Ti ratio of 0.4 presented the finest and the highest number density of nano oxides among the four alloys. The effects of Y/Ti ratio on oxide formation were explored, and the precipitated mechanisms of the nano oxides were interpreted.

The problem of high-temperature creep and long-term strength in the mechanics of materials

The damage parameter is determined as the relative change in volume or density. It is considered that this parameter is an integral characteristic of the damage.

Outstanding compressive creep strength in Cr/Ir-codoped (Mo0.85Nb0.15)Si2 crystals with the unique cross-lamellar microstructure

A (Mo0.85Nb0.15)Si2 crystal with an oriented, lamellar, C40/C11b two-phase microstructure is a promising ultrahigh-temperature (UHT) structural material, but its low room-temperature fracture toughness and low high-temperature strength prevent its practical application. As a possibility to overcome these problems, we first found a development of unique “cross-lamellar microstructure”, by the cooping of Cr and Ir. The cross-lamellar microstructure consists of a rod-like C11b-phase grains that extend along a direction perpendicular to the lamellar interface in addition to the C40/C11b fine lamellae. In this study, the effectiveness of the cross-lamellar microstructure for improving the high-temperature creep deformation property, being the most essential for UHT materials, was examined by using the oriented crystals. The creep rate significantly reduced along a loading orientation parallel to the lamellar interface. Furthermore, the degradation in creep strength for other loading orientation that is not parallel to the lamellar interface, which has been a serious problem up to now, was also suppressed. The results demonstrated that the simultaneous improvement of high-temperature creep strength and room temperature fracture toughness can be first accomplished by the development of unique cross-lamellar microstructure, which opens a potential avenue for the development of novel UHT materials as alternatives to existing Ni-based superalloys.

Creep fracture behaviour of SUS304H steel with vanadium addition based on small punch creep testing

The high-temperature creep fracture behaviour and creep strength of SUS 304H containing a minor addition of V were investigated in this study. A series of small punch (SP) creep and uniaxial creep tests were performed at 700 °C. The load and punch displacement rates obtained from the SP creep tests were used to derive conversion equations to determine the equivalent stress and creep strain rates. A converting equation is suggested in this study so that Norton’s secondary power law creep constants obtained from the SP creep testing can be in agreement with those obtained from the uniaxial tensile creep tests. The creep strength of the modified SUS 304H steel containing V was shown to be superior to that without V based on the current results and other available results for type 304H steel.

First experimental measurement of calcium diffusion in magnesium using novel liquid-solid diffusion couples and forward-simulation analysis

A novel liquid-solid diffusion couple enables effective measurements of composition profiles at high temperatures. A forward-simulation analysis is then performed to extract both interdiffusion and impurity diffusion coefficients. The combined method enables measurement of the first ever set of impurity diffusion coefficients of Ca in Mg and reliable high-temperature diffusion data of Al in Mg. The results show that Ca diffuses faster than the Mg self-diffusion which is faster than the Al impurity diffusion in Mg; thus Ca may not be an ideal element for the enhancement of high-temperature creep strength of Mg alloys as has been often assumed.

Improved creep strength of nickel-base superalloys by optimized γ/γ′ partitioning behavior of solid solution strengthening elements

Solid solution strengthening of the γ matrix is one key factor for improving the creep strength of single crystal nickel-base superalloys at high temperatures. Therefore a strong partitioning of solid solution hardening elements to the matrix is beneficial for high temperature creep strength. Different Rhenium-free alloys which are derived from CMSX-4 are investigated. The alloys have been characterized regarding microstructure, phase compositions as well as creep strength. It is found that increasing the Titanium (Ti) as well as the Tungsten (W) content causes a stronger partitioning of the solid solution strengtheners, in particular W, to the γ phase. As a result the creep resistance is significantly improved. Based on these ideas, a Rhenium-free alloy with an optimized chemistry regarding the partitioning behavior of W is developed and validated in the present study. It shows comparable creep strength to the Rhenium containing second generation alloy CMSX-4 in the high temperature / low stress creep regime and is less prone to the formation of deleterious topologically close packed (TCP) phases. This more effective usage of solid solution strengtheners can enhance the creep properties of nickel-base superalloys while reducing the content of strategic elements like Rhenium.

MTEK 25-35MA

Abstract MTEK 25-35MA is a solid solution microalloyed Fe-Ni-Cr-Nb casting alloy with good high-temperature creep strength. It is intended for use in pyrolysis cracker furnaces. This datasheet provides information on composition, physical properties, and tensile properties as well as creep. It also includes information on high temperature performance as well as casting, heat treating, machining, and joining. Filing Code: SS-1249. Producer or source: MetalTek International. Originally published July 2016, revised September 2016.

MTEK 25-35LS

Abstract MTEK 25-35LS is a solid solution Fe-Ni-Cr-Nb casting alloy with good high temperature creep strength. This datasheet provides information on composition, physical properties, microstructure, elasticity, and tensile properties. It also includes information on casting, heat treating, machining, and joining. Filing Code: SS-1245. Producer or source: MetalTek International.

Microstructural evolution in a new Fe based ODS alloy processed by mechanical alloying

Oxide dispersion strengthened (ODS) ferritic steels containing a high density of nanoscale clusters are produced by mechanical milling show excellent properties such as good irradiation swelling resistance and high-temperature creep strength. Mechanical milling with the aim of a fine dispersion of oxides dissolves in the matrix. The influences of process parameters during mechanical milling of ODS ferritic steels are important for material properties. In this research we investigated different milling time for the morphology and properties of powders. The results show that the powders by a stainless steel grinding media are experienced three stages: the mean powder size increases firstly then decreases. With the extending of the milling time, the mean powder size increases again. Otherwise the grain size decreases initially and then reach a saturation value. The grain size of milled powders was maintained in nanoscale range by TEM observation.

Development of Electron Beam Welding Procedure for Nb-1Zr-0.1C Alloy

Nb-1Zr-0.1C is a Niobium based refractory metal alloy having an excellent combination of high temperature properties like high melting point (∼2500°C), adequate high temperature strength (∼200 MPa) and creep strength for long duration, better compatibility for alkali liquid metal, resistance to thermal shock & radiation and low ductile to brittle transition temperature (DBTT). These properties make this alloy a potential candidate for several structural applications including compact high temperature reactors. Welding of this alloy is a difficult task due to its high melting point and its reactive nature. In comparison to the conventional welding processes, high energy density sources like electron beam and laser beam which can produce deep penetrations with narrow heat affected zones are more suitable to weld the components made of Nb-1Zr-0.1C alloy. This paper describes the development of electron beam welding procedure for Nb-1Zr-0.1C alloy. Welding parameters were developed and weld bead was characterized by metallography and different zones of the weld bead were identified and studied. The results of the welding trials and the tests performed on the welds and the major observations & conclusions drawn from the experience of welding Nb-1Zr-0.1C alloy are presented in this paper.

Improving the high-temperature creep strength of 15Cr ferritic creep-resistant steels at temperatures of 923–1023 K

The precipitation strengthening of creep-resistant 15Cr ferritic steels at temperatures of 923–1023K was improved by nickel addition. The addition of nickel, an austenite-stabilizing element, enhanced the formation of martensitic grains in the ferritic grain boundaries and changed the precipitation behavior during creep at elevated temperatures. The precipitates produced in the creep-ruptured 15Cr steels were intermetallic compounds (Laves phase and χ-phase), carbide (Cr23C6), and Z phase (Cr(V,Nb)N). An increase in the amounts of intermetallic compounds and the preferential precipitation of Cr23C6 carbide on the grain boundaries between the ferritic and martensitic grains were accelerated by the nickel addition. The 15Cr steel after 10,000h at 973 and 1023K demonstrated approximately double the creep rupture strength of 9Cr steel (ASME Grade T92) with a tempered martensitic microstructure. At temperatures of up to 1023K, the 15Cr steel had over 10 times the creep rupture lifetime of T92 steel, primarily because of increased precipitation of intermetallic compounds and Cr23C6 carbide enhanced by nickel addition.

Influence of Mechanical Alloying Time on Morphology and Properties of 15Cr-ODS Steel Powders

Abstract Oxide dispersion strengthened (ODS) ferritic steels are the leading candidates of fuel cladding for Generation IV nuclear reactors due to their excellent properties such as excellent radiation tolerance and high-temperature creep strength. Mechanical milling with the aim of a fine dispersion of oxides in the metal matrix becomes the main process for the production of ODS steels. In order to clarify the influence of milling time on the precursor powders for 15Cr-ODS steel, the morphology and properties of mechanical alloying (MA) powders with different milling time were investigated by scanning electron microscopy (SEM), laser diffraction particle size analyzer, X-ray diffraction (XRD) and Vickers hardness tester. The experimental results showed that the powder was fractured and welded with rotation and vibration of container during mechanical milling. The mean powder size increased (0–1 h) firstly then decreased (2–60 h). Extending milling time to 70 h, the mean powder size increased again. The grain size decreased quickly at the initial stage of milling process (0–2 h) then trended to reach a saturation value. The Vickers hardness increased rapidly at the initial stage of milling, then reached a saturation value.