Higher Creep Rupture Strength Research Articles

Creep Rupture Strength and Phase Stability of the Austenite Phase for KA-SUS304J1HTB

Metallurgical factor causing the heat-to-heat variation in creep rupture strength have been investigated for KA-SUS304J1HTB. In the long-term, there was a maximum difference of 3.5 times in creep rupture time between the heat with low creep strength and the heat with high creep strength. In the heat with low creep rupture strength, most of the creep voids occurred at the matrix/σ phase interface. Moreover, in the heat with low creep rupture strength, the area fraction of σ phase was larger than in the heat with high creep rupture strength. Considering that the difference in the area fraction of σ phase in each heat is related to the difference in phase stability of the austenite phase, the Md value in each heat was evaluated. The Md value is the parameter correlated with phase stability. The creep rupture time of each heat was correlated with the Md value. The smaller the Md value, the longer the creep rupture time. Therefore, the heat-to-heat variation in creep rupture strength is caused by the difference in the phase stability of each heat. In other words, in the heats with low phase stability, creep rupture strength is low because a large amount of σ phase precipitates during the creep test.

Welding, mechanical properties and microstructure of different grades of austenitic stainless steels: A review

Nowadays the power of the industry standard materials has gained a lot with the advancement in technology. The requirement for materials with high creep rupture strength, high thermal conductivity, and strong corrosion resistance rose dramatically when the power plant was operated at greater temperatures and pressures. Creep strength improved steels, nickel based alloys, and austenitic stainless steels are the most often utilised materials in high temperature and high pressure power plants. To meet the strength criteria, reduced cost investment, and service standards must be met, dissimilar welding between Stainless steels is required. This necessitates the combining of different metals. An effective weld among different metals is one which is as strong as that of the weakest of the two different metals being connected, that is, it has enough ductility and tensile strength to prevent the junction from failing during the weld. Such joins may be made from a number of metals and using a range of welding methods. Before selecting the best way to join the metals the evaluation of both the base metals and the filler material is to be done. Here the welding process done between steels is TIG welding. Challenges in the current research are to join the different materials without disturbing the properties of parent metals. Some of the frequently occurring problems are: wider HAZ (heat affected zone), Difference in coefficient of thermal expansions, Difference in chemical composition, Multi pass welds to weld high thickness materials. Current research deals with the Optimization the TIG Welding Process and To Understand the effect of Process Parameters on Mechanical Characteristics of the dissimilar joints and to carry out Numerical Investigation on the dissimilar welding of Austenitic Stainless Steel.

Structural behavior of RC beams strengthened with prestressed near surface mounted technique using basalt FRP bars

This research aimed at studying the flexural behavior of RC beams strengthened with near surface mounted (NSM) technique using basalt FRP (BFRP) bars to assess their efficiency for strengthening RC beams in order to be introduced as an alternative to other types of FRP bars. Moreover, this study investigated a new type of strengthening technique, named prestressed near surface mounted (PNSM) technique, which is expected to offer lots of advantages owing to combining two well-known “external prestressing” and “NSM” techniques. The variables considered in this study were the strengthening effect (unstrengthened and strengthened), the type of the FRP bars (BFRP and GFRP), the strengthening technique (NSM and PSNM), the initial prestressing level (30% and 50% of ultimate tensile capacity of BFRP bars), and the position of NSM FRP bars (bottom and side). The test results demonstrated the efficiency of using BFRP bars for strengthening RC beams with NSM and PNSM techniques. Additionally, BFRP bars offered additional advantage compared to GFRP bars when strengthened with PNSM technique owing to its higher creep rupture strength. Moreover, the test results proved that strengthening of RC beams with PNSM technique offers superior enhancement in the pre-cracking stage compared to NSM technique while sacrificing a great portion of beam’s ductility.

Alloy Design of MARBN for Boiler and Turbine Applications at 650 oC

ABSTRACT Soluble boron stabilises fine distributions of M23C6 carbides near grain boundaries (GBs) in 9Cr steel during creep, enhancing GB precipitation hardening, and suppresses the Type IV fracture in welded joints at 650oC. Excess addition of boron and nitrogen produces BN inclusions at high temperature, degrading creep strength and rupture ductility. A 9Cr-3W-3Co-0.2V-0.05Nb steel with 130 ppm boron and 80 ppm nitrogen, designated martensitic 9Cr steel strengthened by boron and nitrides (MARBN), exhibits not only the much higher creep rupture strength of base metal than Gr.92 but also no Type IV fracture in welded joints at 650oC. The boron and nitrogen concentrations in MARBN are located just inside the phase boundary for BN at normalising heat treatment temperature in the phase diagram, where we can obtain enough soluble boron and soluble nitrogen but no BN inclusion. The oxidation resistance is improved by the pre-oxidation treatment in argon gas.

A critical review on dissimilar welds joint between martensitic and austenitic steel for power plant application

Abstract In this review article, microstructure and mechanical behavior of the dissimilar welded joint (DWJ) between ferritic-martensitic steel and austenitic grade steel along with its application have been summarized in Ultra Super Critical (USC) power plant. Creep-strength enhanced ferritic-martensitic (CSEF/M) P91 steel was developed to sustain at extreme operating conditions of ultra-supercritical (USC) power plants, and later, P92 was developed to achieve better mechanical properties, higher creep-rupture strength and high operating temperature with the reduction in wall thickness as compared to P91 steel. The most common application of P91/P92 material in power plants includes high pressure and high-temperature steam piping, headers, super-heater tubing, and water-wall tubing. The other most commonly used material in the power plants is austenitic stainless steel, i.e., SS 304 L. The austenitic grade stainless steel offers high resistance to corrosion due to the high wt. % chromium and nickel content (18–20 and 8–12, respectively). Due to the low carbon content, the SS 304 L is less sensitive to the sensitization problem and offers excellent weldability. The joining of these dissimilar materials is frequently required in the power generation industry. The current review focuses on the main difficulty associated with dissimilar welding of martensitic P91/P92 and austenitic grade stainless steel. The different chemical composition, mechanical, physical and metallurgical properties of the martensitic P91/P92 and austenitic grade stainless steel leads to the problems such as hot cracking and carbon migration. The other weldability issues are the formation of a brittle intermetallic compound, the formation of soft transaction heat affected zone along with martensitic steel, δ ferrite formation in fusion zone, diffusion related problem, and residual stresses, which necessitates thorough study and qualification of welds. The effect of coarsening of various precipitates such as M23C6 carbides, MX carbonitrides, and effect of laves phase, z-phase, and sigma phase on mechanical property, and creep-rupture strength of DWJ are also discussed in detail. Based on the literature reviewed, it has been found that some of the above-stated problems can be solved by using nickel-based filler wire due to its intermediate physical and mechanical properties. The selection of the proper filler metal is another vital issue in dissimilar welds joint that is also covered in this review article. The reason behind the formation of the unmixed zone, filler deficient region, peninsula, island, beach, migrated grain boundaries, solidified grain boundaries, and solidified subgrain boundaries during DWJ of martensitic P91/P92 and austenitic grade stainless steel is also discussed. The heat treatment is required to eliminate the heterogeneous microstructure during the dissimilar welding. The effect of post-weld heat treatment (PWHT) on the microstructure and mechanical behavior of the DWJ also reviewed. The residual stress developed during the DWJ may cause the premature failure of the components under service, has also been discussed in detail. The effect associated with the residual stress deformation has been reviewed in the different conditions of the DWJ.

Creep Properties of Intercritical Heat Treated Boron Added Modified 9Cr–1Mo Steel

Creep properties of intercritically heat-treated (ICHT) boron free modified 9Cr–1Mo steel (P91-IC) and boron containing modified 9Cr–1Mo (P91B-IC) steel have been studied at 923 K over a stress range of 50–120 MPa. Minimum creep rate of the P91B-IC steel was significantly lower than the P91-IC steel. Stress exponent and threshold stress values in P91-IC and P91B-IC steels were found to be 6.5 and 28 MPa, and 8.7 and 38 MPa respectively. P91B-IC steel has exhibited higher creep rupture life than the P91-IC steel. Higher creep rupture strength in the P91B-IC steel as compared to P91-IC steel has been attributed to the presence of stable M23C6 precipitates at the lath boundaries and other high angle boundaries. The creep rupture strengths of both the ICHT steels were lower than the respective base metal.

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.

A Modified HR3C Austenitic Heat-Resistant Steel for Ultra-supercritical Power Plants Applications Beyond 650 °C

A modified HR3C austenitic steel has been designed by optimizing the chemical composition. Compared with a commercial HR3C alloy, the modified steel has comparable oxidation resistance, yield strength, and plasticity, but higher creep rupture strength and impact toughness after long-term thermal exposure. The results suggest that the modified alloy is a promising candidate for the applications of ultra-supercritical power plants operating beyond 650 °C.

Characterization of microstructure of HAZs in as-welded and service condition of P91 pipe weldments

Steels 9-12% Cr, having the high creep rupture strength are advocated for the modern low polluting thermal power plants. In the present investigation, the P91 pipe weldments have been characterized for microstructural responses in as-welded, post-weld heat treatment (PWHT) and ageing conditions. The PWHT of welded samples were carried out at 760 °C for time of 2 h and ageing at 760 °C for 720 h and 1440 h, respectively. The effect of time has been studied on precipitates size, distribution of precipitates and grain sizes present in various zones of P91 steel weldments. The impact toughness and hardness variation of heat affected zone (HAZ) have also been studied in as-welded condition as well as at different heat treatment condition. A significant change was observed in grain size and precipitates size after each heat treatment condition. The maximum impact toughness of HAZ was obtained after PWHT at 760 °C for 2 h. The main phase observed in weld fusion zone in as-welded, PWHT and ageing conditions were M23C6, MX, M7C3, Fe-rich M3C and M2C. The unwanted Z-phase (NbCrN) was also noticed in weld fusion zone after ageing of 1440 h.

Mechanical properties of T23 steel welded joints without post-weld heat treatment for fossil fired boilers

Abstract

Evolution of phases in P91 steel in various heat treatment conditions and their effect on microstructure stability and mechanical properties

To achieve high thermal efficiency, modern day thermal power plants operate at higher operating temperature and pressure which necessitates use of steels with high creep rupture strength such as modified 9Cr-1Mo steels. In the present study, the evolution of phases in modified 9Cr-1Mo P91 steel and their effects on microstructural stability and mechanical properties have been studied for specimens that were subjected to different thermal heat treatment conditions. The main focus has been to study the effect of heat treatment temperature ranging from 623K to 1033K (350–760°C) on P91 steel. Further, the effect of furnace cooling, water quenching, tempering at 1273K (1000°C) and austenitizing on the mechanical properties and microstructure has been studied. The techniques used for material characterization were scanning electron microscopy (SEM), optical microscopy (OM) and X-ray diffraction. For low tempering temperature, i.e. 623K (350°C), M23C6, M3C, M7C3, and MX precipitates have been observed with high yield strength (YS), tensile strength (UTS), hardness and low toughness. In the high temperature range, 923–1033K (650–760°C), fine MX, M7C3, M23C6, M2X, and M3C precipitates have been observed with low YS, UTS, hardness and high toughness. The steel tempered at 1033K (760°C) was observed to be having best combination of YS, UTS, hardness, toughness and ductility.

Microstructural Degradation and Development of Damage During Creep in 9 % Chromium Ferritic Steels

Creep-rupture properties of grade 9 and 92 steels have been examined for wide range of stresses at 873 K. Both grade 9 and 92 steels obeyed power law in the stress dependence of rupture life and exhibited two-slope behaviour with separate values of stress exponents in the low and high stress regimes. Grade 92 steel exhibited significantly higher creep-rupture strength than grade 9 steel. Grade 9 steel displayed high creep ductility, transgranular fracture and obeyed Monkman–Grant and modified Monkman–Grant relations. On the contrary, grade 92 steel exhibited significant decrease in creep ductility at longer durations associated with secondary cracking originating from decohesion of Laves phase. Grade 92 steel followed generalized form of Monkman–Grant relation and obeyed modified Monkman–Grant relation. High values of creep damage tolerance factor obtained for grade 9 and 92 steels have been correlated with the observed microstructural degradation in terms of decrease in dislocation density and coarsening of precipitates and dislocation substructure.

초초임계 석탄발전 보일러 튜브(SA213 TP347H) 용접부 안정화 열처리 효과

Austenite stainless steel(SA213-TP347H) has widely been used for the superheater & reheater tube in USC(ultra-supercritica) coal boiler because of its high creep rupture strength and anti-oxidation. But recently, the short-term failures have happened frequently in heat affected zone for only 4,000~15,000hours of service. Many investigations have been conducted to understand the failure mechanism. The root cause of failure was comfirmed to strain induce participation hardening crack or reheat cracking. This mechanism often occurred due to weld residual stress and precipitation of the Cr, Nb carbides in the stabilized stainless steel such as TP347H. This paper presents an analysis of failure tube and effect of the sample tubes that conducting stabilizing heat treatment in site after 11,380hours & 16,961hours of service. Visual inspection was performed. In addition, microscopic characteristics was identified by O.M, SEM, and hardness test was carried out to find out the heat treatment effects. Failures seem to happen because of being not conducted stabilizing heat treatment in site. And another cause is inadequate weld parameter such as pass, ampere, voltage, inter-pass temperature. Thus, this paper has the purpose to describe that how to prevent similar failures in those weld-joints.

Creep Behavior in A Newly Developed Heat Resistant Austenitic Stainless Steel

UNS S31035 austenitic stainless steel grade is a newly developed advanced heat resistant material for use in coal fired boilers at metal temperatures up to 700 °C. This new grade that has recently got two AMSE code cases shows good resistance to steam oxidation and flue gas corrosion and high creep rupture strength. This paper will mainly focus on the characterization of long term structure stability and performances such as the creep behaviors at different temperatures for up to 86,000 h at high temperatures. The creep damage mechanisms were studied using electron transmission microscopy, electron backscatter diffraction, and electron channeling contrast image analysis. The results show that the creep strength is related to the intragranular nano particles that act as obstacles for dislocation movements. Plastic deformation and transgranular fracture is the main creep fracture mechanism in the creep test samples of UNS S31035. The material has good creep ductility by formation of twins during the creep test. This material has been installed and tested in several European power plants, and has shown good performance. The material is an excellent alternative for superheaters and reheaters in future high-efficient coal fired boilers with design material temperatures up to 700 °C, instead of more costly nickel based alloy.

Microstructure Stability of V and Ta Microalloyed 12%Cr Reduced Activation Ferrite/Martensite Steel during Long-term Aging at 650 °C

In view of developing novel alloys for applications in supercritical water-cooled reactor fuel cladding and in-core components, a 12%Cr reduced activation ferrite/martensite (RAFM) steel with good corrosion resistance and irradiation performance was developed. V and Ta were added to form fine MX type carbonitrides and enhance the high temperature creep rupture strength. Microstructure stability of the steel during long-term aging at 650 °C was studied experimentally combined with the simulation of Thermo-Calc and DICTRA software. The results show that the precipitates in the steel during long-term aging contain M23C6, MX and Laves phase. M23C6 carbides play a major role in the stabilization of the tempered martensite lath structure by exerting a large Zener pinning force as compared with MX and Laves phase. Adding V and Ta in the steel can not only promote MX precipitation, but also refine M23C6 carbides and thus improve the thermal stability of lath/subgrains, which is beneficial to the improvement of high temperature microstructure stability of the 12%Cr RAFM steel.

Simultaneous Optimization of Multi Performance Characteristics in Dry Turning of Inconel 718 Using NSGA-II

The conventional methods of optimization are unable to handle non differentiable or discontinues functions. Evolutionary Algorithms (EA) are efficient in handling non differential or discontinues functions. Non Dominated Sorting Genetic Algorithm-II (NSGA-II) is an efficient method for solving Multi-Objective optimization problems. Super alloy Inconel 718 is a high temperature alloy capable of having high creep rupture strength at high temperatures to about 650°C to 750°C. It is a difficult to cut material and is widely used in manufacturing of aircraft parts such as turbine disks, turbine blades, etc. A Multi Objective Optimization technique, NSGA-II has been used to predict the optimum values of process variables (Cutting speed, Depth of cut and Feed-rate) in high speed dry turning of Inconel 718 with PVD coated carbide tool by considering Material Removal Rate and Surface Roughness as Multi performance characteristics. According to NSGA-II's Pareto Optimal solutions, the best MRR obtained was 7462.42 mm3/min and the best surface roughness was found to be 0.375μm.

Upper limits to americium concentration in large sized sodium-cooled fast reactors loaded with metallic fuel

Transient analysis of a large sized sodium-cooled reactor loaded with metallic fuel modified by different fractions of americium have been performed. Unprotected loss-of-offsite power, unprotected loss-of-flow and unprotected transient-over-power accidents were simulated with the SAS4A/SASSYS code based on the geometrical model of an IFR1For acronyms and abbreviations, see the Nomenclature at the end of the paper.1 with power rating of 2500MWth, using safety parameters obtained with the SERPENT Monte Carlo code. The Ti-modified austenitic D9 steel, having higher creep rupture strength, was considered as the cladding and structural material apart from the ferritic/martensitic HT9 steel.For the reference case of U–12Pu–1Am–10Zr fuel at EOEC, the margin to fuel melt during a design basis condition UTOP is about 50K for a maximum linear rating of 30kW/m. In order to maintain a margin of 50K to fuel failure, the linear power rating has to be reduced by ∼3% and 6% for 2wt.% and 3wt.% Am introduction into the fuel respectively.Hence, an Am concentration of 2–3wt.% in the fuel would lead to a power penalty of 3–6%, permitting a consumption rate of 3.0–5.1kgAm/TWhth. This consumption rate is significantly higher than the one previously obtained for oxide fuelled SFRs.

High temperature tensile test and creep rupture strength prediction of T92/Super304H dissimilar steel weld joints

After short term tensile test at 823–923 K, microstructure, fractography and fracture mechanism of the T92/Super304H dissimilar steel weld joints were studied. In the high temperature tensile process, the fracture location of all joints is in the T92 base metal; however, the fracture mechanism of these joints changes from a normal mode to a shear mode with the increase of the temperature. Moreover, an equation of the creep rupture strength related to the short term tensile strength of the joints was firstly derived in terms of the Goldenberg model. And then the creep rupture strength of the joints was obtained, = 42·7 MPa.

Effects of Thermal Exposure on Microstructure and Mechanical Properties of Ni Based Superalloy GTD111

The rotating components in the hot sections of land-based gas turbine are exposed to severe environment during several ten thousand hours at above 1100 oC operating temperature. The failure mechanism of the hot gas components would be accompanied by material degradation in the properties of high temperature and creep rupture strength. Many hot gas components in gas turbine are made of Ni-based superalloy because of their high temperature performance. In this work, we surveyed the time and temperature dependent degradation of Ni-based superalloy. We prepared the specimens from GTD111 that are exposed at 871 oC and 982 oC in 1,000 ~ 10,000 hours. We carried out the mechanical test and microstructural observation.

Creep properties and damage behaviour of component-like tubes of Vm12-materials

The gap in the energy supply between current availability and the rising demand for electricity worldwide has to be closed primarily by using modern steam and gas power stations with an increased degree of efficiency and decreased CO2 emissions. Target values for reaching a high degree of efficiency of ≥50% demand increase the steam parameters. The modern creep-resistant steels and their weldments have to have both high creep rupture strength and corrosion resistance.Within the European research programme COST 536, between 2005 and 2009 research and development work in the field of power plant steels had been carried out for conventional applications. The project was focused on the development of appropriate materials, coatings and surface treatments for components in steam power plants with steam inlet temperatures in the turbine of up to 650°C.In framework COST 536, Siempelkamp Pruef- und Gutachter-Gesellschaft mbH (SPG) performed component-like creep tests at pressurized tubes made of martensitic steel VM12. This steel was developed by Vallourec & Mannesmann Tubes with the aim of reaching both sufficient creep strength and increased oxidation resistance and is already used for boiler application in new power plants in Germany.In this paper, the experimental results of uniaxial creep tests, component-like creep tests on tubes with inner pressure and axial loading, metallographic examination and damage characterisation are presented. The tubes are equipped with capacitive high temperature strain gauges for on-line monitoring of strain. All testing data will be implemented as inputs for the numeric FE analysis. The effect of multiaxiality and stress redistribution will be discussed.