Different Grades Of Stainless Steel Research Articles

AUGMENTATION OF DEPTH OF PENETRATION AND PRODUCTIVITY BENEFITS OF ATIG WELDS USING THE AHP

Weldability is an important issue in the fabrication of different grades of stainless steel used in industry. Tungsten inert gas (TIG) welding is widely used in industry for accurate and precision work, but lack of penetration is observed in this process even though current and welding speed can be varied considerably to increase heat input to obtain deeper penetration. Increased heat input adversely affects mechanical properties of the weldment. To overcome this, activated flux TIG welding is often used in industry to achieve higher depth of penetration with relatively less heat input. In this study, activated flux TIG welding is used with input variables such as heat input, welding speed and pulse frequency. The chosen base metal is SS 304L stainless steel, and a hybrid flux mixture containing fluxes of SiO2, MnO2 and MoO3 in a ratio of 1:1:2 is used to obtain the desired depth of penetration. Nine experimental runs are conducted to obtain the optimum depth of penetration. Heat input values are 2.767 kJ/mm, 1.470 kJ/mm and 1.281 kJ/mm and pulse frequency is 160 Hz, 120 Hz and 80 Hz. Welding speed varies from 0.5 mm/s to 1.18 mm/s. A maximum depth of penetration of 4.42 mm is achieved with a heat input of 2.767 kJ/mm, welding speed of 0.5 mm/s and pulse frequency of 160 Hz. The reversed Marangoni effect and arc constriction effect are the mechanisms responsible for the deeper penetration in ATIG welding. In this research, a multi criteria decision making tool, the Analytical Hierarchy Process (AHP), is used to validate the optimum value obtained. The optimal values obtained in the research are in good accordance with those obtained using the AHP. The outcome of the present investigation indicates the applicability of ATIG welding for joining large thickness of stainless steel flats to give enhanced productivity by reducing the number of weld passes.

Research Progress of Cryogenic Materials for Storage and Transportation of Liquid Hydrogen

Liquid hydrogen is the main fuel of large-scale low-temperature heavy-duty rockets, and has become the key direction of energy development in China in recent years. As an important application carrier in the large-scale storage and transportation of liquid hydrogen, liquid hydrogen cryogenic storage and transportation containers are the key equipment related to the national defense security of China’s aerospace and energy fields. Due to the low temperature of liquid hydrogen (20 K), special requirements have been put forward for the selection of materials for storage and transportation containers including the adaptability of materials in a liquid hydrogen environment, hydrogen embrittlement characteristics, mechanical properties, and thermophysical properties of liquid hydrogen temperature, which can all affect the safe and reliable design of storage and transportation containers. Therefore, it is of great practical significance to systematically master the types and properties of cryogenic materials for the development of liquid hydrogen storage and transportation containers. With the wide application of liquid hydrogen in different occasions, the requirements for storage and transportation container materials are not the same. In this paper, the types and applications of cryogenic materials commonly used in liquid hydrogen storage and transportation containers are reviewed. The effects of low-temperature on the mechanical properties of different materials are introduced. The research progress of cryogenic materials and low-temperature performance data of materials is introduced. The shortcomings in the research and application of cryogenic materials for liquid hydrogen storage and transportation containers are summarized to provide guidance for the future development of container materials. Among them, stainless steel is the most widely used cryogenic material for liquid hydrogen storage and transportation vessel, but different grades of stainless steel also have different applications, which usually need to be comprehensively considered in combination with its low temperature performance, corrosion resistance, welding performance, and other aspects. However, with the increasing demand for space liquid hydrogen storage and transportation, the research on high specific strength cryogenic materials such as aluminum alloy, titanium alloy, or composite materials is also developing. Aluminum alloy liquid hydrogen storage and transportation containers are widely used in the space field, while composite materials have significant advantages in being lightweight. Hydrogen permeation is the key bottleneck of composite storage and transportation containers. At present, there are still many technical problems that have not been solved.

Cutting Performance of Austenitic and Duplex Stainless Steels with Drills of Three Cutting Edges

Austenitic and duplex stainless steels are considered be the best in corrosion resistance among different grades of stainless steels. Due to high strength, duplex stainless steels applications are increasingly as an alternative to the austenitic stainless steels. In this sense, the machining study of this materials is an important issue, in order to better understand the performance of the tools and the quality of the parts manufactured for high-demand industries. In this research, the machinability of both stainless steels was evaluated in the drilling operation, using drills with three cutting edges. This type of drill geometry is particularly useful when conventional solid carbide drills fail. The drill point of triple edge is very stable, demonstrating optimal positioning accuracy and better performance in deep bores. Using the same tool geometry, a comparative analysis of drilling performance on austenitic and duplex stainless steels was made. In experimental procedure, external low-pressure cooling or internal high-pressure cooling was applied alternatively. The cutting vibration, the tool wear, the roughness and the hole diameter accuracy were evaluated in the series of holes made. The obtained results show that the most important factor to increase the number of holes made is the use of high-pressure internal cooling. When external cooling is used, AISI 304 have a worse behaviour than duplex stainless steel, due to greater susceptibility to built-up-edge formation and work hardening. The tool deterioration is mainly non-uniform chipping for external cooling and flank wear for internal cooling.

Non-linear behaviour and design of web stiffened battened built-up stainless steel channel sections under axial compression

Cold-formed stainless steel is becoming popular as a structural member with its increased corrosion resistance and durability when compared with carbon steel. Examples of cold-formed steel structures include trusses, wall frames and portal frames. In cases where increased axial strength is required, it is becoming increasingly popular to use battened built-up channel sections, instead of just back-to-back channel sections. For carbon steel battened built-up columns, limited experimental and finite element (FE) results are available in the literature. These results, however, are not applicable to cold-formed stainless steel. This issue is addressed numerically herein. Using the finite element model (FEM) previously reported by the first author of this paper for cold-formed carbon steel, an extensive parametric study, comprising 132 models is described in this paper for stainless steel battened built-up columns. Two different grades of stainless steel, that is, austenitic EN1.4404 and ferritic EN1.4003 have been considered. The effects of slenderness, different cross-sectional geometries and batten depth are investigated.From the results of parametric study, it was found that most of the short, intermediate, and long columns failed by distortional-flexural buckling interaction. For stub columns, distortional buckling governed the failure mode. The axial strengths determined from the parametric study were then compared against the design strengths calculated by the design guidance of carbon steel built-up columns from the American Iron and Steel Institute (AISI (2016)) and Australian and New Zealand Standards (AS/NZS (2018)). This is due to the unavailability of design guidelines for stainless steel battened built-up columns. From the comparison, it was found that the design strengths are 4% conservative on average for stainless steel battened built-up columns. Therefore, a design recommendation is proposed to modify the current design rules of carbon steel built-up columns, which was found to be accurate for stainless steel battened built-up columns.

Nonlinear behavior of cold-formed stainless steel built-up box sections under axial compression

Stainless steel is a highly versatile material, possessing a unique selection of properties that can be exploited in structural (load bearing) applications. In cold-formed steel structures, such as trusses, wall frames and portal frames, the use of built-up cold-formed stainless steel box sections as compression members are becoming popular. The AISI/ASCE Standard, SEI/ASCE-8–02 and AS/NZS do not include the design of stainless-steel built-up channels and very few experimental tests or finite element analyses have been reported in the literature for such face to-face cold-formed stainless-steel channel box sections. Thus, this paper presents a numerical investigation on the behavior of cold-formed stainless-steel built-up channel box sections. Three different grades of stainless steel i.e., duplex EN1.4462, ferritic EN1.4003 and austenitic EN1.4404 were considered. Effect of screw spacing on the axial strength of such built-up channel box sections was investigated. As expected, most of the short and intermediate columns failed by either local–global buckling interaction or just by local buckling, whereas the long columns, failed by global buckling. A comprehensive parametric study was carried out covering a wide range of slenderness and different cross-sectional geometries to assess the performance of the design guidelines by AISI and AS/NZS. In total, 180 finite element models were analyzed. From the results of the parametric study, it was found that the AISI and AS/NZS are conservative by around 15% for duplex grade (EN1.4462) of stainless-steel built-up box columns. However, for ferritic (EN1.4003) and austenitic (EN1.4404) grades of stainless-steel built-up box columns, the AISI and AS/NZS were conservative by only 5%.

A Differential Hall Effect Based Pressure Sensor

This paper presents the design and simulation of a pressure sensor integrated with two identical hall effect sensors and permanent magnets arranged in a differential configuration for measuring pressure in the range of 0–20 bar. The sensor uses the deflection of a circular diaphragm with a simple rigid mechanical structure to convert the applied pressure to a differential hall voltage output. A complete analytical modelling was carried out by assuming the rigid mechanical structure as a central circular boss structure on the circular diaphragm. Numerical simulations were also carried out in COMSOL Multiphysics FEM tool to support the analytical results. Before going for actual fabrication, the optimum sensor dimensions were also fixed from both analytical modelling and numerical simulation analysis. The sensor was planned to be fabricated completely using different grades of stainless steel and hence can be used in high temperature and corrosive environments. The fabricated sensor can be of low cost, self-packaged and the differential arrangement helps in compensating for any ambient temperature variations.

Axial capacity of back-to-back built-up cold-formed stainless steel unlipped channels-Numerical investigation and parametric study

In cold-formed steel structures, such as trusses, wall frames and portal frames, the use of back-to-back built-up cold-formed stainless steel unlipped channels as compression members are becoming popular. The advantages of using stainless steel as structural members are corrosion resistance and durability, compared with carbon steel. Current guidance by the American Iron and Steel Institute (AISI) and the Australian and New Zealand (AS/NZS) standards for built-up carbon steel sections describes a modified slenderness approach, to consider the spacing of the intermediate fasteners. The AISI and AS/NZS do not include the design of stainless-steel built-up channels and very few experimental tests or finite element (FE) analyses have been reported in the literature for such back-to-back cold-formed stainless steel unlipped channel section columns. This paper presents a numerical investigation on the behavior of back-to-back built-up cold-formed stainless steel unlipped channel section columns. Three different grades of stainless steel i.e., duplex EN1.4462, ferritic EN1.4003 and austenitic EN1.4404, were considered. The effects of screw spacing on the axial strength of such built-up unlipped channels were investigated. As expected, most of the short and intermediate columns failed by either local-global or local-distortional buckling interactions, whereas the long columns failed by global buckling. All three grades of stainless-steel stub columns failed by local buckling. A comprehensive parametric study was then carried out covering a wide range of slenderness and different cross-sectional geometries to assess the performance of the current design guidelines of carbon steel built-up sections in accordance with the AISI and AS/NZS. In total, 647 FE models were analyzed. From the results of the parametric study, it was found that the AISI and AS/NZS are conservative by around 14 to 20% for all three grades of stainless steel built-up unlipped channel section columns failed through global buckling. However, the AISI and AS/NZS carbon steel design rules can be un-conservative by around 8 to 13%, when they are used to calculate the axial capacity of those stainless steel built-up unlipped channels which are failed in local buckling.

Microwave heating: Fundasmentals and application in surface modification of metallic materials – A review

Abstract Recently microwave assisted materials processing earningvoguedue to inherent gains in terms of time and energy efficiency, substantially reduced process cycle time, revamp mechanical properties, and beneath environmental hazards. During processing of materials through microwaves, equally electric and magnetic energy is linearly transferred within materials via interfacing interaction between molecules. Microwave processing is pertinent to vast variety of materials including metal matrix composites (MMC), fibre reinforced polymer (FRP), alloys, ceramics, metals, pulverize metallurgy, metal coating/ cladding etc. This paper presents several key fundamentals of microwaves and concisely depicts the studied microwave materials interaction mechanism. Microwave energy are extensively used, notably in the areas like coating, glazing and cladding on various metallic substrate materials like mild steel, and different grades of stainless steel (SS-316, SS-316L, SS-304, AISI-420) etc. Microwave applications in improving the surface properties of metallic materials are briefly reviewed in this paper.

Experimental investigation of Mechanical properties in friction welding parameters for dis-similar metals (SS-304 & SS-410)

This paper mainly concentrated on the joining the different grades of stainless steel by the friction weld process. Stainless steel 304 and Stainless steel 410 are the class of iron based materials which has positive resistance to rusting and to corrosion in specific environments. Due to its good properties, it is the commonly used material in the engineering applications. Maximum of stainless steel welding’s are prepared by fusion welding. The effect of process parameter speed, friction time and forging time were studied. More over the joint strength is measured by the tensile test and Hardness of the joints.

The Critical Chloride Concentration of Austenitic and Duplex Stainless Steel Reinforcing Bars

The critical chloride concentrations for initiation of corrosion in reinforcing bar (rebar) are essential input parameters for service life predictions of reinforced concrete structures. For highway infrastructure, the specified service life is currently 75 to 120 years depending on the jurisdiction. Different techniques have been employed to determine these critical concentrations because of the many factors influencing the corrosion initiation. The current standard methods, e.g., ASTM G109, require weeks or months of testing and are often not logistically feasible for testing large numbers of specimens in different conditions in a reasonable time frame. The authors have proposed a potentiodynamic polarization technique for a rapid throughput assessment of the critical chloride concentration of different grades of stainless steel reinforcing bars in synthetic concrete pore solution corresponding to a specific concrete mixture design. This method was successfully employed in identifying the influence of alloying composition (Ni, Mo, Mn), testing solution, and rebar surface finish on their critical chloride concentration. The critical concentrations of Ni-containing stainless steel grades were higher than the Mn-containing grade by values greater than 13.5 pct by mass of solution. Mo only improved the pitting resistance and resulting critical chloride concentrations of the duplex grades, by 17 pct by mass of solution, but not the austenitic grades. The critical chloride concentrations of tested bars with processing flaw were lower by 13 pct by mass of solution. Sulfate addition and increment, as measured in pore solution expressed from cement pastes, in synthetic pore solution were found to increase the threshold values of these rebar grades by 4 to 8 pct by mass of solution.

The semiconductor properties of passive films and corrosion behavior of stainless steel reinforcing bars in simulated concrete pore solution

The vulnerability of concrete reinforcing steels to corrosion when depassivation occurs, typically in the presence of chloride, makes it important to understand the nature of the steels’ passive films. In Part I of the study, electrochemical techniques and Mott–Schottky analysis were used to investigate these films formed on five different grades of stainless steel and carbon steel reinforcing bars exposed to simulated concrete pore solution. The influence of the steel composition and surface finish on Mott–Schottky plots and the electronic properties are discussed in relation to the steels’ corrosion resistance. A p-type semiconductor behavior was observed in the stainless steel alloys in the cathodic potential regions and an n-type in the anodic potential regions. The n-type behavior is similar to that observed in the carbon steel. The electronic and electrochemical properties of the austenitic grades were superior to the duplex grades. The molybdenum-containing grades, 316LN and 2205, did not show the expected superior properties compared to molybdenum-free grades, 304L and 2304. Also, the replacement of nickel by manganese in the 24100 alloy was not found to be detrimental. The as-received bars showed electronic and electrochemical properties that are more realistic to field conditions than those of the polished cross-sections.

Numerical investigations on the axial capacity of back-to-back gapped built-up cold-formed stainless steel channels

Cold-formed stainless steel is becoming popular as a structural member with its increased corrosion resistance and durability when compared with carbon steel. Examples of cold-formed steel structures include trusses, wall frames and portal frames. In cases where increased axial capacity is required, it is becoming increasingly popular to use back-to-back gapped built-up channels, instead of just back-to-back sections. In such an arrangement, however, the beneficial effect of the gap between the back-to-back cold-formed steel channels is ignored by the current design guidelines (American Iron and Steel Institute and the Australian and New Zealand Standard and Eurocode (EN 1993-1-3)) for both cold-formed carbon as well as cold-formed stainless steel. We have considered this issue both experimentally and numerically through finite element analysis for cold-formed carbon steel; from the results of this work, design recommendations were proposed. These results, however, are not applicable to cold-formed stainless steel. This issue is addressed numerically herein. Using the finite element model previously developed for cold-formed carbon steel, an extensive parametric study, comprising 589 models is described. Three different grades of stainless steel, that is, ferritic EN1.4003, austenitic EN1.4404 and duplex EN1.4462 have been considered. The effect of the gap between the two back-to-back channels, slenderness, link-channel spacing and different cross-sectional geometries are investigated. The finite element results were then used to verify the accuracy of current design guidelines by the American Iron and Steel Institute and the Australian and New Zealand Standard and Eurocode (EN 1993-1-3). It was found that the American Iron and Steel Institute and the Australian and New Zealand Standard and Eurocode (EN 1993-1-3) can be over conservative by as much as 68% when calculating the axial capacity of such built-up gapped stainless steel columns. Therefore, a design recommendation is proposed to modify the non-dimensional slenderness so to consider the gap. This leads to the American Iron and Steel Institute and the Australian and New Zealand Standard and Eurocode (EN 1993-1-3) being within 7% conservative to the finite element results.

Numerical investigation into the buckling behaviour of face-to-face built-up cold-formed stainless steel channel sections under axial compression

Stainless steel is becoming popular as structural members because of its increased corrosion resistance and durability, compared with carbon steel. In cold-formed steel structures, such as trusses, wall frames and portal frames, the use of face-to-face built-up cold-formed stainless steel channel sections as compression members are becoming increasingly popular. In such an arrangement, intermediate fasteners at discrete points along the length prevent the individual channel sections from buckling independently. Current guidance by the American Iron and Steel Institute (AISI) and the Australian and New Zealand (AS/NZS) standards for built-up sections describes a modified slenderness approach, to take into account the spacing of the screws. Not even a single experimental test or finite element analysis, however, have been reported in the literature for such face-to-face built-up stainless steel channel sections, to understand the effect of screw spacing. The issue is addressed numerically herein. This paper presents a finite element investigation on the behaviour of face-to-face cold-formed stainless steel built-up channel sections, subjected to axial compression. Three different grades of stainless steel i.e. duplex EN1.4462, ferritic EN1.4003 and austenitic EN1.4404 have been considered. The effect of screw spacing on axial strength of face-to-face built-up stainless steel channel sections was investigated. A comprehensive parametric study was carried out, covering a wide range of slenderness and different cross sectional geometries to assess the performance of the current design guidelines by AISI and AS/NZS. In total, 160 finite element models were analyzed. From the results of the parametric study, it was found that the AISI & AS/NZS are conservative by around 15% for all stainless steel face-to-face built-up columns failed through global buckling. However, the AISI & AS/NZS are un-conservative by around 5% for face-to-face built-up stainless steel columns failed through local buckling.

Finite element modelling of back-to-back built-up cold-formed stainless-steel lipped channels under axial compression

In cold-formed steel structures, such as trusses, wall frames and portal frames, the use of back-to-back built-up cold-formed stainless-steel lipped channels as compression members are becoming increasingly popular. The advantages of using stainless-steel as structural members are corrosion resistance and durability, compared with carbon steel. The AISI/ASCE Standard, SEI/ASCE-8-02 and AS/NZS do not include the design of stainless-steel built-up channels and very few experimental tests or finite element analyses have been reported in the literature for such back-to back cold-formed stainless-steel channels. Current guidance by the American Iron and Steel Institute (AISI) and the Australian and New Zealand (AS/NZS) standards for built-up carbon steel sections only describe a modified slenderness approach, to consider the spacing of the intermediate fasteners. Thus, this paper presents a numerical investigation on the behavior of back-to-back cold-formed stainless-steel built-up lipped channels. Three different grades of stainless steel i.e., duplex EN1.4462, ferritic EN1.4003 and austenitic EN1.4404 have been considered. Effect of screw spacing on the axial strength of such built-up channels was investigated. As expected, most of the short and intermediate columns failed by either local-global or local-distortional buckling interactions, whereas the long columns, failed by global buckling. All three grades of stainless-steel stub columns failed by local buckling. A comprehensive parametric study was then carried out covering a wide range of slenderness and different cross-sectional geometries to assess the performance of the current design guidelines by AISI and AS/NZS. In total, 647 finite element models were analyzed. From the results of the parametric study, it was found that the AISI & AS/NZS are conservative by around 10 to 20% for cold-formed stainless-steel built-up lipped channels failed through overall buckling, irrespective of the stainless-steel grades. However, the AISI and AS/NZS can be un-conservative by around 6% for all three grades of stainless-steel built-up channels, which failed by local buckling.

Structural behaviour of cold-formed stainless steel bolted connections at post-fire condition

This paper presents an experimental investigation on cold-formed stainless steel single shear and double shear bolted connections at post-fire condition. The connection specimens were fabricated by three different grades of stainless steel. The three different grades of stainless steel are austenitic stainless steels EN 1.4301 (AISI 304) and EN 1.4571 (AISI 316Ti) as well as lean duplex stainless steel EN 1.4162 (AISI S32101). The post-fire connection specimens were heated to different nominal temperatures of 350, 650 and 950°C, respectively, and then cooled down to room (ambient) temperature condition. A total of 82 new connection tests was conducted. The test results of post-fire specimens were compared with those tested without post-fire condition for the same specimen series. Generally, it is found that the single shear and double shear bolted connection specimens cooled down from the nominal temperatures of 350 and 650°C had higher ultimate strengths than those specimens without expose to high temperatures for all three different grades of stainless steel. The specimens cooled down from 950°C generally had lower ultimate strengths than the aforementioned specimens. It is also found that the failure modes of cold-formed stainless steel bolted connection specimens at post-fire condition are similar to those tested without post-fire condition. Finally, design rules are proposed for cold-formed stainless steel bolted connections at post-fire condition for temperature up to 950°C.

Revisiting passivation II: electronic conduction of transition metal oxides, Mott-Schottky plots, and root cause of stainless steel grades and nickel based alloys

The novel views on passivation and passivity are briefly reviewed, with further explanation of some key details. First, the most common polymerisation mechanism directly transforms an insoluble hydroxide into a cross-linked oxide gel. The Al type passivation corresponds to insulating oxides, and this directly leads to a nearly sealed off gel with just a residual access of the electrolyte to the metal surface. In Fe type passivation, the electronic 3d shells are incomplete, which induces a coexistence of bi- and trivalent monomers, and from elsewhere well-known quantum (or magnetic) effects, a complete ordering can suddenly induce an electronic conduction at the interatomic scale. The different grades of stainless steels or Ni based alloys, and a few other paradoxical observations, are thus explained by the required conjunction of magnetic, chemical and electrochemical features. In parallel, any applied polarisation induces a junction potential at the electrolyte boundary, with electric field, ionic space charges, and local electromigration. All are cancelled in the Faraday cage of a conductive gel, but its adaptation to each potential takes some time. Therefore, the alleged “Mott-Schottky” plots do not detect the electronic space charges of a supposedly semiconductor, but just the local ionic space charges induced by artificial polarisations.

Tailoring residual stress profile of Selective Laser Melted parts by Laser Shock Peening

The paper describes a new approach in controlling and tailoring residual stress profile of parts made by Selective Laser Melting (SLM). SLM parts are well known for the high tensile stresses in the as – built state in the surface or subsurface region. These stresses have a detrimental effect on the mechanical properties and especially on the fatigue life. Laser Shock Peening (LSP) as a surface treatment method was applied on SLM parts and residual stress measurements with the hole – drilling method were performed. Two different grades of stainless steel were used: a martensitic 15-5 precipitation hardenable PH1 and an austenitic 316L. Different LSP parameters were used, varying laser energy, shot overlap, laser spot size and treatments with and without an ablative medium. For both materials the as-built (AB) residual stress state was changed to a more beneficial compressive state. The value and the depth of the compressive stress was analyzed and showed a clear dependence on the LSP processing parameters. Application of LSP on SLM parts showed promising results, and a novel method that would combine these two processes is proposed. The use of LSP during the building phase of SLM as a “3D LSP” method would possibly give the advantage of further increasing the depth and volume of compressive residual stresses, and selectively treating key areas of the part, thereby further increasing fatigue life.

Design and testing of piezoelectric resonant pressure sensor

A stainless steel resonant pressure sensor with a new design is proposed with piezoelectric excitation and detection. The sensor consists of a sensing diaphragm, inclined trusses, vertical mounts and a resonating beam. The deflection of the diaphragm is transferred to the resonating beam via specially designed inclined trusses and vertical mounts. The analytical model of the sensor is developed using Ritz method and direct stiffness method for the non uniform sensing diaphragm and resonating beam respectively. The relation between strain due to applied pressure and change in the resonance frequency is derived. The sensor is also modelled numerically using MEMS CAD Tool CoventorWare. The sensor is fabricated with three different grades of stainless steel namely SS 304, SS 431, 15-5 PH, using Electrical Discharge Machining (EDM) and wire cut EDM process. The sensors are tested for its characteristics for an input pressure of 0–25bar. The sensor fabricated using 15-5 PH is found to have good linearity, repeatability, higher sensitivity and low hysteresis compared to the sensors fabricated with SS 304 and SS 431. The sensor design is simple, fabrication involves well known machining process, self packed and hence cost effective.

Weldability assessment and high-temperature properties of advanced creep resisting austenitic steel X6CrNiNbN25-20 (DMV310N/HR3C)

The modern Ultra Super Critical Power Plants (USC PP) applying the 600 °C technology require advanced stainless steels in superheater/reheater systems in order to cope with the increased steam parameters. Different grades of stainless steels have been developed by increasing Cr-contents, alloying with stabilizing and precipitating elements as well as thermomechanical heat treatments resulting in high creep rupture strengths and improved oxidation/corrosion resistance. In the context of a collaborative research project, X6CrNiNbN25-20 (DMV310N/HR3C) has been investigated. The main focus of the research project was on characterization and weldability assessment. As a result, the base metal under investigation was compared with governing code cases and specifications. Base metal chemical composition, microstructures, mechanical properties, reheat cracking sensitivity, and hot ductility as well as creep rupture strengths have been investigated. A weldability assessment, including thermal simulation and welding procedure qualifications, has been performed to establish parameter windows for similar and dissimilar welding. Dissimilar welding between grade 92 and austenitic steel tubes has been performed. The project also took the opportunity to investigate the behavior of a recently developed gas tungsten arc welding (GTAW) P87 consumable for dissimilar welding. Cross-weld creep rupture tests have been conducted for both similar and dissimilar welding, while aging tests addressed microstructural stability.

High temperature tests of cold-formed stainless steel double shear bolted connections

There is currently no design rule on bolted connections of cold-formed stainless steel structures at elevated temperatures. In this study, a total of 194 double shear bolted connection specimens with three different grades of stainless steel were tested, where 106 specimens were tested by steady state test method and 88 specimens were tested by transient state test method. The three different grades of stainless steel are austenitic stainless steel EN 1.4301 (AISI 304) and EN 1.4571 (AISI 316Ti having small amount of titanium) as well as lean duplex stainless steel EN 1.4162 (AISI S32101). The connections were designed with different bolt diameters, number of bolts and arrangement of the bolts. Bearing failure and net section tension failure modes were observed in the double shear bolted connection tests. The test results were compared with the nominal strengths calculated from the design rules in the American Specification, Australian/New Zealand Standard and European Codes for stainless steel structures. In calculating the nominal strengths of the connections, the material properties at elevated temperatures were used in the design equations for room temperature. It is shown that the strengths of the cold-formed stainless steel double shear bolted connections obtained from the specifications are generally conservative at elevated temperatures. The connection strengths decrease as the temperature increases in the similar manner for the steady state tests and the transient state tests as well as the material coupon tests. It is also found that the austenitic stainless steel type EN 1.4571 generally has better resistance than the stainless steel types EN 1.4301 and EN 1.4162 for double shear bolted connections at elevated temperatures.