Characterisations of electrospark deposition Stellite 6 alloy coating on 316L sealed valve used in nuclear power plant

Abstract

In nuclear plant the protection of the H₂BO₄ solution seal areas of AISI 316L austenitic steel is accomplished by Stellite 6 plasma cladding layer. Wear defects created in service required the part to be replaced or repaired. No existing repair technologies were practical. Electrospark deposition (ESD) was to repair defects, enabling the parts to be placed back in service. In this paper the authors report the results obtained depositing directly a layer of Stellite 6 alloy onto a 316L austenitic stainless and Stellite 6 plasma cladding layer by using ESD technique. Electrospark deposition can apply metallurgical bonded coatings without the need of post-heat treatment. Structure, hardness, chemical composition and morphology of the ESD coating have been analysed. By electrochemical measurements it is inferred that the corrosion resistance of the ESD coating is comparable to that of the 316L and Stellite 6 plasma cladding layer. The hardness improvement was ascribed to the refine microstructure and the rapid solidification.     

Increased liquid droplet erosion resistance of steam turbine blades

Abstract

Alloys used for the blades of steam turbines usually do not show a satisfactory resistance to erosion and require surface protection with suitable coatings. Starting from the results obtained with Stellite 6 coatings on AISI 420 stainless steel blades, this research has been developed with the aim to increase erosion resistance by modifying the chemical composition of the coatings. Several samples have been coated by laser cladding and exposed to liquid erosion tests. In particular, concentrations of carbide forming and solid solution strengthening elements have been varied in order to establish their effects against liquid droplet erosion. Results of the tests and EDX analyses on the samples are reported, with a discussion on the various effects on erosion resistance. Two directions have appeared as the most promising, either increasing the concentration of carbide forming elements, or increasing the concentration of nickel in the coatings.     

EFFECT OF VARIOUS DRIVING FORCES ON HEAT AND MASS TRANSFER IN ARC WELDING

Abstract

In this study the heat transfer and fluid flow of the molten pool in stationary gas tungsten arc welding using argon shielding gas were investigated. Transporting phenomena from the welding arc to the base material surface, such as current density, heat flux, arc pressure, and shear stress acting on the weld pool surface, were taken from the simulation results of the corresponding welding arc. Various driving forces for the weld pool convection were considered: self-induced electromagnetic, surface tension, buoyancy, and impinging plasma arc forces. Furthermore, the effect of surface depression due to the arc pressure acting on the molten pool surface was considered. Because the fusion boundary has a curved and unknown shape during welding, a boundary-fitted coordinate system was adopted to precisely describe the boundary for the momentum equation. The numerical model was applied to AISI304 stainless steel and compared with the experimental results.     

Acoustic emission study of deformation behaviour of sensitised 304 stainless steel

Abstract

A systematic study has been undertaken to correlate the changes in acoustic emissions during tensile deformation of sensitised AISI type 304 stainless steel. Samples of a typical 304 stainless steel were sensitised at 700°C for 4, 14 and 24 h after being austenised at 1050°C for 30 min. AE signals were recorded during tensile test by using two sensors with 125 kHz resonant frequency. The results showed significant change in generation of AE during tensile deformation of sensitised AISI 304 stainless steel in compare to solution annealed material. This type of behaviour could be attributed to the microstructural changes in the sensitised specimens especially formation of continuous Cr₂₃C₆ carbides on grain boundaries which lead to increase in shearing by dislocations.     

Localised corrosion processes of austenitic stainless steel bipolar plates for polymer electrolyte membrane fuel cells

This research addresses the problem of localised corrosion of stainless steel PEMFC bipolar plates. The susceptibility to pitting and crevice corrosion of austenitic AISI 304 stainless steel has been investigated both by post-mortem microscopic analysis of the end-plates of a laboratory single-cell and by studies of electrochemically corroded stainless steels, in the presence of specially-designed crevice-formers simulating the operating conditions of a PEMFC. This work is based on optical and scanning-electron microscopies as well as potentiostatic and potentiodynamic measurements. The crevice-formers we considered were: Teflon, graphite and AISI 304. The samples, coupled to the crevice-formers have been tested in aqueous solutions containing Cl⁻, SO₄²⁻ and F⁻. From the E-log i plot, the values of corrosion, pitting, crevice and protection potential have been obtained and perfect and imperfect passivity conditions have been identified.     

Cyclic deformation and microstructural behaviour of reduced activation ferritic–martensitic steels

Abstract

The present paper presents results about cyclic behaviour and the evolution of the dislocation structure of reduced activation ferritic–martensitic steels and commercial martensitic steels AISI 410 and 420. The variation of the free dislocation density within subgrains and subgrain size was mainly analysed during the cyclic softening of EUROFER 97 steel. From the analysis of the flow stress components, the friction and back stresses, and the information of the evolution of the dislocation structure, it could be concluded that the softening of tempered martensitic steels at 20°C is produced by the contribution of the friction stress and aided later by the back stress.     

Electrochemical nitridation of a stainless steel for PEMFC bipolar plates

AISI446 steel has been electrochemically nitrided in 0.1 M HNO₃ + 0.5 M KNO₃ solution at room temperature. XPS analysis revealed surface NH₃ and a deeper nitride layer. The surface layer of the stainless steel modified by electrochemical nitridation was thus composed of a nitrogen-incorporated oxide film. The nitrided steel showed very low interfacial contact resistance (ca. 18 mΩ cm² at 140 N/cm²) and excellent corrosion resistance in simulated PEMFC environments. Electrochemical nitridation provides an economic way to modify the stainless steel’s surface, and is very promising for application to fuel cell bipolar plates.     

Thermal nitridation of chromium electroplated AISI316L stainless steel for polymer electrolyte membrane fuel cell bipolar plate

The electrical and corrosion properties of surface-nitrided AISI316L stainless steel are evaluated to assess the potential use of this material as a bipolar plate for a polymer electrolyte membrane fuel cell. Chromium is electroplated on the surface of the AISI316L stainless steel before nitridation. The nitriding condition is selected so as to form Cr₂N nitride only and the result is compared with that of a CrN + Cr₂N nitride coating. The stainless steels with the Cr₂N nitride protective coating layer exhibit better interfacial contact resistance and corrosion resistance than the as-rolled or (CrN + Cr₂N)-coated AISI316L stainless steels.     

The effect of pH and halides on the corrosion process of stainless steel bipolar plates for proton exchange membrane fuel cells

Stainless steel is attractive as material for bipolar plates in proton exchange membrane fuel cells, due to its high electrical conductivity, high mechanical strength and relatively low material and processing cost. Potentiostatic and potentiodynamic tests were performed in H₂SO₄ solutions on AISI 316L stainless steel bipolar plates with etched flow fields. The effect of pH and presence of small amounts of fluoride and chloride on the corrosion rate and interfacial contact resistance of the stainless steel bipolar plate were investigated. The tests performed in electrolytes with various pH values revealed that the oxide layer was thinner and more prone to corrosion at pH values significantly lower than the pH one expects the bipolar plate to experience in an operating proton exchange membrane fuel cells. The use of solutions with very low pH in such measurements is thus probably not the best way of accelerating the corrosion rate of stainless steel bipolar plates. By use of strongly acidic solutions the composition and thickness of the oxide layer on the stainless steel is probably altered in a way that might never have happened in an operating proton exchange membrane fuel cell. Additions of fluoride and chloride in the amounts expected in an operating fuel cell (2 ppm F⁻ and 10 ppm Cl⁻) did not cause significant changes for neither the polarization- nor the contact resistance measurements. However, by increasing the amount of Cl⁻ to 100 ppm, pitting was initiated on the stainless steel surface.     

SnO2:F coated ferritic stainless steels for PEM fuel cell bipolar plates

Ferrite stainless steels (AISI441, AISI444, and AISI446) were successfully coated with 0.6 μm thick SnO₂:F by low-pressure chemical vapor deposition and investigated in simulated PEMFC environments. The results showed that a SnO₂:F coating enhanced the corrosion resistance of the alloys in PEMFC environments, though the substrate steel has a significant influence on the behavior of the coating. ICP results from the testing solutions indicated that fresh AISI441 had the highest dissolution rates in both environments, and coating with SnO₂:F significantly reduced the dissolution. Coating AISI444 also improved the corrosion resistance. Coating AISI446 steel further improved the already excellent corrosion resistance of this alloy. For coated steels, both potentiostatic polarizations and ICP results showed that the PEMFC cathode environment is much more corrosive than the anode one. More dissolved metallic ions were detected in solutions for PEMFC cathode environment than those in PEMFC anode environment. Sn²⁺ was detected for the coated AISI441 and AISI444 steels but not for coated AISI446, indicating that the corrosion resistance of the substrate has a significant influence on the dissolution of the coating. After coating, the ICR values of the coated steels increased compared to those of the fresh steels. The SnO₂:F coating seems add an additional resistance to the native air-formed film on these stainless steels.     

Thermomechanical modelling of weld microstructure and residual stresses in P91 steel pipe

Abstract

A multipass circumferentially butt welded P91 steel pipe, typically used for high temperature applications in power plants, has been numerically analysed to determine residual stresses, induced by the process of welding, as well as microstructural regions in the weld, caused by thermal cycles. The finite element (FE) method has been applied to simulate residual stresses generated in the weld region and heat affected zone (HAZ), which are then validated by published experimental data. The axisymmetric FE simulation incorporates solid state phase transformation by allowing for volumetric changes and associated changes in yield stress and hardening behaviour due to austenitic and martensitic transformations. The thermal cycles during welding cause different microstructural regions to emerge within the weld metal and HAZ. Columnar and equiaxed microstructural zones have been numerically modelled in the weld region of the pipe. The predicted FE microstructural regions have been corroborated by columnar and equiaxed zones that have been mapped out on a cross-sectional macroimage of the weld.     

Influence of fuel ashes on corrosion of surface coatings cladded by CMT method

The aim of the present research was to investigate the influence of different biomasses and sewage sludge ash (at 650°C for 1000 h and 2000 h, respectively) on the surface of weld cladded (Cold Metal Transfer technique, CMT) stainless steels 309 and 310. The biomass ashes were rich in K₂O, CaO, SiO₂, whereas sewage sludge ash in P₂O₅, CaO, Fe₂O₃, and SiO₂. Characterization of the stainless steel cross-section and surface after the corrosion process inducted by the presence of ash were carried out by scanning electron microscopy (SEM) with EDAX analysis. Phase analysis of corrosion products was made by X-ray diffraction (XRD). Chromium, nickel, and iron oxides were the main oxides which occurred after the corrosion test. It has been proved that biomass ashes have the corrosive properties (because of their chemical composition) and are more aggressive than the sewage sludge ash for that kind of materials.     

Corrosion behavior of TiN-coated stainless steel as bipolar plate for proton exchange membrane fuel cell

Stainless steel is a potential material to be used as the bipolar plate for proton exchange membrane fuel cell (PEFC) because of its suitable physical and mechanical properties. Several coating techniques have been applied to improve its corrosion resistance. But seldom study is focused on the microstructure evolution with corrosion. In the present study, the use of TiN-coated stainless steel as the bipolar plate is evaluated. Two surface coating techniques, pulsed bias arc ion plating (PBAIP) and magnetron sputtering (MS), are adoped to prepare the TiN-coated stainless steel. Their corrosion resistances and electrical conductivities of the coated substrates are evaluated. The performance shows strong dependance on microstructural characteristics. The corrosion of SS304/Ti₂N/TiN prepared by MS mainly occurs on the grain boundary. The corrosion of SS304/TiN prepared by PBAIP mainly takes place from the large particles on the coating. The Ti₂N/TiN multilayer coating provides superb corrosion protective layer for stainless steel. Both the TiN and Ti₂N/TiN coatings provide low contact resistance.     

Effect of substrate material on the corrosion of TiN-coated stainless steels in simulated anode and cathode environments of proton exchange membrane fuel cells

A physical vapor deposition (PVD) TiN coating has been used to increase the corrosion resistance of two stainless steel materials for bipolar plate application in proton exchange membrane fuel cells (PEMFCs). Our earlier studies had shown that a TiN coating on SS410 and SS316L increased the corrosion resistance of SS410 and SS316L significantly. In this study, we examine how the substrate affects the corrosion of TiN-coated stainless steel in the simulated anode and cathode environments. Potentiodynamic and contact resistance test results show that the polarization resistance and contact resistance of TiN-coated SS410 and TiN-coated SS316L are almost the same. However, in the simulated anode condition, the corrosion current density of TiN-coated SS410 is positive and the corrosion current density of TiN-coated SS316L is negative. Inductively coupled plasma optical emission spectrometry (ICP-OES) test results also show that the metal ion concentration is much higher for TiN-coated SS410 at the anode side. At the cathode side, the potentiostatic and ICP-OES tests show that the corrosion of TiN-coated SS410 and TiN-coated SS316L are in the same range. Therefore, the substrate has an effect on corrosion in the simulated anode working conditions of PEMFCs. In order to be the suitable bipolar plate materials, both the coating and substrate need to have a higher corrosion resistance.     

Carbon film-coated 304 stainless steel as PEMFC bipolar plate

Carbon film-coated stainless steel (CFCSS) has been evaluated as a low-cost and small-volume substitute for graphite bipolar plate in polymer electrolyte membrane fuel cell (PEMFC). In the present work, AISI 304 stainless steel (304SS) plate was coated with nickel layer to catalyze carbon deposits at 680°C under C₂H₂/H₂ mixed gas atmosphere. Surface morphologies of carbon deposits exhibited strong dependence on the concentration of carbonaceous gas and a continuous carbon film with compact structure was obtained at 680 °C under C₂H₂/H₂ mixed gas ratio of 0.45. Systematic analyses indicated that the carbon film was composed of a highly ordered graphite layer and a surface layer with disarranged graphite structure. Both corrosion endurance tests and PEMFC operations showed that the carbon film revealed excellent chemical stability similar to high-purity graphite plate, which successfully protected 304SS substrate against the corrosive environment in PEMFC. We therefore predict CFCSS plates may practically replace commercial graphite plates in the application of PEMFC.     

Temperature distribution and residual stresses due to multipass welding in type 304 stainless steel and low carbon steel weld pads

Welding is a reliable and efficient metal-joining process widely used in industry. Due to the intense concentration of heat in the heat source of welding, the regions near the weld line undergo severe thermal cycles, thereby generating inhomogeneous plastic deformation and residual stresses in the weldment. Plates of different thickness are used in industry and these plates are normally joined by multipass welding. In a multipass welding operation, the residual stress pattern developed in the material changes with each weld pass. In the present experimental work, thermal cycles and transverse residual stresses due to each pass of welding have been measured in the weld pads of AISI type 304 stainless steel and low carbon steel with 6, 8 and 12 mm thickness. X-ray diffraction method was used for residual stress measurements. The welding process used was the Manual Metal Arc Welding (MMAW) process. In this paper, the peak temperatures attained at different points during deposition of weld beads in stainless steel and low carbon steel weld pads are compared. The residual stress patterns developed, the change in the peak tensile stress with the deposition of weld beads, and the relation between the peak temperatures and the residual stresses in the weld pads are discussed.     

Ferritic stainless steels as bipolar plate material for polymer electrolyte membrane fuel cells

Both interfacial contact resistance (ICR) measurements and electrochemical corrosion techniques were applied to ferritic stainless steels in a solution simulating the environment of a bipolar plate in a polymer electrolyte membrane fuel cell (PEMFC). Stainless steel samples of AISI434, AISI436, AISI441, AISI444, and AISI446 were studied, and the results suggest that AISI446 could be considered as a candidate bipolar plate material. In both polymer electrolyte membrane fuel cell anode and cathode environments, AISI446 steel underwent passivation and the passive films were very stable. An increase in the ICR between the steel and the carbon backing material due to the passive film formation was noted. The thickness of the passive film on AISI446 was estimated to be 2.6 nm for the film formed at −0.1 V in the simulated PEMFC anode environment and 3.0 nm for the film formed at 0.6 V in the simulated PEMFC cathode environment. Further improvement in the ICR will require some modification of the passive film, which is dominated by chromium oxide.     

Corrosion fatigue behaviour of 317LN austenitic stainless steel in phosphoric acid

The corrosion fatigue crack-growth behaviour of AISI 317LN stainless steel was evaluated in air and in 85% phosphoric acid at 20 °C. Austenitic stainless steels with high molybdenum content have high corrosion resistance and good mechanical properties. However, this increase in the molybdenum content and other elements such as nitrogen can also modify the microstructure. This leads to a modification of its mechanical properties. The corrosion fatigue crack-growth rate was higher in phosphoric acid immersion than in air. Austenitic stainless steels with a fully austenitic microstructure were more ductile, tough, and behave better against corrosion fatigue. The higher resistance to corrosion fatigue was directly associated to its higher resistance to corrosion.     

304不锈钢管凝汽器腐蚀原因研究

进行了不锈钢管、水处理药剂和酸洗液的腐蚀性能试验。凝汽器304不锈钢管发生点蚀的主要原因是不锈钢管质量不合格和使用的酸洗液不当。不锈钢酸洗液腐蚀性能的评价不能采用腐蚀失重法。订合同时应写入按照ASTM A249中S7条款检验不锈钢管焊缝质量。     

Charpy energy–lateral expansion relations for a wide range of steels

Charpy energy (C_V)–lateral expansion (LE) relations are useful in situations where a facility for measurement of LE is not available. In this note, after presenting the data for various steels in a single plot to get a global picture and discussing the dependence of the C_V–LE plot on the type of steel, the data for particular classes of steels (austenitic stainless steels (SSs), AISI 403 martensitic SS, A533B steel, 9Cr–1Mo steels, etc.) are replotted in individual plots, and the mean quadratic fit to the plot is given as well as (where appropriate) the 95% confidence lower bound curve. Finally, C_V normalized by yield stress has been shown to bring all the data into a single band and a quadratic fit curve equation has been derived and shown to predict LE from C_V with acceptable conservatism for engineering purposes. A 95% lower-bound curve equation has also been given.