Cast Stainless Steel Research Articles

Modelling of coupling flow and temperature fields in molten pool during twin-roll strip casting process

Abstract In twin-roll strip casting process, metal flow and temperature distribution in the molten pool directly affect the process stability and the quality of products. In this paper, a 3D coupling temperature-flow finite element method (FEM) simulation has been carried out and an inverse method was used to determine the boundary conditions between the roll and molten pool. For twin-roll casting of stainless steel, the influences of key processing parameters such as the pouring temperature and the height of liquid level on the temperature and flow fields are discussed. Calculated strip surface temperatures for different pouring temperatures are compared with the measured values, which shows they are in agreement.

OS3-2-4 Non-destructive characterization of laser-peened materials with synchrotron radiation of SPring-8

Laser peening without coating (LPwC) is an emerging surface technology to enhance fatigue strength and prevent stress corrosion cracking (SCC) of metallic materials. LPwC was applied to SUS304 austenitic stainless steel and AC4CH cast aluminum alloy samples with a Q-switched and frequencydoubled Nd:YAG laser. Residual stress depth profile over the top surface of the SUS304 sample was precisely measured by a constant penetration depth (CPD) method, which is in the framework of the sin^2Ψ method and can strictly control the X-ray penetration depth into the sample material. By combining the CPD method with high energy X-ray of SPring-8, the depth profile was evaluated non-destructively before, during and after heat treatment up to 673 K (400 ℃). The residual stress in the top surface of the SUS304 sample is entirely compressive despite the direct irradiation of intense laser pulses on the material and the thermal loading. High-cycle fatigue testing was performed for AC4CH samples with and without LPwC, which demonstrated that LPwC significantly prolonged the fatigue life. Replication technique was applied to evaluate the propagation behavior of fatigue cracks on surface. Three-dimensional (3D) images of fatigue cracks were reconstructed by micro tomography (μCT) with phase contrast effect using highly parallelized coherent X-ray of SPring-8. The retardation of crack growth was confirmed in the material with LPwC in the images of μCT.

Microstructure characterization of austenitic Fe–25Mn–22Cr–2Si–0.7N alloy processed by twin roll strip casting

Near net shape casting of austenitic stainless steel containing high Mn and N was explored by using a twin roll strip caster. Microstructural observation and analysis of the as-cast strip by XRD, SEM, and TEM demonstrated that the formation of δ-ferrite and carbides had been prohibited due to the high solidification cooling rate of more than 14,000 K/s during strip casting. Tensile tests indicated that the as-cast strip possessed reasonably good mechanical properties, with a yield strength of 500 MPa and elongation of about 15%. The elastic modulus of the as-cast strip was higher than that of solution treated steel by about one order of magnitude. From the TEM observations, it was found that the as-cast strip contained more dislocations and numerous interstitial elements (N and C) dissolved in grains than the solution treated material, in which dislocations were scarcely observed and precipitates were formed along grain boundaries. Fracture morphology indicates that the as-cast strip has undergone ductile rupture during tensile testing. Deformation twins were observed in the deformed region of the sample by the in-situ tensile test under SEM and TEM observations. The formation of deformation twins were believed to be responsible for high plasticity of the as-cast strip.

Evaluation of Low-Cycle Fatigue in Simulated PWR Environment

For developing fatigue design curve of cast stainless steel that is used in piping material of nuclear power plants, a low-cycle fatigue test rig was built. It is capable of performing tests in pressurized high temperature water environment of PWR. Cylindrical solid fatigue specimens of CF8M were used for the strain-controlled environmental fatigue tests. Fatigue life was measured in terms of the number of cycles with the variation of strain amplitude at 0.04%/s strain rates. The disparity between target length and measured length of specimens was corrected by using finite element method. The corrected test results showed similar fatigue life trend with other previous results.

Correlation Between Molten Vanadium Salts and the Structural Degradation of HK-Type Steel Superheater Tubes

HK steels are among the most used heat-resistant cast stainless steels, being corrosion-resistant and showing good mechanical properties at high service temperatures. These steels are widely used in reformer furnaces and as superheater tubes. During service, combustion gases leaving the burners come in contact with these tubes, resulting in corrosive attack and a large weight loss occurs due to the presence of vanadium, which forms low melting point salts, removing the protective oxide layer. In this work the external surface of a tube with dramatic wall thickness reduction was analyzed using light microscopy, scanning electron microscopy, and transmission electron microscopy. The identification of the phases was achieved by energy dispersive spectroscopy (EDS) analyses. The results showed oxides arising from the external surface. In this oxidized region vanadium compounds inside chromium carbide particles were also observed, due to inward vanadium diffusion during corrosion attack. A chemical reaction was proposed to explain the presence of vanadium in the metal microstructure.

Effect of stress relief at 350 °C and 550 °C on the impact properties of duplex stainless steels

The production of massive duplex stainless steel castings weighting over 2 t, with thicknesses exceeding 5 in. represents a major challenge for the foundry industry. The difficulty in manufacturing such castings lies in the fact that thick sections experiment low cooling rates during the solidification process and during the solution annealing and water quenching heat treatment. As a result, intermetallic phases such as sigma phase (σ), Chi phase (χ), G phase, R phase, and complex carbides may precipitate, causing the material to be extremely brittle [Martins M, Casteletti LC. Effect of heat treatment on the mechanical properties of ASTM A890 grade 6A super duplex stainless steel. J ASTM Int 2005;2(1) [January]. [1]]. After solution annealing and water quenching, the steel is, in principle, free of intermetallic precipitates, but will contain residual stresses resulting from rapid cooling on quenching. During and after machining, these stresses may produce dimensional distortions in the casting, which can be avoided or at least reduced with stress relief heat treatments at intermediary temperatures, taking care to prevent the loss of mechanical properties, mainly impact toughness. The purpose of this study was to investigate the behavior of CD4MCu and CD4MCuN duplex stainless steels in impact tests under the conditions of solution annealing and water quenching and stress relief at 350 °C for 4 h and at 550 °C for 2 h. Compared to CD4MCu the high nitrogen content of CD4MCuN stainless steel has a more balanced microstructure with similar ferrite and austenite contents, providing it with higher energy-absorbing capacity in impact tests. CD4MCuN fracture surfaces have predominantly fibrous structures typical of high toughness materials, while the CD4MCu steel's fracture surface shows cleavage facets typical of low toughness materials. The stress relief heat treatments reduced the impact toughness of the CD4MCu alloy but did not affect the CD4MCuN alloy.

The Effect of the Process Temperature on the Bondability in Transient Liquid Phase (tlp) Diffusion Bonding of AISI 430 Ferritic Stainless Steel with Nodular (Spheroid) Cast Iron Using a Copper Interlayer

Abstract The effect of the process temperature on the bondability in the transient liquid phase (tlp) was studied. The diffusion bonding of AISI 430 with nodular cast iron using a copper interlayer was investigated experimentally under a protective atmosphere at various temperatures and at a defined constant blow pressure which would cause macro deformation. Microstructure examinations were carried out with the help of optical microscopy, SEM and EDS. Hardness values were measured by HV scale under a load of 150 g. At the bond interface, decarburized and dechromised regions were observed on the ductile cast iron side and stainless steel side respectively. Best mechanical and metallurgical properties were observed for the specimen bonded at 1100°C for 20 min. As a function of an elevation of the process temperature, an increase in the amount of graphite atoms that migrated toward the interface and a reduction at the nodular graphite diameters was noticed.

KUBOTA HH II

Abstract Kubota alloy HH II is a heat-resistant fully austenitic stainless steel casting for multiple furnace parts. This datasheet provides information on composition, physical properties, elasticity, and tensile properties as well as creep. It also includes information on high temperature performance as well as casting and joining. Filing Code: SS-980. Producer or source: Kubota Metal Corporation, Fahramet Division.

Assessment of the load-bearing capacity of a primary pipeline

High-alloyed Cr–Ni-based two-phase stainless steel (SS) cast alloys are commonly used in nuclear power plants. The mechanical equipment in these facilities can contribute to a reduction in its resistance to stable crack growth as a result of extended operating times and high temperatures. The toughness of these materials strongly depends on their delta (δ) ferrite content, which spinodally decomposes into two phases with different ratios of Cr and Ni at a relatively low (slightly above 300 °C) temperature. This temperature is similar to the operating temperature of the vital parts, for example, the coolant system. The formation of two phases with the same crystal structure but different lattice parameters causes internal elastic stresses that result in a hardness increase and an impact-toughness decrease. The result is an increased risk of crack formation in the stress–concentration zones such as the critical regions of different welded joints (e.g. “L, T, K and X” shapes). The values of the critical stress intensity factor change according to its position along the crack contour. Therefore, the aim of our study was to assess the influence of the materials’ changes on the crack extension and the decrease of the primary pipeline’s bearing capacity by taking account of the increased temperature and time of operation for the given loading conditions. The SINTAP (European Structural Integrity Assessment Procedure) was used for this assessment.

CF8M과 SA508 용접재의 열화에 따른 파괴인성에 관한 연구

In a primary reactor cooling system(RCS), a dissimilar weld zone exists between cast stainless steel(CF8M) in a pipe and low-alloy steel(SA508 cl.3) in a nozzle. Thermal aging is observed in CF8M as the RCS is exposed for a long period of time to a reactor operating temperature between 290 and 330℃, while no effect is observed in SA508 cl.3. The specimens are prepared by an artificially accelerated aging technique maintained for 300, 1800 and 3600 hrs at 430℃, respectively. The specimens for elastic-plastic fracture toughness tests are according to the process in the thermal notch is created in the heat affected zone(HAZ) of CF8M and deposited zone. From the experiments, the JIC value notched in HAZ of CF8M presented a rapid decrease up to 300 hours at 430℃ and slowly decreased according to the process in the thermal aging time. Also, the JIC value presented a lower value than that of the CF8M base metal. And, the JIC of the deposited zone presented the lowest value of all other cases.

Fracture Toughness with Thermal Aging in CF8M/SA508 Welds

In a primary reactor cooling system, a dissimilar weld zone exists between cast stainless steel (CF8M) in a pipe and low-alloy steel (SA508 cl.3) in a nozzle. Thermal aging is observed in CF8M as the RCS is exposed for a long period of time to a reactor operating temperature between 290 and 330, while no effect is observed in SA508 cl.3. The aged specimens are prepared by an artificially accelerated aging technique maintained for 300, 1800 and 3600 hrs at 430, respectively. The specimens for elastic-plastic fracture toughness tests are prepared two types, which notch is created in the center of deposited zone and the heat affected zone of CF8M. From the experiments, the plastic-elastic fracture toughness values (JIC) with the increase of aging time decrease as the notch is created in the HAZ of CF8M, while that is different slightly as the notch is created in the deposited zone. Also, JIC values in the deposited zones are smaller than the HAZ of CF8M at all aged specimens.

Modern small-capacity unit for the vacuum-oxygen refining of steel

A vacuum-oxygen refining unit has been installed in the pouring and furnace bays of electric steelmaking shop No. 2 at the Zlatoust Metallurgical Plant. The shop specializes in the production and casting of stainless, chromium-nickel, and complex-alloyed steels, alloyed high-speed and tool steels, and nickel alloys. The unit can be used to perform both vacuum degassing and vacuum oxygen refining.

Microstructure and pitting corrosion resistance of annealed duplex stainless steel

The transition from metastable to stable pitting was studied in 0.5 M NaCl water solution for two cast duplex stainless steels under different microstructural conditions achieved by annealing in the range from 900 °C to 1200 °C. The ensuing microstructural changes in heat treated steels were defined and correlated with established pitting potentials ( E p) and sites of corrosion damage initiation. The variations in E p have been discussed in terms of secondary phases precipitation. The critical condition for pit stability was quantified and used to select an appropriate microstructural state, resulting in the higher potential at which stable pit growth is first observed.

Manufacture, testing and quality assurance of cast stainless steel headers for the CIEL pump limiter of Tore Supra

The Tore Supra tokamak is a fusion device which has been in operation since 1988. Its main features are its superconducting toroidal magnet and its actively cooled first wall. Tore Supra is devoted to the study of physics and technology of long plasma discharges. Its actively cooled first wall is able to remove the energy deposited from the plasma during a very long pulse regime (several hundred seconds). A major upgrade of the Tore Supra in-vessel components, the CIEL project (French Acronym of Composants Internes Et Limiteur), has been implemented and is in operation since 2001. This paper describes the experience gained from the manufacturing of cast stainless steel hydraulic structures supporting the toroidal pump limiter, the main component of the CIEL project, as well as the technical description of this structure, the associated specifications, the difficulties encountered during manufacture and the repair possibilities.

Use of Copper Cast Alloys To Control Escherichia coli O157 Cross-Contamination during Food Processing

The most notable method of infection from Escherichia coli O157 (E. coli O157) is through contaminated food products, usually ground beef. The objective of this study was to evaluate seven cast copper alloys (61 to 95% Cu) for their ability to reduce the viability of E. coli O157, mixed with or without ground beef juice, and to compare these results to those for stainless steel. E. coli O157 (NCTC 12900) (2 x 10(7) CFU) mixed with extracted beef juice (25%) was inoculated onto coupons of each copper cast alloy or stainless steel and incubated at either 22 degrees C or 4 degrees C for up to 6 h. E. coli O157 viability was determined by plate counts in addition to staining in situ with the respiratory indicator fluorochrome 5-cyano-2,3-ditolyl tetrazolium. Without beef extract, three alloys completely killed the inoculum during the 6-h exposure at 22 degrees C. At 4 degrees C, only the high-copper alloys (>85%) significantly reduced the numbers of O157. With beef juice, only one alloy (95% Cu) completely killed the inoculum at 22 degrees C. For stainless steel, no significant reduction in cell numbers occurred. At 4 degrees C, only alloys C83300 (93% Cu) and C87300 (95% Cu) significantly reduced the numbers of E. coli O157, with 1.5- and 5-log kills, respectively. Reducing the inoculum to 10(3) CFU resulted in a complete kill for all seven cast copper alloys in 20 min or less at 22 degrees C. These results clearly demonstrate the antimicrobial properties of cast copper alloys with regard to E. coli O157, and consequently these alloys have the potential to aid in food safety.

Microstructural changes caused by yttrium addition to NbTi-modified centrifugally cast HP-type stainless steels

Centrifugally cast heat-resistant HP stainless steels are particularly suitable for applications where service conditions comprise high temperatures and aggressive environments; thus, they are extensively used in reformer furnaces, in which hydrogen production takes place. The demand for better performance has motivated developments in these steels. The addition of Nb and Ti as microstructural modifiers has proved successful in providing a more stable microstructure. In this work yttrium was added to centrifugally cast NbTi-modified HP steels. It was observed that its presence increased the level of fragmentation of the chromium carbides, a positive aspect for creep resistance. The main cause of the fragmentation is the formation of yttrium carbides, which serve as heterogeneous nucleation sites for the other carbides. One of the tubes, with a lower titanium content, showed the best creep performance among those tubes studied owing to the presence of a smaller volume fraction of the deleterious G phase.

5th European Continuous Casting Conference

(2006). 5th European Continuous Casting Conference. Ironmaking & Steelmaking: Vol. 33, No. 2, pp. 82-91.

Evaluation of the Degradation Characteristics of CF-8A Cast Stainless Steel Using Indentation Techniques and EDS

Cast austenitic stainless steel piping pump, valve casings and elbows are susceptible to reductions in toughness and ductility because of long term exposure at the operating temperatures in the LWR (light water reactor). In this study, the three classes of thermally aged CF-8A cast austenitic stainless steel specimens were prepared using an artificially accelerated aging method for 0, 2,679 and 3,572 hours at 400oC. An indentation technique was applied to evaluate of the thermal aging of CF-8A cast austenitic stainless steel. We have performed indentation tests (automated ball indentation and nano-indentation) and EDS (energy dispersive spectroscopy) in order to characterize the micro-structural changes of the phase with aging. The fracture toughness of aged CF-8A cast stainless steel was determined by standard fracture toughness tests and automated ball indentation techniques.

Avaliação da resistência à corrosão em aço inoxidável martensítico do tipo Cr-Ni em meio marinho sintético simulando atividades em águas profundas

Esse trabalho estuda a resistência à corrosão do aço inoxidável martensítico fundido em meio marinho sintético saturado com CO2 e também faz uma comparação com solução aerada. A liga foi usada na condição temperada e revenida. A corrosão foi avaliada usando a técnica de polarização potenciodinâmica cíclica. As medidas de polarização foram obtidas em meio marinho sintético com concentrações de cloreto de 20000ppm e 80000ppm na temperatura ambiente. As curvas de polarização cíclica obtidas mostram uma região de passivação definida para as condições adotadas para o ensaio, sendo possível para a identificação dos parâmetros eletroquímicos, o potencial de pite, o potencial de proteção e a susceptibilidade ao pite. Em solução saturada com CO2 e concentração de cloreto de 20000ppm, o potencial de proteção aumentou e a susceptibilidade ao pite diminuiu, em comparação com a solução aerada. Em contraste, os potenciais de pite e de proteção decresceram em solução saturada com CO2 e concentração de cloreto de 80000ppm, indicando que a resistência ao pite é influenciada por um sinergismo entre a concentração de cloreto e a solução saturada com CO2.

Yttrium addition to heat-resistant cast stainless steel

Yttrium addition to HP-type (where H stands for heat-resistant and P is related to the Ni content) heat-resistant cast stainless steel resulted in increased fragmentation of the primary chromium carbide network and improved high temperature mechanical properties. The improvement was caused by yttrium carbide particles acting as nucleation sites for other carbides, as indicated by transmission electron microscopy and energy dispersive X-ray spectroscopy analysis. Thermodynamic considerations support these conclusions.