Materials Science Forum Vols. 654-656

Abstract: The AISI 316L grade austenitic steel after glow discharge nitriding at a temperature of T = 598 K and for duration of  = 10,8 ks, for different variants of specimen arrangement in the glow-discharge chamber was investigated. In the first variant specimens were placed on the cathode and in the second variant specimens were positioned also on the cathode, but shielded with a booster screen. In order to assess the effectiveness of nitriding process variants a profile analysis examination of obtained surface layers, surface hardness tests and surface layer hardness profile examination, analysis of surface layer structures and corrosion resistance tests were carried out. It was found that application of a booster screen effects in a nitrogen diffusion depth increment into the 316L austenitic steel, resulting in a surface layer thickness escalation.

Abstract: The high vanadium high-speed steel (HVHSS) with about 9wt% vanadium and different carbon contents were prepared using casting process. The effects of carbon on wear properties of HVHSS were studied using pin-on-ring tester, and the failure behaviors were investigated via SEM. Results show the optimal wear resistance is obtained when HVHSS possesses moderate carbon content (2.58wt.%). The cause is that the matrix microstructure of moderate carbon HVHSS is mainly low-carbon lath martensite with good toughness and high hardness, and it can effectively resist micro-cutting and figure wear at the same time, so the role of high-hardness vanadium carbides (VC) can be played enough because of the strong support of matrix. If carbon content is too low, the wear failure of HVHSS is mainly caused by severe micro-cutting and adhesive wear on contact surface because the matrix microstructure of high speed steel is ferrite with very low hardness, which leads to poor wear resistance. While, the matrix microstructure is mainly composed of high carbon martensite with poor toughness when carbon content is too high, therefore, it possesses very poor resistance to cycle fatigue and thermal fatigue, resulting in decrease of wear resistance.

Abstract: Shot peening is a surface treatment that improves the performance of engineering components. In conventional shot peening, the medium consists of small spheres, which are usually made of high-carbon cast steel; the diameter of the spheres is in the range from 0.3 to 1.2mm. More recently, however, a new type of microshot has been developed to enhance the peening effect. The diameter of the spheres in the new medium is in the range from 0.02 to 0.15mm. In the present study, the effect of microshot peening on the surface characteristics of spring steel was investigated. The injection method of the microshot was of the compressed air type. The microshots of 0.1mm diameter were high-carbon cast steel and cemented carbide, and the workpiece used was the commercially spring steel JIS-SUP10. The surface roughness, hardness and compressive residual stress of the peened workpieces were measured. The surface layer of the workpieces was sufficiently deformed by microshot peening. A high hardness or residual stress was observed near the surface. The use of hard microshots such as cemented carbide was found to cause a significantly enhanced peening effect for spring steel.

Abstract: The research aims to evaluate the microstructure, mechanical properties and marine corrosion resistance of P-bearing ULCB steels. P produced stronger solution strengthening effect, but had less effect on the low temperature impact toughness of ULCB steel. With increasing P content to 0.09(mass)%, the immersion corrosion rate of ULCB steel in 3.5% NaCl obviously decreased in comparison with those steels without or with lower P content. This is attributed to that the complex effect of Cu and P promoted the formation of dense amorphous oxyhydroxide in the inner scale, which inhibited further corrosion from NaCl solution. Mo in ULCB steel played an important role in avoiding pitting corrosion in Cl- environment.

Abstract: Lean duplex stainless steels have been developed in Korea for the purpose of being used in the seawater systems of industries. The flow velocity of some part of seawater systems in nuclear power plants is high and damages of components from corrosion are severe. Therefore, this environment requires using high strength and high corrosion resistant steels. The newly developed lean duplex stainless steels STS329LD(20.3Cr-2.2Ni-1.4Mo) and STS329J3L(22.4Cr-5.7Ni-3.6Mo) are evaluated for the compatibility in seawater systems of nuclear power plants. In this study, the physical & mechanical properties and corrosion resistance of two alloys were quantitatively evaluated in comparison with commercial stainless steel 316L. Microstructures and mechanical properties of them were analyzed and the electrochemical properties related to corrosion resistance were measured such as pitting potential, passive current density, and corrosion rates from Tafel analysis. Critical pitting temperatures were measured in accordance with ASTM G48E method. The pitting initiation time and lifetime for replacement were predicted from the PRE values of test alloys and empirical equations that have been formulated from the condenser tubes of a nuclear power plant.

Abstract: The surface cracking behaviors of Cu-bearing steels were studied under similar conditions of the direct hot charging processes. Several specimens with various Cu contents from 0.11 to 0.38% were prepared, the oxidation tests were performed at the temperature of 1100, 1150 and 1200°C, and the hot compression tests were conducted at 1050°C using the Gleeble 3500 in order to examine the Cu induced surface cracks. The surface cracks increased gradually as the Cu-rich phase above the critical level increased. The critical level of Cu-rich phase for the specimens of high Cu contents (0.38%) reaches with a thin scale layer. However, the specimens with lower Cu content (0.13%) the critical level of Cu-rich phase occurred with a thick scale layer. The oxidation potentials of steel were affected by temperature, time, atmosphere and other elements. The oxidation rates of Cu bearing steels decreased with increasing Cu contents. It is suggested that the oxidized scale thickness is the one of important factors inducing the surface cracks by Cu contained steels in direct hot charging process.

Abstract: A thermodynamic model for the prediction of interfacial tension of liquid iron, inclusion and solid oxide substrate/refractory was evaluated. The combined Good’s and Young’s equations were used for high temperature liquid metal-solid oxide substrate-inclusion system to evaluate the interfacial tensions. The study predicts the liquid silicon (as model inclusion/impurity) adherence on the solid oxide substrate/refractory (MgO) in a liquid iron melt. The calculated results for interfacial tension between liquid iron-MgO values decreased from 1798 to 1026 ergs/cm2 as the temperature increases from 1823 to 1933 K, respectively. The Gibbs energy of adhesion for liquid silicon-MgO substrate was calculated shows that silicon adhesion to MgO substrate increases with increasing surface tension of liquid Fe/MgO and with decreasing temperature.

Abstract: With advances in mold instrumentation, high performance mold fluxes, better reliability maintenance procedures, and improved operating practices, there has been a significant decline in the number of unplanned caster breakouts experienced at various production facilities. The typical breakouts of stickers and flux entrapments that were frequently observed in the past is often detected using embedded thermocouples in the mold and automatic slowdowns are initiated which inhibit excessive tearing of the partially solidified shell and prevent subsequent breakouts. However, in-mold events still occur resulting in caster downtime. Many of these occasional events have been linked to bleeders along the corners of the slabs, slab joint defects during tundish changes, and longitudinal face cracking. Considering the potential costs associated with unplanned caster breakouts, improvements have been made in the existing breakout prevention system. Major modifications of this existing breakout prevention system included, in particular, the addition of bleeder thermocouples or edge thermocouples which made possible the detection of shell containment loss near the corners of the slab. These bleeder thermocouples can also be utilized in detecting bad tundish joints related to excessive cooling and corner contraction that lead to tundish change joint defect type breakouts. In addition, changes in the location of the existing embedded thermocouples to a staggered-design extended the detection range of the breakout prevention system without increasing the total number of thermocouples.

Abstract: Coal fired power generation should be amalgamated with efficiency improvement technology of electric-power production and treatment technology of greenhouse gas in green economic era. The efficiency of fossil power plant can be achieved by increment of temperature and pressure of steam, and the performance improvement of the components. The improvement of temperature and pressure of live steam can be supported by the materials with high temperature strength and the excellent resistance to oxidation. Specially, verified materials have to be used in large quantities for both repair and replacement of steam facing components. The efficiency of newly constructed Pulverized Coal – Thermal Power Plant[PC-TPP] is higher than 42% at Ultra Super Critical[USC] conditions of 250~300 bar and 600~610oC. Recently, material technology has been developed to build PC-TPP of the steam condition of advanced USC with more than 46% efficiency. However, it is expected to have problems to economically manufacture in large quantities. Therefore, the paper explains the current the state of the art of materials technologies and the issues with the steam.

Abstract: In this work, Ti5Si3 and TiC particle reinforced titanium aluminide matrix composite sheet was fabricated by rolling and reaction annealing using the starting materials of SiCp/Al composite and pure titanium sheet. The deformation compatibility of both starting materials and microstructure evolution during reaction synthesis were studied. The results show that titanium has the similar deformability with SiCp/Al composite via the introduction of SiC particles and the selection of proper rolling temperature. Titanium aluminide matrix composite reinforced by Ti5Si3 and TiC was synthesized by reactions during the annealing. The reactions include the formation of titanium aluminide matrix by the diffusion synthesis between titanium and aluminum, as well as reinforcements (Ti5Si3 and TiC) by in-situ reaction between SiC and titanium.