ASSISTED CRACK GROWTH IN MARAGING STEEL WELDS BY GASEOUS HYDROGEN

Abstract— The effects of environmental hydrogen content on fatigue crack growth rates (FCGRs) in T-250 maraging steel plates and laser welds were investigated. The influence of ageing treatments on fatigue characteristics of the alloy was also studied. Experimental results revealed that the accelerated FCGRs in the presence of hydrogen were always associated with changes in fracture modes that appear in compact-tension specimens. Even for overaged specimens with excellent resistance to gaseous hydrogen embrittlement, such an acceleration of crack growth in hydrogen could not be avoided. The crack path of underaged specimens in hydrogen was found mainly along prior austenite boundaries for steel plates and along coarse columnar boundaries for welds. In gaseous hydrogen, peak-aged welds exhibited intergranular and quasi-cleavage mixed fracture modes, compared to mainly quasi-clevage for similar aged steel plates. Hence, the enhancement of crack growth in hydrogen was more pronounced for the welds. Overaged welds showed higher FCGRs than the same aged steel plates only in hydrogen and for Δ K values greater than 20MPa√m.     

Molecular cloning and characterization of fengycin synthetase gene fenB from Bacillus subtilis

A fengycin synthetase gene, fenB, has been cloned and sequenced. The protein (FenB) encoded by this gene has a predicted molecular mass of 143.6 kDa. This protein was overexpressed in Escherichia coli and was purified to near homogeneity by affinity chromatography. Experimental results indicated that the recombinant FenB has a substrate specificity toward isoleucine with an optimum temperature of 25 degrees C, an optimum pH of 4.5, a Km value of 922 microM, and a turnover number of 236 s(-1). FenB also consists of a thioesterase domain, suggesting that this protein may be involved in the activation of the last amino acid of fengycin.     

Investigations of surface structure for thermally evaporated silicon on a Cu(111) surface

The surface structure and composition of semiconductor/Cu(111) films prepared by thermal evaporation in an ultrahigh vacuum condition have been investigated. As Si atoms were deposited on a Cu(111) surface, diffused spots were observed up to 2 monolayers while 1 × 1 spots become dimmer as revealed using low-energy electron diffraction technique. Because of a larger electron affinity of Si than that of Cu, the Cu L₃M₄₅M₄₅ Auger line shifts to a lower kinetic energy. Annealing treatments at 425 K causes a splitting of the Cu L₃M₄₅M₄₅ line. This shows the interdiffusion at the Si/Cu interface and the formation of a Cu-rich surface layer. After annealing treatments, the domains grow and aggregate to form larger domains as revealed by the decreasing full-width at half maximum of diffraction spots. Ge/Cu(111) shows 1 × 1 structure as annealing up to 500 K. Lack of a dominant structure and a large valence diameter of Ge result in different structures as compared to Si/Cu(111).     

Hydrogen embrittlement of a Ti-strengthened 250 grade maraging steel

Constant extension-rate tensile tests are performed to investigate the effects of strain rate and environmental hydrogen concentration on the tensile properties of various aged T-250 specimens. The 426 °C (800 °F) underaged specimens are very sensitive to strain rate; the 482 °C (900 °F) peak-aged specimens exhibit a reduced ductility under low strain rates; and the 593 °C (1100 °F) overaged specimens are insensitive to strain rate when tested in air. The excellent resistance to embrittlement of the overaged specimens in gaseous hydrogen could be associated with the extensive formation of reverted austenite and the incoherent Ni₃Ti precipitates. The tensile-fractured surfaces of such specimens reveal a ductile dimple fracture. However, the peak-aged specimens are susceptible to gaseous hydrogen embrittlement, and the embrittled region shows a primary fracture mode of quasi-cleavage. The least resistant to hydrogen embrittlement of the underaged specimens is characterized by a more brittle fracture appearance, that is, intergranular fracture, under a low strain rate or in the gaseous hydrogen environment.     

Gaseous hydrogen embrittlement of T-250 laser welds

The tensile properties of laser-welded T-250 maraging steel are measured, with attention paid to the influence of strain rate and gaseous hydrogen on the fracture behavior of welded specimens. Post-weld heat treatments are performed on laser-welded specimens to obtain underaged (WU), peak-aged (WP), and overaged (WO) specimens. Hydrogen embrittlement (HE) affects the tensile fracture behavior of the welded specimens; HE changes not only the fracture mode but also the fracture location. Without the influence of hydrogen, the fracture location is at the softest region, the weld metal (WM), and the fracture appearance reveals a ductile dimple fracture. For welds sensitive to HE, the fracture is initiated at the heat-affected zone (HAZ) with coarse grain size, and the associated fracture surface exhibits intergranular and quasi-cleavage fractures that are brittle in nature. In addition, the HAZ with coarse grain size is more prone to HE, as compared to other regions in the welded specimens. The WU specimens are susceptible to HE in air under a low strain rate, while the WP specimens are only susceptible to gaseous hydrogen embrittlement (GHE). However, the WO specimens are immune to GHE and insensitive to strain rate.     

A study on instantaneous cutting force coefficients in face milling

In this paper, the characteristics of instantaneous cutting force coefficients in face milling are studied. In order to estimate instantaneous cutting force coefficients in face milling, the relationships between instantaneous cutting force coefficients and measured cutting force signals are derived. A series of experiments are then conducted to study the natures of instantaneous cutting force coefficients. The relationships between instantaneous cutting force coefficients and other cutting parameters are also established. It is found that the normal force coefficient is mainly affected by chip thickness and cutting speed; the vertical force coefficient is mainly affected by chip thickness, cutting edge length and cutting speed; and that the horizontal force coefficient is not only affected by chip thickness, cutting speed and length of cut, but also the variation rate of chip thickness.     

Hydrogen-assisted fatigue crack growth of AISI 316L stainless steel weld

The fatigue crack growth behaviors of AISI 316L stainless steel (SS) welds in air and gaseous hydrogen were evaluated, and further compared with the base plate. In air, the fatigue crack growth rate (FCGR) of the weld after heat-treatment at 1050 ^oC/1 h was similar to that of the base metal. Furthermore, all specimens became susceptible to hydrogen-accelerated crack growth. Mainly quasi-cleavage fracture related with the strain-induced martensite accounted for the accelerated crack growth in hydrogen. A smaller amount of martensite in the weld was responsible for the decreased susceptibility to hydrogen-enhanced fatigue crack growth relative to the base metal.     

EFFECTS OF BINARY MIXTURE CONDENSATION ON TURBULENT MIXED CONVECTION HEAT AND MASS TRANSFER IN A VERTICAL CHANNEL

Abstract

A numerical analysis was carried out to study the detailed heat and mass transfer processes between a condensation liquid film and mixed turbulent moist airflow. Results show that the condensation latent heat transfer is more important for a system with higher inlet relative humidity or lower inlet Reynolds number of a moist airstream. The heat and mass transfer coefficients are higher for a system with higher inlet relative humidity and inlet Reynolds number of moist air. In addition, the aiding-buoyancy forces cause diminution in heat and mass transfer results compared with the corresponding results of forced convection.     

An improved tabu search for economic dispatch with multiple minima

This paper develops an improved tabu search algorithm (ITS) for economic dispatch (ED) with noncontinuous and nonsmooth cost functions. ITS employs a flexible memory system to avoid the entrapment in a local minimum and developed the ideal of "distance" to the fitness to accelerate optimization. The new approach extends simple tabu search algorithm (STS) to real valued optimization problem and applies parallelism to weaken the dependence of the convergence rate of modified tabu search algorithm (NITS) on the initial condition. Effectiveness of the method was compared with many conventional methods. Results show that the proposed algorithm can provide accurate solutions with reasonable performance and has a great potential for other applications in the power system