Effects of heat treatment processes on microstructure and properties of 2·25Cr–1Mo–0·25V HSLA steels

Abstract

In the present paper, the effects of the heat treatment processes with two conditioning treatments and four quenching–tempering processes on the mechanical properties of 2·25Cr–1Mo–0·25V high strength low alloyed (HSLA) steel are investigated. The results show that the conditioning treatments have obvious effects on the low temperature impact energy but little effect on the tensile strength. The elevation of the final austenitising temperature increases the strength, whereas it results in the decrease in the low temperature impact energy due to the coarse microstructure. The results of the fracture surfaces analysis further make sure that the fracture surfaces of tensile specimens all exhibit ductile characters with a lot of dimples. However, the fracture surfaces of impact specimens exhibit two typical fracture characters, i.e. the ductile and brittle fracture surface corresponding to the fine and coarse microstructures respectively. In addition, the elongation and reduction in area seem to be insensitive to the heat treatments. Meanwhile, the impact fracture mode is more sensitive to the grain size and not to the low temperature impact energy.     

Thermophysical properties of liquid Co–Cu–Ni alloys

Abstract

Within the Coolcop project, funded by ESA, the thermophysical properties of selected liquid Cu–Co–Ni alloys were investigated. Using the new high-temperature oscillating cup viscometer at DLR, the viscosity of this alloy along two perpendicular cuts in the ternary phase diagram was measured as a function of temperature. Along the same cuts also surface tension and density were determined by using electromagnetic levitation and image processing.     

Weld brazing of copper thick plates

Abstract

The weld brazing of 6 mm thick copper plates without preheating was investigated. The weld metal of weld brazing is composed of α-Cu solid solution and Cu–Ag–P eutectic structure, and the α-Cu solid solution in the weld metal of weld brazing is larger than that in the brazing weld. The average hardness values within the weld are 60 HV (50 g) lower for weld brazing than for brazing. In all weld brazing specimens tested, the failure was located in the heat affected zone with a tensile strength slightly lower than the base metal but similar to the arc welding joints.     

Temperature influence on the physicochemical surface properties and adhesion behaviour of Enterococcus faecalis to glass and silicone

The interest in studies on the physicochemical surface properties of bacteria has increased because they are related to the causes of the initial adhesion of microorganisms to biomaterials and the subsequent biofilm formation on indwelling medical devices. The determination of physicochemical parameters such as hydrophobicity or surface tension is usually done at room temperature, not taking into account the real temperature at which bacteria cause infection inside the human body. In this work, the influence of the experimental temperature on the surface physicochemical characteristics and adhesion behaviour of Enterococcus faecalis ATCC29212 to glass and silicone has been studied. Water, formamide and diiodomethane contact angles on bacterial lawns changed when the experimental temperature was increased from 22°C to 37°C. Moreover, hydrophobicity, as determined by water contact angle, increases with temperature, in agreement with the higher and lower adhesion to silicone and glass, respectively, observed at 37°C, with respect to the results at 22°C. Also, when the formamide and diiodomethane contact angles are considered, the changes in the adhesion behaviour to glass and silicone are predicted by the sum of Lifshitz-van der Waals and acid-base interaction free energies if the measurement temperature is the same as the bacterial growth temperature, i.e. 37°C.     

Corrosion of stainless steels under heat transfer conditions in nitric acid

Abstract

Novel test rigs are described for the study of the corrosion of metal specimens under controlled heat fluxes. In the corrosion of stainless steels in nitric acid, tests at various heat fluxes with steel surface temperature kept constant have shown that the cooler acid present at the surface under higher heat fluxes leads to slightly smaller corrosion rates than under isothermal conditions. Crevice corrosion can develop under the gasket sealing the stainless steel specimen to the test cell. This crevice corrosion can produce enhanced corrosion rates (by factors up to 100), not only on surfaces within the crevice, but also on those external to the crevice. The factors influencing the development of crevice corrosion are discussed.     

Effect of lanthanum rare earth addition on low temperature plasma nitriding

Abstract

The effect of lanthanum additions on the plasma nitriding of nanocrystalline steel 3J33 is reported. Nitriding was carried out in 20N₂ + 80H₂ at a pressure of 400 Pa at 350 and 410°C. Glow discharge spectrometry shows that although its atomic radius is larger than that of Fe, La can diffuse into the surface layer to a significant depth; the La addition increases the compound layer thickness. Based on the microhardness profile, the diffusion layer thickness also increases due to La addition. XRD analyses indicate that the presence of La did not change the phase structure of the surface, i.e. a compound layer of γ′-Fe₄N type. The incorporation of La not only enhances the microhardness but also improves the toughness of the nitrided layer. The atomic and electron structures of phases γ′-Fe₄N and γ′-(Fe,La)₄N were calculated using the plane wave pseudopotential method. The (Fe,La)₄N phase is more stable than Fe₄N and the shear/bulk modulus ratio G/K of (Fe,RE)₄N is much smaller. This finding provides a possible mechanism for the increased microhardness and improved toughness of nitrided layers containing La.     

Structure property relations of copolymerised polypropylene /SiO2 nanocomposites prepared by solid state shear milling (S3M) method

Abstract

In the present paper, solid state shear milling (S³M) method was used to prepare the copolymerised polypropylene (Co-PP)/nano-SiO₂ composites under the conditions of solvent free and non-organic treatment. The change of phase morphology and arrangement of SiO₂ under mechanical force were firstly observed in the complex multiphase and multicomponent Co-PP/nano-SiO₂ composite. The effect of milling cycles on the phase structure and mechanical properties was studied. It was found that the S³M method was an effective way which can adjust the phase structure of composite through change of milling cycles. After 20 milling cycles, the composite can form a structure with a lot of SiO₂ particles around the ethylene–propylene copolymer phase, the Charpy notched impact strength of the composite with 4 wt-%SiO₂ particles can be largely improved from 24·2 to 38·2 kJ m^–2. While after 30 milling cycles, the composite can form a structure with SiO₂ particles dispersed more uniformly in the Co-PP matrix. However, with this structure, the prepared composite has higher stiffness but the notched impact strength could not be greatly improved. The mechanism of the toughening effect is discussed and the structure property relation established.     

Electrical and thermal properties of carbon nanotubes/PMMA composites induced by low magnetic fields

Abstract

Ni particles supported on carbon nanotubes (CNTs) were dispersed in a polymethyl methacrylate (PMMA) matrix by solution blending and then cast onto an electrode to get composite films under low magnetic fields. The orientation of CNTs in the films was characterised by scanning electron microscope and optical microscope. Multimeter and high resistance meter were used to study the electrical behaviour of the nanocomposites. The glass transition temperature T _g of PMMA was determined by differential scanning calorimetry. The results show that the alignment of the CNTs dispersed in the PMMA was achieved under a low magnetic strength below 0·5 T. Because of the ferromagnetism of Ni particles, the magnetic alignment of CNTs susceptibly changed. The magnetic alignment units in this work were rod-like CNTs aggregates instead of single CNTs, which took part in the buildup of a specific CNTs network structure in PMMA matrix. The network structure played a key role in significantly improving electrical conductivity and T _g of the nanocomposites.     

A study of zinc phosphate coatings on 12·5Cr–21·0Ni stainless steel

Abstract

12·5Cr–21·0Ni stainless steel was chemically treated with zinc phosphate in order to find the most suitable phosphate solution and its operating parameters. The phosphate coatings were tested for their corrosion protection of stainless steel using three methods: the salt spray test, the humidity cabinet test and the brine immersion test. The phosphate coatings were also mechanically tested using a tensile test for determining their mechanical properties. Results clearly show that phosphate coatings with a uniform appearance and full coverage can give high corrosion protection to 12·5Cr–21·0Ni stainless steel by forming a physical barrier against the corrosive environment. The 12·5Cr–21·0Ni stainless steel after coating with zinc phosphate still retains reliable mechanical properties, thereby providing valuable applications in the engineering field.     

Sliding wear characteristics of grey cast iron as influenced by sliding speed,load and environment

Abstract

The present investigation pertains to the observations made during sliding of a grey cast iron against a steel counterface over a range of sliding speeds, applied loads and test environments. The nature of the environment was altered through the presence of oil and suspended graphite particles therein. The presence of oil improved the wear characteristics of the samples in terms of lower wear rate and decreased frictional heating in general. An additional presence of suspended graphite particles in the oil lubricant brought about a further improvement in the wear response of the samples in all the test conditions except at the highest speed at high applied loads; the trend reversed in the latter case. Increasing speed and load led to deterioration in the wear behaviour. The behaviour of the material has been explained in terms of specific response of different microconstituents such as pearlite, ferrite and graphite and corroborated with the observed features of wear surfaces, subsurface regions and debris particles.