E. Stagno

Deformation and recrystallization of a jewellery white gold alloy

Most nickel-containing white gold alloys are susceptible to phase separation and excessive hardening when annealed during fabrication. This article shows how basic metallurgical principles may be applied in seeking to define, for a specific alloy, an annealing schedule which will yield structures suitable for cold-working.

Sinter hardening of low-alloy steels: influence of part geometry and physical properties of the material

Assuming that the cooling rate of parts depends on thermal conductivity of the material and on the surface/volume ratio, a two-ways study has been carried out. Firstly, the isothermal surfaces have been calculated by a numerical method on parallelepipeds of constant cross section and varying height and weight. Microstructures, depending on isothermal surfaces predicted by the numerical model, agree with microhardness and mechanical properties. A maximum weight of about 200 grams has been observed to be apt to a full sinter-hardening process and the surface/volume ratio seems to be the critical driving factor. Numerical analysis and experimental tests show that the thermal diffusivity of sintered steels increases as porosity increases.

Sintering of Carbon Steels: Controlled Atmospheres, Equipment, Practical Results

The influences of carbon on microstructures and properties of wrought steels are discussed. The basic metallurgical knowledge has showed for a long time that the optimum properties depending on alloy additions can be achieved only by a correct control of carbon content. From the comparison between porous and fully dense steels it comes out that carbon control is always a critical factor for achieving the highest properties. The thermodynamic bonds concerning carbon equilibria during sintering are discussed and the possible interactions with various controlled atmospheres are examined. Some atmospheres can cause carbon depletion or enrichment, whereas other gas compositions do not modify the equilibria. In case of chemical changes during sintering, involving carbon, the temperature profile may be a critical item. The microstructures after cooling obviously depend on thermal gradients within certain temperature ranges. The requirements to be fulfilled on furnaces, to control sintered properties, are discussed. Some common schemes of equipment are analysed and their suitability to a correct carbon and microstructure control during sintering is surveyed. Sintered samples produced at different plants under industrial conditions and having various chemical compositions have been observed under the optical microscope and at the SEM. The observations show that microstructures can span from ferrite-pearlite mixes to austenite-bainite ones, even when copper is the only alloy addition besides carbon. Diffusion-bonded powders containing also nickel and molybdenum allow to get hard phases even at relatively low cooling speeds. The strength levels are suitable to fulfil exacting demands and can promote new advanced applications of P/M parts.

Nominally equivalent powders for P/M steels : Analysis of response to sintering and differences at various C content

Globalization enables P/M part makers to choose powders from different sources. Raw materials produced by a given process and having equal chemical composition are supposed to be equivalent. The differences in sintering behavior, in industrial equipment, have been investigated on P/M steels obtained from four diffusion-bonded powders (Fe + Ni + Cu + Mo) on atomized iron base, at the same alloy contents. Two levels of carbon and two sintering conditions have been investigated. Dimensional changes, C content, hardness, microhardness pattern, universal hardness, fractal analysis, pore features, microstructure features, and rupture strength have been compared, to characterize different raw materials. An index of homogeneity of microstructures, based on a specific statistical approach, does not agree completely with observed microstructures and pore feature. The results show that the claimed equivalence is not confirmed by experimental data. Analyses of microhardness and microstructure distributions seem powerful tools to rate the real equivalence of so claimed powders. P/M part makers may use the proposed approach to assess detectable differences on performances among powders that are declared as “true” substitute, so confirming or contradicting any even not negligible cost difference. The comparison methods here described and applied can also help powder producers to improve the suitability of their products to sintering processes.

Mechanical Properties of Large Plastic-Mold Steel Blooms

Molds for plastic automotive components such as bumpers and dashboards are usually machined from large pre-hardened steel blocks. Due to the large size, the blooms undergo a slack quench, so that mixed microstructures occur throughout, both after quench and after the tempering stages. Mechanical properties that are both not homogeneous in the section and everywhere lower (particularly in fracture toughness) than those of correctly quenched and tempered alloy steel specimens are obtained. Successive machining to form molds may be so deep that any of the microstructure occurring at different positions in the original bloom can be found at the mold face, where notch effects are commonly present. Welding, for local shape alterations, may yield further defects.

Metallographic techniques in gold alloy study: Delamination in a white gold alloy

Abstract The structure and phase transformations that occurred during the working and heat treatment of an eighteen-carat gold alloy used in jewel making were studied. A delamination phenomenon encountered during rolling was attributed to the formation of a discontinuous globular phase, clearly observable in the microscope, but chemically indistinguished from the matrix. It appeared that this embrittling phase was precipitated by a critical combination of deformation and intermediate annealing treatment.