M.R. Pinasco

Microstructural and mechanical characterisation of some sinter hardening alloys and comparisons with heat treated PM steels

Abstract Four grades of sinter hardening materials have been compared, using industrial equipment. Three powder types were completely prealloyed; the last one was a hybrid, combining prealloying and diffusion bonding. Different amounts of Cu have been added by mixing. The lubricated mixes, containing 0.6% graphite, have been compacted at different pressures, to form gears at green densities ≥7.0 g cm−3 ; the compacts have been sintered at 1120°C, under endogas from methane and fast cooled (at least 7 K s−1 within the range 850–400°C). The final step has been stress relieving, at 180°C, for 1 h. Material properties have been investigated, focusing on porosity, pore shape, hardness, microhardness, microstructure, local chemical composition and mechanical properties. For comparison, other gears, compacted in the same tool and at the same density level, but manufactured according to a more conventional cycle, i.e. starting from less alloyed powders and adding carbonitriding, quenching and stress relieving, have been used. The analysis of the different experimental results enabled the authors to find out and outline some criteria suitable for selecting sinter hardening materials and for choosing more reliable manufacturing conditions to fulfill specific application requirements.

Dilatometric Analysis for Investigating Age Hardening in a Commercial Precious Metal Dental Alloy

Age hardening in a commercial precious metal dental alloy based on the Au–Cu–Ag system with platinum addition was investigated by light optical microscope, scanning electron microscope, energy dispersion x-ray system microanalysis, image analysis, hardness measurements, and dilatometric tests. Dilatometric tests are known to give important information for optimizing the heat treatment parameters of metallic materials but are seldom used in studying the hardening of precious alloys. Nonisothermal dilatometric tests were used to point out the temperature range within which the microstructural hardening transformations occur, while isothermal tests were adopted to study the kinetics of each transformation. The macrohardness and microhardness measurements contributed to the understanding of the nature of the observed transformations, together with the microstructural analysis. We concluded that hardening is mainly due to order-disorder transformations taking place in the matrix. The identified optimal parameters were used for the heat treatment of a lost wax casting alloy prepared according to the traditional procedures for prosthesis manufacturing.

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.

Study of the age-hardening in an Au-Cu-Ag dental alloy with Pt and Pd additions with dilatometry, hardness measurements and microstructural analysis

The age-hardening behaviour in a commercial gold dental alloy based on the Au-Cu-Ag system with platinum and palladium addition has been investigated with light optical microscope, scanning electron microscope, EDXS, image analysis, hardness measurements and dilatometric tests. The dilatometer, although seldom used in studying the hardening of precious alloys has proved useful to give good information on the heat treatment parameters. The most important hardening of the homogenised alloy takes place after isothermal treatments at temperatures between 150°C and 350°C. The metallographic analysis has shown that the microstructure is modified with the presence of discontinuous precipitation on the grain boundaries only after treatments at temperatures starting from 260°C.

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.

Jominy test applied to PM steels for heat treatment

Abstract The hardenability of some PM steels designed for heat treatment has been evaluated on sintered steel specimens by applying the procedure standardised with the Jominy end quench test. The samples have been produced using two types of commercial powders, frequently selected for exacting applications. Manufacturing process includes admixing with graphite and lubricant, compaction at about 7·0 g cm−3 density and sintering at 1393 K for 30 min in industrial equipment. In order to investigate the thermal behaviour of the tested steels, during the cooling after austenitising, the temperatures inside the Jominy samples at different positions have been recorded. Hardness values and cooling rates have been correlated to an in depth microstructural analysis and microhardness profiles, finding a good agreement. The results can be utilised for a more reliable applications.

Ion nitriding of a sintered iron molybdenum alloy

The behaviour of a sintered alloy Fe Mo 1.5 C 0.01 in two density states when subjected to an ion nitriding process is analysed. The thermochemical treatment was carried out with three different combinations of parameters (time, nitriding atmosphere). The analyses of the material after treatment included dimensional measurements, macro- and micro hardness determinations, diffractometric analysis and microstructural examination with an optical microscope and a scanning electron microscope equipped with an EDX system. All treatments led to satisfactory results for the material in the two density states: the volume mass is not a discriminative parameter in this case.

Experimental Investigation of Microstructural and Mechanical Properties of New Steels for Large Blooms

In the automotive world, more and more components are produced by polymer injections using steel moulds reaching very large sizes, up to 1 m × 1 m sections or even more. These steels should have good thermal conductivity, good weldability, high machinability (especially for deep drilling), and good disposition to polishing and photoengraving. Moreover, high toughness, hardness, wear, and fatigue resistance across the whole thickness of the mould are strongly required. For satisfying these demands, chemical composition and heat treatment cycles of the steel shall be properly designed and optimized. In this paper two mould steels, which were recently developed, were experimentally investigated. Samples were machined from the surface and the core of each bloom and a full mechanical and microstructural analysis was carried out. The tensile properties at room temperature and at high temperature were investigated, together with the fracture toughness. All the obtained results were compared with the traditional and reference ISO 1.2738 mould steel. Moreover, dilatometric tests were performed to investigate the specific microstructural transformations during continuous cooling and isothermal treatments. The obtained results show good mechanical strengths for the whole thickness of the blooms associated with adequate toughness parameters. Finally, results were related to microstructural.