Prosper Matković

The effect of tempering temperature on mechanical properties and microstructure of low alloy Cr and CrMo steel

Two low alloy Cr and CrMo steels with similar levels of carbon, manganese and chromium have been studied to determine the effect of tempering temperature on the mechanical properties and microstructure. The quenching and tempering of steels were carried out using a high-speed dilatometer. The steels were quenched at the average cooling rate of 30 K s-1 in the temperature range from 1123 to 573 K by flowing argon and tempered at 673, 823 and 973 K. The martensite of steels formed during quenching was of entire lath morphology with 2 vol% retained austenite. It was found that after tempering at 973 K the Cr steel contained only orthorhombic cementite, while the CrMo steel contained the cementite and hexagonal Mo2C particles in the ferrite matrix. At the same tempering conditions, the CrMo steel shows higher strength but lower ductility as compared to those of Cr steel. It is shown that this difference results from finer prior austenite grain, substructure within matrix and precipitate dispersion strengthening, primarily by Mo2C. Transmission electron microscopy (TEM) bright- and dark-field micrographs as well as selected area diffraction pattern analysis of orientation relationship showed that the cementite precipitated from the ferrite matrix. Fractography analysis showed that the morphology fracture surface was changed by increasing tempering temperature. Tempering at 973 K obtained ductile fracture by the microvoid coalescence mechanism.

A new intermetallic phase in the Pd-Pb-As system

Abstract A new ternary compound was identified in the Pd-Pb-As system. The Pd 3 Pb 2 As phase crystallizes with an orthorhombic structure in the space group Cmc 2 1 , a = 1.1857(2) nm, b = 0.6861 (1) nm and c = 0.5808(1) nm, Z = 4, although the crystals show quasi-hexagonal symmetry. This structure shows a complex similarity to the NiAs type of structure. The palladium atoms are octahedrally coordinated by four lead and two arsenic atoms; the lead and arsenic atoms are (six) coordinated by palladium atoms in the form of a distorted trigonal prism. The trigonal prisms are connected by sharing edges to form a framework. The Pd 3 Pd 2 As structure was determined from single-crystal X-ray diffraction data and refined by full-matrix least-squares methods to R = 0.073 for 309 reflections.