Effect of molybdenum on the fracture characteristic of high-speed steel quenched matrix

The fractures of three model alloys, imitating by their chemical composition the matrixes of the quenched high-speed steels of various Mo: W relations were analyzed. According to the measurements of the stress intensity factor K_Ic and the differences in the precipitation processes of carbides it was found out that the higher fracture toughness of the matrix of the molybdenum high-speed steels than on the tungsten ones is the results of the differences in the kinetics of precipitation from the martensite matrix of these steels during tempering. After tempering at 250 and 650°C the percentage of the intergranular fracture increases with the increase of the relation of Mo to W in the model alloys of the high-speed steel matrix. This is probably the result of higher precipitation rate of the M₃C carbide (at 250°C) and the MC and M₆C carbides (at 650°C) in the privileged regions along the grain boundaries. The change of the character of the model alloy fractures after tempering at 450°C from the completely transgranular one in the tungsten alloy to the nearly completely intergranular one in the molybdenum alloy indicates that the coherent precipitation processes responsible for the secondary hardness effect in the tungsten matrix begin at a lower temperature than in the molybdenum matrix. After tempering for the maximum secondary hardness the matrix fractures of the high-speed steels reveal a transgranular character regardless the relation of Mo to W. The higher fracture toughness of the Mo matrix can be the result of the start of the coherent precipitation processes at a higher temperature and their intensity which can, respectively, influence the size of these precipitations, their shape and the degree of dispersion. The transgranular character of the fractures of the S 6-5-2 type high-speed steel in the whole range tempering temperatures results from the presence of the undissolved carbides which while cracking in the region of stress concentration can constitute flaws of critical size which form the path of easy cracking through the grains. The transgranular cracking of the matrix of the real high-speed steels does not change the adventageous influence of molybdenum upon their fracture toughness. On the other hand, the carbides, undissolved during austenitizing, whose size distribution in the molybdenum steels from the point of view of cracking mechanics seems to be unsatisfactory, influence significantly the fracture toughness of these steels.     

The effect of carbides on fracture toughness of steels of ferritic matrix

The authors evaluated the effect of the volume fraction and the dispersion rate of cementite on fracture toughness of ferrite. The investigations were performed at -196°C on five types of carbon steels containing 0.028–1.22% of C in which cementite was coagulated at 700°C for 1–8 h from the quenched state. It was determined that the fracture toughness of steel increases very strongly up to the content of carbides of about 7% by volume. At the same time, hardness and strength of these steels grow. First of all, this is the result of size reduction of ferrite grains by fine carbides. These carbides, distributed almost exclusively on grain boundaries, can only participate in the transmission of the crack to the neighbouring grain. At larger contents of carbides, their dispersion rate decreases while their number in the grain volume grows. Fine carbides from inside of the grains set the path of easy cracking on the boundaries with the ferritic matrix while the coarse carbides crack in front of the fracture. As a result, the steel fracture toughness decreases. The fracture development by means of carbides is less harmful than on the carbide/matrix boundaries.     

Basic principles of thermal degradation and thermal stabilization of poly(vinyl chloride). Mathematical model of the action of PVC thermal stabilizers

A model of the action of chemically homogeneous thermal stabilizers of poly(vinyl chloride) is shown. Based on the described computer program and experimental data, the proposed model is confirmed and rate constants of the reaction of stabilizers with HCI and the effect of the products of this reaction on the kinetics of PVC degradation are calculated.     

The effect of molybdenum on the transformations in the quenched matrix of high-speed steels during tempering

The analysis of the transformations during tempering was carried out at the model alloys imitating by their chemical composition the matrixes of quenched high-speed steels of various Mo: W relations. Basing on the results of dilatometric and magnetic tests the transformations during tempering with special concern paid to the precipitation processes from martensite and to the retained austenite destabilization were analysed.     

Statistical and mathematical analysis of the catalytic hydrogenating depolymerization of benzene-insoluble coal extract fraction

This Paper presents a statistical and mathematical analysis of hypotheses concerning the autoclave hydrogenating depolymerization of coal extract fractions insoluble in benzene. From comparison of theoretical and experimental data the most probable reaction mechanism was selected.     

Organic sulphur in the hard coal of the stratigraphic layers of the Upper Silesian coal basin

The paper presents the results of investigation of organic sulphur occurrence in the coal of Upper Silesian coal basin. Six hundred coal samples from 52 mines were tested. It has been found that the content of organic sulphur in the coal depends on the stratigraphic layer, coalification grade and geographic location in the Upper Silesia region. The highest mean content of organic sulphur has been found in the most recent (ca. 0.85%) and in the oldest (ca. 0.5%) strata. The lowest mean contents of organic sulphur have been found in the intermediate strata. In the case of the most recent coals, the content of organic sulphur decreases with increasing of coalification grade. When the carbon content (C^daf) exceeds 82%, the increase of coalification grade causes no further decrease of the mean content of organic sulphur. The share of organic sulphur in the total sulphur is lower in the case of more recent (<50%) and higher in the case of older coals (>50%).