Papers by Author: Ignacy Wierszyłłowski

Abstract: The paper presents results of comparative studies of tempering processes of quenched and conventionally treated tool steel vs. the same steel quenched and cryo-treated. Also ageing processes of Al-4.7%Cu alloy after conventional and cryo-treatments are analyzed and the results presented. It has been observed that in both cases cryogenic treatment has an effect on the development of the tempering and ageing processes. The mechanisms of the influence are different in each case. In the cryo-treated steel, the tempering processes already begin in the process of reaching ambient temperature from 196°C and accelerate the increase in hardness because of tempering. In conventionally treated Al-Cu alloy the processes of dissolving precipitates and the resulting decrease in hardness occur earlier than in the cryo-treated alloy. Reazons of such changes have been analyzed.

Abstract: The study analyses of influence of cryogenic treatment on the resultant structures, the development of changes during tempering and the charcteristic obtained. It was observed that cryo treatment affects the changes through: a significant reduction in retained austenite content, gradual reduction in value of martensite tetragonality and an increase in the number of carbon atoms in transformations during tempering. The consequence of the transformations taking place during cryogenic treatment is precipitation of η – carbide during low tempering instead of ε – carbide. The number of η – carbides precipitated is higher than that of ε – carbide. This affects the functional properties of steel products. Key words: cryogenic processing, supersaturation, dilatometry, DSC, ageing.

Abstract: Studies of dilatometry, DSC, hardness and XRD have shown that a low temperature cryogenic treatment immediately after supersaturation affects changes of structure and properties taking place during ageing after supersaturation. The third peak visible on DSC diagrams for samples conventially treated disappears on DSC diagrams for deep cryogenically treated samples, and the intensity of the two first peaks on DSC diagrams of deep cryogenically treated samples is a few times lower than on diagrams for conventionally treated samples. The dissolution process for precipitation of Al2Cu in conventionally treated samples progresses faster. Hardness of samples aged after a deep cryogenic treatment within the range of 175-220 oC for more than 60 minutes was higher than of those conventionally treated; at higher ageing temperatures (275-350 °C) the differences in hardness were smaller.

Abstract: The paper presents the influence of the grain size and a little higher Cr content on the kinetics of austenite phase transformations during continuous cooling of hypo-eutectoid steel. The kinetics of austenite phase transformations during continuous cooling were determined by means of analysis of the dilatometric curves and structure investigations. The influence of the austenite grain size and the higher Cr content was analysed in two hypoeutectoid steels containing about 0.4% C. One of them had, Cr content higher, by about 1%. In both steels, the austenite grain size was changing insignificantly up to the austenitising temperature of about 950fl. Above that temperature, the austenite grain size in carbon steel grew much quicker than that in the steel with Cr addition. The austenite grain in the Cr enriched steel was smaller than that in carbon steel and, in spite of that, the duration of cooled austenite transformations were several times longer. This means that the phase transformations are much more strongly influenced by the addition of chromium slowing down carbon diffusion in austenite, than by the austenite grain size. For each phase transformation in the examined steels, the activation energy of the transformation has been determined. The activation energy of all the phase transformations varied slightly with the increase of austenitising temperature. On the basis of the obtained results, curves of true isothermal transformations have been developed for the beginning of the phase transformations in both steels, related to infinitely quick cooling down to the transformation temperature.

Abstract: A significant increase in durability of cryogenically treated tools after quenching was reported by a number of publications [1, 2]. As research studies show [4, 5, 6, 7], the main reason for this is the kind of carbides precipitated during tempering at temperature range of 150 –200 0C, which is different than in the case of conventional treatment. These carbides are finer and more evenly distributed in the matrix of steel. The number of carbides is higher than in conventionally treated steels because of higher fraction of martensite in cryogenically treated steels produced by retained austenite transformation at cooling to deep cryogenic temperatures. The number of carbides precipitated from martensite at low temperatures of tempering is proportional to shrinkage produced at the same temperatures of tempering. Calculations on the basis of dilatometric experiments show that the shrinkage difference between the same D2 steel cryogenically and conventionally treated is higher than that which results from the increased fraction of martensite in cryogenically treated steel. The XRD studies of cryogenically treated steel show a presence of two kinds of martensites differing in tetragonality. Low temperature tempering of cryogenically treated steel produced two types of carbides – ε carbide and η carbide. The conventionally treated steel consists of one kind of tetragonal martensite and one kind of carbide - the ε carbide. The hardness of cryogenically treated samples was somewhat higher than in conventionally treated ones, while fracture toughness of conventionally treated samples was somewhat higher than in cryogenically treated ones. The results obtained were discussed in reference to literature data.

Abstract: Studies of transformation kinetics during ageing of Al-4.7 % Cu were performed with use of dilatometric and DTA methods. The dilatometric method was used for studies of isothermal ageing, DTA for studies of ageing during isochronal heating. In order to determine isothermal transformation kinetics the JMA (Johnson – Mehl – Avrami) equation was applied, for isochronal heating transformation kinetics Kissinger method was used. Ageing of Al- Cu alloys consist of precipitation processes and dissolution processes that overlap each other. These processes, during isothermal ageing were distinguished with help of the time exponent n of JMA equation that is very sensitive to change of the transformation mechanism. During isochronal ageing, precipitation processes produce heat, dissolution processes consume heat. That helps to distinguish them. Results obtained during isothermal studies were in agreement with isochronal heating studies. Activation energies for precipitation processes 324-530 K temperature range varied from about 50 kJ/mol to about 100 kJ/mole and complied with the values obtained before. For precipitation processes at 650- 735 K temperature range, activation energies varied from 226 to 300 kJ/mole. The results obtained were discussed with literature data.