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The object of the investigation was to explain the effect of various tempering treatments on the creep resistance of carbon and low-alloy steels and to elucidate the probable behavior of such steels during long-time service at high temperature. It was suspected that the problem was related to precipitation hardening since creep tests and tension tests carried out over a range of temperature showed that an increase or decrease in resistance to deformation could be brought about by different tempering treatments.
To follow up these preliminary tests true stress-strain tension tests were carried out on a series of low-carbon steels containing varied amounts of manganese and molybdenum. Similar steels killed with aluminum were also tested. In the absence of manganese and molybdenum a maximum in the stress for a given strain was obtained at about 200 C. When manganese was added a second maximum appeared at about 300 C, and with molybdenum at about 500 C. These maxima in stress are attributed to the precipitation of carbides during the process of straining, the type of carbide formed depending on the testing temperature and the alloying elements present. The influence of tempering is also indicated.
To each maximum in stress there corresponds a minimum in ductility. This suggests that the strain preceding rupture is also related to the precipitation of carbides, in this case at the grain boundaries.
From these tests it was possible to explain many of the effects of alloying elements on the creep resistance and ductility of steels under various conditions of testing. It was concluded that the factor which largely determined such effects was precipitation.
Metallurgist, Research Department, Messrs. Colvilles Ltd., Motherwell,