Influence of Various Heat Treatment Cycles on the Embrittlement of Blackheart Malleable Iron

Abstract

To clarify the embrittling mechanism of blackheart malleable iron, Charpy impact tests have been carried out for the specimens in the temperature range from -196 to 150°C, and also relationships between embrittled 50% energy transition temperature and various embrittling heat treatment have been investigated. IMA analyses have also been carried out for the brittle manner fracture of embrittled and nonembrittled blackheart malleable irons.
The results obtained are as follows:
(1) Considerable degree of embrittlement is recognized in specimens rapidly heated and then water quenched from 450°C without holding at this temperature.
(2) There seems to be an embrittling temperature range above 375°C and a de-embrittling temperature range below this temperature.
(3) Water quenching from 300°C after 1 h holding at this temperature shows some degree of embrittlement when separately treated, while this treatment after rapidly cooled from 450°C shows remarkable degree of de-embrittlement.
(4) The brittleness resulted from water quenching from 450°C without holding at this temperature disappears easily by slow cooling from 450°C without holding at this temperature.
(5) Remarkable segregation of phosphorus on the grain boundary of the brittle iron is recognized by IMA analysis, while no segregation of silicon, carbon and manganese is recognized.
(6) Consequently it has been concluded that the segregating behavior of carbon having interaction with that of phosphorus on grain boundary seems to contribute to the embrittlement of blackheart malleable iron.