The transformation behaviour of medium manganese rail steels and its influence on their welding characteristics.

Isothermal and continuous cooling transformation diagrams have been produced for six medium-manganese rail steels. In the isothermal experiments, specimens were heated for 15 minutes at 850°C and the progress of transformation followed by dilatometric means and microscopic examination. The continuous cooling transformation diagrams were determined under thermal conditions simulating those occuring in the heat affected zone of arc welds; specimens were heated to a maximum temperature of about 1340°C and subsequently cooled at various rates. The transformation characteristics during cooling were studied by means of a strain-gauge dilatometer, operating a "function plotter". The results have been related to the weldability of the steels with low-hydrogen electrodes. It has been shown that for the steels investigated: (a) the safe limit of average peak hardness in the heat-affected zone is much higher than reported by previous investigators. (b) the relationship between the incidence of hard-zone cracking and end-of-trans- formation temperature, established for low alloy steels, does not apply to medium-manganese rail steels. (c) a 200°C preheat, under thermal conditions occuring in the C.T.S weldability test (T.S.N. 6 -heat input 40 x 10[3] joules/in.) reduces the cooling rate sufficiently to almost completely avoid the formation of martensite, and leads to a hardness not exceeding. 350 V.P.N. in the heat-affected zone. (d) cracking in the C.T.S. test occurs only under welding conditions giving almost completely martensitic structure in the hard-heat-affected zone. (e) provided the cooling curves during welding and in the rapid dilatation tests, can be accurately duplicated, quantitative estimates concerning microstructure and hardness of the heat-affected zone can be derived from the continuous cooling transformation diagrams