The base metal in the neighbourhood of weld is subjected to a rapid rise, followed by a somewhat slower fall in temperature. The nearer the weld, the higher maximum temperature attained.
Physical properties in the adjacent part of weld are influenced by the combined effect of thermal stress and rise of temperature that are caused by the welding heat.
As explained in the first report, the rate of increase of temperature is greater than that usually met in heat treating practice and the subsequent cooling is faster than but still comparable with that experienced in normal air cooling, and not so rapid as an oil or water quenching treatment. Therefore a part of constituent dissolved in ferrite at high temperature is retained in supersaturation.
The author shows in this paper that the value of impact resistance and hardness of the adjacent part of weld and the relation between the maximum heating temperature and impact resistance of metal that has been found by the short time local heating test. Photographs reveal the microstructure of specimens locally heated at various temperature.
From this experiment, specimens locally heated from 750°C to 1200°C show the refined structure that gives the good result in impact resistance. It is because that the refined structure characterized by a finer grain size than the original unweld plate; especially, pearlite is finely distributed.
On account of the grain growth by overheating, the specimen heated above 1200°C shows a poor impact resistance and Widmanstaten structure appears in the heated part.
Hardness and microstructure are not dearly change in specimens heated below 700°C but the appreciable change of impact resistance is apt to occur in them. On the impact test of material heated below 700°C the maximum point is found at heated temperature about 600°C and two mininum point exist at 700°C and in range from 300°C to 500°C respectively. It is believed that the former minimum point due to the precipitation of supersaturated constituent andd the latter is produced by the combined effect of thermal stress and precipitation of the supersatureted matter excited by the rise cf temperature and the thermal stress.
Sensitivity of this heat treatment is varied by the composition of low carbon steel. Impact resistance of the steel contained the suitable quantity of Si or killed steel is hardly affected by welding heat when the material is locally heated below 700°C.
This paper contains the following articles:
1 Thermal stress, phenomena of aging and short time local heating.
2 Impact resistance and hardness of base metal affected by welding heat.
3 Impact resistance, hardness and microstructure of specimen treated by short time local heating.
4 Theoretical consileration on brittleness of base metal treated by short time local heating.

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