Abstract
A transformation from r to a and a microstructure in synthetic weld heat affected zone of Nb, Zr or Ti bearing steels have been studied in detail by measuring cooling rate-temperature curves of weld thermal cycles. The results are as follows: (1) When γ→α transformation begins, a cooling rate decreases gradually and it decreases greatly with growth of boundary ferrites. When ferrite side plates or intergranular fine grained ferrites precipitate following boundary ferrite precipitation cooling rate shows a minimum value. (2) Two types (I and II) of the cooling rate-temperature curves are observed. The cooling rate in type I during γ→α transformation decreases rapidly and begins to increase as soon as it reaches a minimum value with progressing of transformation. In type II, however, the cooling rate shows a constant value over a certain temperature range, keeping minimum value. (3) The cooling ratetemperature curves in specimens containing Nb, Zr and low N-Ti show type I, on the other hand, specimens with high N-Ti and high N-high Zr showing type II. (4) The intergranular fine ferrites precipitate in specimens with type II. The volume fraction of the fine grained ferrite has deep relation with the temperature range which shows a minimum cooling rate
