Weld HAZ Toughness and Its Improvement in Low Alloy Steel SQV-2A for Pressure Vessels (Report 3) : Toughness Improvement by PWHT(Materials, Metallurgy & Weldability)
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The effect of PWHT conditions on the toughness of the weld HAZ of SQV-2A steel was investigated to determine the optimum PHWT condition. The mechanism of toughness improvement was discussed on the basis of TEM observation and fractography. The results showed that the toughness after PWHT was strongly influenced by cooling time Δt_<8/5> of weld HAZ and PWHT conditions. Provided that PWHT time was 28.8ks at about 893K and Δt_<8/5> was shorter than 40s, the toughness was greatly improved. However, when the PWHT temperature was 723K or lower, embrittlement occurred. When Δt_<8/5> was longer than 80s, toughness was increased for PWHT temperatures from 523 to 823K. When the PWHT temperature was about 893K, toughness was increased considerably, regardless of cooling time Δt_<8/5>. The toughness was also affected by PWHT time. By increasing the cooling time to 7.2ks, the toughness was clearly increased. Therefore, a PWHT at 893K for 7.2ks could greatly improve the weld HAZ toughness of SQV-2A steel. If the cooling time Δt_<8/5> of weld HAZ was shorter than 20s, the toughness could be improved to the level of as received material. Metallurgical analysis showed that in the case of Δt_<8/5> of 6s, after PWHT at 893K for 7.2ks, recovery phenomenon occurred to martensite laths, and the matrix was significantly softened. For this reason, the toughness was improved. In the case of Δt_<8/5> of 150s, the M-A constituent was decomposed, but there was no obvious change in ferrite lath of upper bainite after a PWHT at 893K for 7.2ks. Because the ferrite lath was too coarse, the improvement in toughness was limited.
- Transactions of JWRI
Transactions of JWRI 23(2), 223-230, 1994-12