Delayed hydride cracking (DHC) tests were conducted on a cold-worked Zr-2.5Nb tube with hydrogen at different test temperatures ranging from 100 to 350". Striation spacing corresponding to a critical hydride length increased with temperature below 300°C, demonstrating that hydride cracking resistance is enhanced with temperature. Nevertheless, the crack growth rate had positive temperature dependence below 300°C. This fact indicates that below 30(fC, the crack growth rate in DHC of the Zr-2.5Nb tube is governed by hydride growth which is a slower process than hydride cracking. At higher temperatures above 300°C, the crack growth rate had negative temperature dependence. Thus, it is suggested that this rapid drop of the crack growth rate above 300°C is due to enhanced resistance to hydride cracking which becomes a rate-determining process above 300°C. This suggestion is evidenced by experiments where the threshold stress intensity factor or Km increased sharply above 300"C.
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