I have some questions regarding heat treatment processes as per NB/T47015
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I’m very confused by one sentence in Table 5 of NB/T47015-2011 P107, which outlines the recommended specifications for post-weld heat treatment. I would appreciate it if an experienced expert could help me understand it. In item 6 of the notes in Table 5, it is stated that \"when the temperature of the welded part is 650°C or higher, the cooling rate should not exceed 55°C/h; once the temperature drops below 650°C, rapid cooling is required, with a cooling rate sufficient to prevent embrittlement.\" ” What exactly does this sentence mean? If the heat treatment retention temperature is above 650°C, must the cooling rate be less than 55°C/h? And must it be cooled rapidly when the temperature is below 650°C? This contradicts item 4.6.12 on page P109 later on, as well as 9.3.3.4 in GB/T16507.5. Is this statement incorrect, or is there something wrong with my understanding? Also, what is rapid cooling?
Note 6) This is only a special case, applicable only to the materials marked with “6)” in Table 5
Thank you so much! I didn’t notice the small labels in the table above; I thought what was in the box below were the overall notes!
Note 6: The Fe7 category refers to ferritic stainless steels. The temperature range of 650–730 degrees is intended to relieve stress and dissolve chromium carbides. Rapid cooling below 650 degrees is done to avoid brittleness at 475 degrees; strictly speaking, brittleness occurs before 400–500 degrees, but the decrease in brittleness is most noticeable at 475 degrees