Regarding the minimum holding temperature for post-weld heat treatment and the allowable deviation of the heat treatment temperature
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Table 5 of NB/T47015-2011, the recommended specifications for post-weld heat treatment, specifies the minimum holding temperature for various materials. When carrying out actual post-weld heat treatment, it is necessary to specify the allowable temperature deviation range. Among them, the lowest holding temperature for Fe-1 type materials is 600°C. Assume that the normal temperature control range for the heat treatment furnace is ±13°C, with an allowable temperature deviation range of ±15°C. Question: Should the heat treatment temperature be 600±15°C or 615±15°C?
Generally, a minimum insulation temperature is specified, and it should not be lower than this value. Some materials also require a limit on the maximum temperature
Generally, the values specified in standards/specifications refer to nominal values, without taking deviations into account. The allowable deviation of the heat treatment holding temperature should refer to the temperature uniformity of the specified heat treatment furnace. In many foreign countries, PWHT is carried out at 595°C directly, without considering any deviations.
The so-called constant temperature also has a range, and the temperatures at various measurement points should also be kept within an effective range; it is not possible for them to be a constant value.
The design is carried out in accordance with standards, and the heat treatment temperature specified by these standards represents the minimum value (Table 5 of NB/T47015: Minimum holding temperature)； ASME VIII-1 UCS-56: Normal Holding Temperature, Minimum. Different heat treatment equipment and measurement methods will result in varying errors, and the standards do not specify specific temperature tolerance ranges. The temperature suggested by our heat treatment engineers is also 615±15°C; I recommend writing it as 600°C +30/-0°C, as this better reflects compliance with the standards. 600℃ with a lower deviation of 0℃ is the value specified in the standards; the upper deviation of +30℃ is determined by the heat treatment engineer based on the specific heat treatment conditions.