Post-Weld Heat Treatment Holding Time and Ramp Rates

The full post-weld heat treatment cycle, which is considerably longer than the hold everyone quotes.

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Checked against published worked examples from: Gulf Coast Combustion.

Details, formula, and sources

ScopeHolding time comes from the governing thickness at a code rate, commonly an hour an inch with a stated minimum -- and then the controlled heating and cooling either side of it frequently exceed the hold itself, so a schedule written on the hold alone is wrong by a factor of two or more. THE RATES MATTER AS MUCH AS THE HOLD AND THEY ARE THE PART MOST OFTEN VIOLATED. Above a threshold temperature the permitted heating and cooling rates are inversely proportional to the thickness: a thin part can be brought up quickly, a thick one cannot. The reason is not caution but mechanism -- heating or cooling a heavy section too fast puts a thermal gradient through exactly the material the treatment exists to relieve, which ADDS residual stress rather than removing it. Breaking the rate produces nothing visible at the time, costs no scrap, and saves real schedule, which is precisely why it is the requirement that goes. Thickness therefore compounds three times over. On a linear hold rate, doubling the governing thickness doubles the hold (ASME VIII UCS-56 P-No. 1 flattens to 2 h plus 15 min per inch past 2 in), halves the permitted heating rate, and halves the permitted cooling rate, so the cycle grows roughly with the thickness in every one of its three parts. That is the number a fabricator needs before committing a furnace or a bank of pads to a schedule. THE GOVERNING THICKNESS IS DEFINED BY THE CODE RATHER THAN BY MEASUREMENT. For a joint between unequal thicknesses it is not simply the thicker member, and the definition differs between codes and between joint configurations -- so getting it wrong moves the hold and both rate limits at once, in the same direction, and a cycle run on the wrong thickness is wrong throughout. This computes a cycle from entered code parameters. It does not supply the holding temperature, the rate constants, the threshold temperature, or the governing-thickness definition for any code, determine whether PWHT is required or exempt (which is a code question turning on material, thickness and service), address the heated band width, gradient control band, or insulation, specify thermocouple number and placement or the temperature tolerance across the band, evaluate the effect on material properties -- PWHT softens some materials and can embrittle others -- or address local versus furnace treatment. ASME Section VIII, ASME B31.1 or B31.3, AWS D1.1 or the applicable code, the written procedure, and the responsible engineer govern.

Formula or tableholding time = governing thickness x a code rate (commonly 1 h per inch) to 2 in, then 15 min per inch beyond for P-No. 1 (ASME VIII UCS-56), against a stated minimum; above a threshold temperature the heating and cooling rates are limited to a constant divided by the thickness, under a ceiling; the total cycle is the hold plus both controlled ramps.

Source and editionPost-weld heat treatment cycle arithmetic. Every parameter -- rates, minimum, threshold, constants, ceiling and the code's definition of GOVERNING THICKNESS -- is entered from the applicable code, which for a joint between unequal thicknesses does not simply take the thicker member.

Free accessArithmetic on entered code parameters; no code table is reproduced.

What governsEstimate. AHJ and licensed professional govern.

API field namesgoverning_thickness_in, hold_rate_hr_per_in, minimum_hold_hr, holding_temp_f, rate_threshold_temp_f, heating_rate_constant_f_hr_in, cooling_rate_constant_f_hr_in, rate_ceiling_f_hr, actual_heating_rate_f_hr, alt_thickness_in, p1_over_2in_rule, holding_time_hr, max_heating_rate_f_hr, max_cooling_rate_f_hr, heating_hours, cooling_hours, total_cycle_hr, alt_total_cycle_hr

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