Interpass Temperature Window and Time Between Passes

ScopePreheat and interpass are a window rather than two separate rules, and the welder lives inside it: below the preheat minimum, hydrogen cracking risk rises and the procedure is violated, and above the interpass maximum the cooling time stretches, grain growth costs toughness, and the procedure is violated in the other direction. Both limits come from the welding procedure specification. The two outputs answer the two questions that actually arise on a heavy weldment. The idle allowance is how long a welder can stop -- for a fit-up, for a grind, for a break -- before the joint drops below preheat, and it is usually shorter than people expect. The required wait is the other case: a pass ran hot, the joint is at or over the interpass maximum, and welding cannot resume until it comes down. A joint with a measured time constant of 12 min in a 70 F shop, on a procedure calling for 200 F preheat and a 500 F interpass maximum, gives about 6.8 min of idle after a pass that ended at 300 F, and a pass that ended at 600 F, over the 500 F maximum, waits about 2.5 min to come back to 500 F -- waiting all the way to the 200 F preheat floor would take 16.9 min and then need reheating. Seven minutes is shorter than a lot of fit-up interruptions, which is why heavy weldments get blankets and why a torch stays lit next to the work; and 16.9 minutes of standing still, on a joint with twenty passes, is over five and a half hours of lost production if it happens every time. The time constant is a property of the specific joint -- its mass, its section, its exposure -- and it must be measured rather than assumed: timing one cooling interval with a contact pyrometer gives it, and the whole schedule follows. A scheduling aid; the welding procedure specification and a measured joint temperature govern.

Formula or tableT(t) = ambient + (start - ambient) x exp(-t / tau); time to fall from T1 to T2 = tau x ln((T1 - ambient) / (T2 - ambient)). The idle allowance runs from the current temperature down to the preheat minimum; the required wait runs from the current temperature down to the restart temperature, and is zero when the joint is already at or under the interpass maximum.

Source and editionNewtonian cooling of a weld joint toward ambient, applied to the preheat-minimum and interpass-maximum window the welding procedure specification sets, by name. The time constant is a property of the specific joint and is MEASURED, not bundled. The WPS and a measured joint temperature govern.

Free accessFirst-order cooling is public physics; the window limits come from the reader's own procedure and the time constant from their own measurement.

What governsEstimate. AHJ and licensed professional govern.

API field namestau_min, ambient_f, preheat_min_f, interpass_max_f, current_temp_f, restart_temp_f, elapsed_min, idle_allowance_min, required_wait_min, temp_at_elapsed_f

• One window preheat minimum and interpass maximum are two ends of the same window, both from the WPSwelding procedure practice
• Time constant is measured tau belongs to the specific joint's mass, section, and exposure; time one cooling interval with a contact pyrometerwelding practice
• First-order cooling exponential approach to ambient; a joint under a blanket or in wind has a different tauheat transfer first principles