Welding Interpass Temperature Calculator Guide

Per ASTM A923 Method C (ferrite–austenite balance verification) and ISO 17632:2013, the maximum interpass temperature for UNS S32205 is typically limited to 225°C. Exceeding this threshold risks excessive ferrite formation (>70%) and precipitation of sigma, chi, or nitride phases—especially in the heat-affected zone—degrading corrosion resistance and impact toughness. The 225°C upper limit aligns with EN 10149-2 and ASME BPVC Section IX QW-403.11, which mandate interpass control to preserve the target 40–60% austenite/ferrite balance. Our calculator defaults to 225°C but allows adjustment within 100–350°C; however, exceeding 225°C requires formal procedure qualification per AWS D1.6/D1.6M and documented metallurgical validation.

The cooling rate constant (k, in s⁻¹) in our calculator approximates Newtonian cooling behavior—i.e., dT/dt = −k(T − Tₐmbient)—which is empirically validated for thin-to-moderate section duplex welds under controlled conditions. While full transient FEA (e.g., SYSWELD or Thermo-Calc) offers higher fidelity, k provides rapid, field-deployable estimation aligned with ISO 17632 Annex B guidance on simplified thermal assessment. For UNS S32205, k = 0.01 s⁻¹ reflects typical air-cooled, 6–12 mm plate welding with moderate heat input (1–2 kJ/mm). Values outside 0.001–0.1 s⁻¹ require calibration via thermocouple arrays or IR thermography per AWS G1.8, as excessive k underestimates dwell time in critical 800–300°C range where sigma phase nucleates.

No—reducing heat input alone does not justify exceeding 225°C for UNS S32205. Elevated interpass temperature prolongs exposure in the critical 700–900°C range where sigma phase precipitates at rates exponentially accelerated above 225°C, regardless of heat input. ASTM A923 explicitly prohibits interpass temperatures >225°C unless qualified by Charpy V-notch testing at −46°C (minimum 45 J avg.) and ASTM G48 Method A pitting corrosion testing (≤10 mg/cm² mass loss). Even with low heat input, sustained >225°C interpass increases ferrite content beyond 65%, reducing austenite-mediated stress-corrosion cracking resistance. Always verify compliance via ferritoscope readings (target 45–55% ferrite) and metallography per ASTM E562.

The calculator outputs are ±5°C accurate under calibrated conditions—provided inputs reflect actual measured values. Base metal temperature must be verified with a contact thermocouple (ASTM E220) or Class 1 infrared camera (IEC 62133), not surface paint or visual cues. Time must represent true cooling duration from peak weld temperature to next pass start—not arc-on time. The model assumes uniform section thickness and ambient convection; accuracy degrades for thick sections (>25 mm), restrained joints, or drafts >1 m/s. For critical applications (e.g., offshore piping per NORSOK M-650), validate outputs with at least three thermocouple measurements per weld pass and cross-check against ISO 17632’s recommended 200–225°C window using a certified thermal profiler.

Preheat does not increase the maximum allowable interpass temperature—it only shifts the starting point of the thermal cycle. UNS S32205 is rarely preheated (typically 0–50°C), as excessive preheat (>100°C) promotes ferrite retention and slows austenite reformation during cooling, increasing susceptibility to embrittlement. Per AWS D1.6, preheat is generally discouraged unless mitigating hydrogen cracking in heavily restrained joints—but even then, interpass must still cap at 225°C. The calculator’s base metal temperature input accommodates preheat, but the upper limit remains fixed by metallurgical constraints, not thermal history. Always measure interpass temperature on the weld groove face—not the backing bar or adjacent parent metal—to avoid false-low readings that risk exceeding the 225°C threshold.

Exceeding 225°C interpass temperature directly degrades pitting corrosion resistance by promoting chromium nitride (Cr₂N) and sigma phase precipitation along grain boundaries and in ferrite-rich regions. These phases deplete adjacent matrix of Cr and Mo, lowering the critical pitting temperature (CPT) by up to 20°C per 5°C overage—verified per ASTM G150 electrochemical testing. In seawater service (ISO 15156-3), CPT < 25°C indicates unacceptable degradation. Microprobe analysis shows Cr depletion zones <18 wt% adjacent to Cr₂N in over-heated welds. Maintaining ≤225°C ensures balanced austenite/ferrite re-precipitation and preserves the alloy’s PREN ≥34. Field verification requires ASTM G48 Method A testing on transverse weld specimens with 100 h exposure at 22°C.

The calculator itself is not an ASME Section IX-compliant tool—but its output (≤225°C) satisfies QW-403.11’s requirement for interpass temperature limits as an essential variable. To qualify a WPS, you must document the actual interpass temperature used during PQR testing—not just the calculated value. ASME BPVC Section IX mandates recording min/max interpass temps per pass, verified by calibrated instrumentation traceable to NIST. The calculator aids in establishing rational upper bounds and optimizing parameters, but final WPS approval requires empirical validation: tensile, bend, Charpy (−46°C), and ferrite testing per QW-182. Using the calculator without PQR verification violates QG-105 and invalidates Code compliance.

Dropping below 25°C is not inherently harmful—but uncontrolled cooling to ambient risks excessive thermal gradients, increasing residual stress and potential hydrogen-assisted cracking in high-diffusible-hydrogen environments (e.g., SMAW with non-low-hydrogen electrodes). More critically, excessively low interpass (<50°C) may suppress austenite reformation kinetics in the HAZ, leading to ferrite-dominated microstructures with reduced ductility and SCC resistance. ASTM A923 recommends maintaining ≥50°C minimum interpass for structural integrity, though no strict lower bound exists. The calculator’s default base metal temperature of 25°C reflects ambient reference—not a minimum. Use preheat or controlled cooling (e.g., ceramic blankets) to hold 50–100°C when ambient is cold, especially for thick sections or high restraint, per AWS D1.6 Clause 6.2.2.