Standard field practice: (1) Record arc voltage and amperage from the machine display or use a calibrated clamp meter at the work cable (not the power supply display — cable drop causes discrepancy). (2) Time the welder over a known distance (scribe two marks 6–12 inches apart on the joint) with a stopwatch, arc-on only: travel speed = distance ÷ arc-on time. (3) Plug V, A, S, and η into the heat input formula and compare the result to your WPS minimum and maximum limits. For ASME Section IX welds, the PQR-qualified heat input limits are typically expressed as ±10–25% of the tested value per QW-409. Document all measurements in the weld traveler or weld data sheet — QC inspectors and AI/CWIs verify this against the WPS on final inspection.

ASTM A516 Gr. 70 (normalized or as-rolled) is a common pressure vessel plate steel with no specified maximum heat input in the base standard — the limit is established by your PQR (Procedure Qualification Record). However, widely used industry guidance (from published PQR data) places the practical upper limit around 65–90 kJ/in (SMAW/GMAW) for normalized material targeting good CVN toughness at service temperatures above −20°F. For low-temperature service (MDMT below −20°F per ASME Section VIII UCS-66), heat input must be kept to the PQR-tested range, and impact testing per QW-403.6 becomes mandatory. If your WPS was qualified at 40 kJ/in and you want to run at 70 kJ/in, you must re-qualify a new PQR at that heat input level and perform new Charpy impact testing on HAZ samples.

Preheat temperature is a separate controlled variable from heat input: it sets the starting temperature of the base metal before the arc is struck, which directly affects the cooling rate curve — higher preheat = slower cooling = less martensite and lower HICC risk. The heat input formula does not mathematically include preheat, but both variables are controlled in tandem in any code-compliant WPS. AWS D1.5 (bridge welding) and API 1104 (pipeline) specify minimum preheats by carbon equivalent (CE ≥ 0.40 commonly requires 200–300°F preheat). Using this calculator with a higher preheat scenario: if you raise preheat from 150 to 300°F, you may be permitted to reduce heat input (faster travel) while still staying above the HICC threshold — this can increase productivity without sacrificing metallurgical quality. The combined effect is captured by the Graville diagram and cooling rate curves, not the simple heat input formula alone.

Heat input (kJ/in) is a measure of energy per unit length, but the resulting HAZ cooling rate also depends on: (1) Material thickness — thicker plate acts as a heat sink, accelerating cooling for the same heat input. A 1.5” plate with 40 kJ/in cools faster than a 0.25” plate at the same input (more conduction paths). (2) Steel carbon equivalent (CE) — higher CE means the martensite start temperature (Ms) is lower, making hardening more likely at the same cooling rate. ASTM A514 (high-strength Q&T, CE ~0.65+) requires vastly lower heat input than A36 (CE ~0.25) to preserve its 100 ksi yield strength — excessive heat completely destroys Q&T temper in the intercritical HAZ. (3) Thermal conductivity and diffusivity — stainless steel (316L) has ~1/3 the thermal conductivity of carbon steel, so the HAZ stays hot much longer at the same heat input, leading to carbide sensitization (chromium carbide precipitation) if the 800–1500°F sensitization range is traversed slowly. For stainless, the goal is low heat input to get through that range quickly.