Δt8/5 Cooling Time

Calculates Δt8/5 (cooling time from 800°C to 500°C) via Rosenthal's formula — a 2D (thin-plate) form for plate thickness below 23 mm and a 3D (thick-plate) form at or above it, both driven by the effective heat input (η×Q). Effective heat input can be entered directly, or computed from a nominal heat input and an efficiency factor — never both, to avoid double-applying efficiency.

Engineering calculator — results require technical review before production use.

Source & assumptions

IWE Module 2 – Materials and Their Behavior during Welding, Chapter 5, Section 4 (Cooling Rate Δt8/5)

Assumptions

The source states that structure in structural steel depends strongly on cooling time from 800°C to 500°C, and uses Rosenthal's formula to compute it.
2D formula (S < 23 mm): Δt8/5 = (1000 − T0) × 4.3×10⁵ × ((η×Q)/S)² × [1/(500−T0)² − 1/(800−T0)²], seconds.
3D formula (S ≥ 23 mm): Δt8/5 = (1340 − T0) × 5 × η×Q × [1/(500−T0) − 1/(800−T0)], seconds.
T0 = preheat/interpass temperature (°C), S = plate thickness (mm), Q = nominal heat input (kJ/mm), η = efficiency factor (dimensionless).
Both formulas use the effective heat input η×Q, not Q alone — see the Path A / Path B distinction below.
Path A: a nominal heat input Q (entered directly, or derived as Q = (U×I)/(v×1000) from voltage/current/travel speed) is combined with a required efficiency factor η to compute η×Q internally.
Path B: an already efficiency-adjusted effective heat input (η×Q) is entered directly and used as-is, with no further multiplication by any efficiency factor. Path A and Path B are mutually exclusive — supplying fields from both throws, to prevent silently double-applying or ignoring efficiency.
When Q is derived from welding parameters, travel speed is normalized to mm/s via the same Q = (U×I)/(v×1000) relationship already used by this app's Heat Input / Arc Energy (FR-001) and Deposit Coefficient (FR-005) calculators.
The 3D formula has no dependence on plate thickness once S ≥ 23 mm is established — this is a property of the source formula itself, not a simplification introduced here.

Limitations

This calculator computes only Δt8/5 (an estimated cooling time) — it does not by itself constitute a full weldability decision.
It does not approve, recommend, or validate a specific preheat temperature — the user supplies T0 as an input; the calculator does not determine whether that T0 is adequate for any given steel or application. This is not a preheat approval tool.
It does not predict HAZ hardness, and does not perform a hydrogen cracking risk assessment.
It does not compute T100 or relative interpass rise (RIR).
It does not implement Schaeffler/DeLong diagrams, ferrite number, or structure prediction.
It does not approve a WPS — it does not replace WPS qualification, production test, or engineering review.
The T0 < 500°C hard validation is derived from the formula's own algebra (division by (500 − T0)), not a separately stated source limit.
The 0 < η ≤ 1 efficiency-factor bound is reused from this app's Heat Input / Arc Energy (FR-001) convention for consistency — it has not been independently confirmed against Chapter 5, Section 4 specifically.
The 'unusually high' sanity-check warnings (voltage, current, travel speed, effective heat input, plate thickness, preheat temperature) are implementation-level heuristics, not sourced from IWE Module 2.
Results require technical review before production use.