Question 1

Why is t8/5 (the 800–500°C cooling time) important in welding?

Accepted Answer

The t8/5 parameter controls microstructural transformation in steel HAZ. The temperature range 800–500°C covers austenite decomposition into bainite, martensite, or ferrite. Short t8/5 (fast cooling) produces hard martensite, increasing hydrogen cracking risk. Long t8/5 (slow cooling) produces softer ferrite but may cause HAZ softening in high-strength steels. AWS D1.1 and EN 1011 welding codes specify t8/5 limits based on carbon equivalent and base metal thickness.

Question 2

What is the difference between the Rosenthal thin-plate and thick-plate solutions?

Accepted Answer

The Rosenthal thin-plate (2D) solution assumes no temperature gradient through the thickness: ΔT ∝ Q/(2πkt) × K₀(vr/2α) × exp(vx/2α). The thick-plate (3D) solution models a semi-infinite body: ΔT ∝ Q/(2πkr) × exp(-v(r+x)/2α). The thin-plate model applies when plate thickness t is smaller than the thermal diffusion length α/v. This tool uses the 2D model, which is most accurate for thin plates (≤10 mm) under high heat input.

Question 3

How does increasing preheat temperature T₀ affect t8/5?

Accepted Answer

Increasing preheat temperature T₀ slows cooling, increasing t8/5. From the 2D Rosenthal relation: t8/5 ∝ HI × [1/(500-T₀)² - 1/(800-T₀)²]. Raising T₀ from 25°C to 150°C increases t8/5 by approximately 1.5 to 2 times. Preheat is effective for preventing hydrogen-induced cracking but introduces trade-offs: higher distortion, HAZ softening, and reduced productivity. For high-strength steels (≥ 780 MPa), preheats of 80–150°C are typically required.

Question 4

What thermal efficiency values should I use for different welding processes?

Accepted Answer

Typical arc efficiency values are: SAW (submerged arc) η = 0.90–0.99 (highest), GMAW/MIG η = 0.75–0.85, SMAW η = 0.70–0.80, GTAW/TIG η = 0.50–0.65, and laser welding η = 0.25–0.45 for aluminium or 0.50–0.65 for steel. η multiplies directly into effective heat input HI_eff = η·I·V/v, so the same electrical parameters can produce over 50% difference in actual heat input depending on process. Values vary with shielding gas, electrode type, and travel speed.

Welding Heat Input & Cooling Rate (Rosenthal Solution)

Welding Process

Welding Parameters

Base Metal & Plate

Thermal Cycle T(t) — Temperature History at Observation Point r

Isotherm Contour (Top View) — Weld Pool & HAZ Boundary

Peak Temperature T_peak vs Distance from Weld Line r