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t8/5 Thick Plate Weld Cooling

Calculate the critical t8/5 cooling time for thick-plate steel welds per EN 1011-2 to predict weld microstructure and prevent hydrogen-induced cold cracking.

Thermal Geometry Data

Heat Input (Q) (kJ/mm)

Preheat Temperature (T0) (°C)

Thermal Efficiency (η)

⚠️ THERMO-MECHANICAL DIAGNOSIS: The time it takes a weld to cool from 800°C to 500°C dictates the microstructure of the steel. If t8/5 is too fast (under 5 seconds), martensite forms, making the weld brittle and extremely susceptible to catastrophic hydrogen-induced cold cracking.

Critical Cooling Time (t8/5)

0.0 Sec

Time spent crossing the 800°C to 500°C threshold.

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What is Weld Cooling Metallurgy: Why t8/5 Controls Everything?

The t8/5 value is the time in seconds a weld takes to cool from 800°C to 500°C. This specific temperature window is where austenite in carbon steel transforms into either tough pearlite/bainite (slow cooling) or brittle martensite (fast cooling). Getting this number wrong causes catastrophic weld failure — delayed cold cracking that can appear hours or days after welding.

Mathematical Foundation

EN 1011-2 Thick Plate 3D Cooling

t_{8/5} = (6700 - 5T_0) \cdot Q \cdot \eta \cdot \left(\frac{1}{500 - T_0} - \frac{1}{800 - T_0}\right)

t_{8/5}

= Critical cooling time from 800°C to 500°C (seconds)

T_0

= Preheat temperature (°C), capped at 450°C

Q

= Arc heat input (kJ/mm) = (V × I × 60) / (1000 × travel speed mm/min)

\eta

= Thermal efficiency: TIG = 0.60, MIG/SMAW = 0.80, SAW = 1.00

Laws & Principles

t8/5 < 5 seconds = Martensite: Too fast. The weld forms hard, brittle martensite. Hydrogen trapped in the lattice causes delayed cold cracking (HICC) — cracks that appear hours or days after welding.
t8/5 = 8-25 seconds = Target Zone: Bainitic/fine pearlite microstructure with optimal toughness and strength. This is the target range for structural weld procedures.
t8/5 > 25 seconds = Grain Coarsening: Too slow. Excessive grain growth reduces mechanical properties. The weld passes test but may have reduced impact toughness at low temperatures.
Preheat is the Primary Lever: Raising preheat from 20°C to 150°C can more than double the t8/5 time. Preheat is the single most effective tool for controlling cooling rate in the field.
Thermal Efficiency (η): TIG = 0.60 (focused arc). MIG/SMAW = 0.80. SAW = 1.00 (entire arc submerged under flux — a perfectly efficient closed thermal system).

Step-by-Step Example Walkthrough

" MIG welding 25mm A36 plate at 28V, 230A, 300 mm/min travel speed, 100°C preheat. "

1. Heat input Q = (28 × 230 × 60) / (1000 × 300) = 1.29 kJ/mm.
2. MIG thermal efficiency η = 0.80.
3. Temperature factor: (6700 - 500) = 6200.
4. Reciprocal difference: 1/(500-100) - 1/(800-100) = 0.00107.
5. t8/5 = 6200 × 1.29 × 0.80 × 0.00107 = 6.86 seconds.

Final Result: t8/5 = 6.86 seconds — above the 5s martensite threshold but below the 8-25s target zone. Increase preheat to 150°C to push t8/5 above 10 seconds.

Quick Answer: What Is t8/5 and Why Does It Matter?

t8/5 is the cooling time in seconds from 800°C to 500°C — the critical temperature range where steel's microstructure is determined. Below 5 seconds = brittle martensite (cracking risk). Between 8-25 seconds = optimal bainite (tough, strong). Above 25 seconds = grain coarsening (weakened properties). This calculator uses the EN 1011-2 thick plate formula to predict t8/5 from your voltage, amperage, travel speed, and preheat temperature.

t8/5 Outcome Zones

t8/5 Range	Microstructure	Properties	Risk
< 5 seconds	Martensite	Very hard, very brittle	⚠ Cold cracking (HICC)
5-8 seconds	Partial martensite + bainite	Hard, elevated cracking risk	Elevated — preheat needed
8-25 seconds	Bainite / fine pearlite	Tough, strong, ductile	✓ Target zone
> 25 seconds	Coarse pearlite + ferrite	Reduced toughness	Low-temp impact failure

These zones are approximate for carbon and low-alloy steels. High-alloy steels have different CCT diagrams and transformation temperatures. Always consult the specific CCT curve for the exact grade being welded.

Cooling Rate Failures

The Cold Morning Crack

A structural welder tack-welds 20mm A572 Gr 50 plate at 7:00 AM when ambient is 5°C. No preheat. Parameters: 25V, 200A, 350 mm/min MIG. Heat input: 0.86 kJ/mm. t8/5 = (6700 - 25) × 0.86 × 0.80 × [(1/495) - (1/795)] = 3.5 seconds. Deep in the martensite zone. By noon, a transverse crack runs the full length of the tack weld. The hydrogen from overnight humidity in the base metal diffused into the martensite and fractured the lattice. A 100°C preheat would have pushed t8/5 to 8+ seconds and prevented the crack entirely.

The Engineered Preheat

A welding engineer uses the calculator to determine preheat for thick pressure vessel shell plates (40mm SA-516 Gr 70, CE = 0.43). At 20°C ambient with a planned heat input of 1.5 kJ/mm (SMAW), t8/5 = 5.2 seconds — borderline martensite. She iterates the preheat temperature in the calculator: 100°C → t8/5 = 9.8s, 150°C → t8/5 = 14.2s. She specifies 150°C minimum preheat on the WPS. The welds pass radiography with zero indications and Charpy impact testing at -30°C exceeds the 20 ft-lb minimum by 3×.

Pro Tips for Cooling Rate Control

Do This

✓Preheat is your most powerful lever. Raising T₀ from ambient (20°C) to 150°C nearly triples the t8/5 time. When t8/5 is borderline, always specify preheat first — it is cheaper than changing parameters or process.
✓Verify preheat with a contact pyrometer, not hand feel. A surface that feels warm to the touch could be 40°C or 80°C — a huge difference in t8/5 outcome. Use a calibrated contact pyrometer or tempilstik at the specified temperature.

Avoid This

✗Don't assume cold cracking appears immediately. Hydrogen-induced cold cracking (HICC) has a time delay. Cracks can appear 4 to 72 hours after welding. A weld that passes visual inspection immediately may crack overnight. This is why critical welds require a minimum holding period (often 48 hours) before final NDE.
✗Don't use the thick plate formula for thin sheet. This calculator uses the EN 1011-2 3D (thick plate) model. For thin plate where heat flows primarily in 2D (typically < 10mm), the cooling formula changes — thin plates cool faster because there's less mass to absorb heat. Using the thick plate formula on 3mm sheet will underestimate the cooling rate.

Frequently Asked Questions

Why specifically 800°C to 500°C?

At 800°C, carbon steel is fully austenitic — all the iron atoms are in a face-centered cubic (FCC) crystal structure. As the steel cools through this range, the austenite transforms into different microstructures depending on cooling speed. At 500°C, the transformation is complete. Fast cooling through this window forces the FCC lattice into a body-centered tetragonal (BCT) structure — martensite — which is extremely hard but brittle. Slow cooling allows the organized transformation into bainite or pearlite, which are tough and ductile.

What is hydrogen-induced cold cracking (HICC)?

HICC requires three simultaneous conditions: susceptible microstructure (martensite), hydrogen in the weld, and tensile stress. Hydrogen enters the weld from moisture in electrodes, flux, or atmosphere. In martensite, hydrogen atoms are small enough to diffuse through the lattice and accumulate at stress concentrations (the HAZ boundary). Over hours, the hydrogen pressure builds until the lattice fractures. This is why HICC is delayed — the diffusion takes time. Controlling t8/5 to avoid martensite formation removes one leg of the triad.

Does carbon equivalent affect t8/5?

Carbon equivalent (CE) does not directly appear in the cooling rate formula — t8/5 is purely thermal. However, CE determines HOW SENSITIVE the steel is to a given cooling rate. A low-CE steel (CE = 0.30) might tolerate t8/5 = 5 seconds without forming martensite, while a high-CE steel (CE = 0.50) requires t8/5 above 15 seconds. Use the Carbon Equivalent calculator first to assess hardenability, then use this calculator to engineer the cooling rate that avoids the critical transformation zone for that specific CE value.

Can I use this formula for stainless steel?

No — this formula assumes carbon steel thermal properties and the austenite→ferrite/pearlite/martensite transformation. Austenitic stainless steels (304, 316) do not undergo this transformation — they remain austenitic at all temperatures. Their cooling rate concern is sensitization (carbide precipitation at grain boundaries between 450-850°C), which requires FAST cooling, the opposite of carbon steel. Duplex stainless has its own ferrite-austenite balance window. Each requires different thermal analysis.