Welding Heat Input Calculator — Arc Energy, Cooling Rate, Carbon Equivalent & HAZ Hardness
Calculate welding heat input (kJ/mm), cooling rate (t8/5), carbon equivalent (CE), and predicted HAZ hardness for arc welding processes.
Quick Answer
For SMAW at 24V, 180A, travel speed 200mm/min, thermal efficiency 0.8: Heat Input = 1.04 kJ/mm. For 12mm plate, 2D cooling: t8/5 ≈ 6.8 seconds. Carbon equivalent CE = 0.42 for AISI 4140 — preheat to 200°C recommended to avoid martensite in HAZ.
Why Heat Input Controls Weld Quality
Heat input determines cooling rate, which controls microstructure. Cool too fast and you get hard, brittle martensite. Cool too slow and you get coarse grain, low toughness. The sweet spot depends on the steel.
1. Heat Input Formula
HI = (V × I × 60 × η) / (TS × 1000) kJ/mm, where η = process efficiency (SMAW: 0.75-0.85, GMAW: 0.80-0.90, GTAW: 0.60-0.80, SAW: 0.90-1.0). Higher heat input = slower cooling = softer HAZ. Lower = faster cooling = harder HAZ.
2. Cooling Time t8/5
The time to cool from 800°C to 500°C — the critical temperature range for austenite transformation. t8/5 <3s: martensite forms (avoid for carbon steels >0.25%C). t8/5 >20s: coarse ferrite/pearlite, reduced toughness. Target: 5-15s for C-Mn steels, 10-20s for low-alloy.
3. Carbon Equivalent (CE)
CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15 (IIW formula). CE predicts hardenability: CE <0.40: low risk, no preheat needed for t<30mm. CE 0.40-0.60: moderate risk, preheat 100-200°C. CE >0.60: high risk, preheat 200-350°C, consider PWHT.
Common Mistakes
- Using wrong thermal efficiency (η) — Each process has different efficiency: SMAW 0.75-0.85 (stick), GMAW 0.80-0.90 (MIG/MAG), FCAW 0.80-0.90, GTAW 0.60-0.80 (TIG), SAW 0.90-1.0. This is NOT a “pick the middle” parameter — using SMAW efficiency for GMAW underestimates actual heat input by 15%.
- Not checking cooling rate against CCT diagram — Heat input gives t8/5, but you need the steel’s CCT (Continuous Cooling Transformation) diagram to know what microstructure forms. Martensite may form even at “acceptable” t8/5 for some steels.
- Using only CE for preheat decision — CE is a guide, not a guarantee. Restraint level (joint rigidity) and hydrogen input matter equally. A CE=0.38 steel in a highly restrained joint may crack, while CE=0.45 in an unrestrained joint won’t.
- Assuming all heat goes into the plate — The arc radiates heat to the environment. η accounts for this. For out-of-position welding, η drops slightly because the welder moves slower but the arc still radiates at the same rate.
- Forgetting that multiple passes raise interpass temperature — The 2nd pass starts on hot metal. Heat input per pass stays the same, but the effective cooling rate is much slower (the plate is already hot). Control interpass temperature — typical max 250°C for most steels.
Frequently Asked Questions
What t8/5 value should I target?
C-Mn structural steel (S355, A572): 5-15s. Low-alloy Q&T steel (S690, A514): 10-25s. Microalloyed (S460M): 8-20s. Stainless (304/316): not critical for t8/5 but control interpass <150°C. For each steel grade, the supplier provides recommended t8/5 range in the welding procedure datasheet.
How does heat input affect HAZ hardness?
Higher heat input → slower cooling → softer HAZ → lower hardness. But higher heat input → coarser grain → lower toughness. It’s a tradeoff. For 4140: HI=0.5 kJ/mm → HAZ hardness ~450 HV, HI=1.5 kJ/mm → ~350 HV, HI=3.0 kJ/mm → ~280 HV. Target hardness below 350 HV for most structural applications.
Can I weld without preheat by increasing heat input?
Partly — doubling heat input has similar effect to 50-100°C preheat. But: high heat input causes distortion, coarse grain, and reduced toughness. Preheat is preferred because it’s a uniform temperature soak without metallurgical penalty. Use heat input control as a supplement, not replacement.
What is hydrogen cracking and how does heat input prevent it?
Hydrogen from moisture in flux/coating diffuses into HAZ and accumulates at inclusions — when stress exceeds local strength, crack. Prevention: low-hydrogen processes (GMAW > SMAW), preheat (drives H out), maintain heat input (slow cooling lets H diffuse out). Our Weld Strength Calculator covers joint design.
How do I measure actual heat input on the shop floor?
The simplest method: record V and I from the welding machine display (or tong meter for I), travel speed = weld length / time (stopwatch + tape measure), then apply formula. For WPS qualification: calibrated instruments, strip chart recorder or data acquisition system. Modern machines log all parameters automatically.
What happens if heat input is too low?
Rapid cooling → martensite in HAZ → high hardness → hydrogen cracking risk. For carbon steels >0.25%C: hardness >380 HV almost guarantees cracking without preheat. Minimum heat input specs exist for a reason — going below them is dangerous, not conservative.
How do I calculate heat input for pulsed GMAW?
For pulsed MIG: use mean values, not peak. HI = V_avg × I_avg × 60 × η / (TS × 1000). The arc energy per pulse is not the same as average arc energy — the background current period adds little heat. Modern synergic machines display average V and I directly.
Does heat input affect corrosion resistance of stainless welds?
Yes — critical for stainless. High heat input + slow cooling = chromium carbide precipitation at grain boundaries (sensitization, 450-850°C range) → intergranular corrosion. For austenitic stainless: control heat input <1.5 kJ/mm, interpass <150°C, or use low-carbon grades (304L, 316L) or stabilized grades (321, 347).