Welding preheat: temperature chart by steel and thickness (and why 4140 is different)
Preheat ranges for A36, A572, A514, 4140, and 4340 by thickness, the IIW carbon equivalent bands, and the canonical answer to the 4140 preheat question: 400–600°F, low-hydrogen rod, slow cool.
Short answer: preheat when the steel's carbon/alloy content or its thickness makes fast cooling dangerous. For plain carbon steel (A36, 1018, mild plate), most welders start preheating at 3/4"–1" thickness — thinner than that, no preheat is typically needed beyond warming cold steel to 50°F minimum. Mild steel at 1" and up: 150–225°F. The steel everyone actually asks about is 4140: preheat 400–600°F, interpass roughly 400–700°F, and slow-cool after welding (wrap it or bury it in vermiculite). 4340 runs hotter still: 500–700°F. These are typical starting points, not a procedure — the governing document is always your WPS, AWS D1.1 Table 3.3, or the steel maker's datasheet.
Why preheat works
Preheat does one thing: it slows the cooling rate of the weld and the heat-affected zone (HAZ). Slower cooling gives dissolved hydrogen time to diffuse out of the joint instead of getting trapped, and it keeps the HAZ from quenching itself into hard, brittle martensite against the cold mass of the surrounding steel. Those two effects together are what prevent hydrogen-induced cold cracking — the delayed cracking that shows up hours or days after the weld looked fine. The risk rises with two variables at once: chemistry (more carbon and alloy = more hardenable) and thickness (a thick section is a heat sink that pulls heat out of the weld faster).
Quick chart: typical preheat by steel and thickness
Typical starting points — the governing document is your WPS, AWS D1.1 Table 3.3, or the steel maker's datasheet. Every number below is a range for a reason.
Steel Thickness Typical preheat Notes
────────────────── ───────────── ────────────────── ─────────────────────────
A36 / mild steel < 3/4" Usually none 50°F min if steel is
(1018, A1011...) cold; never weld frosty
or wet steel
A36 / mild steel 3/4" – 1.5" 150–225°F Heat-sink effect starts
to matter here
A36 / mild steel > 1.5" 225°F+ Per AWS D1.1 Table 3.3
category
A572 Gr 50 all Per code D1.1 Table 3.3 by
thickness category
A514 / T-1 (Q&T) all Per code ALSO has interpass MAX
(~400–500°F) — see below
4140 (annealed) all 400–600°F Interpass ~400–700°F,
slow-cool after
4140HT (prehard) all 400–600°F Highest cracking risk;
low-hydrogen filler,
consider post-weld temper
4340 all 500–700°F More hardenable than
4140; treat with respect
Cast steel / varies Use CE formula Chemistry unknown =
unknown steel + err hotter assume it's hardenableTwo thumb rules from the trade that hold up: plain carbon steel over 3/4"–1" thick — start preheating. Any chromoly, any Q&T steel, any "mystery steel" that sparks funny on the grinder — preheat, and figure out the chemistry before you trust the joint.
The carbon equivalent formula
When someone on Eng-Tips asks how to decide whether a steel needs preheat from its mill cert, this is the screening tool: the IIW carbon equivalent. It collapses the chemistry into one number that tracks hardenability.
CE = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15
Interpretation (screening values, NOT a WPS):
CE < 0.40 Generally weldable without preheat on thin
sections; watch thickness anyway
CE 0.40–0.60 Preheat 200–400°F depending on thickness
and restraint; low-hydrogen consumables
CE > 0.60 Preheat 400–700°F + controlled interpass
+ often post-heat / slow cool required
Example — A36 (typical heat):
C 0.20, Mn 0.80, trace everything else
CE = 0.20 + 0.80/6 = 0.33 → no preheat on thin plate
Example — 4140 (nominal):
C 0.40, Mn 0.88, Cr 0.95, Mo 0.20
CE = 0.40 + 0.88/6 + (0.95+0.20)/5 = 0.78 → serious preheatThose bands are screening values for planning, not qualification. Thickness and joint restraint shift the answer: a CE of 0.45 in 1/4" sheet is a different animal than the same chemistry in a 3" restrained corner joint. When the CE lands in the middle band, thickness decides which end of the 200–400°F range you start at.
Preheat for 4140 (the question everyone asks)
"Preheat temp for 4140?" gets asked — nearly word for word — on Practical Machinist, the Miller forum, the AWS forum, Home Shop Machinist, the MIG Welding Forum, and Garage Journal. Here is the canonical answer:
- Preheat: 400–600°F, soaked through the section — not a surface flash. Thicker or more restrained = higher end.
- Interpass: hold roughly 400–700°F while welding. Don't let it cool to ambient between passes.
- Filler: low-hydrogen only. E7018 for general repair where you accept an undermatched (but tougher) weld; ER80S-D2 or an ER100-class wire when you need strength closer to the base metal.
- Rod must be dry. 7018 that has sat in an open box absorbs moisture — wet rod pumps hydrogen straight into the joint and defeats the entire point of preheating. Rod oven or fresh hermetic can.
- Slow-cool after the last pass: wrap in welding blanket or bury in vermiculite or dry sand. Air-cooling a 4140 weldment is how you get the midnight ping.
- 4140HT (pre-hardened): everything above, plus consider a post-weld temper ~50°F below the original tempering temperature. The weld and HAZ of prehard are untempered martensite until you deal with them.
Why 4140 is different from mild steel: at ~0.40% carbon plus chromium and molybdenum, its carbon equivalent lands around 0.75+ — deep in the "hardens itself in air" territory. The HAZ of an un-preheated 4140 weld quenches against the cold parent metal and forms fresh, brittle, hydrogen-charged martensite. Preheat plus slow cooling is not optional finesse; it is the whole crack-prevention strategy.
How to actually measure it
A preheat number you didn't measure is a guess. Two tools do the job:
- Temperature-indicating crayons (Tempilstik) — buy one at your target temp and one ~50°F above. Stroke the steel: first one melts, second doesn't = you're in the window. Cheap, no calibration, code inspectors trust them.
- IR gun — fast, but emissivity on shiny or mill-scale steel lies. Check it against a temp stick the first time on a new material.
- Measure ~3" from the joint, and on the opposite side of the plate from the torch where access allows — that proves heat soaked through the thickness instead of skating across the surface.
- Measure after a soak, not the instant you pull the torch or rosebud away. Surface flash readings on a cold-cored plate can read 400°F while the mid-thickness is still at 150°F.
Interpass temperature: the other half
A recurring AWS-forum question: when do you take interpass temperature? Immediately before striking the arc for the next pass, at the same ~3"-from-the-joint location. The rules:
- Interpass minimum = the preheat temperature. If the joint cools below preheat between passes, you've lost the protection — reheat before continuing.
- Interpass maximums exist too. Quenched-and-tempered steels (A514/T-1, 4140HT in service condition) typically cap around 400–500°F, because too-slow cooling over-tempers the HAZ and wrecks the Q&T properties the steel was bought for. More heat is not automatically safer.
- Heat input is the third leg. Preheat, interpass, and heat input together set the cooling rate — a WPS controls all three, and so should you.
Heat input, per pass:
HI (kJ/in) = (Volts × Amps × 60) / (travel speed, ipm) / 1000 × η
η = process efficiency:
~0.8 SMAW / MIG / FCAW
~0.6 TIG (GTAW)
~1.0 SAW
Example — SMAW, 22 V, 110 A, 5 ipm:
gross arc energy = 22 × 110 × 60 / 5 / 1000 = 29.0 kJ/in
actual HI = 29.0 × 0.8 = 23.2 kJ/inDo it on your phone
WelderCalc has a carbon equivalent calculator (punch in the mill-cert chemistry, get CE and the preheat band), preheat guidance by material, and a heat input calculator that applies process efficiency — most free calculators report gross arc energy, which overstates TIG heat input by ~40%; WelderCalc applies η so the number is the one your cooling rate actually sees. 100% offline, free on iOS and Android.
Related
- Heat input limit for SA-516 Gr 70 root pass
- MIG amperage cheat sheet (carbon steel)
- All trade calculators
- WelderCalc — welder's calculator on iOS + Android
Note: This article gives typical ranges for planning and sanity-checking. On any code job, structural weld, or engineered repair, the WPS and the engineer of record govern — always defer to them over any chart, including this one. Reviewed 2026-07.