Welding 4140 Steel: Preheat, Filler and Postweld Practice

Why 4140 needs care

4140 (42CrMo4, SCM440) is a medium-carbon chromium-molybdenum steel with a carbon equivalent typically around 0.75–0.85 — high enough that the heat-affected zone (HAZ) readily forms hard, brittle martensite as the weld cools. Nearly all 4140 welding failures trace back to one of three causes: too little preheat, hydrogen in the deposit, or no postweld treatment.

Preheat: sized by CEV and thickness

Preheat slows the cooling rate so the HAZ stays below the hardness range where hydrogen cracks form. Working ranges used in practice:

Combined thickness	Typical preheat
Under ~12 mm	150–200 °C
12–25 mm	200–250 °C
25–50 mm	250–300 °C
Over 50 mm	300–350 °C

Thicker, more highly restrained joints sit at the top of each range. Preheat the full joint area — roughly 75 mm either side of the seam — and verify with a temperature crayon or pyrometer, not by colour. Maintain the temperature through the whole joint; if welding is interrupted, let the joint cool slowly under insulation and re-establish preheat before restarting.

Consumables and technique

• Use low-hydrogen processes and electrodes (basic SMAW rods properly baked, or solid-wire GMAW with dry, clean wire). Cellulosic electrodes have no place here.
• Filler strength is a design choice: matching fillers (e.g. 110–120 ksi class, or ER80S-D2-type for moderate strength) suit structural repairs, while a softer, ductile deposit such as a 309-type austenitic or nickel-based filler is sometimes chosen for build-up and crack-tolerant repairs — at the cost of joint strength.
• Keep heat input moderate. Very low heat input increases cracking risk; very high heat input widens the softened and hardened zones and costs toughness.
• Avoid craters and abrupt starts/stops — crater cracks are a classic 4140 defect.

Temper bead and PWHT

For repairs where a full furnace cycle is impractical, a temper bead sequence — each pass tempering the pass beneath, finished with a final tempering layer that is later ground off — can substitute partially, but it must be a qualified procedure, not improvisation.

As a rule, welded 4140 should receive postweld heat treatment: a stress-relief temper roughly between 550 °C and the original tempering temperature of the base material (never above it, or strength is lost). Many shops temper the welded part shortly after welding without letting it cool fully to room temperature, which suppresses hydrogen cracking during the wait.

Practical checklist

1. Clean to bright metal; remove oil, scale and cracked material completely — grind out cracks and check with dye penetrant before welding.
2. Establish and hold preheat; monitor interpass temperature.
3. Low-hydrogen consumables, modest heat input, no weaving extremes.
4. Slow cool under insulation, then PWHT or temper.
5. Wait at least 48 hours before final crack inspection on critical work — hydrogen cracks can be delayed.