Can 4140 steel be welded?
Yes, with preheat of about 200-300 °C (400-575 °F), low-hydrogen filler, slow cooling and a post-weld temper. Never weld hardened 4140 without re-heat-treating; the heat-affected zone quenches itself to a brittle 55 HRC.
4140 welds, but it is a hardenable steel with about 0.40% carbon and a carbon equivalent near 0.8, and everything about welding it follows from that. The heat-affected zone reaches austenitizing temperature and then cools fast into the surrounding cold steel, which is an oil quench in effect: a bead laid on cold 4140 leaves a band of untempered martensite at 50-57 HRC beside the fusion line. That band is brittle, it holds the residual stress of the weld, and any hydrogen picked up from moisture, rust or a non-low-hydrogen electrode collects in it. The result is hydrogen-assisted (cold) cracking, sometimes hours after the weld cools. Preheat slows the cooling rate so the zone transforms to bainite and tempered structures instead, and it lets hydrogen diffuse out.
The practical recipe: preheat to 200-300 °C (400-575 °F) depending on section (the top of the range for plate over about 25 mm (1") and for restrained joints), hold the interpass temperature between the preheat and about 350 °C (650 °F), and use a low-hydrogen process: GTAW with ER80S-D2 or ER80S-B2, GMAW with the same wires, or SMAW with E7018 or E8018-B2 electrodes taken from a sealed container or a rod oven. Weld the joint to completion without letting it go cold. After welding, either cool slowly under insulation, or better, post-heat at 200-300 °C for 1-2 hours to bake out hydrogen, then stress-relieve or temper at 590-650 °C (1100-1200 °F), staying about 30 °C (50 °F) below the tempering temperature the base metal was given, so the pre-hard properties are not lost. Matching-strength filler is only needed if the joint must carry pre-hard 4140 loads; a softer E7018 weld with a hard base is common on repairs and fixtures.
The condition of the 4140 changes the rules. Annealed 4140 (about 200 HB) is the easiest case, and the whole weldment can be quenched and tempered afterwards to a uniform hardness. Pre-hard 4140 at 28-32 HRC is welded routinely with the recipe above and a post-weld temper that does not soften the base. Hardened 4140 above about 40 HRC should not be welded as a repair: the heat-affected zone will be tempered soft on one side and quenched hard on the other, the part loses its strength locally, and cracks follow. If a hardened part must be welded, anneal or temper it first, weld, and re-quench and temper the whole part. Keep chromium-molybdenum filler out of joints with plain carbon steel unless the design needs it, and never weld 4140 to 4140 with a plain 70 ksi wire and no preheat and expect it to survive impact.
Welding for 4140 parts: where to send them
What to specify
- Base metal condition at welding: annealed, normalized, or pre-hard 28-32 HRC, and the required condition after welding
- Preheat and interpass: minimum 200 °C (400 °F) for sections under 25 mm (1"), 250-300 °C (500-575 °F) above, interpass maximum 350 °C (650 °F), measured by contact pyrometer or temperature stick
- Filler: ER80S-D2 or ER80S-B2 (AWS A5.28) for GTAW/GMAW, E7018 or E8018-B2 (AWS A5.1 / A5.5) for SMAW, low-hydrogen storage and handling
- Post-weld treatment: hydrogen bake at 200-300 °C (400-575 °F) and stress relief or temper at 590-650 °C (1100-1200 °F), not above the base-metal tempering temperature; or full re-quench and temper for annealed weldments
- Procedure qualification: WPS and PQR per ASME IX or AWS B2.1; 4140 is not a prequalified base metal under AWS D1.1
- Inspection: magnetic particle (ASTM E709) or dye penetrant no sooner than 48 hours after welding to catch delayed cracking; UT for full-penetration joints
- Post-weld machining allowance and the final hardness or mechanical requirement of the joint
Pitfalls
- Welding cold 4140 "just a tack": the tack sits in a 55 HRC quenched zone and cracks, often after the part is in service
- Damp E7018 or a rusty, oily joint: hydrogen goes straight into the martensite and cracks the weld toe a day later
- Preheat measured by guess: an infrared gun on a dark, hot surface reads low; use a contact pyrometer or a temperature stick
- Post-weld temper above the original tempering temperature: the whole pre-hard part softens and the drawing hardness is lost
- Welding a part that is already hardened above 40 HRC: it loses strength beside the weld and cracks; anneal, weld, re-heat-treat
- Inspecting immediately after welding: delayed hydrogen cracks appear over 24-72 hours, so inspect late and again if critical
Frequently asked questions
- What preheat does 4140 need for welding?
- About 200-300 °C (400-575 °F), higher for thick or restrained sections, with interpass held below about 350 °C (650 °F). Preheat slows the cooling so the heat-affected zone does not quench to brittle martensite and gives hydrogen time to escape.
- What filler rod is used to weld 4140?
- ER80S-D2 or ER80S-B2 for TIG and MIG, E7018 or E8018-B2 for stick. All are low-hydrogen. A 70 ksi filler gives a weld softer than pre-hard 4140; use the 80 ksi or a matching chromium-molybdenum filler where the joint carries full load.
- Can hardened 4140 be welded?
- Not without ruining the hardness beside the weld and risking cracks. Anneal or temper the part first, weld with preheat and low-hydrogen filler, then quench and temper the whole part again. A weld repair on a hardened part is a scrap-avoidance measure, not an engineering joint.
- Does 4140 need heat treatment after welding?
- At minimum a slow cool and a hydrogen bake at 200-300 °C. For anything loaded, a stress relief or temper at 590-650 °C (1100-1200 °F) below the base metal temper. For annealed weldments, a full quench and temper afterwards is the cleanest result.
