Can 4340 steel be welded?
Only with a qualified procedure: 260-370 °C (500-700 °F) preheat, low-hydrogen filler and immediate post-weld heat treatment. Weld it annealed, then harden.
4340 is a nickel-chromium-molybdenum steel with about 0.40% carbon and one of the highest hardenabilities of the common alloy steels. Its carbon equivalent is around 0.8-0.9, and it hardens substantially in air: even normalized 4340 comes out near 360 HB. For welding, that means the heat-affected zone becomes hard martensite, around 50-57 HRC, whatever the preheat. With 4140, preheat slows the cooling enough to soften the zone; with 4340 it mainly reduces stress and gives hydrogen time to escape, and the zone still needs a post-weld heat treatment to temper it. At the strengths 4340 is bought for, tolerance to hydrogen cracking is very low, so the whole procedure is built around keeping hydrogen out and never letting the joint sit cold and untempered.
The workable procedure: weld in the annealed or normalized condition; clean the joint to bright metal; preheat to 260-370 °C (500-700 °F), the upper half for sections over about 25 mm (1") and for restrained joints; hold the interpass in the same band; and use low-hydrogen GTAW or GMAW, or SMAW with H4-designated electrodes from a rod oven. Filler follows what happens next. If the weldment will be quenched and tempered to full strength, use a heat-treatable 4340-type filler so the weld hardens with the base metal. If the joint will only be stress relieved, a high-strength low-hydrogen filler such as ER100S-1 to ER120S-1 (AWS A5.28) or E11018-M / E12018-M (AWS A5.5) is used, accepting that the weld will be the weaker zone. After welding the part must not cool to room temperature untempered: either transfer it hot into a furnace for an anneal or normalize followed by a full quench and temper, or let it cool only to about 95-150 °C (200-300 °F) so the zone transforms, then stress relieve immediately at 595-650 °C (1100-1200 °F).
Hardened 4340 is a different question. Landing gear, heavy shafts and high-strength fasteners run at 40-54 HRC, and welding at those hardnesses leaves an overtempered soft band beside a brittle untempered one; a post-weld temper that fixes the hard band softens the rest of the part. Weld repair of hardened 4340 is done only under approved procedures in aerospace, or by annealing, welding and re-heat-treating the whole part. Pre-hard 4340 at 28-32 HRC can be welded with the procedure above and a post-weld temper about 30 °C (50 °F) below its original tempering temperature. If a design needs welds, the better answer is usually a different steel: 4130 for welded, heat-treated structures, or a weldable quenched-and-tempered plate, with 4340 kept for machined parts joined by bolts, pins or shrink fits.
Welding for 4340 parts: where to send them
What to specify
- Base-metal condition at welding (annealed or normalized preferred) and the required condition after welding, for example "quench and temper weldment to 36-40 HRC"
- Preheat and interpass: 260-370 °C (500-700 °F), upper range above 25 mm (1") or on restrained joints, verified by contact pyrometer or temperature stick
- Filler: heat-treatable 4340-type wire when the weldment is re-hardened; otherwise ER100S-1 to ER120S-1 (AWS A5.28) or E11018-M / E12018-M H4 (AWS A5.5), with low-hydrogen handling
- Post-weld sequence written out: no cooling below about 95 °C (200 °F) before heat treatment; stress relief at 595-650 °C (1100-1200 °F), or anneal, then quench and temper
- Procedure and welder qualification: WPS and PQR per ASME Section IX, or AWS D17.1 for aerospace; 4340 is not a prequalified base metal under AWS D1.1
- Inspection after final heat treatment: magnetic particle (ASTM E1444) no sooner than 48 hours after welding, UT or radiography on full-penetration joints, and a hardness traverse across the weld
Pitfalls
- Letting the weld cool to room temperature before post-weld heat: untempered martensite plus hydrogen cracks the toe within hours
- Relying on preheat alone: 4340 still forms martensite with preheat, and without a post-weld temper the joint is brittle
- Welding hardened 4340 as a repair: a soft band next to a brittle band, and a part that no longer meets its drawing
- Using ER70S-6 or E7018 and expecting 4340 strength: the weld is about 480 MPa (70 ksi) against roughly 1000-1900 MPa (145-275 ksi) for heat-treated 4340, and it does not harden in a later quench
- Arc strikes and unpreheated tack welds: each one is a small quenched spot and a crack starter; most aerospace specs reject them outright
- Post-weld tempering in the 260-370 °C (500-700 °F) range: tempered-martensite embrittlement cuts toughness while hardness looks normal
Frequently asked questions
- Is 4340 weldable?
- Yes, but it is one of the harder alloy steels to weld. It needs a 260-370 °C (500-700 °F) preheat, strict low-hydrogen practice and a post-weld heat treatment every time, and the best results come from welding it annealed and heat treating the whole weldment afterwards.
- Can 4340 be welded to mild steel?
- Yes. Set the preheat and post-weld heat treatment by the 4340 side, and use a low-hydrogen filler that matches the mild steel, such as E7018 or ER70S-6. The joint strength is then set by the mild steel and the filler, and the 4340 heat-affected zone still needs tempering.
- Should I use 4130 or 4340 for a welded part?
- Usually 4130. Its lower carbon (0.28-0.33%) and lower hardenability make thin sections weldable with little or no preheat, and a welded 4130 assembly can still be heat treated afterwards. Keep 4340 for heavy, highly stressed machined parts that are not welded.
