4130 vs 4340 Alloy Steel

Updated October 14, 2026

4130 vs 4340 side by side

4340 is a nickel-chromium-molybdenum steel with more carbon and nearly 2 percent nickel; 4130 is a leaner chromium-molybdenum steel. The extra alloy buys 4340 hardenability and strength in thick sections, and costs it weldability. Every number below names its source and whether it is typical or a minimum.

4130 composition, AMS 6345 and 6346 figures and the weldability statement come from the Aircraft Materials 4130 data page; 4340 composition from the Aircraft Materials E-4340 page; hardenability, AMS 6414 minimums, heat treat ranges and welding statements from the Service Steel Aerospace AMS 6414 data page. Distributor figures labeled typical are a guide; acceptance values come from the spec and size range on your PO.

Hardenability and section size

Section size is usually the deciding factor. Hardenability is how deep a steel transforms to martensite when quenched. A shallow-hardening steel such as 4130 reaches full strength at the surface of a thick part but not at its center, so the core is softer than the drawing assumes. 4340's nickel and higher carbon push that depth much further.

• 4130 evidence. The Aircraft Materials page lists hardness values for AMS 6345 plate normalized and heat treated that fall steadily with thickness: 42 at 0.249 to 0.375 inch, 38, 32, 29, 27 and 26 at 0.875 to 1.5 inch. The page does not state the hardness scale, but the trend is the point: thicker 4130 hardens less.
• 4340 evidence. Service Steel Aerospace states AMS 6414 4340 has a through-hardening capability up to 3.5 inches (89 mm) nominal thickness at heat treatment, and lists Jominy minimums of 53 HRC at 12/16 inch and 32 HRC at 20/16 inch from the quenched end.
• Rule of practice. Thin wall tube, sheet brackets and welded frames do not need 4340's hardenability, so 4130 is the efficient choice. Shafts, pins, axles and fittings over about an inch that must be uniformly strong through the section point to 4340.
• Rough machine before heat treat. Hardenability is judged at the section size at the time of heat treatment, so roughing a large bar close to final shape first gives a more uniform result.

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Heat treat ranges and strength at typical tempers

Both steels are austenitized, quenched and tempered, and the tempering temperature sets the strength. Published practice for 4340 to AMS 6414, from Service Steel Aerospace:

The hardness values are labeled approximate and were taken after austenitizing at 1550 F, oil quenching and double tempering for 2 hours each. Aircraft Materials gives a similar route for the 260 ksi minimum level: normalize 1650 F, austenitize 1500 F, oil quench, double temper at 475 F.

For 4130 our sources give the normalized sheet figures and the 125 ksi typical quenched and tempered bar figure in the first table; we did not find a public tempering table we could cite, so specify 4130 heat treat by final strength or hardness and let the heat treat spec set the cycle.

Toughness trades against strength. Higher strength lowers toughness and stress corrosion resistance. NASA MSFC-STD-3029 states that carbon and low alloy steels below 180 ksi ultimate are generally resistant to stress corrosion cracking; the rating drops above that level, as tabulated on our stainless and specialty steels page. Service Steel Aerospace credits the remelted AMS 6414 product with excellent transverse ductility and toughness at high strength, which is why VAR 4340 is the default for highly stressed parts.

Specify the result, not the cycle: heat treat per AMS 2759/1 with a tensile range or hardness range. The heat treatment page explains the slash sheets and pyrometry behind that callout.

Weldability: 4130 welds, 4340 needs a plan

Choose 4130 for welded structure. Aircraft Materials describes it as widely used because of its weldability, ease of fabrication and mild hardenability, which is why it is the steel of welded tube frames and ground support equipment. Lower carbon is what makes it forgiving: the heat affected zone forms less hard, crack-prone martensite.

4340 can be welded, but treat it as a special case. Service Steel Aerospace states it can be welded by gas or arc fusion methods, that it is capable of air hardening, and that it should be annealed or normalized and tempered after welding. In practice that means:

• Preheat, interpass control and post-weld heat treatment written into a qualified procedure, not left to the welder.
• Welding before final heat treatment where possible, then heat treating the weldment to the required strength.
• No welding of 4340 in the hardened condition unless engineering has approved a specific repair procedure.
• Magnetic particle inspection after welding and after heat treatment.

Heat treated 4130 weldments need the same thinking: a frame welded from normalized tube and then quenched and tempered will distort, so plan fixtures and machining allowances. Welding procedure and inspection detail is on the sheet metal and aerospace welding page.

Hydrogen embrittlement and the plating bake

At high strength, 4340 is one of the steels most likely to fail from hydrogen, so plating and cleaning must be planned with the heat treat. An ASM International failure case (ASM Failure Analysis Case Histories: Air and Spacecraft, 2019) describes brittle intergranular fracture of an AISI 4340 landing gear beam from a hydrogen-induced delayed failure: corrosion at damaged protective coating released hydrogen that diffused into the steel under sustained tensile stress, with a cluster of non-metallic inclusions as a second factor. The same case lists cadmium plating, acid pickling and steelmaking as sources of hydrogen on part surfaces.

• Tell the plater the strength. AMS 2759/9 relief baking depends on final strength or hardness, and the purchaser must supply it. The bake rules and timing are on the plating and cadmium alternatives page.
• Avoid acid pickling of hardened parts; use abrasive cleaning where the spec allows.
• Protect the coating in service. The ASM case shows that damage to the finish, not just the plating process, can start the failure.
• Clean steel matters. Inclusions contributed to the landing gear failure, which is one reason remelted AMS 6414 is specified for highly stressed parts.

4130 at moderate strength is far less sensitive, but the bake rules still apply once a part's strength crosses the threshold in the plating or bake spec. For cadmium versus zinc-nickel trade-offs on high-strength steel, see cadmium vs zinc-nickel plating.

Decarburization and surface condition

Decarburization is loss of carbon at the surface during hot working or heat treatment in an oxidizing atmosphere. The surface layer is softer and weaker than the core, and on a high-strength part it lowers fatigue strength exactly where cracks start. Both steels are affected; 4340 parts at high strength have the most to lose.

• Bar stock arrives with a decarburized skin from the mill. Leave machining stock to remove it on every surface that matters, or state the allowable depth on the PO.
• Heat treat in controlled atmosphere or vacuum when parts are hardened after finish machining, and require the decarburization check the heat treat spec calls for. The heat treatment page covers how that check is made.
• Grinding after hardening can remove a shallow decarb layer but can also burn the surface; call out the inspection for grinding damage where the drawing requires it.
• Inspect. Magnetic particle inspection is standard for both steels; see nondestructive testing.

Where each is used: GSE, landing gear and fittings

4130 dominates welded and formed structure; 4340 dominates highly stressed machined parts. Typical uses:

The last row comes from the same ASM case history: eyebolts used to lift a light aircraft broke by brittle intergranular fracture, likely hydrogen-induced, and both were 10 Rockwell C points harder than the 25 HRC specified, with bending applied to the threads. For lifting hardware, verify hardness on the heat treat cert and test results, not just the material. See lifting points and hoist rings and our ground support equipment service.

Specifications for each form

SAE titles and revisions below were read on SAE Mobilus in October 2026; product forms marked distributor come from Aircraft Materials listings. Confirm the current revision on ASSIST or SAE before release.

Note that AMS 6484 covers 4340 normalized and tempered, a supply condition, not a final quenched and tempered strength. Final strength still comes from your heat treat callout.

Cost and availability drivers

Expect 4340 to cost more than 4130, and VAR 4340 to cost more again. We do not quote prices; these are the drivers:

• Alloy content. 4340's 1.65 to 2.00 percent nickel is the main cost difference in the chemistry.
• Melt practice. AMS 6414 requires vacuum consumable electrode remelting, an extra process step. Air melt AMS 6415 or AMS 6484 product costs less where the design allows it.
• Form. 4130 is the deep market for aircraft sheet and tube; 4340 is mainly bar and forgings. Asking for the less common form of either narrows the pool.
• Heat treat and finishing. High-strength 4340 adds controlled atmosphere heat treat, straightening limits, magnetic particle inspection, bake after plating and often shot peening, each with its own cert and lead time.
• Specialty metals. Both steels contain more than 0.25 percent chromium, so DFARS 252.225-7009 applies on contracts that carry it. See DFARS specialty metals.
• Certified surplus. Remnant AMS bar and plate with traceable certs can shorten lead time when the cert shows the right spec, revision and melt practice.

Callouts for the drawing and PO

Name the alloy, spec, form and final strength, and state who heat treats. Examples:

• GSE frame: MATL: STEEL 4130 TUBE PER AMS 6360. WELD PER AWS D17.1 (or the program's weld spec). Add normalize or stress relieve after welding if engineering requires it.
• Formed bracket: MATL: STEEL 4130 SHEET PER AMS 6345, NORMALIZED.
• Clevis pin: MATL: STEEL 4340 BAR PER AMS 6414. HEAT TREAT PER AMS 2759/1 TO 160 TO 180 KSI. Keep below 180 ksi for salt air with sustained stress.
• Highly stressed fitting: MATL: STEEL 4340 VAR PER AMS 6414. HEAT TREAT TO 260 TO 280 KSI. MAGNETIC PARTICLE INSPECT. Add plating spec and bake requirement with the strength stated.

On the PO require the spec and revision, the mill cert with heat number and melt practice, the heat treat cert showing the cycle and tensile or hardness results, the bake record for plated parts, the inspection report, and DFARS 252.225-7009 compliance where it applies. Our certs and traceability page explains what to check. For stainless comparisons see 17-4 vs 15-5 PH and 304 vs 316.

Get 4130 or 4340 material quoted

Send the spec, form, size and final strength through a certified material request. We source AMS 4130 sheet, plate and tube and AMS 4340 bar and forgings from qualified suppliers with traceable mill certs, confirm melt practice and condition on the certs before shipment, and can add machining, heat treat, plating with the bake and inspection on the same PO. A person replies within one business day. See also bar and rod, aerospace tubing and the materials hub.

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Related

• Stainless and Specialty Steels for Aerospace Parts
• Aerospace Heat Treatment: Pyrometry, Process Specs and Callouts
• Aerospace Plating and Cadmium Alternatives: Specs and Callouts
• Sheet Metal Forming and Aerospace Welding

Sources

• Aircraft Materials, Alloy 4130 data: composition, AMS 6345, 6346, 6350, 6370, 6371, weldability
• Aircraft Materials, AISI 4340 / E-4340 data: composition, AMS 6359, 6414, 6415, 6484, heat treatment
• Service Steel Aerospace, 4340 AMS 6414 VAR data: hardenability, tempering, minimum properties, welding
• SAE Mobilus, AMS6414N 4340 vacuum consumable electrode remelted bars, forgings, tubing
• SAE Mobilus, AMS6415 nickel chromium molybdenum steel
• SAE Mobilus, AMS6484 4340 bars, forgings and tubing, normalized and tempered
• SAE Mobilus, AMS6345 4130 sheet, strip and plate
• SAE Mobilus, AMS6350 chromium molybdenum plate, sheet and strip
• SAE Mobilus, AMS6360 chromium molybdenum tubing
• ASM International, Hydrogen Embrittlement of Aircraft Components, in ASM Failure Analysis Case Histories: Air and Spacecraft (2019)
• NASA MSFC-STD-3029 Revision A, section 4.4.2 ferrous alloys
• DFARS 252.225-7009 specialty metals