Stainless and Specialty Steels for Aerospace Parts

Steel families and what they are good for

Aerospace steel callouts fall into five groups. Strength comes from different mechanisms in each, which is why heat treatment, finishing and stress corrosion behavior differ so much between them.

Almost all of them are DFARS specialty metals. DFARS 252.225-7009 covers steel with more than 0.25 percent of chromium, nickel, molybdenum, cobalt, niobium, titanium, tungsten or vanadium, so every grade in the table needs a qualifying melt source on defense contracts that carry the clause.

Stress corrosion limits by grade and condition

NASA MSFC-STD-3029 is the clearest single guide to which steel conditions are safe in salt environments. Its ratings are tied to strength level or aging condition, not just alloy name.

The pattern is that higher strength means lower stress corrosion resistance. Table II materials are also excluded from high installation stress applications such as springs and fasteners on NASA hardware. If a part is in salt air and has sustained tensile stress, 15-5PH at H1025 or H1150 or a 4340 tempered below 180 ksi is a safer default than 17-4PH at H900 or a maraging steel.

17-4PH and 15-5PH: choosing the H condition

Both alloys are machined in Condition A (solution treated) and then aged at a temperature named in the condition: H900 is aged at 900 F, H1150 at 1150 F. Lower aging temperatures give higher strength and lower toughness and SCC resistance.

The minimums above are from a producer's wire data sheet; bar and plate specifications set their own values by size, and published sources differ slightly on H1150 yield, so check the controlling spec. 15-5PH (AMS 5659 for bar and forgings) is the remelted cousin of 17-4PH, made by vacuum arc or electroslag remelting. Producers credit it with an essentially ferrite-free structure and better transverse toughness and ductility than 17-4PH. Published H1025 values for 15-5PH show the same 155 ksi ultimate, but sources disagree on yield, so again rely on the spec.

Dimensional change on aging. PH stainless shrinks when it is aged. For 17-4PH, published data gives a contraction of 0.0004 to 0.0006 in/in from Condition A to H900, and roughly 0.0008 to 0.0012 in/in to H1150, with sources disagreeing on the exact H1150 figure. On a 4 inch dimension that is up to about 0.002 inch at H900. Shops either finish machine after aging or size the Condition A part to land on tolerance after shrinkage. Tell the supplier which approach the drawing assumes.

A286 for heat and nonmagnetic service

A286 is an age-hardenable iron-base superalloy with about 24 to 27 percent nickel and 13.5 to 16 percent chromium. Producer data describes useful strength from -320 F to 1000 F for long-time service, oxidation resistance to 1500 F continuous, and aqueous corrosion resistance comparable to 316 stainless. It is a standard material for high-temperature fasteners, springs and nonmagnetic cryogenic hardware.

The lower solution temperature in AMS 5737 trades creep-rupture strength for higher room temperature strength and ductility. Choose by service temperature, not by habit. Density is 0.303 lb/in3 per the same producer data.

4130, 4340 and maraging steels

4130 is the chrome-moly steel of welded tube structures and GSE frames, usually bought normalized to AMS 6345 for sheet, strip and plate or annealed to AMS 6350. Distributor data lists typical normalized plate around 90 ksi ultimate and 70 ksi yield. It welds well in thin sections, and heat-treated weldments need a post-weld heat treatment plan.

4340 is the high-hardenability quench and temper steel for shafts, pins and high-strength fittings. Aerospace grades such as AMS 6414 are vacuum arc remelted, which improves transverse ductility and toughness. 4340 heat treated to 260 to 280 ksi is the baseline specimen material in ASTM F519 hydrogen embrittlement testing, which tells you how sensitive it is at that level. Keep 4340 parts below 180 ksi ultimate where salt air and sustained stress are present, per the SCC table above.

Maraging steels (AMS 6512 for grade 250, AMS 6514 for grade 300) are machined in the soft solution annealed condition and aged at about 900 F for several hours, with very little distortion. Supplier data for grade 300 lists about 290 ksi ultimate and 280 ksi yield after aging. The tradeoff is the Table III stress corrosion rating, so maraging parts need protective finishes, controlled stress and a documented rationale on NASA programs.

Hydrogen embrittlement and plating

High-strength steels absorb hydrogen during acid cleaning, pickling and electroplating, and the hydrogen can cause delayed cracking under sustained load. The controls engineers specify:

• A stress relief bake before plating for parts with grinding or heavy machining stresses, per ASTM B849 or the plating spec.
• A hydrogen embrittlement relief bake after plating, started within the time limit the spec sets, per ASTM B850 or the plating spec.
• Low-embrittlement processes such as LHE zinc-nickel or cadmium where required, with ASTM F519 test coupons to qualify the process.
• Avoid acid pickling of high-strength parts; use abrasive cleaning where the spec allows.

Put the strength level or hardness range on the drawing so the plater knows which bake applies. A bake requirement that depends on hardness is useless if the plater does not know the hardness.

Passivation: AMS 2700 and ASTM A967

Passivation removes free iron and other surface contamination from stainless steel so the chromium-rich passive film can form. AMS 2700 is the aerospace spec and superseded the canceled QQ-P-35; ASTM A967 is the commercial equivalent. Both cover nitric acid and citric acid methods.

• Numbering differs. AMS 2700 Method 1 is nitric acid with Types 1 through 8; Method 2 is citric. ASTM A967 numbers nitric treatments 1 through 4 and citric 1 through 5. A drawing calling for nitric Type 6 or 7 is referencing AMS 2700, not A967.
• Verification tests include water immersion, high humidity, salt spray, copper sulfate and potassium ferricyanide-nitric acid tests. Specify which test applies, or the processor will choose.
• Grade matters. Martensitic and some PH grades can be etched by aggressive baths. Passivate PH parts after final aging, and flow the alloy and condition to the processor.
• Do not passivate over sensitized welds and expect corrosion resistance. Austenitic stainless held between about 900 and 1500 F forms chromium carbides at grain boundaries; L grades limit that in welds.

More process detail is on the surface finishing page.

Get steel parts quoted

Steel jobs depend on getting the condition, heat treat, bake and passivation chain right across several suppliers. Send the drawing through a CNC quote request and we will flow down the H condition, bake and passivation requirements, and check the mill, heat treat and finishing certs before shipment. See also the materials hub, nickel superalloys and certs and traceability.

Questions

Related

• Aerospace Materials: Selection, Specs and Paperwork
• Nickel Superalloys and Nickel-Copper Alloys
• Aerospace Aluminum Alloys and Tempers
• Material Certs, Specialty Metals and Lot Traceability

Sources

• NASA MSFC-STD-3029A, Tables I-A, II-A and III-A ferrous alloys
• Ulbrich, 17-4PH (UNS S17400) data sheet with H condition minimums
• PMPA, Dimensional Contraction of 17-4 PH Stainless Steel
• Smith Metal, 15-5PH data sheet
• Rolled Alloys, A-286 data sheet (AMS 5732, 5737)
• Aircraft Materials, 4130 (AMS 6345, 6350) data
• ASTM F519, mechanical hydrogen embrittlement evaluation
• GS Alloy, maraging steel 250, 300, 350 bars and forgings
• Gleco, citric acid passivation to ASTM A967 and AMS 2700
• SAE 304 stainless steel, sensitization and L grades
• DFARS 252.225-7009 specialty metals