Steels for strength and machinability

February 19, 2004

Two precipitation-hardenable steel alloys developed for use in aerospace applications bring benefits to other metal-using industries.

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Company comes of age with aerospace

Properties of Custom 456 Stainless Steel

Properties of 70+ Steel Alloy

Speeds and feeds for machining 70+ steel

An aircraft shock strut is made of Project 70+ stainless steel, which is easier to machine than conventional 15Cr-5Ni steel.

The engine powering the Wright Flyer contained components made of Carpenter Technology steels.

Comparing Custom 465 to other PH steel alloys shows it has the highest ultimate tensile strength

Theodore Kosa
David E. Wert
Carpenter Technology Corp.
Reading, Pa.

Precipitation-hardenable (PH) stainless steels are turning up in more and more aerospace applications because they can be age hardened, sometimes after being cold worked, to various strength levels and used in severe environments. Two in particular, Custom 465 stainless and Project 70+ 15Cr-5Ni alloy, offer competitive advantages in strength and machining that could be useful in any industry that uses steel.

Custom 465

Custom 465 stainless (S46500) is the strongest of all commercially available PH stainless alloys. Its ultimate tensile strength, if peak aged (H900 condition), is about 260 ksi (1,795 MPa). In this condition, it still has high notch tensile strength and fracture toughness. It was developed in response to demands from the aerospace industry for materials that will keep aircraft flying for 30 years or more with minimum maintenance. It has been used for structural components such as flap tracks, actuators, engine mounts, and landing-gear hardware.

Custom 465 is a melted, martensitic stainless steel with strength and fracture toughness that resists stress-corrosion cracking better than other high-strength PH alloys. Its chemical composition is: carbon 0.02% max, manganese 0.25% max, phosphorus 0.015% max, sulfur 0.01% max, silicon 0.25% max, chromium 11 to 12.5%, nickel 10.75 to 11.25%, titanium 1.5 to 1.8%, molybdenum 0.75 to 1.25%, the balance is iron. Aging the alloy to H900 through H1000 conditions brings out its strength and is suggested for most applications.

In general, Custom 465 is less sensitive to process parameters than other high-strength PH steels. Its relatively low annealed yield strength, along with a low cold-work hardening rate, make the alloy well suited for making fasteners and drawing small-diameter wires. Custom 465 stainless can be cold drawn in excess of 90% reduction, with no intermediate anneal. On the other hand, drawn and aged Custom 465 wire's strength is over 300 ksi (2,070 MPa). The alloy is also thermally stable after long-term exposure to elevated temperatures.

Custom 465's corrosion resistance approaches that of Type 304 stainless. Tests show that exposure to 5% neutral salt spray at 95°F (35°C) (per ASTM B117) causes little or no rusting after 200 hr, regardless of aging. Double-cantilever-beam tests conducted in 3.5% NaCl (pH 6) at room temperature showed overaged Custom 465 has good resistance to stress corrosion cracking (SCC). And its SSC resistance improves as aging temperatures increase. By comparison, Custom 465 is significantly stronger than PH 13-8 Mo and Custom 455 alloys formulated with the same SSC resistance. Comparing these alloys when heat treated to the same longitudinal fracture toughness (100 ksi (in.^1/2) or 110 Mpa (m^1/2)), their yield strengths rank as follows: Custom 465 stainless at 230 ksi (1,585 MPa), PH 13-8 Mo at 205 ksi (1,413 MPa), and Custom 455 at 200 ksi (1,380 MPa).

To make Custom 465 resist chloride, metalworkers age it at 950°F (510°C) or higher. The resulting metal is stronger than other PH steels with high levels of chloride resistance. Indeed, the ultimate tensile strength of Custom 465 exceeds that of any commercial PH steel aged between H900 and H1150.

Custom 465 stainless is fully solution annealed when held for 1 hr at 1,800°F (982°C) ±15°F, then rapidly cooled and annealed at -100°F. Cold treating it at 100°F reduces the sensitivity of the aged properties to prior processing.

Custom 465 is strengthened by holding it at a selected temperature between 900 and 1,150°F (482 to 621°C) for 4 hr then cooling it in ambient air. The choice of aging temperature depends on what combination of strength, toughness, and SCC resistance is needed. The flexibility that aging gives this alloy, along with the corresponding range in properties, make it a candidate for applications within and beyond the aerospace industry.

Cold working Custom 465 then aging it at 900°F (482°C), for example, gives it maximum strength of over 300 ksi for parts with diameters less than 0.75 in. This is useful for making surgical and dental instruments and needle wire for the medical industry. With the right combination of strength, hardness, and toughness, the material can be used for cutting edges on hand tools, and for tool bodies themselves.

Engineers could use Custom 465 to upgrade cadmium and chromium plated steel-alloy parts. It might also be a replacement for components made of 15Cr-5Ni and Custom 630 (17Cr-4Ni) PH steels that need more strength, a smaller size, or lower weight.

Project 70+

A while back, many shops making parts from 15Cr-5Ni couldn't keep pace with production schedules because of the material's relatively poor machinability. Its machining properties were improved by modifying its composition and keeping in mind the relationship between chemistry and modern steel-processing methods. The result was Project 70+ 15Cr-5Ni, a significant improvement on and drop-in replacement for conventional 15Cr-5Ni stainless, a steel alloy used for decades in aerospace. Project 70+ meets all aerospace composition, corrosion resistance, and mechanical requirements, including Aerospace Materials Specification 5659 covering bars, wire, forgings, rings and extrusions, and ASTM A564.

Project 70+, a vacuum remelted, precipitation-hardening, martensitic stainless steel, lets shops push machining speeds and feeds to higher levels. Machinability can be further improved with assistance from the alloy developer. But even without technical assistance, shops trying out the alloy can begin machining at their own comfort level, then ramp up to find out how much cycle times can be reduced and tool times extended.

In addition to improved machinability, the alloy can be headed, rolled, and otherwise formed by cold working. And it can be forged by uniformly heating work pieces at between 1,950 and 2,050°F (1,066 to 1,121°C) for 1 hr before forging in accordance with a recommended schedule. Project 70+ can also be shield-fusioned and resistance welded.

The alloy's corrosion resistance approaches that of 304 stainless, and is similar to Carpenter's 15Cr-5Ni and Custom 630 (17Cr-4Ni) stainless steels. It resists SSC after age hardening at temperatures of 1,025°F (552°C) and higher. The alloy also resists oxidizing to approximately 1,100°F (593°C). Higher aging temperatures improve toughness and ductility with decreasing strength.

Like conventional 15Cr-5Ni, Project 70+ shows little or no rusting after 200 hr of exposure to ASTM B117's salt-fog test when solution-annealed or age-hardened at temperatures from 900 to 1,150°F (482 to 621°C). However, optimal corrosion resistance requires chemical passivation. Both nitric acid and citric acid treatments are suitable.

It is worth noting that in H925 condition, Project 70+ can have excellent longitudinal and transverse fracture toughness and meet frequently specified 180 to 200-ksi ultimate tensile strength requirements. Due to the alloy's homogeneity, there is little difference between longitudinal and transverse properties.

Analysis shows Project 70+ is comprised of: carbon 0.07% max, manganese 1% max, phosphorus 0.03% max, sulfur 0.015% max, silicon 1% max, chromium 14 to 15.50%, nickel 3.5 to 5.5%, molybdenum 0.5% max, copper 2.5 to 4.5%, columbium 0.15 to 0.45%, tantalum 0.05% max, iron makes up the balance.

In one highly successful application, Sturm Ruger & Co. Inc., Southport, Conn., used Project 70 to replace conventional 15Cr-5Ni for the barrel of its Super Redhawk .454 Casull, one of the most powerful commercially available revolvers in the world. Conventional 15Cr-5Ni stainless steel met design requirements for strength and corrosion resistance, but was difficult to machine. It took Ruger 28 min to gun drill a 0.480-in.-diameter hole in a 1-in. OD X 19-in. long bar at the rate of 0.71 ipm. (The bars are later hammer-forged over a mandrel and cut into three 7-in.-long gun barrels). After switching to Project 70+, gun drilling cycle times went down by 20% and tool life increased. And Project 70+'s transverse mechanical properties let the barrels resist the high stresses produced by the cartridge fired through them.

Other results with aerospace applications have been equally impressive. One manufacturer improved productivity by 145% for machining a part that required milling, boring, turning, drilling, and thread rolling. Another company, doing a simple turning operation, cut cycle times by a third and increased tool life by 50%. Still another parts producer, in milling Project 70, tripled speeds and feeds, and increased cut depth by a factor of 5 without burning the cutting tools.

Company comes of age with aerospace

With corporate roots reaching back to 1889, Carpenter Technology Corp. in Reading, Pa., had just enough time to build a reputation for metallurgic achievement to capture the attention of Wilbur and Orville Wright.

At Kitty Hawk, N.C., on Dec. 17, 1903, the Wright brothers confidently staked their lives on an engine operating with components made of Carpenter steels. On that day, controlled flight first took wing, proving the feasibility of heavier-than-air flight.

Following the 1913 invention of "rustless steels," later known as stainless steels, Carpenter and four other American producers purchased minority interests in the American Stainless Steel Co., ushering in the age of stainless steels. And the first two heats of Carpenter stainless, melted in 1917, went into components for the Liberty engines that powered American fighter planes and bombers in World War I.

On May 8, 1926, Carpenter again helped make history in powered flight. Lt. Comdr. Richard E. Byrd and his pilot, Floyd Bennett, circled the North Pole in the Josephine Ford, a Fokker monoplane powered by three Wright Aeronautical engines. All the gears, shafts, and fasteners of the nine-cylinder Wright "Whirlwind" J-5 engines were made from Carpenter steels.

A year later, on May 21, 1927, Charles Lindbergh put his mark on aviation history with a solo transatlantic flight. His plane, Spirit of St. Louis, was powered by a J-5 engine with components fabricated of Carpenter steels.

Shortly after U.S. entered World War II, Carpenter joined other companies in supplying badly needed materials and items for the war effort. Carburetors, magnetos, superchargers and machine-gun parts for bombers and fighters were made of stainless Types 416, 302, 303SE, and 347. Fasteners made from 6140 alloy steel held fuselages together. Type 416 stainless was machined into numerous instruments, including gyrocompasses and the famous Norden bomb-sights and. And High Permeability "49" was used extensively in radio equipment.

Near the end of the Korean War, Carpenter expanded its line of aerospace alloys that included high-temperature alloys such as A-286, 19-9 DL, and DX. The company also began investing aggressively in vacuum melting technology, which was key to producing high-purity, high-temperature alloys for jet aircraft and space applications.

In 1968, reflecting its ongoing development of specialty alloys, the company changed its name from the Carpenter Steel Co. to Carpenter Technology Corp. After all, technology had always been the heart and soul of the company, and "steel" was no longer a fitting description of the company or its product line. And by now, the company's metals have played important roles in exploring the Moon, Mars, Venus, and outer space.

Properties of Custom 456 stainless steel
Condition	Orientation	0.2%Yield strength (ksi)	Ultimate tensile strength (ksi)	Elongation (% in 4D)	Reduction of area (%)	Notch tensile strength (ksi)	Hardness (Shore Brinell)	Hardness (Shore Brinell)
H9000	L T	238 234	257 257	13 12	58 52	350 338	1.36 1.32	50
H950	L T	235 230	254 250	14 12	63 53	362 355	4.43 1.42	49
H1000	L T	217 211	231 227	15 15	65 61	352 349	1.52 1.54	47
H1050	L T	198 196	215 213	18 17	67 63	326 324	1.52 1.52	45
L(ongitudinal), T(ransverse) This chart shows typical room-temperature properties of Custom 465 stainless steel in forged bars measuring 4.5 X 1.5 in. and 4.5 X 2.75 in.

Properties of 70+ steel alloy
Condition	Orientation	Ultimate tensile strength		0.2% Yield strength		Elongation (% in 4D)	Redusction of area (%)	Hardness (HRC)	Impact strength		Fracture toughness (K_ic)
Condition	Orientation	ksi	MPa	ksi	MPa	Elongation (% in 4D)	Redusction of area (%)	Hardness (HRC)	Charpy ft-lb	V-notch J	Ksi in.²	MPa m²
A	L	146	1007	118	814	16	72	30	-	-	-	-
A	T	146	1007	119	820	14	61	30	-	-	-	-
H900	L	197	1358	184	1269	15	56	43	13	18	55	61
H900	T	195	1344	175	1207	14	50	43	10	14	50	55
H900-4	L	195	1344	180	1241	15	54	42	20	27	81	89
H900-4	T	194	1338	176	1213	14	51	42	20	27	75	83
H925	L	190	1310	177	1220	15	58	41	49	66	110	121
H925	T	188	1296	172	1186	14	53	41	30	41	86	95
H1025	L	166	1145	158	1089	17	67	36	100	136	<146(K_q)	<154(K_q)
H1025	T	164	1131	156	1076	15	59	36	53	72	115(K_q)	127(K_q)
H1150	L	141	972	126	869	21	71	31	130	176	-	-
H1150	T	140	965	124	855	18	62	31	79	107	-	-
H1150M	L	130	896	91	627	23	74	25	-	-	-	-
H1150M	T	129	889	89	613	21	64	25	-	-	-	-
L(ongitudinal), T(ransverse)

Speeds and feeds for machining 70+ steel
FOR SINGLE-POINT AND BOX TOOL
Cut depth (in.)	HIGH-SPEED TOOLS			Tool Material	CARBIDE TOOLS Speed (fpm) (Brazed throw away coated)			Feed (ipr)
Cut depth (in.)	Too material	Speed (fpm)	Feed (ipm)	Tool Material	CARBIDE TOOLS Speed (fpm) (Brazed throw away coated)			Feed (ipr)
SOLUTION TREATED
0.15	M2, T15	110	0.015	C6	3410	340	440	0.019
0.025	M2, T15	125	0.01	C7	240	290	390	0.009
AGED
0.15	T15, M42	85	0.013	C6	190	240	340	0.013
0.025	T15, M42	105	0.008	C7	240	290	390	0.006

FOR CUT-OFF AND FORM TOOLS
TOOL MATERIAL		Speed (fpm)	-------------- FEED (IPR) --------------
High-speed tools	Carbide tools		Cut-off tool width (in.)				Form tool width (in.)
High-speed tools	Carbide tools		0.0625	0.125	0.25	0.5	1	1.5	2
SOLUTION TREATED
T15	C6	80	0.0017	0.0020	0.0025	0.0028	0.0022	0.0019	0.0017
T15	C6	260	0.0021	0.0024	0.0029	0.0032	0.0024	0.0021	0.0019
AGED
T15	C2	70	0.0014	0.0017	0.0022	0.0025	0.0019	0.0016	0.0014
T15	C2	240	0.0016	0.0016	0.0024	0.0027	0.0020	0.0017	0.0015