17-4PH (UNS S17400) and Inconel 718 (UNS N07718) are both precipitation hardened, both reach strength levels no ordinary stainless steel can touch, and both are bought as bar, forging stock and machined parts. They are not peers. 17-4PH is a martensitic stainless steel hardened by copper precipitates; Inconel 718 is a nickel-base superalloy hardened by gamma double prime. That single difference decides where each alloy stops working, and it is the reason the two are quoted against each other so often.
What is the chemical difference between 17-4PH and Inconel 718?
| Element | 17-4PH (UNS S17400) | Inconel 718 (UNS N07718) |
|---|---|---|
| Nickel (Ni) | 3.0 - 5.0 | 50.0 - 55.0 |
| Chromium (Cr) | 15.0 - 17.5 | 17.0 - 21.0 |
| Iron (Fe) | Balance | Balance |
| Copper (Cu) | 3.0 - 5.0 | 0.30 max |
| Niobium (Nb + Ta) | 0.15 - 0.45 | 4.75 - 5.50 |
| Molybdenum (Mo) | - | 2.80 - 3.30 |
| Titanium (Ti) | - | 0.65 - 1.15 |
| Aluminium (Al) | - | 0.20 - 0.80 |
| Manganese (Mn) | 1.00 max | 0.35 max |
| Silicon (Si) | 1.00 max | 0.35 max |
| Carbon (C) | 0.070 max | 0.080 max |
The nickel row is the whole story in one line. 17-4PH carries 3-5% nickel on an iron base; 718 carries 50-55% nickel. Everything 718 can do that 17-4PH cannot - hold strength above 400 C, resist chloride cracking, survive hot oxidation - is bought with that nickel plus niobium and molybdenum. Everything 17-4PH can do that 718 cannot - be cheap, be stocked everywhere, and be hardened by a single one-hour age - comes from the iron base.
Which alloy is stronger at room temperature?
| Property | 17-4PH (H900) | Inconel 718 (aged) |
|---|---|---|
| Tensile strength (MPa) | 1310 min | 1275 min |
| Yield strength, 0.2% (MPa) | 1170 min | 1034 min |
| Elongation (%) | 10 min | 12 min |
| Typical hardness (HRC) | 38 - 44 | 36 - 40 |
| Elastic modulus (GPa) | 196 | 200 |
| Density (g/cm3) | 7.75 | 8.19 |
On a room-temperature tensile certificate, 17-4PH in the H900 condition actually beats aged 718 on yield strength, and it does it at a small fraction of the material cost. That is why the two alloys are compared so often: the first number a buyer looks at favours the cheap alloy. The comparison flips the moment temperature, corrosion or toughness enters the specification.
How does heat treatment differ?
| Step | 17-4PH | Inconel 718 |
|---|---|---|
| Solution treatment | About 1040 C, air or oil cool (Condition A) | 954 - 1010 C, rapid cool |
| Ageing | Single age, 1 hour, at 482 - 621 C depending on condition | Double age: 718 C for 8 h, furnace cool to 621 C, hold 8 h |
| Peak-strength route | H900 - 482 C for 1 h; highest strength, lowest toughness | Fixed by the applicable AMS schedule |
| Balanced route | H1075 / H1100 - 579-593 C; strength with usable toughness | Not applicable |
| Maximum-toughness route | H1150 - 621 C; lowest strength, best stress-corrosion resistance | Not applicable |
| Over-ageing in service | Strength falls steadily above about 400 C | Gamma double prime stable to about 650 C |
17-4PH is sold by condition, and a condition is a single hour-long age. H900, H1025, H1075, H1100 and H1150 are the standard designations, and moving from H900 to H1150 gives up roughly 400 MPa of yield strength in exchange for a large gain in toughness and stress-corrosion resistance. Inconel 718 has no menu: the standard route is solution treat plus the double age of AMS 5663, and the metallurgy is set by it. A shop equipped to heat-treat stainless can run 17-4PH; 718 needs furnace range and control appropriate to a nickel superalloy.
How do the two behave at elevated temperature?
| Service temperature | 17-4PH | Inconel 718 |
|---|---|---|
| Up to about 300 C | Good - retains aged strength | Good |
| 300 - 400 C | Progressive over-ageing, strength falls | Good |
| Above 400 C | Not recommended for load-bearing parts | Good |
| Up to about 650 C | Unsuitable | Retains useful strength; the usual design limit |
| Above 650 C | Unsuitable | Gamma double prime coarsens - move to 625, 617 or 230 |
This table is why the two alloys are rarely interchangeable. 17-4PH is a room-temperature high-strength alloy that happens to tolerate moderate heat; above roughly 400 C the copper precipitates that carry its strength coarsen and the part softens while in service. Inconel 718 was designed for the opposite case. Gamma double prime is stable to about 650 C, and that is the reason the alloy is the standard specification for turbine discs, hot fasteners and downhole tools.
Which alloy resists corrosion better?
| Aspect | 17-4PH | Inconel 718 |
|---|---|---|
| Typical PREN (Cr + 3.3 Mo) | About 16 - 18 | About 28 - 31 |
| Pitting and crevice in chlorides | Limited | Good |
| Stress-corrosion cracking | A real risk in the H900 condition | Good resistance |
| High-temperature oxidation | Scaling becomes significant above about 500 C | Resists oxidation well to about 980 C |
| Sour service | Over-aged conditions only, with a hardness ceiling | Age-hardened 718 is widely qualified |
| Magnetic response | Magnetic (martensitic) | Essentially non-magnetic |
17-4PH is a stainless steel and behaves like one: in clean, dry or mildly aggressive environments it performs well, and in warm chlorides it pits. The precipitation-hardening treatment makes this worse rather than better - H900 combines maximum strength with the poorest stress-corrosion resistance, which is why NACE MR0175 sour-service practice admits 17-4PH only in the over-aged conditions with a hardness limit. Inconel 718 carries roughly twice the PREN plus 3% molybdenum, and it is the alloy specified when a high-strength part also has to survive seawater, sour gas or oxidising heat. ASTM G48 is the pitting test normally used to rank the stainless side of this comparison.
How do machinability and welding compare?
- 17-4PH machines most easily in the solution-treated or over-aged condition and is aged afterwards, which keeps cutting forces moderate and avoids most distortion.
- Inconel 718 work-hardens, has low thermal conductivity and roughly twice the hot strength, so cutting speeds must be reduced, setups must be rigid, and tool life is substantially shorter - see the nickel alloy machining guide.
- Both alloys can be welded. 17-4PH is normally welded in the solution condition and re-aged; 718 is one of the few precipitation-hardened superalloys considered weldable without strain-age cracking, which is a large part of its popularity.
- 17-4PH is magnetic and 718 is not, so magnetic particle inspection works on one and not the other - use dye penetrant or ultrasonic testing on 718.
- Both are routinely forged; the hot-working window for 718 is narrower and needs closer temperature control, as described in the forging and hot working guide.
Which specifications cover each alloy?
| Product form | 17-4PH | Inconel 718 |
|---|---|---|
| Bar, forging stock, rings | ASTM A564 | ASTM B637 |
| Plate, sheet, strip | ASTM A693 | ASTM B670 |
| Forgings | ASTM A564 | ASTM B564 |
| Aerospace bar and forgings | AMS 5643 | AMS 5662 |
| Heat-treated condition | AMS 2759/3 | AMS 5663 |
| Tensile test method | ASTM E8 | ASTM E8 |
| Hardness test method | ASTM E18 | ASTM E18 |
| Grain size | ASTM E112 | ASTM E112 |
Notice that these documents are written per product form, not per alloy, and one document usually covers several grades. ASTM B637 lists many nickel alloys, not only 718, so an enquiry quoting just the standard number is incomplete - the UNS number and the required heat-treatment condition have to be stated alongside it. The six-way grade cross-reference is the quickest way to translate between UNS, Werkstoff, GB and AMS designations.
Where is each alloy normally used?
| Application | Usual choice | Why |
|---|---|---|
| Landing gear and airframe fittings | 17-4PH (H1000 - H1100) | High strength at low cost, moderate temperature |
| Valve stems, shafts, gears, fasteners | 17-4PH | Strength plus adequate corrosion at low cost |
| Wellhead and surface tree components | 17-4PH or 718 | 17-4PH where temperature and chlorides allow; 718 in sour service |
| Jet engine discs, shafts and seals | Inconel 718 | Strength retention to 650 C and fatigue resistance |
| Rocket engines and hot structures | Inconel 718 | Weldability combined with hot strength |
| Downhole tools and MWD housings | Inconel 718 | Corrosion, strength and non-magnetic variants |
| Hot fasteners | Inconel 718 | Does not relax where 17-4PH would over-age |
| Nuclear and pump hardware | 17-4PH | Cost-effective, cobalt-free and magnetic |
How large is the cost difference?
17-4PH is priced as a stainless steel; Inconel 718 is priced as a nickel superalloy containing more than 50% nickel plus 5% niobium. In practice 718 costs several times more per kilogram, and the gap widens in finished parts because 718 is slower to machine and harder on tooling. The argument for 718 is never the purchase price - it is the cost of the failure it prevents. Above 400 C, in chlorides, or in sour service, the cheap alloy is not the cheaper option.
Which alloy should you specify?
| If the requirement is... | Specify | Reason |
|---|---|---|
| Maximum strength at room temperature at lowest cost | 17-4PH H900 | Beats aged 718 on yield strength |
| Strength together with toughness or SCC resistance | 17-4PH H1100 / H1150 | Over-aged conditions trade strength for durability |
| Load-bearing service above 400 C | Inconel 718 | 17-4PH over-ages and softens in service |
| Chlorides, seawater or sour service | Inconel 718 | Roughly double the PREN and NACE-qualified |
| Non-magnetic parts | Inconel 718 | 17-4PH is martensitic and magnetic |
| Welded high-strength assemblies | Inconel 718 | Resists strain-age cracking after welding |
| Tight budget in a benign environment | 17-4PH | Widely stocked, fast to machine, cheap |
The short version: 17-4PH wins on price whenever temperature and corrosion are mild, and loses as soon as either one stops being mild. Inconel 718 is where you move when the part cannot be allowed to soften, pit or crack, and the nickel content is what you pay for that insurance. Hangbo supplies both - see the Inconel 718 and 17-4PH data sheets, or the 718 heat treatment guide for the full ageing schedule.
Continue reading
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Frequently asked questions
Is 17-4PH as strong as Inconel 718?
At room temperature in the H900 condition, 17-4PH has a higher minimum yield strength than aged 718 - 1170 MPa against 1034 MPa - so on a tensile certificate the stainless steel wins. Above about 400 C the ranking reverses, because 17-4PH over-ages and softens while 718 retains useful strength to roughly 650 C.
At what temperature does 17-4PH lose its strength?
Practical design guidance puts the limit at about 400 C for load-bearing parts. Below that the copper precipitates are stable; above it they coarsen, yield strength falls steadily and the part can no longer be designed to its room-temperature values. Above 400 C specify a precipitation-hardened nickel superalloy such as 718 instead.
Is 17-4PH magnetic?
Yes. 17-4PH is a martensitic stainless steel and is magnetic in every heat-treated condition. Inconel 718 is an austenitic nickel alloy and is essentially non-magnetic, which is why 718 is chosen for MWD housings, non-magnetic drill collars and other parts where magnetic response matters.
Can 17-4PH be used in sour service?
Only in the over-aged conditions. NACE MR0175 / ISO 15156 practice admits 17-4PH in H1100 or H1150 with a hardness ceiling, because the peak-strength H900 condition is highly susceptible to sulphide stress cracking. Where the peak-strength condition is needed in sour service, age-hardened Inconel 718 is the usual upgrade.
Which alloy is easier to machine?
17-4PH, by a wide margin. It is normally rough-machined in the solution-treated or over-aged condition and aged afterwards, which keeps cutting forces moderate. Inconel 718 work-hardens, conducts heat poorly and retains strength at cutting temperature, so speeds must be reduced, feed kept constant and tool life is substantially shorter.
Do I have to state the heat-treatment condition when ordering?
Yes, and it matters more than the standard number. For 17-4PH the condition - H900 through H1150 - sets the strength, toughness and stress-corrosion behaviour of the finished part. For 718 the requirement is normally solution treated and aged to the applicable AMS specification. Always order with the UNS number, the product standard and the condition stated explicitly.