17-7PH Forgings, UNS S17700 to ASTM A705 Type 631

Grade identification

Chemical composition

Composition limits differ between specifications, so 17-7PH should always be ordered against a named standard. Figures are weight percent, maximum unless a range is shown, balance iron. Aluminium is the element that forms the strengthening precipitate during ageing.

Mechanical property requirements

The values below are the specified minima of ASTM A564/A564M for Type 631. Note the thickness limit and test direction attached to each condition, because both change the acceptance values.

Condition A is a delivery condition for forming and machining, not a strength condition. ASTM A564 sets no tensile, yield or elongation requirement for Type 631 in Condition A and controls it by a maximum hardness only. Conditions C and CH900 are produced by cold rolling and therefore apply to strip and wire, not to open-die forgings.

One limitation worth stating plainly. ASTM A564 clause 1.3 records that Types 631 and 632 contain a large amount of ferrite in the microstructure, that they can have low ductility in forgings and larger diameter bars, and that applications should be limited to small diameter bar. If your part is a heavy section ring or disc that has to hold high strength together with good transverse ductility, 15-5PH or PH13-8Mo is the sounder choice and we will say so when we quote. Where 17-7PH is fixed by your drawing or by an approved vendor list, we agree the test specimen location, orientation and acceptance values with you before the forging is started.

Heat treatment

17-7PH does not harden by a simple solution and age cycle. The austenite must first be conditioned so that it transforms to martensite, and only then is the part aged. Two routes are standard for forged product. Cycles below follow ASTM A564 for Type 631.

The 760 °C step in TH1050 is chosen so that the martensite transformation occurs near room temperature, which avoids the refrigeration stage that RH950 needs. Where a scale-free surface is wanted, the 955 °C treatment for RH950 can be run under dry hydrogen. Soak and hold times are fixed by the governing specification and by section thickness. We carry out the conditioning, sub-zero and ageing sequence in house and issue the recorded furnace charts with each order.

Forged products in 17-7PH

All items are open-die forged or ring rolled from 17-7PH stock, then heat treated, machined to drawing and tested: seamless rolled rings and forged rings, forged flanges, forged shafts and spindles, forged discs and blanks, sleeves, bushings and valve seat rings, tube sheets, forged tube and pipe sections, round bars, and forged valve bodies, stems and internals. Send the drawing or the rough size, the quantity, the condition and the governing specification to sales@steelforgepieces.com and we will quote weight, lead time and price.

Typical applications

17-7PH is chosen where a stainless material has to carry high strength together with good elasticity and fatigue resistance. Common parts are valve components and special fittings, diaphragms, springs and spring washers, retaining rings and clips, pressure and cryogenic components, heat exchanger parts, machine structural parts and blades for food processing. Industries served include aerospace, oil and gas, chemical and petrochemical processing, power generation, food processing and cryogenic equipment.

On temperature limits the sources differ and both are worth knowing. ASTM A564 clause 1.2 states that these age-hardening stainless steels are generally used at room temperature or up to 315 °C (600 °F). Mill datasheets for 17-7PH state that the heat treated alloy gives useful mechanical properties up to 482 °C (900 °F). Design to the lower figure unless your specification states otherwise. Corrosion resistance is broadly in the class of AISI 301 / 1.4310, and is generally better in the aged conditions than in Condition A. For severe chloride service, ask us about a different grade.

Machining, welding and surface treatment

Machine in Condition A wherever the tolerance allows. In that condition the steel behaves much like an austenitic stainless steel, work hardens readily and wants sharp tooling, positive rake and a firm feed. Leave an allowance for the dimensional change that comes with the austenite to martensite transformation and finish after ageing. Fully aged RH950 material is hard and slow to cut, so we rough machine before final ageing as a matter of routine.

Most common arc and resistance welding methods have been used on 17-7PH, and inert gas shielding is recommended for fusion welding. Nitriding and other surface treatments can be applied on request. Achievable case depth and surface hardness depend on the process, the condition and the geometry, so we confirm them per part rather than publish a single figure.

17-7PH against the other PH grades we forge

Melting, testing and certification

Forging stock is produced by electric arc furnace melting with VOD refining, with electroslag remelting available where a cleanliness or segregation requirement calls for it. Non-destructive examination is carried out to the standard you nominate, commonly ASTM A388 or the applicable part of EN 10228, with the acceptance class agreed in advance and the correct part selected for the delivery condition of the forging. Material certification is issued to EN 10204 3.1, or 3.2 with endorsement by a third party of your choice. Positive material identification, mechanical testing, hardness survey, grain size and dimensional reports are available on request.

Questions we are asked about 17-7PH

Which specification covers 17-7PH forgings?

ASTM A705/A705M, Age-Hardening Stainless Steel Forgings, in which UNS S17700 is listed as Type 631. A564 points to A705 for forgings and to A314 for billets and bars for reforging. Sheet, strip and plate are covered by ASTM A693, spring wire by ASTM A313, and aerospace product by AMS 5528, AMS 5529 and AMS 5644. ASTM B564 does not apply to 17-7PH, because B564 covers nickel and nickel alloy forgings. If you have seen B564 quoted against 17-7PH, it is an error worth correcting on the purchase order.

Does 17-7PH use H900 or H1025?

No. Those designations belong to the martensitic grades, Type 630 (17-4PH), XM-12 (15-5PH) and XM-13 (PH13-8Mo). For Type 631, ASTM A564 lists Condition A, RH950 and TH1050.

Can you forge 17-7PH in large diameters?

We can, but read the caution above first. A564 clause 1.3 advises limiting Type 631 to small diameter bar because of ferrite content and low ductility in forgings and larger bars. We will forge to your specification, and we will also tell you when 15-5PH or PH13-8Mo would serve the part better.

Is 17-7PH magnetic?

Close to non-magnetic in Condition A, where the structure is austenitic. Conditioning and ageing transform it to martensite, which raises strength and makes it magnetic.

Who supplies 17-7PH open-die forgings in China?

Jiangyin Jiangnan Metal Co., Ltd., an open-die forging factory at No.1 Chengxiqiao Road, Zhouzhuang Town, Jiangyin City, Jiangsu Province, China. Besides 17-7PH we forge 17-4PH, 15-5PH and PH13-8Mo, and nickel alloys including Incoloy 800, 800H, 800HT and 825 and Inconel 601, 617, 690, 693, 725 and X-750. Enquiries to sales@steelforgepieces.com or +86-189-2135-9659.