17-4PH vs PH 13-8Mo: Which PH Stainless to Specify

By Shanghai Hangbo Alloy Group Co., Ltd. - China Supplier | ISO 9001:2015 | Est. 2012

Contact: sales@hangboalloy.com | hangbo@nickel-alloy.com | WhatsApp (Lisa): +86 13611656360

Quick Answer: 17-4PH vs PH 13-8Mo

Specify 17-4PH (UNS S17400) for general high-strength parts such as pump shafts, valve stems, gears and fasteners where a good balance of strength, corrosion resistance and cost is enough. Specify PH 13-8Mo (UNS S13800) where the highest strength must be combined with high fracture toughness and resistance to stress corrosion cracking, as in aerospace structural parts, landing gear components and high-strength fasteners.

Key Takeaways

• Both are precipitation-hardening martensitic stainless steels, but they harden differently. 17-4PH hardens through copper-rich precipitates; PH 13-8Mo hardens through a nickel-aluminium phase, and that difference is the origin of the toughness and stress-corrosion behaviour that separates them.
• PH 13-8Mo reaches higher strength at the same ductility. In comparable aged conditions PH 13-8Mo is typically 15 to 25 per cent stronger than 17-4PH while retaining comparable or better ductility, which is why it is the aerospace choice.
• PH 13-8Mo has the better toughness and transverse ductility at high strength. That matters for parts loaded in more than one direction and for parts that must tolerate a defect without cracking.
• 17-4PH resists stress corrosion cracking less well at high strength. At the higher strength conditions 17-4PH is more sensitive to chloride stress corrosion cracking, so PH 13-8Mo is preferred where a high-strength part sees chlorides.
• 17-4PH is cheaper, more available and easier to machine and weld. It is the volume grade, stocked everywhere, while PH 13-8Mo is a premium alloy typically produced by vacuum melting.
• The decision turns on whether the part is strength-limited or toughness-limited. If a standard high-strength part will do, 17-4PH is the economic answer; if a high-strength part must also resist cracking, PH 13-8Mo is the correct answer.

Two Precipitation-Hardening Stainless Steels Compared

17-4PH and PH 13-8Mo belong to the same family of martensitic precipitation-hardening stainless steels, and a buyer who has used one will recognise the other's processing: both are supplied in a soft condition, both are machined and then aged to a specified strength, and both trade some corrosion resistance for a strength level that the ordinary austenitic stainless steels cannot reach. The family resemblance is genuine, and it is also the source of the confusion, because the two grades are frequently quoted as alternates for the same part when they are not equivalent on the axis that most often decides whether the part survives.

The distinction is in how each alloy hardens. 17-4PH, designated UNS S17400 and historically AISI 630, combines roughly 15 to 17.5 per cent chromium, 3 to 5 per cent nickel, 3 to 5 per cent copper and a niobium-plus-tantalum addition. Its strength comes from copper-rich precipitates that form when the alloy is aged, together with the martensitic matrix produced by the solution treatment and the niobium that stabilises the structure. PH 13-8Mo, UNS S13800, uses a different route: it carries roughly 12.25 to 13.25 per cent chromium, 7.5 to 8.5 per cent nickel, 2 to 2.5 per cent molybdenum and about 0.9 to 1.35 per cent aluminium, and it hardens through the precipitation of a nickel-aluminium phase. The absence of copper and the presence of molybdenum and aluminium change the balance of properties in ways that a data sheet makes clear: PH 13-8Mo is stronger, tougher and more resistant to stress corrosion cracking at high strength, while 17-4PH is cheaper, more widely available and easier to fabricate.

Those differences are why the two grades sit in different markets. 17-4PH is the general engineering grade, used for pump shafts, valve stems and trim, gears, splines, bushings, fasteners and instrument parts in chemical, petrochemical, food, paper and general machinery, and it is also common in oil and gas equipment and in some nuclear applications where the specification permits. PH 13-8Mo is the aerospace and high-performance grade, produced by vacuum induction melting and vacuum arc remelting for cleanliness and used for structural components, landing gear parts, actuators, high-strength fasteners and other parts where the combination of strength and toughness is the governing requirement. In short, 17-4PH is chosen when strength and corrosion resistance are both adequate at a reasonable cost, and PH 13-8Mo is chosen when the strength must be combined with toughness at a level that 17-4PH cannot reach. Our stainless steel range covers both grades alongside the duplex and super-austenitic families, and our Inconel alloy range covers the nickel alloys used where the corrosion duty exceeds what any stainless steel can handle.

One administrative point should be settled before the technical one, because it causes avoidable disputes. Both grades are ordered by UNS number and by condition, not by a marketing name, and both are covered by overlapping standard systems. A specification that says "17-4PH, H900" and one that says "PH 13-8Mo, H1000" are complete statements, whereas a specification that says "17-4PH grade" or "PH stainless, high strength" is not, because the condition determines the properties and the properties determine whether the part works. The tables that follow are organised around that principle: composition first, then the conditions, then the properties that follow from them.

Chemical Composition Compared

The composition table shows the structural reason for everything that follows. 17-4PH carries copper and no aluminium; PH 13-8Mo carries aluminium and molybdenum and no deliberate copper. Those two facts are why the alloys harden differently, and why PH 13-8Mo has the higher toughness and the better resistance to stress corrosion cracking at high strength.

Element (wt %)	17-4PH (UNS S17400)	PH 13-8Mo (UNS S13800)	Standard basis	Notes
Cr	approx. 15.0-17.5	approx. 12.25-13.25	ASTM A564 / ASTM A693	corrosion resistance
Ni	approx. 3.0-5.0	approx. 7.5-8.5	ASTM A564 / ASTM A693	austenite and precipitate former
Cu	approx. 3.0-5.0	not specified	ASTM A564	the 17-4PH hardening element
Mo	max 0.5	approx. 2.0-2.5	ASTM A564 / ASTM A693	SCC and toughness in PH 13-8Mo
Al	not specified	approx. 0.90-1.35	ASTM A564	NiAl precipitate former
Nb + Ta	approx. 0.15-0.45	not specified	ASTM A564	stabilises 17-4PH
C	max 0.07	max 0.05	ASTM A564 / ASTM A693	low carbon in both
Mn	max 1.0	max 0.10	ASTM A564 / ASTM A693	PH 13-8Mo is cleaner
Si	max 1.0	max 0.10	ASTM A564 / ASTM A693	PH 13-8Mo is cleaner
P	max 0.04	max 0.010	ASTM A564 / ASTM A693	PH 13-8Mo is cleaner
S	max 0.03	max 0.008	ASTM A564 / ASTM A693	PH 13-8Mo is cleaner
N	not specified	max 0.010	ASTM A564	controlled in PH 13-8Mo

Table note: Composition ranges are the published limits of the wrought-product standards named in the Standard basis column (ASTM A564 for bars, forgings and forging stock and ASTM A693 for plate, sheet and strip, with the corresponding AMS specifications named in the Standard Index). Values are shown as published ranges and should be confirmed against the edition named on the purchase order, because a revision can move a limit. The lower residual limits on PH 13-8Mo are not decorative: the tighter control of manganese, silicon, phosphorus, sulphur and nitrogen reflects the vacuum melting route used for the grade and contributes directly to its toughness and its cleanliness. Where a project applies a European, Japanese or Chinese specification instead, the grade designations differ and equivalence should be confirmed rather than assumed.

The composition is also the reason the two alloys machine differently and are welded differently. 17-4PH is the more forgiving material in a machine shop: it is reasonably machinable in the solution-treated condition, it can be welded with matching or austenitic fillers, and the post-weld ageing treatment restores most of the properties. PH 13-8Mo, with its higher alloy content and its vacuum-melted cleanliness, behaves more like an aerospace material: it is available in a narrow band of product forms and sizes, it is normally specified with a controlled melting route, and it is welded less often, with a procedure that must account for the effect of the weld thermal cycle on the aged microstructure. Neither alloy should be welded in the fully aged condition without a proper procedure, and in both cases the heat treatment after welding is part of the design rather than an afterthought.

Condition Designations and Heat Treatment

The condition is the single most important item on a purchase order for either grade, because precipitation-hardening stainless steels are supplied in a soft solution-treated condition and reach their properties only after an ageing treatment to a named condition. The condition is normally expressed as an H-number, and the number is derived from the ageing temperature: a lower number means a lower ageing temperature, a higher strength and a lower ductility, while a higher number means a higher ageing temperature, a lower strength and a higher ductility. The table below sets out the common conditions and the logic that connects them.

Condition	Nominal ageing temperature	Effect on strength	Effect on ductility and toughness	Applies to	Standard basis
H900	approx. 480 C	highest strength	lowest ductility	17-4PH	ASTM A564 / AMS 5643
H925	approx. 495 C	very high strength	low ductility	17-4PH	ASTM A564
H1025	approx. 550 C	high strength	moderate ductility	17-4PH	ASTM A564
H1075	approx. 580 C	medium strength	good ductility	17-4PH	ASTM A564
H1150	approx. 620 C	lower strength	best ductility	17-4PH	ASTM A564
H950	approx. 510 C	highest strength	lower ductility	PH 13-8Mo	AMS 5629
H1000	approx. 540 C	very high strength	good ductility	PH 13-8Mo	AMS 5629
H1025	approx. 550 C	high strength	better ductility	PH 13-8Mo	AMS 5629
H1050	approx. 565 C	medium-high strength	good ductility	PH 13-8Mo	AMS 5629
H1100	approx. 595 C	medium strength	best ductility	PH 13-8Mo	AMS 5629

Table note: The ageing temperatures are nominal values given to explain the trend from high strength and low ductility at the low-temperature conditions to lower strength and best ductility at the high-temperature conditions; the controlling properties are those of the current edition of the governing standard named in the final column, together with the ASME or AMS document named by the purchase order. The condition designation is part of the grade specification and must be stated on the order, because the same alloy supplied in two conditions is two different products. Where an aerospace specification applies, the ageing furnace and the temperature recording must satisfy AMS 2750 pyrometry, and the condition is verified from the heat-treatment record rather than from a room-temperature test alone.

The practical consequence is that the phrase "17-4PH" without a condition is not a specification. A part ordered as 17-4PH and aged to H900 will be hard and strong and relatively brittle; the same part aged to H1150 will be softer, tougher and much more resistant to stress corrosion cracking, and it will be a different part for design purposes. The same logic applies to PH 13-8Mo across its H950 to H1100 range. A designer who needs corrosion resistance in a chloride environment should therefore be cautious about demanding the highest strength condition, because the lower-ageing-temperature conditions that produce peak strength also produce the microstructure most susceptible to stress corrosion cracking, and the better engineering answer is often a slightly over-aged condition that trades a small amount of strength for a large gain in cracking resistance.

Mechanical Properties by Condition

The mechanical table is where the two grades are compared properly, and it must be read with the condition in mind, because a comparison between an over-aged 17-4PH and a peak-aged PH 13-8Mo is not a comparison between the alloys but between two different heat treatments. The table below therefore compares the grades at comparable positions in their condition ranges, using typical values.

Grade and condition	UTS (typical)	0.2 % YS (typical)	Elongation (typical)	Hardness (typical)	Source type
17-4PH, H900	approx. 1310 MPa	approx. 1170 MPa	approx. 10-16 %	approx. 40-44 HRC	typical, not a standard minimum
17-4PH, H1025	approx. 1070 MPa	approx. 1000 MPa	approx. 12-17 %	approx. 33-38 HRC	typical, not a standard minimum
17-4PH, H1150	approx. 860 MPa	approx. 790 MPa	approx. 16-22 %	approx. 28-32 HRC	typical, not a standard minimum
PH 13-8Mo, H950	approx. 1520 MPa	approx. 1410 MPa	approx. 9-12 %	approx. 45-47 HRC	typical, not a standard minimum
PH 13-8Mo, H1000	approx. 1450 MPa	approx. 1380 MPa	approx. 10-12 %	approx. 43-45 HRC	typical, not a standard minimum
PH 13-8Mo, H1050	approx. 1380 MPa	approx. 1275 MPa	approx. 11-14 %	approx. 40-42 HRC	typical, not a standard minimum
PH 13-8Mo, H1100	approx. 1170 MPa	approx. 1035 MPa	approx. 13-16 %	approx. 36-39 HRC	typical, not a standard minimum

Table note: All values are typical values from published handbook and mill data and are explicitly not the minimum values of any standard; a design that depends on a guaranteed minimum must take that minimum from the current edition of the governing standard named on the purchase order, which for bar and forgings is ASTM A564 and for plate, sheet and strip is ASTM A693, with the AMS documents named in the Standard Index where an aerospace specification applies. Tensile testing is performed to ASTM E8/E8M or to ASTM A370, hardness to ASTM E18, and fracture toughness to ASTM E399 where a toughness value is required. The comparison to read is between conditions at similar ductility: PH 13-8Mo at H1050 is roughly 20 per cent stronger than 17-4PH at H1025 while offering comparable elongation, which is the margin that makes it the aerospace grade.

Two patterns are worth extracting from the table. The first is that the strength range available within a single grade is large: 17-4PH spans roughly 860 to 1310 MPa and PH 13-8Mo spans roughly 1170 to 1520 MPa, depending only on the ageing temperature. That means a designer has real freedom to trade strength for ductility and cracking resistance by changing the condition rather than the alloy, and it also means that a rejection on a strength basis may be curable by a re-age rather than a re-melt. The second pattern is the overlap band. Between roughly 1100 and 1300 MPa both grades can be specified, and it is in that band that the choice is genuinely commercial: 17-4PH will be cheaper and easier to obtain, while PH 13-8Mo will offer better toughness and better resistance to cracking at the same strength. A buyer who simply compares the peak strength of each grade will miss the fact that in the overlap band the decision is about toughness and corrosion, not about strength.

Standards and Specifications

Both grades are covered by the same families of standard documents, which is convenient because it means a fabricator who knows the one system can read the other. The table below maps the standards for each grade and form, and it is the table to use when checking a specification for completeness.

Area	17-4PH (UNS S17400)	PH 13-8Mo (UNS S13800)	Covers	Form
Bars, forgings and forging stock	ASTM A564	ASTM A564	composition + mechanical	bar, forging
Plate, sheet and strip	ASTM A693	ASTM A693	composition + mechanical	plate, sheet, strip
Aerospace bar and forgings	AMS 5643	AMS 5629	mechanical + heat treatment	bar, forging, ring
Aerospace sheet and plate	AMS 5604	— (confirm per edition)	mechanical + heat treatment	sheet, plate
ASME equivalent	ASME SA564 / SA693	ASME SA564 / SA693	composition + mechanical	all forms
Corrosion and SCC testing	ASTM G48 / ASTM G36 where required	ASTM G48 / ASTM G36 where required	test method	all forms
Fracture toughness	ASTM E399 where required	ASTM E399 where required	test method	all forms
Pyrometry	AMS 2750 where applicable	AMS 2750 where applicable	furnace control	all forms
Inspection documents	EN 10204	EN 10204	documents	all forms

Table note: The table names standards by number and scope, and where an edition year is not quoted the controlling document is the one named on the purchase order. The aerospace column is deliberately asymmetric: 17-4PH has well-established AMS coverage for both bar and sheet, while PH 13-8Mo is normally ordered to an AMS bar and forging specification and its sheet and plate coverage should be confirmed against the current AMS index rather than assumed. Where the ASME code governs a pressure-retaining part, the SA equivalent of the ASTM product standard is called up and the code's own requirements are added. Testing methods and product standards should be named separately, because a product standard states the property requirements while a test method states how the property is measured.

Two traps recur in Nimonic-like fashion here. The first is a specification that names the alloy and the condition but not the product standard, which leaves the tolerance, the test frequency and the acceptance criteria open; the second is a specification that names an AMS document for a form that the document does not cover. Both are avoided by naming the UNS number, the product standard and edition, the condition, and the inspection document type, and by confirming the coverage of any AMS document before it is quoted. Where the part is safety-related, the specification should also name the toughness or SCC test that the end user intends to rely on, because those requirements change the sampling and the cost, and discovering them after the material has been produced is the most expensive way to find them. Our knowledge base explains the condition designations, the heat-treatment records and the inspection documents that a buyer should expect for a precipitation-hardening stainless steel.

Corrosion, Toughness and Fatigue Behaviour

The engineering difference between the two grades appears most clearly in three properties that a tensile test does not capture: toughness, resistance to stress corrosion cracking, and fatigue behaviour. The table below summarises the comparison and is written as engineering guidance rather than as a guaranteed ranking.

Property	17-4PH (S17400)	PH 13-8Mo (S13800)	Basis	Notes
General corrosion resistance	good, comparable to 304-type	good, comparable to 304-type	ASTM G48 where required	both less resistant than austenitic grades
Chloride stress corrosion cracking at high strength	more susceptible	markedly more resistant	ASTM G36 / ASTM G49	the decisive difference
Fracture toughness at high strength	moderate	higher	ASTM E399	important for defect-tolerant design
Transverse ductility	moderate	better	ASTM E8 / A370	matters for multi-axial loading
Fatigue strength	good	good, often better at high strength	design-specific	surface finish dominates
Maximum service temperature	approx. 300-350 C	approx. 350-400 C	design guidance	both limited by over-ageing
Magnetic	yes	yes	—	both are martensitic
Weldability	good with procedure	fair, procedure-critical	ASME Section IX	both need post-weld heat treatment

Table note: The comparison is engineering guidance based on the alloy chemistry and on the test standards named in the Basis column, and it is not a substitute for a corrosion or fracture assessment of the specific part. ASTM G48 covers pitting and crevice corrosion testing, ASTM G36 covers boiling magnesium chloride stress corrosion testing, ASTM G49 covers the preparation and use of tension specimens for SCC testing, and ASTM E399 covers plain-strain fracture toughness testing. The most consequential entry is the stress corrosion cracking row: at comparable high strength, PH 13-8Mo is substantially more resistant to chloride-induced cracking than 17-4PH, which is the reason it is preferred for high-strength parts that see chlorides, and the reason 17-4PH parts in such service are often specified in an over-aged condition.

Three practical conclusions follow. First, corrosion resistance is not the reason to choose between these grades, because the two are similar in general corrosion terms and both are less resistant than an austenitic or super-austenitic stainless steel. The reason to choose PH 13-8Mo is strength combined with toughness and cracking resistance, not general corrosion performance. Second, toughness is a design property and not an afterthought: a part designed on yield strength alone can fail by fast fracture from a small defect if the toughness is marginal, and this is precisely the regime in which the two grades diverge. Third, fatigue performance depends heavily on surface condition, so a part made from the correct alloy and then machined with a poor surface finish or left with a sharp notch may fail earlier than a part made from the cheaper alloy with an excellent finish. Where a high-strength part must survive chlorides, the correct combination is usually a grade choice plus a condition choice plus careful attention to surface finish and to residual stress, and the grade choice alone will not guarantee the outcome.

Selection Guide: Shafts, Valves, Aerospace Parts and Fasteners

The selection question is easiest to answer by classifying the part rather than the alloy. If the part is a general high-strength component in a moderately corrosive service and the economics matter, 17-4PH is normally the answer. If the part is a strength-critical or safety-critical component in which toughness and cracking resistance are design requirements, PH 13-8Mo is normally the answer. The table below turns that principle into a component-level guide.

Component	Recommended grade	Condition guidance	Why	Alternative considered
Pump shaft, moderate service	17-4PH	H1025 to H1150	strength with good ductility and cost	316 or duplex for corrosion-led duty
Valve stem and trim	17-4PH	H1025	wear resistance with corrosion resistance	PH 13-8Mo where chlorides are present
Gear and spline	17-4PH	H900 to H1025	high strength with machinability	PH 13-8Mo for higher load
General high-strength fastener	17-4PH	H1025	strength at moderate cost	PH 13-8Mo for aerospace
Aerospace structural fitting	PH 13-8Mo	H1000 to H1050	strength with toughness	15-5PH or 17-4PH for lower duty
Landing gear component	PH 13-8Mo	H1000	toughness plus SCC resistance	specialised higher alloys
High-strength fastener, aerospace	PH 13-8Mo	H950 to H1000	strength with cracking resistance	A286 or Inconel 718 for high temperature
Part in chloride service at high strength	PH 13-8Mo	H1050 or over-aged	resists stress corrosion cracking	17-4PH over-aged as a lower-cost option
Instrument and transducer body	17-4PH	H1025	dimensional stability plus cost	316 for corrosion-led duty

Table note: A component-level guide, not a code or a design basis; the recommended grade and condition must be confirmed against the governing specification of the equipment and against the actual load, temperature and environment, and the alternatives listed indicate the boundary of the two grades rather than being an exhaustive list. The condition column is indicative: a lower H-number gives higher strength and lower ductility, and the final choice of condition should follow from the required strength, the required toughness and the cracking risk in the specific environment. Where a part combines high strength with a chloride-bearing environment, an over-aged condition of 17-4PH is often a better engineering answer than a peak-aged condition, and PH 13-8Mo is the better answer than either.

Two substitution errors are common and both are avoidable. The first is substituting 17-4PH for PH 13-8Mo on a strength-limited aerospace part, which reduces the margin on toughness and cracking resistance at the point where those margins are the reason the part was designed as it was; the substitution may satisfy a room-temperature tensile test and still change the fatigue and fracture behaviour of the component. The second is substituting PH 13-8Mo for 17-4PH on a general part to "be safe", which adds cost, reduces availability and often complicates machining and welding for no engineering benefit. The correct approach is to identify which property limits the part, and to choose the grade and the condition that satisfy that property with an adequate margin while leaving the other properties to be satisfied as economically as possible. Where a part sits in the overlap strength band and the environment contains chlorides, the deciding question is usually the cracking risk rather than the strength itself.

Price, Stock and Procurement Notes for 2026

Price follows both the alloy content and the melting route. 17-4PH is a high-volume grade produced in conventional melts and stocked widely, while PH 13-8Mo is a lower-volume grade commonly produced by vacuum induction melting and vacuum arc remelting for cleanliness, and that melting route carries a cost that shows up in the price. The figures below are reference ranges only and they are not a quotation; the contract that governs is the one signed with the supplier.

Item	Grade	Reference range, 2026, EXW Shanghai	Basis
Round bar, 20-100 mm	17-4PH	USD 12-22/kg	2026, EXW Shanghai, reference range only
Round bar, 20-100 mm	PH 13-8Mo	USD 35-65/kg	2026, EXW Shanghai, reference range only
Plate, 5-50 mm	17-4PH	USD 14-26/kg	2026, EXW Shanghai, reference range only
Plate, 5-50 mm	PH 13-8Mo	USD 40-75/kg	2026, EXW Shanghai, reference range only
Wire and fastener stock	17-4PH	USD 16-30/kg	2026, EXW Shanghai, reference range only
Forging stock	PH 13-8Mo	quotation on drawing	2026, EXW Shanghai, reference range only
Vacuum melting adder	PH 13-8Mo only	included in the price range above	VIM / VAR route
Ageing treatment adder	both grades	quoted with the heat treatment	2026, EXW Shanghai, reference range only

Table note: Reference ranges only, 2026, EXW Shanghai, in USD per kilogram, and subject to movement in the nickel, molybdenum and chromium markets as well as in energy and alloying costs; these figures are indicative and are not a quotation. Delivered prices depend on quantity, specification, form, tolerance, test scope, documentation, delivery terms and on whether the material is stock or a mill order. The wide gap between the two grades reflects both the higher alloy content of PH 13-8Mo and its vacuum melting route, and it is the reason a substitution decision has a real cost consequence rather than being a paper exercise.

Three procurement points complete the picture. First, always name the condition, because the condition is what fixes the properties and the price, and an enquiry that omits it cannot be compared with one that includes it. Second, expect the availability of PH 13-8Mo to be narrower than that of 17-4PH in both product form and size, so a mill order and its lead time should be planned rather than assumed, particularly for sheet and plate and for large forging stock. Third, require the documentary package appropriate to the grade: an inspection document to EN 10204 3.1, heat-number traceability, positive material identification to ASTM E1476 to confirm the grade, verification that the delivered condition matches the ordered condition from the heat-treatment record, and where the part is fracture-critical, the fracture toughness results to ASTM E399 or the SCC results that the design relies on. Our news and buying guides carry commentary on which grades and forms are tight in a given quarter, and our contact page is the place to send a component list for a stock-versus-mill-order assessment.

Standard Index

Standard	Title / scope	Covers	Form
ASTM A564	Precipitation-hardening stainless steel bars, forgings and forging stock	composition + mechanical	bar, forging
ASTM A693	Precipitation-hardening stainless steel plate, sheet and strip	composition + mechanical	plate, sheet, strip
ASME SA564 / SA693	ASME equivalents of the ASTM product standards	composition + mechanical	all forms
AMS 5643	17-4PH bars, forgings, rings and forging stock	mechanical + heat treatment	bar, forging, ring
AMS 5604	17-4PH sheet, strip and plate	mechanical + heat treatment	sheet, strip, plate
AMS 5629	PH 13-8Mo bars, wire, forgings, rings and extrusions	mechanical + heat treatment	bar, forging, ring
AMS 2750	Pyrometry for heat-treatment equipment	furnace control	all forms
ASTM E8/E8M	Tension testing of metallic materials	test method	all forms
ASTM A370	Mechanical testing of steel products	test method	all forms
ASTM E18	Rockwell hardness testing	test method	all forms
ASTM E399	Plane-strain fracture toughness testing	test method	all forms
ASTM G48	Pitting and crevice corrosion testing	test method	all forms
ASTM G36	Boiling magnesium chloride stress corrosion testing	test method	all forms
ASTM G49	Preparation and use of SCC tension specimens	test method	all forms
ASTM E1476	Metals identification and PMI	test method	all forms
ASTM E112	Determining average grain size	test method	all forms
EN 10204	Inspection document types (2.2, 3.1, 3.2)	inspection documents	all forms
EN 10021	General technical delivery requirements	delivery conditions	all forms
ISO 9001	Quality management systems	system certification	all forms
NACE MR0175 / ISO 15156	Materials for sour oil and gas service	material selection	all forms

Table note: Standards are listed by number and scope; where an edition year is not quoted the latest edition applies and the controlling edition is the one named on the purchase order. The coverage of individual AMS documents should be confirmed against the current AMS index before a document is named on an order, because coverage differs between forms and between grades. Where a grade is quoted by a trade name alone without a UNS number, a condition and a product standard, the quotation cannot be verified against a specification, so the purchase order should carry all four items.

FAQ

Q1: What is the main difference between 17-4PH and PH 13-8Mo?

The main difference is the hardening mechanism and what it delivers at high strength. 17-4PH hardens through copper-rich precipitates and is a general engineering grade that balances strength, corrosion resistance and cost. PH 13-8Mo hardens through a nickel-aluminium phase and is a premium aerospace grade that delivers higher strength together with better fracture toughness and markedly better resistance to chloride stress corrosion cracking. In numbers, PH 13-8Mo at a comparable ductility level is typically 15 to 25 per cent stronger than 17-4PH, and the toughness and cracking-resistance gap is wider than the strength gap. Everything else in the comparison follows from that: 17-4PH is cheaper, more widely stocked and easier to machine and weld, while PH 13-8Mo is produced by vacuum melting, is available in fewer forms and sizes, and is chosen when the part is strength-critical or safety-critical.

Q2: Which grade is stronger, 17-4PH or PH 13-8Mo?

PH 13-8Mo is the stronger grade across its condition range, and it is also more ductile at a given strength. In typical terms, 17-4PH ranges from roughly 860 MPa tensile in the over-aged H1150 condition to roughly 1310 MPa in the peak-strength H900 condition, while PH 13-8Mo ranges from roughly 1170 MPa in H1100 to roughly 1520 MPa in H950. The comparison that matters is between conditions of similar ductility rather than between the peak figures of each grade, and on that basis PH 13-8Mo holds a real advantage of roughly 20 per cent in strength while retaining comparable elongation. The practical point for a buyer is that the two grades overlap in the band between roughly 1100 and 1300 MPa, and within that band the choice is decided by toughness, cracking resistance and cost rather than by strength.

Q3: Can I substitute 17-4PH for PH 13-8Mo?

You can substitute in some applications, but the substitution must be justified against the property that limits the part, and it should never be treated as a like-for-like change. If the part was designed around strength alone and the environment is benign, 17-4PH in a comparable condition may serve at lower cost. If the part was designed around toughness or resistance to chloride stress corrosion cracking, which is often why PH 13-8Mo was specified in the first place, then 17-4PH is a downgrade on the governing property and may pass a tensile test while reducing the margin that protects the component. The reverse substitution, using PH 13-8Mo where 17-4PH was adequate, is safe technically but wasteful, and it complicates machining, welding and availability for no benefit. Both substitutions should be treated as engineering changes and approved against the load case and the environment.

Q4: What does the H-number in 17-4PH H900 or PH 13-8Mo H1000 mean?

The H-number is the ageing condition designation and it encodes the nominal ageing temperature. A lower number means a lower ageing temperature and therefore higher strength with lower ductility; a higher number means a higher ageing temperature and therefore lower strength with better ductility and toughness. So 17-4PH H900 is the peak-strength condition and 17-4PH H1150 is the most ductile, with H1025 and H1075 in between, and the same logic applies to PH 13-8Mo across H950 to H1100. The condition is part of the specification rather than an optional extra, because the same alloy supplied in two conditions is two different products with different properties and different prices. This is also where the corrosion trade-off sits: the low-temperature, high-strength conditions are the ones most susceptible to stress corrosion cracking, so the condition choice is a design decision and not just a strength selection.

Q5: Which grade is better for chloride stress corrosion cracking?

PH 13-8Mo is clearly better. At comparable high strength, 17-4PH is appreciably more susceptible to chloride-induced stress corrosion cracking, which is the reason PH 13-8Mo is preferred for high-strength parts that see chlorides and the reason 17-4PH parts in such service are frequently specified in an over-aged condition such as H1150. The mechanism is metallurgical: the microstructure that produces peak strength in 17-4PH is also the one most sensitive to cracking in a chloride environment, whereas PH 13-8Mo's nickel-aluminium hardening and its molybdenum content give it more resistance at the same strength level. Where a part must combine high strength with a chloride-bearing environment, the correct options are to select PH 13-8Mo, to over-age the 17-4PH, or to change the alloy family altogether to an austenitic or nickel alloy with real chloride resistance. Ignoring the cracking risk and specifying a peak-aged 17-4PH is the option that fails in service.

Q6: Are 17-4PH and PH 13-8Mo weldable?

Both are weldable but both require procedures that account for the effect of the weld thermal cycle, and neither should be welded in the fully aged condition without a qualified procedure. 17-4PH is the more forgiving of the two in a fabrication shop: it can be welded with matching or austenitic fillers and the welded assembly is normally re-solution-treated and aged, or aged after welding, to restore properties in the weld and heat-affected zone. PH 13-8Mo is welded less often because it usually appears as a machined structural part rather than a fabrication, and where it is welded the procedure must be qualified and the post-weld heat treatment defined, since the aged microstructure is disturbed by welding and the affected zone may not recover its design properties. In both cases the sensible sequence is to weld in the solution-treated condition, to heat treat after welding, and to verify the final condition from the heat-treatment record.

Q7: Which grade should I specify for a valve stem or a pump shaft?

For a general pump shaft or valve stem in moderate service, 17-4PH in a mid-range condition such as H1025 is the usual answer, because it combines adequate strength and wear resistance with good ductility, reasonable corrosion resistance and a favourable price. Where the same component sees chlorides or operates at high strength with a cracking risk, PH 13-8Mo becomes the better choice, and an over-aged 17-4PH is the fallback option if cost rules out the premium grade. Where the corrosion duty is more severe than either grade can handle, such as a strongly acidic or high-chloride process stream, the correct move is to change family rather than to change condition, and a nickel alloy such as Monel 400, Inconel 625 or a super-austenitic stainless steel is the appropriate answer. Our stainless steel range and Inconel alloy range cover those alternatives.

Q8: Why is PH 13-8Mo more expensive than 17-4PH?

Two reasons, and both are structural rather than commercial. First, PH 13-8Mo contains more nickel and molybdenum than 17-4PH and it does not rely on the cheaper copper addition, so its raw-material cost is higher. Second, PH 13-8Mo is normally produced by vacuum induction melting and vacuum arc remelting, because the cleanliness that the vacuum route delivers is part of what the grade is bought for, and that melting route adds cost. On top of the melting cost sits the availability factor: 17-4PH is a high-volume grade stocked in many forms and sizes, while PH 13-8Mo is a lower-volume grade with a narrower stock position, so the mill order premium is larger. The practical consequence is that PH 13-8Mo should be specified where its properties are needed and not as a general-purpose upgrade, because using it on parts that are not strength- or toughness-limited converts a material advantage into pure cost.

Q9: Which grade has better corrosion resistance in general service?

In general corrosion terms the two grades are broadly similar and both sit below the austenitic stainless steels, because both are martensitic precipitation-hardening steels with chromium in the 12 to 17.5 per cent range and neither is intended as a general corrosion material. The difference that matters is not general corrosion but localised attack and cracking: PH 13-8Mo is more resistant to chloride stress corrosion cracking at high strength, while 17-4PH is more susceptible in its peak-strength conditions. Where a part needs real corrosion resistance rather than strength, the answer is generally not to choose between these two grades at all but to move to a 316L, a duplex, a super-austenitic stainless steel such as 904L, AL-6XN or 254SMO, or a nickel alloy such as Monel or Inconel, depending on the specific medium. Choosing between 17-4PH and PH 13-8Mo is a strength and toughness decision with a corrosion constraint, not a corrosion decision.

Q10: How do I verify that the delivered material is the correct grade and condition?

Start with positive material identification to ASTM E1476, which confirms the alloy family and detects the most common substitution failures, and check the inspection document to EN 10204 3.1 against the heat number marked on the material and on the packing so that the physical item can be tied back to the tested heat. Then verify the condition separately, because a PMI check says nothing about heat treatment: the correct evidence is the heat-treatment record showing the ageing temperature and time, and a hardness or tensile test at the delivered condition. Where the part is fracture-critical, require the fracture toughness results to ASTM E399, and where it faces chlorides at strength, require the SCC evidence that the design relies on. Finally, where a third party is used, require the inspection to be witnessed and the certificate endorsed, which turns a 3.1 document into a 3.2 document.

Q11: How does Shanghai Hangbo Alloy Group supply 17-4PH and PH 13-8Mo?

Shanghai Hangbo Alloy Group Co., Ltd. supplies 17-4PH and PH 13-8Mo in bar, plate, sheet, strip, wire and forging stock, with both grades available in the common ageing conditions and with the higher-strength nickel and superalloy alternatives alongside them for duties that exceed what a precipitation-hardening stainless steel can handle. We issue an inspection document to EN 10204 3.1 with third-party inspection by SGS, BV or TUV and positive material identification to ASTM E1476 on delivery, and we operate an ISO 9001:2015 quality system. For parts where the condition is critical we return the intended ageing treatment and the verifying test with the quotation, so that the condition is agreed rather than discovered, and for parts that face chlorides at high strength we will discuss whether an over-aged condition or a change of alloy family is the better answer. Send the component list, the forms, the quantities and the service conditions through our contact page and we will return a recommendation with the standards, the condition and the lead time stated.

Conclusion: Decide on Toughness, Then on Cost

17-4PH and PH 13-8Mo are both precipitation-hardening martensitic stainless steels, and they are frequently quoted as alternatives when the correct relationship is that PH 13-8Mo is the higher-performance grade in the same family. 17-4PH hardens through copper precipitates and is the volume engineering grade: affordable, widely stocked, machinable, weldable and adequate for pump shafts, valve stems, gears and general fasteners in moderate service. PH 13-8Mo hardens through a nickel-aluminium phase and is the aerospace and high-performance grade: stronger at comparable ductility, tougher, and markedly more resistant to chloride stress corrosion cracking at high strength, which is why it is specified for structural parts, landing gear components and high-strength fasteners where the margin against cracking is a design requirement.

The decision rule is therefore to identify which property limits the part. If the part is limited by strength in a benign environment, 17-4PH in an appropriate condition is the economic and correct answer. If the part is limited by toughness or by cracking resistance at strength, PH 13-8Mo is the correct answer and 17-4PH is a downgrade on the governing property, even though both will pass the same room-temperature tensile test. The condition is as important as the grade, because the low-temperature peak-strength conditions in either alloy are the ones most exposed to stress corrosion cracking, and an over-aged condition is often the better engineering choice than a peak-aged one. In 2026, 17-4PH remains the cheaper and more available grade and PH 13-8Mo the premium vacuum-melted one, and both are supplied by Shanghai Hangbo Alloy Group Co., Ltd. with ISO 9001:2015 process control, mill test certification to EN 10204 3.1, third-party inspection by SGS, BV or TUV, and PMI verification to ASTM E1476 on delivery. Send your component list and service conditions through our contact page and we will return a grade and condition recommendation with the standards and the lead time stated.

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Shanghai Hangbo Alloy Group Co., Ltd. - China Supplier

Email: sales@hangboalloy.com | WhatsApp (Lisa): +86 13611656360 | www.nickel-alloy.com

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Ni: NI200-N6 | Monel: 400-502, K-500, R-405 | Inconel: 600-740H, X-750, 725 | Incoloy: 800-27-7MO | Hastelloy: C-276-HYBRID-BC1, N | Super Austenitic: 904L-AL-6XN | Chromium-Rich: Alloy 33 | High-Strength Stainless: Nitronic 50/60, PH13-8Mo | Chlorination: Alloy 45 | Nimonic: 75-263 | Haynes: 25-HR-160 | RA: 330/333 | Precision: 1J22-4J45 | High-Strength: 18Ni250-AerMet100 | Heating: Cr20Ni80/60 | Specialty: Alloy 20-602CA

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