Precipitation Hardening Stainless Steel: Grades and Temper Designations
Send an enquiryPrecipitation hardening (PH) stainless steels are a family of stainless steels strengthened by a low-temperature ageing treatment rather than by quenching. The steel is supplied in a soft, machinable condition, machined to finished geometry, and then aged at a low temperature, which across most of the family falls between 480 and 620 degrees Celsius. During ageing, dissolved elements come out of solution as sub-microscopic particles that obstruct dislocation movement, raising the strength of the steel while producing only a small, predictable dimensional change.
This sequence distinguishes the family from conventional quench-hardened steels. Quench hardening a martensitic grade transforms the crystal structure, which involves a volume change and thermal shock and therefore distortion, so finished dimensions must be recovered by grinding after hardening. Precipitation hardening grows particles inside a structure that is already present, so a part can be machined once, in the soft condition, and hardened afterwards with minimal distortion.
Mechanism
The process comprises three steps.
- Solution treatment. The steel is held at a temperature high enough for the hardening elements, which are copper, aluminium, titanium or niobium depending on the grade, to dissolve fully into the matrix. Cooling then retains them in an unstable solid solution. This state is designated Condition A, in which the steel is soft, machinable and dimensionally stable.
- Machining. The part is cut to its finished geometry while the steel remains in the soft condition.
- Ageing. The part is held at a low temperature, typically between 480 and 620 degrees Celsius, for a few hours. The dissolved elements precipitate as sub-microscopic particles distributed through the matrix, and these particles block the movement of dislocations, producing a large increase in strength. Because the temperature is low and no quench is involved, distortion is minimal and the dimensional change is small and predictable.
Two secondary effects follow from the low ageing temperature. Scale and discolouration are minimal, so a finished surface is preserved through the hardening step. The dimensional change on ageing is also repeatable, which allows it to be compensated for in the pre-age dimensions rather than corrected by rework afterwards.
Grades and classification
The Laxcon grade register carries nine precipitation hardening grades. They are classified by the precipitating element and by the matrix structure, and this classification determines most of the behaviour of each grade.
| Grade | Type | Hardening element | Cr % | Ni % | Also |
|---|---|---|---|---|---|
| 17/4 PH | Martensitic | Copper | 15.0 to 17.5 | 3.0 to 5.0 | Cu 3.0 to 5.0, Nb plus Ta 0.15 to 0.45 |
| DIN 1.4542 | Martensitic | Copper | 15.0 to 17.0 | 3.0 to 5.0 | Cu 3.0 to 5.0, tighter sulphur at 0.015 maximum |
| 15/5 PH | Martensitic | Copper | 14.0 to 15.5 | 3.5 to 5.5 | Cu 2.5 to 4.5. A ferrite-free refinement of 17/4 |
| Custom 450 | Martensitic | Copper and niobium | 14.0 to 16.0 | 5.0 to 7.0 | Cu 1.25 to 1.75, Mo 0.5 to 1.0 |
| Custom 455 | Martensitic | Titanium and copper | 11.0 to 12.5 | 7.5 to 9.5 | Ti 0.9 to 1.4, Cu 1.5 to 2.5 |
| 13/8 Mo PH | Martensitic | Aluminium | 12.25 to 13.25 | 7.5 to 8.5 | Al 0.90 to 1.35, Mo 2.0 to 2.5, C 0.05 maximum |
| 17/7 PH | Semi-austenitic | Aluminium | 16.0 to 18.0 | 6.5 to 7.75 | Al 0.75 to 1.50 |
| 15/7 Mo PH | Semi-austenitic | Aluminium | 14.0 to 16.0 | 6.5 to 7.75 | Al 0.75 to 1.50, Mo 2.0 to 3.0 |
| A 286 | Austenitic | Titanium | 13.5 to 16.0 | 24.0 to 27.0 | Ti 1.90 to 2.35, Mo 1.0 to 1.5, boron |
The three structural types
Martensitic PH grades transform to martensite on cooling from solution treatment and are then aged. 17/4 PH and 15/5 PH together account for most of the family's use. These grades are magnetic, machine reasonably in Condition A, and have corrosion resistance between that of grade 410 and grade 304.
Semi-austenitic PH grades remain austenitic after solution treatment, and can therefore be formed and bent in that condition. A conditioning treatment then transforms them to martensite before ageing. The additional step provides formability that the martensitic grades lack. 17/7 PH is the usual choice for springs and strip parts.
Austenitic PH grades do not transform. A 286 remains austenitic and hardens on titanium precipitates. It is the member of the family that retains its strength at high temperature and remains non-magnetic. Its nickel content of 24.0 to 27.0 percent makes it correspondingly expensive.
Grade names and specified ranges
Grade names are nominal. 17/4 PH is named for approximately 17 percent chromium and 4 percent nickel, but the specification permits chromium from 15.0 to 17.5 percent and nickel from 3.0 to 5.0 percent, so a heat at 15.2 percent chromium is compliant 17/4 PH. The same applies to 304 and its nominal 18 percent chromium. The certified analysis, not the grade name, states the actual composition.
Temper designations
The published heat treatment conditions are solution annealing, H900, H925, H1025, H1075, H1100, H1150, H1150D and H1150M. The number in each H designation is the ageing temperature in degrees Fahrenheit, and it sets the position of the material on the strength-toughness curve. The H scheme belongs to the martensitic grades of the family, 17/4 PH and 15/5 PH and their relatives. The semi-austenitic grades reach an aged condition only after the conditioning treatment described above, and are ordered under their own condition designations; austenitic A 286 does not transform at all and ages on a schedule of its own, at a higher temperature and for longer than the H conditions in the table.
| Condition | Ageing temperature | Strength | Toughness and ductility | Typically chosen for |
|---|---|---|---|---|
| Condition A | none, solution treated only | Lowest | Low, and not a service condition | The condition parts are machined in |
| H900 | 900 F, about 482 C | Highest | Lowest | Maximum strength and hardness where the part is not impact loaded |
| H925 | 925 F, about 496 C | Very high | Slightly better | A small step back from H900 |
| H1025 | 1025 F, about 552 C | High | Improving | General engineering compromise |
| H1075 | 1075 F, about 579 C | Moderately high | Good | Shafts and pressure parts |
| H1100 | 1100 F, about 593 C | Moderate | Good | Where some impact duty exists |
| H1150 | 1150 F, about 621 C | Lowest of the aged conditions | Best of the single-stage conditions | Toughness-led applications |
| H1150D | 1150 F, applied twice | Lower still | Better still | Where a code requires impact values a single age cannot reach |
| H1150M | a higher first stage, then 1150 F | Lowest | Maximum | Maximum toughness, including low-temperature service |
Each step up in ageing temperature trades strength for toughness, and the two double treatments, H1150D and H1150M, extend the range beyond the lowest-strength single-stage condition. An order for a PH grade without a stated condition is incomplete, since the condition determines the mechanical properties of the delivered material.
For oil and gas service, NACE MR0175 restricts the conditions acceptable for sour service on martensitic PH grades. The double-aged conditions exist in part because that specification demanded them, and where sour service applies the condition is a requirement rather than a preference.
Applications
Published applications are oil and gas, power, offshore, chemical, nuclear, food, aerospace, pulp and paper, high-pressure pump and valve components, measuring and control, and mechanical and welding applications.
The common feature is a part that is geometrically complicated and highly loaded, such as a valve stem, a pump shaft, an impeller or an aerospace fitting. Producing such a part in a quench-hardened grade requires machining, hardening, and then grinding back to size on every critical feature to recover the distortion. Producing it in a PH grade requires machining once, followed by ageing.
Comparison with other stainless families
| Compared with | PH grades win on | PH grades lose on |
|---|---|---|
| Austenitic, such as 304 and 316 | Strength, by a wide margin. Hardness | Corrosion resistance, weldability, formability, cost |
| Martensitic, such as 420 and 431 | Distortion on hardening, toughness at strength, corrosion resistance | Cost, and outright hardness for wear surfaces |
| Duplex | Absolute strength, and the machine-then-harden route | Chloride resistance, and cost per tonne on a structural section |
Ordering requirements
A complete order for PH stainless bar specifies five items.
- The grade, against a standard. Published standards are AMS 5642, AMS 5622, ASTM A564 and DIN or EN 10088-3.
- The condition. Condition A where the purchaser will carry out the ageing, or the H designation where the material is to be supplied aged.
- Diameter and tolerance class, from the relevant bar product. PH grades are supplied across the bright bar, PSQ and hot rolled ranges.
- Any code requirement, such as NACE MR0175 for sour service, which constrains the acceptable conditions.
- Testing. Hardness after ageing, impact testing at temperature, and ultrasonic inspection must each be named where required.
Frequently asked questions
What does PH stand for in stainless steel?
PH stands for precipitation hardening. It names the strengthening mechanism, in which sub-microscopic particles formed by a low-temperature ageing treatment obstruct deformation. It is not a grade or a family of chemistry, which is why nine grades of quite different composition share the label.
What is the difference between 17-4 PH and 15-5 PH?
15/5 PH is a ferrite-free refinement of 17/4 PH. Its chromium range of 14.0 to 15.5 percent is slightly lower than the 15.0 to 17.5 percent of 17/4 PH, and its nickel range of 3.5 to 5.5 percent is slightly higher than 3.0 to 5.0. The removal of delta ferrite improves transverse toughness, which matters on larger sections.
What is Condition A?
Condition A is the solution treated, unaged state. It is the softest and most machinable condition, and it is the condition in which PH bar is normally supplied so that the purchaser can machine the part and then age it. It is not a finished condition for a loaded part.
Is 17-4 PH the same as grade 630?
Grade 630 is the UNS-derived designation used for the same steel in several standards, and 1.4542 is the European number. The Laxcon grade register publishes the composition under each name it appears in, and the windows are not always identical: 1.4542 holds sulphur to 0.015 percent maximum, against 0.030 percent for the AISI form.
Can PH stainless be welded?
PH stainless can be welded, which is one of its advantages over the high-carbon martensitic grades. Carbon is held to 0.07 percent maximum in the copper-hardened grades and 0.05 percent in 13/8 Mo PH, so the heat-affected zone does not harden into brittleness. The part is normally re-aged after welding to restore properties in the weld zone.
How corrosion resistant is 17-4 PH?
17-4 PH is more corrosion resistant than a straight martensitic grade and less so than 304. At 15.0 to 17.5 percent chromium with no molybdenum it will pit in chlorides, so it is not a seawater material. Where a duty combines high strength with substantial chloride exposure, a super duplex grade is the better choice.
Sources
Grades, standards and the full list of heat treatment conditions are the published supply conditions on the precipitation hardening steels product page. Every composition window is the published specification in the grade reference, which carries nine PH grades. The temper designation scheme, the ageing temperature ranges and the NACE MR0175 restriction are published standard practice rather than Laxcon measurements. Bar size ranges and tolerances are on the individual product pages.
