Precipitation hardening

What defines the precipitation hardening family?

Precipitation hardening stainless steel is a stainless steel family strengthened by precipitating a copper, aluminium or titanium-rich phase through a martensitic or austenitic matrix. It is supplied in a solution-treated condition that permits machining. A single ageing treatment at a few hundred degrees Celsius develops full strength. The family combines strength approaching that of alloy steel with corrosion resistance in the region of the 300 series.

The ageing treatment does not involve quenching and uses a low temperature. A finished part therefore remains close to its original dimensions. Martensitic stainless steel can reach similar strength, but it requires quenching from above 1000 degrees Celsius. This treatment can distort a finish-machined part. A precipitation hardening grade can be machined to completion before ageing, which removes the need for a correction operation.

The Laxcon Steels grade reference lists nine precipitation hardening grades with their full composition. The table below carries every one of them that has a cross-standard equivalent on record.

17-4 PH, 15-5 PH and 17-7 PH

17-4 PH, 15-5 PH and 17-7 PH cover most applications within the family. They differ in precipitate composition and matrix structure.

17-4 PH contains 15.0 to 17.5 percent chromium, 3.0 to 5.0 percent nickel, 3.0 to 5.0 percent copper and 0.15 to 0.45 percent niobium plus tantalum. It has a martensitic matrix and is strengthened by copper-rich precipitates. It is the most widely stocked and specified grade in the family. It is generally considered first unless an application excludes it.

15-5 PH is a re-balanced form of 17-4 PH that eliminates the residual ferrite that can occur in 17-4 PH. It contains 14.0 to 15.5 percent chromium and 3.5 to 5.5 percent nickel, with the same copper and niobium additions. Its fully martensitic structure provides better transverse toughness and through-thickness properties. These properties make it preferable when a heavy forging or thick plate is loaded across the grain.

17-7 PH contains 16.0 to 18.0 percent chromium, 6.5 to 7.75 percent nickel and 0.75 to 1.5 percent aluminium. It is a semi-austenitic grade. The material is initially austenitic and formable. A conditioning heat treatment or cold work transforms it to martensite. Ageing then forms aluminium-rich precipitates. This sequence permits springs, diaphragms, clips and formed sheet parts to be shaped in a soft condition and strengthened afterward.

The family also includes more specialised compositions. 13-8 Mo PH is a very clean, low-residual grade for aerospace use. It contains 2.0 to 2.5 percent molybdenum and has tight limits on silicon, phosphorus and sulphur. 15-7 Mo PH is the molybdenum-bearing counterpart of 17-7 PH. It provides better corrosion resistance in the same semi-austenitic form.

Ageing conditions and specifications

A precipitation hardening grade requires a specified condition. The H designations from H900 through H1150 indicate ageing temperatures in degrees Fahrenheit. Different conditions produce different properties from the same composition. H900 uses the lowest temperature and produces the highest strength and hardness. Each increase in ageing temperature reduces strength and increases toughness, ductility and resistance to stress corrosion cracking. H1150 M and double-aged variants provide still greater toughness.

Condition selection is an engineering decision. High-strength conditions can be susceptible to hydrogen and to stress corrosion cracking in chloride environments. Oil and gas and subsea specifications therefore commonly prohibit low-temperature ageing conditions. They instead require a double-aged or overaged condition with a maximum hardness limit. Selection of H900 solely for its higher strength can cause failure in service.

ASTM A564 covers hot-rolled and cold-finished age-hardening stainless bar and shapes. The specified condition accompanies this bar specification. ASTM A693 covers equivalent sheet, strip and plate.

Questions about precipitation hardening grades

Pitting resistance across the precipitation hardening family

Computed by Laxcon Steels from its own grade reference using PREN = Cr + 3.3Mo + 16N, the relationship published in Practical Guidelines for the Fabrication of Duplex Stainless Steels (International Molybdenum Association, third edition, 2014). The relationship is published for austenitic and duplex stainless steels, so these figures extend it, and the same source says the number is useful for ranking grades within one family rather than across families. Read the pitting resistance reference before acting on any of these figures: it states what the number does not predict.

Precipitation hardening equivalents across standards

Basis "verified" means every value in that row was checked against the governing standard on 2026-08-18; "inherited" means it came from the previous website and has not been checked. The complete mapping table is on the equivalents reference.

Where the precipitation hardening family is not the right answer

Precipitation hardening stainless steels are unsuitable for chloride environments when used at high strength. Low-temperature ageing conditions are susceptible to chloride stress corrosion cracking and hydrogen embrittlement. Overaged conditions improve resistance to both mechanisms but sacrifice much of the strength associated with the family. A duplex grade or nickel alloy is more suitable for a severe chloride environment.

The family is unsuitable for high-temperature service. Its strengthening precipitate is a low-temperature structure. Service above the ageing temperature continues the ageing process and softens the part.

The family is also unsuitable when corrosion resistance, rather than strength, is the primary requirement. A 300 series grade provides better performance in most aqueous environments at substantially lower cost. A grade designation without a condition is also incomplete because it defines the chemistry but not the strength, toughness or environmental resistance.

Precipitation hardening grades in detail

Product forms Laxcon lists in precipitation hardening steel