HOT D. SCOTT MACKENZIE, PH.D.

, FASM

SEAT
SENIOR RESEARCH SCIENTIST–METALLURGY   
QUAKER HOUGHTON INC.

Precipitation hardening stainless steels

The top alloy groups have seen increased formation. Aging at temperature allows the precipitation hardening
mechanism to occur.
growth in application because of corrosion
resistance, strength, and low distortion upon AUSTENITIC PRECIPITATION HARDENING
STAINLESS STEELS
precipitation hardening treatment. This group of alloys has lower mechanical properties than the other
two groups of precipitation hardenable stainless steels but has good

I n this column, we will discuss precipitation hardening steels and

their physical metallurgy.
Precipitation hardened stainless steels are a class of stainless steels
creep resistance and holds their properties to temperatures to as high
as 704°C (1,300°F). Precipitation occurs during aging at 650°-750°C.

that can be hardened to significant strength levels by heat treatment. PHYSICAL METALLURGY
These alloys were first introduced in 1946 [1] to fill the need of high- There are predominantly two different crystal structures for
strength, corrosion-resistant alloys that would be capable of operating precipitation hardening stainless steels – ferrite and austenite.
at elevated temperatures. Since their initial
creation, numerous different alloys have Common Type Typical Chemical Analysis %
been created. These alloys are now widely Name C Mn Cr Ni Mo Cu Al Ti Others
used in aerospace, marine, automotive,
A 286 Austenitic 0.04 1.45 15.25 26.00 1.25 – 0.15 2.15 V 0.25,
paper, nuclear, petrochemical, and other
B 0.007
applications. These alloys are used whenever
a combination of high strength, corrosion 17-10PH Austenitic 0.07 0.75 17.20 10.80 – – – – P 0.28
resistance, and toughness is required. 13-8Mo Martensitic 0.05 0.10 12.75 8.00 2.25 – 1.25 – –
Precipitation hardening is achieved by the
addition of copper, molybdenum, aluminum, 17-4PH Martensitic 0.05 0.75 16.50 4.25 – 4.25 – – Nb 0.3
and titanium. These alloys are generally Custom 455 Martensitic 0.05 0.50 11.50 8.50 0.50 2.00 – 1.10 Nb + Ta 0.4
solution heat treated at the mill, fabricated at
17-7PH Semi-austenitic 0.06 0.70 17.25 7.25 – – 1.25 – –
shop (forming and machining), then aged to
achieve the desired mechanical properties. AM-350 Semi-austenitic 0.09 0.80 16.50 4.30 – 2.75 – – –
The age hardening step then precipitates PH 15-7 Mo Semi-austenitic 0.06 0.70 15.50 7.25 2.60 – 1.30 – –
the hard intermetallics that significantly
increase hardness and strength. Table 1: Typical compositions of some precipitation hardening stainless steels.
Precipitation hardening stainless steels
are divided into three main groups of alloys: martensitic; semi- Ferrite is a body centered cubic structure while austenite is a face
austenitic; and austenitic. Typical chemistries of common alloys in centered cubic structure. Chromium, molybdenum, vanadium, and
each group are shown in Table 1. niobium are ferrite stabilizers. Nickel, manganese, copper, and cobalt
are austenite stabilizers.
MARTENSITIC PRECIPITATION HARDENING Solubility of the alloying elements increases at higher tempera-
STAINLESS STEELS tures, meaning that the martensite start and finish temperatures
These stainless steels are typically used as bar or forging stock, but can can be controlled by the solution heat-treating temperatures. At high
be available as castings, sheet, or plate. Cold forming of these alloys is solution heat-treating temperature, the alloy content of austenite is
difficult because of the untampered martensitic structure developed increased, and the martensite start temperature is depressed. At lower
during solution heat treatment. Alloys in this condition have relative solution heat-treating temperatures, the austenite is leaner in alloy
low ductility and high strength. Hardening by a single aging treatment content (less in solution) and upon cooling transforms to martensite.
will produce yield strengths from 1,170 MPa to 1,376 MPa (170-200 Ksi). These alloys are called semi-austenitic precipitation hardenable alloys.
These alloys can be used at temperatures up to 482°C (900°F). The effect of alloying content on the type of alloy is shown in Figure 1.
The primary precipitation hardening elements in these stainless
SEMI-AUSTENITIC PRECIPITATION HARDENING steels are aluminum, titanium, and copper. There are three basic steps
STAINLESS STEELS to hardening these alloys. While there are differences between the
These alloys are predominantly produced as sheet because the aus- different groups, they all follow the same scheme.
tenitic structure after solution heat treatment provides excellent First there is solution heat treatment. As with aluminum alloys,
formability. Mechanical deformation after solution heat treatment this temperature is chosen to dissolve all the solute atoms in solution.
transforms the austenite present from solution heat treatment to a The amount of time, and the temperature are chosen depending on
martensitic structure. Refrigeration can also drive martensite trans- the alloying elements present. As indicated previously, time and
26     gearsolutions.com
 “Condition A.” In this condition, the material is readily machined to
the desired shape. After machining, the part is aged to the desired
properties. One of the advantages of precipitation hardening stainless
steel is the ability to readily machine parts in Condition A, and age
them at moderate temperatures to the final strength. The typical con-
traction from hardening this group of alloys during aging is extremely
small. Aging from Condition A to 900°F, the resulting contraction is
0.0004-0.0006 inches per inch. The contraction from aging Condition
A at 1,150°F is 0.0009-0.0012 inches per inch [2].

SEMI-AUSTENITIC PRECIPITATION HARDENING

STAINLESS STEELS
These steels are austenitic in the solution-treated condition, and trans-
Figure 1: Effect of alloying content on the transformation temperature of formed to martensite either by mechanical working or by thermal
precipitation hardening stainless steels [1]. treatment. Additional strengthening occurs during aging. An addition-
al austenite conditioning step is required before aging. Refrigeration at
-100°F after rapid cooling from the conditioning step helps to achieve
the desired peak strength. These alloys have a much more complex
heat-treatment cycle and mechanical working to achieve the very high
strengths. Space does not allow a full discussion of the interactions
between austenite conditioning, aging, and mechanical deformation.

AUSTENITIC PRECIPITATION HARDENING STEELS

In these alloys, sufficient austenite stabilizers are present (such as
nickel) to maintain an austenitic structure. The martensite start
temperature is lowered, so that it is room temperature or below. This
group of alloys contains titanium and aluminum, and hardens by the
Figure 2: Effect of aging temperature on the yield strength of a typical martensitic formation of Ni3 (Al,Ti) during precipitation hardening.
precipitation hardening stainless steel [3]. The heat treatment of this group of alloys consists of solution treat-
temperature can be adjusted to change the martensitic transformation. ment at 982°C (1,800°F), and cooling rapidly (typically an oil or polymer
Secondly, a supersaturated solid solution must be created. This is quench) to Condition A. The alloy is then fabricated, and subsequently
accomplished by quenching. If the alloy transforms to martensite upon aged at 704°C (1,300°F) for 16 hours, then air cooled [4]. These alloys
quenching, greater supersaturation occurs because of limited solubility have a much lower yield strength (620 MPa) than the martensitic or
of the alloying elements in martensite. This means the surrounding semi-austenitic grades.
matrix has a greater alloying content, and greater supersaturation.
This martensite reaction can be driven to completion by refrigeration CONCLUSIONS
to below the martensite finish temperature. In this column we introduced the different types of precipitation
The last step in the process is precipitation hardening or aging. hardening stainless steels, and briefly discussed the thermal
Since the diffusion rate is so slow because of the temperatures treatments and physical metallurgy of the three alloy groups within
involved, natural aging does not occur. Upon application of elevated the broader precipitation hardening grades of stainless steels.
temperature, diffusion occurs, and fine precipitates occur along crys- These alloys have seen increased growth in application because
tallographic planes of the matrix. A highly strained matrix results, of corrosion resistance, strength, and — importantly — low distortion
with a mismatch occurring between the matrix and the precipitate. upon precipitation hardening treatment. 
As in aluminum, the size and morphology of the precipitates can be
controlled by different times and temperatures. Long exposure at low REFERENCES
temperatures creates many fine precipitates which, in turn, increases [ 1 ] C. J. Slunder, A. F. Hoenie and A. M. Hall, "Thermal and Mechanical Treatment
the strength. Intermediate temperatures can still achieve maximum for Precipitation-Hardening Stainless Steels," NASA, Washington D.C., 1967.
strengthening, but a shorter cycle. At high temperatures, the precipi- [ 2 ] AK Steel International, ARMCO PH 13-8 MO Stainless Steel Product Data
tates lose coherency with the matrix. The precipitates shear, reducing Bulletin, Barcelona, Spain, 2019.
the strain between the matrix and the precipitate, and the structure [ 3 ] Allegheny Technologies International, Stainless Steel 17-4 Precipitation
becomes overaged. Hardening Alloy, Pittsburgh, PA, 2006.
[ 4 ] L. Zubeck, "A Technical Review of Precipitation Hardening Steel Grades,"
MARTENSITIC PH STAINLESS STEELS Springs, no. January, pp. 14-16, 2006.
This material is typically solution heat treated at 1,037°±13°C (1,900°F)
[ 5 ] Allegheny Technologies, AM350 Technical Data Sheet, Pittsburgh, PA, 2012.
and quenched at the mill. Product forms are bar, plate, sheet, and billet.
This solution heat treated and quenched thermal treatment is called [ 6 ] AK Steel, 17-7 PH Stainless Steel, West Chester, OH, 2020.

ABOUT THE AUTHOR D. Scott MacKenzie, Ph.D., FASM, is senior research scientist-metallurgy at Quaker Houghton Inc. For more
information, go to https://home.quakerhoughton.com/

July 2021     27