Precipitation hardening stainless steels

The top alloy groups have seen increased growth in application because of corrosion resistance, strength, and low distortion upon precipitation hardening treatment.

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In this column, we will discuss precipitation hardening steels and their physical metallurgy.

Precipitation hardened stainless steels are a class of stainless steels that can be hardened to significant strength levels by heat treatment. These alloys were first introduced in 1946 [1] to fill the need of high-strength, corrosion-resistant alloys that would be capable of operating at elevated temperatures. Since their initial creation, numerous different alloys have been created. These alloys are now widely used in aerospace, marine, automotive, paper, nuclear, petrochemical, and other applications. These alloys are used whenever a combination of high strength, corrosion resistance, and toughness is required. Precipitation hardening is achieved by the addition of copper, molybdenum, aluminum, and titanium. These alloys are generally solution heat treated at the mill, fabricated at shop (forming and machining), then aged to achieve the desired mechanical properties. The age hardening step then precipitates the hard intermetallics that significantly increase hardness and strength.

Precipitation hardening stainless steels are divided into three main groups of alloys: martensitic; semi-austenitic; and austenitic. Typical chemistries of common alloys in each group are shown in Table 1.

Table 1: Typical compositions of some precipitation hardening stainless steels.

Martensitic Precipitation Hardening Stainless Steels

These stainless steels are typically used as bar or forging stock, but can be available as castings, sheet, or plate. Cold forming of these alloys is difficult because of the untampered martensitic structure developed during solution heat treatment. Alloys in this condition have relative low ductility and high strength. Hardening by a single aging treatment will produce yield strengths from 1,170 MPa to 1,376 MPa (170-200 Ksi). These alloys can be used at temperatures up to 482°C (900°F).

Semi-Austenitic Precipitation Hardening Stainless Steels

These alloys are predominantly produced as sheet because the austenitic structure after solution heat treatment provides excellent formability. Mechanical deformation after solution heat treatment transforms the austenite present from solution heat treatment to a martensitic structure. Refrigeration can also drive martensite transformation. Aging at temperature allows the precipitation hardening mechanism to occur.

Austenitic Precipitation Hardening Stainless Steels

This group of alloys has lower mechanical properties than the other two groups of precipitation hardenable stainless steels but has good 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.

Physical Metallurgy

There are predominantly two different crystal structures for precipitation hardening stainless steels – ferrite and austenite. Ferrite is a body centered cubic structure while austenite is a face centered cubic structure. Chromium, molybdenum, vanadium, and niobium are ferrite stabilizers. Nickel, manganese, copper, and cobalt are austenite stabilizers.

Solubility of the alloying elements increases at higher temperatures, meaning that the martensite start and finish temperatures can be controlled by the solution heat-treating temperatures. At high solution heat-treating temperature, the alloy content of austenite is increased, and the martensite start temperature is depressed. At lower solution heat-treating temperatures, the austenite is leaner in alloy content (less in solution) and upon cooling transforms to martensite. These alloys are called semi-austenitic precipitation hardenable alloys. The effect of alloying content on the type of alloy is shown in Figure 1.

Figure 1: Effect of alloying content on the transformation temperature of precipitation hardening stainless steels [1].
Figure 1: Effect of alloying content on the transformation temperature of precipitation hardening stainless steels [1].

The primary precipitation hardening elements in these stainless steels are aluminum, titanium, and copper. There are three basic steps to hardening these alloys. While there are differences between the different groups, they all follow the same scheme.

First there is solution heat treatment. As with aluminum alloys, this temperature is chosen to dissolve all the solute atoms in solution. The amount of time, and the temperature are chosen depending on the alloying elements present. As indicated previously, time and temperature can be adjusted to change the martensitic transformation.

Secondly, a supersaturated solid solution must be created. This is accomplished by quenching. If the alloy transforms to martensite upon quenching, greater supersaturation occurs because of limited solubility of the alloying elements in martensite. This means the surrounding matrix has a greater alloying content, and greater supersaturation. This martensite reaction can be driven to completion by refrigeration to below the martensite finish temperature.

The last step in the process is precipitation hardening or aging. Since the diffusion rate is so slow because of the temperatures involved, natural aging does not occur. Upon application of elevated temperature, diffusion occurs, and fine precipitates occur along crystallographic planes of the matrix. A highly strained matrix results, with a mismatch occurring between the matrix and the precipitate.

As in aluminum, the size and morphology of the precipitates can be controlled by different times and temperatures. Long exposure at low temperatures creates many fine precipitates which, in turn, increases the strength. Intermediate temperatures can still achieve maximum strengthening, but a shorter cycle. At high temperatures, the precipitates lose coherency with the matrix. The precipitates shear, reducing the strain between the matrix and the precipitate, and the structure becomes overaged.

Martensitic PH Stainless Steels

This material is typically solution heat treated at 1,037°±13°C (1,900°F) and quenched at the mill. Product forms are bar, plate, sheet, and billet. This solution heat treated and quenched thermal treatment is called “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 contraction 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 transformed to martensite either by mechanical working or by thermal treatment. Additional strengthening occurs during aging. An additional 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 formation of Ni3 (Al,Ti) during precipitation hardening.

The heat treatment of this group of alloys consists of solution treatment at 982°C (1,800°F), and cooling rapidly (typically an oil or polymer quench) to Condition A. The alloy is then fabricated, and subsequently aged at 704°C (1,300°F) for 16 hours, then air cooled [4]. These alloys have a much lower yield strength (620 MPa) than the martensitic or semi-austenitic grades.

Conclusions

In this column we introduced the different types of precipitation hardening stainless steels, and briefly discussed the thermal treatments and physical metallurgy of the three alloy groups within the broader precipitation hardening grades of stainless steels.

These alloys have seen increased growth in application because of corrosion resistance, strength, and — importantly — low distortion upon precipitation hardening treatment. 

References

  1. C. J. Slunder, A. F. Hoenie and A. M. Hall, “Thermal and Mechanical Treatment for Precipitation-Hardening Stainless Steels,” NASA, Washington D.C., 1967.
  2. AK Steel International, ARMCO PH 13-8 MO Stainless Steel Product Data Bulletin, Barcelona, Spain, 2019.
  3. Allegheny Technologies International, Stainless Steel 17-4 Precipitation Hardening Alloy, Pittsburgh, PA, 2006.
  4. L. Zubeck, “A Technical Review of Precipitation Hardening Steel Grades,” Springs, no. January, pp. 14-16, 2006.
  5. Allegheny Technologies, AM350 Technical Data Sheet, Pittsburgh, PA, 2012.
  6. AK Steel, 17-7 PH Stainless Steel, West Chester, OH, 2020.