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A Comprehensive Guide to Precipitation Hardening Stainless Steel
Definition of PH Stainless Steel
Precipitation hardening (PH) stainless steels are a group of corrosion-resistant alloys. Some of these alloys can be heat treated to gain tensile strengths ranging from 850 MPa to 1700 MPa and yield strengths from 520MPA to over 1500 MPa. This is significantly higher than the strength of austenitic stainless steels like 304 SS or 316 SS. PH stainless steels are commonly utilized in the nuclear, oil and gas, and aerospace industry fields, where a combination of high strength, good corrosion resistance, and acceptable toughness is needed. The precipitation hardening process involves the addition of copper, molybdenum, aluminum, and titanium, individually or in combination.
The History of PH Stainless Steel
The history of precipitation hardening stainless steel may be traced back to the early 20th century. In 1929, William J. Kroll of Luxembourg made a pivotal discovery, using titanium to pioneer the concept of precipitation hardening in stainless steel. Kroll is famous for inventing the Kroll process, which transformed the refining of titanium and zirconium.
In the 1940s, the use of precipitation hardening in metals became widespread. The first-generation martensitic precipitation-hardened stainless steel was developed during this time. In 1948, Armco’s innovative work resulted in the creation of 17-4 PH steel. This type of steel was used in aircraft landing gear, fasteners, and engine components. The development of 15-5 PH stainless steel was influenced by a deliberate decrease in chromium content and an increase in nickel content in the 17-4 PH steel matrix. Thanks to advancements in materials science, PH stainless steel has made significant progress in recent years. Ferrium® S53®, a product of this era, has confirmed exceptional capabilities. It has been used in aerospace applications, showcasing the ongoing advancement of precipitation hardening in stainless steel.
Typical Process of PH Stainless Steel
The precipitation hardening process consists of three main steps, each requiring a finely controlled approach.
Solutionizing
Solutionizing, regarded as “solution treatment,” starts the precipitation-hardening process. This step includes dissolving precipitates and minimizing potential alloy segregation. To obtain this, the material is heated to its solidus temperature and held there to foster the development of a uniform solid solution. Once this uniformity is achieved, the material is removed from the heat source in preparation for the subsequent phase.
Quenching
The next step of the process involves rapidly cooling or quenching the alloy. During this phase, the material’s cooling rate is so fast that it creates a supersaturated solid solution with excess copper components. This rapid transformation prevents the diffusion of nucleation sites, causing the quenching to happen so quickly that precipitates cannot form on the alloy.
Aging
During the aging stage, the final step in the precipitation hardening process, the material is heated again, but below the solidus temperature. This controlled heating causes atoms to move short distances, forming finely dispersed precipitate layers within the material. This process effectively strengthens the alloy by limiting dislocation movement.
Classifications of PH Stainless Steel
Precipitation hardening stainless steels are categorized into three groups based on their properties after heat treatments: martensitic, semi-austenitic, and austenitic.
Austenitic Alloys
Austenitic stainless steels maintain their structure after the heat treatment process. The alloy should undergo annealing, reheating, and hardening treatments. The steel should be heated to no more than 2050 degrees Fahrenheit during the annealing procedure. Precipitation occurs during the reheating process, which increases the hardness and strength of the steel.
Martensitic Alloys
Martensitic stainless steels should be heated to a maximum of 1950 degrees Fahrenheit during the annealing process. During the cooling process, this material transforms from austenite to martensite.
Semi-Austenitic Alloys
Semi-austenitic alloys are a type of precipitation hardening steels with relatively low hardness and can be cold-worked. Unlike other types of precipitation hardening steels, austenitic alloys maintain their original structure at room temperature but transform to martensite when exposed to extremely low temperatures.
Typical Chemical Composition of PH Stainless Steel
| Element | Percentage(%) |
| C | 0.0-0.07 |
| Mn | 0.0-1.5 |
| Si | 0.0-0.7 |
| P | 0.0-0.04 |
| S | 0.0-0.02 |
| Cr | 15.0-17.0 |
| Ni | 3.0-5.0 |
| Cu | 3.0-5.0 |
| Nb | 0.0-0.45 |
| Mo | 0.0-0.6 |
| Fe | Balance |
The Properties of PH Stainless Steel
Typical mechanical properties for stainless steel alloy 17-4PH
| 17-4PH | Annealed | Cond 900 | Cond 1150 |
| Tensile Strength (MPa) | 1100 | 1310 | 930 |
| Proof Stress 0.2% (MPa) | 1000 | 1170 | 724 |
| Elongation A5 (%) | 15 | 10 | 16 |
Typical physical properties for stainless steel alloy 17-4PH
| Property | Value |
| Density | 7.75 kg/m3 |
| Electrical Resistivity | 0.8 x 10-6Ω.m |
| Modulus of Elasticity | 196 GPa |
| Thermal Expansion | 10.8 x 10-6/K |
| Thermal Conductivity | 18.4 W/m.K |
Some other properties
Strength
Yield strengths of precipitation-hardening stainless steels range from 515 to 1415 MPa, with tensile strengths ranging from 860 to 1520 MPa. Elongation percentages fall between 1 and 25%. Cold working before aging can be utilized to achieve even higher strengths.
Heat Treatment
The properties of precipitation hardening stainless steels are influenced by heat treatment. After these steels’ solution treatment or annealing, a single low-temperature “age hardening” stage is used to achieve the desired properties. Since this treatment is carried out at a low temperature, there is no distortion and only superficial discoloration. During hardening, a mild decrease in size occurs. This shrinkage is approximately 0.05% for condition H900 and 0.10% for H1150. The typical mechanical properties achieved for 17-4 PH after solution treating and age hardening are provided in the following table. Condition designations are based on the age hardening temperature in °F.
Mechanical property ranges after solution treating and age hardening
| Cond. | Hardening Temp and time | Hardness (Rockwell C) | Tensile Strength (MPa) |
| A | Annealed | 36 | 1100 |
| H900 | 482°C, 1 hour | 44 | 1310 |
| H925 | 496°C, 4 hours | 42 | 1170-1320 |
| H1025 | 552°C, 4 hours | 38 | 1070-1220 |
| H1075 | 580°C, 4 hours | 36 | 1000-1150 |
| H1100 | 593°C, 4 hours | 35 | 970-1120 |
| H1150 | 621°C, 4 hours | 33 | 930-1080 |
Corrosion Resistance
Precipitation hardening stainless steels offer moderate to good corrosion resistance in various environments. They provide a better strength and corrosion resistance balance than the heat-treatable 400 series martensitic alloys. Their corrosion resistance is similar to that of grade 304 stainless steel.
In chloride-rich environments, 17-4 PH is prone to pitting and crevice corrosion. Aging at 550°C or higher enhances its resistance to stress corrosion cracking, with even better resistance at higher temperatures.
The solution-treated (annealed) condition has low corrosion resistance, so it should not be used before heat treatment.
Heat Resistance
17-4 PH offers good oxidation resistance. To maintain mechanical properties, avoid exceeding the precipitation hardening temperature. When critical for ambient temperature toughness, prolonged exposure to 370-480°C should be avoided.
Fabrication
Using tools dedicated to stainless steel materials is essential when working with stainless steel. Alternatively, if using shared tools, thoroughly clean the tooling and work surfaces before use. These precautions are necessary to prevent cross-contamination of stainless steel by easily corroded metals, which could discolor the surface of the fabricated product.
Cold Working
Cold forming processes, such as rolling, bending, and hydroforming, can only be performed on 17-4PH in the fully annealed condition. After cold working, re-ageing at the precipitation hardening temperature can improve stress corrosion resistance.
Hot Working
The hot working of 17-4 PH should be conducted at temperatures ranging from 950°C to 1200°C. Following the hot working process, a complete heat treatment is necessary. This includes annealing and cooling the component to room temperature or lower. Subsequently, the component must undergo precipitation hardening to achieve the desired mechanical properties.
Machinability
In the annealed state, 17-4 PH exhibits outstanding machinability, like that of 304 stainless steels. Machining becomes difficult after hardening heat treatment, but it is still feasible.
Carbide or high-speed steel tools are typically used with standard lubrication. When stringent tolerance limits are necessary, the dimensional changes resulting from heat treatment must be considered.
Welding
Precipitation hardening stainless steels can be easily welded using procedures like those employed for the 300 series of stainless steels.
Grade 17-4 PH can be welded successfully without the need for preheating. Post-weld heat treatment can impart the same properties to the weld metal as the parent metal. The recommended filler rod grade for welding 17-4 PH is 17-7 PH.
Materials Subjected to PH Process
Precipitation heat treatment is applicable to a wide variety of metal alloys, including age-hardening aluminum (2024, 6061, and 7075), precipitation hardening stainless steel (17-4 PH, 15-5 PH), titanium alloys, and magnesium alloys.
Age hardening is not limited to traditional metals but also applies to superalloys. Nickel-based superalloys such as Inconel 718, Waspaloy, and René 41 are also amenable to precipitation heat treatment.
Overall, age-hardening alloys offer a wide range, providing engineers with significant flexibility in their work.
Applications of PH Stainless Steel
Precipitation hardening alloys are utilized in a wide range of applications. They are ideal for applications that require rigid materials that do not flex under stress. Components such as valves, gears, shafts, engine parts, turbine blades, ball bearings, bushes, dies, and fasteners are commonly precipitation-hardened. Here are some of its core applications:
Aerospace: Precipitation-hardened alloys are commonly used to manufacture aircraft structural components such as landing gear, engine components, and fasteners. For example, Al-6061 T6 is considered space-grade due to its aerospace applications.
Automotive: Automotive components, such as engine, suspension, and chassis, undergo age-hardening processes to enhance strength and durability.
Medical: Various implant and surgical instrument manufacturers prefer precipitation-hardened alloys because of their hardness and durability.
Tooling: Cutting tools used for machining are often constructed from precipitation alloys because of their enhanced strength and resistance to wear.
Advantages & Disadvantages
Advantages:
- Improved strength: The strength can be increased by up to four to five times for many materials. This offers numerous benefits, including smaller parts, reduced costs, lighter weight, and safer operation.
- Improved hardness:Particle hardening increases the durability of metals, making them less likely to wear out easily.
- Improved corrosion resistance:Age hardening can improve corrosion resistance depending on the alloy, particularly in components frequently exposed to corrosive compounds like chemicals and seawater.
- Improved ductility:Precipitation hardening can enhance a product’s ductility, improving resilience and preventing cracking and breakage.
- Formability: Many grades of semi-austenitic stainless steel can be formed without undergoing any heat treatment. In contrast, martensitic stainless steels are strong both before and after the heat treatment process and are not easily shaped.
- Weldability: Unlike other stainless steel alloys, precipitation hardening steels can be easily welded using standard fusion and resistance methods. However, special care must be taken during the heat treatment process to ensure that the best mechanical properties for weldability are achieved.
- Cost Effectiveness: Precipitation heat treatment reduces production costs. The enhanced mechanical properties often outweigh the added cost of heat treatment by eliminating the need for an expensive alloy with similar properties.
Disadvantages:
- Precipitate Homogeneity: Achieving uniform precipitate size and distribution is crucial for successful age hardening, but it can be challenging. These issues arise when heat is unevenly supplied to the material or when quenching is not efficient enough to freeze all portions of the material’s volume uniformly. This challenge is particularly pronounced in large and complex parts.
- Over-Aging: Over-aging commonly occurs in precipitation heat treatment when the material is aged at very high temperatures or for too long, causing coarse precipitates and a subsequent reduction in strength.
- Residual Stresses: When a material undergoes rapid cooling followed by thermal expansion, it can lead to the development of residual stresses, causing issues such as dimensional distortion, cracking, and uneven properties. To address this, engineers can minimize thermal gradients during heating and cooling processes and use stress-relieving treatments like cold compression.
Precipitation Hardening VS Annealing and Quenching
Heat treatment encompasses various processes, including precipitation hardening, annealing, and quenching, which are the most common. These processes involve exposing alloys to specific temperatures for defined periods, which can lead to confusion due to their similarities.
Precipitation Hardening vs Annealing
Annealing is primarily a stress-relieving process that enhances the material’s ductility and machinability. This contrasts with age hardening, which aims to make a material stronger and harder.
Although the initial stages of both processes are similar, the subsequent steps differ. In both processes, the furnace raises the alloy’s temperature to around its recrystallization temperature. However, in precipitation hardening, the hot, solid solution undergoes rapid cooling (quenching). On the other hand, in annealing, the heated alloy is allowed to cool down at a much slower, controlled rate in a less severe cooling medium, such as still air.
Precipitation Hardening vs Quenching
Quenching has been previously mentioned in this discussion as a sub-process in the precipitation hardening procedure.
During the process of quenching, the material is heated to a very high temperature and then rapidly cooled to room temperature, causing its microstructure to freeze in its current state. This process induces hardness and brittleness in the material.
It fundamentally differs from precipitation hardening, which involves heating the material, quenching it, and then reheating it to form hardening precipitates. Therefore, precipitation hardening has more steps than a simple quenching process.
Summary
Precipitation hardening is undeniably one of the most valuable heat treatment processes in manufacturing, significantly improving strength, hardness, and dimensional stability.
