17-4PH stainless steel (ASTM) is a martensitic precipitation-hardening type, equivalent to the national standard 05Cr17Ni4Cu4Nb. This type of stainless steel has a low carbon content and high Ni and Cr content, making it highly weldable and corrosion-resistant. Additionally, the steel contains a high level of alloying elements such as Cu and Nb. These elements precipitate ε-Cu, NbC, and M23C6 phases during heat treatment, enhancing the material's strength and hardness. Due to these advantages, 17-4PH martensitic precipitation-hardening stainless steel is widely used in the aviation, aerospace, chemical, and nuclear industries. The mechanical properties of precipitation-hardened stainless steel are significantly influenced by its heat treatment state. The conventional heat treatment process for 17-4PH martensitic precipitation-hardening stainless steel involves solution treatment followed by aging treatment. By adjusting the microstructure and controlling the precipitation of phases, the strength, hardness, and corrosion resistance can be improved. Currently, research on the heat treatment processes of 17-4PH stainless steel has reached a high level of maturity. This article summarizes and briefly discusses the performance and mechanisms under different heat treatment processes.

1.Heat treatment of 17-4PH stainless steel

17-4PH The martensitic transformation point of stainless steel is above room temperature. After solution treatment, the matrix structure is basically martensitic and its strength has been very high. Different aging treatments can be carried out on the basis of solution treatment to improve the strength of the material and meet the needs of various production practices.

The chemical composition of 17-4PH stainless steel (by mass fraction,%) is as follows: ≤0.07% carbon (C), ≤1.0% 00Mn,≤1.00Si, ≤0.023% phosphorus (P), ≤0.03% sulfur (S), 15.50 to 17.50% chromium (Cr), 3.00 to 5.00% nickel (Ni), 3.00 to 5.00% copper (Cu), and 0.15 to 0.45% niobium (Nb). The primary precipitation hardening elements are copper and niobium, with some cases including aluminum and titanium. These elements are used to achieve the strengthening process by utilizing their solubility. When 17-4PH stainless steel is heated to its austenite temperature, the higher solubility of these strengthening elements in austenite and lower solubility in martensite leads to the formation of a supersaturated martensitic structure with copper and niobium. The martensite itself has high strength and toughness, providing a certain level of strengthening. After aging treatment, the supersaturated copper and niobium dissolve into the matrix, further enhancing the material's strength. Therefore, various performance requirements can be met through different heat treatment processes.

1. Solid Solution Treatment Solid solution treatment is an essential heat treatment process for 17-4PH steel. During solid solution treatment, the heating temperature should ensure that carbon and alloy elements are fully dissolved into austenite, but it should not be too high. For 17-4PH steel, Ac1 is approximately 670℃, Ac3 is about 740℃, Ms is 80 to 140℃, and Mf is about 32℃. Therefore, the standard recommends a solid solution treatment temperature of 1020 to 1060℃. Different solid solution temperatures result in different microstructures and properties. Zhao Liping, Du Daming, and others studied the microstructure and properties of 17-4PH steel at different solid solution temperatures, selecting treatment temperatures of 1000,1040, and 1080℃. The study found that after a 1040℃ solid solution treatment, the samples had the highest hardness. This is because when the solid solution treatment temperature is low, the austenite formed during heating is uneven, and the dissolved alloy carbides are minimal, leading to lower martensite hardness after quenching. When the solid solution treatment temperature is high, the grains become coarser, and more alloy carbides dissolve into austenite, increasing austenite stability and lowering the martensite transformation point. As a result, the amount of martensite decreases after quenching, the amount of residual austenite increases, and the hardness decreases. Additionally, excessively high heating temperatures can lead to a higher content of ferrite in the solid solution structure, affecting the final strengthening effect. Therefore, it is essential to select the appropriate solid solution treatment temperature to ensure the desired properties. Due to the presence of chromium and nickel in 17-4PH steel, it can form martensite when air-cooled. However, to achieve a finer solid solution structure, better strengthening, and improved ductility and toughness, oil cooling is commonly used in production. The microstructure after solution treatment consists of low-carbon bainitic plates containing supersaturated copper and niobium. Sometimes, due to insufficient quenching or excessively high heating temperatures, a small amount of residual austenite and ferrite may remain.

17-4PH steel should be heat-treated according to the required performance, with the heating temperature and holding time determined accordingly. Studies have shown that after solution treatment at 1040℃, as the aging temperature increases, martensitic structures undergo tempering, and precipitates continuously form. At 450℃, copper and niobium precipitates begin to form. By 470-480℃, the precipitates are fine and widely distributed within the grains, resulting in the highest hardness of the material. As the aging temperature continues to rise, the hardness and strength decrease, while plasticity and toughness increase. Since the changes in hardness and strength follow similar patterns, for parts with specific requirements for hardness and strength, the aging temperature should be strictly controlled to meet the usage requirements. The changes in strength and plasticity during the aging process of 17-4PH steel are similar to those in 0Cr15Ni5Cu2TiC precipitation-hardening stainless steel. Aging above 510℃ is considered over-aged. Hou Kai et al. studied the impact toughness of 17-4PH steel under over-aged conditions and found that as the aging temperature increases, the material's impact toughness gradually improves. To ensure the full formation of precipitates and effective aging, the holding time at the aging temperature should generally be no less than 4 hours, followed by air cooling. At the same aging temperature, different holding times result in different final properties. Figure 1 shows the hardness curve of 17-4PH steel at 350℃ aging temperature, with the changes over time. It is evident that as the holding time increases, the hardness of the samples gradually increases. In the early stage of aging treatment, the increase in hardness is relatively slow; after 6000 h of aging, the increase in hardness accelerates; around 9000 h of aging, the hardness reaches its peak; after this point, as the aging time continues to extend, the hardness begins to decline rapidly. Peng Yanhua et al. conducted a detailed study on the relationship between long-term aging and tensile properties of 17-4PH steel. The results indicate that after 350℃ of long-term aging, the yield strength and tensile strength increase with extended aging time, while the reduction in area and elongation decrease; the fracture surface transitions from fine to coarse pit structures. The study also found that after long-term aging, the microstructure of 17-4PH steel changes, with spinodal decomposition beginning at grain boundaries, and the precipitated ε-Cu particles gradually growing, along with the formation of a small amount of reversed transformation austenite. As aging time extends, spinodal decomposition gradually shifts from grain boundaries to within the grains, and a large number of oriented fine G phases precipitate in the matrix, while the matrix structure remains bainitic. The embrittlement behavior of 17-4PH steel under 350℃ long-term aging was studied using the oscillographic impact method. The oscillographic impact test provides various transient information on the energy-time, load-time, and deflection-time of the deformation and fracture process during the impact fracture of the sample, which is essential for understanding the deformation and fracture behavior of materials under dynamic loading conditions. The results show that the crack initiation work (Ei), crack propagation work (Ep), total impact work (Et) and dynamic fracture toughness (KId) of 17-4PH steel decrease with the extension of long-term aging time at 350℃

The conventional heat treatment for 17-4PH stainless steel involves solution and aging. Recent studies have shown that performing an adjustment treatment before aging can significantly alter the material's mechanical and corrosion resistance properties. The purpose of this adjustment treatment is to adjust the Ms and Mf transformation points of the steel, hence it is also known as a phase transformation treatment. After adding the adjustment treatment, the impact toughness of the material more than doubles at the same solution and aging temperatures, and its corrosion resistance is also significantly enhanced. Yang Shiwei and colleagues used methods such as chemical immersion, polarization curves, cyclic polarization curves, and electrochemical impedance to study the corrosion resistance of 17-4PH steel in artificial seawater under conditions of solution aging and solution + adjustment + aging. The study found that after 17-4PH stainless steel undergoes an adjustment treatment followed by aging, the self-corrosion potential and pitting potential increase, while the annual corrosion rate decreases, significantly improving its seawater corrosion resistance compared to samples aged directly. This is because the adjustment treatment effectively prevents the formation of chromium-poor areas, which are crucial for maintaining good corrosion resistance. Additionally, the martensite structure becomes finer, enhancing the uniformity of the material's microstructure. The microstructures after solution aging and solution + adjustment + aging are shown in Figure

2. It can be seen that the microstructure after adjustment treatment has clearer grain boundaries, uniformly fine martensite plates, and a clear orientation relationship. In contrast, the microstructure after solution aging alone shows coarse martensite plates and a large amount of white precipitates at the grain boundaries. After the adjustment treatment, the martensitic structure "genetically" inherits the characteristics of miniaturization in the adjusted state. The grain boundaries are connected to form a network, and the grains mainly composed of martensite and residual austenite are encapsulated in it. This kind of structure is related to the production of more reverse transformation austenite in steel.

Many researchers have also studied the effects of adjusting the processing time and temperature. The studies found that while the adjustments to time and temperature had a limited impact on the material's microstructure, as the adjustment time increased, the martensite structure became finer and more uniform. As the processing temperature increased, the material's strength gradually increased, but its plasticity and toughness decreased. After the 816℃ adjustment treatment, as the aging temperature increased, the material's strength gradually decreased, while its plasticity and toughness gradually increased.

217-4PH stainless steel heat treatment strengthening mechanism.

During the solid solution treatment of 17-4PH martensitic stainless steel, copper and niobium dissolve into the austenite grains. Upon cooling, this process results in a supersaturated martensite containing copper and niobium, leading to the first strengthening. During the aging process, the supersaturated elements precipitate from the grains, resulting in a second strengthening of the matrix. This is the primary strengthening mechanism for 17-4PH steel.

Different heat treatment processes can produce different microstructures and properties, but the strengthening mechanism is the same: it is related to the precipitation of precipitates. The distribution of precipitates such as ε-Cu, NbC, and M23C6 varies, leading to different material properties. In precipitation-hardened alloys, the yield strength is determined by the effect of strengthening phases on dislocations. When the strengthening phase particles are extremely fine and dispersed, they form a dense layer that blocks dislocation lines, preventing them from passing through these particles, thus increasing the alloy's yield strength and ultimately causing embrittlement. Conversely, when the strengthening phase particles are larger and less densely distributed, dislocations can bypass these particles according to the Owrrone mechanism, preventing dislocation line blockage and reducing the alloy's yield strength. This is why, in aged 17-4PH steel, when there are many inverse transformation austenite grains, the ε-Cu particles in the inverse transformation austenite are finer and more sparsely distributed than those in martensite, providing little or no obstruction to dislocations, which reduces the alloy's yield strength. Generally, after quenching, 17-4PH steel retains a small amount of residual austenite, which are very fine particles that become the core of the inverse transformation austenite during tempering. Therefore, the more residual austenite in the alloy, the more inverse transformation austenite is generated during aging. Therefore, when the content of elements that promote martensite formation (such as C) in the alloy decreases, while the content of elements that stabilize austenite (such as N) is too high, more residual austenite will form after quenching and more reverse transformation austenite will form after tempering, thereby reducing the yield strength of the alloy. As the aging temperature increases, reverse transformation austenite begins to form and grow, leading to an increase in the amount of residual austenite at room temperature and a decrease in strength. Therefore, for materials with strength requirements, it is essential to develop a reasonable heat treatment process and strictly control the amount of reverse transformation austenite in the microstructure. ε-Cu is the primary strengthening phase in 17-4PH steel. In recent years, research on its morphology has increased. Foreign countries started earlier, while domestic research, particularly at Harbin Turbine Factory, has been more thorough. It was generally believed that "in all cases, ε-Cu is spherical." However, Harbin Turbine Factory found that ε-Cu phases precipitated from the martensitic matrix are smooth short rods, whereas those precipitated from austenite (reverse transformation austenite) are spherical. This is because both austenite and ε-Cu phases have face-centered cubic lattices, and their interfacial energy is very low, resulting in spherical ε-Cu phases. In contrast, martensite has a body-centered cubic lattice, which differs significantly from the face-centered cubic lattice of ε-Cu phases, leading to a high interfacial energy and rod-like ε-Cu phases. Zhang Hongbin et al. also studied the morphology of ε-Cu phases in 17-4PH steel and found that ε-Cu phases precipitated from the martensitic matrix are nearly spherical