Knowledge

17-4PH stainless steel (ASTM) is a martensitic precipitation-hardening stainless steel, equivalent to the national standard 05Cr17Ni4Cu4Nb. This type of stainless steel has a low carbon content, high Ni and Cr content, good weldability and excellent corrosion resistance. At the same time, the content of alloying elements such as Cu and Nb in this steel is also relatively high. These alloying elements can precipitate and age hardening phases such as ε-Cu, NbC, and M23C6 during heat treatment, endowing the material with high strength and hardness. Due to these advantages, 17-4PH martensitic precipitation-hardening stainless steel is widely used in aerospace, chemical, and nuclear industries, among others. The mechanical properties of precipitation-hardening stainless steel are significantly related to the heat treatment state. The conventional heat treatment process for 17-4PH martensitic precipitation-hardening stainless steel is solution treatment + aging treatment, which improves strength, hardness, and corrosion resistance by adjusting the microstructure and controlling the precipitated phases. Currently, the research on the heat treatment process of 17-4PH stainless steel is quite mature. This article summarizes and briefly describes its performance and mechanism under different heat treatment processes. ​​​​​​​

 

Heat treatment of 17-4PH stainless steel

The martensite start transformation point of 17-4PH stainless steel is above room temperature. After solution treatment, the matrix structure is basically martensite, and its strength is already very high. Different aging treatments based on solution treatment can increase the material's strength to meet various production needs.

The chemical composition of 17-4PH stainless steel (mass fraction, %) is: ≤0.07C, ≤1.00Mn, ≤1.00Si, ≤0.023P, ≤0.03S, 15.50 - 17.50Cr, 3.00 - 5.00Ni, 3.00 - 5.00Cu, 0.15 - 0.45Nb. The main precipitation hardening elements are copper and niobium, and some are aluminum, titanium, etc. The strengthening process is achieved by utilizing the solubility of these elements. When 17-4PH stainless steel is heated to the austenite temperature, due to the large solubility of these strengthening elements in austenite and the small solubility in martensite, when cooled to the martensite temperature, a supersaturated copper and niobium martensite structure is obtained. Martensite itself has high strength and toughness, thus achieving a certain degree of strengthening. After aging treatment, the supersaturated copper, niobium and other elements dissolved in the matrix structure precipitate, further strengthening the material. Therefore, various performance requirements can be met through different heat treatment processes.

 

1.1 Solution Treatment:

Solution treatment is an indispensable heat treatment process for 17-4PH steel. During solution treatment, the heating temperature should ensure that the carbon and alloy elements in the steel are fully dissolved in the austenite, but it should not be too high. The Ac1 of 17-4PH steel is approximately 670°C, Ac3 is about 740°C, Ms is 80-140°C, and Mf is approximately 32°C. Therefore, the recommended solution treatment temperature in the standard is 1020-1060°C. Different solution treatment temperatures result in different final microstructures and properties. Zhao Liping, Du Daming, et al. studied the microstructure and properties of 17-4PH steel at different solution treatment temperatures, choosing 1000°C, 1040°C, and 1080°C. The research found that the hardness of the sample was the highest after solution treatment at 1040°C. This is because when the solution treatment temperature is low, the austenite obtained after heating is not uniform, and the dissolved alloy carbides are also few, resulting in a lower hardness of the martensite after quenching; when the solution treatment temperature is high, on the one hand, the grains are coarse, and on the other hand, too many alloy carbides dissolve into the austenite, increasing the stability of the austenite and lowering the martensite transformation point. Therefore, the amount of martensite decreases and the amount of retained austenite increases after quenching, reducing the hardness. At the same time, excessively high heating temperatures may also cause a relatively large amount of ferrite to exist in the solution-treated microstructure, affecting the final strengthening effect. Therefore, the solution treatment temperature must be reasonably selected to ensure the required properties. Since 17-4PH steel contains elements such as chromium and nickel, it can obtain martensite through air cooling. However, to achieve a finer microstructure after solution treatment, obtain better strengthening effects, and improve plasticity and toughness, oil cooling is often used in actual production. The microstructure after solution treatment is low-carbon lath martensite containing supersaturated copper and niobium. Sometimes, due to insufficient quenching or excessively high heating temperatures, there may be a small amount of retained austenite and ferrite.

 

1.2 Aging Treatment: 

The aging treatment of 17-4PH steel should be determined based on the performance requirements, including the heating temperature and holding time. Research indicates that after solution treatment at 1040°C, the martensitic structure of 17-4PH steel undergoes tempering and continuously precipitates as the aging temperature increases. At 450°C, copper, niobium and other precipitates have already formed. When the aging temperature reaches 470-480°C, the intragranular precipitates are fine and evenly distributed, and the material's hardness is at its peak. As the aging temperature further increases, the hardness and strength decrease, while the ductility and toughness improve. Due to the similar variation patterns of hardness and strength, for parts with clear requirements for hardness and strength, the aging temperature should be strictly controlled to meet the application requirements. The variation patterns of strength and ductility during the aging process of 17-4PH steel are similar to those of 0Cr15Ni5Cu2TiC precipitation-hardening stainless steel. Aging of 17-4PH steel at temperatures above 510°C is considered overaging. Hou Kai et al. studied the impact toughness of 17-4PH steel during overaging. The research indicated that as the aging temperature increases, the material's impact toughness gradually improves. To ensure the full precipitation of the precipitates and the aging effect, the holding time at the aging temperature is generally no less than 4 hours, and air cooling can be used after holding. At the same aging temperature, different aging holding times result in different final properties. Figure 1 shows the variation curve of hardness with aging time for 17-4PH steel at an aging temperature of 350°C. It can be seen that as the holding time increases, the hardness of the sample continuously rises. In the early stage of aging treatment, the hardness of the sample increases relatively slowly; after aging for 6000 hours, the hardness of the sample increases rapidly; around 9000 hours of aging, the hardness of the sample reaches its maximum value; thereafter, as the aging time continues to increase, the hardness begins to drop 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 showed that after long-term aging at 350°C, as the aging time increases, the yield strength and tensile strength increase, while the reduction of area and elongation decrease; the fracture surface changes from fine dimples to coarse dimples. The study also found that after long-term aging, the microstructure of 17-4PH steel changes, spinodal decomposition begins at the grain boundaries, the precipitated ε-Cu particles gradually grow, and a small amount of reverse transformation austenite is produced. As the aging time increases, spinodal decomposition gradually shifts from the grain boundaries to the grain interiors, and a large number of fine, oriented G phases precipitate in the matrix, while the matrix structure remains as lath martensite. Wang Jun et al. used the oscilloscope impact method to study the embrittlement behavior of 17-4PH steel during long-term aging at 350°C. The oscilloscope impact test can provide various transient information such as energy-time, load-time, and deflection-time during the impact fracture process of the sample, providing conditions for a deeper understanding of the deformation and fracture behavior of materials under dynamic loading. The research indicated that during long-term aging at 350°C, the crack initiation energy (Ei), crack propagation energy (Ep), total impact energy (Et), and dynamic fracture toughness (KId) of 17-4PH steel decrease with the increase of aging time. This indicates that after long-term aging, the material's toughness decreases and embrittlement occurs.

 

1.3 Adjustment Treatment:

The conventional heat treatment for 17-4PH stainless steel is solution treatment followed by aging. Recent research has found that if an adjustment treatment is carried out before aging, the mechanical properties and corrosion resistance of the material will change significantly. The purpose of the adjustment treatment is to adjust the martensite transformation points Ms and Mf of the steel, so it is also called phase transformation treatment. After adding the adjustment treatment, for the same solution and aging temperatures, the impact toughness of the material will increase by more than twice, and the corrosion resistance will also be significantly improved. Yang Shiwei et al. studied the corrosion resistance of 17-4PH steel in artificial seawater in the direct aging state after solution treatment and in the solution + adjustment + aging state by means of chemical immersion, polarization curves, cyclic polarization curves, and electrochemical impedance. The research shows that after the adjustment treatment, the self-corrosion potential and pitting potential of 17-4PH stainless steel increase, and the annual corrosion rate decreases. The seawater corrosion resistance is far superior to that of the direct aging sample. The reason is that after the adjustment treatment and then aging, the formation of chromium-depleted zones can be effectively avoided, and chromium is the key to ensuring good corrosion resistance of metals. At the same time, the martensite structure becomes fine, and the microstructure uniformity of the material is improved. The microstructures after direct aging after solution treatment and after solution + adjustment + aging are shown in Figure 2. It can be seen that the microstructure after adjustment treatment has a clearer grain contour, fine and uniform martensite laths, and a clear orientation relationship. The microstructure after direct aging after solution treatment has coarse martensite laths, and a large amount of white precipitated phases are distributed along the grain boundaries. The martensite structure after adjustment treatment and then aging "inherits" the fine characteristics of the adjustment treatment state, and the grain boundaries are interconnected in a network, enclosing the grains mainly composed of martensite and retained austenite. This microstructure is related to the formation of more reverse transformation austenite in the steel.

 

Many researchers have also studied the adjustment treatment time and temperature. The research found that adjusting time and adjusting temperature do not have a significant impact on the material's microstructure morphology. However, as the adjustment time increases, the martensite structure becomes finer and more uniform; as the adjustment treatment temperature rises, the material's strength gradually increases, while its plasticity and toughness gradually decrease; after adjustment treatment at 816 ℃, as the aging temperature increases, the material's strength gradually decreases, while its plasticity and toughness gradually increase.

 

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