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17-4 PH Stainless Steel Investment Castings Aging Treatment

17-4PH stainless steel is a martensitic precipitation hardening stainless steel, which is equivalent to the national standard 05Cr17Ni4Cu4Nb. This type of stainless steel has low carbon content, high Ni and Cr content, good weldability and good corrosion resistance.

At the same time, the content of alloying elements such as Cu and Nb in the steel is also relatively high. During the heat treatment, these alloying elements can precipitate age-hardening phases ε-Cu, NbC, M 23 C 6 , etc., so that the material has higher strength and hardness. Based on the above advantages, 17-4PH martensitic precipitation hardening stainless steel is widely used in aviation, aerospace, chemical and nuclear industries and other fields.

The mechanical properties of precipitation hardening stainless steel have a great relationship with the heat treatment state. The conventional heat treatment process of 17-4PH martensitic precipitation hardening stainless steel is solution + aging treatment, which can improve the strength, hardness and corrosion resistance by adjusting the structure and controlling the precipitated phase. At present, the research on the heat treatment process of 17-4PH stainless steel is quite mature, and the performance and mechanism under the same heat treatment process are summarized.

What Is 17-4PH Stainless Steel

17-4PH high-strength steel, whose grade is 0Cr17Ni4Cu4Nb, is a low-carbon martensitic precipitation-hardening stainless steel developed on the basis of Cr17 martensitic stainless steel. Due to the low carbon content in the matrix, corrosion resistance and The weldability is superior to other martensitic stainless steels, and close to some austenitic stainless steels, and has good comprehensive mechanical properties, and can be widely used in high-tech fields such as aviation, aerospace, chemical and nuclear industries. Our company produces die forging products of this material, and the delivery status is solution aging heat treatment after forging, which is mainly used for the production and processing of aviation high temperature resistant and high pressure resistant fan blades.

17-4PH Stainless Steel Chemical Composition (%)

  • Carbon C: ≤0.07
  • Manganese Mn: ≤1.00
  • Silicon Si: ≤1.00
  • Chromium Cr: 15.5~17.5
  • Nickel Ni②: 3.0~5.0
  • Phosphorus P: ≤0.04
  • Sulfur S: ≤0.03
  • Copper Cu: 3.0~5.0
  • Niobium+tantalum Nb+Ta: 0.15~0.45

17-4PH Mechanical Properties

  • Tensile strength σb (MPa): aged at 480°C, ≥1310; aged at 550°C, ≥1060; aged at 580°C, ≥1000; aged at 620°C, ≥930
  • Conditional yield strength σ0.2 (MPa): aged at 480°C, ≥1180; aged at 550°C, ≥1000; aged at 580°C, ≥865; aged at 620°C, ≥725
  • Elongation δ5 (%): aging at 480°C, ≥10; aging at 550°C, ≥12; aging at 580°C, ≥13; aging at 620°C, ≥16
  • Reduction of area ψ (%): aging at 480°C, ≥40; aging at 550°C, ≥45; aging at 580°C, ≥45; aging at 620°C, ≥50
  • Hardness: solid solution, ≤363HB and ≤38HRC; 480℃ aging, ≥375HB and ≥40HRC; 550℃ aging, ≥331HB and ≥35HRC; 580℃ aging, ≥302HB and ≥31HRC; 620℃ aging, ≥277HB and ≥28HRC

17-4PH Stainless Steel Aging

  • After the 17-4PH stainless steel has undergone solid solution + stabilization treatment, it is subjected to aging treatment at different temperatures. After aging at 568 °C, the fatigue limit of the sample is significantly improved compared with that aged at 605 °C.
  • Compared with aging at 605 °C, aging at 568 °C increases the fatigue crack growth threshold of 17-4PH stainless steel, increases the fatigue crack growth resistance and reduces the fatigue crack growth rate.
  • After the 17-4PH stainless steel has undergone solid solution + stabilization treatment and aged at 568 °C and 605 °C respectively, the fatigue fracture morphology of the 17-4PH stainless steel shows a river-like pattern, which is a dissociation step, and the flow direction of the river is a crack The direction of expansion belongs to the cleavage fracture.

Aging Treatment for 17-4 PH Stainless Steel Investment Castings

17-4 PH stainless steel is a precipitation-hardening stainless steel that is commonly used in various industries, including aerospace, oil and gas, and medical applications. It offers a combination of high strength, good corrosion resistance, and excellent mechanical properties.

To enhance the properties of 17-4 PH stainless steel investment castings, an aging treatment is typically performed. Aging treatment involves heating the castings to a specific temperature and holding them at that temperature for a certain period of time, followed by cooling.The aging treatment of 17-4PH stainless steel investment castings refers to the aging treatment of 17-4PH stainless steel at different temperatures after solid solution + stabilization treatment to achieve the effect of improving its fatigue limit.

The purpose of the aging treatment is to promote the precipitation of fine particles within the stainless steel matrix, which improves the strength and hardness of the material. The aging process helps to transform the stainless steel from a solution-treated condition to a stronger, more durable state.There are two primary types of aging treatments used for 17-4 PH stainless steel investment castings:

Aging Treatment – H900 Condition:

  • Castings are heated to around 900°C (1650°F) for a specified period, typically 1 to 4 hours.
  • The castings are then rapidly cooled, often in water or air.
  • This treatment achieves maximum strength and hardness while maintaining good toughness.

Aging Treatment – H1150 Condition

  • Castings are heated to around 620°C to 650°C (1150°F to 1200°F) for a specified period, typically 4 to 6 hours.
  • The castings are then air-cooled or cooled in still air.
  • This treatment provides a balance between strength, hardness, and toughness, making it suitable for certain applications.
  • It’s important to note that the exact aging treatment parameters may vary depending on the specific requirements of the application, the size and geometry of the castings, and the standards or specifications being followed. It is recommended to consult the material supplier or a metallurgical expert to determine the optimal aging treatment conditions for 17-4 PH stainless steel investment castings in your specific case.

Also, the aging treatment allows for a high degree of customization. By varying the temperature and duration of the treatment, manufacturers can precisely control the final properties of the 17-4 PH stainless steel, making it adaptable to specific application requirements.

17-4PH Stainless Steel Martensitic Transformation Point

The martensite transformation point of 17-4PH stainless steel is above room temperature. After solution treatment, the matrix structure is basically martensite structure, and its strength is already very high. On the basis of solid solution treatment, different aging treatments can improve the strength of the material and meet the actual needs of various production. The chemical composition (mass fraction, %) of 17-4PH stainless steel 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, niobium, and some are aluminum, titanium, etc. The solubility of these elements is used to achieve the strengthening process. When 17-4PH stainless steel is heated to austenite temperature, because these strengthening elements have a large solubility in austenite and a small solubility in martensite, when it is cooled to martensite temperature, it will be supersaturated The martensite structure of copper and niobium, the martensite itself has high strength and toughness, so it can be strengthened to a certain extent; after aging treatment, the supersaturated copper, niobium and other elements dissolved in the matrix structure are precipitated, so that the material can be further strengthened . Therefore, different heat treatment processes can be used to meet various performance requirements.

The Solution Treatment Of 17-4PH Steel

Solution treatment is an indispensable heat treatment process for 17-4PH steel. During solid solution, the heating temperature should ensure that the carbon and alloying elements in the steel are fully dissolved in the austenite, but it should not be too high. The Ac 1 of 17-4PH steel is about 670 °C, the Ac 3 is about 740 °C, the Ms is 80-140 °C, and the Mf is about 32 °C. Therefore, the solid solution temperature recommended in the standard is 1020-1060 °C. The solution temperature is different, and the final structure and properties are also different.

The microstructure and properties of 17-4PH steel at different solution temperatures were studied, and the selected solution treatment temperatures were 1000, 1040 and 1080 °C. The study found that after solution treatment at 1040 ℃, the hardness of the sample is the highest. This is because when the solution treatment temperature is low, the austenite obtained by heating is not uniform, and the dissolved alloy carbides are also very few, resulting in the low hardness of the martensite obtained after quenching;

When the solid solution temperature is high, on the one hand, the grains are coarse, and on the other hand, the alloy carbide is too much dissolved into the austenite, the stability of the austenite will increase, and the martensite transformation point will decrease, so after quenching The amount of martensite decreases, the amount of retained austenite increases, and the hardness decreases. At the same time, too high heating temperature may also cause a large amount of ferrite in the solid solution structure, which will affect the final strengthening effect. Therefore, the solid solution temperature must be selected reasonably to ensure the required properties.

Since 17-4PH steel contains elements such as chromium and nickel, it can obtain martensite when it is air-cooled. Adopt oil cooling method. The microstructure after solution treatment is low-carbon lath martensite containing supersaturated copper and niobium. Sometimes due to insufficient quenching or high heating temperature, there will be a small amount of retained austenite and ferrite.

Aging treatment The aging treatment of 17-4PH steel should determine the heating temperature and holding time according to the performance requirements. After the 17-4PH steel was in solid solution at 1040 ℃, the martensitic structure was tempered with the increase of the aging temperature, and the precipitate phase was continuously precipitated. When aging at 450 ℃, copper, niobium and other precipitates have been precipitated. When the temperature reaches 470-480 ℃, the intragranular precipitates are fine and dispersed, and the hardness of the material is the highest at this time.