1. What categories do these four types of stainless steel belong to? What’s the core difference?
17-4PH is a martensitic precipitation hardening stainless steel. Its unique feature is that through solid solution treatment followed by aging heat treatment, nanoscale copper-rich phases and carbides are precipitated, thereby obtaining ultra-high strength, and the strength can be flexibly adjusted within a wide range.
F51 is a duplex stainless steel with a microstructure composed of approximately half ferrite and half austenite. This dual-phase structure combines the high strength of ferritic stainless steel with the good toughness of austenitic stainless steel, and has excellent resistance to corrosion cracking.
SS431 is a high-chromium martensitic stainless steel with a chromium content of 15%-17%. It obtains high strength and hardness through quenching and tempering, and its corrosion resistance is significantly better than ordinary martensitic stainless steel.
SS420 is a basic martensitic stainless steel with a high carbon content (about 0.15%-0.40%). It can obtain extremely high hardness and excellent wear resistance through heat treatment, but its corrosion resistance is relatively limited among the four materials.
2. How to rank strength and hardness? How to select materials based on mechanical properties?
From the perspective of achievable hardness and strength potential, the ranking is roughly: 17-4PH ≥ 420 > 431 > F51, but each has a different focus.
The 17-4PH strength provides the most flexibility. By selecting different aging temperatures, its tensile strength can be adjusted between 930MPa and 1310MPa. For example, when aged at low temperature at 480°C, the hardness can reach HRC 40-45 while maintaining good plasticity and toughness. It is the first choice for aerospace and heavy-duty structural parts.
The advantage of SS420 is its hardness. After quenching and low-temperature tempering, the hardness can reach HRC 50-55 or even higher, and the tensile strength can reach over 1500MPa. However, the price is low plasticity, which is suitable for working conditions where wear resistance is the main failure mode.
The quenching and tempering hardness of SS431 is usually between HRC 35-45, and the tensile strength is about 850-1000MPa. Its comprehensive mechanical properties are better than ordinary carbon steel and low alloy steel, and it is suitable for making shafts and fasteners under high load conditions.
The yield strength of F51 is more than twice that of conventional austenitic stainless steel (such as 304, 316). The yield strength at room temperature can reach 450-550MPa, and the tensile strength is about 620-800MPa. Although the absolute hardness is not as good as that of martensitic steel, its specific strength (strength/density) advantage is obvious, and the wall thickness and structural weight can be significantly reduced during design.
Material selection suggestions: Choose 17-4PH when pursuing ultimate strength and load-bearing capacity; choose 420 when wear resistance is the primary conflict; choose 431 when a certain strength and good corrosion resistance are required; choose F51 for large structural parts that face the risk of chloride corrosion.
3. How does corrosion resistance compare? What are the shortcomings of each?
In conventional atmosphere and weakly corrosive media, the corrosion resistance ranking is: F51 > 17-4PH ≈ 431 > 420.
F51 has the strongest corrosion resistance. Its chromium content reaches 21%-23%, molybdenum content is 2.5%-3.5%, and nitrogen is added. The pitting corrosion resistance equivalent (PREN) is usually above 35. It has excellent resistance to pitting, crevice corrosion and stress corrosion cracking in chloride environments, making it the first choice for marine and chemical environments. However, it should be noted that F51 may precipitate harmful intermetallic phases after long-term service at high temperatures (>300°C), resulting in a decrease in toughness and corrosion resistance.
The corrosion resistance of 17-4PH is equivalent to that of 304 stainless steel, and it has good resistance to atmosphere, fresh water, dilute acid and salt environments. Its shortcomings are: it is not recommended for use in high-temperature environments above 300°C (over-aging softening will occur), and it is not suitable for strong reducing acids or high-temperature and high-pressure media containing halogens. Additionally, the corrosion resistance of the weld heat-affected zone may be slightly lower than that of the base metal.
Due to its high chromium content of 15%-17%, SS431 has good resistance to nitric acid, organic acids and most neutral salt solutions, and its corrosion resistance is significantly better than 420. Its shortcoming is that its nickel content is low and its performance in strongly oxidizing media is not as good as austenitic steel.
The corrosion resistance of SS420 is relatively average and is only suitable for atmosphere, fresh water and weakly corrosive media. Spot rust is prone to occur in humid or weakly acidic environments, and the surface condition needs to be optimized through passivation treatment after heat treatment. Not suitable for long-term contact with salt water, acidic solutions or high temperature and humidity environments.
4. What application scenarios are each suitable for?
17-4PH occupies the high-end manufacturing field with its ultra-high strength and good comprehensive performance. In aerospace, it is used in gas turbine compressor blades, turbine disks and aircraft structural connectors; in the energy field, it is used in high-pressure valves, nuclear waste storage and transportation barrels and deep-sea oil production equipment components; in general machinery, it is used in high-strength transmission shafts, gears and molds. Typical scenarios are parts that are required to withstand extreme loads and work in certain corrosive media for a long time.
The advantage of F51 lies in the combination of structural strength + corrosion resistance. In the oil and gas industry, it is used to transport oil and gas pipelines, downhole tubing hangers and Christmas trees containing H₂S and CO₂; in the chemical industry, it is used in heat exchangers, reactors and chemical storage tanks; in the paper industry, it is used to replace easily corroded carbon steel in the manufacture of digesters and bleaching equipment; in the seawater desalination and shipbuilding industries, it is used in pump casings, propellers and seawater cooling systems. For structural parts that are exposed to chloride-containing media over a large area and need to be loaded, F51 is often the optimal solution.
The application of SS431 is between martensitic steel and austenitic steel. In machinery manufacturing, it is used for high-load pump shafts, compressor connecting rods, transmission gears and valve stems; in the automotive industry, it is used for high-performance engine valves, crankshafts and transmission half-shafts; in general industry, it is widely used as high-strength bolts, pins and wear-resistant bushings. Its typical characteristics are that it requires both a certain surface hardness and no obvious rust in a humid atmosphere.
SS420 adheres to the positioning of “hard and wear-resistant”. It is used in surgical blades, bone drills and dental instruments in medical devices; in the field of cutting tools it is used in kitchen knives, industrial scissors and food processing blades; in precision machinery it is used in bearing cages, valve sealing surfaces and injection mold cores. In conditions where corrosive environments are mild but abrasive or adhesive wear is severe, 420 provides excellent service life at a relatively low price.
5. Conclusion
TWS understands that “working conditions determine material selection”. For each butterfly valve order, we accurately match the valve shaft material from 17-4PH, F51, SS431, SS420 and other stainless steels based on the medium, pressure, temperature and corrosive environment provided by the customer – with the optimal combination of strength, hardness and corrosion resistance, each butterfly valve exactly fits its working conditions.
Post time: Nov-07-2026
