Two Very Different Stainless Steels
17-4 PH, also designated UNS S17400 and SUS630 in Japan, is a precipitation-hardening martensitic stainless steel, while 304, or UNS S30400, is an austenitic stainless steel. Although both are called stainless, they belong to different metallurgical families with different crystal structures, heat treatments and property profiles. The choice between them is not about corrosion resistance alone; it is about whether the part needs extremely high strength in a corrosion-resistant material.
Comparing Mechanical Properties
The strength difference between the two grades is the defining distinction:
304 annealed: yield strength about 205 MPa minimum, tensile strength about 515 MPa minimum, elongation about 40 percent.
17-4 PH condition H900: yield strength about 1170 MPa, tensile strength about 1310 MPa, with hardness around 40 HRC or higher.
17-4 PH condition H1025: yield strength about 1000 MPa, tensile strength about 1070 MPa, hardness around 33 to 38 HRC.
17-4 PH condition H1150: yield strength about 725 MPa, tensile strength about 930 MPa, with maximum toughness for the grade.
The aging condition of 17-4 PH is chosen to balance strength, toughness and stress corrosion resistance for the specific application.
Why 17-4 PH Is So Much Stronger
17-4 PH contains about 15 to 17.5 percent chromium, 3 to 5 percent nickel, 3 to 5 percent copper and small additions of niobium and tantalum. After solution treatment and cooling, the martensitic structure is aged at a temperature between 480 and 620 degrees Celsius, which precipitates fine copper-rich particles that harden the matrix. This precipitation hardening is what lifts the yield strength far above the level of any austenitic stainless such as 304, and the process is reversible, so the bar can be re-solution-treated and re-aged if needed.
Corrosion Resistance and Other Differences
In the annealed and lightly aged conditions, 17-4 PH offers corrosion resistance broadly similar to 304 in atmospheric and mild chemical service, but it is less resistant in strongly reducing or chloride environments, and its toughness is lower than austenitic 304. 304 remains the better choice where deep forming, very high toughness or superior general corrosion resistance is required. 17-4 PH also has higher hardness, which improves wear resistance but makes machining more demanding, and it is magnetic, while annealed 304 is essentially non-magnetic.
Where 17-4 PH Round Bar Is Used
Aerospace fasteners, actuator shafts and landing gear components.
Pump shafts, valve stems and high-pressure fittings in the oil and gas and chemical industries.
Marine hardware such as propeller shafts and underwater fasteners.
Medical instruments and surgical tools where strength and corrosion resistance combine.
Paper mill and food processing shafts that face corrosive washdown environments.
Frequently Asked Questions
Q: How much stronger is 17-4 PH than 304?
In the H900 aged condition, 17-4 PH has a yield strength of about 1170 MPa, roughly five times the 205 MPa minimum yield of annealed 304.
Q: What does PH mean in 17-4 PH?
PH stands for precipitation hardening, the heat treatment that precipitates copper particles in the martensitic matrix to reach high strength.
Q: Is 17-4 PH corrosion resistant like 304?
In many environments it is broadly similar, but 304 generally has better general corrosion resistance and toughness, while 17-4 PH wins on strength and hardness.
Q: Which condition of 17-4 PH should be ordered?
It depends on the application: H900 for maximum strength, H1025 for a balance of strength and toughness, and H1150 for maximum toughness and stress corrosion resistance.
Q: What are the common designations of 17-4 PH?
UNS S17400, AISI 630, SUS630 in Japan and the trade condition names H900 through H1150 for the aged states.