What Kind of Stainless Steel Is SUS630? 17-4PH Grade Explained

What SUS630 Is

SUS630 is the designation used in Japanese industrial standards for a martensitic precipitation hardening stainless steel. The same material is widely known as grade 630 and as 17-4PH, and its unified numbering system identity is UNS S17400. It is one of the most widely used precipitation hardening stainless steels because it offers high strength together with corrosion resistance that is clearly better than that of the standard 12 percent chromium martensitic grades.

The hardening mechanism is what distinguishes this family from ordinary martensitic stainless steel. Instead of relying on carbon to form hard phases during quenching, SUS630 is solution treated to produce a low carbon martensitic structure and then aged at a moderate temperature. During aging, fine copper rich precipitates form and raise the strength. Because the aging temperature can be varied, the same steel can be supplied in several strength and toughness combinations without changing its composition.

Chemistry and Equivalent Designations

The composition is balanced around roughly 17 percent chromium and 4 percent nickel, with copper and niobium additions that form the strengthening precipitates during aging. Carbon is kept low, typically at or below about 0.07 percent, so that the corrosion resistance and toughness of the aged material are not degraded by carbide formation. Elements such as sulfur and phosphorus are held to low levels to maintain toughness in the transverse direction.

Heat Treatment and Strength Levels

The standard treatment is a solution anneal at approximately 1040 degrees Celsius followed by cooling, and then aging at a temperature chosen to suit the required properties. Lower aging temperatures give higher strength and lower ductility; higher aging temperatures give lower strength with improved toughness. Typical minimum property values for aged conditions, as specified for bar and forging product forms, are shown below.

These figures apply to round bar and forgings. Product forms such as cold drawn tube, wire and sheet have their own requirements in the relevant product specification, so the applicable standard for the form being purchased should always be stated on the order.

Corrosion Behaviour and Temperature Limits

SUS630 resists atmospheric corrosion and attack by dilute acids and salts at a level that is superior to the 12 percent chromium martensitic grades and broadly comparable to common austenitic grades of the 304 family in mild environments. It is not, however, a substitute for a molybdenum bearing austenitic or duplex stainless steel in chloride rich service, where pitting and crevice corrosion resistance is the deciding factor.

Two temperature effects must be respected. Prolonged service above roughly 300 degrees Celsius causes overaging and a progressive loss of strength, so the material is normally limited to that level for load bearing duties. At the other end of the range, low temperature toughness is limited and the grade is not intended for cryogenic service. Because the material is martensitic and high strength, it can also be sensitive to hydrogen embrittlement, so it is not the first choice for sour oilfield service.

Applications

High strength shafts, valve stems, spindles and pump components that must resist both load and mild corrosion.

Last stage moving blades and other structural parts of steam turbines operating below the temperature limit.

Fasteners, couplings and precision machined parts where a strength level above that of austenitic stainless steel is required.

Components in chemical and food processing plant exposed to dilute acids, alkalis and salt solutions in non chloride critical service.

Aircraft and general engineering parts requiring a combination of high strength and good atmospheric corrosion resistance.

Fabrication Points

SUS630 is machinable in the solution treated condition and is usually aged after machining. Welding is possible with matching filler metals, but the weld and heat affected zone must be re-aged after welding to restore strength, and the resulting properties depend on the joint geometry and the section thickness. Deep cold forming is difficult in the aged condition and should be planned before aging rather than afterwards. Where tolerance is critical, the slight dimensional change that occurs during aging should be allowed for in the machining sequence. Finally, if corrosion resistance is the main reason for choosing the grade, surfaces should be kept clean and free from embedded iron or scale, since surface condition frequently governs performance in service.

FAQ

Q: Is SUS630 stainless steel?
Yes. It is a martensitic precipitation hardening stainless steel containing roughly 17 percent chromium, which gives it stainless properties with corrosion resistance better than the plain chromium martensitic grades.

Q: What is the difference between SUS630 and 17-4PH?
They are the same material. SUS630 is the Japanese standard designation and 17-4PH is the widely used trade description, with UNS number S17400.

Q: What strength can SUS630 reach?
It depends on the aging condition. In the H900 condition typical minimum values are around 1310 MPa tensile and 1170 MPa yield, while the H1150 condition gives roughly 930 MPa tensile and 725 MPa yield with better toughness.

Q: Can SUS630 be used at high temperature?
Load bearing service is normally limited to about 300 degrees Celsius, because higher temperatures cause overaging and a permanent loss of strength.

Q: Is SUS630 better than type 316 stainless steel?
It is considerably stronger, but its pitting and crevice corrosion resistance in chloride environments is lower than that of molybdenum bearing austenitic grades. The choice depends on whether strength or chloride resistance governs the application.

Q: What happens to welded SUS630?
The weld and heat affected zone lose strength during welding, so a post weld aging treatment is normally required to restore the properties of the base material.