17-4 PH

What is 17-4 PH?

17-4 PH is a martensitic precipitation-hardening stainless steel. It contains 15.0 to 17.5 percent chromium, 3.0 to 5.0 percent nickel, 3.0 to 5.0 percent copper, and 0.15 to 0.45 percent niobium plus tantalum. Its maximum carbon content is 0.07 percent. The designation refers to the nominal chromium and nickel contents of 17 percent and 4 percent. Copper produces the precipitation-hardening response.

The grade is supplied solution treated in Condition A. In this condition, it has a martensitic structure and remains suitable for machining. A single low-temperature ageing treatment develops its strength. During ageing, copper leaves the supersaturated solution and forms a fine dispersion of copper-rich particles. These particles restrict dislocation movement. Ageing occurs between about 480 and 620 degrees Celsius. It causes no significant distortion. Predictable shrinkage is approximately 0.05 percent in the hardest condition and 0.10 percent in the softest condition.

This treatment distinguishes 17-4 PH from the other martensitic grades considered here. A long, slender shaft can be finish machined in the softer condition and then hardened to more than 1,300 MPa tensile strength without subsequent straightening. This sequence is not possible with AISI 410, 420, or 431.

Laxcon Steels lists 17-4 PH in its grades reference as a precipitation hardening grade. The same steel is written 17-4PH, 17/4 PH, 630, SUS 630, 1.4542 and S17400.

What is the chemical composition of 17-4 PH?

Composition limits in weight percent are carbon 0.07 maximum, manganese 1.0 maximum, sulphur 0.03 maximum, phosphorus 0.04 maximum, silicon 1.0 maximum, chromium 15.0 to 17.5, nickel 3.0 to 5.0, copper 3.0 to 5.0 and niobium plus tantalum 0.15 to 0.45.

What is 17-4 PH equivalent to in other standards?

Commonly listed under its AISI type number, 630.

Equivalence means nearest counterpart, not identity: each standards body sets its own composition window, so check the limits of the standard actually named on the order before substituting.

What is the PREN of 17-4 PH?

Computed by Laxcon Steels from the composition above as PREN = Cr + 3.3Mo + 16N, the relationship published in Practical Guidelines for the Fabrication of Duplex Stainless Steels (International Molybdenum Association, third edition, 2014). The published relationship is given for austenitic and duplex stainless steels, so this figure extends it: read it against other precipitation hardening grades rather than across families. The first figure uses the minimum specified chromium, molybdenum and nitrogen, so it is the floor for a conforming heat rather than a typical value; the second uses the maxima, so the two together are the width of the band one compliant grade allows. PREN ranks resistance to chloride pitting and nothing else: it does not predict crevice corrosion, stress corrosion cracking or service life, and it is not a substitute for a corrosion test. The pitting resistance reference carries the full table and the caveats.

Ageing conditions and mechanical properties

The condition designations identify the ageing temperature in degrees Fahrenheit. ASTM A564 specifies bar in these conditions. The published minima from that standard vary with the selected treatment. Condition A is solution treated without ageing. It typically gives about 1,105 MPa tensile strength, 1,000 MPa proof stress, and 15 percent elongation, with a maximum hardness of 38 HRC. H900 requires ageing at 480 degrees Celsius for one hour. It requires 1,310 MPa tensile strength, 1,170 MPa proof stress, 10 percent elongation, and 40 HRC. H925 requires ageing at 495 degrees Celsius for four hours. It requires 1,170 MPa tensile strength and 1,070 MPa proof stress at 38 HRC.

Higher ageing temperatures reduce strength and hardness. H1025 requires ageing at 550 degrees Celsius for four hours. It gives 1,070 MPa tensile strength and 1,000 MPa proof stress at 35 HRC. H1075 at 580 degrees Celsius gives 1,000 MPa tensile strength and 860 MPa proof stress at 32 HRC. H1100 at 595 degrees Celsius gives 965 MPa tensile strength and 795 MPa proof stress at 31 HRC. H1150 at 620 degrees Celsius gives 930 MPa tensile strength, 725 MPa proof stress, 16 percent elongation, and 28 HRC. Before ageing, solution treatment requires a hold at 1,040 degrees Celsius for half an hour, followed by air cooling to a maximum of 30 degrees Celsius.

Tensile strength decreases by roughly 30 percent between H900 and H1150. Elongation increases from 10 to 16 percent. The ageing condition also affects resistance to cracking.

Stress corrosion and sulphide stress cracking

17-4 PH has high resistance to stress corrosion cracking when aged at 550 degrees Celsius or higher. Resistance improves as the ageing temperature rises. Condition A has lower ductility and lower resistance to stress corrosion cracking. It should not generally be used for service in that condition, even when its hardness is satisfactory.

Resistance to sulphide stress cracking also requires a highly aged condition. NACE MR0175 permits 17-4 PH only in a double age-hardened condition. H900 provides maximum strength but lies at the less resistant end of the range for both cracking mechanisms. H1025 or H1150 reduces strength and increases environmental cracking resistance.

Corrosion behaviour

17-4 PH has a pitting resistance equivalent number of 15.0 at the specified composition minima and 17.5 at the maxima. Its general corrosion resistance is good in a wide range of environments and approaches that of AISI 304. It can also reach 1,310 MPa tensile strength. Like AISI 304, it is susceptible to pitting and crevice corrosion in warm chloride environments. The grade has been used for boat shafting in fresh water. Its use in seawater requires cathodic protection.

Machining, welding, and temperature limits

17-4 PH is normally supplied solution treated because Condition A permits machining. Its machinability in this condition is similar to that of AISI 304. All standard methods can weld the grade without preheating. Post-weld heat treatment can give the weld properties comparable with those of the parent metal. Designs should avoid concentrations of weldment stress, as with other high-strength steels.

The grade has good oxidation resistance. Its practical temperature limit depends on the ageing treatment rather than scaling. Service above the applied ageing temperature causes continued ageing and loss of strength. Prolonged exposure between 370 and 480 degrees Celsius should also be avoided when ambient-temperature toughness is critical.

Applications and designations

Applications include high-strength pump shafts, boat propeller shafts, valve components, gears, bolts, plastic moulding dies, aerospace components, engine components, and oilfield hardware. The grade is also used for parts that require high strength or hardness with corrosion resistance.

The UNS designation is S17400, and the AISI type number is 630. Older tables therefore identify the grade as Type 630. The nearest European counterpart is EN material number 1.4542, with the name X5CrNiCuNb16-4. The Japanese designation is SUS 630. Purchaser designations include 17-4PH, 17/4 PH, 630, SUS630, and SS174PH.

In which product forms does Laxcon list 17-4 PH?

Taken from the published specification on each product page. A form that is not listed here is not a form this reference can confirm the grade in; the enquiry route settles what can actually be rolled to an order.

Where 17-4 PH is not the right choice

17-4 PH is unsuitable for service in Condition A. Solution-treated but unaged material has reduced ductility and materially lower resistance to stress corrosion cracking. It should not enter service in this state, even when its hardness is adequate. A drawing that does not identify the required condition does not fully specify the part.

The H900 condition is unsuitable where chlorides and tensile stress occur together. Maximum strength corresponds to the lowest cracking resistance in the ageing range. A highly stressed H900 component is therefore vulnerable in marine or sour environments. Ageing at 550 degrees Celsius or higher improves cracking resistance. Sour service requires the double age-hardening condition specified by NACE MR0175. This change reduces tensile strength by roughly one fifth.

17-4 PH is unsuitable for seawater service without cathodic protection. Its PREN range of 15.0 to 17.5 sits below that of AISI 304, which is 18.0 to 21.6. Super duplex 2507 is an alternative where high strength and seawater resistance are both required. It has a PREN of 37.7 to 47.6 and a minimum yield strength of 550 MPa. The grade is also unsuitable above its ageing temperature or for prolonged exposure between 370 and 480 degrees Celsius, where toughness decreases. Where transverse toughness in heavy sections is critical, 15-5 PH is a refinement of the same alloy developed specifically to improve that property.

Other precipitation hardening grades