Precipitation-hardening stainless steels, often designated with a suffix PH, are types of stainless steels that get their strength from precipitation reactions. They contain elements that will form fine precipitates when heat treated. Hardening of most of these steels is also accomplished by the formation of martensite.
They are grouped into:
- Semi-austenitic, and
17-4PH is a martensitic precipitation hardened stainless steel having high tensile strength, corrosion resistance, toughness, and good machinability up to 600°F (316°C). This grade is also available in semi-austenitic type, where there is a slight change in the chemical composition as shown in the below table.
Delivery Specification & chemical composition of 17-4PH Steel
ASTM A693 (Grade 630 UNSS17400), AMS 5604
The nominal compositions of 17-4PH grades are given in the below table:
Mechanical properties vs. heat treatment conditions:
Mechanical properties of 17-4PH stainless steel depending on precipitation treatment. Table 2 shows the various precipitation treatment temperatures and their corresponding mechanical properties of 17-3PH grade material.
Microstructures of hot rolled 17-4PH stainless steel (martensitic type) & 17-7PH stainless steel (Semi-austenitic type)
17-4PH Welding Consideration:
- When welding 17-7PH types, higher heat input is advised to obtain high dilution. By this, 17-7PH steel will get a reasonable heat treatment response due to base metal dilution.
- A short arc during welding shall be used to minimize the oxidation, loss of chromium, and nitrogen pickup from the atmosphere.
17-4PH Metallurgy consideration:
MS Temperature: 132°C (270°F),
Mf Temperature: 32°C (90°F)
The martensitic and semi-austenitic precipitation-hardening stainless steels solidify as primary ferrite in either the FA or F mode as shown in below figure.
It can be seen that they all solidify as virtually 100% ferrite. The ferrite largely transforms to austenite during cooling, but some residual ferrite in the microstructure at ambient temperatures should be expected. But during welding, much of the austenite will transform to martensite.
Welding of PH steels shall be done in the solution-treated condition before precipitation takes place. In this condition, the martensitic PH stainless steels are somewhat hard but still moderately ductile. Because the weld metal
cools rapidly, virtually no precipitation normally occurs in the weld metal, so that in the as-welded condition, the weld metal is not unlike the solution-annealed base metal in terms of its microstructure and properties.
Welding consumables for 17-4PH:
17-4PH can be successfully welded using SMAW, GTAW & GMAW processes. E630 types electrode which matches the base metal chemistry of 17-4PH steel is recommended for welding 17-4PH and 17-7PH types stainless steels. Welding shall be followed by full solution heat treatment and aging (Precipitation treatment).
The table below shows the welding electrodes and filler wires combinations:
|Designation||UNS Number||SMAW Electrode||Solid wire (GTAW/GMAW)||Dissimilar welding of 17-4PH to SS/ CS|
|Martensitic types PH-steel|
|17-4PH or 15-5PH||S17400/ S15500||E630 (17-4PH) or E308||17-4PH or ER308||E/ ER309 or E, ERR309 Cb|
|Semi-Austenitic types PH-steel|
|17-7PH||S17700||E630 (17-4PH) or E308||17-4PH||E/ ER310, ENiCrFe-2 or ERNiCr-3|
Preheating for 17-4PH steel Welding
Preheating in the range of 150 to 200 degree C is recommended to avoid to any weld cracking issues.
PWHT after welding:
the strengthening treatments for the PH stainless steels can be quite complex and require careful control to optimize properties. The martensitic alloys, after welding, are usually given a single PWHT in the range 480 to 620°C (900 to 1150°F). This heat treatment both tempers the martensite and promotes precipitation strengthening. At high PWHT temperatures [over 540°C (1000°F)], some austenite may re-form in the final structure.