How to weld 17-4PH stainless steel

Knowing how to weld 17-4PH stainless steel is very important as this alloy requires careful welding consideration, right welding filler wire & PWHT for desirable weldment properties.

What is Precipitation Hardened Stainless Steel?

Precipitation-hardening stainless steels, often designated with the suffix PH, are types of stainless steels that get their strength from precipitation reactions.

17-4PH steel is alloyed with controlled elements that form fine precipitates when heat treated. Hardening of most of these steels is also accomplished by the formation of martensite.

They are grouped into:

  1. martensitic,
  2. Semi-austenitic, and
  3. austenitic.
How to weld 17-4PH stainless steel

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). You can find17-4PH in semi-austenitic grade also & this is obtained by minor chemistry change in the alloy.

Delivery Specification & chemical composition of 17-4PH Steel

The material specification for 17-4PH is ASTM A693 (Grade 630 UNSS17400), AMS 5604. The UNS Number of 17-4PH is UNSS17400. 17-4PH is an unclassified material, hence no P-Number as per ASME Section IX.

The nominal compositions of 17-4PH grades are given in the below table:

Martensitic type
C%Si%Mn%P%S%Cr%Ni%Cu%Nb% 17.503-53-5.015-.045
Non-Martensitic type
C%Si%Mn%P%S%Cr%Ni%Al%Cu/ Nb 186.5-7.750.75-1.5000
Note: Single values are the maximum allowed.

Mechanical properties vs. heat treatment conditions:

The mechanical properties of 17-4PH stainless steel depend on precipitation treatment. Table 2 shows the various precipitation treatment temperatures and their corresponding mechanical properties of 17-3PH grade material.

The hardness of 17-4PH also depends on the heat treatment. The maximum hardness of40- 48 HRC for 17-4PH is obtained by precipitation treatment at 482 Deg C after solution annealing.

A detailed hardness chart for 17-4PH at various heat treatment temperatures is given below.

How to weld 17-4PH stainless steel
Table 1: Various heat treatments for 17-4PH stainless steel

Microstructures of hot rolled 17-4PH stainless steel (martensitic type) & 17-7PH stainless steel (Semi-austenitic type)

How to weld 17-4PH stainless steel

17-4PH Welding Consideration:

  1. 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.
  2. 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 the below figure.

Pseudobinary Thermocalc® diagram for 17-4PH
Pseudobinary Thermocalc® diagram for 17-4PH

It can be seen that they all solidify as virtually 100% ferrite. During solidification, ferrite converts to austenite however slight residual ferrite can remain in the microstructure after solidification. Although, during welding of 16-4PH material, most of the austenite will convert to martensite.

Welding of PH steels shall be done in the solution-treated condition before precipitation takes place. At this tage, the martensitic type PH stainless steels are little hard but still somewhat 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.

Best Welding consumables/ rod for 17-4PH:

The best welding rod for 17-4 ph stainless steel is an E630 stick welding or SMAW rod and ER17-4PH / ER308 is used for TIG/ MIG welding of 17-4PH steel. 17-4PH can be successfully welded using Stick welding (SMAW), GTAW & GMAW processes.

Usually, after welding, 17-4PH steel is heat-treated by full solution heat treatment and aging (Precipitation treatment).

Stick Welding or SMAW welding of 17-4PH is carried out using an E630 rod. For TIG and MIG Welding of 17-4PH, the suitable welding rod/ filler wire is ER17-4PH and ER308.

The table below shows the welding electrodes and filler wires combinations:

DesignationUNS NumberSMAW ElectrodeSolid wire (GTAW/GMAW)Dissimilar welding of 17-4PH to SS/ CS
Martensitic types PH-steel
17-4PH or 15-5PHS17400/ S15500E630 (17-4PH) or E30817-4PH or ER308E/ ER309 or E, ER309 Cb
Semi-Austenitic types PH-steel
17-7PHS17700E630 (17-4PH) or E30817-4PHE/ ER310, ENiCrFe-2 or ERNiCr-3
 Table 2: Welding consumables for 17-4PH/ 15-5PH stainless steel

Welding 17-4 PH to carbon steel & SS304, SS316L

Welding of 17-4PH to carbon steel is carried out using an E309L type stick welding rod. TIG/ MIG welding of carbon steel to 17-4PH can be done using ER309L.

Stick Welding or TIG/ MIG Welding of 17-4PH to SS304L or SS316L is carried out using Er309L or ENiCrFe-2 or ERNiCr-3 type welding rod and filler wire.

Preheating for 17-4PH Steel Welding

Preheating in the range of 150 to 200 degrees C is recommended to avoid any weld cracking issues.

17-4PH PWHT after welding:

Improving the strength of 17-4PH steel is a critical process and it needs precise control of the heat-treatment process. The martensitic alloys, after welding, are usually given a single PWHT in the range of 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.

17-4 PH Stainless Steel heat treatment

The main heat treatment for 17-4PH is precipitation hardening. This heat treatment is carried out after solution annealing of the material. The detailed precipitation treatment temperature and corresponding mechanical properties of 17-4PH material are given in the below table for reference.

How to weld 17-4PH stainless steel
Table 1: Various heat treatments for 17-4PH stainless steel

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