Can 17-4 PH stainless be welded?
Yes. Weld in Condition A with matching 17-4 PH (AMS 5825) filler, then solution treat and age the whole part. Welding an aged part softens the joint unless it is re-aged.
17-4 PH welds well by TIG, MIG, electron beam and laser. Its martensite is low in carbon (0.07% max), so it does not harden and crack the way a martensitic 410 or 420 does, and preheat is not normally needed. The right sequence is what matters. Weld the part in Condition A (solution treated), then heat treat the assembly to the final condition. The weld metal and heat-affected zone respond to the same aging cycle as the base metal, and the joint comes out with properties close to the parent, typically 90% or better of the H1025 or H1150 strength.
The matching filler is 17-4 PH itself, sold as AMS 5825 wire (also called ER630 under AWS A5.9). It gives a weld that hardens on aging. Where the weld will not be heat treated, or where the joint needs ductility more than strength, ER308L or ER309L austenitic fillers are used, at the cost of a soft weld about 550-620 MPa (80-90 ksi) tensile that will not match the aged base metal. Welding a part that has already been aged softens the heat-affected zone back toward Condition A and, if the part is not re-aged, leaves a weak, tough band beside the weld; re-aging restores most of the strength, but re-solution treatment is needed to make the properties uniform.
Practical points: keep interpass temperature under about 120 °C (250 °F), use argon shielding and a back-purge, and clean off the oxide before heat treatment. Heavy weldments benefit from a stress relief or a full solution treat before aging so that residual welding stresses do not add to the (small) aging contraction. Corrosion resistance across the weld is good in H1025 and H1150; H900 welds are more prone to stress-corrosion cracking and are avoided in chloride service.
Welding for 17-4 PH parts: where to send them
What to specify
- Weld in Condition A per AMS 5643 or ASTM A564 Type 630, and heat treat after welding to the specified condition (H1025 or H1150 are the usual choices for weldments)
- Filler: 17-4 PH (AMS 5825 / ER630) for heat-treated joints; ER308L or ER309L only where the weld will be left soft, and state which
- Post-weld heat treatment: solution treat at 1040 °C (1900 °F) and age, or age only, with the resulting hardness range
- Interpass temperature limit (120 °C / 250 °F) and argon back-purge on full-penetration welds
- Procedure qualification to AWS D1.6 or AMS 2680 / AWS D17.1 for aerospace, with a weld map
- Hardness test across the weld and heat-affected zone on a first-article or coupon
- Passivate after heat treatment and before assembly
Pitfalls
- Welding an H900 or H1025 part and shipping it without re-aging, leaving a soft heat-affected zone
- Using an austenitic filler and then expecting the weld to reach base-metal strength after aging
- Aging a welded assembly without a solution treat and getting a hardness gradient across the joint
- Choosing H900 for a welded part in chloride service; stress-corrosion cracking starts at the weld
- Ignoring the small aging contraction on a large weldment with finished bores
- Poor shielding leaving heavy oxide that scales and pits during the heat-treat cycle
Frequently asked questions
- What filler is used to weld 17-4 PH?
- Matching 17-4 PH wire, AMS 5825 (AWS ER630), when the assembly will be heat treated afterward. ER308L or ER309L austenitic fillers are used only for joints that will stay soft.
- Should 17-4 PH be welded before or after heat treatment?
- Before. Weld in Condition A, then solution treat and age the assembly. Welding after aging softens the heat-affected zone, and a re-age recovers only part of the strength.
- Does 17-4 PH need preheat for welding?
- Not usually. Its low-carbon martensite is not crack-sensitive like 410 or 420. Keep interpass temperature under about 120 °C (250 °F) and let the post-weld age do the rest.
