Weld Then Machine, or Machine Then Weld?

September 21, 2026

A welded and machined stainless assembly has more ways to go wrong than either operation alone. Most of them are avoidable by deciding the sequence deliberately rather than by default.

Welding moves the part

A weld is a local melt that shrinks as it solidifies and cools. That shrinkage pulls the surrounding material, and austenitic stainless amplifies it. Its coefficient of thermal expansion is roughly 50 percent higher than carbon steel and its thermal conductivity is about a third, so heat concentrates and the expansion is larger.

The practical consequence is that a welded stainless frame can move by millimetres. Machining tolerances are measured in hundredths. Any dimension finished before welding is a dimension you will have to check and probably rework afterwards.

So the default sequence is: fabricate, weld, stress relieve if needed, then machine the features that matter.

The exceptions are real but narrow. Machine before welding when a feature will be inaccessible afterwards, when the part is too large to fit the machine once assembled or when a surface has to be flat before a mating part is welded to it. Say on the drawing which it is, because a shop cannot infer your intent from a finished dimension.

Sensitisation and weld decay

Chromium is what makes stainless stainless, and carbon competes for it. Hold the material between roughly 450 C and 850 C and chromium carbides form at grain boundaries. The chromium locked into those carbides is no longer protecting the boundary, so a narrow band either side of the weld corrodes preferentially. In service the attack runs along grain boundaries and the part can fail while still looking sound.

Weld heat puts a band through that temperature range on every pass. That is why the L grades exist.

Specify 316L or 304L on anything that will be welded, including parts that will only be welded once, during installation, by somebody else. There is more detail in 316, 316L and 316Ti compared.

For sustained high temperature service the L trick is not enough, because carbide can form over months rather than seconds. That is the case for a stabilised grade such as 316Ti or 321.

Heat tint is not cosmetic

The straw, blue and purple colours next to a weld are oxide layers, and under them the metal is chromium depleted. Heat tint measurably reduces corrosion resistance, and in food, pharmaceutical and high purity work it is a rejection item.

Removing it takes one of:

• Pickling, usually a nitric hydrofluoric paste or bath. Effective and it removes the depleted layer as well as the colour.
• Mechanical cleaning, abrasive or brushing. Must use stainless or non metallic media, because a carbon steel brush embeds iron.
• Electrochemical cleaning, for localised work.

Back purging with argon during welding prevents most of it forming in the first place, which is cheaper than removing it. Say in the RFQ whether the internal surface of a tube or vessel is product contact, because that decides whether back purging is required.

After cleaning, passivate. See citric against nitric.

Distortion control, and what you can ask for

You are not specifying the welding procedure, but a few requests are reasonable and they change the outcome.

Ask for stress relief where flatness matters. On austenitic grades a solution anneal fully resets the structure but it is a high temperature operation that distorts thin parts and is impractical on large assemblies. A lower temperature stress relief is more common in practice and it is a compromise. Discuss it rather than writing it blind.

Allow machining stock. Tell the shop how much stock to leave on critical faces after welding. Without a number, they guess.

Give a datum scheme that survives welding. If your datums are on a face that will be welded to, the inspection is meaningless. Put datums on machined features that exist after the assembly is complete.

Do not over constrain flatness on a large weldment. Flatness on a 600 mm welded frame is expensive and often unnecessary. Specify it locally where the mating part sits.

Duplex and precipitation hardening grades

Both need more care.

Duplex. The phase balance between austenite and ferrite is set by the mill and weld heat can destroy it, taking the corrosion resistance with it. Welding duplex needs a qualified procedure and controlled heat input. If your 2205 part is welded, say so prominently in the RFQ.

17-4 PH. Weld in the solution treated condition and age afterwards, otherwise the weld heat alters the temper locally. Say which condition applies to the finished assembly. See 17-4 PH conditions.

What to put in the RFQ

1. The sequence you expect, or a statement that the shop should propose one
2. Grade, with the L designation where the part is welded
3. Weld requirements: process, filler, whether a procedure qualification or welder certification is required
4. Whether back purging is required, and on which surfaces
5. Heat tint removal method, and the acceptance criterion
6. Stress relief, if any, and at what stage
7. Machining stock to leave on critical surfaces
8. Datums on features that exist after welding
9. Passivation after all of it, with the standard and test

This is a longer list than a single machined part needs, and that is the point. A welded stainless assembly quoted from a drawing that says weld and machine will come back with three very different prices. The RFQ checklist covers the common ground.

Frequently asked questions

Figures in this guide are published specification limits or published typical values for the alloy, not measurements of any part. Machinability is rated against free-machining carbon steel B1112 at 100 and PREN is chromium plus 3.3 times molybdenum plus 16 times nitrogen. The conventions are set out in thereference section and the editorial standards.

Keep reading

• Choosing a Passivation Method Under ASTM A967
• Passivation and Electropolishing: What Each One Does