Heavy Forging Machining and Nitriding

Nitriding is one of the few surface treatments a buyer can specify and then discover, at delivery, that it was applied in the wrong sequence — with the part irrecoverable. The reason is that a nitrided surface cannot be machined again without losing the case. Every decision about machining, allowance and masking therefore has to be made before the part goes into the furnace, not after.

This guide covers how nitriding fits into a machined forging, what it does and does not change about the part, and what to put on the drawing so the sequence is right the first time.

Why Forgings Behave Differently From Plate

A forging is not a shaped piece of plate. The grain structure follows the shape of the die, so the material has directional properties and a denser, more continuous structure through highly stressed regions. For a component such as a shaft, a hub, a gear blank or a heavily loaded lever, this is the reason a forging is specified in the first place.

Two consequences matter for machining:

The as-forged surface is not a machining datum. Forging tolerances are broad, and scale and decarburisation sit on the outside. The first machining operation has to get below that layer before any dimension is meaningful.
There may be more stock than expected. A near-net forging leaves less to remove; a conventional forging can leave several millimetres. This affects both the number of passes and whether the finished part still sits inside the original grain flow.

A useful question to ask at quotation is whether the supplier has allowed for the decarburised layer when they set the roughing cut. If they have not, hardness readings on the finished surface can come back low for a reason that has nothing to do with the steel.

What Nitriding Actually Does

Nitriding diffuses nitrogen into the surface of the steel at a temperature well below the transformation range — typically in the region of 500–580 °C depending on the process. Because the part never becomes austenitic, there is no quench and therefore no quenching distortion. That is the main reason nitriding is chosen for finished, accurate components.

What it gives you:

High surface hardness without a change of core properties — the part stays tough underneath while the surface resists wear, galling and abrasion.
Low distortion compared with case hardening or induction hardening, so it can be the last operation on an already-machined part.
Improved fatigue resistance on parts subject to cyclic loading, from the compressive residual stress in the case.
No quench cracks, because there is no quench.

What it does not give you:

It does not correct dimensions. The part has to be at final size, or very close to it, before nitriding.
It does not make a soft core strong. Case hardness and core strength are separate decisions; the core has to be right from the forging and the pre-treatment.
It is not a corrosion coating. Nitrided steel offers some resistance but it does not replace painting, plating or stainless selection.

The Dimensional Change Nobody Warns You About

Nitriding grows the surface. The compound zone that forms at the outermost layer adds material, and the diffusion zone beneath it changes the volume of the case. On a ground shaft this shows up as a measurable increase in diameter.

The growth is small — commonly described in the range of a few micrometres to a few tens of micrometres depending on process, time and material — but on a tolerance measured in hundredths of a millimetre it is not negligible. Two things follow:

Either the part is machined undersize by the expected growth, and the supplier has to know that figure for their own process.
Or the part is ground or honed after nitriding to bring it back to final size, which removes part of the case and has to be accounted for in the specified case depth.

Which route is used is a process decision, not a drawing decision — but the drawing has to say which dimensions are critical, so the supplier knows where the growth matters. A shaft with a bearing seat at tolerance and a free end at general tolerance does not need the same treatment on both.

Machining Sequence: Where Nitriding Sits

The sequence that works in practice, for a machined and nitrided forging:

Rough machine — remove scale and decarburised material, establish the main geometry with allowance left on all nitrided surfaces.
Stress relief if required — for heavily reduced forgings, or where the part will be held to tight limits.
Semi-finish machine — bring non-nitrided features to size, leave the nitrided surfaces with allowance.
Finish machine — cut the nitrided surfaces to final size minus the expected growth. Threads, keyways and sealing faces are completed here.
Mask and nitride — protect anything that must not be hardened.
Grind or hone only where specified — for seats that must hold a tolerance tighter than the growth allows.
Inspect — dimensions, case depth by microhardness, and visual check of the surface.

The trap in this sequence is step 4. If a drawing asks for a nitrided bore to hold a tolerance and does not say whether grinding after nitriding is permitted, the supplier has to choose, and the two choices give different results.

Masking: The Detail That Ruins Parts

Nitriding hardens everything it can reach. Features that must stay soft, or that would be damaged by a hard brittle case, are protected by masking — stopping off with a coating, mechanical shielding, or by leaving extra stock that is removed afterwards.

Features that usually need masking or special handling:

Threads — a nitrided thread becomes brittle and can chip or crack under load; it also grows, which changes the fit.
Sealing faces and O-ring grooves — hardness is not the issue, but surface finish and the compound zone can be.
Bores for press fits — growth changes the interference.
Any surface that will be welded later — welding onto a nitrided surface is a defect waiting to happen.
Areas where the compound zone must not exist — for parts subject to impact or where a brittle white layer could spall.

Masking is a workshop decision, but which features need it is a design decision. If the drawing does not identify them, the supplier is guessing — and a guess that goes wrong is not recoverable by re-machining.

What to Put on the Drawing

The base material and its condition — grade, and whether it is supplied normalised, quenched and tempered, or in another condition. Nitriding response depends on the steel; not every grade nitrides well.
The nitriding process, if it matters — gas, plasma, salt bath or a named proprietary process, if you have a reason to require one.
Case depth and how it is measured — total case depth, or depth to a specified hardness, and the test method (usually Vickers microhardness at a stated load). Case depth without a measurement method is not a specification.
Surface hardness and where it is measured — a value at the surface is easy to hit and easy to fake by measuring on a lucky spot; a value at a stated depth is a real requirement.
The compound zone (white layer) — allowed, controlled, or removed. A brittle white layer is acceptable on some parts and unacceptable on others. If you have no preference, say what the part does and let the supplier recommend.
Which surfaces are nitrided and which are not — this is the masking instruction, and it belongs on the drawing.
Which dimensions are critical — so growth and post-nitriding grinding are applied where they matter.
Whether grinding or honing after nitriding is permitted.
Inspection requirements and the report — what is measured, at what depth, and in what form it is supplied.

Inspection After Nitriding

A nitriding report that only states a surface hardness figure is close to worthless, because the surface is the easiest place to get a good number. What a buyer should expect:

Case depth by microhardness traverse, measured on a test coupon processed with the batch, or on a sacrificial section of the part.
Hardness at a stated depth, not only at the surface.
Dimensional inspection after nitriding, because growth has occurred since the last machining operation.
Visual and, where relevant, metallographic check of the compound zone.
Which surfaces were masked, so the buyer can confirm the instruction was followed.

For a component that will be assembled into equipment, the dimensional record after nitriding is the one that matters — it describes the part as it will actually be used.

Where This Fits With Machining and Welding

Forgings that are nitrided are usually components rather than weldments, but they frequently meet welded structure: a nitrided shaft in a fabricated housing, a hardened pin in a welded bracket. Two rules apply at that interface. Never weld onto a nitrided surface, and never assume a nitrided part can be adjusted on site — it can only be replaced or re-made.

For the machining side of heavy components, see CNC machining for large and heavy steel components. For how tolerances are established and verified on large parts, see tolerance verification on large components and heavy fabrication tolerances.

Common Questions

Should a part be nitrided before or after final machining?

After. Nitriding is normally the last process on a machined part, because the case cannot be cut afterwards without losing it. The part is finish-machined to final size minus the expected growth, then nitrided. Some features are ground or honed after nitriding where the tolerance is tighter than the growth allows, but this is a planned exception rather than the general rule. Anything that still needs machining after nitriding has been sequenced wrongly.

How much does nitriding change the dimensions of a part?

The change comes from two effects: the compound zone adds material at the surface, and the diffusion zone beneath it changes volume. In practice the growth is small — a few micrometres to a few tens of micrometres depending on process, time and material — but it is real and it is directional on an outside diameter. On a tolerance held to hundredths of a millimetre it cannot be ignored. The supplier should know the growth figure for their own process and machine to it; the buyer should identify which dimensions are critical so the growth is managed where it matters.

Can a heavy forging be machined and then nitrided?

Yes, and that is the normal sequence for a forged component that needs a hard wear surface. The forging is rough machined to remove scale and decarburised material, stress relieved if required, then finish machined and nitrided. The size of the forging is not the constraint — the constraint is whether the workshop can handle and measure the part accurately, and whether the furnace can take it. Those are the two questions worth asking a supplier before assuming a large nitrided forging is feasible.

What steel grades are suitable for nitriding?

Nitriding response depends on the steel, and not every grade nitrides well. The grades usually chosen are those alloyed with elements that form stable nitrides — chromium, aluminium, molybdenum and vanadium. A common approach is to use a quenched and tempered alloy steel that both carries the load and responds to nitriding, rather than a plain carbon steel. If a drawing specifies nitriding on a grade that does not respond to it, the result is a part with the right dimensions and the wrong surface, which is usually discovered at the hardness test. The grade and the treatment have to be chosen together.

What is the white layer, and should it be removed?

The white layer, or compound zone, is the outermost region of the nitrided case where nitrogen concentration is high enough to form nitrides as a distinct surface layer. It is very hard and wear resistant, but it is also brittle. Whether it should be present, limited in thickness, or removed depends on what the part does: on a sliding wear surface it is often useful, on a part subject to impact or bending it can be a source of spalling. There is no universal answer, which is why the drawing should either state a requirement or state the application and let the supplier recommend.

Working With VHOFAB

VHOFAB machines heavy components to customer drawings and can arrange surface treatments including nitriding, with dimensional checks after treatment. Machining, welding and assembly are carried out in one workshop, so the sequence between machining and treatment is planned rather than passed between suppliers.

To discuss a forged or machined component and the treatment it needs, send the drawing and technical requirements through our contact form for a manufacturing review. Capabilities are listed on the capabilities page.