QUALITY · August 14, 2026 · 6 MIN READ

Surface Roughness Ra and Rz for Forgings: What the Process Delivers and What Needs Machining

As-forged surfaces typically measure Ra 6.3–12.5 µm, with Ra 3.2 µm achievable on a well-polished die and Ra 1.6 µm requiring machining. Rz runs about 4–7 times Ra depending on the process, so a drawing that specifies Ra 3.2 and Rz 6.3 simultaneously is usually asking for two different things — Rz 6.3 corresponds to roughly Ra 1.2–1.6.

WHERE THIS APPLIES AT FORGEPRESS CHINA

ForgePress China supplies Ra 6.3–12.5 µm as-forged and machines functional surfaces to Ra 1.6 µm or better when the drawing requires it. Capacity: HKM3 hot-die forging line 1,000–5,000 t, part weight 0.1–500 kg, maximum envelope 1,200 × 800 mm, as-forged to ISO 8062 class C with critical dimensions machined to ±0.2 mm, process control to ISO 9001, first article 4–8 weeks from die design release.

KEY NUMBERS

As-forged Ra: 6.3–12.5 µm typical; 3.2 µm on a polished impression; 1.6 µm requires machining.
Die impression finish is normally Ra 0.8–1.6 µm — the forging is always rougher than the die.
Rz ≈ 4–7 × Ra depending on process; specifying both independently often creates a contradiction.
Scale and decarburisation add 0.1–0.3 mm of surface layer that must be removed before measurement.

Two parameters, two different measurements

Ra is the arithmetic mean deviation of the profile from its mean line over the sampling length — an average, and therefore insensitive to a single deep scratch. Rz is the average of the maximum peak-to-valley heights within each sampling length, so it is dominated by the worst features.

That difference matters in sealing and fatigue applications. A surface with a good Ra but a few deep grooves will leak and will initiate cracks, and only Rz will show it. For general fits, Ra is adequate and cheaper to measure.

Process	Typical Ra (µm)	Typical Rz (µm)	Where it appears
As-forged, normal die	6.3–12.5	40–80	General forged surfaces
As-forged, polished die	3.2–6.3	20–40	Cosmetic or sealing surfaces
As-forged, die with worn land	12.5–25	80–150	End-of-life tooling
Rough machining	3.2–6.3	16–32	First machining pass
Finish machining	1.6–3.2	8–16	Functional surfaces
Grinding	0.4–0.8	2–4	Bearing seats, sealing faces

Typical values for carbon and low-alloy steel forgings. Stainless and heat-resistant alloys tend to run one grade rougher because of higher flow stress and heavier scale.

Why the forging is always rougher than the die

The die impression is polished to Ra 0.8–1.6 µm, yet the part comes out at Ra 6.3–12.5 µm. Three effects explain the gap. Scale forms on the billet in the furnace and is pressed into the surface. The metal does not perfectly replicate the die because flow stress at 1,000 °C is high and the contact time is milliseconds. And thermal cycling roughens the die itself — heat checking on the impression is transferred to every part.

This is why polishing the die beyond about Ra 0.8 µm produces diminishing returns: the scale and the flow behaviour set the floor, not the die finish.

The Ra/Rz ratio and the contradiction it creates

For most processes Rz is 4–7 times Ra. Turning tends toward 5–7, grinding toward 4–5, and forging toward 6–8 because the profile is dominated by scale pits and die marks. A drawing that asks for Ra 3.2 µm and Rz 6.3 µm is self-contradictory: Rz 6.3 corresponds to about Ra 1.2–1.6 µm, and meeting the Rz requirement will force machining that the Ra requirement did not ask for.

Specify one parameter and let the process own the other. For as-forged surfaces, specify Rz if the function is sealing or fatigue, and Ra if it is appearance or a general fit.

Measuring roughness on a forged surface

Remove the scale and the decarburised layer first, or the stylus measures the oxide, not the metal.
Use a cut-off length of 0.8 mm for Ra below 2 µm and 2.5 mm above; the default 0.8 mm on a coarse forged surface under-reads.
Measure at least three locations and report the worst, not the average, when the surface is functional.
Record the measurement direction — a forged surface is anisotropic and readings across the flow differ from readings along it.

Frequently asked questions

What surface finish does hot forging produce?

Ra 6.3–12.5 µm as-forged is typical, with Rz around 40–80 µm. A polished die can reach Ra 3.2 µm. Anything below Ra 1.6 µm requires machining or grinding after forging.

What is the relationship between Ra and Rz?

Rz is roughly 4–7 times Ra for most processes, and 6–8 times for as-forged surfaces where scale pits dominate. The ratio is not a constant, which is why specifying both parameters independently can create an unmeetable requirement.

Can surface roughness be improved without machining?

Shot blasting or vibratory finishing will remove scale and improve appearance, typically bringing Ra from 12.5 to 6.3 µm, but it rounds edges and is not a substitute for machining where a functional surface is required.