MATERIALS · September 13, 2026 · 7 MIN READ

Mechanical Properties of Forged Steel: What UTS, Yield, Elongation and Impact Actually Tell You

A material certificate that lists only tensile strength tells you almost nothing about whether a forging will survive service. Four numbers matter together: ultimate tensile strength, 0.2 % proof stress, elongation at fracture and Charpy impact energy at the service temperature. For a 42CrMo forging in the quenched and tempered condition you would expect roughly 1,000 MPa UTS, 850 MPa proof stress, 12 % elongation and 40 J at 20 °C — and if any one of those is missing, the specification is incomplete.

WHERE THIS APPLIES AT FORGEPRESS CHINA

ForgePress China supplies material certificates to EN 10204 3.1 with tensile and impact results for the heat actually used, on request with the quotation. 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

Proof stress (Rp0.2) matters more than UTS for design: it is the stress at which the part stops springing back.
UTS can be estimated as roughly 3.3–3.5 × Brinell hardness for steel, valid only between about 200 and 450 HB.
Charpy impact must be quoted with a temperature; 40 J at 20 °C and 40 J at −40 °C are different materials.
Forging improves fatigue life mainly through grain flow, not through tensile strength.

The four numbers and what each one is for

Ultimate tensile strength is the peak of the stress-strain curve and the number most often quoted, but it is the least useful for design because a part that reaches UTS is already permanently deformed and about to fail. Proof stress (Rp0.2) is the stress at 0.2 % permanent strain, and it is what a designer compares against working stress with a safety factor.

Elongation at fracture measures ductility — the ability to deform before breaking, which is what absorbs overload and prevents brittle failure. Charpy impact measures toughness: energy absorbed by a notched bar at a specified temperature, which is the number that predicts sudden fracture in cold weather or under shock load.

Grade and condition	UTS (MPa)	Rp0.2 (MPa)	Elongation %	Charpy J (20 °C)
45, normalized	600–650	355	16–18	30–45
45, quenched and tempered	700–850	500–600	12–15	25–40
40Cr, quenched and tempered	850–1,000	650–800	10–13	25–35
42CrMo, quenched and tempered	950–1,100	800–900	11–14	35–50
42CrMo, quenched and tempered, low-temp service	850–950	700–800	13–16	27 J at −40 °C

Typical ranges for forgings in the 20–200 mm section range after proper austenitising and tempering. Exact values depend on section size and tempering temperature — always take them from the actual test certificate.

Why the certificate and the drawing disagree

A drawing that says "42CrMo, 1,000 MPa" is not a specification, because 1,000 MPa can be reached at several tempering temperatures with very different ductility and toughness. The same steel tempered at 560 °C gives high strength and modest toughness; tempered at 620 °C it gives 150 MPa less strength and much better impact energy.

The fix is to specify the tempering temperature range or the hardness range, and to require the impact test. A hardness specification of 285–321 HB on 42CrMo pins the strength to about 950–1,050 MPa and leaves the heat treater enough freedom to meet the toughness requirement at the same time.

Grain flow: the reason to forge at all

A machined part from bar has grain flow running in one direction, so properties are strongly anisotropic: strong along the bar, weaker across it. Forging redirects grain flow to follow the part contour, which typically improves fatigue life by 20–40 % at fillets and raises transverse ductility.

That benefit is destroyed by cutting the part from plate rather than forging it, and it is partly destroyed by excessive machining that cuts through the flowed surface. When a drawing specifies "forged", it is usually the grain flow that is wanted, not the shape.

The hardness-to-strength shortcut and its limits

For steel, UTS in MPa is approximately 3.3–3.5 times Brinell hardness, which is convenient for a quick shop-floor check. It is valid from roughly 200 to 450 HB and only for steels whose microstructure is tempered martensite or pearlite; it fails for austenitic stainless, for cast iron, and for any grade where hardness comes from cold work rather than from microstructure.

Use it as a screening tool. A part that reads 250 HB when the certificate says 300 HB has a heat treatment problem worth investigating, whatever the conversion table says.

Frequently asked questions

Should I specify UTS or hardness on a forging drawing?

Hardness, plus a tensile test on a representative sample. Hardness is cheap, fast and can be checked on every part; tensile testing is destructive and sampled. A hardness range of 285–321 HB on 42CrMo pins UTS to roughly 950–1,050 MPa without over-constraining the heat treater.

Why is Charpy impact quoted at a temperature?

Because toughness falls steeply as temperature drops, and the ductile-to-brittle transition can happen over a 40 °C range. A steel that absorbs 40 J at 20 °C may absorb 8 J at −30 °C, which is the difference between bending and shattering. Always specify the test temperature alongside the energy.

Does forging make steel stronger than machining from bar?

Not in tensile strength — the alloy and heat treatment set that. Forging improves fatigue life and transverse ductility by aligning grain flow with the part contour, typically 20–40 % better fatigue life at fillets. That is a real functional benefit, but it is not a strength increase you can read on a tensile certificate.