MATERIALS · September 4, 2026 · 7 MIN READ

Carbon and Alloy Steel Grade Equivalents: GB, AISI and JIS for Forging

45 steel is AISI 1045 and JIS S45C; 40Cr is 5140 and SCr440; 42CrMo is 4140 and SCM440. Those three substitutions are safe. The one that is not is 20CrMnTi, which is routinely quoted as "equivalent to 8620" but contains no nickel and will not reproduce 8620's case depth or core toughness — a difference that shows up as tooth-root cracking after carburising.

WHERE THIS APPLIES AT FORGEPRESS CHINA

ForgePress China forges 45, 40Cr, 42CrMo, 35CrMo and 20CrMnTi with mill certificates traceable to the heat, and flags any grade substitution in 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

45 / 1045 / S45C: carbon 0.42–0.50 %, the default medium-carbon forging grade.
42CrMo / 4140 / SCM440: 0.9–1.2 % Cr, 0.15–0.25 % Mo; hardenability to about 60 mm section.
20CrMnTi is NOT equivalent to 8620: no nickel, and case-core toughness differs after carburising.
Hardenability, not chemistry, decides whether a grade can be through-hardened at your section size.

Why chemistry tables are not enough

Two steels with the same carbon and chromium can behave completely differently in heat treatment because hardenability depends on the combined effect of carbon, manganese, chromium, molybdenum and grain size. A grade that through-hardens a 25 mm section may show ferrite at the centre of a 60 mm section, and that soft core is what fails in fatigue.

When you substitute an equivalent grade, check the hardenability band — the Jominy curve or the ideal critical diameter — not just the composition table.

Working equivalence table

GB	AISI / SAE	JIS	C %	Typical forging use
20#	1020	S20C	0.17–0.24	Case-hardened parts, low load
35#	1035	S35C	0.32–0.40	General shafts, medium strength
45#	1045	S45C	0.42–0.50	The workhorse: shafts, gears, rods
40Cr	5140	SCr440	0.37–0.44	Higher strength shafts, axle parts
42CrMo	4140	SCM440	0.38–0.45	Connecting rods, cranks, high-load parts
35CrMo	4135	SCM435	0.32–0.40	Bolts, studs, moderate sections
20CrMnTi	8620 (not equivalent)	—	0.17–0.23	Gears, carburised parts — verify case depth
65Mn	1566	—	0.62–0.70	Springs, clips, wear plates

AISI/SAE and JIS columns give the closest standard grade. Carbon ranges shown are GB limits; ASTM and JIS limits may differ by 0.01–0.02 %.

The 20CrMnTi problem

20CrMnTi is the standard Chinese carburising gear steel: 0.17–0.23 % C, 1.0–1.3 % Mn, 0.04–0.10 % Ti. It is cheap, grain-refined by the titanium, and it carburises well. It is also frequently described as "the Chinese 8620", which is misleading — 8620 contains 0.4–0.7 % nickel, and that nickel is what gives 8620 its core toughness.

For a lightly loaded gear, the substitution is often fine. For a gear that sees shock load or a thin rim, the reduced core toughness shows up as tooth-root cracking after carburising and quenching, and it appears months into service rather than at first article. If the drawing says 8620 and the quotation says 20CrMnTi, ask for a fatigue assessment or a core hardness comparison before accepting.

Choosing between 45, 40Cr and 42CrMo

45 steel is the default: cheap, forgeable, machinable, and adequate up to roughly 600 MPa UTS in a normalized or quenched-and-tempered condition. It loses hardenability above about 20 mm section, so a 60 mm shaft in 45 will have a soft core.

40Cr adds chromium for hardenability and roughly 15 % strength at the same hardness. 42CrMo adds molybdenum, which suppresses temper embrittlement and pushes usable section size to about 60 mm — which is why connecting rods and cranks are 42CrMo and not 40Cr, despite the price difference.

Section under 20 mm, moderate load, cost-sensitive: 45 steel.
Section 20–40 mm, needs through-hardening: 40Cr.
Section above 40 mm, fatigue or impact loading: 42CrMo.
Carburised wear surface with tough core: 20CrMnTi, or 8620 if the drawing demands it.

Frequently asked questions

Is 45 steel the same as 1045?

Very close: both are 0.42–0.50 % carbon plain steel, and for most forged parts they are interchangeable. JIS S45C is the closest of the three. Differences appear in residual elements and hardenability band rather than in nominal composition.

Can I replace 42CrMo with 40Cr?

Only at smaller section sizes. 40Cr through-hardens to roughly 40 mm; 42CrMo to roughly 60 mm and also resists temper embrittlement better. Above 40 mm section the substitution typically produces a soft core and lower fatigue life even though the surface hardness is the same.

What does the H suffix mean on Chinese drawings?

H means the steel is supplied to a guaranteed hardenability band rather than only to a composition range, for example 42CrMoH. It costs more and it is worth specifying for any part that is through-hardened and fatigue-loaded.