JIS G4051 Carbon Steels for Machine Structural Use — Full Grade Comparison

JIS G4051 covers eleven plain carbon steel grades — from the soft, case-hardenable S10C up to the high-carbon S58C — plus a parallel set of free-cutting variants. Each grade is defined by its carbon content, which governs strength, hardenability, and weldability. This reference covers the full grade table, international equivalents, and a practical guide to which tier of grade belongs to which type of application.

The JIS G4051 Designation System

Fig. 1 — The two-digit number encodes the approximate mean carbon content in hundredths of a percent.

Free-cutting variants (e.g., S45C-D, SUM22) add a suffix to indicate sulfur or lead additions that improve machinability at the cost of weldability and fatigue strength. This article covers the standard (non-free-cutting) grades only.

Full Grade Table — S10C through S58C

Grade	C (%)	Mn (%)	Tensile normalized (MPa)	Yield (MPa)	Elongation (%)	Hardness (normalized, approx.)
S10C	0.08 – 0.13	0.30 – 0.60	≥ 310	≥ 205	≥ 33	~90HBW
S15C	0.13 – 0.18	0.30 – 0.60	≥ 370	≥ 225	≥ 30	~105HBW
S20C	0.18 – 0.23	0.30 – 0.60	≥ 400	≥ 245	≥ 28	~115HBW
S25C	0.22 – 0.28	0.30 – 0.60	≥ 440	≥ 270	≥ 27	~125HBW
S30C	0.27 – 0.33	0.60 – 0.90	≥ 470	≥ 295	≥ 26	~137HBW
S35C	0.32 – 0.38	0.60 – 0.90	≥ 510	≥ 305	≥ 23	~149HBW
S40C	0.37 – 0.43	0.60 – 0.90	≥ 540	≥ 315	≥ 21	~159HBW
S45C ★	0.42 – 0.48	0.60 – 0.90	≥ 570	≥ 325	≥ 20	~167HBW
S50C	0.47 – 0.53	0.60 – 0.90	≥ 610	≥ 365	≥ 18	~179HBW
S55C	0.52 – 0.58	0.60 – 0.90	≥ 650	≥ 390	≥ 17	~192HBW
S58C	0.55 – 0.61	0.60 – 0.90	≥ 650	≥ 390	≥ 17	~197HBW

★ S45C highlighted as the most widely used grade. Si: 0.15–0.35 % for all grades; P and S: ≤ 0.030 % and ≤ 0.035 % respectively. Values above are for hot-rolled / normalized condition per JIS G4051.

Grade Groups — Three Tiers of Application

Tier 1 — Low Carbon (S10C – S25C): Case-Hardening Grades

Carbon content below 0.28 % is insufficient to form useful amounts of martensite by through-hardening or induction hardening. These grades are used as the core material for carburizing — a process that enriches the surface to 0.7–0.9 % C, which is then hardened while the low-carbon core remains tough and ductile. Typical applications: small gears, pins, cams, and bearing races in automotive and precision machinery.

Grade	Primary heat treatment	Typical application	AISI equivalent
S10C	Carburize + quench	Small precision gears, pins	1010
S15C	Carburize + quench	Cam followers, small sprockets	1015
S20C	Carburize + quench	Light-load gears, bolts (class 4.8)	1020
S25C	Carburize or normalize	Structural brackets, light shafts	1025

Tier 2 — Medium Carbon (S30C – S50C): Machine Structural Grades

The workhorse tier. Carbon content 0.30–0.53 % enables meaningful response to through-hardening (Q&T) and induction hardening. Weldability decreases as carbon rises. S45C is the dominant grade in this tier — widely available, well-characterized, and the industry default for shafts, keys, and general machine parts.

Grade	Primary strength	Best use case	AISI equivalent
S30C	470 MPa (normalized)	Weldable structural parts, levers	1030
S35C	510 MPa (normalized)	Weldable shafts, bolts, light gears	1035
S40C	540 MPa (normalized)	General shafts, intermediate grade	1040
S45C ★	570 MPa (normalized)	Standard machine parts, induction hardened shafts	1045
S50C	610 MPa (normalized)	Higher surface hardness, springs, high-strength shafts	1050

Tier 3 — High Carbon (S55C – S58C): High-Strength / Spring Grades

Above 0.52 % C, strength increases further but weldability becomes very difficult and quench cracking risk rises. These grades are used where high surface hardness or spring properties are the design driver — not as general-purpose machine structural steel.

Grade	Primary use	Notes	AISI equivalent
S55C	Springs, high-strength bolts, saw blades	Requires care in welding and quenching	1055
S58C	Same as S55C; slightly higher carbon for harder induction surface	Induction surface to 63–65HRC; crack risk if geometry is complex	1059 / 1060

International Equivalents — JIS vs. AISI / DIN / EN

JIS G4051	AISI / SAE	DIN / EN (EN 10083-2)	BS (UK)
S10C	1010	C10 / 1.0301	040A10
S15C	1015	C15 / 1.0401	050A15
S20C	1020	C20 / 1.0402	050A20
S25C	1025	C25 / 1.0406	070M26
S30C	1030	C30 / 1.0528	080A30
S35C	1035	C35 / 1.0501	080A35
S40C	1040	C40 / 1.0511	080A40
S45C	1045	C45 / 1.0503	080A45
S50C	1050	C50 / 1.0540	080A50
S55C	1055	C55 / 1.0535	070M55
S58C	1059 / 1060	C60 / 1.0601	080A57

Equivalents are approximate — verify before substituting Composition limits between standards overlap but are not identical. Manganese ranges differ (JIS G4051 allows 0.60–0.90 %; AISI sometimes allows up to 1.00 %), and P/S limits vary. For non-critical applications, substitution is generally safe. For fatigue-critical or fracture-toughness-governed applications, request a material certificate and confirm actual composition and mechanical properties — do not rely on designation matching alone.

Practical Selection Guide

Which JIS G4051 grade for your application?

Part will be carburized: S10C – S20C (low carbon core, carbon-enriched surface)
Part needs welding, moderate load: S30C – S35C (CE ≤ 0.44, weldable without preheat in thin sections)
General shaft / gear / key, induction hardened: S45C (the industry default — widest availability, best documentation)
Needs harder induction surface than S45C provides (> 60HRC): S50C or S55C
Spring application, high-cycle fatigue: S55C – S58C (or alloy spring steels SUP6/SUP7)
Large section (> 50 mm) requiring through-hardened Q&T properties: None of the above — step up to SCM440 (Cr-Mo alloy steel)

Practical note: Why S40C is rarely specified

ObservationS40C sits between S35C and S45C in both carbon content and properties, but is rarely seen in production drawings or stock lists.

WhyS40C offers neither the weldability advantage of S35C nor the proven induction hardening response of S45C. It occupies an awkward middle ground that neither design requirement optimally targets. Procurement also disfavors it: S40C commands a similar price to S45C but is held in much smaller stock quantities, leading to longer lead times without meaningful performance benefit.

LessonWhen in doubt between two adjacent grades, default to the grade with the larger installed base — typically S35C or S45C. Mid-range grades like S40C should only be specified when a design requirement genuinely falls between the standard grades and cannot be met otherwise.

Summary

JIS G4051 defines 11 plain carbon steel grades from S10C (0.08–0.13 % C) to S58C (0.55–0.61 % C).
Tier 1 (S10C–S25C): case-hardening grades — used as carburizing base materials for small gears and pins.
Tier 2 (S30C–S50C): machine structural grades — the dominant range for shafts, gears, bolts, and keys; S45C is the most widely used.
Tier 3 (S55C–S58C): high-carbon grades for springs and high-hardness induction applications; difficult to weld.
International equivalents (1010–1060 in AISI; C10–C60 in DIN) are close but not exact — always verify by mill certificate for critical applications.
No JIS G4051 grade provides adequate hardenability above ~50 mm cross-section — upgrade to SCM440 or SNCM439 for large-section through-hardening requirements.