S45C Steel Explained: Properties, Heat Treatment, and Practical Selection Guide

S45C is the workhorse medium carbon steel of Japanese manufacturing — and by extension, much of Asian industry. Standardized under JIS G4051, it offers a well-balanced combination of strength, machinability, and hardenability that makes it the default choice for shafts, gears, keys, and general machine structural parts. This guide covers chemical composition, international equivalents, heat treatment options, and — critically — the selection traps that catch engineers off guard.

What Is S45C? — JIS Designation Breakdown

S45C belongs to the JIS G4051 family of carbon steels for machine structural use. The three-character designation encodes specific information about composition and steel type:

Fig. 1 — Reading the JIS G4051 designation

The number “45” represents the approximate mean carbon content in hundredths of a percent (0.45 % C). The suffix “C” indicates a plain carbon steel — no intentional alloying elements such as Cr or Mo. This distinguishes it from alloy steels like SCM440, where the “SCM” prefix denotes a chromium-molybdenum alloy.

Chemical Composition — S45C vs. AISI 1045 vs. DIN C45

S45C has well-recognized international equivalents. The table below compares composition limits across JIS, AISI/SAE, and DIN/EN standards.

Element	S45C (JIS G4051)	1045 (AISI / SAE)	C45 (DIN EN 10083-2)
C (%)	0.42 – 0.48	0.43 – 0.50	0.42 – 0.50
Si (%)	0.15 – 0.35	0.15 – 0.35	0.17 – 0.37
Mn (%)	0.60 – 0.90	0.60 – 0.90	0.50 – 0.80
P (%)	≤ 0.030	≤ 0.040	≤ 0.030
S (%)	≤ 0.035	≤ 0.050	≤ 0.035
Cr (%)	≤ 0.20 (residual)	—	≤ 0.40 (residual)
Ni (%)	≤ 0.20 (residual)	—	≤ 0.40 (residual)

Substitution note S45C, 1045, and C45 are frequently treated as interchangeable — and in most applications they are. JIS applies tighter limits on P and S, while DIN allows slightly higher residual Cr and Ni. These differences rarely affect practical performance. When substituting between standards, confirm via mill certificate rather than relying solely on the designation.

Mechanical Properties

S45C properties depend heavily on heat treatment condition. The chart and table below show key values across three common delivery or processing states.

Condition	Tensile Strength (MPa)	Yield Strength (MPa)	Elongation (%)	Hardness
Normalized (820–860 °C, AC)	≥ 570	≥ 325	≥ 20	~170HBW
Quenched & Tempered (Oil, 530–600 °C)	690 – 780	≥ 490	≥ 17	200 – 235HBW
Induction Hardened (surface layer)	—	—	—	55 – 62HRC

Hardness after induction hardening applies to the surface layer only (typically 1–3 mm effective hardened depth). The core remains at the pre-treatment condition (~170–235HBW) — this is by design, giving the part a hard wear surface and a tough interior.

Heat Treatment Options

① Normalizing

Heating to 820–860 °C followed by air cooling (AC). Used to relieve rolling or forging stresses and produce a uniform pearlitic microstructure. Most commercial S45C bar stock is supplied in this condition. Delivers the baseline mechanical properties shown in the table: ≥ 570 MPa tensile, ~170HBW.

② Quenching and Tempering (Q&T)

Austenitize at 820–860 °C, quench in oil or water, then temper at 530–600 °C. This raises tensile strength to the 690–780 MPa range while preserving adequate ductility and toughness. Tempering temperature controls the final balance:

Lower tempering (400–480 °C) → higher hardness, lower toughness
Higher tempering (560–620 °C) → sacrifices ~50 MPa strength for better impact resistance

Critical hardenability limit S45C has moderate hardenability — reliable up to approximately 40–50 mm cross-section diameter. Beyond that, the core may not fully transform to martensite during quenching. Actual tensile properties across the full section will fall short of the specification. For parts exceeding ~50 mm that require uniform Q&T properties, switch to SCM440 (Cr-Mo alloy steel).

③ Induction Hardening

A high-frequency induction coil rapidly heats only the surface layer, which is then immediately quench-cooled. The result: a hard martensitic surface (55–62HRC) over a tough normalized or Q&T core. Ideal for shafts and gears where wear resistance at the contact surface is critical but bulk toughness must be retained. After induction hardening, a low-temperature temper at 150–200 °C is applied to relieve quench stresses and reduce the risk of surface cracking.

Common Applications

Shafts & Axles

The most common S45C application. Normalized or Q&T bar is machined to final form. Journal bearing contact zones are often induction hardened, adding wear resistance without distorting the full shaft geometry.

Gears (Light to Medium Load)

Spur and helical gears for moderate-load applications. Heavy-duty or high-speed gears typically call for carburized alloy steels (SCM415, SCr420) to achieve deeper case depth and higher core toughness than S45C can provide.

Keys, Pins & Couplings

Good machinability makes S45C a practical choice for small transmission components produced in high volumes. Hardness can be boosted selectively by induction hardening if wear is a concern.

Bolts & Fasteners

Q&T S45C corresponds roughly to ISO 898-1 property class 8.8 in tensile performance. Verify thread root fatigue strength individually if cyclic or dynamic loading governs the design.

Jigs & Fixtures

Predictable dimensional behavior after heat treatment and consistent machinability make S45C a stable, economical choice for tooling components and production fixtures.

Hydraulic Cylinder Rods

Often induction hardened then hard chrome or electroless nickel plated. Confirm surface finish requirements (Ra ≤ 0.2 µm is typical for seal interfaces) before finalizing the process sequence.

Machinability and Weldability

Machinability

In the normalized or annealed condition, S45C machines reliably. As a rough benchmark, its machinability index is approximately 65–70 % relative to AISI B1112 free-machining steel (index = 100 %). Starting parameters for turning on conventional tooling:

Tool material	Cutting speed (m/min)	Feed (mm/rev)	Depth of cut (mm)
HSS	60 – 90	0.10 – 0.30	1 – 4
Uncoated carbide	150 – 250	0.10 – 0.35	1 – 5
Coated carbide (TiCN / Al₂O₃)	250 – 400	0.10 – 0.40	1 – 5

Weldability

S45C is weldable but requires care. Its carbon equivalent (CE, IIW formula) typically falls around 0.45–0.50 %, placing it in the “conditionally weldable” zone. Without preheating, the heat-affected zone (HAZ) forms hard martensite on rapid cooling — brittle, and prone to hydrogen-assisted cold cracking that can appear 12–48 hours after welding when dissolved hydrogen diffuses to stress concentrations.

Trouble Spot: HAZ cold cracking in an S45C repair weld

SituationA repair weld was performed on a normalized S45C shaft using low-hydrogen electrodes, without preheating, in a 15 °C shop environment.

What happenedHairline cracks appeared in the HAZ roughly 18 hours after welding was completed. The part was scrapped at final inspection.

Root causeCarbon content near 0.45 % promotes martensite formation when the HAZ cools rapidly to ambient temperature. Even “low-hydrogen” electrodes retain some diffusible hydrogen, which migrates to stress concentrations at the martensite boundary and drives cracking.

PreventionPreheat to 100–200 °C before striking the arc. Maintain interpass temperature throughout. Apply post-weld heat treatment (PWHT) at 550–600 °C for structural or fatigue-critical joints. If the geometry makes preheating difficult, consider switching to S35C (lower CE ≈ 0.38) which welds without preheat in most section sizes.

S45C vs. Similar Steels — Selection Guide

Steel	C (%)	Tensile normalized (MPa)	Hardenability	Weldability	Relative cost
S35C	0.32 – 0.38	≥ 510	Low	Good (CE ~0.38)	≈ S45C
S45C	0.42 – 0.48	≥ 570	Moderate	Conditional (CE ~0.47)	Reference
S50C	0.47 – 0.53	≥ 610	Moderate–High	Requires preheat	≈ S45C
SCM440	0.38 – 0.43	≥ 930 (Q&T)	High (Cr + Mo)	Conditional (CE ~0.75)	+30 – 50 %

Choose S35C when:

The part requires welding without preheating, or when strength requirements are moderate (≤ 510 MPa tensile) and fatigue loading is not critical. Common in lightly loaded brackets, levers, and fixtures where machinability and weldability matter more than hardened surface properties.

Choose S45C when:

You need a well-characterized, widely available general-purpose machine steel for shafts, keys, and moderate-load gears. S45C responds predictably to normalizing and induction hardening for section sizes up to ~50 mm. When in doubt for a new shaft design, S45C is typically the right starting point.

Choose S50C when:

Surface hardness after induction hardening is paramount. The higher carbon content pushes the achievable surface range toward the upper end (60–64HRC). The trade-off: even less weld-friendly than S45C, and more susceptible to quench cracking if section geometry is complex.

Choose SCM440 when:

Cross-section exceeds 50 mm and uniform through-hardness is required
Fatigue or impact loading is severe (SCM440 Q&T delivers ≥ 930 MPa tensile with far better toughness than a Q&T carbon steel)
The part is a critical structural component where the 30–50 % material cost premium is justified by performance and reduced failure risk

Trouble Spot: S45C Q&T specified for a 100 mm shaft — properties not met

SituationA designer specified “S45C, Q&T, tensile strength ≥ 690 MPa” for a 100 mm diameter pump shaft. The material passed incoming inspection on a standard test bar.

What happenedHardness mapping on the actual shaft showed a soft core (~130–150HBW). Cross-section tensile specimens cut from the shaft center failed to meet the 690 MPa requirement.

Root causeS45C’s Jominy end-quench hardenability band drops below 25HRC at approximately 25 mm from the quenched face — far too shallow to harden the center of a 100 mm section. The test bar (typically 25 mm) met spec; the production shaft did not.

Correct approachSwitch to SCM440 (or SCM435). The Cr-Mo additions shift the Jominy curve significantly to the right, allowing full through-hardening of sections up to 150 mm in oil quench — and actual through-section tensile properties that meet the 930 MPa Q&T specification.

S45C Specification Checklist — Before You Finalize the Drawing

Is the maximum cross-section ≤ 50 mm? If not, evaluate SCM440 for hardenability.
Does the part require welding? If yes, confirm preheat temperature (100–200 °C) is feasible in production, or switch to S35C.
Is induction hardening specified? Confirm effective hardened depth (1–3 mm typical) and post-hardening temper temperature (150–200 °C) are called out on the drawing.
Is the heat treatment condition called out explicitly on the drawing? (Normalized / Q&T / Induction hardened — leaving it blank invites material mix-ups.)
Has the mill certificate carbon content been checked against the JIS G4051 range (0.42–0.48 % C)? Heat-to-heat variation within the range affects achieved hardness after heat treatment.

Summary

S45C (JIS G4051) is a medium carbon steel equivalent to AISI 1045 and DIN C45, with C: 0.42–0.48 %.
Mechanical properties: ≥ 570 MPa tensile (normalized), 690–780 MPa (Q&T), 55–62HRC surface (induction hardened).
Hardenability is limited to ~50 mm cross-section — use SCM440 for larger or higher-load applications.
Welding requires 100–200 °C preheat due to CE ~0.47; for freely weldable designs, consider S35C.
Selection rule: S35C → weldability first; S45C → balanced general use; S50C → harder surface; SCM440 → large section or high fatigue load.