C45 vs 42CrMo4 Steel: Carbon or Alloy — Which to Choose?

📋 C45 Datasheet → · 42CrMo4 Datasheet → — chemical composition, section-dependent mechanical properties and international equivalent tables.

What Are C45 and 42CrMo4?

Both are engineering steels that can be hardened by heat treatment (quench & temper), but they belong to different classes:

• C45 (mat. no. 1.0503, AISI 1045): unalloyed medium-carbon steel (~0.45% C)
• 42CrMo4 (mat. no. 1.7225, AISI 4140): chrome-molybdenum quenched & tempered alloy steel (~0.42% C + 1% Cr + 0.2% Mo)

The decisive difference is the alloying elements: the chromium and molybdenum in 42CrMo4 dramatically increase hardenability and strength. In short: C45 is economical and easy to machine; 42CrMo4 is far stronger and hardens fully even in thick sections.

Chemical Composition Comparison

Element	C45 (%)	42CrMo4 (%)
Carbon (C)	0.42–0.50	0.38–0.45
Silicon (Si)	≤ 0.40	≤ 0.40
Manganese (Mn)	0.50–0.80	0.60–0.90
Chromium (Cr)	≤ 0.40	0.90–1.20
Molybdenum (Mo)	≤ 0.10	0.15–0.30
Phosphorus (P)	≤ 0.030	≤ 0.025
Sulfur (S)	≤ 0.035	≤ 0.035

Use our Chemical Composition tool to compare analyses by grade.

Mechanical Properties (quenched & tempered, ≤16 mm section)

Property	C45 (+QT)	42CrMo4 (+QT)
Yield strength (Rₑ)	min. 490 MPa	min. 900 MPa
Tensile strength (Rₘ)	700–850 MPa	1100–1300 MPa
Elongation (A)	min. 14%	min. 10%
Hardenability	Limited (thin sections)	Deep (thick sections)
Weldability	With preheat	Limited (preheat + stress relief required)

The Decisive Difference: Alloying → Hardenability

C45 and 42CrMo4 have a similar carbon content, but the difference comes from chromium and molybdenum:

• C45, being unalloyed, hardens fully only in thin sections; in thick parts the core stays soft (low hardenability). In return it is cheaper, easy to machine and suitable for flame/induction surface hardening.
• 42CrMo4, thanks to chrome-moly, hardens right through even in large sections, offering higher strength and fatigue resistance. However, its high carbon equivalent requires 200–300 °C preheat and post-weld stress relief when welding.

Use the Hardness Converter to convert between HRC/HB/Rm.

When to Choose Which?

Choose C45 — if:

• Moderate strength is sufficient and cost matters
• The part is thin/medium section (shafts, pins, bolts, general machine parts)
• Good machinability and surface hardening (induction) are desired

Choose 42CrMo4 — if:

• High strength, toughness and fatigue resistance are required
• The part is thick section and must harden to the core (crankshafts, axles, gears, high-strength bolts)
• You produce critical components under heavy loads

Applications

Sector	C45	42CrMo4
Machinery	Shafts, pins, keys, general parts	Heavily loaded shafts, gears, axles
Automotive	Simple axles, links	Crankshafts, suspension parts
Oil & gas	Auxiliary components	Valve bodies, connectors
Construction/machine	Bolts, studs	High-strength bolts (10.9/12.9)

Name equivalents: C45 ≈ AISI 1045 ≈ Ck45, 42CrMo4 ≈ AISI 4140 ≈ 708M40. See our Steel Grade Equivalent tool for all cross-standard equivalents.

Supply from Aktif Çelik

At Aktif Çelik we stock both C45 and 42CrMo4 as hot-rolled bar, cold-drawn (bright) bar and forged bar across a wide diameter range. With EN 10204 3.1 certificates and normalized (+N) or quenched & tempered (+QT) delivery conditions, we recommend the grade best suited to your project. Contact our team for technical advice and cut-to-length service.