40Cr vs 42CrMo – Composition, Heat Treatment, Properties, and Applications

Introduction

40Cr and 42CrMo are two widely used medium‑carbon steels in engineering applications where a balance of strength, toughness, and cost is required. Engineers and procurement teams commonly choose between them for shafts, gears, forgings, and heavily stressed machine parts; the selection decision typically balances achievable strength and toughness (and section‑size hardenability) versus material cost and downstream processing (welding, heat treatment, machining).

The principal metallurgical distinction is that 42CrMo contains molybdenum as a deliberate alloying addition while 40Cr is primarily a chromium‑bearing medium‑carbon steel without intentional Mo. That difference increases the hardenability and tempering resistance of 42CrMo, influencing section thickness limits for quench and tempering, toughness at equivalent strength levels, and welding/heat‑treatment requirements. These two grades are therefore often compared when a design needs better through‑hardening or higher fatigue/toughness performance without moving to more expensive alloy steels.

1. Standards and Designations

• GB/T (China): 40Cr, 42CrMo (common national designations).
• EN (Europe): 42CrMo4 is the common EN equivalent to 42CrMo (often specified as 1.7225); 40Cr has rough equivalents in EN and is sometimes treated as similar to medium‑carbon chromium steels in the EN system.
• AISI/SAE (US): approximate equivalents — 40Cr ≈ SAE 5140; 42CrMo ≈ SAE 4140 (commonly used shorthand in industry).
• JIS (Japan): 42CrMo is commonly compared to SCM440; 40Cr maps to similar SCr/SCM medium‑carbon chromium steels.

Classification:
- 40Cr — medium‑carbon chromium steel (alloyed carbon steel).
- 42CrMo — medium‑carbon chromium‑molybdenum steel (low‑alloy steel with higher hardenability).

(Notes: equivalence between national standards is approximate — always verify specification chemical and mechanical limits for procurement and inspection.)

2. Chemical Composition and Alloying Strategy

Typical composition ranges (weight %) for commonly supplied commercial grades. Values are representative typical ranges from industrial specifications; check specific mill certificates for exact composition.

Element	40Cr (typical range)	42CrMo (typical range)
C	0.37–0.44	0.38–0.45
Mn	0.50–0.80	0.60–0.90
Si	0.17–0.37	0.10–0.40
P	≤0.035	≤0.035
S	≤0.035	≤0.035
Cr	0.80–1.10	0.90–1.30
Ni	— (trace)	— (trace)
Mo	— (trace)	0.15–0.30
V, Nb, Ti, B, N	typically not intentional in standard grades; present only as residuals or in microalloyed variants	typically not intentional in standard grades; present only as residuals or in microalloyed variants

How alloying affects properties: - Carbon (C): primary strengthening element; higher C increases achievable hardness and strength but reduces weldability and ductility. - Chromium (Cr): improves hardenability, strength, wear resistance, and tempering resistance. - Manganese (Mn) and Silicon (Si): deoxidizers and strengtheners; Mn also increases hardenability. - Molybdenum (Mo): significantly increases hardenability and tempering resistance, improves high‑temperature strength and toughness, and helps reduce quench sensitivity. The Mo addition in 42CrMo is the key reason it hardens more deeply and tends to retain toughness after tempering compared with 40Cr.

3. Microstructure and Heat Treatment Response

Typical microstructures: - In the as‑rolled or normalized condition, both steels present ferrite + pearlite microstructures; prior austenite grain size and cooling rate determine pearlite spacing and strength. - After quenching (from austenitizing) and tempering, both steels typically form tempered martensite; tempering temperature and time determine final hardness, strength, and toughness.

Heat treatment behavior: - Normalizing: refines grain size and produces homogeneous ferrite/pearlite; beneficial before forging or machining. - Quench & temper (Q&T): both grades respond well; 42CrMo achieves higher hardenability, meaning thicker sections can be fully martensitic after quench compared with 40Cr at the same cooling severity. - Thermo‑mechanical processing: both can be hot‑forged and subsequently normalized/quench‑and‑tempered to obtain desired properties. The presence of Mo in 42CrMo increases resistance to temper softening and improves toughness at elevated tempering temperatures.

Practical consequence: for large cross sections or components requiring high core strength/toughness, 42CrMo provides more consistent through‑hardening and avoids soft cores that can happen with 40Cr unless specialized quenching or alloying is used.

4. Mechanical Properties

Mechanical properties depend strongly on heat treatment, section size, and tempering. The values below are typical ranges for quenched and tempered conditions commonly used in engineering practice — treat these as representative examples.

Property (Q&T typical)	40Cr (representative range)	42CrMo (representative range)
Tensile strength (MPa)	700–1000	900–1150
Yield strength (MPa)	500–800	700–1000
Elongation (%L0)	10–18	10–15
Impact toughness (Charpy V, J)	20–60 (depends on tempering & section)	30–80 (generally higher toughness at comparable hardness)
Hardness (HRC or HB)	HRC ~20–50 (HB ~180–520 depending on condition)	HRC ~22–55 (HB ~200–560 depending on condition)

Interpretation: - 42CrMo typically attains higher tensile and yield strengths after Q&T for the same heat‑treatment parameters, and it maintains better toughness at equivalent hardness because Mo improves hardenability and tempering resistance. - 40Cr can be tempered to comparable hardness in thin sections but will show reduced through‑hardening and possibly lower core toughness in large cross sections. - Ductility (elongation) is comparable at similar tempering levels; however, for a given nominal strength, 42CrMo often enables a tougher combination.

5. Weldability

Weldability is governed by carbon content, equivalent carbon, and hardenability. Use empirical indices to assess preheat and post‑weld heat treatment needs.

Common weldability formulas (qualitative use): $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

Interpretation: - Both grades have medium carbon and moderate alloying, so neither is highly weldable without controls. The presence of Mo in 42CrMo raises the $CE_{IIW}$ and $P_{cm}$ indices relative to 40Cr, indicating greater susceptibility to hard, brittle martensitic HAZ and a higher risk of hydrogen‑induced cold cracking if welding controls are not applied. - Practical welding guidance: preheat to reduce cooling rate, control heat input, use low‑hydrogen consumables, and perform post‑weld heat treatment (PWHT, stress‑relief or tempering) on higher thicknesses or when mechanical property retention is critical. 42CrMo typically requires more careful preheat/PWHT practices than 40Cr for comparable thicknesses because of higher hardenability.

6. Corrosion and Surface Protection

• Neither 40Cr nor 42CrMo is stainless; both are susceptible to general and localized corrosion in aggressive environments. Standard protection strategies apply: painting, oiling, plating, galvanizing (where appropriate), or conversion coatings.
• For environments requiring elevated corrosion resistance or pitting resistance, stainless families should be considered — PREN (pitting resistance equivalent number) is not applicable to 40Cr/42CrMo. Example PREN formula for stainless alloys: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
• When specifying surface treatments for 42CrMo, consider that molybdenum can influence coating adhesion and conversion coating behavior; surface cleaning and pre‑treatment are important before plating or painting.

7. Fabrication, Machinability, and Formability

• Machinability: in annealed or normalized condition, both grades machine reasonably well. 40Cr often machines slightly more easily due to marginally lower hardenability and lower tempering resistance. 42CrMo in higher‑strength (Q&T) conditions is harder and tougher, leading to increased tool wear and more challenging machining.
• Formability and cold forming: as received (annealed/normalized), both can be formed and bent, but higher carbon and alloy content limit deep drawing and severe cold forming compared with low‑carbon steels. Preheating for bending thicker sections is sometimes recommended.
• Grinding and finishing: when hardened, both require high‑quality grinding practices; 42CrMo may require more aggressive abrasives due to higher toughness and hardness.

8. Typical Applications

40Cr — Typical Uses	42CrMo — Typical Uses
Automotive shafts, bolts, gears (moderate sections), couplings, crankshafts (where appropriate), general forgings	High‑strength shafts, heavy gears, hydraulic components, high‑stress fasteners, large‑section forgings, oilfield equipment, heavy machinery components
Machine parts where cost control is important and section sizes are moderate	Components requiring deeper through‑hardening, higher fatigue strength and better toughness at comparable hardness

Selection rationale: - Choose 40Cr for cost‑sensitive parts where sections are moderate and Q&T plus surface treatment meets performance targets. - Choose 42CrMo when parts have larger cross sections, require higher core strength or fatigue life, or where superior toughness at elevated strength is critical.

9. Cost and Availability

• Cost: 42CrMo is typically more expensive than 40Cr because of the molybdenum addition and processing for consistent Mo chemistry. The cost delta varies with market prices for Mo and steelmaking.
• Availability: both grades are widely available in bar, plate, forgings, and cold‑drawn forms from major steel mills and distributors. 42CrMo may be more commonly specified in Europe under EN designations (42CrMo4), while 40Cr is common in regions using GB/AISI designations.
• Lead times: comparable for standard stock sizes; special chemical variants or tight chemical tolerances can increase lead times.

10. Summary and Recommendation

Summary table (qualitative):

Attribute	40Cr	42CrMo
Weldability	Better (but still requires controls for C levels)	More demanding (higher preheat/PWHT risk)
Strength–Toughness balance	Good for moderate sections	Superior for through‑hardening and toughness at high strength
Cost	Lower	Higher

Conclusions: - Choose 40Cr if: - You need a cost‑effective medium‑carbon chromium steel for moderate cross‑section parts. - Component sections are small to moderate and full through‑hardening is not required. - Welding, machining, and heat treatment budgets are limited and you can accept modest hardenability.

• Choose 42CrMo if:
• The component requires deep hardening (large cross section) or higher fatigue/toughness for a given strength.
• Through‑hardening, better tempering resistance, and improved high‑strength toughness are design priorities.
• You are prepared to apply stricter welding procedures, preheat, and PWHT as needed and accept a higher material cost.

Final practical note: always specify the required heat‑treatment condition, mechanical property targets, and any NDT or hardness mapping needed for acceptance. When in doubt for critical rotating or fatigue‑sensitive components, perform section‑size trials or consult the mill for hardness vs. depth data for the proposed quenching medium; for welding, compute carbon equivalents and follow qualified welding procedures that include preheat and PWHT recommendations.