40Cr vs 45Cr – Composition, Heat Treatment, Properties, and Applications

Introduction

40Cr and 45Cr are two commonly specified quenched-and-tempered alloy/carbon steels used for shafts, gears, axles, and other mechanical components. Engineers and procurement professionals frequently weigh the trade-offs between higher alloy content (for hardenability and tempering resistance) and higher carbon content (for as-quenched hardness and strength). Typical decision contexts include whether a part must be through‑hardened or surface‑hardned, whether weldability or toughness is a priority, and the allowable material and processing cost.

The main practical distinction between these grades is that 40Cr is formulated with intentional chromium to improve hardenability and tempering behavior, while 45Cr emphasizes higher carbon (and sometimes slightly different Cr levels) to achieve higher strength after heat treatment. Because of that, they are often compared where strength, toughness, and hardenability are all design drivers.

1. Standards and Designations

• Common national/industry standards and equivalents (informative, check contract/spec for exact limits):
• GB/T (China): 40Cr (alloy structural steel), 45Cr (higher-carbon chromium steel in some catalogs) — many suppliers reference GB grades.
• AISI/SAE: 40Cr ≈ AISI 5140 family; 45Cr ≈ AISI 5145 family (equivalents vary by supplier).
• EN (European): no exact direct numeric equivalent—closest are medium‑alloy steels such as 41Cr4/42CrMo variants; check EN spec for chemical limits.
• JIS: Similar family steels exist in Japanese standards; verify exact designation.
• Classification: both grades are alloyed carbon steels (not stainless). They are used as structural/alloy steels suitable for heat treatment; they are neither plain carbon only nor HSLA in the strict sense.

2. Chemical Composition and Alloying Strategy

• Table: typical nominal composition ranges (expressed as weight %; actual spec limits vary by standard and mill). Always verify the mill certificate for contract compliance.

Notes: - The table gives representative ranges: 40Cr conventionally includes around 0.8–1.1% Cr to increase hardenability and tempering resistance. “45Cr” formulations can vary—some suppliers position 45Cr close to a higher‑carbon chrome steel (C ≈ 0.45%) but with lower chromium than 40Cr; others treat 45Cr as a chromium‑bearing version of 45# (0.45%C) steel. Always confirm the exact chemical certificate for the batch you purchase. - How alloying affects behavior: increasing C raises achievable hardness and strength but reduces weldability and ductility. Chromium boosts hardenability, improves depth of hardening in thicker sections, and enhances tempering resistance and wear performance.

3. Microstructure and Heat Treatment Response

• Typical microstructures:
• As‑hot‑rolled or normalized: ferrite + pearlite microstructure; 40Cr may show finer pearlite and more retained carbide dispersion due to Cr.
• After quenching: martensite (plus retained austenite at very high C); 45Cr (higher C) will form a harder, more brittle martensite if quenched to the same hardness level.

• After tempering: tempered martensite; Cr-containing steels (40Cr) generally develop tempering resistance—retaining strength at higher tempering temperatures and giving a favorable toughness–strength balance.

• Heat treatment routes:

• Normalizing: refines grain and removes banding; commonly used as an intermediate step for either grade.
• Quenching and tempering (Q&T): the principal hardening route. 40Cr achieves deeper hardening at the same quench severity due to Cr; 45Cr reaches higher core hardness in thinner sections primarily due to higher carbon.
• Thermo-mechanical processing: controlled rolling and accelerated cooling can further refine microstructure and improve strength/toughness; benefits apply to both but alloying content determines achievable hardenability.

4. Mechanical Properties

• The following are typical property directions (actual values depend on heat treatment, section size, and testing standard). Use the mill test report for procurement decisions.

Explanation: - Strength: 45Cr’s higher carbon enables higher strength/hardness for the same heat treatment; however, the presence of chromium in 40Cr allows better hardenability for larger sections and improved toughness after tempering. - Toughness: alloying (Cr) helps maintain toughness at higher strengths because it reduces the rate of embrittlement during tempering. - Ductility: higher carbon typically reduces ductility, so for applications requiring elongation or fatigue resistance, 40Cr may be preferable at a given strength level.

5. Weldability

• Key factors: carbon content and carbon equivalents drive preheat/postheat and consumable selection. Hardness and risk of cold cracking increase with higher C and high hardenability alloys.
• Common carbon equivalent formulas used to assess weldability:
• IIW carbon equivalent: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$
• International Pcm: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
• Interpretation:
• 45Cr (higher C) typically shows higher CE and Pcm, implying greater preheat and interpass temperature requirements and higher susceptibility to hydrogen-induced cold cracking.
• 40Cr’s added chromium increases CE somewhat (because Cr appears in the numerator of the CE formula) but its hardenability effect means that thick sections require careful weld procedure control. In practice, both grades require appropriate preheat, controlled interpass temperatures, low hydrogen consumables, and post-weld heat treatment (PWHT) depending on thickness and final service conditions.

6. Corrosion and Surface Protection

• These steels are not stainless; corrosion resistance is limited. Protective options:
• Painting, powder coating, oiling, or hot-dip galvanizing for general corrosion protection.
• For components expected to operate in severe environments, consider surface treatments such as nitriding, carburizing (for wear surfaces), or plating.
• PREN (pitting resistance equivalent number) is not applicable to non-stainless structural alloy steels, but the example formula is: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
• Practical point: small Cr contents in 40Cr/45Cr do not make them corrosion-resistant; chromium here serves metallurgical—not corrosion—purposes.

7. Fabrication, Machinability, and Formability

• Machinability:
• Higher carbon content (45Cr) generally reduces machinability due to higher hardness after heat treatment; in the annealed condition machinability is acceptable but poorer than low-carbon steels.
• 40Cr with slightly lower C and higher Cr behaves similarly; machining grades often require softer (annealed) condition and appropriate tooling.
• Formability and bending:
• In annealed/normalized condition both grades can be formed; higher carbon reduces ductility—plan forming operations in softer condition.
• Finishing:
• Surface finish and grinding: both grades can be ground and polished; chromium can influence abrasive wear of tooling.
• Heat treatment distortion: both will experience distortion during quenching; 40Cr’s higher hardenability can reduce distortion in thicker sections if properly quenched.

8. Typical Applications

Selection rationale: - Choose 40Cr when you need better hardenability for thicker sections, improved tempering stability, and a better strength–toughness balance. - Choose 45Cr when higher carbon content (higher achievable hardness and strength) in moderate section sizes is the priority and acceptable trade-offs in ductility/weldability are managed.

9. Cost and Availability

• Relative cost: material costs vary by mill and market, but:
• 40Cr is typically slightly more expensive than equivalent plain carbon steels due to alloying (Cr) and associated processing.
• 45Cr’s higher carbon content may be similar or marginally lower cost than 40Cr depending on chromium levels; availability of either grade in common bar/forging sizes is good from major steel mills.
• Product forms: both are widely available as round bars, forgings, and cold‑finished bars. Lead times and price volatility depend on chromium supply and global steel market conditions.

10. Summary and Recommendation

Recommendation: - Choose 40Cr if you need good through‑hardening in larger sections, improved tempering resistance, and a better toughness–strength balance for heavily loaded components. - Choose 45Cr if your design calls for maximized strength/hardness in moderate sections and you can accept the trade-offs in weldability and ductility (or mitigate them with appropriate heat treatment and processing).

Final note: exact chemical and mechanical limits differ among standards and suppliers. For procurement and design, always specify the standard (mill certificate required), required heat treatment route, and relevant mechanical/property acceptance criteria.