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

Introduction

Engineers, procurement managers, and manufacturing planners often face the choice between plain-carbon steels and low-alloy steels for rotating shafts, gears, pins, and machine components. 45# (commonly designated as a medium carbon plain steel) and 40Cr (a chromium-alloyed medium carbon steel) are frequently compared because they occupy adjacent composition space but deliver different hardenability, strength, and heat-treatment responses.

The fundamental distinction is that 40Cr’s chromium addition increases hardenability and attainable strength after quenching and tempering, while 45# relies on carbon content and section size to achieve hardness and strength. This difference drives selection where through-hardening, section thickness, and post-heat-treatment mechanical targets are critical.

1. Standards and Designations

• 45#: Often found as GB/T grade “45#” (China). Equivalent Western grades: roughly AISI/SAE 1045 (medium carbon steel). Classified as a plain carbon steel (non-alloy).
• 40Cr: Found in GB/T as “40Cr.” Rough equivalents: AISI/SAE 5140/4140 family (low-alloy chromium steel). Classified as a low-alloy steel.

Other relevant standard systems that may cover comparable steels: - ASTM/ASME: SAE/AISI series (e.g., 1045, 4140). - EN: EN 8 / C45 (closest for 45#); 40Cr approximates EN 19/42CrMo4 variants depending on exact chemistry. - JIS: JIS shows similar medium carbon and alloy steels under different numeric codes. - GB: Chinese GB/T specifications for 45# and 40Cr.

2. Chemical Composition and Alloying Strategy

Table: typical composition ranges (wt%). Actual spec limits depend on standard and mill.

Element	45# (typical)	40Cr (typical)
C	0.42–0.50	0.37–0.44
Mn	0.50–0.80	0.50–0.80
Si	0.17–0.37	0.17–0.37
P	≤0.035	≤0.035
S	≤0.035	≤0.035
Cr	— (trace)	0.80–1.20
Ni	— (trace)	≤0.30 (may be absent)
Mo	— (trace)	≤0.08 (small or absent)
V, Nb, Ti, B, N	trace/controlled	trace/controlled

How the alloying influences properties: - Carbon: primary hardenability and room-temperature strength; higher C increases achievable hardness but reduces toughness and weldability. - Chromium (in 40Cr): raises hardenability and tempering resistance, improves strength and wear resistance after quench & temper; also refines carbide structure. - Manganese and silicon: deoxidation and strength; Mn contributes to hardenability. - Phosphorus and sulfur kept low to maintain toughness and machinability.

3. Microstructure and Heat Treatment Response

Typical microstructures: - 45#: In the annealed or normalized condition, microstructure is ferrite + pearlite with medium pearlite fraction consistent with ~0.45%C. Quench + temper produces martensite tempered to a desired hardness, but because 45# lacks strong alloying elements its hardenability is limited—core martensite is achievable only in relatively small cross-sections. - 40Cr: In normalized condition, ferrite + pearlite with alloy carbides; after quench it is capable of forming martensite in larger sections compared to 45# due to Cr. Tempering produces tempered martensite with better strength–toughness balance and improved tempering resistance.

Effects of common processing: - Normalizing: both grades refine grain size and produce a predictable ferrite/pearlite microstructure; 40Cr may form finer carbide dispersions. - Quench & tempering: 40Cr achieves higher strength and toughness in thicker sections; 45# can reach comparable hardness in small sections but will require careful control to avoid brittle behavior. - Surface hardening (induction, carburizing): Both grades are suitable; 40Cr is preferred when a tough core is required with a hardened surface, and it can be carburized for enhanced surface wear resistance.

4. Mechanical Properties

Note: mechanical properties vary strongly with heat treatment and section size. Values below are typical ranges used for engineering comparison rather than definitive spec values.

Property (typical ranges)	45#	40Cr
Tensile strength (MPa)	520–750	600–1100
Yield strength (MPa)	300–500	400–950
Elongation (%)	10–18	8–16
Impact toughness (Charpy V, J)	15–60 (heat-treatment dependent)	20–80 (better in tempered quenched state)
Hardness (HB or HRC)	HB 160–250 (HRC ~15–30)	HB 180–320 (HRC ~18–36)

Interpretation: - Strength: 40Cr typically can achieve higher tensile and yield strengths after quench & temper due to Cr-enhanced hardenability and tempered martensite stability. - Toughness: When properly tempered, 40Cr often provides a better strength–toughness balance in larger sections. In small sections or annealed condition, 45# can show comparable toughness. - Ductility: 45# annealed tends to show slightly higher elongation; 40Cr after high-strength heat treatments will be less ductile.

5. Weldability

Weldability depends on carbon content, alloying elements, and thickness (hardening tendency). Useful severity indices:

• International Institute of Welding carbon equivalent: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$

-ustenitic weldability parameter (Pcm): $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

Qualitative interpretation: - 45#: With C ≈ 0.45% and very low alloy content, the $CE_{IIW}$ is moderate; preheating and controlled cooling are recommended for thicker sections to avoid cracking, but overall weldability is better than alloyed steels of the same hardness because there are fewer hardenability-promoting elements. - 40Cr: The chromium raises the $CE_{IIW}$ and $P_{cm}$ contributions via the $(Cr+Mo+V)$ term; thus 40Cr has a higher tendency for hard, martensitic HAZ in thicker sections and typically requires preheat, interpass temperature control, or post-weld heat treatment (PWHT). Use of low-hydrogen electrodes and controlled welding procedures is advised.

Recommendation: For critical welds or thick sections, choose procedures that account for higher hardenability of 40Cr; for small components or when welding is occasional, 45# is easier to weld.

6. Corrosion and Surface Protection

• Neither 45# nor 40Cr is stainless or corrosion-resistant by chemical composition. Corrosion resistance is similar in bulk unless specific alloying (e.g., higher Cr or Mo) is present.
• Common protections: painting, oiling, phosphating, and galvanizing for atmospheric exposure; plating or coatings for wear-corrosion environments. For 40Cr parts that are heat-treated, select coatings compatible with post-heat-treatment processing.
• PREN (pitting resistance equivalent number) is not applicable to these non-stainless steels; for reference: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ but this index is relevant only for stainless alloys with significant Cr, Mo, and N.

7. Fabrication, Machinability, and Formability

• Machinability: 45# (1045) in the annealed condition machines reasonably well; higher carbon raises tool wear when cutting hardened material. 40Cr tends to be tougher and may be more abrasive on tools when hardened; in normalized or annealed conditions machining is manageable.
• Formability/bending: 45# annealed is more formable; 40Cr in normalized condition will be less ductile than annealed 45# and requires larger bend radii, or preheating for forming if in a hardened condition.
• Surface finishing: both respond well to grinding, turning, and polishing when properly heat-treated; selection of cutting speeds and tool materials must account for hardness and tempering.

8. Typical Applications

45# (typical uses)	40Cr (typical uses)
Shafts, axles, studs, pins, crank components in low-to-moderate duty	Heavily loaded shafts, gears, crankshafts, high-strength pins, gear blanks
General machined parts where moderate strength is required and cost matters	Parts requiring deeper hardening and higher fatigue strength in larger sections
Bolts and broached components after quench/temper in small sections	Forged components, carburized/quenched components for wear resistance

Selection rationale: - Choose 45# where cost, moderate strength, and simpler heat-treatment or welding are priorities and cross-sections are small. - Choose 40Cr where higher through-hardening, better tempering stability, and higher load-bearing capacity—especially for larger cross-sections—are required.

9. Cost and Availability

• Cost: 45# is typically less expensive per tonne than 40Cr because it lacks alloying additions. 40Cr carries a premium for Cr content and for being specified as an alloy steel.
• Availability: Both grades are commonly available worldwide in bar, plate, forgings, and round stock. 45# is ubiquitous for general-purpose stock; 40Cr is widely stocked for engineering applications and is commonly offered in normalized, quenched & tempered, and forged conditions.
• Lead times: Standard metric bars and forgings are readily available; special chemistries or tight-tolerance forgings can increase lead time.

10. Summary and Recommendation

Summary table (high-level qualitative):

Characteristic	45#	40Cr
Weldability	Good (with preheat for thick sections)	More challenging (higher preheat/PWHT often needed)
Strength–Toughness (post HT)	Moderate	Higher (better through-hardening & tempering response)
Cost	Lower	Higher

Choose 45# if: - You need a cost-effective medium-carbon steel for small to moderate cross-sections. - Welding ease and simpler heat-treatment procedures are priorities. - Applications require reasonable strength with good machinability and formability (after anneal or normalization).

Choose 40Cr if: - You require higher hardenability and greater strength/toughness after quench & temper, especially in larger sections. - Parts are subject to higher fatigue loads, heavier service, or require a tougher core with a hardened surface. - You are specifying components where predictable performance after heat treatment and better temper resistance matter enough to justify higher material cost.

Concluding note: Final selection should be driven by required mechanical targets, section thickness, heat-treatment capability, welding requirements, and total lifecycle cost. When in doubt, specify required mechanical properties and heat-treatment state rather than grade alone; a materials engineer can then select the most economical grade and process to achieve those targets.