45 Steel, 40Cr, and 42CrMo are three commonly used medium-carbon steels, each with distinct properties and suitable applications. To help you gain a thorough understanding of them, this article will conduct a detailed comparison from the aspects of chemical composition, mechanical properties, heat treatment performance, application scenarios, and finally provide material selection guidelines.

1. Comparison of Chemical Composition (wt%)

The chemical composition is the fundamental factor that determines the properties of these steels. The specific components and their core differences are shown in the following table:

Analysis of Chemical Composition

45 Steel: As a carbon steel, its properties are almost entirely determined by the carbon content, with no intentional addition of other alloying elements.
40Cr: Based on 45 Steel, chromium (Cr) is added. The main function of chromium is to enhance the steel's hardenability, ensuring that the performance of the entire cross-section of the part is more uniform during heat treatment, thereby achieving better comprehensive mechanical properties.
42CrMo: On the basis of 40Cr, molybdenum (Mo) is further added. Molybdenum plays a crucial role:

It further improves hardenability, making it particularly suitable for large-sized parts.
It refines grain size, enhancing the steel's strength and toughness.
It inhibits temper brittleness, allowing the steel to obtain better toughness after high-temperature tempering.
It improves heat resistance, enabling the steel to maintain high strength at elevated temperatures.

2. Comparison of Mechanical Properties (After Quenching and Tempering)

Quenching and tempering (quenching + high-temperature tempering) is the most commonly used heat treatment process for these three steels, aiming to obtain a tempered sorbite structure with excellent strength-toughness balance.

Analysis of Mechanical Properties

45 Steel: It has average comprehensive mechanical properties, with relatively low strength and toughness.
40Cr: Its strength is much higher than that of 45 Steel, but it sacrifices a certain degree of plasticity. However, its toughness is better than that of 45 Steel.
42CrMo: It achieves a perfect combination of high strength and high toughness. Its comprehensive mechanical properties are the best, especially its impact toughness and fatigue strength, which are quite outstanding.

3. Comparison of Heat Treatment Performance

The heat treatment performance directly affects the application scope of the steel, especially in terms of the size of the parts that can be manufactured.

Heat Treatment Characteristic 45 Steel 40Cr 42CrMo
Hardenability Poor. The critical diameter for water quenching is approximately 15-20mm, and only 5-10mm for oil quenching. Large cross-section parts cannot be fully quenched, resulting in poor core performance. Medium. The critical diameter for oil quenching is approximately 20-30mm. Suitable for quenching and tempering of medium-sized parts. Excellent. The critical diameter for oil quenching can reach 40-60mm or even higher. Large cross-section parts can also obtain uniform and high performance.
Temper Stability Poor. Hardness decreases rapidly during tempering. Good. Very Good. Due to the content of Mo, hardness decreases more slowly at the same tempering temperature, enabling higher toughness.
Tendency of Water Quenching Cracking High. Strict control of water temperature and operation is required. Medium. Oil quenching is generally adopted. Low. Oil quenching is usually used, with high safety.

Analysis of Heat Treatment Performance

Hardenability is the core difference among the three steels, and it directly determines the size of the parts that the material can be used to manufacture.

45 Steel: It has the worst hardenability and can only be used to manufacture small-sized parts that bear low loads.
40Cr: Its hardenability is significantly improved, and it can be used to manufacture important medium-sized parts.
42CrMo: It has the highest hardenability and can be used to manufacture key large cross-section parts that bear heavy loads.

4. Comparison of Application Scenarios

Different properties determine the different application directions of the three steels in industrial production.

Grade Typical Application Scenarios Examples
45 Steel Used to manufacture parts with low load, simple shape, and small size. Shafts, connecting rods, gears, sleeves, pins, lead screws and other general-grade parts.
40Cr Used to manufacture important parts that bear medium loads and operate at medium speeds. It is one of the most widely used quenched and tempered steels. Important shafts, gears, connecting rods, bolts, worms, main shafts, pin shafts, etc. Important parts in automobiles and machine tools.
42CrMo Used to manufacture key parts that require high strength, high toughness, bear heavy loads, and have large cross-sections. Drive shafts of heavy vehicles, gearbox gears, large motor shafts, oil drilling joints, connecting rods, aircraft structural parts, wind power shaft parts, etc.

5. Material Selection: Summary and Guidelines

When selecting among these three materials, a core principle should be followed: under the premise of meeting the performance requirements, choose the material with the lowest cost. You can follow the following decision-making process:

Step 1: Determine the Load and Criticality of the Part

Light load, non-critical parts: 45 Steel is the first choice. It has the lowest cost and good machinability. Examples include fixtures, brackets, handwheels, and light-load lead screws.
Medium load, critical parts: 40Cr is the first choice. It has the highest cost-effectiveness and can meet the needs of most mechanical parts. Examples include machine tool main shafts, automobile half shafts, gears, and connecting rods.
Heavy load, impact load, large cross-section, critical parts: 42CrMo must be selected. Although its cost is high, it has reliable performance and good safety. Examples include gear shafts of heavy machinery, main shafts of large water turbines, and bolts of high-pressure boilers.

Step 2: Consider the Cross-Section Size of the Part

Small size (< Φ30mm): All three materials can be considered, and the selection depends on the load.
Medium size (Φ30mm ~ Φ60mm): This is the advantage range of 40Cr, while the core of 45 Steel may not be fully quenched.
Large size (> Φ60mm): 42CrMo is a better choice, as it can ensure the core performance. 40Cr may have insufficient performance, and 45 Steel is not applicable.

Step 3: Consider the Cost

Price order: 45 Steel < 40Cr < 42CrMo.
Rough estimation: The price of 40Cr is approximately 1.5 times that of 45 Steel, and the price of 42CrMo is approximately 1.5-2 times that of 40Cr. Unless necessary, do not blindly choose high-performance materials to avoid waste.

One-Sentence Summary

For low cost: Choose 45 Steel (note that the size cannot be too large).
For balance: Choose 40Cr (a versatile option suitable for most scenarios).
For high performance: Choose 42CrMo (suitable for heavy load, large parts, and high reliability requirements).