The hardenability of steel is the key factor that determines the quality of quenching and tempering treatment. For steels with different hardenability, the depth of the hardened layer formed after quenching and the internal microstructure will vary, ultimately leading to uneven distribution of mechanical properties across the workpiece section. The higher the requirements for the mechanical properties of the workpiece, especially when the section size is large, the more significant the impact of hardenability will be.

Based on the level of hardenability, quenched and tempered steel can be divided into four categories: steel with the lowest, lower, higher, and highest hardenability.

1. Quenched and Tempered Steel with the Lowest Hardenability
This type of quenched and tempered steel is medium-carbon non-alloy steel (medium-carbon carbon steel), with a critical diameter of 15–23mm when water-quenched at 20℃. Typical grades include 35 steel, 40 steel, 45 steel, and 50 steel. Among them, 45 steel (code U20452) is the most widely used and consumed quenched and tempered steel in China. Limited by its low hardenability, 45 steel is only suitable for the quenching and tempering treatment of small workpieces with a section size less than 25mm.
However, for a long time, many enterprises have often ignored hardenability—the core quality index—in the quenching and tempering processing of 45 steel. Many enterprises apply the quenching and tempering process as pre-heat treatment for workpieces with a section size of 60–100mm, and even for large shaft parts with a diameter of 300–500mm.
The larger the workpiece size, the lower the hardness after quenching, the shallower the hardened layer, and the lower the comprehensive mechanical properties. In addition, rough turning usually leaves a margin of 3–5mm, so it is easy to cut off most or even all of the hardened layer of the workpiece during finish turning.
When the workpiece diameter exceeds 60mm, the performance after quenching and tempering is similar to that after normalizing; when the diameter is greater than 100mm, a martensitic structure cannot even be formed on the workpiece surface. Therefore, 45 steel workpieces with large section sizes are more suitable for use under normalized or surface-quenched conditions.

2. Quenched and Tempered Steel with Lower Hardenability

This type of steel is medium-carbon low-alloy steel (the mass fraction of alloying elements is usually not more than 2.5%), with a critical diameter of 30–54mm when water-quenched at 20℃ and 19–40mm when oil-quenched. Common grades include 35Cr, 40Cr, 45Cr, 35Mn2, 40Mn2, 45Mn2, etc. Among them, 40Cr steel (code A20402) is the most consumed alloy quenched and tempered steel in China, and is mostly used to make quenched and tempered workpieces with medium section sizes and higher mechanical property requirements than carbon steel.

3. Quenched and Tempered Steel with Higher Hardenability

This type of medium-carbon low-alloy steel has a critical diameter of 42–85mm when water-quenched at 20℃ and 30–60mm when oil-quenched. Representative grades include 30CrMnSi, 35CrMnSi, 35CrMo, 42CrMo, 40CrNi, etc.
35CrMo (code A30352) and 42CrMo (code A30422) are the most widely used grades. They have high strength, good toughness, and better hardenability than 40Cr steel. Meanwhile, they possess excellent high-temperature creep strength and rupture strength, and can work stably for a long time at 500℃.

4. Quenched and Tempered Steel with the Highest Hardenability

This type of steel includes medium-carbon low-alloy steel and medium-carbon medium-alloy steel, with a critical diameter of 60–126mm when water-quenched at 20℃ and 46–114mm when oil-quenched. The main grades include 40CrNiMoA (code A50403), 40CrMnMo (code A34402), 37CrNi3 (code A42372), etc.
Due to their excellent hardenability and mechanical properties, they are often used to manufacture high-strength parts with large sections that bear impact loads, such as the drive eccentric shafts of horizontal forging machines, the crankshafts of forging presses, high-strength tie rods, and anchor rods.

The above four types of quenched and tempered steel only cover the commonly used grades in the field of general machinery manufacturing. Due to the differences in chemical composition, workpiece section size, and heat treatment process of various steels, there is no strict boundary between the four types of quenched and tempered steel.

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