Differences Between 45 Steel and 40Cr

Chemical Composition is Different

Spark Identification Method

This method utilizes a physical-chemical phenomenon during the grinding process of steel. Different chemical compositions result in different spark characteristics, which can be used to distinguish steel grades. The entire spark stream of carbon steel is yellow. The presence of chromium in chromium steel makes the spark stream brighter. One can practice grinding 40Cr and 45 steel on a grinder in the workshop, carefully observing the difference in sparks. Through practice, one can quickly master the distinction between 40Cr and 45 steel.

Service Performance

Starting from the wear resistance of 45# steel and 40Cr steel, their tribological properties under different heat treatment conditions were studied and compared. The results show that:

In the quenched and tempered condition, the friction coefficient of both 45# steel and 40Cr steel when sliding against GCr15 steel is basically the same, but the wear loss of 45# steel is lower than that of 40Cr steel. If the strength meets the requirements and the workpiece shape is not too complex, it is recommended to use 45# steel.

In the quenched and low-temperature tempered condition, both the friction coefficient and wear loss of 45# steel are higher than those of 40Cr steel. When emphasizing the strength and wear resistance of the workpiece, it is recommended to use 40Cr steel.

Under high-frequency quenching plus low-temperature tempering and ion nitriding treatments, both the friction coefficient and wear loss of 45# steel are close to those of 40Cr steel. If strength is guaranteed, 45# steel can be considered to reduce production costs.

Machinability

Machinability ranking: 45# steel > 40Cr > 20Cr.

20Cr and 40Cr have relatively good toughness, but in terms of machinability, they are worse than 45 steel. However, after quenching and tempering, their mechanical properties become better than those of 45 steel. Generally speaking, steel containing Cr/Mn has increased toughness, leading to higher temperatures during cutting, which can easily burn the tool tip and cause severe work hardening.

Overview of Steel Grade Designation Systems

Carbon Structural Steel

Composed of Q + Number + Quality Level Symbol + Deoxidation Method Symbol. The steel grade is prefixed with “Q”, representing the yield point of the steel. The following number indicates the yield point value in MPa. For example, Q235 indicates a yield point (σs) of 235 MPa.

Symbols for quality level and deoxidation method can be added after the steel grade if necessary. Quality level symbols are A, B, C, D. Deoxidation method symbols: F denotes rimmed steel; b denotes semi-killed steel; Z denotes killed steel; TZ denotes special killed steel. Killed steel (Z) and special killed steel (TZ) can be omitted. For example, Q235-AF represents Grade A rimmed steel.

Carbon steels for specific purposes (e.g., bridge steel, shipbuilding steel) basically adopt the designation method of carbon structural steel, but a letter indicating the purpose is added at the end of the steel grade.

High-Quality Carbon Structural Steel

The first two digits of the steel grade indicate the average carbon content in parts per ten thousand. For example, steel with an average carbon content of 0.45% has the grade “45”. It is not a serial number and should not be read as “Number 45 steel”.

For high-quality carbon structural steel with higher manganese content, the manganese element should be indicated, e.g., 50Mn.

Rimmed steel, semi-killed steel, and high-quality carbon structural steel for specific purposes should be specially marked at the end of the steel grade, e.g., 10b for semi-killed steel with an average carbon content of 0.1%.

Carbon Tool Steel

The steel grade is prefixed with “T” to avoid confusion with other steel types.

The number in the steel grade indicates the carbon content in parts per thousand. For example, “T8” indicates an average carbon content of 0.8%.

For higher manganese content, “Mn” is marked at the end of the steel grade, e.g., “T8Mn”.

The phosphorus and sulfur content of high-grade quality carbon tool steel is lower than that of ordinary quality carbon tool steel. The letter “A” is added at the end of the steel grade to distinguish them, e.g., “T8MnA”.

Free-Cutting Steel

The steel grade is prefixed with “Y” to distinguish it from high-quality carbon structural steel.

The number after “Y” indicates the carbon content in parts per ten thousand. For example, free-cutting steel with an average carbon content of 0.3% has the grade “Y30”.

For higher manganese content, “Mn” is also marked after the steel grade, e.g., “Y40Mn”.

Alloy Structural Steel

The first two digits of the steel grade indicate the steel’s carbon content in parts per ten thousand, such as 40Cr.

Main alloying elements in the steel are generally expressed as a percentage, except for individual microalloying elements. When the average alloy content is <1.5%, generally only the element symbol is marked in the steel grade, not the content. However, in special cases where confusion may occur, the number “1” can be marked after the element symbol. For example, the grades “12CrMoV” and “12Cr1MoV”: the former has a chromium content of 0.4-0.6%, the latter 0.9-1.2%, and other components are identical. When the average alloy content is ≥1.5%, ≥2.5%, ≥3.5%…, the content should be indicated after the element symbol, represented as 2, 3, 4… respectively. For example, 18Cr2Ni4WA.

Alloy elements such as Vanadium (V), Titanium (Ti), Aluminum (Al), Boron (B), Rare Earth (RE), etc., in steel are microalloying elements. Although the content is very low, they should still be indicated in the steel grade, e.g., 20MnVB steel contains V 0.07-0.12% and B 0.001-0.005%.

High-grade quality steel is marked with “A” at the end of the steel grade to distinguish it from ordinary quality steel.

For alloy structural steel for special purposes, the steel grade is prefixed (or suffixed) with a symbol representing the intended use of the steel. For example, 30CrMnSi steel specially used for rivets and screws is designated as ML30CrMnSi.

High-Strength Low-Alloy Steel

The designation method is basically the same as that for alloy structural steel.

For high-strength low-alloy steel for professional use, this should be indicated at the end of the steel grade. For example, for 16Mn steel, the special grade for bridges is “16Mnq”, for automobile beams “16MnL”, and for pressure vessels “16MnR”.

Spring Steel

Spring steel can be divided into carbon spring steel and alloy spring steel based on chemical composition. Their steel grade designation methods are basically the same as those for high-quality carbon structural steel and alloy structural steel, respectively.

Bearing Steel

The steel grade is prefixed with the letter “G”, indicating the bearing steel class.

The carbon content is not indicated in the grade of high-carbon chromium bearing steel; the chromium content is expressed in parts per thousand. For example, GCr15. The designation method for carburizing bearing steel is basically the same as that for alloy structural steel.

Alloy Tool Steel and High-Speed Tool Steel

For alloy tool steel, when the average carbon content is ≥1.0%, it is not indicated; when the average carbon content is <1.0%, it is expressed in parts per thousand. For example, Cr12, CrWMn, 9SiCr, 3Cr2W8V.

The method for indicating the content of alloying elements is basically the same as that for alloy structural steel. However, for alloy tool steel grades with low chromium content, the chromium content is expressed in parts per thousand, and a “0” is added before the number indicating the content to distinguish it from the method indicating general element content as a percentage. For example, Cr06.

The carbon content is generally not indicated in the grade of high-speed tool steel; only the average percentage of various alloying elements is indicated. For example, the grade of tungsten-based high-speed steel is “W18Cr4V”. Steel grades prefixed with the letter “C” indicate that their carbon content is higher than the common steel grade without the “C” prefix.

Stainless Steel and Heat-Resistant Steel

The carbon content in the steel grade is expressed in parts per thousand. For example, the average carbon content of “2Cr13” steel is 0.2%. If the carbon content is ≤0.03% or ≤0.08%, the steel grade is prefixed with “00” and “0” respectively, e.g., 00Cr17Ni14Mo2, 0Cr18 Ni9.

The main alloying elements in the steel are expressed as a percentage, while Ti, Nb, Zr, N… are indicated using the method for microalloying elements in alloy structural steel mentioned above.

Electrode Steel

Its steel grade is prefixed with the letter “H” to distinguish it from other steel classes. For example, stainless steel welding wire is “H2Cr13”, which can be distinguished from stainless steel “2Cr13”.

Electrical Silicon Steel

The steel grade consists of letters and numbers. The leading letters DR indicate hot-rolled silicon steel for electrical purposes, DW indicate cold-rolled non-oriented silicon steel for electrical purposes, and DQ indicate cold-rolled grain-oriented silicon steel for electrical purposes.

The numbers after the letters represent 100 times the iron loss value (W/kg).

The letter “G” added at the end of the steel grade indicates inspection at high frequency; without “G”, it indicates inspection at a frequency of 50 Hz. For example, the steel grade DW470 indicates that the maximum unit weight iron loss value of the cold-rolled non-oriented silicon steel product for electrical purposes is 4.7 W/kg at 50 Hz.

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