Alloy steel

What defines the alloy steel family?

Alloy steel is steel with deliberate additions of chromium, nickel, molybdenum, vanadium or boron, usually totalling a few percent. These additions primarily increase hardenability. Carbon steel and alloy steel with the same carbon content can reach the same surface hardness after quenching. Alloy steel can also reach that hardness at the centre of a thick section.

Hardenability is the capacity to form martensite through a section during cooling. It differs from hardness. Carbon determines the possible hardness of martensite. Alloying elements reduce the cooling rate required for martensite to form instead of ferrite and pearlite. The centre of a large bar cannot cool rapidly, regardless of the quenchant. Alloy steel is therefore required when a heavy section must have strength through its thickness. Longer soaking or a more severe quench does not provide equivalent hardenability.

The Laxcon Steels grade reference lists 136 alloy steel grades with their full composition. The table below carries every one of them that has a cross-standard equivalent on record.

Through-hardening and case-hardening grades

Carbon content distinguishes through-hardening alloy steels from case-hardening alloy steels.

Through-hardening grades typically contain 0.35 to 0.5 percent carbon. This carbon content permits hard martensite to form wherever the quench produces the required cooling rate. EN 19, EN 24, SAE 4140 and SAE 4340 are through-hardening grades. A part is machined, quenched and tempered to a specified strength. The resulting properties are uniform across the section. Typical applications include shafts, axles, connecting rods, heavy fasteners and components subjected to bending or torsion through the body.

Case-hardening grades typically contain 0.12 to 0.23 percent carbon. Carburising adds carbon at the surface. SAE 8620 and EN 36C are case-hardening grades. The treated component has a hard, wear-resistant case over a tough and comparatively soft core. This structure gives a gear tooth contact fatigue resistance at the surface and shock resistance beneath it. A case-hardening grade remains soft when supplied and used without carburising. This condition is not a steel defect.

The two types are not interchangeable. A component drawing must identify the applicable hardening regime. Through-hardening grades are specified by strength or hardness. Case-hardening grades are specified by effective case depth, surface hardness and core hardness. A grade designation alone does not define these properties.

SAE and BS 970 designations

The SAE four-digit system and BS 970 are the principal designation systems for these steels. In the SAE system, the first two digits identify the alloy series. The number 41 denotes chromium-molybdenum steel, 43 denotes nickel-chromium-molybdenum steel, 86 denotes low nickel-chromium-molybdenum carburising steel, and 92 denotes silicon-manganese spring steel. The final two digits give the mean carbon content multiplied by one hundred.

In BS 970, the first three digits identify the group. The 500 to 999 range covers alloy steels. The 700 and 800 bands cover chromium-molybdenum and nickel-chromium-molybdenum steels respectively. The letter indicates whether the steel is purchased by analysis, hardenability or mechanical properties. The final two digits give the mean carbon content multiplied by one hundred.

A composition specification guarantees the constituents of the steel. A hardenability specification guarantees hardness at stated distances along a Jominy end-quench bar. This test directly measures the depth of hardening. The H versions in the SAE system and the H letter in BS 970 identify this requirement. H grades typically permit a wider composition range than corresponding plain grades. The mill may adjust the analysis if the hardenability curve remains within the specified band.

Grade selection and specified conditions

Section size, required strength, toughness and surface wear determine selection within the family. EN 8 carbon steel may provide the required properties in a light section under ordinary duty. EN 19 or SAE 4140 applies to medium sections that require strength through the section. EN 24 or SAE 4340 adds nickel for heavy sections or applications requiring toughness at strength. Carburising grades apply where the surface must resist wear and the body must absorb shock. SAE 8620 covers ordinary duty. EN 36C applies where the section is heavy or loading is severe.

Formal specifications commonly identify the supply condition. ASTM A193 Grade B7 covers quenched and tempered chromium-molybdenum bolting with a minimum tempering temperature of 593 degrees Celsius. ASTM A320 Grade L43 covers the nickel-bearing equivalent for low-temperature service and includes a Charpy requirement at minus 101 degrees Celsius. These requirements distinguish the low-temperature role of the nickel-bearing grade.

Questions about alloy steel grades

Alloy steel equivalents across standards

Basis "verified" means every value in that row was checked against the governing standard on 2026-08-18; "inherited" means it came from the previous website and has not been checked. The complete mapping table is on the equivalents reference.

Where the alloy steel family is not the right answer

Alloy steel is unsuitable where corrosion resistance is part of the duty. These grades do not form a passive film. Passive film formation requires roughly ten and a half percent chromium in solution, while these grades contain at most one or two percent. Wet service therefore requires paint, plating or oil. Plating can introduce hydrogen into high-strength steel.

Alloy steel is generally unsuitable for welded fabrication. Carbon equivalents typically range from about 0.5 in the leanest carburising grades to nearly 0.9 in nickel-chromium-molybdenum steels. Preheating and low-hydrogen practice become mandatory at roughly 0.45. Each welded joint therefore requires a procedure. A weldable structural steel is usually more suitable for structures containing many welds.

Alloy steel is also unsuitable for light sections at ordinary strength when carbon steel can produce the same properties at the centre. In that case, the alloy content provides no functional benefit.

Alloy steel grades in detail