Gear Steel Grade Cross-Reference: GB, DIN, JIS, AISI Conversion and Substitution Risks

The same gear: a Chinese drawing specifies 20CrMnTi, a German customer’s drawing specifies 16MnCr5 or 20MnCr5, a Japanese drawing specifies SCM420, and an American drawing specifies 8620 — the meanings of these four grades are close, but they cannot be directly equated. Grade cross-referencing means mapping a steel grade under one standards system to a relatively close grade in another system based on chemical composition ranges and hardenability requirements; it is an approximate correspondence. Substitution (using another grade to replace the material specified on the drawing) requires, on top of that, re-checking the upper and lower composition limits, end-quench data, inclusion requirements, and heat treatment parameters. Below, the four-system cross-reference tables for the three common categories of gear steel — carburizing steel, quenched and tempered steel, and nitriding steel — the mechanical property ranges of GB grades, and the five items that must be checked before substitution are set out clearly.

I. First, distinguish clearly between three terms#

Grade cross-reference: Based on approximate relationships of chemical composition ranges and mechanical properties, corresponding a steel grade under standards system A to a relatively close grade in system B. It gives a candidate range, not a substitution permit.

Substitution (material replacement): The customer’s drawing specifies a certain grade, and the supplier produces with another grade. This step requires re-confirming composition, hardenability, cleanliness, and heat treatment process, and usually requires the customer’s written confirmation.

Hardenability: The ability of steel to obtain a hardened layer of a certain depth during quenching, measured by the end-quench test (Jominy), implemented domestically according to GB/T 225. Two grades with the same or similar names may have different hardenability curves — this is one of the items where problems occur most often in substitution.

II. Four-system cross-reference for carburizing gear steel#

The “—” positions in the table indicate that the system has no common grade that can be directly corresponded, which does not mean that German carburizing steel cannot be used to make the same gear; rather, it requires going through the step of checking hardenability and composition, and substituting after confirmation by both parties. Conversely, when a customer specifies German-standard carburizing steels such as 16MnCr5 or 20MnCr5, the supplier must also first look at the cross-section size and the required case depth before deciding which one to choose.

III. Four-system cross-reference for quenched and tempered gear steel#

The correspondence for quenched and tempered steel is tidier than for carburizing steel, because chromium-containing and molybdenum-containing quenched and tempered steels have mature series in each system. But note: although the correspondence between 45 steel and C45, S45C is common, 45 steel itself has limited hardenability; once the cross-section is large, the hardness decay along the cross-section is more pronounced than with 42CrMo, so using 45 steel for quenching and tempering of large cross-section parts should be done with caution.

IV. Cross-reference for nitriding gear steel#

Nitriding steels have few grades in each system, and the range of choice is smaller than for carburizing steel. The advantage of nitriding treatment is small distortion and high tooth flank hardness, often used for precision parts that are inconvenient to grind after heat treatment; the cost is a thin hardened layer, making it unsuitable for strong impact conditions.

V. Mechanical property ranges of common GB grades#

The data is used for quickly judging the grade tier; specific values are subject to the steel mill’s quality certificate and incoming re-inspection.

From this table one line can be seen: the strength of carburizing steel is obtained from the surface hardened layer, with core hardness held at 30~45 HRC to retain toughness; the strength of quenched and tempered steel is obtained from through-hardening, with hardness falling in the 240~320 HB range. The basis for choosing between the two routes is whether tooth flank contact stress or tooth root bending stress is more critical, not which grade sounds more advanced.

VI. Hardness is the common unit for cross-system comparison#

Drawings from different systems may be marked in HRC, HB, or HV respectively, and must be converted when the same gear is checked together. Common tiers (approximate values):

Common hardness requirement tiers for gears:

The conversion relationships hold only within an approximate range: when HRC is below 20, the HRB scale should be used instead; when HB is below 450, HRC and HB can be approximately estimated as HRC ≈ (HB − 200) / 10. In regions with hardness below 20 HRC, measurement with the Rockwell C scale gives large errors, so the inspection scale should be agreed upon at acceptance.

VII. Five items to check before substitution#

VIII. Practical insights#

For export orders, first ask the customer which standard they inspect against. The drawing says 20CrMnTi, but the customer in Germany inspects according to the German standard; the problem is often not whether it can be made, but against what values it is submitted for inspection. Adding a sentence in the technical agreement with the material standard number and composition range saves trouble compared with arguing afterwards.

End-quench data is more trustworthy than the grade name. The same grade from different steel mills has different hardenability bandwidths. For parts with hardenability requirements, write the J values into the purchasing requirements, and check them against them at incoming re-inspection — this is more reliable than only looking at the grade on the quality certificate.

Changing material requires changing the heat treatment parameters along with it. After substitution, only changing the drawing annotation while calling up the carburizing process as before is a common source of batch anomalies — when the nickel content changes, the case depth obtained with the same carbon potential and time will also change.

Write the grade, standard number, and heat treatment state in full on inspection reports. During export inspection, if the report only writes information such as “alloy steel, HRC58”, the customer cannot correspond it to his drawing requirements; writing all three in full closes the traceability chain.

From material grade to finished gear, several stages must be passed in between: machining and inspection of precision cylindrical gears , turning, milling, and heat treatment of shaft-type transmission parts , and precision machining and gear grinding processes . The material for standard series parts (such as racks) is written directly into the part number: in GY-CSTGH-020-10-S, the material position C represents S45C and M represents SCM440; the specification code 020 is module M2, and 10 is length 1000mm. If you have drawings that require an engineering review based on material and heat treatment, you can contact us .