Comparing Gear Steel Specifications Between GB, DIN, JIS and AISI: What Conversion Can and Cannot Tell You

Comparing Gear Steel Specifications Between GB, DIN, JIS and AISI: What Conversion Can and Cannot Tell You

Take a single gear as the example. One drawing, of Chinese origin, specifies 20CrMnTi. The German buyer’s drawing names 16MnCr5 or 20MnCr5. From Japan comes SCM420, and from the United States, 8620. Meaning-wise these four sit in the same neighbourhood, and yet no one of them may be treated as identical to another. When we cross-reference grades, what we actually do is take a grade defined in one standards system and line it up against the closest counterpart in a second system, using chemical composition windows and hardenability demands as our basis. An approximate correspondence is all that comes out of it. Substitution is a separate matter — making the part in a grade other than the one the drawing names — and it multiplies the workload: composition ceilings and floors, end-quench curves, inclusion limits and heat treatment settings each demand fresh verification. Below you will find four-system cross-reference tables covering the three usual gear steel families (carburizing, quenched and tempered, nitriding), the mechanical property bands of GB grades, and the five checks that precede any substitution.

I. Three terms that must be kept apart

Grade cross-reference. Using approximate relationships of chemical composition ranges and mechanical properties, a grade under standards system A is corresponded to a comparatively near grade in system B. What this yields is a candidate range, not a permit to substitute.

Substitution (material replacement). The customer’s drawing names one grade and the supplier produces in another. Composition, hardenability, cleanliness and the heat treatment route all have to be confirmed again at this stage, and in most cases the customer must put the approval in writing.

Hardenability. How deep a hardened layer a steel can develop during quenching defines this property; the end-quench test (Jominy) is the measuring method, and GB/T 225 is the domestic implementation. Two grades whose names match or nearly match can still follow different hardenability curves — a frequent point of failure when substitution is attempted.

II. Carburizing gear steel: four-system cross-reference

• China GB 20CrMnTi: — (no direct equivalent)
  • Japan JIS: SMK22
  • USA AISI/SAE: —
  • Common applications: Automotive gearbox-type carburized gears, large domestic usage
• China GB 20CrMo: 20CrMo5 / 1.7320
  • Japan JIS: SCM420
  • USA AISI/SAE: 4118 / 4120
  • Common applications: Small and medium module carburized gears
• China GB 20CrNi2Mo: 20NiCrMo2-2 / 1.6523
  • Japan JIS: SNCM420
  • USA AISI/SAE: 4320
  • Common applications: Heavy-duty carburized gears
• China GB 20CrNiMo: 20NiCrMo2-2 / 1.6523
  • Japan JIS: SNCM220
  • USA AISI/SAE: 8620
  • Common applications: General-purpose carburized gears

Where the table shows “—”, the system has no common grade that can be corresponded directly. That does not mean German carburizing steel cannot make the same gear; it means the work must pass through the hardenability and composition check, and the substitution is made only after both parties confirm it. The reverse case matters too: when a customer names German-standard carburizing steels such as 16MnCr5 or 20MnCr5, the supplier should first look at the cross-section size and the required case depth before deciding which of the two to take.

III. Quenched and tempered gear steel: four-system cross-reference

• China GB 40Cr: 41Cr4 / 1.7035
  • Japan JIS: SCr440
  • USA AISI/SAE: 5140
  • Common applications: Medium-load quenched and tempered gears
• China GB 42CrMo: 42CrMo4 / 1.7225
  • Japan JIS: SCM440
  • USA AISI/SAE: 4140 / 4142
  • Common applications: General-purpose high-strength gears, gear shafts
• China GB 35CrMo: 34CrMo4 / 1.7220
  • Japan JIS: SCM435
  • USA AISI/SAE: 4135
  • Common applications: Medium cross-section gears
• China GB 45 steel: C45 / 1.0503
  • Japan JIS: S45C
  • USA AISI/SAE: 1045
  • Common applications: Low-speed light-load gears
• China GB 40CrNiMo: 40CrNiMo6 / 1.6511
  • Japan JIS: SNCM439
  • USA AISI/SAE: 4340
  • Common applications: High-strength gear shafts
• China GB 34CrNiMo: 34CrNiMo6 / 1.6582
  • Japan JIS: —
  • USA AISI/SAE: —
  • Common applications: Large cross-section quenched and tempered gears

Quenched and tempered steel corresponds more tidily than carburizing steel, since the chromium-bearing and molybdenum-bearing families have mature series in every system. One caution applies. The correspondence of 45 steel with C45 and S45C is common enough, but 45 steel on its own has limited hardenability; once the cross-section grows large, hardness decays across the section more sharply than with 42CrMo. Using 45 steel for quenching and tempering of large cross-section parts therefore deserves care.

IV. Nitriding gear steel: cross-reference

Every system offers few nitriding steel grades, so the range of choice is narrower than for carburizing steel. Nitriding brings small distortion and high tooth flank hardness, which suits precision parts that are inconvenient to grind after heat treatment. The trade-off is a thin hardened layer, which rules it out for strong impact conditions.

V. Mechanical property ranges of common GB grades

These figures serve for a quick judgement of the grade tier. Specific values are subject to the steel mill’s quality certificate and to incoming re-inspection.

One line can be read off this table. Carburizing steel draws its strength from the surface hardened layer while core hardness is held at 30~45 HRC to keep toughness; quenched and tempered steel draws its strength from through-hardening, with hardness landing in the 240~320 HB band. Which route to take depends on whether tooth flank contact stress or tooth root bending stress is the more critical, not on which grade sounds more advanced.

VI. Hardness as the shared unit for cross-system comparison

A drawing may carry HRC, another HB, a third HV, so whenever one gear has to be judged against all three, a conversion step is unavoidable. The tiers met most often (values approximate):

• HRC 25: 255
  • HV: 260
  • Typical correspondence: Medium carbon steel, quenched and tempered
• HRC 30: 285
  • HV: 290
  • Typical correspondence: Quenched and tempered steel
• HRC 40: 370
  • HV: 380
  • Typical correspondence: Medium carbon steel, quenched and tempered
• HRC 48: 455
  • HV: 470
  • Typical correspondence: 40Cr induction hardened
• HRC 52: 510
  • HV: 525
  • Typical correspondence: Alloy steel, quenched
• HRC 58: 580
  • HV: 610
  • Typical correspondence: 20CrMnTi, carburized and quenched
• HRC 62: 640
  • HV: 670
  • Typical correspondence: High-speed steel / carburizing steel

Common hardness requirement tiers for gears:

• Gear type Carburized and quenched gear: Carburize + quench + low temper
  • Common hardness range: 58~62 HRC
  • Description: Hard tooth flank, tough core, impact resistant
• Gear type Quenched and tempered + induction hardened gear: Quench and temper + tooth flank induction hardening
  • Common hardness range: 48~55 HRC
  • Description: Hard tooth flank, core retains toughness
• Gear type Quenched and tempered gear: Quench and temper
  • Common hardness range: 280~320 HB
  • Description: Medium strength, used for medium and low speeds
• Gear type Nitrided gear: Quench and temper + gas/plasma nitriding
  • Common hardness range: ≥50 HRC (nitrided layer)
  • Description: Small distortion, wear resistant
• Gear type Through-hardened gear: Through harden + low temper
  • Common hardness range: 45~55 HRC
  • Description: Small module gears

These conversion relationships hold only within an approximate range. Below 20 HRC the Rockwell C scale gives large errors and the HRB scale should be used instead; below 450 HB, HRC and HB can be approximately estimated as HRC ≈ (HB − 200) / 10. Because measurement error grows in regions below 20 HRC, the inspection scale should be agreed upon at acceptance.

VII. Five items to check before substitution

• Chemical composition: Each standard has differences in the upper and lower limits for Si, Mn, Cr, Ni, Mo; check element by element
• Hardenability: Compare according to end-quench data, do not infer from the grade name; the larger the cross-section, the more critical
• Cleanliness: Each standard has different requirements for S and P content and inclusion levels
• Heat treatment process: After changing material, carburizing temperature, carbon potential, quenching medium, and tempering parameters need to be re-confirmed
• Customer specification: When the drawing explicitly specifies a standard and grade, purchase according to the specification; changes require customer confirmation

VIII. Practical insights

• For export orders, establish first which standard the customer inspects against. The drawing may say 20CrMnTi while the German customer inspects to the German standard; the difficulty is usually not whether the part can be made, but against which values it is submitted for inspection. A sentence in the technical agreement giving the material standard number and composition range saves more trouble than an argument afterwards.
• Trust the end-quench curve ahead of the grade label. Steel mills ship the same nominal grade with differing hardenability bandwidths. Where hardenability is a requirement, put the J values into the purchase specification and verify them during incoming re-inspection; that beats reading the grade alone off the certificate.
• A material change drags the heat treatment parameters with it. Changing only the drawing annotation while calling up the carburizing process as before is a common source of batch anomalies — when the nickel content shifts, the case depth obtained at the same carbon potential and time shifts as well.
• Inspection reports should carry the grade, the standard number and the heat treatment state in full. On export inspection, a report that says only “alloy steel, HRC58” leaves the customer unable to match it to the drawing requirements; writing all three in full closes the traceability chain.

Between material grade and finished gear sit several stages: machining and inspection of precision cylindrical gears, turning, milling, and heat treatment of shaft-type transmission parts, and precision machining and gear grinding processes. For standard series parts such as racks, the part number itself encodes the material: in GY-CSTGH-020-10-S, position C stands for S45C and M for SCM440, while 020 denotes module M2 and 10 denotes length 1000mm. Where a drawing requires an engineering review covering material and heat treatment, contact us.

PinyiGear supplies precision gears and transmission parts to drawing, with material and heat treatment documentation traceable to the standard named on the order.