Quick answer: AISI 8620 and 42CrMo4 (commonly compared with AISI 4140) solve different heat-treatment problems. AISI 8620 is a low-carbon nickel-chromium-molybdenum steel normally selected for carburizing when a hard, wear-resistant case and a tougher core are required. 42CrMo4 is a higher-carbon chromium-molybdenum steel normally selected for quenching and tempering when strength and toughness are needed through a larger part of the section. The drawing, final heat-treatment route, section size and acceptance properties should decide the grade—not the alloy name alone.
The Main Difference Is the Required Hardness Profile
The most useful comparison starts with the finished component. If the design needs a very hard surface for wear or contact fatigue while retaining a tougher interior, a carburizing grade such as AISI 8620 is a logical starting point. If the design needs a stronger, tempered structure through the section, 42CrMo4 is usually the more relevant family to evaluate.
This is not a promise that either material will reach one universal hardness or strength. Results depend on the exact chemistry, bar diameter, prior microstructure, furnace cycle, quench severity, tempering conditions, machining allowance and the location where properties are measured. The purchase specification should therefore define both the raw material and the final processing route.
| Selection point | AISI 8620 | 42CrMo4 / 1.7225 |
|---|---|---|
| Typical design route | Carburize, quench and temper to create a hard case over a tougher core | Quench and temper for strength and toughness through the section, subject to section-size effects |
| Carbon level | Low carbon; carbon is added at the surface during carburizing | Medium carbon; responds directly to quenching and tempering |
| Target property distribution | Deliberately different case and core properties | More uniform strength than a carburized case/core design, within the achievable hardening depth |
| Surface-hardening option | Carburizing is the defining route for many applications | Induction hardening may be considered when a hardened surface is required on a suitable base condition |
| RFQ focus | Case depth, case hardness, core properties, distortion allowance and heat-treatment responsibility | Delivery condition, section size, required mechanical properties, hardness location and impact requirements |
Composition Explains the Different Heat-Treatment Routes
AISI 8620 is a nickel-chromium-molybdenum alloy steel with a relatively low carbon range. The low-carbon core supports toughness, while carburizing increases the carbon content near the surface before quenching. Nickel contributes to toughness and hardenability; chromium and molybdenum support hardenability and tempering response.
42CrMo4 is a chromium-molybdenum steel with substantially more carbon. That chemistry makes quenching and tempering the normal route for developing a balance of strength and toughness. It can also be surface hardened by processes such as induction hardening, but that route is not the same as carburizing a low-carbon case-hardening steel.
| Element | AISI 8620 typical specified range, mass % | 42CrMo4 EN ISO 683-2 range, mass % |
|---|---|---|
| Carbon | 0.18–0.23 | 0.38–0.45 |
| Chromium | 0.40–0.60 | 0.90–1.20 |
| Molybdenum | 0.15–0.25 | 0.15–0.30 |
| Nickel | 0.40–0.70 | Not an intentional alloying range in the cited 42CrMo4 designation |
The ranges above are a comparison aid, not a purchase specification. Final limits depend on the governing grade standard and edition. A material certificate should be checked against the exact standard named on the order.
Is 42CrMo4 the Same as AISI 4140?
42CrMo4 and AISI/SAE 4140 are commonly treated as comparable chromium-molybdenum grades, and many buyers use both names during preliminary sourcing. They should not be treated as automatically identical on a purchase order. The designation systems, permitted chemistry, product standards, heat-treatment condition and mechanical-property requirements can differ.
When a drawing states 4140 but a quotation offers 42CrMo4—or the reverse—the responsible engineer should approve the substitution against the actual specification and required properties. Writing “42CrMo4/4140 equivalent” without an acceptance basis can create disputes at certificate review.
When AISI 8620 Is the Better Starting Point
AISI 8620 is worth evaluating when the component needs a controlled wear-resistant case and a core that remains tougher than the surface. Typical design examples include gears, splined parts, pins, bushings and other components exposed to repeated surface contact. These are application examples only; the correct grade still depends on load, geometry, required case depth, distortion limits and validation by the designer.
The carburizing route must be planned before final machining. Case depth, grinding stock, masked areas, distortion allowance and post-heat-treatment finishing can all affect the blank size and manufacturing sequence. If the buyer orders only “8620 bar” without defining who controls the final heat treatment, the raw material order does not establish the finished component performance.
When 42CrMo4 Is the Better Starting Point
42CrMo4 is worth evaluating when the component needs a quenched-and-tempered balance of strength and toughness across the working section. Shafts, couplings, high-strength fastener components and heavily loaded machine parts are common design contexts. The required properties must be tied to the actual diameter or ruling section because a thicker part does not cool like a small test coupon.
Ovako’s published 42CrMo4 data describes the grade as a quenching-and-tempering steel and explicitly notes section-size limits on full through hardening. That is an important procurement lesson: “through hardening” is a design objective, not a guarantee that every diameter will have identical microstructure from surface to center.
Do Not Compare Mechanical Properties Without the Condition
A table that lists one tensile strength or one hardness for each grade can be misleading. AISI 8620 may be supplied annealed, normalized or in another machining condition before carburizing. 42CrMo4 may be supplied untreated, normalized, annealed or quenched and tempered. The same grade name can therefore appear with very different reported values.
For an engineering comparison, state the material condition, product form, section size, test location and heat-treatment route. For procurement, state the properties that must be certified on the delivered bar and separately identify any properties required after component heat treatment. Our broader alloy steel heat-treatment guide explains how annealing, normalizing, quenching, tempering and case hardening change the result.
RFQ Checklist for 8620 or 42CrMo4 Bar
| RFQ field | What to state | Why it matters |
|---|---|---|
| Grade and standard | AISI/SAE 8620 or 42CrMo4/1.7225, plus the governing material and product standard | Prevents “equivalent” grades from being accepted without a defined basis |
| Product form and dimensions | Round, flat or forged blank; diameter or section; length; tolerances | Section size influences hardenability, machining stock and test results |
| Delivery condition | Annealed, normalized, quenched and tempered, or another specified condition | Controls machining response and the properties of the delivered bar |
| Final heat treatment | Carburizing, quench-and-temper or induction-hardening route, with responsible party | Separates raw-material supply from finished-component performance |
| Acceptance properties | Required hardness, tensile, impact, case depth or core-property criteria and test location | Makes quotations and certificates comparable |
| Surface and machining allowance | Black, peeled, turned or ground surface; finished size; allowance and decarburization limits if applicable | Avoids ordering a nominal bar size that cannot reach the finished component |
| Inspection and documents | Certificate type, traceability, inspection method, sampling and acceptance criteria | Prevents undefined requests such as “full testing” |
| Commercial information | Quantity, destination, packing restrictions and requested schedule | Supports a quotation against the actual order scope |
For grade-specific product information, review the AISI 8620 alloy steel bar and 42CrMo4 / 1.7225 alloy steel bar pages. Available condition, dimensions, inspection scope and commercial details should be confirmed in the actual quotation. For a broader material-family overview, see Alloy Steel Composition, Types, Advantages and Limitations.
Frequently Asked Questions
Can 42CrMo4 replace AISI 8620?
Not as a direct substitution. The grades use different carbon levels and heat-treatment strategies. A change from a carburized case/core design to a quenched-and-tempered design requires engineering review of wear, fatigue, toughness, distortion and manufacturing requirements.
Which grade is better for gears?
Neither grade is universally better. AISI 8620 is commonly evaluated for carburized gears that need a hard case and tough core. 42CrMo4 may be evaluated for gears or shafts requiring a different strength and surface-hardening route. The load spectrum, tooth geometry, case-depth requirement and finishing process control the choice.
Is 42CrMo4 always through hardened?
No. The achievable hardened depth depends on hardenability, section size, quench conditions and prior structure. Large sections can show different microstructures and properties between surface and center, so acceptance requirements should identify the test location.
Should an RFQ use “4140 equivalent to 42CrMo4”?
Use that phrase only as a sourcing reference. The order should name the accepted grade, governing standard, condition, dimensions and required properties. Any substitution should be approved against those requirements.
Request a Grade-Specific Quotation
Send the drawing or dimensional list together with the required grade, standard, delivery condition, final heat-treatment route, acceptance properties, quantity and destination. SASA Alloy can review the requested bar scope and quotation fields; final material selection and finished-component performance remain subject to the purchaser’s engineering requirements and the agreed order specification.
Post time: Oct-30-2026