Same family, different heat treatment path
8620 and 8640 share the same nickel-chromium-molybdenum alloy family and nearly identical alloying elements, the real difference is carbon content, and that one difference determines how each gets heat treated and where it's used.
| Property | 8620 Alloy Steel | 8640 Alloy Steel |
|---|---|---|
| Category | Alloy Steel | Alloy Steel |
| Carbon (C) | 0.18 to 0.23% | 0.38 to 0.43% |
| Chromium (Cr) | 0.4 to 0.6% | 0.4 to 0.6% |
| Nickel (Ni) | 0.4 to 0.7% | 0.4 to 0.7% |
| Molybdenum (Mo) | 0.15 to 0.25% | 0.15 to 0.25% |
| Hardness | 149 HB | Not published |
| Density | 7.85 g/cm³ | 7.70 to 8.03 g/cm³ |
| Group | Nickel-chromium-molybdenum case-hardening steel | Nickel-chromium-molybdenum alloy steel |
Confirm against the mill test certificate before specifying. Chat with us if you need help matching a grade to a job.
8620: low carbon, built for carburizing
8620 runs a low 0.18 to 0.23% carbon, deliberately kept low because the steel is meant to be carburized, a process that adds carbon to the surface layer to create a hard, wear-resistant case while the low-carbon core stays tough and impact-resistant. That's the classic combination for gears: a tooth surface that resists wear, backed by a core that won't crack under shock loading. In the annealed condition, 8620 runs around 530 MPa tensile strength, 385 MPa yield strength, and 149 Brinell hardness, numbers that describe the core before carburizing, not the hardened case. Thermal conductivity is around 46.6 W/mK. It's also known by DIN 1.6523, JIS SNCM 21, or British Standard 805 H20/M20, useful cross-references when a customer specifies by a standard other than AISI.
8640: higher carbon, through-hardens instead
8640 carries 0.38 to 0.43% carbon, high enough to through-harden by conventional quench and temper rather than needing a carburizing step. Where 8620 needs a case, 8640 is chosen when the application wants consistent hardness and strength through the whole cross-section, without the extra processing time and equipment carburizing requires. Density runs slightly wider than most alloy steels, 7.7 to 8.03 g/cm³ depending on processing, and it's referenced under DIN 1.6546 internationally.
Choosing between them for a forging job
If a customer is making a gear or a component with a wear surface that needs to survive repeated contact while the core absorbs shock, quote 8620 and confirm they have carburizing capability, several Lahore and Karachi forging shops already offer it in-house alongside their forging and CNC gear hobbing work. If the part just needs to be strong and hard through its whole section, no case required, 8640 skips a processing step and gets there directly, which can mean a shorter turnaround and lower processing cost for the customer even though the raw material itself is comparable in price.
A note on machinability
8620's published machinability rating, based on the common 100-point scale where AISI 1212 steel is the reference, is around 65, meaning it machines reasonably well in the annealed condition before carburizing. That's worth mentioning to customers weighing 8620 against a higher-carbon alternative purely on machining ease, the case-hardening approach doesn't cost much in terms of how easily the steel cuts before heat treatment.
In the Pakistani forging trade
Both sit in the same PAAPAM-linked forging and gear-manufacturing supply chain as 4140, stocked by shops that also do CNC gear hobbing and in-house heat treatment. Worth quoting both side by side when a customer asks for "case hardening steel" without specifying a grade, the answer depends on whether they actually need carburizing or just want the phrase, and a quick question about whether the part gets a separate hardening process after forging usually clears it up.
Need help specifying which one fits your part? Talk to us with the application and we'll recommend a grade.