Can 8620 steel be case hardened?
Yes. 8620 is the standard carburizing steel: a 58-62 HRC case, typically 0.5-1.5 mm (0.020-0.060") deep, over a tough core of roughly 25-45 HRC.
8620 exists to be carburized. Its 0.18-0.23% carbon keeps the core tough, and its small additions of nickel (0.40-0.70%), chromium (0.40-0.60%) and molybdenum (0.15-0.25%) give it enough hardenability that both the high-carbon case and the low-carbon core harden in an oil quench. That is the difference from 1018: plain carbon steel needs a water quench and still leaves a soft core, while 8620 gives a hard case on a strong core with less distortion. It is the default material for carburized gears, pinions, splined shafts, camshafts, pins and bearing races in industrial and automotive work; 9310 and similar high-nickel grades take over in aerospace gears, and 4320 or 4820 where heavier sections need more core hardenability.
The cycle: carburize at 900-955 °C (1650-1750 °F), most often 925 °C (1700 °F), in an endothermic gas atmosphere or by low-pressure vacuum carburizing, until the surface carbon reaches about 0.8-0.9%. Roughly 4-8 hours at 925 °C gives about 1 mm (0.040") of effective case, and deeper cases take disproportionately longer because depth grows with the square root of time. The parts are then either direct-quenched in oil from about 845 °C (1550 °F), or cooled and reheated to 815-845 °C (1500-1550 °F) and oil quenched, which refines the grain. Temper at 150-200 °C (300-400 °F). Typical results: surface 58-62 HRC, effective case depth to 50 HRC of 0.5-1.5 mm (0.020-0.060"), and a core of roughly 25-45 HRC, highest in small parts and falling toward the low 20s HRC in heavy sections.
Most of the engineering is in details the heat treater cannot guess. Case depth scales with the load and the part: 0.25-0.5 mm (0.010-0.020") on small pins and fine-pitch gears, 0.75-1.25 mm (0.030-0.050") on typical industrial gears, more on coarse-pitch gears; too deep a case through-hardens thin teeth and makes them brittle. Threads, holes drilled later and areas to be welded are masked with copper plate or stop-off paint, or left oversize so the case can be machined off before the quench. Gas carburizing leaves 10-20 µm of intergranular oxidation at the surface, which low-pressure vacuum carburizing avoids and grinding removes; high surface carbon leaves retained austenite that lowers hardness and can grow the part over time, controlled by carbon potential or a sub-zero treatment. Shot peening after hardening adds fatigue strength at gear roots and fillets. AMS 2759/7 governs aerospace-grade carburizing, and AGMA 923 sets metallurgical quality grades for carburized gears.
Carburizing for 8620 parts: where to send them
What to specify
- Surface hardness after temper, typically 58-62 HRC, and the temper: 150-200 °C (300-400 °F)
- Effective case depth range to 50 HRC per SAE J423, stated on the finished surface, for example 0.75-1.0 mm (0.030-0.040") after grinding
- Core hardness range and where it is measured, for example 30-40 HRC at the tooth center or at mid-radius
- Masking: threads, holes and weld areas to remain soft, by copper plate or stop-off, or oversize stock machined off before the quench
- Grinding stock of 0.1-0.25 mm (0.004-0.010") per side on ground surfaces, and the distortion allowed (runout, flatness) after heat treatment
- Microstructure limits on critical gears: maximum retained austenite, no continuous carbide network, maximum intergranular oxidation depth, per AMS 2759/7 or the AGMA 923 grade
- Material: 8620 or 8620H (hardenability band per SAE J1268) to ASTM A29 / A322, with the mill cert
Pitfalls
- Case depth specified before grinding: grinding takes 0.1-0.25 mm and the finished case comes out too thin
- Too deep a case on small or fine-pitch teeth: the tooth through-hardens and snaps instead of bending
- Unmasked threads and splines that must be assembled or machined later: brittle, and uncuttable except by grinding
- Distortion on thin rings, long shafts and asymmetric gears: plan press quenching, straightening and grind stock
- High surface carbon with no sub-zero treatment: retained austenite lowers hardness, and the part can grow in service
- Tempering above about 200 °C (400 °F) to cure brittleness: the surface drops below 58 HRC and wear life falls
Frequently asked questions
- How hard does carburized 8620 get?
- About 58-62 HRC at the surface after oil quenching and a 150-200 °C (300-400 °F) temper, with a core of roughly 25-45 HRC depending on section size. The surface hardness comes from the added carbon; the core hardness from the alloy content.
- What case depth should I specify on 8620?
- It depends on load and size: about 0.25-0.5 mm (0.010-0.020") for small pins and fine-pitch gears, 0.75-1.25 mm (0.030-0.050") for typical industrial gears and shafts, and more for coarse-pitch gears. Specify it as effective depth to 50 HRC on the finished surface.
- Should I use 8620 or 4140 for gears?
- 8620 carburized for high contact stress and wear, where a 58-62 HRC surface is needed. 4140 through-hardened, induction hardened or nitrided for moderate loads, where lower cost and less distortion matter more than maximum surface hardness.
- Can 8620 be hardened without carburizing?
- Only partly. With about 0.20% carbon it quenches to roughly 35-45 HRC in thin sections and less in heavy ones, with no wear surface. It is bought to be carburized; for through-hardened parts use 4140 or 4340.
