Propriedades do aço 8620: composição, dureza e aplicações

8620 (AISI/SAE 8620) is a low-carbon nickel-chromium-molybdenum alloy steel made for case hardening by carburizing. It is one of the most widely used carburizing grades, chosen for gears, shafts, pins, and other parts that need a hard, wear-resistant surface over a tough, ductile core. In Chinese standards, it corresponds to 20CrNiMo.

8620 is not a tool steel. It is a case-hardening engineering steel. Its carbon content is only about 0.20 percent, so it cannot be through-hardened to tool-steel hardness. Even if fully hardened, martensite at this carbon level reaches only about 40-45 HRC. Useful surface hardness is achieved through carburizing, which enriches the outer layer with carbon before quenching.

Composição Química

Elemento	AISI 8620
Carbono ©	0.18–0.23%
Manganês (Mn)	0.70–0.90%
Silício (Si)	0.15–0.35%
Níquel (Ni)	0.40–0.70%
Cromo (Cr)	0.40–0.60%
Molibdênio (Mo)	0.15–0.25%
Fósforo (P)	0,035% máx.
Enxofre (S)	0,040% máx.

Nickel gives the core its toughness. Chromium and molybdenum provide the hardenability the carburized case needs to reach full surface hardness on an oil quench, and molybdenum also reduces the risk of temper embrittlement. Together, the alloy content maintains a fine grain structure throughout the long carburizing soak.

Notas equivalentes

Padrão	Designação
AISI / SAE	8620
GB (China)	20CrNiMo
JIS (Japão)	SNCM220
DIN / EN	21NiCrMo2 (1.6523)

How 8620 Is Used

8620 is almost always supplied and used in the carburized condition. The finished part has two distinct zones: a hard case for wear resistance and a soft, tough core for impact and bending loads.

After a full carburizing and tempering cycle, the core has a hardness of 30–42 HRC with good toughness and ductility. The carburized surface reaches 58–63 HRC, with a case depth typically 0.5-2.0 mm, depending on carburizing time and temperature.

A typical heat treatment sequence is:

Carburize at about 925°C in a carbon-rich atmosphere, for several hours depending on the target case depth.
Cool, then reheat to about 840–870°C to refine the grain.
Têmpera em óleo.
Temper at about 150–200°C to keep the case hard.

Aplicativos

8620 suits parts that need a hard surface and a tough core at the same time:

Automotive transmission gears
Differential gears and pinions
Camshafts and cam followers
King pins and wrist pins
Bushings and bearings
Medium-duty shafts
Chain links and pins

The tough core carries impact and bending loads, while the hard case resists wear and improves surface fatigue strength. The carburized layer also carries beneficial compressive residual stress, which further increases fatigue life.

8620 vs Tool Steel

8620 and tool steel solve different problems. The table below compares 8620 with D2, a common through-hardened cold-work tool steel.

Aspecto	8620 (carburized)	Aço para ferramentas D2
Teor de carbono	~0.20%	~1.50%
Método de endurecimento	Carburize then quench	Through-hardening (air)
Surface hardness	58–63 HRC	58–60 HRC
Core hardness	30–42 HRC	58–60 HRC (same as surface)
Wear depth limit	Case depth (~2 mm max)	Through the full section
Regrindability	Limited, the case is consumed	Good, hard through the section
Robustez	Excellent, tough core	Moderado
Primary use	Gears, shafts, pins	Dies, punches, cutting tools
Custo relativo	Inferior	Mais alto

Why 8620 Cannot Replace Tool Steel

The hardness of an 8620 part is confined to the thin carburized case, limiting its use. It cannot be reground repeatedly, because each regrind removes part of the case, and once the case is gone, the part wears out quickly. A through-hardened tool steel can be reground many times. For the same reason, 8620 is not suitable for cutting tools: the edge is only as hard as the case, so as the case wears, the edge is lost. It is also unsuited to hot work, where the case overtempers and the core lacks hot strength, and to high-compression dies, where even a 60 HRC case is too thin to support heavy forming loads. The soft core deforms under the case, causing the case to crack or spall.

When 8620 Is the Right Choice

8620 is the correct material for gears and shafts that carry bending and impact loads and need a genuinely tough core. It works well where wear is concentrated on the surface, where a complex shape would distort badly under through-hardening, and where high fatigue resistance matters, since the carburized case adds compressive residual stress.

Common Misconceptions

“8620 can be hardened like tool steel.” It cannot. At about 0.20 percent carbon, conventional quenching cannot form martensite of useful hardness. The surface must be carburized first.

“8620 is a cheaper alternative to O1 ou A2 for dies.” It is not. The soft core and thin case make it unsuitable for dies or cutting tools, and an 8620 die would fail quickly.

“8620 and D2 are similar because both contain chromium.” They are not. D2 contains about 12 percent chromium, which forms carbides that enhance wear resistance. 8620 carries 0.4 to 0.6 percent chromium only for hardenability. Same element, very different purpose and level.