Understanding 8620 Carburizing Steel

In the field of mechanical manufacturing, carburizing steel is the core material of high-load parts such as gears, bearings, and transmission shafts. Among the many carburizing steel grades, 8620 steel (SAE/AISI 8620) is known as the “king of carburizing steel” for its excellent balance of comprehensive performance. Behind this title is its unique advantages in composition design, process adaptability and industrial application.

8620 Steel Chemical Composition-Designed For Carburizing

8620 steel belongs to low-carbon alloy steel, and its typical element ratio and design logic is:

Carbon:0.18-0.23% —— Low carbon matrix ensures core toughness and provides space for carburizing.
Nickel:0.40-0.70% —— Improves toughness and low-temperature impact resistance.
Chromium:0.40-0.60% —— Enhances hardenability and form high-hardness carbides.
Molybdenum:0.15-0.25% —— Inhibit temper brittleness and improve high-temperature strength.

This combination makes it form a “hard surface and tough core” structure after carburizing:

Surface:carbon content can reach 0.8-1.2%, hardness can reach HRC 58-62, and wear resistance is excellent;
Core:maintain the high toughness and fatigue resistance of low-carbon steel,absorbing and buffering impact loads to prevent brittle fracture.

8620 Steel Golden Partner:Carburizing + Quenching + Low-Temperature Tempering

This is the core surface hardening process for 8620 steel.Through carburizing, the low-carbon matrix accumulates carbon on its surface.After quenching, high-carbon martensite forms on the surface, resulting in high hardness and excellent wear resistance. Meanwhile, the core, due to its low carbon content, retains good toughness and can withstand impact loads.

8620 Steel Core Process
Carburizing	Quenching	Tempering
900-930°C	830-850°C/Oil	150-200°C

What has changed for 8620 steel?

Area	Status Before Quenched & Tempered	Effect After Quenched & Tempered	Advantages
Surface carburized layer	High-carbon martensite, HRC 62-65, hard and brittle.	Tempered martensite, HRC58-62, maintains high hardness while improving toughness.	Excellent wear resistance
Low-carbon core area	Low-carbon martensitic/bainitic, HRC35-40, insufficient strength and toughness.	After tempering, HRC28-38 achieves an optimized balance of strength and toughness.	Good toughness and impact resistance

8620 Steel-Irreplaceable Industrial Application

The “kingly status” of 8620 steel stems from its use in manufacturing critical transmission structural components that withstand heavy loads, impacts, and alternating stresses, primarily in automotive and heavy machinery gears and shafts.

Automobile industry:This is its largest and most core application area. 8620 steel is mainly used in the manufacture of automotive gearbox gears, differential gears, half shafts, camshafts, etc.
Heavy machinery:8620 steel is used to manufacture heavy-duty gears, gear shafts, and pins for heavy machinery.

8620 steel can be found wherever the surface of the parts needs to be wear-resistant while the core can withstand impact.

8620’s Battle with Similar Materials

8620 vs 1018/1020：The biggest difference is that 1018/1020 is carbon steel, while 8620 has significantly better hardenability, core strength/toughness than carbon steel. 1018/1020 is cheaper but has limited load-bearing capacity, making it suitable for lightly loaded, non-critical components.
8620 vs 4320：8620 exhibits better tempering resistance and fatigue resistance due to its higher Mo content. 4320, with a higher nickel content (1.65-2.00%), offers superior core toughness and is suitable for applications requiring higher impact resistance.
8620 vs 18CrNiMo7-6：18NiCrMo7-6 has a higher nickel content (1.4-1.7%) than 8620 (0.4-0.7%), resulting in greater core toughness and load-bearing capacity, making it suitable for high-end automotive/aerospace heavy-duty components.

8620 steel’s title of “King of Carburizing Steel” comes from the perfect closed loop of its composition, process and application. Whether it is traditional fuel vehicle gears or aerospace precision parts, it has always been an irreplaceable “ace material” in the hearts of engineers with its characteristics of “hard but not brittle, strong and durable”. With the advancement of technology, this “king” may continue to write a new legend.