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MechDatum

Steel 8620 properties

AISI 8620 steel is a low-hardenability nickel-chromium-molybdenum alloy steel, formulated primarily for carburizing. It combines excellent core toughness with high surface hardness after thermochemical treatment, making it a standard choice for components requiring surface wear resistance and high internal ductility. Its versatility in forging, machining, and hardening processes positions it as a reference material in the automotive, agricultural, and machinery industries.

Chemical Composition

Section titled “Chemical Composition”

8620 steel has a controlled chemical composition, with a carbon content between 0.18% and 0.23% that facilitates carbon diffusion during carburizing and provides a tough core after hardening. Alloying elements such as nickel, chromium, and molybdenum increase hardenability and overall mechanical strength. The following table specifies the standard compositional ranges for this grade.

Element Content (%)
Iron, Fe 96.895 – 98.02
Manganese, Mn 0.700 – 0.900
Nickel, Ni 0.400 – 0.700
Chromium, Cr 0.400 – 0.600
Carbon, C 0.180 – 0.230
Silicon, Si 0.150 – 0.350
Molybdenum, Mo 0.150 – 0.250
Sulfur, S ≤ 0.0400
Phosphorus, P ≤ 0.0350

Physical Properties

Section titled “Physical Properties”

The density of 8620 steel is 7.85 g/cm³ (0.284 lb/in³). The physical properties of this alloy are consistent with those of other low-alloy steels. The modulus of elasticity is typically between 190 GPa and 210 GPa, while Poisson’s ratio ranges from 0.27 to 0.30.

Property Metric System Imperial System
Density 7.85 g/cm³ 0.284 lb/in³
Modulus of Elasticity 190 – 210 GPa 27 557 – 30 458 ksi
Bulk Modulus (typical for steel) 140 GPa 20 300 ksi
Shear Modulus (typical for steel) 80 GPa 11 600 ksi
Poisson’s Ratio 0.27 – 0.30 0.27 – 0.30

Mechanical Properties in Annealed Condition

Section titled “Mechanical Properties in Annealed Condition”

In the annealed condition, 8620 steel reaches a tensile strength of 530 MPa (76 900 psi) and a yield strength of 385 MPa (55 800 psi). Machinability in this state, compared to AISI 1212 steel as a reference of 100%, is evaluated at 65%.

Property Metric System Imperial System
Tensile Strength 530 MPa 76 900 psi
Yield Strength 385 MPa 55 800 psi
Brinell Hardness 149 149
Rockwell B Hardness (converted from Brinell) 80 80
Izod Impact Strength 115 J 84.8 ft·lb
Machinability (reference AISI 1212 = 100 %) 65 65

Mechanical Properties in Quenched and Tempered Condition

Section titled “Mechanical Properties in Quenched and Tempered Condition”

After oil quenching from approximately 845 °C and low-temperature tempering, 8620 steel can develop a tensile strength exceeding 800 MPa in small sections. Final properties depend on the tempering temperature and part geometry, but generally a balance between high strength and good toughness is sought.

Property Metric System Imperial System
Tensile Strength (quenched and tempered) 780 – 930 MPa 113 000 – 135 000 psi
Yield Strength (quenched and tempered) 570 – 710 MPa 82 700 – 103 000 psi
Rockwell C Hardness (core, estimated) 25 – 38 HRC 25 – 38 HRC
Elongation (in 50 mm) 18 – 22 % 18 – 22 %

Mechanical Properties in Carburized and Hardened Condition

Section titled “Mechanical Properties in Carburized and Hardened Condition”

8620 steel is designed for carburizing. After a carburizing cycle at 925 °C, direct quench or re-quench from 845 °C, and tempering between 150 °C and 200 °C, the surface hardness typically reaches 60 HRC, while the core maintains values between 25 HRC and 35 HRC. This combination provides maximum wear resistance on the surface with excellent bending fatigue strength.

Property Metric System Imperial System
Surface Hardness (carburized) 58 – 62 HRC 58 – 62 HRC
Core Hardness (carburized and hardened) 25 – 35 HRC 25 – 35 HRC
Effective Case Depth (typical) 0.5 – 1.5 mm 0.020 – 0.060 in

Thermal Properties

Section titled “Thermal Properties”

The thermal conductivity of AISI 8620 steel is 46.6 W/m·K (323 BTU·in/h·ft²·°F). This property is relevant for the design of heat treatment cycles and for the service behavior of components subjected to thermal gradients.

Property Metric System Imperial System
Thermal Conductivity 46.6 W/m·K 323 BTU·in/h·ft²·°F
Specific Heat (typical for steel) ~ 475 J/kg·K ~ 0.114 BTU/lb·°F

Equivalent Designations

Section titled “Equivalent Designations”

AISI 8620 steel is marketed under a wide range of international standards, ensuring interchangeability in global projects. The most common equivalents include AMS and ASTM series specifications as well as European and Japanese designations.

Standard / Specification Designation
AISI / SAE 8620
UNS G86200
ASTM A29, A322, A506, A507, A519, A829
AMS 6274, 6276, 6277, 6375
DIN 1.6523 / 21NiCrMo2
AFNOR 20 NCD 2 / 22 NCD 2
JIS SNCM 21 / SNCM 21 H
BS 805 H 20 / 805 M 20

Applications

Section titled “Applications”

8620 steel is predominantly used for carburized parts operating under high load cycles and requiring a wear-resistant surface. Its balance of properties makes it suitable for mechanical transmission components.

Among its most frequent applications are pinions and transmission gears, crankshafts and camshafts, chain pins and bushings, splined shafts and universal joint crosses, shift forks and steering components, as well as forged and machined parts for the heavy machinery, agricultural equipment, and construction sectors.

Comparison with Similar Materials

Section titled “Comparison with Similar Materials”

Compared to lower-grade carburizing steels such as AISI 8620H, the standard variant offers a narrower compositional range that ensures a more predictable hardening response. In comparison with chromium-manganese steels like 5115, 8620 provides greater core toughness due to the presence of nickel. Regarding higher-hardenability alloys like AISI 4320, 8620 is a more economical alternative while maintaining sufficient performance for most medium-duty applications. When compared to through-hardening steels like AISI 4140, 8620 is distinguished by its ability to be carburized, achieving much harder surfaces without sacrificing core ductility.

Frequently Asked Questions (FAQ)

Section titled “Frequently Asked Questions (FAQ)”

What is the maximum surface hardness obtainable by carburizing 8620 steel?

Section titled “What is the maximum surface hardness obtainable by carburizing 8620 steel?”

The maximum surface hardness after carburizing and low-temperature tempering is between 58 HRC and 62 HRC when the case reaches an eutectoid carbon content.

Section titled “What tempering temperature is recommended to obtain a tensile strength of 800 MPa in the core?”

A tempering between 150 °C and 200 °C after hardening allows obtaining core tensile strengths close to 800 MPa in carburized parts.

How much carbon does the surface of 8620 steel absorb during a standard carburizing?

Section titled “How much carbon does the surface of 8620 steel absorb during a standard carburizing?”

During the carburizing phase at 925 °C with controlled carbon potential, the surface absorbs approximately between 0.80% and 1.00% carbon.

What is the hardness difference between the carburized case and the core in a treated gear?

Section titled “What is the hardness difference between the carburized case and the core in a treated gear?”

The carburized case typically reaches 60 HRC, while the core remains between 25 HRC and 35 HRC, representing a difference of up to 35 Rockwell C points.

What carburized case depth is specified in automotive pinion applications?

Section titled “What carburized case depth is specified in automotive pinion applications?”

The effective case depth commonly specified for transmission pinions ranges from 0.5 mm to 1.5 mm, depending on the gear module.

What is the advantage of using 8620 over a plain carbon steel for wear parts?

Section titled “What is the advantage of using 8620 over a plain carbon steel for wear parts?”

8620 steel develops a surface hardness exceeding 58 HRC after treatment, doubling the wear resistance of a plain carburized carbon steel, which rarely exceeds 45 HRC effective in service.

Sources

Section titled “Sources”

References

Section titled “References”