Chrome Moly JIS SNCM220H Alloy Steel Bars

Product Description

JIS SNCM220H is a premium Nickel-Chromium-Molybdenum (Ni-Cr-Mo) triple-alloy structural steel standardized under JIS G4052. It is a specialized, case-hardening (carburizing) steel engineered specifically for application profiles requiring precise control over heat-treatment behavior.

The critical suffix "H" indicates that this material belongs to a dedicated Hardenability Band classification. When you specify SNCM220H bar stock rather than standard SNCM220, the mill guarantees that the steel's depth-of-hardening profile will fall within tightly restricted, mathematically predictable limits during a standard Jominy end-quench test.

1. Chemical Formula (JIS G4052)

To maintain consistent hardenability limits from batch to batch, the chemical breakdown of SNCM220H bars is tightly regulated:

2. Why Specify the "H" Grade? (Guaranteed Jominy Performance)

In standard precision machining (such as automotive mass-production), small variations in alloy chemistry between steel batches can cause unpredictable distortion or varying core hardness after a quench line.

By utilizing SNCM220H, you ensure the steel's cooling-rate-to-hardness ratio is guaranteed by the mill. During testing, the bar is heated to its austenitizing temperature and quenched from one end with water. The hardness profile measured at fixed distances from that quenched end must fit within a strict, standardized curve. This allows heat-treatment facilities to run highly automated, repeatable cycles with zero variation in final component performance.

3. Surface Carburization & Core Dual-Mechanics

Because SNCM220H has a relatively low base carbon content ($\sim0.20\%$), it is built to undergo a carburizing heat treatment process after machining:

1. Carburizing: The machined bars are held at $880^\circ\text{C} - 950^\circ\text{C}$ in a carbon-rich atmosphere, allowing carbon to diffuse into the surface layer to a depth of 0.5 mm to 2.0 mm.

2. Quenching & Tempering: The part is oil-quenched and low-temperature tempered ($\sim150^\circ\text{C} - 200^\circ\text{C}$).

• The Structural Yield: This forms a glass-hard surface case of 58 to 63 HRC for premium wear and contact-fatigue resistance, supported by a highly resilient structural core with a tensile strength ranging from 750 to 1100 MPa (depending on cross-sectional bar diameter).

4. Key Engineering Applications

SNCM220H bars are heavily utilized for components subjected to intense surface friction combined with continuous shock loading or high torsional forces:

• Automotive Powertrains: Differential pinions, crown wheels, transmission planetary gears, universal joint journals, and drive shafts.

• Heavy Industrial Equipment: Splined shafts, heavy-duty bushings, link pins, piston pins, and steering knuckles.

• Drilling & Mining: Heavy rock drills, high-pressure valve components, and specialized gear boxes.

5. Direct Global Cross-References

When tracking down equivalent grades from alternative regional supply lines, SNCM220H maps directly to:

• United States: AISI 8620H / SAE 8620H / UNS H86200

• Europe / Germany: 20NiCrMo2-2 (1.6523) / 21NiCrMo2 (with specified hardenability)

• China: GB 20CrNiMoH

• United Kingdom: BS 805H20

• France (AFNOR): 20NCD2 (H-grade variants)

6. Sourcing & Fabrication Notes

• Machinability: In its supplied Soft Annealed (+A) condition, the material sits $\le$ 212 HBW. It chips cleanly and exhibits excellent tool life on CNC turning and milling centers before case hardening.

• Weldability: Due to its low initial carbon level, it is significantly easier to weld than high-carbon structural alloys like SCM440 (AISI 4140). However, due to its Cr-Ni-Mo alloy contents, preheating heavy bar sections to $150^\circ\text{C} - 200^\circ\text{C}$ is highly recommended to completely mitigate hydrogen cracking in the Heat-Affected Zone (HAZ).