Bearing Heat Treatment: Why Through-Hardening Beats Case-Hardening for Needle Roller Bearings

time 2026-06-02

Through-hardening produces uniform HRC 60-65 hardness from surface to core in GCr15 needle bearings, ideal for standard rolling contact fatigue. Case-hardening (carburizing) gives a hard surface (HRC 58-62) over a tougher core (HRC 30-45), preferred for impact loading or shock applications. DW Bearing uses through-hardening for HK/BK/NK series as the optimal choice for most rolling fatigue conditions.

1. Two Approaches to Bearing Hardness

Through-Hardening (Martensitic Quenching)

The entire bearing component is heated above austenitizing temperature, then rapidly quenched in oil to form martensite throughout the cross-section. Used with 1% carbon steels (GCr15, SUJ2, 100Cr6, 52100). Results in uniform hardness from surface to core.

Case-Hardening (Carburizing)

Low-carbon steel (e.g., 20CrMo or 16MnCr5) is held in a carbon-rich atmosphere at 900-950°C, allowing carbon to diffuse into the surface. Subsequent quench produces a hard high-carbon surface (typically 0.7-1.0% C, depth 0.5-2 mm) over a softer low-carbon core (0.2% C, HRC 30-40).

2. Microstructure Comparison

Aspect	Through-Hardened GCr15	Case-Hardened 20CrMo
Surface hardness	HRC 60-65	HRC 58-62
Core hardness	HRC 60-65 (same)	HRC 30-45
Case depth (effective)	N/A (full hardened)	0.5-2.0 mm
Surface compressive stress	Minimal	200-400 MPa (beneficial)
Impact toughness	Low (10-15 J)	High (40-80 J at core)
Rolling contact fatigue life	Excellent	Very good
Cost	Lower (1 step)	Higher (slower process, gas control)

3. When to Choose Each

Use Through-Hardening (DW Standard)

Pure rolling contact, steady load (HK transmissions, NK gearboxes)
Most automotive, power tool, and industrial applications
Cost-sensitive applications where impact is not significant
Where bearing space is constrained (no need for extra section thickness)

Use Case-Hardening

High impact loading (cam followers, rocker arm bearings)
Heavy shock applications (mining equipment, rock crushers)
Where through-section fatigue is a concern
Where bearings are exposed to surface contact stresses + bending stresses

4. DW Bearing Heat Treatment Process for HK/NK Series

K Series radial needle roller and cage assembly.png

Step	Temperature	Time	Atmosphere
Austenitizing	835-845°C	30-45 min	Controlled CP (carbon potential 1.0)
Oil quench	60-80°C oil	5-10 min	Quench oil (Houghton K oil or equivalent)
Sub-zero treatment (optional)	-80°C	1-2 hr	Dry ice / methanol
Tempering	150-180°C	2 hr	Air
Final hardness	HRC 60-65	-	Verified per piece sampling

5. Quality Control After Heat Treatment

Surface hardness (Rockwell C): 100% piece-by-piece for thrust washer; 5/lot for needles and races
Retained austenite measurement (X-ray diffraction): 1/lot, target 6-10%
Microstructure (50x to 500x): 1/lot, photographed and archived
Decarburization depth: 1/lot, max 0.05 mm allowed
Magnetic particle inspection: 100% for quench cracks

6. FAQ

Q1: Why does my bearing crack during quenching?
Quench cracks come from too-fast cooling, geometric stress concentrations, or improper austenitizing temperature. DW uses oil quench (slower than water) and controlled temperature to minimize this.

Q2: What is retained austenite and why does it matter?
Untransformed FCC iron remaining after quench. Some retained austenite (6-10%) improves toughness; too much (>15%) causes dimensional instability over time as it slowly transforms to martensite.

Q3: Can you do case-hardened needle bearings on request?
Yes. For impact-loaded applications, we offer carburized variants. Lead time and cost are 30-50% higher than standard through-hardened.

Q4: What is sub-zero treatment for?
Cools the part to -80°C to transform residual retained austenite to martensite. Improves dimensional stability for precision bearings.