How Improper Pre-Incarburizing Preparation Causes Uneven Case Depth Failures in Gears

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How Improper Pre-Incarburizing Preparation Causes Uneven Case Depth Failures in Gears

Why a "Seemingly Simple Pre-Treatment" Determines Gear Lifespan?

Incarburizing Starts Before Furnace Loading—Not When the Furnace Ignites

In the gear manufacturing industry, a well-recognized truth holds: "Half the success of carburizing hinges on pre-treatment." Many on-site carburizing quality issues—local soft spots, inconsistent case depth, premature pitting, sudden drops in contact fatigue life, and more—can ultimately be traced not to furnace malfunctions or flawed chemical formulas, but to errors in pre-incarburizing preparation.

Uneven case depth is one of the most critical hidden hazards for gears. Its consequences go far beyond mere hardness inconsistencies:

Local soft spots → High susceptibility to premature pitting
Inconsistent case depth → Imbalanced contact stress distribution
Insufficient case depth at tooth roots → Reduced bending fatigue life
Uneven surface structure → Increased risk of "white layers" or burning during subsequent gear grinding
Elevated noise and unstable meshing → Deteriorated NVH (Noise, Vibration, Harshness) performance

In short: Uneven case depth is a ticking time bomb for early gear failure.

Three Overlooked Pre-Treatment Issues That Determine Carburizing Outcomes

1. Incomplete Degreasing → Surface Carbon Potential Shielding and Asymmetric Carburization

Degreasing removes oil stains, coolant residues, hand perspiration, cutting fluid deposits, and other contaminants. Inadequate degreasing leads to:

Oil films blocking carbon potential transmission
Reduced local carburizing rates
Shallow case depth or even "white spots" and "soft spots"

These problematic areas are particularly prone to pitting in high-contact-stress applications such as orbital gearboxes.

2. Unremoved Scale → Formation of Carbon Barrier Layers

Forged gear blanks typically have thick oxide scale that, if not fully eliminated, causes:

Carbon-blocked zones even in vacuum carburizing processes
20%–50% reduction in case depth
Uneven surface microstructure
"Reverse carburization" (carbon enrichment in deeper layers alongside surface carbon depletion)

Gears with this defect are highly susceptible to pitting after grinding—insufficient surface hardness combined with internal hardness creates dangerous stress concentrations.

3. Improper Furnace Loading → Obstructed Local Carburizing Pathways

Furnace loading is far more complex than simply "placing gears inside." It directly influences:

Furnace gas circulation patterns
Furnace gas contact area
Uniformity of carbon potential exposure across all gear surfaces

Improper loading results in:

Local dead zones → Shallow case depth
Overlapping or shielding between gears → Sheet-like soft spots
Overcrowding → Disrupted furnace gas flow
Mixed loading of small and large gears → Temperature inconsistencies due to differing thermal capacities

These issues occur much more frequently on-site than commonly assumed.

Microscopic Nature of Uneven Case Depth: Structural Differences from Uneven Carbon Potential

The core principle of carburizing is:Carbon atoms → Diffuse into the steel surface → Achieve target concentration and depth

When degreasing, descaling, or loading deficiencies reduce the surface's ability to absorb carbon:

Carbon diffusion slows down
Carbon potential reactions are hindered
Local carbon-depleted zones form
Surface martensite content decreases
Hardness drops by 50–150 HV
Case depth is insufficient by 0.1–0.3 mm
Surface residual compressive stress is reduced

Ultimately, gears exhibit early-stage failures including:

Pitting
Spalling
Microcracks
Increased meshing noise
Significantly reduced fatigue life (typically 30–60% shorter)

Common Characteristics of Gear Failures Caused by Uneven Case Depth

Pitting concentrated in specific tooth surface areas (not random distribution)
Obvious hardness inconsistencies (e.g., HRC 60 vs. HRC 54)
Significant case depth differences between left and right tooth surfaces
Step-like or abrupt changes in case depth profile
Metallographic analysis reveals increased surface ferrite content
Hardness distribution lacks a gradual gradient (showing abrupt jumps or collapses)

These signs all point to one core issue: Inadequate pre-treatment leading to uneven carburizing efficiency.

How to Prevent Uneven Case Depth?

1. Establish Strict Degreasing Standards

Regular testing of degreasing fluid concentration
Ultrasonic cleaning (highly recommended)
Mandatory hot water rinsing
Controlled drying temperature
"Water film test" for surface cleanliness verification

2. Standardize Scale Removal Processes

Adopt appropriate methods:

Sandblasting (SA2.5 standard recommended)
Tandem pickling + neutralization
Mechanical grinding
Laser derusting (high-end solution)

Goal: Achieve a fully metallic surface with no residual deep oxide scale.

3. Formalize Furnace Loading Procedures

Develop enterprise-specific SOPs (Standard Operating Procedures):

Maximum X pieces per layer
Prohibit direct tooth-to-tooth contact
Ensure unobstructed furnace gas circulation
Separate loading of small and large gears
Utilize standard clamping fixtures

4. Verify Carburizing Consistency with Test Specimens

Recommendations:

Standard test bars (Ø20×20 mm)
Synchronous furnace loading with production gears
Hardness and metallographic comparison
Data-driven production optimization

Pre-Incarburizing Preparation: The Starting Line for Gear Quality

Carburizing is one of the most critical gear manufacturing processes, but the "small, easily overlooked steps" that precede it truly determine case quality:A single drop of residual oil, a trace of oxide scale, a single blocking point, or an incorrect loading angle—any of these can halve the service life of a batch of gears.

Remember: Carburizing quality begins not when the furnace ignites, but with pre-treatment preparation. Investing in proper pre-processing builds the foundation for the long-term reliability and performance of gears.

Pub Time : 2025-11-18 10:07:26 >> News list

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