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Metallographic Examination of Gears: Principles, Methods And Key Knowledge

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Gears are core components of mechanical transmission, and their material properties and heat treatment quality directly affect service life and reliability. Metallographic examination, through microscopic analysis of gear materials, evaluates key indicators such as heat treatment processes, case hardening depth and grain size, serving as a crucial quality control method.

Core Objectives and Detection Items

The primary goal of gear metallographic examination is to ensure product performance by assessing critical parameters:

Case hardening depth: A key indicator for wear resistance of carburized/quenched gears (as required by ISO 6336 standard).
Grain size: Influences gear strength and toughness (graded per ASTM E112).
Microstructure: Morphologies of martensite, retained austenite and carbides determine fatigue performance.
Surface defects: Detects grinding burns and cracks (compliant with AIAG CQI-9 standard).

Basic Microstructural Constituents

Ferrite (α): Body-centered cubic (BCC) structure, soft and tough with low hardness (~80HV), common in low-carbon steel and pure iron.
Austenite (γ): Face-centered cubic (FCC) structure, high plasticity and non-magnetic, present in high-temperature or high-alloy steel like 304 stainless steel and high-manganese steel.
Cementite (Fe₃C): Orthorhombic crystal system, hard and brittle (~800HV) and enhances wear resistance, found in white cast iron and high-carbon steel.
Martensite: Body-centered tetragonal (BCT) structure, high hardness (500~1000HV) obtained through quenching, used in quenched steel and tool steel.

Common Microstructural Morphologies

Microstructure Type	Formation Conditions	Performance Characteristics	Typical Applications
Pearlite	Slow cooling (eutectoid transformation)	Balanced strength and toughness	Rail steel, gear quenching and tempering
Bainite	Medium-temperature isothermal quenching	Higher strength and toughness than pearlite	Springs, high-strength bolts
Sorbite	Tempered martensite (500~650℃)	Excellent comprehensive properties	Shafts, connecting rods

Testing Process and Standard Methods

Sampling and Sample Preparation

Sampling positions: Tooth top (evaluates surface hardening effect), tooth root (analyzes microstructure in stress concentration areas), cross-section (measures case hardening gradient).
Key preparation steps: Cutting → Mounting → Grinding → Polishing → Etching → Microscopic observation.
Mounting: Use epoxy resin for edge protection (cold mounting recommended to avoid thermal impact).
Polishing: Polish to 0.05μm mirror finish with diamond polishing paste to prevent scratch interference.

Etchant Selection

Material Type	Recommended Etchant	Effect
Carburized steel	4% Nital (nitric acid-alcohol)	Clearly displays martensite/austenite
Nitrided steel	Picric acid + detergent	Highlights nitride layer (e.g., γ'-Fe₄N)
Stainless steel gears	Oxalic acid electrolytic etching (10V, 20s)	Distinguishes σ phase and carbides

Key Testing Equipment

Optical Microscope (OM)

Application: Basic microstructure observation (e.g., grain size grading).
Configuration requirements: 500×~1000× magnification, equipped with image analysis software (e.g., Olympus Stream).

Scanning Electron Microscope (SEM)

Advantages: High-resolution observation of non-metallic inclusions (e.g., MnS) and composition analysis via EDS.
Case example: Intergranular cracks caused by sulfur segregation detected in wind power gearbox fracture analysis.

Microhardness Testing

Method: Vickers hardness (HV0.3~HV1) gradient testing to plot case hardening curves.
Standard: ISO 2639 defines case hardening depth as the distance from surface to substrate at 550HV1.

Microstructure Analysis

Normal Microstructures

Heat Treatment Process	Ideal Microstructure
Carburizing and quenching	Fine acicular martensite + <10% retained austenite
Induction hardening	Cryptocrystalline martensite + uniform transition zone
Quenching and tempering	Tempered sorbite (uniform carbide distribution)

Common Defects and Causes

Excessive carburization: Network carbides on surface, increasing brittleness and risk of tooth surface spalling.
Grinding burn: Temper colors revealed by pickling (ASTM E1257), prevented by controlling feed rate and using CBN grinding wheels.
Quenching cracks: Intergranular propagation with sharp ends (confirmed by SEM).

Defect Name	Microscopic Characteristics	Causes and Impacts
Widmanstätten structure	Acicular ferrite invading grains	Overheating leads to reduced toughness
Banded structure	Alternating layers of ferrite and pearlite	Casting-rolling segregation causes anisotropy
Overheating	Grain boundary oxidation or melting	Excessively high heating temperature results in total scrapping