Industry Insight: Mechanism of Microcrack Propagation Caused by Grinding Burn on Gear Tooth Surfaces

Gear grinding is the final "finishing" step in gear manufacturing, determining the tooth surface's form accuracy, roughness, and load-bearing capacity. However, improper control of the grinding process can lead to thermal damage on the tooth surface, commonly known as grinding burn in the industry. This issue is far from a minor flaw; it is a potentially fatal defect that threatens gear reliability.

The essence of grinding burn lies in the combination of instantaneous high temperature and rapid cooling (quenching). During grinding, the instantaneous temperature at the contact area between the grinding wheel and the tooth surface can exceed 800–1200°C. Insufficient coolant supply, dull grinding wheels, or excessive feed rates may trigger a series of harmful reactions: first, the surface layer undergoes instantaneous austenitization, leading to secondary hardening (re-quenching) or annealing, which forms coarse-grained structures such as tempered martensite (light yellow or brown) or quenched martensite (blue); subsequent rapid cooling by coolants generates brittle martensite with poor durability; finally, thermal stress arises—when the contracted surface layer is restricted by the underlying tissue, tensile stress initiates microcracks. In summary, grinding burn is essentially the synergy of thermal damage, thermal stress, and brittle tissue that gives rise to microcracks.

These microcracks are invisible to the naked eye and can only be detected using metallographic microscopes (100–1000×) or SEM. Their formation follows a distinct path: brittle surface tissue combined with residual stress triggers crack initiation; thermal damage causes carbide segregation and grain boundary embrittlement, enabling cracks to propagate rapidly along grain boundaries; due to localized shear stress concentration from grinding, cracks form at a specific angle to the grinding direction; though typically 10–100 μm deep, these cracks are lethal enough to expand rapidly under high-cycle bending stress during gear operation.

During service, microcracks propagate through three key mechanisms. First, contact stress-driven propagation (Mode II and III): periodic shear stress from gear meshing causes cracks to "crawl" along the shallow layer of the tooth surface. Second, inadequate protection from elastohydrodynamic lubrication (EHL) oil films: cracks disrupt the oil film, forming local dry friction zones that accelerate crack growth. Third, surface hardness failure leading to fatigue spalling: grinding burn reduces tooth surface hardness by 50–250 HV, severely compromising load-bearing capacity. This ultimately results in pitting, which expands into flaking, leaving "pockmarks" on the tooth surface and, in severe cases, large-scale surface detachment—a typical chain failure process of "burn → hidden crack → propagation → spalling".

To identify grinding burn during processing, several reliable methods are available: nitric acid-alcohol etching (the most common method, revealing blackened areas, abnormal reflectivity, or disorganized surface patterns); magnetic noise testing (for rapid stress anomaly detection, widely used in high-end foreign gears); X-ray residual stress measurement (showing elevated tensile stress and abrupt stress gradients in burned areas); hardness distribution testing (indicating lower surface hardness in tempered burns or higher hardness in quenched burns); and microstructural inspection (revealing coarse martensite, uneven quenched tissue, and quench crack paths).

Preventing grinding burn requires comprehensive process control: optimize grinding parameters (reduce feed rate, maintain wheel sharpness, adjust linear speed, and use heat-resistant materials like CBN); enhance cooling systems (direct coolant to the grinding zone, increase flow rate/pressure, and optimize nozzle angle to avoid air film blockage); regularly dress grinding wheels to prevent friction-induced heating from dullness; implement in-process monitoring (tracking current waveforms, magnetic noise, and temperature); and optimize carburized hardening layer stress (ensure sufficient thickness and gradient to resist burn).

Grinding is the critical final step in gear manufacturing, and neglecting grinding burn can prematurely determine a gear's service life. Behind the seemingly simple metal teeth lies the intersection of materials science, heat treatment, grinding technology, lubrication, and contact fatigue. Mastering the mechanism and control of grinding burn is essential to ensuring gears operate reliably throughout their service life.