Metallurgical and Mechanical Impacts: Grinding Burn and Residual Stress Mechanisms

Heat generated during the grinding process does not merely elevate the temperature of the material; it possesses destructive energy capable of physically rearranging the crystalline structure of the workpiece’s outermost surface. Specifically, when grinding steel, if the maximum temperature rise (ΔT_max) derived in the previous post exceeds the material’s intrinsic transformation points, the material undergoes irreversible metallurgical changes. This process is primarily divided into two regimes: the Over-tempering mechanism and the Re-hardening mechanism.

1.1. Mechanism of Grinding Burn and Formation of Oxide Films

Visually identifiable “Grinding Burn” is the result of an oxide film formed when the machined surface is exposed to high temperatures and reacts with atmospheric oxygen. The resulting coloration serves as an indicator that the surface temperature has reached specific intervals between approximately 200°C and 700°C. This range is indicative and varies with steel grade, exposure time, and oxidation conditions. However, from an engineering perspective, the critical concern is not the surface color itself, but the collapse of the microstructure occurring beneath it. Therefore, color should be treated as a qualitative warning sign rather than a quantitative thermometer. As the temperature rises, carbides within the steel migrate and recombine, serving as the starting point for fundamental degradation of surface integrity, which determines component longevity.

1.2. Softening of Martensite: The Over-tempering Mechanism

When grinding hardened steel, if the localized temperature remains below the austenite transformation point (A_c1) but exceeds the material’s original tempering temperature, the Over-tempering mechanism is activated. In this regime, carbon precipitates from the hardened martensitic structure, leading to localized softening of the microstructure.

1.3. Formation of White Layer and the Re-hardening Mechanism

When the temperature surpasses the A_c3 transformation point, the machined surface temporarily transitions into an austenitic state. Subsequent rapid quenching by the grinding fluid or rapid heat conduction into the bulk material causes a re-transformation into martensite, known as the Re-hardening mechanism. The resulting layer appears white when exposed to etchants, hence the term ‘White Layer.’

This layer is extremely hard yet highly brittle, promoting the development of micro-cracks and significantly lowering the fatigue limit of the workpiece. Consequently, in precision grinding, it is essential to control the process to satisfy the following thermal condition:

Residual Stress remaining on the surface after grinding is a key indicator that determines the fatigue strength and operational lifespan of a component. The final state of residual stress is governed by a complex interaction mechanism between two opposing energy sources: mechanical deformation caused by the physical scratching of abrasive grains and thermal deformation induced by frictional heat.

2.1. Mechanical Deformation and the Formation of Compressive Residual Stress

In ideal precision grinding, compressive residual stress is formed on the surface. This occurs as the abrasive grains induce plastic deformation of the workpiece surface. As the grains press and pass across the surface, the top layer attempts to elongate, but the undeformed bulk material beneath constrains this movement, leaving a pulling force that acts inward. This compressive stress serves as a “protective shield,” hindering the propagation of micro-cracks and enhancing the durability of the component.

2.2. Thermal Expansion and the Stress Reversal Mechanism

Problems arise when the thermal load overwhelms the mechanical load. Rapid temperature rise in the grinding zone induces localized thermal expansion of the surface layer. Since the cold bulk material constrains this expansion, intense compressive stress is generated temporarily, leading to plastic compressive deformation. During the subsequent cooling process, the surface layer—which has already been plastically compressed—attempts to shrink beyond its original volume, ultimately remaining in a state of tension exerted by the underlying material.

2.3. Risks of Tensile Residual Stress and Crack Initiation

Tensile residual stress generated by thermal mechanisms weakens the bonding force of the material and acts as a conduit for immediate micro-crack propagation when external loads are applied. In particular, when the “White Layer” mentioned in Section 1.3 combines with tensile stress, it can lead to grinding cracks—delayed failures that appear long after the machining process is complete. Therefore, modern deterministic grinding control focuses on limiting heat flux to ensure the stress state does not transition into the tensile regime.

Grinding burn and tensile residual stress are not accidental occurrences but inevitable physical phenomena that manifest when the input energy exceeds the material’s critical thermal capacity. Therefore, the key to high-quality machining lies in establishing a deterministic control mechanism that maintains the heat flux (q_w) entering the workpiece below the threshold where microstructural transformation begins.

3.1. Critical Energy Density and Conditions for Burn Avoidance

To prevent metallurgical degradation on the workpiece surface, the energy input per unit time and area must be strictly controlled. Engineers utilize the Critical Heat Flux (q_bc) model for this purpose. Metallurgical integrity can only be guaranteed when the actual heat flux under specific machining conditions remains lower than this critical threshold.

3.2. Temperature Control via Grain Loading and Dressing Mechanisms

While the critical heat flux condition (q_w < q_bc) defines the thermal limit for surface integrity, its practical enforcement in grinding operations depends fundamentally on the condition of the grinding wheel.

The condition of the grinding wheel directly influences the heat generation mechanism. As abrasive grains become dull (dulling), energy consumption due to friction—rather than cutting—surges, causing the heat flux to rise sharply. It is essential to expose sharp grains through proper dressing to lower the specific energy and ensure sufficient porosity in the machining zone for maximized cooling efficiency. This represents a sophisticated control strategy that selectively reduces thermal load while maintaining mechanical efficiency.

The Metallurgical and Mechanical Impacts discussed in this report are not independent phenomena but a series of chain reactions resulting from the input of machining energy. This mechanism, which serves as a core pillar of our 30-part series, is summarized as follows:

• Control of Metallurgical Thresholds: The moment grinding temperature exceeds A_c1, structural softening initiates. Upon cooling after surpassing A_c3, the material passes through the M_s-M_f range, leading to the formation of a lethal “White Layer.” Preventing this is the primary objective.
• Optimization of Stress States: It is critical to maximize compressive stress (-) induced by mechanical burnishing while suppressing the generation of tensile stress (+) caused by thermal expansion and phase transformation. Tensile stress acts as a “time bomb” for potential component failure.
• Monitoring of Specific Energy (e_s): When specific energy exceeds 1.5 times its baseline (approx. 70–80 J/mm³) due to wheel wear, the mechanical cutting mechanism collapses, and heat generation becomes dominant. This represents the physical limit of the process.

Ultimately, a successful grinding process design involves performing preemptive dressing before the wheel dulls and specific energy reaches its threshold, thereby restoring the machining system to its initial “Sharp” state.

Productivity is not simply about increasing feed rates; it is about achieving precision process optimization by ensuring stable compressive residual stress (-) within the physical critical temperature (A_c1). This deterministic approach is the only way to guarantee the quality of core components for semiconductors, aerospace, and electric vehicles, where high reliability is paramount.

The collision mechanism between mechanical compressive stress (-) and thermal tensile stress (+) can be quantitatively interpreted via the McDowell Algorithm. This model provides the core equations for predicting final residual stress by calculating the plastic deformation and recovery based on the grinding temperature gradient. Below is a simplified engineering form that captures the yield-versus-thermal-stress criterion.

2. Engineering Interpretation of the Algorithm

The core of this algorithm is determining ‘whether the thermal expansion attempt exceeds the material’s yield strength.’ If the thermal stress generated during heating surpasses the yield strength, plastic compressive deformation occurs. Upon cooling, this manifests as tensile residual stress (+) on the surface.