Analysis of Common Causes of Cracks in Diamond Saw Blade Substrates

Diamond saw blade substrates primarily use medium-to-high carbon spring steel. Non-metallic inclusions, uneven microstructure, micro-cracks, and surface defects in these steels can become potential sources of fatigue cracks, leading to premature substrate failure.

1. Material Factors Affecting Crack Formation

1.1 Non-Metallic Inclusions

Non-metallic inclusions such as Al₂O₃ and TiN are produced during the steelmaking process. Their impact on fatigue performance depends on their type, quantity, size, shape, and distribution. Large brittle inclusions with weak bonding to the matrix and spherical non-deformable inclusions are most harmful. The higher the strength level of medium-to-high carbon spring steel, the more significant the detrimental effect of inclusions on the fatigue limit.

1.2 Surface Defects

Surface quality issues fall into three categories:

• Rolling defects — folds, laps, and scratches caused by outdated rolling equipment and inadequate pass design
• Surface cracks — longitudinally distributed cracks caused by residual billet cracks and subsurface defects
• Surface scratches and peeling — related to tooling conditions and improper handling during packaging and transportation

While small pits, scratches, and spots may be within acceptable standards, they represent weak points in the material that can become breakthrough points for cracking when overall material ductility is poor.

1.3 Band Segregation

Negative segregation bands (white bright bands) can occur during continuous casting of medium-to-high carbon spring steel due to improper electromagnetic stirring or unstable cooling. These bands have lower carbon content and form ferrite bands that, after heat treatment, create martensite with lower carbon content than the surrounding structure.

The presence of band segregation significantly affects the fatigue life of saw blade substrates because the relatively weak banded structure easily generates fatigue crack sources under cyclic loading.

2. Heat Treatment-Induced Cracks

Improper heat treatment is the main cause of crack formation in diamond saw blade substrates.

2.1 Oxidation and Decarburization

Heating in oxidizing atmospheres causes surface oxidation, increasing roughness and reducing precision. Surface oxide scale often causes quenching soft spots and cracking, reducing substrate strength.

Decarburization is a common surface defect. A 0.1mm decarburization layer significantly reduces the fatigue limit, and the presence of ferrite in the decarburized layer can reduce the fatigue limit by 50%. Surface hardness reduction under alternating stress easily leads to crack formation and premature fatigue failure.

2.2 Quenching Deformation

Diamond saw blade substrates are thin sheets with large diameters (Φ300–2200mm) and small thicknesses (2–10mm). Quenching deformation and cracking control is the key challenge in substrate heat treatment.

Deformation causes include:

• Heat convection impact during heating
• Uneven cooling across the cross-section causing thermal stress
• Phase transformation stress during martensite transformation

2.3 Overheating and Burning

Excessive heating temperature or holding time causes austenite grain coarsening, resulting in coarse needle-like martensite after quenching. This significantly reduces strength, especially impact toughness and ductility, making brittle fracture more likely.

Burning — where austenite grain boundaries partially melt or oxidize — is an irreversible defect that requires scrapping the substrate.

2.4 Tempering Embrittlement

Medium-to-high carbon spring steel is susceptible to tempering embrittlement in the 350–550°C range. The addition of 1–2% Mn increases susceptibility to overheating and tempering embrittlement. Proper tempering temperature control, holding time, and cooling rate are critical to avoiding embrittlement.

3. Improper Quenching Medium Selection

When carbon content is below 0.20%, lath martensite forms with good plasticity and strength. At 0.60% carbon, needle-like martensite forms — hard and brittle. Between 0.20–0.60%, a mixed structure forms.

The appropriate quenching medium should achieve maximum cooling speed at the nose of the TTT curve while maintaining moderate cooling speed below 300°C to prevent pearlite or bainite formation.

If the cooling rate is too fast below the martensite start temperature, fine needle-like martensite appears and quenching deformation increases sharply. The volume expansion from martensite transformation creates tensile stress on the surface layer, which can cause longitudinal cracks when exceeding the steel’s yield strength.

4. Conclusions

• Non-metallic inclusions, surface defects, and band segregation in medium-to-high carbon spring steel are the main factors affecting saw blade substrate service life and the primary crack sources during use.
• Serious oxidation and decarburization, excessive quenching deformation, overheating, burning, and tempering embrittlement are the main causes of substrate cracking.
• Correct selection of quenching medium and method, based on substrate material, original microstructure, and technical requirements, is critical to substrate quality and subsequent processing difficulty.

SINO SEIKO（Sino Qtech Industries Co., Ltd.） provides diamond saw blades and cutting tools for industrial applications. Contact us for product specifications and technical consultation.