Even with its high hardenability, there is a limiting section size for through-hardening. As bar diameter increases:
Cooling Rate at the Core Decreases: The center of a thick bar cools much slower than the surface during quenching.
Risk of Non-Martensitic Transformation: Beyond a certain diameter, the core cooling rate may fall below the critical cooling rate required for 100% martensite formation. Softer transformation products like bainite or even pearlite may form.
Effect on Properties: This results in a gradient of properties.
Surface: High hardness/strength (martensite).
Core: Lower hardness/strength (mixture of martensite and bainite/pearlite). The impact toughness at the core can also be affected.
"Through-Hardening" Diameter: For 42CrMo oil-quenched, the typical maximum diameter for reasonably uniform through-hardening (to a predominantly martensitic structure) is often considered to be 80mm to 100mm. For larger diameters, special quenching media (more severe than oil) or modified chemistries (e.g., 34CrNiMo6) may be required. The producer's data sheets usually provide property guarantees for different ruling sections.