Leeb Hardness

Leeb hardness is a dynamic rebound hardness test for metals where a spring-driven impact body strikes a surface. The hardness value (HL) is calculated from the ratio of rebound velocity to impact velocity, expressed as HL = 1000 × (vr/vi). Harder materials return more energy, yielding higher HL values. This test is defined by ISO 16859 and ASTM A956, and is commonly used for large or immobile parts.

In a CNC cell or fabrication bay, Leeb testing is employed when a part is too large, heavy, or awkward for bench testers like Rockwell. The portable probe is pressed against a prepared surface, measuring impact and rebound velocities internally. The resulting HL number is often converted to Rockwell, Brinell, or Vickers using standardized tables. On machined steel, it serves for incoming material verification, heat-treatment checks, and post-machining spot tests. For aluminum or softer alloys, caution is needed as surface condition and mass effects can distort readings. The test is non-destructive, allowing hardness verification directly on fixtures, welded assemblies, large plates, shafts, or fabricated structures without cutting coupons. This speed and portability make it invaluable for quality control on the shop floor, especially when parts cannot be moved to a lab.

• ⚠️Insufficient part mass: A light or thin part absorbs impact energy through movement, lowering the rebound reading and making a good part appear soft.
• ⚠️Rough surface finish: Scale, machining marks, or weld spatter disrupt the impact event, causing erratic or low rebound values that do not reflect true hardness.
• ⚠️Device mismatch: Using different impact devices or converting HL without accounting for the device suffix leads to incorrect comparisons and acceptance errors in incoming inspection.