Strained Zone
Definition
The region of cold-worked, plastically deformed metal lying beneath an original stamp. The crystal lattice is distorted, dislocation density is elevated, and the strained metal dissolves at a different rate from the surrounding unstrained metal under chemical or electrolytic attack.
- Location
- Beneath an original stamped marking
- Cause
- Cold-worked plastic deformation
- Microstructure effect
- Elevated dislocation density
- Exploited by
- Chemical or electrolytic etching
Common questions
Why does the strained zone let erased markings be restored?+
The distorted lattice beneath the original impression dissolves at a faster rate than the surrounding unstrained metal when an etchant is applied, so the original digits or letters reappear as a visible pattern even after grinding or filing removed the surface marks.
Does grinding away the surface destroy the strained zone?+
Not necessarily. Cold work extends some depth beneath the stamp, so unless the surface is removed past that depth, or the metal is subsequently annealed to relieve the strain, the deformation record can survive well below what the eye can see.
Related terms
- Davis Reagent
- Variant etchant for steel using cupric ammonium chloride (Cu(NH4)2Cl4) in dilute HCl. Slower acting than Fry's, sometimes preferred when contrast on Fry's...
- Fry's Reagent
- Classical etchant for plain carbon and low-alloy steel: 40 g cupric chloride (CuCl2) + 90 mL concentrated HCl + 100 mL water....
- Heat-Tinting
- Restoration by controlled heating of the polished surface. Strained metal oxidises at a different rate from unstrained metal, producing colour contrast (straw...
- Magnetic-Particle Method
- Non-destructive restoration for ferromagnetic metals. The polished surface is magnetised and dusted with fine iron particles; the particles cluster at the strained...
- Peak Contrast
- The narrow window during a chemical etch when the original digits are most legible. Continuing past peak contrast overetches the unstrained metal...
- Stamped Marking
- A serial, chassis, engine or part number impressed into metal by a hardened steel die under high force. The visible impression is...
- Villela's Reagent
- Etchant for stainless and high-chromium steels: 1 g picric acid + 5 mL concentrated HCl + 100 mL ethanol. Targets prior austenite...