Perfection Annealing is a heat treatment process that produces the highest permeability shielding.
Stress Annealed and Perfection Annealed
Our MuMETAL® brand is typically stocked in a semi-finished, stress annealed condition to allow further fabrication, stamping, severe forming or bending. During fabrication, the material is cold worked and the crystal grain structure is distorted. After final fabrication processes are completed, grain structure is then modified by controlled atmospheric Perfection Annealing to relieve stress in the material, resulting in larger grain structure, softer temper, and ultimate shielding performance. The permeability of the cold worked material compared to Perfection Annealed condition is shown in the plot below.
Perfection Annealing Processing for Optimum Magnetic Properties
After fabricating a magnetic shield, Perfection Annealing is generally required to re-crystallize the material, which improves shielding efficiency. Extra care must be taken to avoid dropping or denting the final annealed parts as shock may degrade annealed shield's magnetic properties. MuMETAL® magnetic shields are Perfection Annealed (fully annealed in a controlled atmosphere) to Magnetic Shield Corporation's exacting standards.
Optimum properties are obtained in batch furnace with a pure, dry hydrogen atmosphere. Dissociated ammonia provides good results in continuous furnaces, as does a vacuum atmosphere in batch operations. In all cases, the dew point inside the furnace should be less than -40°C. Optimum magnetic properties of CO-NETIC® and MuMETAL® materials are obtained by annealing at temperatures of 2010-2160°F (1100-1180°C) for a period of time. After annealing, the cooling rate is adjusted for the permeability. To avoid excessive distortion of light gauge shields, an annealing temperature of 1950°F (1065°C) will provide acceptable magnetic properties. Correspondingly, magnetic tests may disclose that an accelerated cooling rate can be adopted without adverse effect.
To insure your shield is annealed properly, we can measure attenuation (a shield's ability to absorb magnetic energy) in our ISO 9001 certified Quality Assurance Lab using one of our Helmholtz coils. Our fully annealed MuMETAL® offers attenuation properties that are considered the best available for most applications worldwide. If you would like a quote for perfection annealing please contact us.
(a) Stress Annealed
(b) Perfection Annealed
Figure 1 The above images compare the crystalline grain structure of Stress Annealed (a) and Perfection Annealed (b) MuMETAL® samples at 44x magnification. The grains are larger in the Perfection Annealed sample so the boundaries are visible. The maximum grain size of the Perfection Annealed sample is about .014" [0,36 mm], which is the material thickness.
