Ferromagnetic Annealing and In-Process Heat Treating
Hiperco 50 and Hiperco 50 HC
It is important to avoid any contamination of the finished parts during the heat treatment. All parts must be cleaned thoroughly to remove any surface contaminants prior to annealing.
Batch heat treating in a sealed retort or welded box-type furnace is recommended. Thoroughly degreased and cleaned laminations can usually be stacked without an insulating media separation. To obtain the best degree of lamination flatness, a light weight can be placed on top of the stack. It may be necessary to determine the correct amount of weight to assure that there is no sticking of the laminations within the stack height employed.
A dry hydrogen atmosphere or a high vacuum is recommended to minimize oxide contamination of the parts during annealing. When hydrogen is employed, the entry dew point should be dryer than -60°F (-51°C) and the exit dew point dryer than about -40°F (-40°C) when the inside retort temperature is above 900°F (482°C).
Anneal parts at 1300/1600°F (704/871°C) for 2 to 4 hours in dry hydrogen or vacuum and cool at 250/400°F (139/222°C) per hour until 600°F (316°C) is reached. After any cooling rate can be employed. The exact heat treat temperature to be employed will depend upon the application and the desired compromise between magnetic and mechanical properties. With increasing temperature from 1300° to 1600°F (704° to 871°C), the magnetic properties improve while the yield and tensile strengths decrease. The temperature at no time should exceed 1600°F (871°C) as an upper limit, as the soft magnetic characteristics start to decline due to formation of an austenitic phase.
For certain A.C. applications, improved magnetic characteristics and/or lower core loss are realized by creating a thin oxide layer on the surface of the annealed laminations. The surface oxide layer can be achieved by heating in an oxygen bearing atmosphere in the range of 600° to 900°F (316° to 480°C) for about 30 to 60 minutes. The exact baking parameters must be determined for the annealing facility employed and the thickness of oxide layer desired.
Anneal parts at 1575/1600°F (857/871°C) for 2 to 4 hours in dry hydrogen or vacuum and cool at 150/350°F (83/194°C) per hour until 600°F (316 °C) is reached, after which any cooling rate can be employed.
It is important to avoid any contamination of the finished fabricated parts during the heat treatment. All parts must be cleaned thoroughly to remove any surface contaminants prior to being placed in an air-tight retort.
A dry hydrogen atmosphere or a high vacuum is recommended to minimize oxide contamination of the parts during annealing. When hydrogen is employed, the entry dew point should be dryer than -60°F (-51 °C) and the exit dew point dryer than about -40°F (-40 °C) when the inside retort temperature is above 900°F (482°C).
- Centerless Ground Bars Less Than 1.250″ (31.75 mm) Diameter
This duplex practice is only recommended for centerless ground bars in the size range given. Larger centerless ground bars, shapes, or hot rolled bars may be subject to cracking if quenched in this manner. This is a two-step treatment where the first step conditions the material for best machinability while the second develops optimum magnetic properties.
Preheat treat 2 to 3 foot lengths at 1850°F (1010°C) for about 30 minutes in air or protective atmosphere and quench bars in cold agitated water with their long axes in the vertical position. This preheat treatment develops a coarse acicular martensitic type structure which imparts ductility to the material and improves machining of the finish part.
The second part of the duplex treatment involves the employment of the standard treatment as described earlier. At no time should the temperature of the second treatment exceed 1600°F (871°C) because of the formation of a nonmagnetic austenitic phase which transforms upon cooling, thereby degrading magnetic properties due to transformation stresses.
- For Products Produced from Any Size Bar
This practice can be applied to all parts produced from bar products, regardless of their size since no rapid quench practices are required. It provides improved capability magnetic properties on especially large cross sections compared to those of the standard heat treatment. Note, however, it is a lengthy process and, therefore, more expensive.
Heat the finish machined part in dry hydrogen or high vacuum to 1700°F (927°C) (heating rate is not critical) and hold at 1700°F (927°C) for three to four hours. Cool at 20°F (11°C) maximum per hour to 1350°F (732°C), then cool to 950°F (510°C) at 200°F (111°C) per hour. Cool at any rate thereafter.