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This chunk of Armco iron (i.e. extremely low carbon ferrite)
was struck heavily with a hammer after being cooled to the temperature
of liquid nitrogen (-196C). The photomicrograph at left is at
200X with a Nital etch. These are deformation twins, also called
Neumann bands. Note their ragged edges. Their presence is a
sign of high strain rate, low temperature deformation, such as by
impact loading on cryogenic or arctic equipment. Armco iron was made by deliberate oxidation of molten low carbon steel so as to decarburize it. That is what causes the presence of so much oxide as inclusions. It is magnetically soft and once was used for solenoid cores and other devises where silicon iron (Fe- 3% Si) can't be used. It has become an anachronism, like wrought iron. |
| Move on to Specimen 8. |