Microstructures
by George Langford, Sc.D., Massachusetts Institute of Technology, Cambridge, MA, 1966
Copyright©2005 by George Langford
Low Alloy Steels - Lesson 1 - Third specimen - Part II
Slag stringer in wrought iron at 1000X The  two photomicrographs on this page are of the dark masses in the specimen on the preceding page, but were each taken at 1000X.

The material is wrought iron ... quite an anachronism even in 1960 when this specimen was obtained by M.I.T.

The first photomicrograph on the preceding page is a longitudinal section (note the aspect ratios of the slag stringers) while the second is a transverse section.

Wrought iron was made by forging or "puddling" a spongy ball of mixed slag and pig iron, which squeezes out excess oxide and oxidizes the carbon out of the pig iron at the same time as the iron is being consolidated. 

Slag in wrought iron at 1000X Eventually a red hot mass of low carbon iron is obtained, once no more slag can be exuded.  Hot working stretches the oxide stringers while the workpiece is being elongated.  The properties are therefore quite anisotropic, the fracture of such a piece being as fibrous as a piece of green wood.

On this page, the upper photomicrograph is a longitudinal section, while the one at left is a transverse section. 

Why aren't these microstructures directional also ?

Pause to formulate an answer and then take a look.

























The slag is a two phase mixture of iron oxide and silica, and it recrystallizes easily at the hot working temperature.
Proceed to Specimen 4.