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This is a free machining, low carbon steel with 0.1% carbon,
0.2% lead, 0.2% sulfur and 0.6% manganese. The lead cannot be
seen in the 200X image at left, etched with Nital. Much higher
magnifications would be needed to detect the tiny lead globules. This is a normal structure. The lead is introduced by pouring it into the stream of molten steel as it is being teemed into the ingot mold. Gravity segregation of the much denser lead globules is a serious problem, and only Stokes Law enables the steel to be made successfully. Sometimes, large portions of the bottom of the ingot must be cropped, opposite to the usual practice of removing the pipe at the top of the ingot. |
| The manganese sulfide stringers
reduce the ductility of the chips formed by the cutting tool during
machining, while lead lubricates the tool - chip interface, reducing
the contact pressure there, reducing the ductility of the chip still
further. Manganese sulfide has the same crystal structure as sodium chloride (which plastically deforms in the earth to form the salt domes that sometimes trap oil and gas deposits). There are enough slip systems in this structure to emable substantial ductility of the MnS stringers at hot working temperatures. Also, the surface energy of the MnS with respect to iron is such that the MnS forms discrete islands. Iron sulfide, which would form in the absence of manganese, instead wets the austenite grain boundaries, has only one slip plane, and melts at much lower temperatures than does MnS, so it ruins the ductility of the austenite during hot working, making it hot short. All steels have at least 0.2% to 0.4% manganese to avoid the formation of FeS. Only free machining steels have as much manganese as this specimen has, however. This one has been deliberately resulfurized. |
| SUMMARY:
Do you understand how these microstructures were produced and how some
of them got out of control ? |