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Tool steel bars like this one (1% carbon, 0.2% silicon, 0.2%
manganese and 0.2% vanadium) are always delivered in the spheroidized
(i.e., annealed) condition. However, there is pearlite present
here (100X, Nital etch at left) which does not machine as well as
spherodite (ferrite plus large, spheroidal cementite particles). In the middle photomicrograph below at 500X you can resolve the microstructure better. There is a mixture of proeutectoid ferrite and pearlite which formed from austenite of near eutectoid composition. Evidently, the carbon content is subnormal in this part of the specimen, and the cooling rate during annealing was too rapid to permit the pearlite to spheroidize while it was forming. |
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Ordinarily, at this (intended) carbon content there would be
spheroidized proeutectoid cementite, because the annealing treatment is
started from just above the eutectoid temperature rather than from the
all austenite phase field. The lower starting temperature avoids
any precipitation of excess cementite on the austenite grain boundaries |
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The last photomicrograph from this specimen shows a
different place on the specimen where the microstructure is more nearly
normal: proeutectod cementite plus partially spheroidized pearlite at
500X. The last specimen in this lesson is a tool steel which was abusively cut with an abrasive disk. |