The effect of austenite grain size on microcracking in martensite of an Fe-1.22C alloy

Austenitic grain sizes of ASTM No. 9 and coarser were produced in an Fe-1.22 pct C alloy austenitized by immersion in molten lead at 1640†F (893°C), a temperature just above theA _cm for this alloy, for periods between 20 s and 1 h. Microcracking sensitivity,Sv, measured as crack area/unit volume martensite, was determined as a function of grain size in brine quenched specimens. Two locations of microcracks were observed in this investigation: 1) intragranular, resulting from the impingement of one martensite plate with another, and 2) grain boundary or intergranular resulting from the impingement of martensite plates at prior austenite grain boundaries. Intragranular microcracking sensitivity, the subject of previous investigations, increased and became the dominant type of cracking with increasing grain size, and reached a constant level for grain sizes of ASTM No. 4.5 and coarser. Total microcracking sensitivity, consisting of both intragranular and grain boundary microcracks, also increased with increasing grain size, then decreased to approach the intragranular value for grain sizes coarser than ASTM No. 3.5. On the other end of the scale, grain boundary microcracking made up a much larger proportion of the total microcracking in the fine grained specimens.