Fig. 4. shows another OM observation of the alloy steel after solution heat treatment at 1373 K, reheating, and isothermal holding at 973 K for 100 h. As the OM shows, quite a few grain boundary precipitates, Widmanstätten side-plates and intragranular plates exist in the austenite. The major grain boundary precipitates are in the form of continuous films which may thicken and appear as allotriomorphs and completely outline the grain boundaries. Widmanstätten side-plates proceed by extrusions from the allotriomorphs and the intragranular Widmanstätten plates develop by separate nucleation and growth of the cementite in the austenite [11]. Most of these precipitates have been identified by TEM as M3C carbide. Therefore, we have observed that the proeutectoid carbide exhibits various morphologies. There are at least five forms of M3C carbide. 3 forms are at grain boundaries: separate grains, continuous films and allotriomorphs. 2 forms are in the austenite matrix: Widmanstätten side-plates and intragranular plates. All these five morphologies of M3C carbide in the Mn steels have been investigated in the literature [11], [12] and [13].