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Multiple interface structures of M23C6 carbides precipitated at grain boundary in 100Mn13 high carbon high manganese steel, including the interface between the carbide particle and the matrix together with the interface between two adjacent carbides, were studied after 1050 °C solution treatment and subsequently after 475 °C aging treatment by high resolution transmission electron microscope (HRTEM) based on traditional transmission electron microscope (TEM). A growth model describing the adjacent M23C6 precipitations was proposed. The results show that precipitated carbide type is M23C6, which grows into both sides of austenite by a step growth mechanism. When two adjacent carbides precipitate along grain boundary, there are two cases as follows: If the two carbides have the same orientation relationship with the same austenite grain, they would grow independently to contact with each other to coalesce into a larger size carbide with the same orientation relationship. If the two carbides have different orientation relationships with the same austenite grain, and there is an original twinning in one carbide, they would grow independently to contact with each other to form a precipitated twinning.

The distribution and the microstructure of carbides and pearlite, orientation relationships of pearlitic component phase in 100Mn13 steel by solution treatment at 1050 C and aging treatment at 525 C were investigated by optical microscope and transmission electron microscope. The results show that there is a large number of carbides and pearlite forming within the austenite grain and at the grain boundaries. The carbides in the pearlite of 100Mn13 steel have two kinds of morphologies, namely flaky and short rod. The type of carbides is M 7 C 3 . The carbides with different morphologies in the pearlite have different crystallographic orientation relationships with the ferrite. The orientation relationships between the flaky carbides and the ferrite are ð011Þ aÀFe kð213Þ M7C3 , ½100 aÀFe k½173 M7C3 , while the orientation relationships between the short-rod carbides and the ferrite are ð011Þ aÀFe kð203Þ M7C3 , ½100 aÀFe k½674 M7C3 . The short-rod carbides distribute directionally and orderly on the ferrite, which is similar to the morphology of the interphase precipitate. Therefore, the class-interphase precipitate mechanism is proposed in this study.

After 798 K aging treatment, lamellar pearlite connecting with grain boundary proeutectoid phase occurs in 100Mn13 steel. The proeutectoid phase is M23C6 carbide, which is different from pearlitic carbide, cementite. However, the lamellar pearlite has an orientation relationship (OR) with proeutectoid M23C6. Simultaneously, the pearlite also keeps a new OR with austenite where it occurs, (10)α//( 1)M3C//(1 )γ, [111]α//[113]M3C//[211]γ, indicating nucleation and growth of pearlite both are carried out in coherent austenite. Therefore, it is considered that pearlite does not develop from proeutectoid M23C6 but nucleates on proeutectoid M23C6 independently. The pearlite nucleation is probably affected by elastic deformation caused by M23C6 carbide, and pearlite growth mainly relies on new OR between pearlite and austenite. Under new OR, both ferrite and cementite can form by simple atomic movement based on the austenite lattice.

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