Phase transformations of a 20Mn-1.6Al-1C steel
| Autor: | Yu-Cheng Li, 李育誠 |
|---|---|
| Rok vydání: | 2011 |
| Druh dokumentu: | 學位論文 ; thesis |
| Popis: | 99 We have studied the phase transformations of a Mn-Al steel at various low temperatures. The composition of the steel is Fe-19.8 Mn-1.64 Al-1.03 C (wt.%). We investigated the effects of adding Mn and Al elements to the M3C and M23C6 pearlites in the high manganese steel. The heat treatment of the steel is heating at 1100℃ for solution heat treatment and holding isothermally at temperatures below 850℃ for 100 hours. The constituent phase of the steel is single austenite at temperatures between 1100 and 800℃. Irregular martensitic plates appear in the austenitic matrix after the solution heat treatment. The martensitic plates are FCC micro-twins. Irregular martensitic plates still exist in the austenitic matrix after the isothermal holding processes. At temperatures below 800℃, we discovered the coexistence of M3C and M23C6 carbides in the forms of either grain boundaries or pearlitic colonies. The upper temperature limit for the existence of these two carbides is between 800 and 775℃. At temperatures below 650℃, we found two different pearlites: one is M3C pearlite which is lamellae of M3C and ferrite, and the other is M23C6 pearlite which is lamellae of M23C6 and ferrite. These two pearlites are from two separate eutectoid reactions, i.e. the supersaturated austenite has decomposed into two separate colonies of ferrite and carbide. The upper temperature limit for the existence of the two various pearlites is between 675 and 650℃. The nose temperature for appearance of both pearlites is near 525℃. We have investigated the orientation relationships between different crystals, such as ferrite and carbides, and some results are shown as follows. A variant orientation relationship has been found between M3C carbide and ferritic grains: [010]C // [11-1]α; (10-3)C // (011)α, (10-1)C // (112)α. In M23C6 pearlite, we also discovered the well-known N-W orientation relationship between M23C6 carbide and ferrite: [-101]C6 // [001]α；(111)C6 // (110)α. |
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