The study of two eutectoid reaction in a Mn-Al steel

• Main Authors: Hsin-yu Chen, 陳欣佑
• Other Authors: Wei-Cheng Cheng
• Format: Others
• Language: zh-TW
• Published: 2011
• Online Access: http://ndltd.ncl.edu.tw/handle/js6394

碩士 === 國立臺灣科技大學 === 機械工程系 === 99 === We have studied the phase transformations of a Mn-Al steel after it was heated at 1100℃ for solution heat treatment and followed by isothermal holding for 100 h at temperatures between 800 and 500℃. The composition of the steel is Fe-14.8 Mn-3.4 Al-1.0 C (wt%). The constituent phase of the steel at temperatures between 1100 and 725℃ is single austenite. Irregular martensitic plates exist in the austenitic matrix after cooling. They are FCC micro-twins. At a temperature of 700℃, grain boundary M3C carbide precipitates in the austenitic grains. Thus, the upper temperature for the appearance of M3C carbide is between 725 and 700℃. We found that kappa carbide appears at the austenitic grain boundaries for the steel being isothermally held at 675℃. The appearance of the kappa phase is probably due to the high content of Al in the Mn-Al steel. For the steel held at the same temperature, we observed two different pearlites coexisting in the austenitic matrix. One pearlite is composed of lamellar grains of ferrite and M3C carbide, and the other is composed of lamellar grains of ferrite and M23C6 carbide. They are from two separate eutectoid reactions, decomposing supersaturated austenite into lamellae of ferrite and carbide. The carbide could be either M3C or M23C6. We discovered several orientation relationships between two adjacent grains with different crystal structures. Between M3C and ferrite grains: 1. [010]C // [1 ¯11]α, (103)C // (110)α; 2. [311 ¯]C // [1 ¯11]α, (022)C // (110)α; 3. [100]C // [11 ¯2]α, (022)C // (110)α; 4. [101 ¯]C // [01 ¯1]α, (121)C // (011)α; 5. [010]C // [113 ¯]α, (103)C // (11 ¯0)α; 6. [121]C // [012]α, (303 ¯)C // (121)α; 7.[111]C // [110]α, (022)C // (110)α. Between M23C6 and ferrite grains: [01 ¯1]C6 // [001]α, (111)C6 // (110)α; and [01 ¯1]C6 // [1 ¯11]α, (111)C6 // (110)α.