洛阳理工学院

1. Iron-Iron Carbide Phase Diagram Fe-Fe3C平衡相图 洛阳理工学院

2. 2.3 The Iron-Iron Carbide (Fe-Fe3C) Phase Diagram 同素异晶 • The allotropy of pure iron • Allotropy is the ability of some elements to exist in different • physical forms (differing in color ,hardness, melting point etc.). • Iron is allotropic, at room temperature pure iron exists in the • Body Centered Cubic crystal form but on heating transforms to a Face • Centered Cubic crystal. The temperature that this first transformation • takes place is known as a critical point and it occurs at 912 degrees • Celsius. This change in crystal structure is accompanied by a shrinkage • in volume, sine the atoms in the face centred crystal are more densely • packed together than in the body centred cubic crystal. • At the second critical point the F.C.C crystal changes back to an • B.C.C crystal and this change occurs at 1394 degrees Celsius. 1 洛阳理工学院

3. The allotropy of pure iron • Iron can exist in three forms: • α-Ferrite（铁素体）... BCC crystal, exists at temperatures up to 912℃ • γ-Ferrite（奥氏体）... FCC crystal, exists at temperature range 912℃ to 1394℃ • δ-Ferrite（高温铁素体）... BCC crystal, exists at temperature range 1394℃ to 1538 ℃ • The maximum solubility of C in Fe is 0.09% at 1495℃. Pure iron is soft • (softer than aluminium), Iron has no real practical significance in • engineering； 但由于有高的磁导率，主要用于制造仪表的铁磁心 等于 2 洛阳理工学院

4. Photo of pure iron Photo and Mircrostructure of Pure Iron 3 洛阳理工学院

5. Microstructure of Ferrite 2.3 The Iron-Iron Carbide (Fe-Fe3C) Phase Diagram • (2) The phase diagram includes four solid phases • 1)Ferrite [‘ferait]铁素体, 用符号“F”或“α”表示 • Ferrite is the interstitial solid solution of carbon in αiron. • Structure: BCC crystal structure. • Maximum Solubility of Carbon in Iron: • 0.0218% at 723 ℃, at 0 ℃ temperature • the solubility falls to 0.008%. • Properties: HBS=50~80 • δ=30%~50% soft and tough • Microstructure Appearance: • equiaxial grain 呈明亮的多边形晶粒，晶界曲折 4 洛阳理工学院

6. 2.3 The Iron-Iron Carbide (Fe-Fe3C) Phase Diagram • 2) Austenite[‘ɔstə .nait]奥氏体 , 用符号 “A” 或“γ”表示 • The interstitial solid solution of carbon in γ-Fe • a. Structure: face-centered cubic lattice • b. Maximum Solubility: 2.1% at 1148℃ • The solubility decreases to 0.77% at 723 ℃. • This has a FCC crystal structure with a high solubility for carbon • compared with α-Fe.  The difference in solubility between the austenite • and α Ferrite is the basis for the hardening of steels. • c. Properties: HBS=170~220 δ=40%~50%  soft and tough too • d. Microstructure Appearance: equiaxial grain 5 洛阳理工学院

7. 2.3 The Iron-Iron Carbide (Fe-Fe3C) Phase Diagram A显微组织特征：与F相似，呈多边形粒，但晶界较F平直， 塑性好，变形抗力小，易于锻造成型 6 洛阳理工学院

8. 2.3 The Iron-Iron Carbide (Fe-Fe3C) Phase Diagram • 3) Cementite[si‘mentait] 渗碳体, 用符号 “Fe3C” 或“ Cm”表示 • This is an intermetallic compound which contains 6.69% C and 93.3% Fe by weight.   • Cementite is a hard brittle compound with an orthorhombic crystal structure • It forms directly from the melt in the case of white cast iron. In carbon steel, it either forms from austenite during cooling or from martensite during tempering.Cementite mixes with ferrite, the other product of • austenite, to form lamellar structures called pearlite and bainite. 金属间化合物 [ɔ:θə‘rɔmbik]斜方晶系的 7 洛阳理工学院

9. 2.3 The Iron-Iron Carbide (Fe-Fe3C) Phase Diagram • properties：hard and brittle. （~1000HV，或800HBW）；塑性很差、韧性几乎为零；硬而脆，强度很低，耐磨性好，熔点为1227℃，是钢中的主要强化相 • Fe3C 是铁和碳形成的复杂结构的间隙化合物，是一个亚稳定的化合物，在一定温度下可分解为铁和石墨 Photomicrograph of Pearlite Structure (Dark bands are cementite) 8 洛阳理工学院

10. 2.3 The Iron-Iron Carbide (Fe-Fe3C) Phase Diagram Fig 2.21 Fe-Fe3C Phase Diagram （备用） 9 洛阳理工学院

11. 2.3 The Iron-Iron Carbide (Fe-Fe3C) Phase Diagram Introduction A In their simplest form, steels are alloys of iron (Fe) and carbon (C), so we will be looking at phase transformations using the Fe-C phase diagram. This is a fairly complex phase diagram but we are only interested in the steels part of the diagram (up to around 6.69% carbon). Steels have been important to engineers for so many years that each phase has inherited a name as well as a Greek letter, so both terms will be used. 11 洛阳理工学院

12. 2.3 The Iron-Iron Carbide (Fe-Fe3C) Phase Diagram A 温度坐标轴 D C E A F G s P K 成分坐标轴 12 洛阳理工学院

13. Figure 2.22 shows the equilibrium diagram for combinations of carbon in a solid solution of iron. 1) Iron and carbons combined to form Fe3C at the 6.69%C end of the diagram. 2) The left side of the diagram is pure iron combined with carbon, resulting in steel alloys. 3) Three significant regions can be made relative to the steel portion of the diagram. They are the eutectoid, the hypoeutectoid, and the hypereutectoid. 合成作用 [,i:kwi'libriəm] 部 分 [ju:‘tektɔid] 共析体 [haipə ju:‘tektɔid] 过共析体 [haipou ju:‘tektɔid] 亚共析体 2.3 The Iron-Iron Carbide (Fe-Fe3C) Phase Diagram 13 洛阳理工学院

14. 一、Fe-Fe3C相图的主要 特性点（参见P10图） H B J N 美国材料信息学会，原美国金属学会 14 洛阳理工学院

15. A D δ B H J N F C G E K P S Q L • 五个单相区 1) ABCD以上 — 液相区（L）； 2) NJESGN — 奥氏体区（A或γ） 3) GPQG — 铁素体区（F或α）； 4) DFKL — 渗碳体区（Fe3C或Cm） 5) AHNA —固溶体区（δ） 15 洛阳理工学院

16. 七个两相区 A D δ B H J N F C G E K P S Q L （两相邻的单相区之间）： L+A，L+Fe3C，F+A，A+Fe3C，F+Fe3C，L+δ, δ+ A 16 洛阳理工学院