相片N, O, P: 淬火回火後. 枝晶偏析仍存在, 但較緩和. H~L 中的致命的鑄態尖角沃斯田鉄已消失. Ｐhoto N ~ P: Quenched and tempered. dendritic segregation is less severe than as-cast condition. Harmful angular Austenite areas has gone into solution and helped form martensite.
N
O
P
A ~ D Quenched & Tempered. No as-cast carbide particles are present. They went into solution of austenite on heating and formed Martensite after quenching. 1: area first to freeze (carbon/alloy poor). 2: area last to freeze (therefore, carbon/alloy rich).
A
B
C
D
E
F~M as-cast AISI 4340; Note: 枝晶結構, 枝晶偏析. dendritic structure.
F
G
小白點群聚處為最後凝固處, 此處溶質(Fe 以外的其他元素)原子濃度最高(solute-rich area), 所以形成碳化物顆粒而群聚. 更正: 此白奌應為含過飽和碳原子的沃斯田鉄(A).但也有碳化物微粒(K)存在, 都是白色 不易區分, 有人稱之為 A+K 島嶼. A=Austenite; K= Carbide.
H
J
K
Carbide- rich area: white angular islands(A+K) very detrimental to toughness and elongation of steel. They are harmful to machinability, too. Please bear in mind that this priciple applys to all kinds of steel, or, should I say, all kind of metals.
L
M
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