二次回火后硬度变化不大，三次回火后，硬度反而降低。
Two times temper has little effect on the hardness, three times temper will decrease the hardness.
淬火态组织为马氏体、碳化物和奥氏体，淬火三次回火态组织为马氏体和碳化物。
The microstructure of quenching are martensite, carbide and retained austenite, and are martensite and carbide by quenching and three times tempering.
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