Quench crack due to different volume expansions from Martensite phase change.
50X
Photo taken in 1984. Linkage plate of chain, 1.0mm in thknss. Chain failed tensile test and submitted for inspection to find out why. Green areas on both upper and lower side of the above photo are carburized areas which are high carbon Martensite (Hi C M) after Q/T. White core in the middle are low carbon (Lo C) Martensite. Note that carburized area is twice as thick as core area. On quenching, Hi C M expands a bit more than Lo C M, therefore outer layer put the core layer in tension. Since this tension is greater than that which can be endured by the core area, core area is torn apart. If core were strong enough, no crack would have happened. Then, core would be in tension state while outer layers would be in compression state. compression stress is good stress and will improve fatigue life of any parts.
By extension from this, induction hardened layer of a stud would have expanded freely if it were not restrained by the much thicker core adjacent to it. This restraining effect put the induction layer under compression and compressive stress is thus "stored" in the layer to counteract tensile stress once the stud is stressed. The magnitude of this beneficial "retained" compressive stress is reversely proportional to the thickness of the induction hardened layer: The thinner the hardened layer, the higher the compressive stress. So, larger studs require thicker induction-hardened layer and smaller studs require thinner hardened layer. A taper shank need a taper induction hardened layer.
1984攝. 厚度~1.00 mm 鏈條拉力試驗不合格, 切片檢驗 發現內部裂痕. 此係因滲碳外皮為高碳麻田散鐵, 相變化時的膨脹量超過了內部低碳麻田散鐵所能承受的量, 內部因而被拉裂.
*注意: 破裂一定自受拉應力處起源, 不會發生在處於壓應力處. 滲碳層厚度通常遠小於工件肉厚, 不易因滲碳導致破裂. 薄鋼片例外.
In most cases, thickness of carburized area on a part is much thinner than core area. This kind of cracking seldom found.
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