檢查結果: 過度酸洗, 蝕刻晶界成凹槽. 隨後的染黑(或磷酸膜處理)再填滿這些凹口.但凹口應力集中效應已經使此軸強度劣化. 此例類似於AISI 420 晶界腐蝕. 都是表面晶界先受損. 形成應力集中位置.
The outer-most grains were somehow marred by over-pickling. This left micro-crevices between grains, resulting in “notch-effect”. Severe damage was already done to the tensile strength at the root area although later blackening process filled up these crevices with iron oxides. These crevices act as “crack initiation sites” when a load was applied on the spline teeth.---- In other words, grains would be opened up a lot easier with these crevices present at root surface. This factor played the largest role in the abrupt fracture of the axle.
下: 晶界上的氧化鉄或磷化鉄不受腐蝕(不溶於浸蝕液), 晶內的相則會溶於浸蝕液, 所以有高低差.
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