Nature of the light-colored areas in the fracture of axle steel
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Light spots found in the fracture area of tensile specimens of axle steel represent the surface of internal brittle fissures formed during the tensile test. Their appearance is accompanied by a considerable impairment of elongation and reduction of area. The tensile and yield strengths are not affected.
These internal flaws appear in connection with the presence in the metal of low-ductility zones due to locally high hydrogen content. Segregation zones are particularly prone to this defect. Removal of hydrogen by prolonged holding at room temperature, or for a shorter time at elevated temperature, restores the ductility.
Non-metallic inclusions also reduce the ductility but their effect is much less than that of hydrogen.
The light spots have the same origin as flakes: in both cases hydrogen is the responsible factor. The difference is that flakes are formed prior to the application of an external force while the light spots appear during testing.
KeywordsBrittle Ductility Yield Strength Tensile Test External Force
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