Ultrasonic Testing Defect Types in Forged Steel

Ultrasonic Test on Forged Round Bar
Classification Kinds of Defects
A Microscopic non-metallic inclusions – Sulfides, silicates, alumina, granular oxides.
B Visible non-metallic inclusions – Pinhole, slag, foreign object.
C Shrinkage Cavity, porosity, overheating, surface crack, quenching crack.


Class A

Class A defects are tiny non-metallic inclusions, which can be found after machining, and are in the form of granules with a size of approximately 0.2mm or less. If these defects are not clustered together, there will not be obvious effect on the strength of steel material.

However, for bearings and rollers, it is considered harmful and should be strictly controlled. This kind of defect is presented as F waveforms in ultrasonic testing.


Class B

Class B defects with larger size, generally categorized as pinhole, slag, foreign object, which are visible during machining.

Pinhole is sometimes called casting non-metallic inclusion, size and shape similar to class A defect, distributed along the metal flow line in a single or multiple forms.

Slag inclusion has the same shape as pinhole, but with larger size, which is caused by external hard mixing during ingot casting.

Foreign object accidentally casting into the steel ingot can be detected by color when machining, and if detected by dye penetrant inspection, the defects are presented in the form of stains.


Class C

Class C defects are crack types, the effective methods for detecting them are ultrasonic testing and dye penetrant inspection.

Shrinkage cavity: a hole-like defect that remains in the center of the ingot without compaction, usually occurs in the center of the ingot. The bottom reflected wave in UT sometimes does not appear due to the shrinkage cavity size.

Porosity: when the forging ratio is not sufficient, it is easy to leave porous defects in the center of forgings.

Segregation: usually can be found in the segregated area or inside the ingot.

Overheating: which caused grain boundary oxidation and coarse grains, appearance is crack on the material surface.

Surface cracks: caused by material overlap in forging, sometimes accompanied by oxidized skin, which can be detected by dye penetrant testing or visual inspection.

Quenching cracks: occur on the surface or inside during the heat treatment process due to rapid temperature changes.

Quality Control