Surface pitting related only to welding line

[coreman73]

I have a finished 3/8" ratchet handle. The head was as-cast from steel grade 4130, which was then friction welded to bar stock of steel grade 1045. What might have caused the clearly visible surface pits found localized along the welding line?

I have sectioned through the weld zone and am not finding anything wrong with the weld quality. It's actually quite good. Material chemistry meets spec.

Attached are some macro/microscopic photos of the pits and weld zone. It should be noted that the pitting was visible on both sides of the weld zone so not related to one material type.

I would appreciate any opinions as to the origin of these defects. This is not an isolated incident since it occurred to an entire batch of handles.

 

[metengr]

Looks like surface artifacts from the friction welding flash zone.

 

[TVP]

I agree with metengr, this is most likely a remnant of the friction welding flash. Depending on the amount of upset in the weld, there can be surface anomalies in this region. The shape does not appear to be a casting defect like gas porosity, microshrinkage, etc.

 

[coreman73]

Thanks guys. That does make sense and is probably the only reasonable explanation.

 

[redpicker]

It should be noted that the pitting was visible on both sides of the weld zone so not related to one material type.

It appears, to me at least, the effect is concentrated on the casting side of the weld. This would make sense, since the casting will contain micro-voids and larger inclusions than the wrought bar. The firction welding flows the material at 90 degrees to the longitudinal axis, which means that imperfections due to stringers in the wrought piece will be deeper near the weld and that micro-voids and inclusions that would be sub-surface in the cast head will not be exposed to the surface.

You could try increasing the upset pressure and time on the friction welding cycle, this will help. Since it has happened to an entire batch, it could indicate either a improper weld set-up or worn grips, which prevent achieving the higher upset forces at the end of the welding cycle.

rp