Cracked welds in EN24T/ Mild steel 1

[CTNMB]

A split ring (made from 25mm section EN24T)has two 10mm M/S lugs welded on either side of the 5mm split. A component is placed in the ring and held in place via an M10 bolt through the lugs - closing the split together. The jig and component are placed in an oven at 165degC for 8 hours. When removing from the oven, the welds have split.
The welds were produced with a 4mm M/S electrode (arc) at approx. 170Amps - penetration seems good, and the weld solid - no cavities or slag in the weld.
Can anybody tell me why please??

 

[Metalguy]

EN24T isn't a carbon steel. Was there any preheat before welding? How high? Did the welds fail in the HAZ?

 

[CTNMB]

Amateur but experienced welder, and no preheat was applied. The welds broke evenly from both the M/S and the EN24T removing approx. 1.5mm material from both welded surfaces. In effect I now have two unjointed pieces of steel and one perfectly formed bead of weld not attached to anything.

 

[metengr]

CTNMB;
The EN24 is a British grade of AISI Type 4140 alloy steel. If you did not use preheat, I would suspect that the cause of the weld failure was underbead cracking due to the formation of martensite (locally hardened zone) in the 4140 alloy steel HAZ.

A preheat of 200 deg C should correct this problem provided the filler metal is compatible for this application.

 

[arunmrao]

EN24 is a medium carbon steel with alloying elements of Cr, Ni,Mo. This is normally used in hardened condition. Were the rings in hardened or in normalised condition. Also welding rods matching EN 24 grade are available. All said and done you cannot miss out on the preheat process suggested.

 

[Metalguy]

Because the weld failed on BOTH sides, I'm wondering if you have enough weld metal alloy dilution from the low alloy steel to make it transform to untempered martensite just like what is almost certainly happening in the low alloy HAZ.

Is this a single-pass weld? I'll bet it is.