[Springeye]

I'm working with class 8 truck axle housings. some weld a crack, the weld has a center line crack. this is on the top of the axle square tube. this is a high stress area, the axle bracket weld under this.

[weldin mike 27]

When does it crack? Immediately after welding?

[Poland308]

This may require pictures. If the crack was underneath then it may have just come through. Cracks generally need to be ground out and filled in. But you must find the end of the crack.

[weldin mike 27]

A centreline crack that occurs immediately after welding is referred to as a solidification crack. It has to do with the time it takes for the weld to solidify and allow impurities to rise out of the weld pool. Submerged arc welds can sometimes do this, as can vertical down welds. The other main reason is that the weld hasn't got a sufficient throat thickness. If the weld is very flat or concave it may be this reason. Grind out as Josh says, reweld ensuring that your weld has a decent ratio of throat to width. (width to depth ratio) if it's a issue with the metal itself or the design the cracks would appear in a different orientation.