OD dimension of ASME B16.25 1

[doses]

Hi,

Looking ASME B16.25 I noticed that in Table 1 are specified two column for outside diameter (A) of welding ends. One is for cast components and other is for wrought components. Can anyone tell me why OD for welding ends is bigger for cast components than for wrought components? Thanks for answers or comments.

Regards. David

 

[racookpe1978]

Usually, the difference is in the required safety factor for two methods of fabricatioon.

A cast piece cools down and creates a somewhat random crystal structure through the whole part in the mold. Each crystal in the part is randomly aligned, each crystal starts from a different location in each different casting, and cools slightly differently in different part. So the average strength of all of your parts can be predicted, but each different part will have a different strength and toughness and fracture resistance with a larger variation across all of the parts.

A wrought (forged) piece gets "squished" into shape: the mechanical work literally compresses out pinholes, re-alignes crystals and increases the overall strength of the piece. It tends to remove small local stressed points - while the shaping the whole part and compressing and pulling the edges of the part into the final configuration. This massive external shaping removes the small local variations that cause unpredictable results or fractures and stress rises: so the final sigma (variation) between forged parts is smaller, and the requirement to have a large safety margin to accomodate variations is reduced in a wrought/forged part compared in a cast part. (A bad forged part that pushed too far will still fail of course, but every forged piece from a bad manufactoring technique will fail immediately and be won't get sold.)

This is similar to a "rolled thread" in a bolt or screw compared to a "cut thread". Cutting a thread removes the metal from the thread "valleys", and leaves the hills with the rough cut surfaces and the total mass of the threaded bolt reduced. While rolling a thread "pushes" the steel from the valleys of the thread back "up" into the hills and so increases the strength of the high points - the bolt weighs as much after rolling as before.