We do itching test on fillet weld to find out the weld fusion. But is there any standard to check the amount of weld fusion depth needed in correspondence to the fillet weld leg size or mateerial thickness? Please share your knowledge regarding this.

The fillet weld must have fusion to the root, but not necessarily beyond the root if you are working to AWS structural codes. However, if one is investigating skewed T-joints, it is likely one will not have fusion to the root. In that case, one needs to verify the incomplete fusion doesn't exceed the Z-loss listed by the applicable structural welding code.

As for the depth of fusion in the region of the fusion faces, the depth of fusion is not a dimension that is specified, only that the weld be fused to the base metal. Excessive depth of fusion can be detrimental in some cases due to excessive dilution.

Al