Is that a K-weld prep on the tube? meaning you want to weld from the top and from the inside of the hollow tube?
Or a K prep on the bars that go on top of and under the tube? Or are those "bars" actually a complete plate through the tube?
If bars, you'll have very limited shear capacity there.
If a plate, I don't get how you'll be able to weld a K.
I don't understand the question nor your sketch.

Yes, it is acceptable to have fillet welds larger than the thickness of the base metal in the orientation you have shown.

Following up on @dauwerda's response, you should check for tearing of the base metal. If critical, use a longer weld or redesign with a thicker base metal.

The picture below is in agreement with dauwerda,

From SE Solutions, LLC [SE University]



But 18mm (0.71") of weld, how many passes to be made? You might need to check AWS standard, or a welding engineer for procedures.

Quote (retired13)

If tube thickness is 6mm ,then can we use w=1.4t,
1.4x6=8mm weld?

Thanks in advance!!

AWS structural guidance for the maximum size of fillet welds is that

1) Along edges of material less than 1/4" (6mm) thick, weld size (w) shall not be greater than the thickness of material.
2) Along edges of material more than 1/4" (6mm) thick, weld size (w) shall not be greater than the thickness of material - 1/16" (2mm) unless the weld is especially designed to built out to obtain full-threat thickness.

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So for 6mm tube, w can be up to 6mm. For 8mm tube, w can be up to 8mm - 2mm = 6mm unless especially designed to be built out.

Note that the effective throat is the weld size (=leg size) /1.4 for a root opening of 0. For larger root openings, the effective throat is less.
Fillets larger than 3/16" (5mm) may require multiple passes depending on the process and electrode used. Multiple-pass welds may be significantly more costly than single-pass welds or a similar multipass PJP or CJP weld, again depending on the process, electrode or weld position.

jdonville, the parameters that you sited are for the edge of the plate, which is not what was presented in the sketch the OP gave.
See, this previous discussion for further clarification: thread725-428434: Max size fillet weld in T joint.
Specifically, this image:


Typically, up to 5/16" fillets can be done in 1 pass (depending on welding process)

dauwerda,

I stand corrected on the point of maximum weld size for the application shown by the OP.

Yes, number of passes depends on the process and equipment. We normally use E7018 electrodes for our SMAW field welding and I (perhaps conservatively) assume that we can reliable do a 3/16 weld in a single pass. In our shop, the manager (perhaps conservatively) says that we can reliably do 1/4" in a single pass using FCAW.

I also do not like to go bigger than 1/2 the base metal thickness with the leg size when specifying 2-sided fillet welds in T-welds of one plate to another as I like to give my field welders some leeway to provide a larger leg to compensate for root openings larger than 0.

The 5/16" I cited comes out of AWS D1.1, below is a snip of table 3.6, which provides the max single pass fillet weld size. Certainly there is no issue using something smaller if that is what your welder is telling you they do. AISC Steel Construction Manual also provides an approximation of passes based on weld size in part 8, Table 8-12.

It's important to understand why thse maximums exist for the case of a lap joint, its from an inspection point of view so you see the original plate edge and the welder doesn't grind it off (or melt it) and represent a weld that might be the total thickness of the plate vs a smaller size.

The maximum weld size provided by AWS is to prevent distortion of the work piece but also depend on engineering decision, in this case to weld the plate along the length of the tube/pipe is acceptable.

Regards,
Pook