Answer: Yes, but only specific types can be directly welded together; different connection structures have strict limits.

Below is clear classification based on ASME standards, suitable for communicating with overseas customers.

1. Permitted Direct Welding (Common & Standard)

(1) Weld Neck Flange (WN Flange) + Butt-Weld Fittings (ASME B16.9)

✅ Direct butt welding is fully allowed and the most common design

WN flange end: butt-weld hub

Fittings: elbow, tee, reducer, cap (BW type)

Welding form: Butt weld (BW)

Application: High pressure, high temperature critical piping

Specification requirement: Same NPS, matching schedule wall thickness.

Example: NPS6 Class300 WN flange directly butt weld to NPS6 90° LR elbow.

(2) Socket Weld Flange (SW Flange) + Socket Weld Fittings

✅ Direct fillet socket welding

Normally limited to small size ≤ NPS 4

Mainly for instrument piping, low volume small-bore lines

2. Cannot Directly Weld (Wrong Assembly, Do NOT Do This)

(1) Slip-On Flange (SO) / Lap Joint Flange (LJ)

❌ You cannot weld SO / LJ flange body directly to butt-weld fittings

Reason:

SO flange inner hole is only for pipe to slip through; no butt-weld hub.

LJ flange is a loose backing ring; it has no flared sealing face.

Correct assembly:

Pipe runs through SO flange → weld pipe and flange;

Pipe connects to butt-weld fitting.

For LJ system:

Stub End (butt-welded to pipe/fitting) + LJ flange (loose on stub end).

LJ flange itself never gets welded to any fitting or pipe.

(2) Blind Flange (BL)

❌ Blind flange is an end closure plate, not for welding to fittings.

It is bolted onto another flange to seal the pipeline end.

(3) Threaded Flange (TH)

❌ Connects via NPT thread, not welding.

3. Important Practical Rules

Only WN flanges are designed for direct butt welding to BW pipe fittings

If you want to connect SO flange to a BW elbow:

Must add a section of straight pipe between them, cannot weld flange face directly to fitting.

Wall thickness matching:

When butt welding WN flange and BW fittings, transition thickness shall comply with ASME B31.3 to avoid stress concentration.

Lap joint reminder:

Stub end (not LJ flange) is butt welded to pipe or fittings.

Simple Summary for Customer Communication

✅ WN Flange ↔ BW pipe fittings: Direct butt weld — Standard practice

❌ SO / LJ / BL Flange ↔ BW pipe fittings: Cannot weld directly

✅ SW Flange ↔ Socket weld fittings: Direct socket fillet weld (small bore only)

Short English version you can send to clients

Weld Neck flanges can be directly butt-welded to butt-weld pipe fittings (elbow, tee, reducer etc.) as per ASME B16.5 and ASME B16.9.

Slip-on flanges, Lap Joint flanges and Blind flanges cannot be directly welded to pipe fittings.

Lap Joint flanges work with stub ends; only the stub end is butt-welded to fittings or pipes.