Complete professional FAQ article for industrial pipe fitting website,covering two mainstream forming technologies:hydraulic bulging(small&medium size)and hot extrusion/hot pressing(large diameter).

Overview

Seamless tee is integrally formed from one whole seamless pipe blank without any welding seam.It features uniform metal grain flow,high pressure resistance and zero leakage risk,complying with ASME B16.9,GB/T 12459 standards.Two mainstream manufacturing methods are adopted based on diameter,material and working pressure:hydraulic cold bulging and hot extrusion pressing.

1.Universal Pre-Production Steps(for both processes)

Step 1 Raw Material Inspection&Cutting Billet

Adopt qualified seamless steel pipes:carbon steel A234 WPB,alloy steel WP11/WP22,stainless steel 304/316L.

Strict lab test on chemical composition,tensile strength and wall thickness to eliminate defective pipes.

Cut pipe blanks to calculated length with extra machining allowance for later trimming.

Clean surface rust,oil and oxide scale by shot blasting.

Step 2 Pre-Processing Marking&Drilling

Mark the exact branch position on tube blank according to drawing dimension.

Drill a small pilot hole at the branch outlet for metal flow during forming.

2.Process 1:Hydraulic Cold Bulging(DN15~DN400,stainless steel&thin-wall carbon steel)

Suitable for small/medium equal&reducing seamless tees,high dimensional precision,smooth surface.

Mold Loading&Sealing

Place the cleaned tube blank into customized hydraulic forming mold,seal both pipe ends tightly to prevent liquid leakage.

Dual-Axis Axial Compression+Internal High-Pressure Liquid Injection

Two horizontal hydraulic cylinders push both ends of the blank inward to supply metal material axially.Meanwhile,inject high-pressure water/oil into the sealed blank to build internal pressure.

Branch One-Step Bulging Forming

Under combined axial thrust and internal hydraulic pressure,metal flows toward the mold’s branch cavity and extrudes a complete branch pipe in one molding cycle.The shoulder and joint wall naturally thicken for higher pressure resistance.

Mold Release&Rough Trimming

Release hydraulic pressure,take out the formed tee blank,cut excess material on three pipe ends.

Advantages of Hydraulic Bulging

Cold forming without high-temperature oxidation,bright surface for sanitary stainless steel tees.

One-step molding,high production efficiency,stable wall thickness reinforcement on tee shoulders.

Limitation

Requires ultra-large tonnage hydraulic press;not applicable for oversized DN400+thick-wall high-pressure tees.

3.Process 2:Hot Extrusion/Hot Press Forming(DN200~DN1200,thick-wall carbon/alloy steel high-pressure tees)

Mainly for large-diameter,thick-wall seamless tees used in oil,gas,thermal power pipelines.Lower equipment tonnage requirement than hydraulic bulging.

Pre-Flatten Tube Blank

Cold press the round seamless pipe blank into oval shape to reduce radial size for easy heating and extrusion.

High-Temperature Uniform Heating

Send oval blank into resistance heating furnace:

Carbon steel:850–950℃,hold 30–60 mins

Stainless steel/alloy steel:1000–1100℃

Ensure even temperature distribution to avoid local cracking during extrusion.

Hot Extrusion Branch Forming

Transfer hot blank to hydraulic extrusion die quickly.Hydraulic equipment compresses the oval blank radially;high-temperature plastic metal flows outwards through the pre-drilled hole to stretch and form the branch pipe with a supporting mandrel inside to control inner diameter.

Air Cooling After Forming

Cool the hot formed tee blank naturally in air to avoid rapid quenching deformation.

Advantages of Hot Press Forming

Works for super large diameter,extra thick-wall high-pressure seamless tees.

Lower hydraulic tonnage demand,lower production cost for mass large-size orders.

Limitation

High-temperature heating generates oxide scale on surface;extra pickling&sandblasting treatment required afterward.

4.Post-Forming Unified Finishing Procedures

Step 1 Heat Treatment(Mandatory)

Eliminate internal residual stress generated in cold/hot forming,stabilize mechanical performance:

Carbon steel:Normalizing treatment

Alloy steel/stainless steel:Solution annealing

Avoid tee deformation,cracking and strength decline under high pressure&temperature service.

Step 2 CNC Precision Machining

Process three pipe end faces,inner&outer diameter to standard tolerance;machine welding bevel for butt-weld connection.Control perpendicularity of branch pipe within 0.3mm/m.

Step 3 Surface Treatment

Carbon steel tee:Shot blasting+anti-rust primer/3PE anti-corrosion coating

Stainless steel tee:Pickling&passivation;sanitary type adopts mirror polishing

Remove all oxide scale,burrs and surface defects.

Step 4 Strict Quality Inspection

Dimension measurement:OD,wall thickness,branch angle,length tolerance

NDT non-destructive testing:UT ultrasonic wall thickness detection,MT magnetic particle/PT penetrant flaw detection to find internal&surface cracks

Pressure burst test for random sampling to verify pressure bearing capacity

Mark standard,size,material grade,heat number on qualified tees