Types of Stainless Steel Pipes: Manufacturing Routes and Structures

Classification by Rolling Method

The first and most practical classification follows the forming route, because the route fixes the achievable size range, the tolerance and the delivery length. Stainless steel pipe is produced by hot rolling, by hot extrusion, or by cold drawing and cold rolling. Hot working gives the largest diameters and the heaviest walls; cold working gives the tightest tolerances, the best surface finish and the thinner walls.

In the hot route a round billet about one metre long is cut, charged into a furnace at roughly 1200 °C and pierced on a conical roller piercer, which combines high output with good wall quality and accepts a wide range of grades. After piercing, the hollow shell is cross rolled on a three roll mill, continuously rolled or extruded, then detached, sized or reduced, cooled, straightened, pressure tested or examined, marked and placed in store.

In the cold route the pierced shell passes through heading, annealing, pickling and oiling or copper plating before multi pass cold drawing or cold rolling. The finished blank tube is then heat treated, straightened, hydrostatically tested or examined for flaws, marked and stored. Annealing between drawing passes is what restores ductility and allows the large reductions that produce thin wall, close tolerance tube.

Classification by Metallographic Structure

The second classification is by microstructure, and it is this grouping that determines corrosion behaviour, strength and welding response. Five families cover almost all commercial orders.

Family	Typical grades	Characteristics
Austenitic	304, 304L, 316, 316L, 321, 347H	non-magnetic, excellent weldability and general corrosion resistance
Austenitic-ferritic (duplex)	2205 type duplex stainless	roughly twice the yield strength of austenitic, resistant to chloride stress corrosion
Ferritic	430 type	magnetic, lower cost, limited high temperature strength
Martensitic	410, 420, 431 type	hardenable by heat treatment, used for wear and mildly corrosive duty
Semi-ferritic semi-martensitic	transitional low chromium grades	mixed structure with moderate corrosion resistance and strength

Austenitic pipe is the workhorse of chemical, food, pharmaceutical and cryogenic service. Duplex pipe is selected where chloride stress corrosion cracking or high pressure demands a stronger material at the same wall thickness. Martensitic pipe appears in mechanical components and mildly corrosive applications where hardness rather than corrosion resistance is the driver.

Standard Dimensions, Lengths and Size Ranges

Dimensional ranges are fixed by the product standard. Under GB/T 14975, the standard for stainless steel seamless steel tubes for structural use, and its counterparts for fluid transport such as GB/T 14976, the usual delivery lengths are as follows.

Product	Condition	Delivery length
Hot rolled tube	as rolled	1.5-10 m
Hot extruded tube	as extruded	1 m and above
Cold drawn or cold rolled	wall 0.5-1.0 mm	1.0-7 m
Cold drawn or cold rolled	wall above 1.0 mm	1.5-8 m

Size coverage follows the same split. The hot rolled and hot extruded family covers 45 outside diameter sizes from 54 mm to 480 mm, with 36 wall thicknesses from 4.5 mm to 45 mm. The cold drawn and cold rolled family covers 65 outside diameter sizes from 6 mm to 200 mm, with 39 wall thicknesses from 0.5 mm to 21 mm. Buyers should confirm that the size they need sits inside the mill's current table rather than assuming that any combination can be drawn.

Surface Quality, Straightness and End Finish

The inner and outer surfaces may not carry cracks, folds, laps, rolled-in scales, laminations or scabs. Any such defect is completely removed except on pipe intended for machining; after removal the wall thickness and outside diameter must still lie within the permitted negative deviation. Other minor surface imperfections that remain inside the negative deviation limit do not have to be removed.

Straightness is expressed as permitted local bend depth:

Hot rolled and hot extruded pipe with outside diameter not more than 140 mm: bend depth not more than 5% of nominal wall thickness, and not more than 0.5 mm in absolute terms.

Cold drawn and cold rolled pipe: bend depth not more than 4% of nominal wall thickness, and not more than 0.3 mm in absolute terms.

Both ends are cut square with the pipe axis and the burrs are removed, so that the tube can be butted and welded without an artificial gap. A square, burr free end is also what allows reliable automated welding on site.

Manufacturing Route for Seamless Stainless Tube

The two seamless routes can be summarised as follows, and the sequence explains why cold drawn tube costs more and holds tighter tolerances.

Hot rolled or extruded: round billet, heating, piercing, three roll cross rolling, continuous rolling or extrusion, detaching, sizing or reducing, cooling, straightening, pressure test or flaw detection, marking, warehousing.

Cold drawn or cold rolled: round billet, heating, piercing, heading, annealing, pickling, oiling or copper plating, multi pass cold drawing or cold rolling, blank tube, heat treatment, straightening, hydrostatic test or flaw detection, marking, warehousing.

Because the cold route includes annealing, pickling and repeated drawing, the finished tube has a cleaner surface and a much smaller tolerance band than the hot product, which is why instrument tubing, heat exchanger tube and precision mechanical tube are normally ordered cold finished.

Frequently Asked Questions

Q: What are the main types of stainless steel pipe?
They are grouped by rolling method into hot rolled, hot extruded and cold drawn or cold rolled pipe, and by structure into austenitic, duplex, ferritic, martensitic and semi-ferritic semi-martensitic pipe.

Q: Which standard covers stainless steel seamless tubes for structural use?
GB/T 14975, which superseded the older 1994 edition; fluid transport service is normally ordered to GB/T 14976 or to the equivalent international specification.

Q: What lengths are supplied?
Hot rolled tube 1.5-10 m, hot extruded tube 1 m and above, and cold drawn tube 1.0-7 m for walls of 0.5-1.0 mm or 1.5-8 m for thicker walls.

Q: What size range is available?
The hot family covers 54-480 mm outside diameter with 4.5-45 mm walls, and the cold family covers 6-200 mm outside diameter with 0.5-21 mm walls.

Q: How is straightness specified?
As a maximum local bend depth: 5% of nominal wall with a 0.5 mm cap for hot pipe, and 4% with a 0.3 mm cap for cold drawn pipe.

Q: May surface defects be left on the pipe?
No. Cracks, folds, laps, laminations and scabs must be removed completely, and the remaining wall and diameter must stay within the permitted negative deviation.