Stainless Steel Pipe and Tube

Stainless Steel is a versatile material exhibiting a wide range of properties. In addition to its primary property of resistance to both aqueous (wet)  and gaseous (dry) corrosion; it has excellent cleanability, a high hygiene factor, the ability to handle both high and extremely low temperatures, a high resistance to fouling, and the ability to be fabricated by standard processes into components of sound structural integrity.

Such properties result in the application of stainless steel pipe and tube in diverse industry sectors across a wide spectrum of applications.

For example:

  • In the petrochemical, chemical, food and beverage industries to integrate the process vessels and tanks within the factory by the conveyance of liquids or gases between such items of equipment.
  • As integral component parts of fabricated items such as the tube bundle in a heat exchanger, vacuum pipes on filters.
  • As individual items such as hypodermic needles, tube fittings, injector nozzles.

Tube and pipe therefore makes up a very significant component of all stainless steel consumption.

One of the first questions often asked is what is the difference between a pipe and a tube? There is no clearly defined difference. The World Book Dictionary states the following :

“PIPE – A pipe is a tube through which liquid or gas flows.”
“TUBE – A long pipe of metal, glass, rubber, plastic or other material used to carry liquid or gas.”

In actual fact the terms PIPE and TUBE are synonymous, but generally breakdown as follows:

PIPE – A major use of pipe is in pressure piping systems for which the wall thickness relative to the outside diameter(OD) determines the pressure rating. Pipe is produced to standard OD sizes, a peculiarity being that these are designated by a NOMINAL BORE dimension. Each specific Nominal
Bore size has several associated wall thicknesses designated by SCHEDULE numbers, resulting in different outside diameters (OD’s).

TUBE – Tubes are used where precise fitment of the OD is required, e.g. tubes into a tube plate in a heat exchanger. Tube is produced to standard OD sizes, which is the critical factor and specified exactly by the actual dimension. Each OD size has a large range of associated wall thicknesses specified by the exact actual dimension. Tubes tend to have a relatively thinner wall thickness than do pipes.

No difference is intended between PIPE and TUBE, although idiomatic usage sometimes prefers one over the other.

THE MANUFACTURING PROCESSES EMPLOYED IN THE PRODUCTION OF STAINLESS STEEL PIPE AND TUBE

The following processes are used in the production of stainless steel pipe and tube:

  • Seamless
  • Centrifugal Cast
  • Longitudinal Welded
  • Spiral Welded
  • Press Brake Fabricated
  • Roll Formed Fabricated.

SEAMLESS

This was the traditional method of manufacture of pipe/tube. However, with the continued improvements in welding technology since the 1950’s, seamless is being replaced to a greater and greater degree by welded pipe/tube except for the manufacture of heavy wall pipe.

A high capital investment is required for the sophisticated production equipment which must be capable of working with the material at high temperatures in excess of 1000°C.

The 1st stage of Seamless pipe or tube production is by one of two methods:

  • Rotary Piercing
  • Extrusion
  • ROTARY PIERCING. It is the least used production process for the manufacture of Seamless Stainless Steel pipe/tube. The hot workability of the grade of stainless steel being worked has a major influence on the process, viz a narrow hot working temperature range, too great a high temperature strength, a tendency to mechanically rupture during processing.
  • EXTRUSION. This is the most utilized production process for Seamless stainless steel pipe/tube. Refer to Fig 1, above.
    The two main stages in this production method are:
  • A round tube billet is either bored or hot pierced (±1200°C) to give a central hole.
  • The billet is then extruded through a die at high temperatures (±1250°C).

The die controls the OD. A mandrel, held by the ram and extending through the hole in the billet into a central position within the die, controls the ID.

The extruded pipe/tube is either air or water cooled (dependent on the grade of Stainless Steel), and is then subjected to pickling followed by water rinsing. At this stage the pipe/tube is termed “hot finished”, “Hot finished” pipe/tube has certain limitations which often results in the need for subsequent cold work to produce the required final product. The reasons for cold working include:

  • Stipulated by the applicable specification.
  • Closer size tolerances and improved eccentricity.
  • Production of smaller OD (<±30mm), and/or thinner walls (<±3mm), and/or longer lengths.
  • Non-circular shape is required.
  • Better control of mechanical properties.
  • Improved smoother surfaces, both OD and ID.

Various cold working operations are used. These include:

  • Pilgering, which is a cold rolling technique.
  • Plug Drawing (employing either a Fixed or Floating plug) and
  • Mandrel Drawing which change both the diameter and the wall thickness; or
  • Sinking which changes the OD only.

The production of Seamless pipe/ tube is a piece-by-piece process, i.e. each length is individually produced. The production of “hot finished” stainless steel Seamless pipe/tube requires ±17 operations. If further cold finishing is required this can entail up to an additional 20 operations.
Seamless stainless steel pipe/tube is therefore relatively expensive compared to longitudinal welded, and is most cost competitive in the heavier wall thickness sizes.

The size range of seamless stainless steel pipe/tube is between 3-610mm OD, with an infinite range of OD/wall thickness combinations.

Seamless stainless steel pipe/tube has many applications, most of which involve elevated temperature and/or high pressures.

CENTRIFUGAL CAST

This is a piece-by-piece production method.

Molten stainless steel is poured into a cylindrical mould which is rotating at high speed about a horizontal axis. The centrifugal force throws and holds the molten metal against the walls of the mould where it solidifies as a hollow tubular casting.

The OD of the cast pipe is controlled by the ID of the mould. Therefore pipe of any specific OD depends on that size of mould being available.

The ID of the cast pipe is controlled by the amount of molten metal poured into the mould. Therefore for any one OD an infinite number of ID’S can be produced.

The length of the cast pipe depends on the lengths of moulds available. Long moulds require high capacity casting equipment to spin the moulds.

Lengths shorter than the length of the mould can be produced by positioning of “end plates” within the mould.

General availability is within the range of 65-680mm OD and in lengths between 2-6 metres. Heavy wall and large diameters can have a length limitation imposed by the ability to melt sufficient metal and the mechanical capacity of the casting machine.

Centrifugally cast pipe/tube is used extensively in the steel industry as furnace hearth rolls, rollers in galvanizing lines; and in the petrochemical industry e.g. as reformer tubes.

LONGITUDINAL WELDED

As a result of technological advances in the welding processes employed, welded tube and pipe is used to a greater extent than seamless (now over 80%), especially in applications where a thinner wall thickness is required.

This is a continuous welding method of manufacture. Refer to Fig 2.

 

  • Stainless Steel coils of the required thickness (i.e. the wall thickness of the pipe or tube) are continuously cut into strip of the required width (i.e. the circumference for the diameter [OD] of the pipe or tube).
  • The coils of cut-to-width strip are loaded onto the coil holder at the end of the tube mill and fed into the mill. As a coil of strip is used up, the tail is butt welded to the start of the next coil of strip to give a continuity to the production process.
  • The strip then passes through a series of in line rolls which consist of three types; forming rolls, closing rolls and finish pass rolls. The strip is thus continuously curved and finally formed into the required tubular shape.
  • The formed open pipe/tube enters the welding section of the mill where squeeze rolls hold or force the edges into the required contact for the
    welding process which is being used.
  • The welding is done using different automatic welding processes.
  • The welded pipe/tube is passed under a linishing belt which removes the weld bead on the OD, and then through final sizing/shaping rolls.
  • The pipe/tube is then cut to the required length. The ability for both long and/or specific lengths to be cut is a major advantage of the longitudinal Welded production process.

The continuous nature of the Longitudinal Weld enables continuous non-destructive testing, and finishing processes to be done “in-line” on the welded tube as it emerges from the mill.

  • Various non-destructive methods such as Eddy Current testing may be used to detect any flaws or discontinuities in the weld. If detected the affected area is automatically marked and later cut out.
  • Bead Rolling. A close-fitting mandrel is held in position within the bore of the pipe/tube as it emerges from the welding head. As the pipe/tube
    passes over the mandrel, reciprocating contoured rollers roll the longitudinal weld onto the mandrel with sufficient force to completely flatten the inside weld bead consistent to the wall of the pipe/ tube. Therefore, there is no protrusion and the resultant bore is completely smooth. The grain structure of the weld metal is also refined. Refer to Fig 3.

  • Non-Round Sections. Square and rectangular sections can easily be produced “in-line” by drawing the welded pipe/tube through a “turk’s
    head” as it emerges from the mill.
  • Annealing. If this is required an “inline” induction heater coil can be used.

Stainless Steel pipe/tube produced by this method is covered by many international specifications.

SPIRAL WELDED

This is a flexible, continuous welding method of manufacture and is particularly suited to the production of large diameter tube (>100mm) with
relatively thin wall thicknesses. Refer to Fig 4.

Stainless Steel coils of the required thickness (i.e. the wall thickness of the tube) are slit into strip of the required width. The coils of cut-to-width strip are loaded into the coil holder and fed at an oblique angle to a cylinder former and hence to the welding section of the mill.
-The inter-related factors of the width of the strip and the oblique feed angle control the OD of the tube produced.

The welded tube is then cut to the required length. The ability for both long and/or specific lengths to be produced is a major advantage of the Spiral Welded production process.

The use of strip cut from coils enables control of the wall thickness of the tube to close tolerances.

Further the nature of the process results in tubes which are straight, of excellent concentricity, and stiff (resistant to bending over relatively large support spacings).

Large diameter thin wall stainless steel Spiral Welded tube is used in the Pulp and Paper Industry, and for effluent lines in the Petrochemical Industry.

It is also now being increasingly applied in the Food and Beverage Industry as new methods of weld finishing result in a smooth interior surface. It is generally used for noncritical or low pressure applications.

PRESS BRAKE FABRICATED

This is a piece-by-piece production method, particularly suited for the manufacture of large OD relatively thick wall pipe.

  • Stainless Steel plate (or sheet) of the required thickness is sheared to the width for the required OD. The sheared edges may be mechanically dressed  to produce the necessary weld joint preparation.
  • The round tubular shape is developed by using a press brake equipped with the appropriate tooling for the size being made.
  • Following this initial forming, the open tubular shape may be rolled to improve the concentricity
  • It is then jigged for the welding operation.
  • After welding the pipe may be annealed to remove stresses induced by the forming and welding operations.

Limitations of this method of production are imposed by the power of the press brake and the length which it can accommodate.

It can also be an expensive method of production in that the shearing of the width for the required circumference from available standard size plate or sheet can result in a low yield factor

ROLL FORM FABRICATED

This is similar in most respects to the Press Brake Fabricated method. It has an advantage that, if the diameter of the pipe results in a circumference greater than the available width of plate or sheet (e.g. >477mm diameter = >1500mm circumference) such longer dimensions can then be cut from the length of the plate/sheet and processed by this production method.

This is a piece-by-piece production method.

  • Precut plate or sheet of the required thickness is sheared to suit the required OD. The sheared edges may be mechanically dressed to produce the necessary weld joint preparation.
  • The round tubular shape is rolled to the required diameter.
  • It is then jigged for the welding operation.
  • After welding the pipe is sometimes annealed to remove the stresses induced by the forming and welding operations.

A limitation of this process is that the length of the pipe section is restricted by the roll sets or the width of available plate/ sheet to commonly 1000mm, 1500mm or 2000mm.

The short, welded sections (termed CANS) are then joined by circumferential welding to make the required length of pipe.

The increased amount of welding involved, and the testing thereof, if required, contributes to making this an expensive method of manufacture.

A further contribution to the expense is that the shearing of the width/length for the required circumference from available standard size plate or sheet can result in a low yield factor.

WELDING PROCESSES USED IN THE PRODUCTION OF STAINLESS STEEL PIPE AND TUBE

In the manufacturing methods which employ continuous welding, viz Longitudinal Welded and Spiral Welded, automatic welding processes are used, which include

 Electric Fusion Welding (EFW) processes.
 Tungsten Inert Gas Welding (TIG) [also termed Gas Tungsten Arc Welding (GTAW)]  Plasma Arc Welding (PAW), often referred to as Plasma/TIG welding
 Metal Inert Gas Welding (MIG) {also termed Gas Metal Arc Welding (GMAW)]  Laser welding
 Electric Resistance Welding (ERW) process.

The welding of stainless steel is covered in a previous module of the Info Series and this should be referred to for details of the above processes.
Aspects which relate to some of the above welding processes with respect to their use in the manufacture of stainless steel pipe/tube by the continuous welding processes are briefly.

TUNGSTEN INERT GAS WELDING (TIG)

– This is the most utilizedwelding process in the production of Longitudinal Welded stainless steel pipe/ tube, especially for the “Specification
pipe/tube” used in heat exchangers, evaporators, pressure vessels and pressure piping systems.
– In the production of thinner wall pipe/tube (<±3mm) an AUTOGENOUS welding technique is often employed, i.e. no filler wire is used and the weld metal results from the fusion (melting) of the parent metal. For thicker wall thicknesses or if a specific weld metal composition is required a filler wire is utilized.
= The utilization of a filler metal is dependent on the conditions of the specification; some preclude it, in others it is permitted.

METAL INERT GAS WELDING (MIG)

– This welding process enables greater production rates in the welding of the thicker wall pipe/tube (>±4mm), either Longitudinal Welded
or Spiral Welded in that filler wire is continuously fed into the weld pool during the process.

PLASMA ARC WELDING (PAW), OR PLASMA TIG

This process is capable of generating extremely high temperatures which enable faster welding speeds (greater rates of production). It utilize plasma
gases developed in the tungsten welding head and channelled at high velocity to the weld joint. This process normally produces autogenous welds i.e. with no filler material.

Very close control of the process is possible. This enables a weld to be completed with negligible protrusion of the weld bead into the ID of the tube, e.g. Micro-plasma welding of very thin wall (<0,5mm) tubing.

LASER WELDING

Laser welding offers very high production rates with low heat inputs, offering close control of both weld bead geometry and HAZ thermal effects. Laser welding is now considered as the 1st choice for many applications such as exhaust tubing, heat exchanger tubing and balustrading where long production runs and high speed are essential. Also produces autogenous welds.

ELECTRIC RESISTANCE WELDING (ERW)

– The properties of the weld deposit are usually inferior to the various Electrical Fusion Welding processes
– This welding process can only be used for stainless steel tube intended for decorative/non-critical applications (e.g. hand railing, table legs, exhausts) and CANNOT be used for any pressurmised or critical applications (e.g. pressure tubing, heat exchangers).
– In this process the edges of the formed open tube are held in contact in rotary dies

These edges are heated to the required welding temperature due to the natural resistance across the gap to AC electricity of low voltage and
high current. The edges are then forced together to complete the weld.

– High welding speeds are attainable.

There are practical limitations with respect to the amount of weld metal which can be deposited per unit of time in the various processes. Therefore,
the properties of the weld deposit (particularly in the thicker materials) may require that the weld metal be deposited sequentially in multi-pass
welds. The welding station in the mill may then have multiple welding heads to attain both higher rates of throughput (production) and the required weld properties.

In the piece-by-piece manufacturing methods of producing welded pipe and tube, viz Press Brake Fabricated and Roll Form Fabricated; manual,  semi-automatic or automatic welding processes are used which include the various Electric Fusion

Welding processes such as

  • Manual Metal Arc (MMA) [also termed Shielded Metal Arc Welding (SMAW)]
  • Tungsten Inert Gas Welding (TIG)
  • Metal Inert Gas Welding (MIG)
  • Submerged Arc Welding (SAW)

The choice of any one of these welding processes will depend on factors such as:

 The grade and thickness of Stainless Steel,
 The facilities/equipment available,
 The quantity and
 The rate of production required.

FINISHING OPERATIONS EMPLOYED IN THE PRODUCTION OF STAINLESS STEEL PIPE AND TUBE

Various finishing operations may be applied to stainless steel pipe/tube to ensure that the product complies with:

– The normal production standards adopted by the manufacturer for the market acceptance of their product.
– The requirements of the specification.
– The optional/supplementary requirements, either as detailed in the specification or to meet the criteria as required by the end-user.

Further, the finishing operations applied will depend on the method of manufacture, for example

– Seamless pipe/tube may require to be cold worked for the reasons as set out previously.

Such cold working may necessitate that certain finishing operations (e.g. cold drawing, degreasing, annealing, pickling) be carried out more than once.

– Longitudinal Weld pipe/tube can be ordered/supplied Direct Off Mill (DOM), in which case only a minimum of finishing operations are carried
out (degreasing/cleaning, testing by flaring/flanging, visual inspection).

The finishing operations which can be employed include:

DEGREASING/CLEANING

This is usually done as a standard finishing operation to ensure

  • Cleanliness. Grease, oil, grit, chips which could either mar the surface appearance, initiate corrosion or cause contamination in the final application, are removed.
  • No impairment of the corrosion resistance by Carbon(C) pick-up during a subsequent high temperature operation (e.g. welding) resulting in sensitization and thus Intergranular Corrosion.

It is usually carried out in heated alkaline solutions. Special care is needed with small OD pipe/tube to ensure the removal of contamination
from the bore.

HEAT TREATMENT

This is done to ensure that the correct crystal structure, and hence mechanical and corrosion resistant properties are developed.

Annealing is the most commonly employed heat treatment process.

  • Stresses and work-hardening induced by forming and welding, and by any subsequent cold work are removed.
  • Any sensitization which may have occurred is rectified.
  • Annealing may have to be carried out more than once depending on the production/finishing operations employed (e.g. in the cold working of “hot finished” Seamless stainless steel pipe/ tube, inter-process annealing to enable further cold work to be carried out).

– Different furnaces can be used for the annealing process:

 Batch furnaces
 Continuous inert atmosphere furnaces
 Induction heater coils (either separate from, or “in-line”, with the mill).

– An inert atmosphere may be employed to prevent the high temperature oxidation (scaling) of the surface of the Stainless Steel. This is termed ”BRIGHT ANNEALING”, and no subsequent pickling is necessary.

PICKLING

This is done to remove any high temperature oxidation (scale) from the weld HAZ and the surface of the pipe/ tube. Refer to previous modules of the
Info Series for details.

PASSIVATION

This is done to promote the formation and improve the integrity of the passive layer on any freshly created surface (e.g. through mechanical
polishing), and also to dissolve any free iron/steel contamination which may have occurred.

Refer to previous modules on Pickling and Passivation in the Info Series for details.

COLD REDUCTION/FORMING

Two processes are used

  • Drawing, which changes both the OD and the wall thickness.
  • Sinking, which changes the OD only.

Cold reduction/forming is done

  • To overcome the process limitations (as mentioned before) in the manufacture of Seamless pipe/tube.
  • To produce special (nonstandard) sizes of pipe/tube.
  • To produce non-round sections (e.g. square, rectangular pipe/tube).

If cold reduction/forming is carried out it will normally necessitate subsequent annealing

It will also flatten the weld bead on the ID of Longitudinal Welded pipe/tube if cold drawing over a plug or mandrel is employed.

SURFACE POLISHING

  • Mechanical polishing to stipulated standard designations of surface finish.
  • Electro-polishing.

CUTTING AND DEBURRING

  • Cutting is done to produce
     Required length to within stipulated tolerances
    Square ends for subsequent joining.
  • Deburring is done to remove the burrs which would otherwise cause difficulties in the joining process.
  • In certain instances, generally in the case of heavy wall thicknesses, the ends are bevelled at the same time as deburring to produce the necessary weld joint preparation.

STRAIGHTENING

Straightening to conform to the standards specified.
– Straightness is expressed as the maximum deviation from a straight edge of defined length held along the length of the pipe/tube.

The inspection and testing performed may include the following:

VISUAL INSPECTION

Properties which can be checked by  visual inspection include Degreased/ Clean, Ends Cut Square/Deburred, assessment of Straightness, Surface
Defects.

 It is essentially subjective, and therefore dependent on the skill/ experience of the person performing the inspection.
 It may be permissible to rectify detected surface defects by spot conditioning.

CHECKING OF DIMENSIONS

Dimensions which may be checked include the OD, ovality, wall thickness, ID, length and straightness.

– The tolerances applicable to the various dimensions are an important factor.

PRESSURE TESTING

Pressure testing is usually done by hydrostatic testing, but pneumatic testing can also be used.

It is normally confined to nondestructive testing, but in some instances destructive testing is required.

Non-destructive Pressure Testing

  The pipe/tube is hydrostatically or pneumatically pressurised to a prescribed pressure. Any leaks will become apparent either by visual means (e.g. leaks, or bubbles escaping in underwater pneumatic testing), or by loss of pressure over a time period.
 Acoustic Emission Testing may be used during the pressure testing to detect defects which do not result in leaks or loss of pressure. The sound of a defect deforming under pressure is detected by using a very sensitive sound probe in contact with the pipe/ tube. By using two such probes the exact location of the defect can be determined.

Destructive Pressure Testing

– The pipe/tube is hydrostatically pressurised until failure by bursting occurs.
– It is seldom required, usually only when certification of pressure ratings is needed for critical applications.
– Testing is then carried out on a stipulated sample quantity (e.g. 1 length in every 1000).

WELD TESTING

The testing of the weld to ensure the integrity thereof may be either by destructive or non-destructive methods.

Destructive Testing of Welds

It is standard practice that samples are cut at prescribed intervals from Longitudinal Welded pipe/tube and tested by destructive methods. The
testing methods employed include:

– Flaring
– Flanging
– Flattening
– Reverse bending

  • If the properties of the weld are inadequate, these will show as mechanical failure caused by the severe deformation imposed in these testing
    methods.

Non-destructive Testing of Welds

> Radiographic examination.

Some specifications require that the weld be radiographed to detect internal defects or inclusions.

The amount of examination varies from complete (i.e. 100%) to various lesser stipulated spot radiography.

Radiographic examination makes use of X-rays (or similar radiation) which pass through metal. This enables the detection and location of defects/inclusions within the weld and the body of the tube.

> Eddy Current Testing

May be employed in final inspection and testing i.e. on individual lengths of pipe/ tube or continuously during manufacture. It enables the nondestructive testing of weld integrity. In addition, it will also detect any internal or surface defects present, and give a measure of eccentricity for either seamless or welded pipe/tube.

— A primary coil placed around the pipe/tube is used to induce alternating eddy currents into the pipe/tube.

The associated magnetic field (flux) is detected by a secondary coil. Flaws or defects are detected because they alter the pattern of the magnetic field.

MARKING

Marking as required for identification purposes.

PACKING

This is required to prevent damage occurring during handling, storage and transport. The packing will vary depending on the degree of protection
required. Mass limitations may also be imposed.

COMPARISON OF LONGITUDINAL WELDED VS SEAMLESS STAINLESS STEEL PIPE AND TUBE

It has been mentioned previously that as a result of technological advances in welding processes, the use of Longitudinal Welded stainless steel
pipe/tube now exceeds that of Seamless pipe/tube.

There are many more producers of Longitudinal Welded stainless steel pipe/tube than for Seamless. Research and development into welding processes and related production methods is continuously undertaken, which results in on-going improvement of the manufacturing parameters and the related properties of the product The advantages of Longitudinal Welded as compared to Seamless stainless steel pipe/tube include:

SURFACE FINISH

Longitudinal Welded pipe/tube is made from cold rolled strip having a 2B surface finish. This is superior to the finish on Seamless pipe/tube, which if supplied as “hot finished” will have a surface similar to a No 1 surface finish, and also will tend to exhibit light scoring or small laps if subsequently subjected to cold drawing operations.

CORROSION RESISTANCE

The sophisticated and precisely controlled processes which are used to pickle and passivate stainless steel cold rolled strip of 2B finish, result in
the development of optimum corrosion resistant properties. The superior surface finish also plays a part in this.

LENGTH OF PIPE/TUBE

As a result of the continuous nature of the Longitudinal Welded manufacturing process there is a greater flexibility and ability to supply both a variety of specific (or multiple) lengths as well as longer lengths

CONCENTRICITY/ECCENTRICITY

Longitudinal Welded stainless steel pipe/ tube is produced utilizing strip which has a very small variation (tight tolerances) from the specified
thickness. This then becomes the exact and consistent wall thickness of the Longitudinal Welded pipe/tube. In the production of Seamless pipe/
tube the mandrel or plug tends to move (float) which results in variable eccentricity, both with respect to variations of wall thickness and the
location of such variations. Refer to Fig 5.

PRICE

There are several comparable factors which influence price.

  • The capital investment required for plant and equipment to produce Seamless as compared to similar sizes of Longitudinal Welded pipe/tube is in the ratio of approximately 25 : 1.
  • Yields are relatively low in the production of Seamless pipe/ tube. Very high yields are attained in the Longitudinal Welded production
    process.
  • Production costs. The pieceby- piece nature of production associated with Seamless pipe/tube increases the production costs to a marked extent compared to those of the continuous inline production of Longitudinal Welded pipe/tube.
  • Size and Wall Thickness. The production of smaller OD and thinner wall pipe/ tube by the Seamless process can involve several cold drawing,
    annealing and pickling operations which are not necessary in the Longitudinal Welded production of similar dimensions.

Production costs are therefore much lower for Longitudinal Welded pipe/ tube of such dimensions. However, as the OD increases up to and in excess of 150mm, and also as the wall thickness increases, the price differential of Longitudinal Welded versus Seamless becomes less significant.

AVAILABILITY AND DELIVERY

Due to the flexibility, versatility of production, and the greater number of suppliers of Longitudinal Welded stainless steel pipe/ tube, the availability
and delivery, in general, is better than that for Seamless stainless steel pipe/ tube.

Further, if non-round sections (squares and rectangles) are required, these can be easily produced “in-line” as the Longitudinal Welded pipe/tube
emerges from the mill using a TURKS HEAD, with associated better cost, availability and delivery.

To produce such non-round sections as Seamless pipe/tube would require separate operations on each individual length.

THE FORMING OF STAINLESS STEEL PIPE AND TUBE BENDING

Bending is a commonly employed  forming process. Refer to Fig 6.

  • Stainless Steel pipe/tube is not easy to bend. It is suggested that the merits of subcontracting to converters specializing in bending be seriously
    considered.
  • Bending induces both tensile and compressive stress/strain into the pipe/tube being bent.

 The material on the outer radius is in tension, which stretches and thins the material. If excessive, the resultant pressure rating will be too low,
or mechanical failure will occur during bending.
 The material on the inner radius is under compression which thickens the material, and may result in wrinkling and buckling.
 The interaction of the tensile and compressive strains may result in flattening of the pipe/tube.
 To prevent the wrinkling, buckling or flattening it is necessary to apply support to both the inner and outer surfaces during bending.

  • The severity or ease of bending is an inter-related function of the dimensions of the pipe/tube and the geometry of the bend. The greater the
    ratio of the OD to the Wall thickness; the lower the power required to effect the bend, but the greater the tendency for mechanical failure, wrinkling and buckling. The smaller the ratio of the Centre Line Radius to the OD; the greater the severity of the bend.

Consideration of this interrelationship will give an indication of the degree and technique of inner and outer support required during bending, and
if the bending of a pipe/tube of given dimensions to a specific bend geometry is possible or practical.

* The properties of the different classifications of stainless steel affect the bending operation.

– The high ductility of the austenitic stainless steel allow for a greater amount of stretching and thinning.
– The marked response of austenitic stainless steel to workhardening increases the power required to effect bending, and the tendency to
wrinkling and buckling.
– Austenitic stainless steel pipe/tube which has been cold worked during manufacture without subsequent annealing, will be work-hardened. Such
material is more difficult to bend because of its higher strength and lower ductility. The relatively low ductility of ferritic stainless steels makes bending more difficult, with an associated limitation to the production of bends having a lower order of severity.

* Heating stainless steel pipe and tube for bending has many inherent disadvantages, and it is recommended that it is not employed. If it is employed
it must be done with extreme care and under closely controlled conditions by experienced persons.

EXPANDING

Expanding is a cold work process in which the diameter is increased over a relatively short length. It is more usually applied to tube, often for heat exchangers and exhaust components.

The high ductility of austenitic stainless steel renders it eminently suitable for expanding purposes. Expanding includes such operations as:

– Flaring of tube ends to suit ferrule tube fittings, or to make a female “slip-over” joint for the simple joining of tube in non-critical applications.
– Flanging of tube ends is an extension of flaring in which the flange is pressed in a second operation from a preformed flare, to be at 90° to the axis of the tube, thus making a simple integrated flange.
– Expanding of tubes into tube plates in heat exchangers.

EXPANDING OF TUBES INTO TUBE PLATES OF HEAT EXCHANGERS

One major application for stainless steel tube produced to specification is in heat exchangers, mainly in the size range of approximately 15-50mm OD.

The tubes may be fixed into the tube plate by expanding which gives a joint of high efficiency and integrity. Therefore, this method is frequently used.

Certain precautions and requirements are mandatory to ensure the efficiency of an expanded joint.

– The tube to tube sheet hole tolerance must be held to within narrow limits. These tolerances are specified in Standards.
– The tube ends and the hole in the tube plate must be perfectly clean and free from any contamination.
– The surfaces of the hole must be free of any mechanical surface damage such as scratches.
– The tube-end surfaces and the ends of the tube must be free of surface defects such as scratches, nicks and notches.
– Rotary hydraulic torque controlled 4 or 5 roll expanders are recommended in order to ensure consistency during expanding.

= It is essential that the expander tooling be maintained in prime condition.

– The maximum length of tube which is necessary to expand is that of the thickness of the tube plate.
– It is recommended that the amount (degree) of expansion should not exceed a reduction of 6% in the tube wall thickness.
= It is suggested that a few test holes are expanded to set the exact amount of torque required to ensure that a leak free joint is obtained.
= Over-rolling of the tubes reduces the mechanical strength and stability of the expanded joint.

CORROSION RESISTANCE

The Corrosion Resistance of stainless steel and the maintenance thereof by good housekeeping and fabrication practices are covered extensively in other modules of the Info Series.

Contamination of, or mechanical/ thermal damage to the surface will adversely affect the passive film and hence may cause the initiation of corrosion. Any effected areas must be repaired by passivation, pickling, or pickling and passivation to restore the corrosion resistance.

APPLICATIONS OF STAINLESS STEEL PIPE AND TUBE PIPE APPLICATIONS

A major use of stainless steel pipe is in pressure piping systems.

For example, in the Petrochemical Industry the cost component attributable to the piping systems can typically represent 20% of the total capital cost of the installed process plant and equipment. Correct specification of the pipe to be used is therefore critical.

— There is well documented literature and computer programmes for the correct sizing of pipe which is a reflection of the importance of this aspect.
— It is essential that all designers involved in the design of the different sections of the plant use the same basis for the calculation of the pipe sizes, and that this is held constant for all phases of the project. If this is not done huge problems can arise in interfacing one piping system with another.
— As cost is an important factor, advantage should be taken of the higher mechanical properties and high corrosion resistance of stainless steel to allow the use of thinner wall thicknesses
— Much of the stainless steel pipe specified is to Specification ASTM A312, which covers both Seamless and Longitudinal Welded Pipe. It should be noted that no different conditions or stipulations are made for the welded pipe, demonstrating the accepted equivalent performance of the welded  pipe.

TUBE APPLICATIONS

A major application of stainless steel tube produced to specification is in heat exchangers.

Heat transfer in stainless steel

Thermal Conductivity is a physical property of a material which is measures the rate at which heat will flow through
the material from surfaces (or areas) at different temperatures.

Stainless Steels have low thermal conductivities and therefore, on a theoretical basis, would appear to be excluded from applications which involve heat transfer, such as heat exchangers

By referring to Fig 7

it may be seen that the major effect on the heat transfer is exerted by the corrosion scales/fouling on the metal surfaces and the gas or liquid “contact films” which exist.

Stainless Steels have, in fact, major advantages for application as heat exchanger tube.

– As a result of their passivity (high corrosion resistance) their surface remains essentially free of corrosion scales.
– The surfaces therefore remain smooth and hence resist fouling.
– The smoothness of the surface minimizes the thickness of the gas and liquid “contact films”.
– The above three factors suffer little, if any, deterioration with time.

Therefore, the efficiency of heat transfer remains over long periods of operation.

– Because of the high corrosion resistance of Stainless Steels, tubes of thinner wall thickness can be used.

The net effect is that heat exchangers tubed with stainless steel have a high long lasting efficiency, and require shorter lengths of tube to affect the
required heat transfer.

CONCLUSION

Other major uses of stainless steel tube are in the Pharmaceutical, Dairy, Food and Beverage industries. As a result of bead rolling the complete
inside surface has a smooth non-porous surface equivalent to the parent cold rolled 2B finish strip. Therefore there is no uneven surface which  would enable the entrapment and growth of bacteria.

In addition, the ease of cleaning and sterilising further contribute to the excellent hygiene factor which is so important in these industries.
Stainless Steel tube is used for a multitude of other applications (e.g. architectural, structural, conveyance of non pressurised, corrosive fluids),
of both a non-critical (e.g. hand rails) or critical (e.g. hydraulic lines) nature.

Such applications may require either no finishing operations (i.e. DOM tube) or a variety of finishing operations (e.g. polishing, flaring).