Production processes for Welded Pipes
Production processes for Welded Pipes
Steel pipes are long, hollow tubes which are used mainly to convey fluid or fluidized products from one location to another. They are produced mainly by two distinct production processes which result in either a welded pipe or a seamless pipe.
Welded steel pipes are produced with either a longitudinal seam or a spiral (helical) seam. The diameters of these pipes range from around 6 mm to 2,500 mm with wall thicknesses in the range of 0.5 mm to around 40 mm.
The starting material for the production of the welded pipes is rolled flat product which depending on the pipe production process, pipe dimensions and application, can be hot rolled (HR) or cold rolled (CR) steel strip/skelp, and HR wide strip or plate. This starting material can be formed into pipe shape in either hot or cold condition. The forming process can be either a continuous process or a single pipe forming process.
In continuous pipe forming process, uncoiled strip material is taken from an accumulator, with the leading end and the trailing end of the consecutive coils being welded together. In single pipe forming process, the pipe forming and welding operation is carried out in single pipe length.
There are two types of welding processes which are mainly used for the welded pipe production. These are (i) pressure welding processes, and (ii) fusion welding processes. The commonly used pressure welding processes are (i) pressure welding process e.g. Fretz-Moon process, (ii) DC (direct current) electric resistance welding (ERW), (iii) low frequency (LF) electric resistance welding, (iv) high frequency (HF) induction welding, and (v) HF conduction welding. The commonly used fusion welding processes are (i) submerged arc welding (SAW), and (ii) gas shielded welding.
|Tab 1 Welded pipe production processes|
|Forming process||Welding process||Type of welding||Weld type||Pipe size range (OD) in mm|
|Continuous process||High pressure welding||Fretz-Moon||Longitudinal||13-115|
|Electric resistance welding (ERW)||1. Direct current (DC)|
2. Low frequency (LF)
3. High frequency (HF)
|Electric arc welding (fusion welding)||1. Submerged arc (SAW)|
2. Gas metal arc (MAG) (for tack welding) 3. Gas metal arc (TIG, MIG, ERW)*
|Spiral Spiral/ longitudinal||168-2,500|
|1. Single forming operation 2. 3-roll bending machine 3. C-ing press||1. Submerged arc (SAW)|
2.Gas metal arc (TIG, MIG, ERW)*
|Longitudinal||500 and higher|
|1. Single forming operation 2. U/O-ing press||1. Submerged arc (SAW)|
2. Gas metal arc (MAG) (for tack welding)
|*Stainless steel (SS) pipe|
Pressure welding processes
The popular pressure welding process is the Fretz-Moon process.
Fretz-Moon process – Fretz-Moon is the name of the inventor of the process. In this process, steel strip in the form of continuous strip is heated to the welding temperature in a forming and welding line (Fig 1). The rollers continuously form the strip into an open seam pipe. After this, the mating edges are pressed together and welded by a process which is based on the forge welding technique. Pipes from 40 mm to 115 mm outside diameter (OD) can be produced by this method. The welding speed ranges from 200 m/min (metres per minute) to 100 m/min respectively.
These days the endless pipe from the Fretz-Moon plant is directly fed to a stretch-reducing mill. This mill is provided in the run out line for rolling the pipe of the same heat to various diameters down to around 13 mm. The pipe is then cut into individual lengths for placement on the cooling beds. This combination provides the advantage that the Fretz-Moon plant can be used for a single, constant pipe diameter, thus eliminating costly roll changing and resetting work.
Fig 1 Schematics of Fretz-Moon forming and welding process for pipes
The HR coils are used as starting material. The coils are uncoiled at high speed and the strip is stored in loop accumulator. The stored material acts as a buffer during the continuous production process. This enables the tail end of the running strip to be butt welded to the head end of the strip of the next coil. The continuous strip is then taken to a tunnel furnace where it is heated to a high temperature. Laterally arranged burners increase the temperature at the strip edges to a welding temperature which is around 100 deg C to 150 deg C higher than the temperature prevailing at the strip centre. The forming roll stand continuously shapes the incoming strip into an open seam pipe, the circumference of which is slightly reduced (by around 3 %) in the downstream squeeze roll welding stand. The welding stand is offset at 90 degrees to the preceding stand. The welding stand produces the upsetting pressure which causes the edges to be pressed together and welded. The weld structure is further compressed in the downstream reducing roller stands again offset by 90 degrees. These reducing roller stands size the pipe. In case there is no stretch-reducing mill then a flying saw located downstream cuts the endless pipe into individual lengths. These cut pipes are conveyed via cooling bed to pipe finishing section.
ERW pipe production processes
Both the direct current (DC) and the alternating current (AC) are used for ERW pipe production. In AC welding processes either LF current or HF current are being used.
DC process– DC process employs the quasi-direct current effect (square wave system). The main characteristic of a square wave welder is that only high voltage/low current is used until the weld point is reached. Electrically, the main difference between the DC and the square wave is that line current is rectified (through a full wave rectifier) without going through a step-down transformer. DC process is used for the longitudinal welding of small pipes upto 20 mm OD (30 mm OD in special cases), with thin wall thicknesses ranging from 0.5 mm to 2 mm. CR strip is used as starting material for this process because of the tolerance requirements.
The advantage of DC welding compared to the LF and HF welding methods is the smooth finish of the inside pass with small ridging (reinforcement). This is important when the pipe needs a smooth inside weld and where it is not possible to remove the welding flash (example heat exchanger pipes).
The range of application for the DC process is limited by the electrical power which can be transmitted by the disc electrodes used in the process. The welding speed which is obtained in the process ranges from 50 m/min to 100 m/min. Pipes which are produced by the DC process are normally cold stretch-reduced subsequently. Due to it, the thickness of main body of the pipe is increased slightly more than that of the weld zone resulting into pipe showing virtually no internal weld protrusion at all.
LF process –In the LF process, welding is done with AC in the frequency range between 50 Hz (hertz) to 100 Hz. An electrode consisting of two insulated discs of an alloy of copper (Cu) is used not only for the power supply but also as the forming tool and the element which generates the required welding pressure (Fig 2). The process is used to produce longitudinal welded pipes in the diameter range of 10 mm to 115 mm at welding speeds of upto 90 m/min depending on the wall thickness.
Fig 2 Low frequency pressure resistance welding
The electrodes are the critical component of the process since they are to be provided with a groove which matches the diameter of the pipe being produced. Also, this groove is to be constantly monitored for wear during the operation.
The material during the pressure welding process forms an inner and outer flash along the weld zone which is required to be removed in line downstream by internal and external trimmers. The process can produce welds of a high degree of perfection subject to the process being carefully monitored to meet the various requirements.
HF processes – HF electric resistance welding process for the production of pipe was introduced in 1960s. The process involves application of HF AC electric current in the range of 200 kHz (kilo hertz) to 500 kHz. The pipe forming and energy input operations are performed by separate units. The strip is shaped in a roll forming mill or in an adjustable roll stand (natural function forming) into an open seam pipe for a wide range of pipe products. These include line pipes and structural pipes in the size ranges of around 20 mm to 600 mm OD and wall thickness range of 0.5 mm to 16 mm, and pipe blanks for a downstream stretch-reducing mill. The starting material is HR wide steel strip or skelp. Depending on the pipe dimension and application, and particularly in case of precision pipes, the steel strip can undergo an upstream pickling operation, or CR strip is used. The coils are uncoiled at high speed and the strip is stored in loop accumulator. The stored material acts as a buffer during the continuous production process. This enables the trailing end to be butt welded to the leading end of the strip of the next coil. The pipe welding machine operates continuously at a speed ranging from 10 m/min to 120 m/min by drawing strip from the loop accumulator.
Fig 3 shows the principle of a roll forming mill. The roll forming mill is used for pipe diameters upto 600 mm maximum. It normally consists of 8 to 10 largely driven roll forming stands in which the strip is gradually shaped in stages (1 to 7 stands) into an open seam pipe. The last three passes (8-10 stands) are usually finish passes which guide the open seam pipe towards the welding table (11). The forming rolls are to be precisely matched to the final pipe diameter. In case of large diameter pipe, the natural function forming process can also be applied.
Fig 3 Principle of roll forming process
The main features of a forming roll stand is that a number of non-driven internal and external forming rollers, adjustable in a wide product diameter range, are arranged in a funnel shaped forming line which gradually bend the strip into an open seam pipe shape. Only the break down stand at the inlet and the finishing pass stands at the exit end are actually driven. A schematic view of the roll stands in a roll forming mill is given at Fig 4. In the figure certain cross sectional details are shown which indicate the degree of deformation n and the arrangement of the forming rollers at various sections along the line.
Fig 4 Schematic view of the roll stands in a roll forming mill
The welding process uses simultaneously pressure and heat in order to join the strip edges of the open seam together without the use of a filler material. Squeeze and pressure rolls in the welding stand brings the edges of the open seam pipe gradually together and apply the pressure necessary for welding. There are several advantages with the use of HF AC electric power as energy source for generating the heat needed for the welding process. As an example, it has the advantage over normal AC power of generating a very high current density (flux) over the cross section of the conductor. Due to its HF, the HF current has the effect of building up a magnetic field at the centre core of the conductor. The ohmic resistance of the conductor is highest in this field, so that the electron follows the path of least resistance at the outer surface region of the conductor (skin effect). Hence, the current flows along the strip edges of the open seam pipe to the point at which the strip edges adjoin (welding point), and the ensuing concentration, promoted by the proximity of the negative conductor, results in a high level of energy utilization. Below the Curie point (768 deg C), the depth of the current penetration only amount to a few hundredths of a mm. Once the steel is heated above this temperature, it becomes non magnetic and the current penetration depth rises to several tenths of a millimeter at frequencies in the region of 450 kHz.
The welding current can be introduced into the open seam both by the conductive means by using sliding contacts or by inductive means using single or multi wind coils. Accordingly, a distinction is made in the nomenclature between HF induction (HFI) welding and HF conduction welding. A schematic view of the HF welding of pipe is shown in Fig 5.
Fig 5 HF welding of pipes
In recent times, in order to accommodate small production batches, high strength low alloy (HSLA) steel grades, and extreme wall thickness/diameter ratios, a straight edge forming process has been developed. In this process, instead of the bottom forming rollers, roller straight edges are used. This has resulted into substantial reduction in the length of the forming line. This process can form pipes with wall thickness/diameter ratios ranging from 1:8 to 1:100.
The increase of the mill efficiency is achieved by reducing the conversion time from one pipe size to other pipe size through the introduction of centralized tool adjustment (CTA) forming process. All the rollers of the forming line are mounted in a beam and are adjusted through the CTA process by a single motor. This means that throughout the size range no forming roller (tool) changes are necessary. This results in considerable reduction in size conversion and set up time.
Before the entry of the strip in the forming section, the strip is straightened and trimmed to a constant width by a longitudinal edge trimmer. The cut edges can be additionally bevelled for the welding preparation. The strip is then formed into an open seam pipe and with the gap still relatively wide, is fed through the finishing pass stands to the welding table. The overhead finishing rolls, the width of which is tapered towards the welding point determines the gap entry angle and control its central position in the welding table. There the converging strip edges are pushed against each other by the shaped squeeze rolls and then welded by means of the HF electric resistance process. The current can be transferred either inductively through an induction coil arranged around the open seam pipe or conductively through sliding contacts running around the open seam pipe.
The external and internal ridges which occur during the pressure welding of pipes with (ID of around 30 mm and higher), are normally trimmed by planning or scraping the material when it is still hot. The pipe is then rounded and sized in between two to six sizing stands. These stands are usually designed for circumferential reduction. The process also causes straightening effect on the pipe. The addition of a multi-strand shaping roll sizing unit in the pipe run out section of the mill can also enable the round pipe to be directly formed into specialty sections.
The trimmed weld is examined through the non-destructive testing and the pipe is cut into the desired lengths by a flying cut off machine. The cutting of the pipe can be done either of the methods namely (i) by breaking the pipe off at a narrow inductively heated zone, (ii) rotational cutting by the disc type blades, or (iii) by cold or friction parting off saws.
The HF pressure weld can either is left in its as welded condition or subsequently heat treated in the normalizing range depending on the application of the pipe. Partial inductive annealing of the weld can also be performed on the continuous pipe, or the individual pipe can be subjected to a separate heat treatment following its cutting to length.
In the HF induction welding process, welding speeds of upto 120 m/min can be obtained depending on the wall thickness and the application. The process is shown in Fig 5. The HF conduction welding is also known as ‘Thermatool’ process. In this process, welding speeds of upto 100 m/min can be achieved depending on the wall thickness and the application. The process is also shown in Fig 5.
Fusion welding processes
Fusion welded steel pipe is normally used for the production of large diameter pipe for pipeline construction. The pipe forming processes used for in case of fusion welding are (i) the three roll bending process for plate forming, employed as either cold or hot forming process, (ii) the C-ing press process for cold forming of plate, (iii) the U-ing and O-ing press process for cold forming of plate, and (iv) the spiral pipe forming process for cold forming of wide strip or plate. These processes are shown in Fig 6.
Fig 6 Pipe forming processes
Out of the above four processes, the last two processes namely (i) the U-ing and O-ing press process for cold forming of plate, and (ii) the spiral pipe forming process for cold forming of wide strip or plate are the frequently used processes in most of the production facilities of today.
The submerged arc welding (SAW) process or a combination of gas shielded tack welding with downstream submerged arc welding is widely accepted as the standard method for welding of the large diameter pipe. The fusion welding processes are also used in the production of spiral and longitudinal welded pipe of high alloy stainless steels. The product in this case is in the form of thin walled pipe in the diameter range of around 10 mm to 600 mm. Apart from pure TIG (tungsten inert gas welding) process, various combined welding methods are also used. For example, these are TP (tungsten plasma) arc welding + TIG, TP + MIG (metal inert gas) welding, and TP + SAW processes.
SAW process – The SAW process is an electric fusion welding method which is carried out with a concealed arc. In comparison to arc welding with welding electrodes, the arc in the SAW process is hidden under a blanket of slag and flux. One of the characteristics features of the SAW process is its high deposition rate, which essentially stems from the high current strength which is applied combined with a favourable heat balance.
The filler metal used takes the form of coiled, bright welding wire which is fed continuously in the liquid metal pool dictated by the deposition rate. Just above the parent metal (pipe), the welding current is conducted by the sliding contacts into the wire electrode and returned through the ground lead connected to the pipe material (Fig 7)
Fig 7 Submerged arc welding process
The arc causes the incoming wire and the open seam edges to melt. A part of the continuously fed welding flux is also melted by the heat of the arc, causing it to form a liquid covering of slag which shields the weld pool, the melting wire electrode, and the arc itself from the atmospheric influences.
In addition, the welding flux also felicitates formation of the weld bead and serves as a donor of the alloying elements in order to compensate for the melting and the oxidation losses. In many cases, it is also used specifically to alloy the weld metal in order to impart to it specific chemical and mechanical properties. After the movement of the arc, the liquid slag, which is left behind, solidifies. The welding flux which does not melt is recovered by vacuum extraction and reused. The slag is easily removed once it is solidified. The chemical composition of the wire electrode and the welding flux is required to match the material being welded. The SAW welding of pipes is normally done by two pass method (i.e. first run followed by sealing or backing run) and is generally performed with the inside pass first followed by the outside pass second. This ensures that the two passes sufficiently overlap.
The result is a fusion weld which generally does not need any further heat treatment. Welding with SAW can be done both with AC and DC and in multi-wire systems where a combination of AC and DC can be used. The efficiency of the SAW process is given by the rate of the filler metal deposited per unit time (rate of deposition). As a result, a very high welding speed is possible.
The rate of deposition can be increased by increasing the welding current. However, owing to the limited current carrying capacity of the flux, the performance can be enhanced in single wire welding upto a maximum input of around 1200 A (amperes). Any increase in the rate beyond this limit needs the deployment of several wire electrodes. This then allows a higher overall current to be applied for the welding work without the danger of the current carrying capacity of the flux being exceeded at any of the individual wire electrodes. In practical operations, increased performance is obtained by using a multi-wire welding configuration with 2, 3 or 4 electrodes. The higher rate of deposition achieved with multi-wire welding results into a higher welding speed under practical welding conditions.
With the use of high performance fluxes, the three wire welding process is normally sufficiently efficient for wall thickness upto 20 mm. in case of wall thicknesses higher than 20 mm; fourth wire is needed for the maintenance of the welding speed and thus for achieving the production efficiency. A requirement for the cost- effective application of the multi-wire welding is that the process parameters are to be optimized for ensuring reliable achievement of the specified quality requirements of the weld. In practice, the welding speeds ranging between 1 m/min to 2.5 m/min can be achieved depending on the welding process, wall thicknesses, and type of flux used.
Gas shielded arc welding – It is also an electric fusion welding process. In this process, the weld pool is produced by the effects of an electric arc. The arc is quite visible as it burns between the electrode and the work piece. The electrode, arc, and weld pool is protected against the atmosphere by an inert or active shield gas which is constantly fed into the weld area.
The gas shield arc welding processes are classified according to the type of electrodes and the gas used. These are normally divided into two main categories. The categories are (i) gas tungsten arc welding (GTAW) namely TIG, TP, and THG (tungsten hydrogen gas) arc welding, and (ii) gas metal arc welding (GMAW) namely MIG, and MAG (metal active gas) welding. The processes mainly used for the production of pipes are TIG, MIG, and MAG welding processes. TIG and MIG welding processes are mainly used for SS pipe production. In the TIG welding process, the arc burns between a non melting tungsten electrode and the work piece. Any filler metal is fed mainly without any DC input. The shield gas flows from a gas nozzle and protects the electrode, filler metal, and the liquid pool from contact with the atmospheric air.
The shield gas is inert normally argon (Ar), helium (He), or a mixture of these gases. In the MIG and MAG processes, in contrast to the TIG process, the arc burns between the work piece and a melting, consumable electrode which provide the filler metal. The shield gas used in the MIG welding is inert normally Ar, He, or a mixture of these gases. In case of MAG welding process, the shield gas is active and consist of pure CO2 (carbon di-oxide), or of a gas mixture made up of CO2, Ar, and O2 (oxygen). MAG process is increasingly used for tack-welding in the production of large diameter longitudinal and spiral welded pipes. The tack weld also serves as the weld pool backing for the subsequent SAW process. The requirements of an optimum weld are a precise edge preparation (double V butt joint with wide root faces), and a good, continuous tack weld. In large diameter pipe production, the welding speeds for the tack weld ranges from around 5 m/min to 12 m/min.
Production of longitudinal welded pipe (U-ing/O-ing process)
The plates used for the longitudinally welded pipes are formed on presses having open dies for the U-ing and closed dies for the O-ing operations. The process is also called UOE forming process (U-ing, O-ing and expanding) and is used for the production of longitudinally welded large diameter pipe in individual lengths upto 18 m. Modern plants using this process are designed for a pipe diameter ranging from around 400 mm to 1,620 mm, and the wall thicknesses ranging from 6 mm to 40 mm. The starting material is steel plates. The material flow in the process indicating the important operational and inspection stages during the production of large diameter pipes by UOE process of forming is shown in the Fig 8.
Fig 8 Flow of material in the production of large diameter pipes by UOE method of forming
At the start of the welding process, run-in and run-off tabs are welded on the flat plates in order to ensure the lead and the tail phenomena associated with SAW process occur outside the pipe metal. Before the plate is bent into an open seam pipe by the various stages and the forming presses used, the two longitudinal edges undergo machining by a planer machine for ensuring that they are parallel. The welding bevel needed for the concerned plate thickness is also cut.
In the first forming stage, the plate is crimped in the area of its longitudinal edges. The bending ratio corresponds roughly to the diameter of the open seam pipe. Crimping is performed in the special forming presses. In the second stage, the plate is bent into a U shape in one operation involving a circular radius tool pushing the plate down between two supports. Towards the end of the operation, the distance between the supports is reduced in order to apply a small degree of over-bend for countering the spring-back effect. In the third forming operation, the U shape is placed in the O-ing press to produce in a single operation, the round open seam pipe.
The forming processes carried out in the U-ing and the O-ing presses are coordinated so as to ensure that the spring back effect is effectively countered and the open seam pipe is as circular as possible with the longitudinal edges flushing. These operations need high press loads.
The edges of the open seam are then pressed together (eliminating any offset) in tack- welding stands, which is normally designed in the form of roller cages, and then joined by a continuous seam deposited by automatic MAG welding machines. Depending on the thickness of the pipes, the applied welding speeds can be in the range of 5 m/min to 12 m/min.
The tack welded pipes are then conveyed by a roller table and distribution system to the SAW stands, where, at separate lines, they are provided first with the inside and then with the outside pass. These runs are deposited by moving the pipe on a carriage under a stationary welding head. For inside pass, the welding head is mounted on an arm which extends inside the pipe, In order to preclude the possibility of weld offset; both the outside and inside heads are continuously monitored and controlled for perfect alignment to the weld centerline. Any of the multi wire SAW processes can be used, depending on the pipe dimensions (diameter and wall thickness). After welding, the pipes are sent to the finishing section.
The pipes after welding normally do not satisfy the tolerance requirements with respect to diameter and roundness. Hence, in the finishing department, the pipes undergo a thorough inspection, and are sized by cold expansion. This operation is carried out by mechanical or hydraulic expanders. The amount of expansion applied is around 1 %, and this value is considered when determining the initial circumference of the open-seam pipe. The production process is completed in the finishing department with the machining of the pipe ends and carrying out any necessary rework.
The pipes are subjected to a hydraulic test before the pipe end machining operation. Then, a final ultrasonic (US) examination is carried out over the entire length of the weld zone. Indications revealed by this automatic US examination and also the weld regions at the end of the pipes are further checked by X-ray inspection. All the pipe ends are also US inspected for laps and laminations.
Production of spiral pipes
The spiral pipes are also known as helical seam pipes. During the production of spiral pipes, the hot strip or sheet is continuously shaped into a pipe by a spiral forming facility applying a constant bending radius, with abutting strip edges also being continuously welded inline.
In comparison to the longitudinally welded pipe production, in which each pipe diameter needs a certain width of the strip, spiral pipe production is characterized by the fact that pipes with different diameters can be produced from a single strip width. This is because the approach angle of the strip as it is fed to the forming unit can be changed. The smaller is this inlet angle, the larger is the diameter of the pipe from the same strip. The technical optimum ratio of the pipe diameter and the strip width ranges from 1:2 to 1:2.2. There is mathematical dependence between feed angle, strip width, and pipe diameter which apply in case of spiral pipe production. This mathematical dependence is shown in Fig 9.
The range of pipe diameters produced by the spiral welding process technology lies in the range of around 500 mm to 2,500 mm. The starting material used for spiral welded pipe production is normally wide HR strip with upto 20 mm wall thicknesses. For pipe thicknesses in excess of 20 mm, plates in individual lengths upto 30 m are generally needed.
There are two types of facilities associated within the production of spiral pipes. These facilities can consist of (i) integrated forming and SAW pipe processing line, and (ii) separated forming and SAW pipe processing lines (Fig 9).
Fig 9 SAW pipe processing lines for spiral pipe production
Integrated forming and SAW welding line – The integrated forming and SAW welding line is the conventional production facility for the spiral pipe production. In this type of facility, the production process consists of (i) a strip preparation stage, and (ii) a pipe forming operation combined with simultaneous inside and outside pass SAW unit. Apart from the welding the strips together, the strip preparation stage also straightens the strip and trim it to the exact width. The strip edges need to be accurately machined within close tolerances, and a defined edge crimping is also needed to be performed in order to prevent impermissible ridge formation/peaking if pipe formation is to be successful.
The strip being fed in from the uncoiler is joined to the trailing end on the previous coil by SAW process. The weld is deposited along the face which later is going to form the inside surface of the pipe. The outside SAW pass is deposited in a separate line on the finished pipe. The strip then run through a straightening mill and is cut to a constant width by an edge trimmer. Additional device also bevel the edges in preparation for the main SAW welding operation. Before entry in the forming section, the strip edges are crimped in order to avoid ridging/peaking at the join.
In the integrated line, the strip preparation stage is immediately followed by the forming process with simultaneous inside and outside welding by the SAW process. A pinch roll unit feeds the strip at a predetermined entry angle into the forming section of the machine.
The purpose of the forming section is to bend the exactly prepared strip of width ‘A’ at a certain feed angle into a cylindrical pipe of diameter ‘D’ in line with the mathematical relationships as given in Fig 9.
There are many forming methods which are used to produce the spiral weld pipes. Apart from the direct shoe method which has its limitations, the two main spiral welding techniques (Fig 10) which are normally used are (i) three roll bending with an inside diameter roller cage, and (ii) three roll bending with an outside diameter roller cage. In a three roll bending system, several individual and guiding rollers are used rather than a single forming roll.
Fig 10 Two main spiral welding techniques
The roller cage serves to fix the pipe axis and maximize the roundness of the pipe in order to ensure offset free convergence of the strip edges at the welding point. This facilitates attainment of accurate pipe dimensions, so that the pipe leaving from the machine is already produced to within the standardized diameter, roundness, and straightness tolerances. Hence, expansion/sizing of the pipes after welding is not necessary.
In the spiral pipe forming and SAW machine, the converging strip edges are first inside welded at around the 6 o’clock position and then, half a pipe turn further, outside welded in the 12 o’clock position. Welding head alignment to the weld centre and gap control is performed automatically.
The produced pipe string is subsequently cut to length by a flying parting off device. The individual pipe is then sent to the finishing section where the production process is completed by machining of the pipe ends and by the performance of any rework. Before the pipe edge machining, a hydraulic test is conducted on the pipe. The entire weld region is then US inspected, with the weld zone at the pipe ends are also examine by X-ray. In addition, each pipe is US inspected over its full circumference for laps/laminations. If needed, the weld zone and the parent metal are also US inspected before the hydraulic test.
The productivity of this process is determined by the speed of the SAW operation. The pipe formation is normally capable of substantially higher production rates. In order to utilize the efficiency of the spiral pipe forming section, the plants are now designed on the basis of separate forming and SAW lines. In such plants, the forming line features a tack welding facility which has a capability of production speed matching with the speed of the forming line. The SAW line performs the seam welding off line in a number of several separate welding stands which match the forming line productivity.
Separated forming and SAW welding lines – The main feature of plants having separated forming and SAW welding lines is that there are two independent production units consisting of (i) pipe forming with internal tack welding, and (ii) inside and outside SAW operation on separate welding stands.
Apart from higher cost efficiency of such plant achieved due to the faster forming and tacking operation, there are also technical advantages of separating the two operations. These advantages are since both the operations can be individually optimized.
In the spiral pipe forming section, the merging strip edges (one on the already formed pipe section and the second on the incoming strip) are continuously joined by the inside tack welding. The tack welding process is performed by MAG welding at a speed of 12 m/min in the region of 6 o’clock position. CO2 is normally used as shield gas. The weld edges below the welding position run with virtually no gap over a rigidly fixed guide roller.
A parting- off device cuts the tack welded pipe string into the required individual lengths. This pipe cutting process is the last operation performed in the spiral forming line. Since the tack welding speed is high, it is necessary to use high speed plasma torch operating with water injection in place of conventional oxy-acetylene torch. The cut to length pipes are sent for the SAW operation downstream which is carried out in combined two pass SAW stands for final welding.
A special roller table rotates the pipe in precise accordance with its spiral joint, thus enabling SAW heads to perform first the inside welding and then the welding outside. Precise weld centre line alignment control of the inside and the outside welding heads is needed in this operation in order to minimize weld offset. The two wire or three wire methods are used for the inside and outside pass welding operations.
Apart from a few modifications, the subsequent stages of production such as pipe end machining, hydraulic testing as well as non destructive examinations and mechanical tests are in principle the same as those used in the integrated spiral welding lines. The feedbacks of these tests and inspections are immediately given to the individual production stages in order to ensure continuous product quality optimization.