Graham Fry

Welding of austenitic stainless steel – Part 1

There are a number of different types of steels that may be referred to as ‘stainless’; previous articles have considered ferritic and precipitation hardening steels for example. It is therefore advisable to be specific and to refer to the group to which the steel belongs in order to avoid confusion.

Although commonly referred to as ‘stainless steel’, the steels covered in this article should be more correctly referred to as austenitic, 18/8 or chromium-nickel stainless steels.  As with the other types of stainless steels, the austenitic stainless steels are corrosion and oxidation resistant due to the presence of chromium that forms a self-healing protective film on the surface of the steel. They also have very good toughness at extremely low temperatures so are used extensively in cryogenic applications. They can be hardened and their strength increased by cold working but not by heat treatment.

They are the most easily weldable of the stainless steel family and can be welded by all welding processes, the main problems being avoidance of hot cracking and the preservation of corrosion resistance.

A convenient and commonly used shorthand identifying the individual alloy within the austenitic stainless steel group is the ASTM system. This uses a three digit number ‘3XX’, the ‘3’ identifying the steel as an austenitic stainless, and with additional letters to identify the composition and certain characteristics of the alloy eg type 304H, type 316L etc; this ASTM method will be used in this article.

Typical compositions of some of the alloys are given in Table 1. The type 304 grade may be regarded as the archetypal austenitic stainless steel from which the other grades are derived and changes in composition away from that of type 304 result in a change in the identification number and are highlighted in red.

Table 1 Typical compositions of some austenitic stainless steel alloys

The 3XX may followed by a letter that gives more information about the specific alloy as shown in the Table. ‘L’ is for a low carbon austenitic stainless steel for use in an aggressive corrosive environment ; ‘H’ for a high carbon steel with improved high temperature strength for use in creep applications; ‘N’ for a nitrogen bearing steel where a higher tensile strength than a conventional steel is required.

These suffixes are used with most of the alloy designations eg type 316L, type 316LN, type 347H, where the composition has been modified from that of the base alloy.

Austenitic stainless steels are metallurgically simple alloys. They are either 100% austenite or austenite with a small amount of ferrite (see Table 1). This is not the ferrite to be found in carbon steel but a high temperature form known as delta (δ) -ferrite. Unlike carbon and low alloy steels the austenitic stainless steels undergo no phase changes as they cool from high temperatures. They cannot therefore be quench hardened to form martensite and their mechanical properties to a great extent are unaffected by welding. Cold (hydrogen induced) cracking (Job Knowledge No. 45) is therefore not a problem and preheat is not necessary irrespective of component thickness.

Alloying elements in an austenitic stainless steel can be divided into two groups; those that promote the formation of austenite and those that favour the formation of ferrite. The main austenite formers are nickel, carbon, manganese and nitrogen; the important ferrite formers are chromium, silicon, molybdenum and niobium. By varying the amounts of these elements, the steel can be made to be fully austenitic or can be designed to contain a small amount of ferrite; the importance of this will be discussed later.

In 1949 Anton Schaeffler published a constitutional or phase diagram that illustrates the effects of composition on the microstructure. In the diagram Schaeffler assigned a factor to the various elements, the factor reflecting the strength of the effect on the formation of ferrite or austenite; these factors can be seen in the diagram.

The elements are then combined into two groups to give chromium and nickel ‘equivalents’. These form the x and y axes of the diagram and, knowing the composition of an austenitic stainless steel, enables the proportions of the phases to be determined.

Fig 1. Shaeffler diagram (A-austenite; M – martensite; F – ferrite)

Typical positions of some of the commoner alloys are given in Fig.1. Also superimposed on this diagram are coloured areas identifying some of the fabrication problems that may be encountered with austenitic stainless steels.

Although all the austenitic stainless steels are sensitive to hot cracking (see the Job Knowledge article on Solidification Cracking), the fully austenitic steels falling within the vertically blue area in Fig.1 such as type 310 are particularly sensitive.

The main culprits are sulphur and phosphorus. To this end, these tramp elements have been progressively reduced such that steels with less than 0.010% sulphur and phosphorus less than 0.020% are now readily available. Ideally a type 310 or type 317 alloy should have sulphur and phosphorus levels below some 0.003%.

Cleanliness is also most important and thorough degreasing must be carried out immediately prior to welding.

The steels such as type 304, type 316, type 347 that fall within, or close to, the small uncoloured triangular region in the centre of the diagram contain a small amount of delta-ferrite and, whilst not being immune to hot cracking, have improved resistance to the formation of sulphur-containing liquid films. The reasons for this are that a) ferrite can dissolve more sulphur and phosphorus than austenite so they are retained in solution rather than being available to form liquid films along the grain boundaries and b) the presence of quite a small amount of ferrite increases the grain boundary area such that any liquid films must spread over a greater area and can no longer form a continuous liquid film. The 100% austenitic steels do not have this advantage.

One problem that has arisen with very low sulphur steels is a phenomenon known as ‘cast to cast variation’ or ‘variable penetration’. The weld pool in a low sulphur steel (<0.005%) tends to be wide with shallow penetration; a steel with sulphur over some 0.010% has a narrower, more deeply penetrating weld bead.

This is generally only a problem with the use of the fully automated TIG welding process, a manual welder being capable of coping with the variations in penetration due to the differences in sulphur content in different casts of steel. However, automated TIG welding procedures developed on a ‘high’ sulphur steel, when used to weld a low sulphur steel may result in lack of penetration type defects; the reverse situation may result in excessive penetration. Changes to the procedure that have mitigated, but never eliminated, this problem have included slow travel speed, pulsed current, use of Ar/H2 shield gas mixtures.

Other methods include specifying a minimum sulphur of, say, 0.010% or segregating the steels into batches with known penetration characteristics and developing welding procedures to suit. The A-TIG activated flux process has also been found to be of benefit.

The problems of welding the fully ferritic steels that fall into the pink area, where grain growth and embrittlement is a problem, have already been dealt with in Job Knowledge – Welding of ferritic/martensitic stainless steels.

The austenitic stainless steels falling into the yellow area will also embrittle but this is as a result of the formation of hard brittle phases called ‘sigma’ (σ) and ‘chi’ (χ). This embrittlement takes place in the temperature range of approximately 500 to 900°C. It is a sluggish process and is not a problem during welding of the austenitic stainless steels, but can occur in elevated temperature service or if the welded component is stress relieved.

Flux cored arc welding

The previous two  articles dealt with the mechanics of costing a weld: how to calculate the weld volume and how to calculate the amount of welding consumables required to fill a weld preparation.

The final step in costing a weld is to determine the length of time to deposit this weight of weld metal. This is obviously a function of the deposition rate of the process.

The deposition rate is generally expressed as kgs/hr or lbs/hr deposited at a given welding current, welding continuously and without any breaks for electrode changing or deslagging.

The deposition rate will be affected by many factors and it will not be possible within the limitations of these articles to list the precise deposition rates for any specific process or welding current. Such data can be found in publications referenced below or by a web search. The ranges of approximate deposition rates for the commoner arc welding processes are listed in Table 1.

Table 1 Indicative deposition rates – arc welding processes

To obtain an accurate figure for the specific parameters to be used is a relatively simple exercise. Weighing a plate, depositing weld metal using the required parameters on this plate for a fixed time and then re-weighing the plate will give an accurate figure that may be used for estimating purposes.

There is one golden rule for minimising the cost of making a weld and, whilst this may seem to be self-evident, it is worth repeating: deposit the minimum amount of the highest quality weld metal with the largest gauge electrode or wire at the highest current in the shortest possible time. This is obviously the ideal and can seldom be achieved in practice because of limitations on heat input, access etc.

The implications of applying the golden rule are:

1. To deposit the minimum amount of weld metal the designer, aided by the welding engineer, must select the smallest weld preparation that is capable of providing the required weld quality. If the included angle is too narrow then lack of side wall fusion is a possibility with the consequent costs of repair; too wide an angle is wasteful in terms of deposited weld metal. Remember, though, that the cost of providing a weld preparation (by flame-cutting, edge planning, milling etc) must also be included in any costing exercise as must the cost of assembly. Machined weld preparations are more accurate than flame cut edges and this may result in faster set-up times and a reduced weld repair rate. It may be possible to use a square edge preparation by using the deep penetration characteristics of some of the welding processes; electron beam and laser welding are the best examples of this technique. Plasma-TIG and activated flux TIG can penetrate up to 10mm in a single pass; the ‘finger’ penetration of spray transfer MAG welding can penetrate up to 6mm and a submerged arc weld can penetrate up to 15mm. There is also the benefit when using a square edge preparation in that the consumption of filler metal is substantially reduced, the bulk of the weld metal being provided by the parent material. The final option on reducing costs when butt welding is for the designer to specify a partial penetration joint. The most expensive weld pass in any full penetration butt weld is the root pass and if this can be eliminated by using partial penetration joints then substantial savings can be made. However, the decision to use partial penetration welds should not be taken lightly but only if service conditions permit the presence of a large crevice at the weld root.

The designer will therefore need to consider whether fatigue, creep, corrosion etc are likely to occur and must clearly specify where the joints are permitted and the minimum acceptable weld throat.

2. Depositing the highest quality weld metal infers that the weld repair rate will be reduced. Repair weld metal is very costly, particularly if the unacceptable defects are detected late in the fabrication programme; perhaps after final assembly where access is difficult or after post weld heat treatment. Accurate weld preparations and fit-up, easy access for the welder, welds made in the flat position and well trained welders will all help to minimise the weld repair rate.
3. Depositing weld metal with the largest electrode or wire at the highest current will obviously give the highest weld deposition rate and shortest joint completion time. The deposition rate figures in Table 1give the minimum and maximum deposition rates at minimum and maximum welding currents. As an example, a 1.2mm diameter MAG wire at 120amps will deposit around 1.2 kgs/hr, at 380amps around 8 kgs/hr. To enable high welding currents to be used the item must be placed in the flat position and there must be easy access for the welder. One benefit of using the high welding currents is that the number of weld runs to fill the joint will be reduced and this, in most circumstances, will result in less distortion than a large number of low current weld passes. Remedial work to correct distortion can therefore be reduced. A further benefit when welding the ferritic steels is that high current and therefore high heat input may allow any preheat to be reduced or eliminated entirely.

However, there are limitations to this approach to improving productivity. If achieving high toughness is a factor then it is likely that heat input will need to be controlled when welding the ferritic steels, placing a limit on the welding current and travel.

speed. High welding currents also imply a large, fluid weld pool and it may not be possible to control this pool when welding in any other than the flat position – for example, MAG welding cannot be performed using spray transfer (high welding current) in the vertical position due to the absence.

of a flux to hold the pool in place. Using a manual process at such high currents also results in increased welder fatigue resulting in a reduced duty cycle. A solution to this problem is to mechanise or automate the process.

To achieve the most cost effective solution to producing a welded structure is therefore not simply to increase duty cycle or deposition rate but to consider all aspects of fabrication from the design stage to final inspection, involving all members of the team from designer to welder.

This article was written by Gene Mathers.

References:

Procedure Handbook of Arc Welding, Publication –  Lincoln Co.

Standard Data for Arc Welding, Publication – TWI (out of print)

Welding Handbook Vol 2 Welding Processes, Publication –  American Welding Society

Copyright © TWI Ltd 2014

The content of this article was correct at the time of publication. For more information visit: www.twi-global.com

Part 1 of this article, dealt with the methods of determining the weight of deposited weld metal in a joint, enabling the cost of welding consumables to be calculated.

This is obviously the first step towards calculating the cost of actually making a welded joint but there are many other factors that need to be considered but which are beyond the scope of these articles.

The most significant of these costs is the overhead; the cost of providing a welding workshop or site and the costs of managing and running the organisation.

These costs are dependent on the accounting practices of the organisation. They comprise factors such as rent, rates, bank interest, cost of indirect workers, i.e. those not directly involved in fabricating, depreciation of plant etc.

In addition, other accounting decisions (for example, where the costs of machining and assembly are absorbed) may affect the decisions on which is the most cost-effective joining method.

One of the most significant costs is that of labour and this inevitably varies with industry, time and country. The costs mentioned above cannot generally be influenced by the decisions made by a welding engineer. These articles will therefore concentrate on those aspects of welding activities that are not subject to accounting practices, overhead or labour costs.

There are many costs, other than the cost of depositing weld metal that will affect the price of a welded fabrication.

The work done by the designer in designing the most cost-effective joint in an item that can be placed in the most advantageous position for welding will have major effect on costs. For example, the type of joint preparation the designer selects; a single or double-V preparation can be flame cut, a J-preparation must be machined and is generally far more expensive. A machined J-preparation, however, may have less volume than a single-V, depending on thickness; will be more accurate and therefore quicker to assemble within tolerance and may result in a lower repair rate leading to a lower cost than the V preparation.

Costs that are directly affected by welding engineering decisions, in addition to the cost of actually depositing weld metal, are therefore; joint preparation, assembly time (which includes positioning in any jig or fixture and tacking), cleaning and dressing the weld, removal from jigs or fixture, post weld heat treatment, costs of non-destructive testing and cost of repairs. The amount of weld metal deposited is rarely the same as the amount of filler metal purchased. This is the result of losses when, for example, GMAW or submerged arc welding wire is trimmed back to the contact tip, when the wire reel runs out and the length of wire between the drive roll and the contact tip is scrapped or the wire or reel is damaged.

Such losses tend to be quite small but this is not the case with coated electrodes. Damaged flux coatings, incorrectly stored electrodes and the stub ends discarded by the welder all contribute to as much as a third of the purchased weight of manual metallic arc electrodes being scrapped. Some electrode manufacturers’ catalogues give figures for these losses which can vary depending on electrode type and diameter.

To assist in calculating the amount of welding consumables to be purchased Table 1 gives some multiplication factors for the more common arc welding processes. The weight of weld metal in the joint should be multiplied by this factor to give the amount of welding consumable required. These figures assume good housekeeping and shop floor discipline such that consumables are not wasted or scrapped unnecessarily.

Table 1 Multiplication factor. Weight of weld metal to give the weight of filler metal required.

The other consumables in this cost equation are shielding gases or flux.

The conventional shoulder height welding gas cylinder contains approximately 10,000 litres of shielding gas at a pressure of 200bar. As the gas flow rates normally used in production are around 12 to 15 litres per minute, this typical cylinder should provide in the region of 10 to 12 hours of welding time, allowing for losses at the beginning and end of the arcing period.

The rate of flux consumption in submerged arc welding is approximately 1kg of flux for every 1kg of deposited weld metal.

This assumes good housekeeping and an efficient flux recirculation system. Calculation of the amount required (and hence the cost) of these consumables is therefore relatively straightforward.

The cost of the welder’s time to weld a joint does not depend solely on the deposition rate of the process. A most important factor in determining the time required by the welder is what is known as the ‘duty cycle’ or ‘operating factor’. This is a percentage figure giving the amount of time that the arc is burning and weld metal is being deposited versus the total time that the welder is working. Table 2 gives some figures for the more common arc welding processes. Note that these do NOT include set-up or assembly time and individual circumstances can increase or decrease these figures.

Table 2 Duty cycles for arc welding processes

The lost time in this figure can be accounted for by considering all of the other activities that the welder performs. In MMA welding, for example, time is required for tacking, de-slagging and cleaning a weld pass, for changing electrodes, for changing position, for rest breaks and for removal of the item from a fixture. Similar activities need to be performed using the other welding processes.

Increasing the duty cycle is therefore one method of increasing productivity, either by organising the shop floor such that lost time is reduced or by the use of a higher duty cycle process. However, remember that the arcing time may well be only a very small proportion of the total time to manufacture and attention to other aspects of the manufacturing cycle may give better returns than simply increasing the welding duty cycle. Reference to Table 2 also suggests that mechanisation is one method to increase the duty cycle. Caution needs to be exercised, however, if the total (floor to floor) time is to be reduced. For one-off or small batch items the time taken to prepare and set up a mechanised system to weld the item may be longer than that taken to weld using a manual process. Note also that if a mechanised system is used, the duty cycle may in fact decrease, as the welding speed is increased and the weld is completed in a shorter time although the number of items welded per day will increase. It is therefore essential to consider the complete manufacturing cycle to achieve the most cost effective solution.

A previous article from the TWI (number 95), dealt with the methods of determining the weight of deposited weld metal in a joint, enabling the cost of welding consumables to be calculated.

This is obviously the first step towards calculating the cost of actually making a welded joint but there are many other factors that need to be considered but which are beyond the scope of these articles.

The most significant of these costs is the overhead; the cost of providing a welding workshop or site and the costs of managing and running the organisation.

These costs are dependent on the accounting practices of the organisation. They comprise factors such as rent, rates, bank interest, cost of indirect workers, i.e.those not directly involved in fabricating, depreciation of plant etc. In addition, other accounting decisions (for example, where the costs of machining and assembly are absorbed) may affect the decisions on which is the most cost-effective joining method.

One of the most significant costs is that of labour and this inevitably varies with industry, time and country. The costs mentioned above cannot generally be influenced by the decisions made by a welding engineer. These articles will therefore concentrate on those aspects of welding activities that are not subject to accounting practices, overhead or labour costs.

There are many costs, other than the cost of depositing weld metal that will affect the price of a welded fabrication.

The work done by the designer in designing the most cost-effective joint in an item that can be placed in the most advantageous position for welding will have major effect on costs.

For example, the type of joint preparation the designer selects; a single or double-V preparation can be flame cut.

A J-preparation must be machined and is generally far more expensive. A machined J-preparation, however, may have less volume than a single-V, depending on thickness; will be more accurate and therefore quicker to assemble within tolerance and may result in a lower repair rate leading to a lower cost than the V preparation.

Costs that are directly affected by welding engineering decisions, in addition to the cost of actually depositing weld metal, are therefore; joint preparation, assembly time (which includes positioning in any jig or fixture and tacking), cleaning and dressing the weld, removal from jigs or fixture, post weld heat treatment, costs of non-destructive testing and cost of repairs.

The amount of weld metal deposited is rarely the same as the amount of filler metal purchased.

This is the result of losses when, for example, GMAW or submerged arc welding wire is trimmed back to the contact tip, when the wire reel runs out and the length of wire between the drive roll and the contact tip is scrapped or the wire or reel is damaged.

Such losses tend to be quite small but this is not the case with coated electrodes. Damaged flux coatings, incorrectly stored electrodes and the stub ends discarded by the welder all contribute to as much as a third of the purchased weight of manual metallic arc electrodes being scrapped.

Some electrode manufacturers’ catalogues give figures for these losses which can vary depending on electrode type and diameter.

To assist in calculating the amount of welding consumables to be purchased. Table 1 gives some multiplication factors for the more common arc welding processes.

The weight of weld metal in the joint should be multiplied by this factor to give the amount of welding consumable required. These figures assume good housekeeping and shop floor discipline such that consumables are not wasted or scrapped unnecessarily.

Table 1 Multiplication factor. Weight of weld metal to give the weight of filler metal required

The other consumables in this cost equation are shielding gases or flux.

The conventional shoulder height welding gas cylinder contains approximately 10,000 litres of shielding gas at a pressure of 200bar. As the gas flow rates normally used in production are around 12 to 15 litres per minute, this typical cylinder should provide in the region of 10 to 12 hours of welding time, allowing for losses at the beginning and end of the arcing period.

The rate of flux consumption in submerged arc welding is approximately 1kg of flux for every 1kg of deposited weld metal. This assumes good housekeeping and an efficient flux recirculation system.

Calculation of the amount required (and hence the cost) of these consumables is therefore relatively straightforward.

The cost of the welder’s time to weld a joint does not depend solely on the deposition rate of the process. A most important factor in determining the time required by the welder is what is known as the ‘duty cycle’ or ‘operating factor’.

This is a percentage figure giving the amount of time that the arc is burning and weld metal is being deposited versus the total time that the welder is working.

Table 2 gives some figures for the more common arc welding processes. Note that these do NOT include set-up or assembly time and individual circumstances can increase or decrease these figures.

 Table 2 Duty cycles for arc welding processes

The lost time in this figure can be accounted for by considering all of the other activities that the welder performs.

In MMA welding, for example, time is required for tacking, de-slagging and cleaning a weld pass, for changing electrodes, for changing position, for rest breaks and for removal of the item from a fixture. Similar activities need to be performed using the other welding processes.

Increasing the duty cycle is therefore one method of increasing productivity, either by organising the shop floor such that lost time is reduced or by the use of a higher duty cycle process.

However, remember that the arcing time may well be only a very small proportion of the total time to manufacture and attention to other aspects of the manufacturing cycle may give better returns than simply increasing the welding duty cycle.

Reference to Table 2 also suggests that mechanisation is one method to increase the duty cycle.

Caution needs to be exercised, however, if the total (floor to floor) time is to be reduced. For one-off or small batch items the time taken to prepare and set up a mechanised system to weld the item may be longer than that taken to weld using a manual process.

Note also that if a mechanised system is used, the duty cycle may in fact decrease, as the welding speed is increased and the weld is completed in a shorter time although the number of items welded per day will increase.

It is therefore essential to consider the complete manufacturing cycle to achieve the most cost effective solution.

Introduction

Hot gas welding is a fabrication process for thermoplastic materials. The process uses a stream of heated gas, usually air, to heat and melt both the thermoplastic substrate and the thermoplastic welding rod. The substrate and the rod fuse together to produce a weld.

To ensure that welding takes place, adequate temperature and pressure must be applied to the rod, along with the correct welding speed and gun position. The overall welding operation consists of three main phases: substrate and rod preparation, welding and, finally, weld finishing. The final stage is not a necessary requirement and is dependent upon the application.

There are two types of hot gas welding for thermoplastic materials, round nozzle welding and speed welding. In the first, the welding gun is used with a round nozzle attached and in the latter, a second nozzle is attached to the round nozzle and is used to heat the welding rod at the same time as the operator applies welding pressure through the tip. Figure 5 shows a range of welding nozzles.

Fig.5. Plastics welding nozzles (left to right) tacking nozzle, round nozzle, high speed nozzle

Material preparation

The first phase of welding is preparation of the substrate materials and the welding rod. It is important to ensure that both are clean. This is helped by ensuring good storage procedures in a clean and dry environment. Before preparing the substrate it is important to check that any protective film is removed from the surface. Once the film is taken off, a scraper can be used to remove the surface layer of the material in the vicinity of the weld.

When selecting the materials for welding, the rod and substrate materials must be of the same polymer type.

If required, for example if the butt joint is used, an appropriate weld preparation should be added to the edge of the material before the scraping operation. Where the material thickness is less than 6mm, the preparation will generally be a 60° single-V chamfer. If the material thickness is greater than 6mm, then a 60° double-V preparation is used.

When the joint is in a T configuration, it is not necessary to prepare the edges with a chamfer, although it is still important to scrape both the substrate materials in the vicinity of the weld to remove any surface contamination.

Once the materials are prepared, the adjacent surfaces are abutted together and tacked into position using the tacking nozzle. Tacking should be sufficient to hold all the pieces of the fabrication together prior to welding.

It should be noted, however, that tacking alone should not be relied upon as the only method of holding the materials whilst welding. It is important to use additional clamps for this purpose. Once the substrates are assembled, the materials can be permanently welded together using either the round nozzle or speed welding techniques.

Round nozzle welding

In hot gas round nozzle welding, or hand welding as it is sometimes known, the rod is fed into the joint by hand. The nozzle is moved in a pendulum motion along the joint and up the welding rod, heating both the joint and rod as the weld is progressed. Figure 6 shows round nozzle hot gas welding.

When using this method it is important to maintain a constant pressure on the welding rod and a constant welding speed across the substrate. Generally, this can be helped by ensuring that you are in a comfortable position when welding. Round nozzle hot gas welding is generally only used where access to the joint is difficult, for example, around internal corners.

Fig.6. Hot gas round nozzle welding

Speed welding

In the hot gas speed welding technique, the rod is fed into the joint via a tube in the speed-welding nozzle (see Fig.4) attached to the round nozzle on the hot-gas welding gun.

The speed-welding nozzle also introduces hot air to the filler rod and to the joint. With this technique, the toe of the speed-welding nozzle is used to apply the pressure required for welding. Figure 7 shows hot gas speed welding.

Fig.7. Hot gas speed welding

In order to achieve adequate welding pressure, the position of the operator’s hand on the welding gun handle is important. The hand holding the gun should be placed underneath the grip and forced downwards, applying pressure to the nozzle toe as welding progresses.

A light force should also be applied to the welding rod to ensure that it is fed through the tube at a constant speed. As with hand welding it is also important to feed the rod at a constant speed, maintain weld force using the nozzle toe and travel along the weld at a constant speed. Again, this is achieved by adopting a comfortable position when welding.

Producing the weld

In order to achieve a weld, the joint must be sufficiently filled with welding rod and each welding run must be adequately fused to both the next run and the parent material.

Once the materials are tacked (see preparation), the first weld run can be placed into the base of the joint. The temperature of the welding gun should be set in accordance with the guidelines produced by the materials manufacturer.

The manufacturers’ guidelines on welding pressure, airflow, and welding speed should also be followed. The base run should have good root penetration and, in the case of polypropylene, a small ‘side wash’     (see Fig.8)

Fig.8. Section through polypropylene weld showing ‘side wash’

The welding rod should not display signs of degradation (brown spots) or overheating. In the case of PVC, overheating is signified by a shiny smooth surface on the welding rod.

In the case of polypropylene, ‘splattering’ at the edge of the weld indicates overheating of the material during welding.

Between each run, as the joint is filled, a chisel or scraper is used to produce a small V preparation for the subsequent adjacent weld runs to be positioned in. This is shown in Fig.9.

Fig.9. Chisel between hot gas weld runs

The number of runs needed will be determined by the thickness of the substrate being welded. The weld is finished when the top series of runs is above the surface of the parent material. These runs are known as capping runs.

Finishing the weld

In some circumstances it may be necessary to remove the weld capping runs and leave a flush surface, for example where an aesthetic finish to the joint is required. These capping runs can be removed using either a hand grinder or scraper until the weld is flush with the surface of the substrate material. This can be done on both sides of the weld.

Copyright © TWI Ltd 2014

The content of this article was correct at the time of publication. For more information visit: www.twi-global.com

Hot gas welding of plastics:

Part 1 – The basics

 Process background

Hot gas welding is a fabrication process for thermoplastic materials. The process, invented in the mid 20th century, uses a stream of heated gas, usually air, to heat and melt both the thermoplastic substrate material and the thermoplastic welding rod. The substrate and the rod fuse to produce a weld (See Fig 1 and 2).

Fig.1. Hot gas hand welding nozzle motion and rod angle

Fig.2. Hot gas hand welding

To ensure welding takes place, adequate temperature and pressure must be applied to the rod, along with the use of the correct welding speed and gun position.

The weld quality, since this is a purely manual technique, is dependent on the skill of the welder. Typical applications include chemical storage vessels, ventilation ducting and repair of plastic mouldings such as car bumpers.

Welding materials

There are two groups of plastic materials; thermoplastics and thermosets. The hot gas welding technique is only applicable to those plastic materials that can be heated and melted repeatedly, namely thermoplastics. When a thermoplastic is heated, the molecular chains become mobile within the material and allow it to melt and flow. Thermosets are a group of plastic materials in which the molecular chains form cross-links. These cross-links, formed by a chemical reaction, prevent the molecular chains becoming mobile when heat is applied. Although many thermoplastics can be welded by this process, the most common are polypropylene, polyethylene, PVC and some fluoropolymers such as PVDF, FEP and PFA.

Extruded rod and sheet are the most commonly used raw materials for the manufacture of fabricated plastic products. It is of utmost importance when fabricating plastics that the welding rod and the sheet are of identical material and chemical type.

For example, although it is possible to weld polypropylene homo-polymer to polypropylene random block copolymer, the strength of the weld will be reduced significantly. It is also important to check the quality of the welding rod prior to use, since air bubbles within the rod can form during the extrusion process.

These will lead to voids in the weld. Welding rods will typically be either three or four millimetres in diameter.

Welding equipment

The equipment used for hot gas welding consists of an air supply, a handle with sturdy grip, a heating chamber with temperature control to produce the hot gas and a nozzle where the heated gas leaves the welding gun in order to heat the plastic rod and substrate. (Fig.3)

Fig.3. Hot gas welding gun

A fan, either incorporated into the welding gun handle or positioned remotely and connected to the gun, provides the air supply. It is also possible to use compressed gas from bottles, for example, air or nitrogen. Whichever gas supply is used, it is important that it is clean and dry, since dirt and moisture will contaminate the weld.

The gun temperature is set via a dial on the handle, with some welding guns showing the temperature of the air stream on a digital read-out also on the handle. It is good practice to measure the gas temperature consistently using a digital thermometer, for example, with the thermocouple tip placed 5mm inside the welding gun nozzle.

The front end of the welding gun allows interchangeable welding nozzles to be fitted depending on the type of welding needed.

Three nozzle types are most commonly used, the tacking nozzle, the round nozzle and the high-speed nozzle (Fig.4).

Fig.4. Plastic welding nozzles ( left to right) tacking nozzle, round nozzle, high speed nozzle

The tacking nozzle, as the name suggests, is used to tack the materials together before welding. The round nozzle allows the welder to heat the rod and substrate without physical contact with either and is useful for welding in areas with difficult access. This is less commonly used than the high-speed welding nozzle where the toe of the nozzle contacts the welding rod and allows the welder to put pressure on both the rod and the substrate material whilst welding.

Along with correct temperature, the pressure ensures that there is adequate fusion between the welding rod and the substrate material. In addition to the welding gun, several tools are needed. These are a coarse tooth file, router and hand grinder for edge preparation, a scraper for removal of the material surface around the weld and a wire brush for cleaning the nozzle.

Also, wire cutters are required for cutting the welding rod and a jigsaw for cutting the substrate material.

Welding parameters

There are four main welding parameters in the hot gas welding process: temperature, pressure, welding speed and gun position.

Since the process is manual, it is important that the welder has a good understanding of the need to ensure that all four of these parameters are correct and controlled during the welding operation.

Temperature is the most important of the four parameters, since the temperature at the interface between the rod and the substrate is not only controlled by the setting on the gun, but also by the gun travel speed and the gun position with respect to the substrate. Typically, the temperature for welding is set between 80 and 100°C above the melting point of the material being welded.

The gun travel speed is normally between 0.1 and 0.3m/min, again, depending upon the material being welded.

The welding pressure is applied via the toe of the welding nozzle and is achieved by holding the welding gun grip firmly and pushing down into the weld. For round nozzle welding, pressure is applied manually from the welding rod. The correct welding pressure is easier to achieve using welding guns with the fan separate to the gun since a firmer grip around the handle can be achieved. The force applied to the welding rod would typically be between 15 and 30N. Practical welding details are given in Part 2 – welding techniques

Weld quality

As a manual process, weld quality is dependent on skill. There is no recognised non-destructive technique, that conclusively shows the presence of defects in plastic welds that could lead to weld failure. Therefore, it is recommended that good quality welder training is received and that welder certification is adopted.

The European Standard (EN13067) sets out the criteria for plastic welder approval. It details a scheme where the welder undergoes both a theoretical and a practical test and the welder, upon successful completion, is awarded a certificate of approval in the specific material categories taken in the test. Certification lasts for two years with a further two years prolongation, effectively giving the welder an approval certificate for four years before requiring a full retest.