Weld overlaying.... - are we doing the right thing?

TWI Bulletin, July - August 2010

A procedure qualification based on heat input may not necessarily ensure dilution levels.....or corrosion resistance!

In weld overlaying, layers of weld metal are deposited onto components using welding processes to improve the corrosion resistance. One of the major concerns in arc based weld overlaying is the modification of the chemistry of the deposited metal by the base metal (Fig.1). As Vinod Kumar and Chi Lee explain even though some generic information is available on the extent of dilution associated with common arc welding processes, the actual dilution with a particular process can vary over a wide range, depending on the welding parameters adopted.

Fig.1. Definition of weld dilution

In most cases, it is necessary to control the average dilution within close limits because a segmentation in the weld metal can affect the performance of the overlay. For a given process, there are a number of variables which affect dilution including the welding current, arc voltage, current polarity, electrode diameter, electrode extension, weld-bead separation, welding speed, electrode grinding angle, welding position and shielding gas. It is necessary to control each of these variables within close limits to ensure the desired composition of the overlay. So it is also necessary to have a clear understanding of the influence of each of these parameters on dilution. Unfortunately, many of these parameters are not often well controlled.

Codes and standards, such as ASME Section IX for qualification of welding procedures, state that heat input for the first weld layer is an essential variable, ie a change in heat input over 110% of that qualified, requires requalification. The same heat input can be achieved by proportionally varying the welding current and the welding speed, with an entirely different effect on dilution. The extent of overlapping between adjacent beads is also a key variable influencing the dilution, in many cases, more than the heat input.

Arc welding processes for weld overlaying

Weld overlays can be prepared using a number of arc welding processes. Manual metal arc (MMA), submerged arc welding (SAW), metal inert/active gas (MIG/MAG) welding, and tungsten inert gas (TIG) welding (hot wire and cold wire) processes are commonly used. Improved process control has been achieved in new generation MIG welding equipment through digital control, giving the benefits of reduced heat input, a stable arc, and spatter-free welding. There are electronically controlled short-circuit MIG welding variants as well as a number of high-deposition TIG welding variants which could provide high-deposition rates with good control of dilution.

In TIG welding, the arc provides the entire energy required for heating and melting the filler wire. The metal deposition rate is thus limited by the rate at which the heating and melting process can take place. The energy consumption from the arc can be reduced if the wire can be fed into the arc at a higher temperature. In this case the majority of arc energy can be used for melting the parent metal thus facilitating a higher welding speed, or an increased rate of filler wire melting using the same arc energy, compared to conventional welding. This principle is being used in the hot wire TIG welding process. The heating is accomplished by resistive heating of the wire between the feeder nozzle and the molten pool. The electric current required for this preheating is supplied either from the welding power source, itself or from a separate power source. The secondary current circuit is closed so long as the wire is in contact with the molten pool, so there is a limit on the maximum preheat current that can be used.

Another method of increasing the deposition capability of TIG welding is to introduce the filler wire into the inner hot region of the arc. In conventional TIG welding, this is difficult due to the shallow angle used for wire feeding. However if the filler wire is fed through the welding nozzle, it can be introduced through the inner core of the arc where the temperature is highest. An example of this type of system is the Air Liquide TOPTIG welding process.

In MIG welding, the arc is formed in an inert atmosphere between a continuously fed consumable wire electrode and the work piece. The electrode continuously melts forming the weld. The heat input in MIG welding depends on the metal transfer mode, which can be classified into short-circuit transfer, globular transfer, spray transfer, or pulse transfer. The operating parameters such as the arc voltage, current, shielding gas, and electrode wire feed rate control the transfer mode. In conventional short-circuit transfer, the molten metal droplet formed at the tip of the electrode grows, and when it comes into contact with the molten pool, metal transfer occurs.

Fig.2. Examples of weld overlays produced with different welding processes

Spray transfer occurs at relatively high values of arc voltage and current. Under the spray transfer condition, the droplets formed are much smaller resulting in the transfer of molten electrode material in the form of a spray into the weld pool. In pulse welding, the current oscillates between a peak and a background value, making the average current relatively low, whilst facilitating spray transfer. The pulsing of welding current extends the range of the spray transfer region well below the natural transition from dip to spray transfer. Since the current oscillates between the peak and the background value, the average heat input can still be lower than that of a spray transfer process.

Short-circuit transfer greatly reduces the heat input in MIG welding, but conventional short-circuit transfer produces excessive spatter. The digitally controlled power sources allow a higher degree of control over the welding parameters, on a real time basis, than that possible with the traditional methods using analogue controllers. Such technology has been used in systems such as the Lincoln Electric STT^TM (surface tension transfer) technology, Daihen Corporation CBT (controlled bridge transfer) technology and EWM-coldArc^® technology, to achieve controlled short-circuit transfer. The digitisation of the controllers also improved the dynamic response of the power sources, resulting in the generation of self-tuning power sources such as the ESAB QSet^TM. Hence the consistency of weld overlays produced with the MIG welding process has improved substantially in recent years.

Influence of heat input on dilution

The influence of the heat input on dilution is well recognised. It may be logical to believe that a high heat input is associated with a high dilution as the high heat input increases the parent metal melting. However, is the relationship between heat input and dilution that straight-forward? Of course not, a higher heat input can be associated with a higher filler wire melting rate which can result in a lower dilution. Also, how a specified heat input is achieved matters considerably. For example, a specified heat input can be achieved by using a high welding current with a high welding speed or a proportionally lower welding current at a proportionally lower welding speed.

If we consider the former case in TIG welding, for a feasible range of welding speeds without affecting the consistency, a higher current can result in a higher parent metal melting rate. Similarly, a higher welding speed will result in lower filler metal deposition per unit length of the weld for a fixed filler feed rate. Both the above factors result in a higher dilution compared to a procedure which uses a proportionally lower welding current and welding speed. An example of variations in dilutions of the first weld-bead and the third weld-bead of an overlay as a function of welding current and welding speed in TIG welding is shown in Figure 3. These experiments were conducted on a 15mm thick BSIN 10025 2004 355J2+N carbon steel plate using alloy 625 filler wire, and the dilution was measured as the percentage of iron in the resulting weld metal using semi-quantitive energy dispersive X-ray (EDX) analysis. The typical welding parameters used are given in Table 1.

Fig.3. TIG welding - variations in the dilution of the 1st and 3rd bead with

a) welding current, and

b) welding speed, and with other welding parameters remaining constant

Heat input is the ratio of the product of welding current and voltage to the welding speed, multiplied by the arc efficiency factor of (0.6 for TIG welding). Based on the above definition if we combine the data corresponding to the above two graphs, another set of graphs can be generated correlating the heat input and dilution as shown in Figure 4. Graphs with legend 'I' are obtained by changing the welding current at a fixed welding speed, and those with legend 's' are obtained by changing the welding speed at a fixed welding current. As can be seen from Figure 4, for the same heat input entirely different dilution is produced depending on whether the heat input is achieved by varying the welding current or the welding speed. For example, a heat input of 0.75kJ/mm obtained with low welding current and welding speed limits the dilution in the range 16-22% whilst the same heat input obtained with a higher current and higher welding speed result in dilution in the range 48-58%. So a procedure qualification based on only a controlled heat input may not necessarily ensure the required quality in a weld overlay.

Fig.4. TIG welding - variations in the dilution of the 1st and 3rd bead with heat input obtained by varying the welding current denoted by 'I' and welding speed denoted by 's' in the legend, with other welding parameters remaining constant. Process: TIG welding

The influence of dilution on corrosion resistance

The current qualification procedures for weld overlay cladding involve measurement of dilution and ensuring that it is below a maximum level, and also passing a standard generic corrosion test such as ASTM G48. There are two drawbacks to this approach. First, the chemical analysis results only give the average of the volume of metal analysed, locally, higher/lower dilution levels may exist; secondly, the ASTM G48 test is too aggressive and not representative of most service environments and so needs to be inspected carefully.

Thus, these tests provide limited information on the effects of variations in welding procedures/dilution on actual service performance of the weld overlay. Due to the uncertainties involved in the quality of the weld overlay a conservative approach is often taken while specifying the permissible dilution, resulting in substantial productivity losses and increased cost. In order to understand the significance of dilution with respect to service performance, there is a need for testing methods which are able to measure and rank the corrosion resistance of weld deposits with differing degrees of dilution. Ranking the corrosion resistance of individual weld beads with differing degrees of dilution can be achieved using a 'droplet cell corrosion testing' technique. This technique uses a small droplet of solution placed on the test surface, and by carrying out electrochemical anodic polarization within this droplet, it is able to determine corrosion resistance of the material in a small region. This overcomes the problems of measuring the corrosion resistance of a large surface area where the dilution levels can vary significantly.

The results from 'droplet cell' tests on alloy 625 weld overlays on carbon steel with a range of dilution (Fig.5) showed that the corrosion resistance of alloy 625 weld overlay did not decrease significantly until dilution levels greater than those equivalent to defects with 36% iron wire were reached in the selected test environment. This indicated that dilutions levels up to 136%Fe may be tolerated if the service environment is less aggressive than or similar to the test environment. Below these dilution levels, no pitting was observed.

Fig.5. The relationship of breakdown potential against %Fe in weld metal

Furthermore, the results showed a clear step change in corrosion resistance in relation to dilution. It would be informative to repeat these tests in a more aggressive environment to determine if a similar step change still occurs with a shift towards lower dilution levels, or if the change becomes more gradual.

Although iron dilution is often specified for qualification purposes, corrosion resistance is also expressed as pitting resistance equivalent number (PREN) which takes into account all major alloying elements which affect the corrosion resistance. The test results showed a drop in corrosion resistance for PREN less than 33 (Figure 6). This corresponds to the lower end of the specification range for alloy 904L and within the composition range of alloy 825. It should be remembered that the results are from 'droplet cell' tests conducted on weld metal, essentially an as-cast alloy. Nevertheless, it showed that the alloy 625 weld overlays need to reach dilution equivalent to those of the alloy 825 composition range before a significant drop in corrosion resistance occurs in a 10w/v% NaCl at pH3 which was used in this investigation.

Fig.6. Breakdown potential plotted against pitting resistance equivalent number (PREN) of weld beads. The PREN of commonly used wrought grades of corrosion resistant alloys (CRAs) are also shown for comparison

Conclusion

In TIG welding, the same heat input can produce overlays with very different dilutions depending on the actual values of the welding current and welding speed used. Hence a procedure qualification based on heat input may not necessarily ensure the dilution levels and hence the corrosion resistance. Limited local corrosion testing employing 'droplet cell corrosion' testing techniques showed no decrease in corrosion resistance of alloy 625 weld overlays for an iron level up to 36% and a step change in corrosion resistance beyond this level for the 10w/v% NaCl at pH3 environment. This suggests that opportunities exist for relaxing the limiting dilution levels specified for several industrial applications.

Further work

There are a number of digitally controlled MIG welding variants and high-deposition TIG welding variants available to improve the deposition capabilities. Also, variable polarity submerged arc welding provides the capability to achieve different deposition rates and penetration depths without changing the voltage or current. Hence the required dilution limits or deposit height may be achieved with fewer layers than those which were possible with conventional processes. However, at present we do not have a clear comparison on the dilution levels that each of these processes is capable of achieving without compromising on the productivity. Also, there appear to be opportunities for relaxing the present permissible limits for %Fe in weld overlays. A higher limit on dilution would improve the economics associated with weld overlaying.

To address some of these issues, TWI proposes to launch a Group Sponsored Project (GSP) in autumn 2010. The project will examine some of these issues through a review of current manufacturing practices, investigating improved manufacturing processes and procedures, the use of alternative consumables, evaluating the local and bulk corrosion resistance and developing reliable inspection procedures.

For further information on this project, please contact vinod.kumar@twi.co.uk or chi.lee@twi.co.uk

Acknowledgement

This work was funded by the Industrial Members of TWI, as part of the Core Research Programme. The authors acknowledge the support of Chris Hardy and Harry Froment, in carrying out the welding trials, and Sheila Stevens and Ashley Spencer for their support in preparing and analysing a large number of samples. The authors are also grateful to Dr Oliver D Lewis of Sheffield Hallam University for his support in carrying out the 'droplet cell corrosion' tests.