Underwater laser cutting as a decommissioning tool

Ali Khan and Paul Hilton

TWI Granta Park, Great Abington, Cambridge CB21 6AL

Abstract

There are several potential benefits of using laser cutting in aspects of nuclear (and other) decommissioning processes.

These include the speed of the cut, the lightness of the cutting head, the flexibility offered by optical fibre beam

delivery, the minimal reaction force on the part being cut and the high degree of remote automation possible with this

process. Such benefits have already been described, for cutting in air, by TWI and others. This paper will focus on laser

cutting underwater, where it will be shown that some of these benefits are just as applicable underwater as in air and

indeed, particularly for the application of nuclear decommissioning, some additional benefits accrue for the case of

underwater cutting. A cutting head will be described which uses a series of jets of compressed air to create a local dry

zone at the point of interaction of the laser beam and the material being cut, to perform underwater cutting. A series of

trials are described to investigate the effects of parameters used for cutting C-Mn steel and stainless steel underwater,

using a 5kW laser beam. The parameters investigated primarily involved the relative pressures in the gas jets and the

positions of the laser beam focus and the cutting nozzle tip with respect to the surface of the material being cut. The

results indicate that with an appropriately designed underwater cutting head, the cutting performance for both C-Mn

steel and stainless steel is very close to that achievable in air with the same laser power. In underwater cutting, the

height of dross (re-solidified metal and metal oxide) left adhering to the cut edges, was found to be significant,

particularly when cutting C-Mn steel. During size reduction of contaminated material, maximising the dross adhesion

may be important, as it means less radioactive material from the kerf will be released as secondary waste which reduces

the risk of loss of containment of radionuclides during the size reduction operation.

1. Introduction

One motivation for dismantling intermediate and

high level nuclear waste underwater is to significantly

reduce the amount of secondary waste produced during

size reduction operations, escaping into the atmosphere.

Underwater cutting could also eliminate the logistics of

handling and transporting contaminated product from a

pond environment to a dry dismantling facility. There is

an increasing interest in the nuclear sector to acquire an

underwater size reduction technology that could be

versatile enough to dismantle and decommission such

waste. The nuclear industry is very conservative in its

approach to decommissioning and the techniques

currently in use for size reduction encompass abrasive

water jet cutting, diamond wire sawing and mechanical

shearing. The only thermal process which has had some

use is plasma arc cutting. Amongst these size reduction

technologies, laser technology, especially with recent

developments in fibre delivered beams, could offer the

nuclear sector alternative size reduction tool, capable of

cutting various materials both in air and underwater and

has the potential to minimise the production of

secondary waste and reduce the complexity of remote

operation.

Several examples of laser cutting in air (as opposed to

underwater) have been demonstrated in published

literature. The most applicable to actual use in

decommissioning all are more recent and involve the use

of 1 micron laser beams delivered by optical fibre. A

good example is the paper [1] Pilot, which is a review of

research into the use of laser cutting in

decommissioning, for work up to 2010 and references

work conducted in France and Japan.

Since this time, TWI have published several papers on

the use of a 5kW fibre laser and a range of purpose built

decommissioning specific cutting heads, for cutting a

range of stainless steel plate and tube material and C-Mn

steel plate, in air. Ref [2] discusses mainly the potential

for tube cutting, where the laser demonstrates a

particularly useful benefit for decommissioning, in that it

can cut tubes with a linear pass from only one side of the

tube. With a 5kW laser, TWI demonstrated the cutting of

170mm diameter stainless steel tube of 8mm wall

thickness, using this single sided technique. Ref [3]

describes the cutting of C-Mn and stainless steel plate

material, at thicknesses up to 50mm, again with a 5kW

laser source. In this work the relationships between the

cutting gas pressure, the beam focus position and the

distance between the material surface and the tip of the

cutting nozzle, were established for optimum cutting

speeds. The work demonstrated that for applications

where the resultant edge quality is not important, a large

tolerance in stand-off distance is available in producing a

cut, all other cutting parameters remaining constant. For

example, 25mm thickness stainless steel plate was cut at

constant gas pressure, laser power and cutting speed,

while the stand-off distance was changed from 15 to

70mm, in a cyclic fashion, along the cut length. Such

tolerances are important for real applications on active

components which of course must be undertaken

remotely.

The capability to laser cut underwater was first

demonstrated using a CO2 laser many years ago. In 1996

[4] a comparison of underwater cutting using CO2 and

Nd:YAG lasers was published by Alfille et al and more

recently, there has been interest in this subject in Japan

[5], [6], and India[7], using 1 micron sources and fibre

optic delivery. The lasers used were COIL, 4kW

Nd:YAG and 250W average power Nd:YAG,

respectively. The cutting heads for these systems all

employed cutting gas of some sort to create a local water

free region in the area of the cut. It is probable that

various types of underwater plasma torch designs

influenced the design of the early underwater laser

cutting nozzles.

This paper will describe the design and operation of an

underwater cutting head for use with a 5kW fibre laser

system and water depths up to a few metres, in line with

the typical ‘pond’ applications mentioned earlier. It has

been used to cut plates of both C-Mn steel and 304L

Stainless steel, with the laser beam directed vertically

downwards and also with the beam directed horizontally.

The cutting head uses a series of jets of compressed air

to remove water and maintain an effective dry area in the

region of the interaction of the laser beam and the

surface of the material being cut. An additional, more

conventional, central gas jet, is used to blow molten

material from the kerf of the cut. The experimental

parameters investigated primarily involved optimisation

of the relative pressures in the gas jets and the positions

of the laser beam focus and the cutting nozzle tip, with

respect to the surface of the material being cut.

2. The cutting head and its experimental variables

The schematic design of the cutting head is shown in

Figure 1. It utilised conventional high power laser optics

in the form of coated fused silica collimating and

focussing lenses.

Fig. 1. Schematic of the underwater cutting head.

Although not shown in this diagram, a coated fused

silica cover slide was positioned below the beam

forming optics to prevent spatter reaching the focussing

lens. It can be seen from Figure 1, that in this design, the

coupling of the fibre to the collimator (not necessarily

sealed against water in commercially available systems)

does not need to be watertight and in operation, when

pressurised correctly, there is only ever air on the

material side of beam forming optics.

Below the optics the arrangement was such that cutting

gas could be introduced at various pressures through a

somewhat conventional laser cutting nozzle tip, coaxial

with the focussing beam. Around this central gas jet, a

second annular gas jet (henceforth referred to as the

‘secondary gas jet,’) was provided, to support the cutting

gas jet. An additional pressure chamber, bounded by the

outer ‘shroud’ could also be pressurised using a third

independent gas stream, (henceforth referred to as the

‘tertiary gas jet’). In this work, in all experiments, the

gas used was compressed air, as cutting in a nuclear

environment using pure oxygen would not be allowed

and, considering cut quality is not that important, there is

little point in using expensive pure inert cutting gases.

The lower part of the shroud contained an annular steel

wire brush ‘seal’, to slide over the surface of the material

being cut. The design relies on a combination of the

three gas supplies to essentially exclude water from

inside both the shroud and the central primary gas

nozzle.

The exits of the cutting gas nozzle and the secondary

nozzle were kept the same distance above the workpiece.

The air pressure in the tertiary and secondary gas jets

was maintained at 4bar and various primary cutting gas

pressures were used. The laser beam focus was

maintained either at the position of the exit of the cutting

nozzle tip, or 15mm below this position. For each of

these positions, the distance between the exit of the

nozzle and the plate being cut (henceforth known as

‘stand-off distance’), could be changed from 15 to

30mm. This was implemented by employing a series of

spacers in the body of the cutting head, without changing

the position of the laser beam focus with respect to the

surface of the material being cut. For all experiments

reported here, the surface of the wire brush was

maintained on the surface of the material being cut. In

the trials, the laser power used ranged from 2 to 5kW, as

determined using a calibrated Ophir power meter,

positioned below the laser beam focus position. The

laser power calibration was made in air. The cutting

head was mounted to a conventional robot, via a solid

metal arm, so that it could be inserted and removed from

a water tank. The same robot was used to manipulate the

head and support arm, across the material being cut. The

water tank had a volume of about 1m3 and samples to be

cut could be mounted either vertically or horizontally.

3. Materials cut

The materials for the cutting trials consisted of plates

of 15, 20 and 35mm thickness S275JR C-Mn steel.

60mm thickness plates of S355J2+N C-Mn steel, with

their edges machined at 45 degrees, so as to offer

variable thicknesses to the laser beam, were also used as

a relatively efficient way to determine maximum cutting

depth for a single set of experimental parameters.

Additional trials were undertaken on plates of grade

304L Stainless steel, of 6, 12, and 35mm thickness.

Henceforth these materials will be referred to simply as

either ‘C-Mn steel’ or ‘stainless steel’.

4. Underwater cutting performance

4.1. Influence of stand-off distance and primary gas jet

pressure

In this experiment a 60mm thickness plate of C-Mn

steel was used with its edge machined away at 45

degrees, so as to offer a range of thickness to the laser

beam. The beam was traversed into the plate at various

speeds, until it was clear that cutting had stopped. The

maximum cut depth achieved was measured afterwards.

The focus position of the beam was at the level of the

exit of the central cutting nozzle tip, the stand-off

distance was varied between 15 and 30mm, the laser

power was 5kW and the laser beam was pointing

vertically downwards, for all trials. Primary gas jet

pressures of 2, 4 and 8bar were used. The results can be

seen in Figure 2 for stand-off distances of 15 and 30mm.

Fig. 2. Maximum underwater cut depths in C-Mn steel as a

function of set cutting speed for two stand-off of distances

(SD) and three different primary gas jet pressures.

Figure 2 indicates that the major differences in

performance occur at cutting speeds less than

200mm/min, corresponding to cutting depths greater

than about 20mm using the 5kW of laser power

available. Above this speed, the performance is a little

better, in terms of cut depth achieved, at the stand-off

distance of 15mm. In this region, the exact influence of

primary cutting gas pressure is unclear but relatively

constant with changes in cutting speed. Significant

changes can be seen, however, below 200mm/min. The

use of the lowest stand-off distance appears greatly

beneficial at low speeds, regardless of the primary

cutting gas pressure. For all the results with a stand-off

of 30mm, the maximum cut depth achievable is

significantly lower but still not particularly dependent on

primary gas pressure. Generally, a positive effect of

higher gas pressure on maximum cut depth was observed

at the larger stand-off distance but the increase in

performance was relatively small.

4.2 In air/underwater comparison

Using the underwater head but with the outer shroud

and secondary annular nozzle removed, and using gas

only in the central nozzle assembly, a series of cutting

trials were also undertaken in air, for comparison to the

underwater results.

For the comparison trials the same 60mm thickness plate

of C-Mn steel, with its edge machined away at 45

degrees, was used. The focus position of the beam was at

the level of the exit of the central cutting nozzle tip, the

stand-off distance chosen was 15mm, the laser power

was 5kW and the laser beam was pointing vertically

downwards for all trials. Primary gas jet pressures of 2

and 8bar were used. The results can be seen in Figure 3.

Fig. 3. Comparison of cutting performance in air (dry) and

underwater, on C-Mn steel, for two different primary gas jet

pressures.

As expected, the maximum cut depths achieved were

higher when cutting in air but only significantly when

using a pressure of 8bar for the cutting gas. The

difference in achievable cutting speed, for 2 and 8bar

primary gas pressure, was small when underwater

cutting. At the highest cutting speed of 1000mm/min, the

underwater process performance is essentially the same

as the in air performance, which is a significant result

considering the potential heat sink offered by the water

surrounding the sample. At the lowest speeds

investigated, the performance in air, particularly for a

gas pressure of 8bar, is clearly better, corresponding to a

cut depth difference of some 10mm in favour of in air

cutting, when compared to the underwater result.

For a direct comparison of edge quality between in air

and underwater cutting, a 15mm thickness C-Mn steel

plate was chosen. Figure 4 shows the edges of the

resulting samples cut both underwater and in air. The

cutting speed used in air was 380mm/min and the cutting

speed underwater was 250mm/min, the cutting gas

pressure was 8bar and the stand-off was 15mm. It is very

evident that there are significant differences between the

two cut edges, both in terms of the amount of adhering

dro ss on the base of the cut and on the cutting striation

pattern. Further results concerning dross adhesion will

be presented later in the paper.

Fig. 4. Images of the edges on 15mm C-Mn steel, cut

underwater (top) and in air (bottom).

Regarding the general performance of the underwater

head for cutting C-Mn steel, it is clear that the system is

relatively tolerant to parameter change for cutting speeds

greater than 200mm/min, (when using 5kW of laser

power). This speed corresponds approximately to a

cutting depth of 20mm. Below 200mm/min, there was a

definite benefit in cutting at the lower stand-off distance,

(which corresponds in this experiment to the highest

power density on the plate surface), although the

performance seemed independent of primary cutting gas

pressure.

In terms of an in air/underwater comparison of the

cutting head, this work has two significant outcomes.

The first, is that notwithstanding the heat sink provided

by the water bath, the underwater performance was

almost as good as that in air, other than when cutting in

air with 8bar cutting gas pressure, where, particularly at

low cutting speeds, the performance was better with a

maximum cutting depth of 47mm recorded for the 5kW

of laser power in use. In fact at the highest cutting speed

investigated of 1000mm/min, corresponding to plate

thicknesses of the order 6mm, the underwater and in-air

cutting performance was essentially the same for all the

variables investigated. The second is the amount of

adhering dross found when cutting C-Mn steel

underwater. This point will be discussed again later the

following section.

5. Assessment of dross adherence and mass

reduction during cutting

In a nuclear decommissioning application, the

amount of contaminated material, (that has to be

accounted for and disposed of), which is liberated during

the cutting process, should be kept to a minimum. One

way to reduce this released fraction is to choose cutting

parameters or conditions which in fact maximise the

amount of dross which remains adhered to the edges of

the materials being cut or to minimise the mass loss in

the plate during cutting. This section will describe

experiments conducted to analyse mass loss and

adhering dross height, in cutting plates of C-Mn and

stainless steel.

5.1. Dross adherence assessment

In order to assess the influence of the laser cutting

parameters on dross adherence, a series of approximately

150mm long linear cuts were made in 6, 12 and 32mm

thickness C-Mn and stainless steel plates. For laser

powers of 2 and 5kW and a cutting gas pressure of 8bar,

the cutting head was made to traverse horizontally across

a 250 x 250mm plate, mounted vertically in the water

tank, 600mm below the water level. The cutting speed

chosen was governed by the material thickness being

cut. Trials were conducted with the laser beam focus

position both on the surface of the plates and 15mm

above the surface of the plates, while the stand-off

distance between the nozzle and the plate surface was

kept constant at 15mm. After each set of cuts, the height

of the dross attached at the bottom (kerf exit) of the plate

was measured at 10 locations using a digital dial gauge.

For each, an average of 10 readings was taken and the

variation in the dross height was recorded.

5.2. Mass reduction assessment

For the mass reduction assessment, for each

combination of cutting parameters, three linear cuts,

approximately 150mm long, were made and each sample

was accurately weighed before and after the laser cutting

operation. From the data, the mass reduction was

calculated corresponding to a 1m length of cut. For these

trials, cutting was also undertaken in air, to provide a

comparison. A laser power of 5kW was used for all

experiments and the cutting gas pressure was varied

from 2 to 8bar. The stand-off distance was kept constant

at 15mm. For each material thickness, the cutting speed

was kept constant to allow the effects of material and

environment (in air or underwater) to be assessed. For

cutting trials performed with the beam focus on the

surface of the plate, cutting speeds of 2000 and

600mm/min were used, for the 6 and 12mm thickness

plates respectively. For cutting trials performed with the

beam focus position 15mm above the plate surface,

cutting speeds of 800 and 400mm/min were used, for the

6 and 12mm thickness plates respectively. Resulting

dross heights were also measured in the same way as

described for the dross adherence experiments.

5.3. Equipment for metallographic assessment

Selected samples were cross-sectioned, ground,

polished and etched, to reveal the microstructure of the

metal surrounding the laser cut kerfs. This section

included bulk material, melted material and oxides. Each

metallographic sample was examined using light and

scanning electron microscopes (SEM). In addition, using

SEM with energy dispersive X-ray (EDX) analysis,

selected locations of samples were analysed for

elemental composition.

5.4. Results

5.4.1. Adhering dross – influence of cutting speed

Figure 5 shows examples of underwater laser

cuts (beam exit side shown uppermost) in 12mm

thickness C-Mn steel and stainless steel plates. Similar

experimental parameters were used for both materials. It

can easily be seen that the attached dross on the C-Mn

steel plate is significantly higher than that on the

stainless steel plate.

Fig. 5. a)

Beam exit side of underwater cuts in C-Mn steel

Fig.5.b) Beam exit side of underwater cuts in stainless steel

Figure 6a) shows, for underwater cutting, a plot of the

average dross height, as a function of cutting speed,

produced in 6, 12, and 32mm thickness C-Mn steel

plates, for a beam focus position on the surface of the

plate, laser powers of 2 and 5kW, a stand-off distance of

15mm and a cutting gas pressure of 8bar. Figure 6b)

shows the same, for a focus position 15mm above the

plate surface. Points positioned on the abscissa indicate

no cut was achieved for these conditions. In these

graphs, the error bars represent the standard deviation

across 10 measurements made on each sample.

a)

b)

Fig. 6. Influence of underwater cutting speed and laser power

on average dross height, in 6, 12 and 32mm thickness C-Mn

steel plate. a) focus position on plate surface, b) focus position

15mm above plate surface. The stand-off distance was 15mm.

As can be seen from Figure 6, generally, the attached

dross height increased with an increase in steel thickness

and laser power and decreased with an increase in

cutting speed. In addition, the average dross height

produced with the beam focus above the plate was

generally higher than that produced with the beam focus

on the plate. At 5kW power, underwater laser cutting of

32mm thickness C-Mn steel could only be successfully

carried out with the beam focus on the plate surface.

Nevertheless, as expected, the 32mm thickness material

produces the steepest rise in dross height as the cutting

speed decreased.

Figures 7, a) and b), show the same for the underwater

cutting of stainless steel.

a)

b)

Fig. 7. Influence of underwater cutting speed and laser power

on average dross height, in 6, 12 and 32mm thickness 304

stainless steel plate.

a) focus position on plate surface;

b) focus position 15mm above plate surface. The stand-off

distance was 15mm.

For this material it can be seen that average dross height,

certainly for any speed above 500mm/min, is much

lower than that observed when cutting the C-Mn steel.

However, it increases with a decrease in cutting speed,

particularly for higher plate thicknesses but at all speeds

above 250mm/min, is less than that recorded for cutting

C-Mn steel. With the beam focus on the plate surface, an

increase in laser power appears to have little or no

difference on the recorded dross height, whereas with the

beam focus 15mm above the plate surface, when using

5kW laser power, the average dross height is larger for

the 12mm thickness material but at 6mm thickness, there

is no clear indication of a dependence on laser power.

At the very low cutting speeds used to cut the 32mm

thickness stainless steel, both beam focus positions were

able to produce cuts where the average dross height

recorded was large, the largest being for the beam focus

position of 15mm above the plate surface. In this case

the average dross height for stainless steel was in fact

larger than that recorded for C-Mn steel and the same

parameters.

5.4.2. Adhering dross – influence of cutting gas

pressure

Figures 8 and 9 show the average recorded dross

height as a function of cutting gas pressure for C-Mn and

stainless steel, respectively, when cut underwater and in

air. In this experiment the laser power was 5kW and the

cutting speeds were, for 6mm thickness material, 2000

and 600mm/min, with the laser beam focus on the

material surface or 15mm above it, respectively and for

12mm thickness material, 800 and 400mm/min, with the

laser beam focus on the material surface or 15mm above

it, respectively.

a)

b)

Fig. 8. Average dross height as a function of primary cutting

gas pressure for underwater and in air cutting of C-Mn steel. a) underwater; b) in air.

Generally, the adhered dross underneath 6 and 12mm

thickness in air cuts in C-Mn steels is less than that

found on C-Mn steel cut underwater, using identical

cutting parameters. As expected, the thicker 12mm

material showed increased adhering dross. The cutting

gas pressure appears to have very little influence on

dross height for 6 and 12mm thickness C-Mn steel cut in

air. When cutting C-Mn steel underwater, there is an

increase in recorded dross height, compared to the in air

results, for both material thicknesses. Again, the results

show little variation with primary cutting gas pressure. It

could also be argued that the position of the laser beam

focus, both in air and underwater, has had little effect on

the recorded dross height.

a)

b)

Fig. 9. Average dross height as a function of primary cutting

gas pressure for a) underwater and b) in air cutting of stainless

steel.

In contrast to the C-Mn steel results, when cutting 6 and

12mm thickness stainless steel, both in air and

underwater, the recorded dross height appears to

decrease as the cutting gas pressure increases. In air,

there is little difference in dross height as a function of

beam focus position or thickness, apart from at the

lowest pressure of 2bar. The position for underwater

cutting is similar but with slightly higher recorded dross

when cutting the 12mm material. The largest difference

between C-Mn steel and stainless steel, is for the cutting

of 12mm thick material underwater, and particularly at

higher cutting gas pressure.

5.4.3 Mass loss in plate being cut

Figure 10 shows, for the C-Mn steel plate, cut

both underwater and in air, the results of the calculated

mass loss per metre of cut, as a function of cutting gas

pressure, for the two beam focus positions. The cutting

speeds used for the beam focus on the plate surface

were, for 6mm thickness material, 2000mm/min and for

12mm thickness material, 600 mm/min. The cutting

speeds used for the beam focus 15mm above the plate

surface were, for 6mm thickness material, 800mm/min

and for 12mm thickness material, 400 mm/min.

a)

b)

Fig.10. Influence of cutting gas pressure on mass loss when

cutting C-Mn steel of thicknesses 6 and 12mm: a) underwater

and b) in air, for two beam focus positions and at 5kW laser

power. The stand-off distance was 15mm.

Figure 11 shows similar data for the results from the

stainless steel.

From Figure 10 it can be seen that for underwater cutting

of C-Mn steel, of both thicknesses, the observed mass

loss is small and the tolerance to beam focus position

and cutting gas pressure is large. For in air cutting of the

same material, the position is similar to that underwater,

at the lowest gas pressure of 2bar. Above this pressure

the mass loss gets larger and also becomes dependent on

focus position, particularly for the 12mm thickness. The

highest mass loss was recorded for this material with the

beam focus on the material surface.

For cutting the 6mm thickness stainless steel, there is

little variation in mass loss for all cutting gas pressures

and both beam focus positions, both in air and

underwater, although the mass loss is approximately

twice that recorded for C-Mn steel. The picture is

different for the 12mm thickness material however,

where the mass loss is higher, between 80 and 100 g/m

of underwater cut, and between 80 and 140g/m of, in air

cut. In air, there is also a large difference due to beam

focus position with, (as for C-Mn steel) a beam focus on

the plate surface producing the highest mass loss. The

variations seen with cutting gas pressure are relatively

small.

a)

b)

Fig. 11 Influence of cutting gas pressure on mass loss when

cutting stainless steel of thicknesses 6 and 12mm:

a) underwater and b) in air, for two beam focus positions and at

5kW laser power. The stand-off distance was 15mm.

In summary, for both the 6mm thickness C-Mn and

stainless steels, there is no potential benefit, in terms of

minimising mass loss, by cutting underwater. As the

thickness increases to 12mm, when cutting with a gas

pressure of 8bar, there appears a significant benefit, in

terms of minimisation of mass loss, by cutting C-Mn

steel underwater, at either beam focus position. For the

stainless steel at 12mm thickness, the only benefit that

can be seen by cutting underwater occurs with the beam

focus position on the plate surface.

5.4.4. Metallographic assessment

Four metallographic cross-sections were made from

examples of 12mm thickness C-Mn and stainless steel

samples, cut both in air and underwater. All these were

made using the same parameters, consisting of: 5kW

laser power, 15mm stand-off distance, 8bar cutting gas

pressure and the beam focus position 15mm above the

surface of the material. The cutting speed used was

400mm/min. Figure 12 shows the sections and

associated dross microstructures. These sections were

analysed using optical and scanning electron

microscopes.

The dross is believed to consist of re-solidified metal and

metal oxides. EDX analysis on the cross-sections shown

in Figure 12 a) and c) was performed, to obtain

compositional information on the parent materials and

dross.

Fig. 12. Laser cut kerfs produced in 12mm thickness materials.

For each case, the upper image is an optical micrograph and

the lower, a scanning electron micrograph. In d), as dross was

not present at the bottom of the kerf, the SEM image shown is

that of residual melt inside the kerf itself. See text for cutting

parameters. a) Underwater cut C-Mn steel: b) In air cut C-Mn

steel: c) Underwater cut stainless steel: d) In air cut stainless

steel.

Figures 13 a) and b) show the EDX information for

underwater C-Mn and stainless steel cuts. In both

examples it can be seen that the composition of the re-

solidified metal was very similar to the corresponding

parent material. EDX analysis of the C-Mn steel oxide,

selected by contrast on the images, indicated that this

consisted mostly of iron oxide with traces of manganese.

Although not presented in this paper, the oxide

composition observed when cutting in air, contained

slightly higher levels of oxygen compared with that seen

in the underwater cuts. EDX analysis of the stainless

steel oxide suggested chromium oxide was the main

oxide constituent, with manganese and silicon also

present. X-Ray analysis of dross produced in underwater

laser cutting of 316L stainless steel carried out in an

earlier study [6] also showed increased levels of these

elements.

a) b) c)

d)

d)

a)

b)

Fig.13. EDX analysis of parent material, re-solidified metal and metal oxide in underwater laser cutting of

a) 12mm C-Mn steel plate;

b) 12mm stainless steel plate.

Analysis of Figures 12 and 13 showed that:

The kerfs were irregular in shape and contained

re-solidified molten metal and metal oxides in

the form of dross. Dross was present both inside

and underneath the laser kerfs.

The kerfs produced in stainless steel appear

wider than those produced in C-Mn steel.

The underwater kerfs showed increased

amounts of dross attached underneath the

material surface, compared with the in air kerfs.

More metal oxide was present inside the air cut

kerfs. This was particularly notable for the C-

Mn steel.

More residual dross can be seen inside the C-

Mn steel kerfs.

The dross in the C-Mn steel kerfs is in the form

of long filaments of re-solidified metal,

surrounded by metal oxide, compared with

intermittent layers of smaller granular re-

solidified metal and metal oxide, in the stainless

steel kerfs.

For underwater cutting of C-Mn steel, the oxide

surrounding the dross was significantly thinner

than that present when cutting in air.

For underwater cutting, the attached dross

beneath the stainless steel cuts, Figure 12c) had

higher oxide content than those in C-Mn steel,

Figure 12a).

Conversely, for underwater cutting of stainless

steel, the intermittent metal oxide thickness was

larger than that present when cutting in air.

5.5. Discussion on dross formation

Comparing the data in Figures 6 and 7, shows

that the dross height in the region of the kerf exit was

more pronounced for underwater cutting of C-Mn steel,

than for underwater cutting of stainless steel. A

reduction in cutting speed, or an increase in material

thickness, or movement of laser beam focus away from

the plate surface, tended to result in increasing dross

height, as did an increase in laser power (under certain

conditions).

From the analysis performed on the sections shown in

Figure 12, it is apparent that more metal oxide is formed

when cutting in air than when cutting underwater. The

amount and thickness of oxide present inside the kerfs of

both steel types, are also larger when cutting in air than

when cutting underwater. This suggests that the

oxidation process continues to occur even after the laser

material interaction has ceased. This implies that

immediately after cutting, the region of the kerf is hotter

for in air cutting than for underwater cutting.

Furthermore, owing to the higher thermal conductivity

(42.7 W/mK) of C-Mn steel, compared to stainless steel

(21.4 W/mK), it is likely that the cooling rates

experienced by the parent metal and dross in the C-Mn

steel are higher. Underwater cutting is likely to further

increase the quenching effect for C-Mn steel, owing to

the heat extraction by the surrounding water.

Clear differences in the heights of the attached dross can

be seen in the cross-sectional views for the steels cut

underwater and in air. Variations in the distribution of

oxides in the dross can also be seen between the two

cutting environments and steels, especially in the SEM

images. Close examination of the SEM images shows

that a greater volume fraction of oxide is present in the

underwater cut stainless steel sample, compared to the

underwater cut C-Mn steel sample. The stainless steel

dross appears a more intermittent mixture of re-solidified

metal and metal oxide, compared to the C-Mn steel

dross, in which most of the oxide is present on the

melted surface, which appears as long filaments. These

molten metal filaments are likely to re-solidify

significantly faster during underwater cutting and remain

attached to the parent material, potentially resulting in

even higher heat transfer rates. The addition of re-

solidified metal beneath the kerf may result in increasing

the effective thickness of the C-Mn steel plate. It may be

possible that the combination of an irregular kerf shape

and the added length of re-solidified metal, further

obstructs the flow of molten material through the kerf,

promoting rapid quenching and dross attachment. This

proposed mechanism may be more pronounced when

underwater cutting at lower speed or when cutting

thicker material.

In the presence of water, the rate of oxidation of stainless

steel at high temperatures, is believed to be high because

the rate of reaction is greater than for oxidation in just air

[9, 10]. Compared to oxides formed in air, oxides

formed in the presence of water are believed to differ in

morphology and oxidation in the presence of water has

been shown to encourage the formation of more porous

oxides, [11]. Oxidation in the presence of water has also

been found to reduce the plasticity of stainless steel

oxides, but the exact mechanism is unclear. The

reduction in plasticity is believed to make stainless steel

oxides more prone to spallation, as they cannot relieve

the induced stresses [9]. The presence of water can also

affect the mechanical properties of the total oxide

thickness, such as increasing adherence for iron oxides

and reducing it for chromium oxides. The reduced

adherence of chromium oxide could be due to an

increased chromium oxide thickness arising from the

increased kinetics of oxidation [11]. The combination of

reduced plasticity and mechanical strength across the

intermittent mixture of re-solidified metal and metal

oxide of stainless steel dross, coupled with cyclic

thermal and mechanical loadings, induced by the laser

beam and the instability of the gas/water mixture

flowing through the cut kerf, could be a possible

mechanism for detachment of stainless steel dross from

the kerf in underwater cutting.

It is interesting to compare, for underwater cutting, the

results of the dross adherence experiment and the mass

reduction experiment, to test the hypothesis that high

amounts of attached dross correspond to low levels of

mass reduction. From the results, it is possible to

compare 6 and 12mm thickness cutting of both

materials, at both positions of the beam focus, at the

cutting speeds mentioned in section 5.4.3, in terms of

dross height and mass loss per metre of cut. Four

comparisons can be made. The first is for 6mm thickness

C-Mn steel, where the lowest mass loss condition

(FP=15) does not equate to the corresponding highest

average dross height (FP=0). The second is for 12mm

thickness C-Mn steel, where the lowest mass loss

condition (FP=15) does equate to the corresponding

highest average dross height (FP=15). The third is for

6mm thickness stainless steel, where the same is true.

For the fourth, 12mm thickness stainless steel, it could

be argued either way, as the mass loss is effectively the

same for both cutting conditions (FP=0 and FP=15).

This means the exact relationship of what is completely

removed from the kerf and what is left attached to the

sides of the cut, is a complex one, which needs further

attention.

For both steels, it appears that there is a transition to

increased levels of dross height when underwater cutting

below 500mm/min. These trends indicate that in order to

achieve more attached dross, it may be beneficial to cut

slowly, regardless of material thickness. It was also

noted that, especially in C-Mn steel, dross height was

considerably higher, when cutting underwater, compared

with cutting in air, as can be seen in Figures 6 and 7.

EDX analysis of the samples cut underwater indicates

that the elemental composition of the re-solidified metal

was similar to that of the parent material, for both steels.

However, the composition of the C-Mn steel oxide

consists mostly of iron oxide, followed by manganese. In

the case of stainless steel, the oxide consists mostly of

chromium oxide and also high levels of manganese. This

dross enrichment while laser cutting of stainless steel has

been noticed in another earlier study [8], where it was

reported that aerosols produced from in air Nd:YAG

laser cutting were enriched in chromium, manganese,

and nickel. For underwater cutting, an enrichment in

chromium and manganese was seen but for nickel, a

reduction was seen, [3].

The mass reduction study showed that in laser cutting of

both C-Mn and stainless steel, there is the potential to

reduce secondary emissions, in the form of released

dross and fume, for material thicknesses greater than

6mm, by cutting underwater. Generally, a higher plate

mass reduction was associated with cutting stainless

steel. The combination of larger kerfs and fragmentation

of the dross may be the cause of the higher mass loss

noticed in cutting stainless steel underwater. From the

point of view of producing minimal mass loss, it would

be better to laser cut both these steels (in either cutting

environment) with reduced laser power density. (In this

work corresponding to the beam focus position 15mm

above the plate surface). The mass reduction study also

indicates that when laser cutting both steels at 6mm

thickness, the cutting gas pressure and the environment,

have a negligible influence on secondary emissions. It is

anticipated that if laser cutting even lower thicknesses of

material, the influence of focus position, to an extent,

will also be negligible. A main benefit in underwater

laser cutting of both C-Mn and stainless steel, would be

that secondary emissions, in the form of dross and

fumes, would be suspended in water rather than being

airborne.

6. Conclusions

This work on underwater cutting using a 5kW laser

source and optical fibre delivery of the beam, has

allowed the following conclusions to be made:

A cutting head has been successfully developed for

use underwater, which employs separate jets of

compressed air to create a localised dry zone in the

area where the laser beam interacts with the material

being cut and to remove molten metal from the cut

kerf.

This system has been shown to be able to cut C-Mn

steel underwater, at thicknesses up to 40mm. At

cutting speeds above 200mm/min, the process is very

tolerant to changes in stand-off distance and cutting

gas pressure. Below this speed, the use of a high

cutting gas pressure is important.

At high cutting speeds, the underwater performance

is close to that in air. At the lowest cutting speeds,

the in air performance is only significantly better than

the underwater performance, when a high cutting gas

pressure is used.

For underwater cutting, the attached dross height

increased with an increase in steel thickness and laser

power and decreased with an increase in cutting

speed. In addition, the average dross height produced

with the beam focus above the plate was generally

higher than that produced with the beam focus on the

plate.

For underwater cutting of stainless steel, it can be

seen that the average dross height, certainly for any

speed above 500mm/min, is much lower than that

observed when cutting the C-Mn steel. However,

dross height increases with a decrease in cutting

speed, particularly for higher plate thicknesses but at

all speeds above 250mm/min, is less than that

recorded for cutting C-Mn steel.

The cutting gas pressure appears to have very little

influence on dross height for 6 and 12mm thickness

C-Mn steel cut in-air. When cutting C-Mn steel

underwater, there is an increase in recorded dross

height, compared to the in air results, for both

material thicknesses.

In contrast to the C-Mn steel results, when cutting 6

and 12mm thickness stainless steel, both in air and

underwater, the recorded dross height appears to

decrease as the cutting gas pressure increases.

For both the 6mm thickness C-Mn and stainless

steels, there appears no potential benefit, in terms of

minimising mass loss, by cutting underwater. As the

thickness increases to 12mm, when cutting with a gas

pressure of 8bar, there appears a significant benefit,

in terms of minimisation of mass loss, by cutting C-

Mn steel underwater, at either beam focus position.

For the stainless steel at 12mm thickness, the only

benefit that can be seen by cutting underwater, occurs

with the beam focus position on the plate surface.

7. Acknowledgements

The authors would like to thank Frank Nolan, Paul

Fenwick and Matt Spinks who conducted the cutting

trials. The work reported in this paper was part of the

LaserSnake2 collaborative project which is co-funded by

the Technology Strategy Board, the Department of

Energy and Climate Change and the Nuclear

Decommissioning Authority of the United Kingdom of

Great Britain, under grant number 110128.

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