AbstractAustenitic stainless steel is used widely in almost all industries due to their excellent properties. 304SS is one of these types used; however, its pitting resistance is low because of the formation of the MnS inclusions that serve as initiation sites of the pitting corrosion. One can increase the pitting resistance of this steel using some method of reducing or eliminating the amount of MnS inclusions present in the austenite phase. Laser surface melting can do this job. Laser surface melting is one of the laser surface processes used to treat different type of materials to improve their properties. Upon treatment by LSM, 304SS shows increase in pitting resistance due to the formation of the delta-ferrite phase in the melting pool where the solubility of the sulfur is much higher, hence hindering the formation of MnS inclusions. In this experiment, different scanning speeds of LSM have been examined to study the effect on the pitting resistance of the 304SS. The most notable conclusion is that as we increase the scanning speed, the pitting resistance of the 304SS decreases.

IntroductionAustenitic stainless steels are widely used in different industries because of their desirable mechanical and electrochemical properties. AISI 304 SS is widely used in process industries because of the low cost and excellent corrosion resistance properties. However, the pitting corrosion resistance is not high enough for durable service in industries. This is due to the formation of the MnS inclusions in the austenitic stainless steels which serve as initiation sites for pitting corrosion. The pitting corrosion resistance can be increased by introducing the ferrite phase in the austenitic stainless steels because sulfur has much higher solubility in ferrite phase than that for austenite phase.

Laser surface modification processes have become widely used because of the low cost and brilliant control of the processes. Laser Surface Melting is one of these processes which involves rapid melting and rapid solidification of the austenitic stainless steel producing fine-grained microstructure of delta-ferrite phase. This, as discussed above, will modify the pitting corrosion resistance of the AISI 304 stainless steel. One more advantage of using LSM for curing austenitic stainless steels is the redistribution of the Cr atoms in the substrate yielding to better passive film formation, hence, better pitting corrosion resistance [1].

Experiment:The equipment used in the experiment is a Rofin-Sinar 2kW CO2 laser which is coupled to an Aerotech CNC x-y table through a beam delivery system and laser processing head as shown in figure1. After completion of the LSM process the specimens then were sectioned, polished and electrochemically etched with oxalic acid at 1A/cm2 for 15 seconds [2]. After that, the specimens were examined under the optical microscope to note the difference between the specimens before and after LSM treatment.

Table 1 summarizes the six different specimens were tested in the experiment. For the single track experiment, different speeds of the laser beam were examined to study the effects of scanning speed on the melting pool dimensions and the microstructure of the specimens. Similar analysis was conducted for the overlap specimens with overlap ratio of 50%. For the pitting corrosion resistance, two polarization curves for the two different speeds were plotted and compared to the as-received specimen.

Table 1: Laser operating parameters used in the experiment

Results:Figures 2 through 5 show the microstructure of the single track specimens examined under the optical microscope. The melting pool dimensions are summarized in table 2 below. Figure 6 shows the melting pool dimensions as a function of scanning speed. From the table 2 and figure 6 we notice that as we increase the scanning speed; while keeping the spot size and the power constant, the melting pool dimensions decrease in both the depth and the width. This can be interpreted as the melting of the surface of the stainless steel at lower speed is higher because the surface is under effect of the laser beam for a longer time. Notably, the melt width for the specimen at 60 mm/s speed is much higher than that of the remaining specimens. If we exclude the experimental and human errors, it can be inferred that there is a critical speed at which the melting pool dimensions decreased dramatically with scanning speed. More tests should be conducted to reveal the right conclusion as our conclusion depends on one test only.

Table 2: Melting pool dimension of the single track specimens

SpecimenScanning Speed

mm/sMelt Depth, mm Melt Width, mm

S1 60 0.22 2.16S2 90 0.17 1.66S3 120 0.09 1.64S4 150 0.05 1.62

The as-received 304SS specimen when examined under the optical microscope is shown in figure 7. From this figure, we can obviously figure out the austenitic structure of the 304SS as the bulky globular grains. After laser treatment using LSM method, one can notice the difference between the microstructure of the bulk material and the microstructure of the deposit caused by the laser treatment. The bulk stainless steel material is the same as the as-received specimen, austenitic. However, the deposit microstructure, figure 8, is columnar and dendritic grains. Columnar grains forms for the rapid cooling while the dendritic ones form under slow cooling of the melt pool. The phase in the melt pool is δ-ferrite as it is the first phase to form after solidification of the austenitic stainless steel [1]. This delta-ferrite phase is what gives LSM-treated 304SS its desirable and better pitting corrosion resistance as discussed above. The tiny bars observed in the austenite are the undesirable inclusions; MnS among others, that formed in longitudinal bars along the rolling direction of the steel [1]. Final conclusion of this test is that as we increase the scanning speed, the ferrite phase presence decreases also.

Sample No.Laser

Power, W

Laser beam spot size,

mm

Scanning velocity,

mm/sRemarks

S

1

1600 2

60

Single track2 903 1204 150

W1

1600 260

Overlap2 120

To cover large surface area, the laser beam has to pass multiple times over the surface of the stainless steel. Therefore, overlap technique is used. Indeed, to get nice constant depth of the protective layer of the melted surface, overlapping to about 50% is used. This ratio is what was used in our experiment. Figures 9 and 10 show the microstructure of the overlap technique under the optical and scanning electron microscopes respectively. From figure 10, we can see small transformation of the δ-ferrite back to austenite in the reheated zone. This is because the increase in temperature promotes the transformation of ferrite to austenite.

Finally, the electrochemical properties; the pitting resistance in this experiment, increased as a result of the LSM treatment of the AISI 304SS. This is again due to the formation of the ferrite phase in the austenitic matrix of the stainless steel. MnS inclusions are believed to be the initiation sites of the pitting corrosion of the stainless steels. Introducing the ferrite phase increases the solubility of the sulfur inside it and prevents the formation of the MnS inclusions, hence increasing the pitting potential of the stainless steel. Most importantly; as we increase the scanning speed, the pitting potential decreases. This, as pointed above, due to the less amount of ferrite phase formed for higher scanning speeds. The less amount of the ferrite phase means that there are some sulfur not fully dissolved in the ferrite phase and might form the MnS inclusions that would initiate the pitting corrosion of the stainless steel. Polarization curves of the different 304SS specimens are shown in figure 11.

Safety Precautions:The laser produces an intense, highly directional beam of light. The human body is vulnerable to the output of certain lasers and, under certain circumstances; exposure can result in damage to the eye and skin. The eye is almost always more vulnerable to injury than skin. In addition, there are non-beam hazards and electrical hazards where most lasers make use of high voltages that one should be aware of. Any use of the laser equipment should rely under fully addressed standards such as IEC 60825 and ANSI Z136. Report of all incidents during the use of the laser equipments should be reported immediately to avoid further damage of human body and laboratory assets [3].

Conclusions: LSM is becoming more popular surface treatment of the austenitic stainless steels

because of the low cost and excellent results obtained LSM treatment of the AISI 304SS has increased its pitting resistance The LSM treatment produces duplex microstructure with austenite and ferrite MnS inclusions are no more present for the LSM-treated 304SS, because of the

high solubility of the sulfur in ferrite phase As the scanning speed of the LSM increases, the melt pool dimensions decrease Higher melt pool dimensions for 60mm/s scanning speed compared to the other

speeds Overlapping technique in LSM used to cover large surface areas transforms some

of the ferrite phase in the reheated zone to austenite increasing it is susceptibility to pitting corrosion. However, enough amount of ferrite phase can dissolve all the sulfur present

Pitting potential of the stainless steel was increased after LSM treatment. However higher scanning speed lowers the pitting potential; still above the as-received pitting potential; due to the less amount of ferrite formed at higher speeds

References:

[1] Laser Surface Melting Of 304 Stainless Steel For Pitting Corrosion Resistance Improvement; T.S. Seleka and S.L. Pityana; 8th International Corrosion Conference; The Southern African Institute of Mining and Metallurgy

[2] Laser Surface Treatment of AISI 304 Stainless Steel; Ziu Liu; Corrosion and Protection Centre; The University of Manchester, 2007

[3] Laser Safety; [http://www3.imperial.ac.uk/safety/training/presentations], accessed 28th December 2007

Appendix:Figures cross-referenced in the report.

Figure 1: Schematic diagram of the set-up used [2]

Focusing mirror Laser

Laser beam

Workpiece

CNC unitWorkstation

Processing head

Laser beam

Nozzle

Melt pool

Workpiece

Travelling direction

Figure 2: Optical micrograph of single track specimen at scanning speed of 60mm/s (S1) at 5x magnification

Figure 3: Optical micrograph of single track specimen at scanning speed of 90mm/s (S2) at 5x magnification

Figure 4: Optical micrograph of single track specimen at scanning speed of 120mm/s (S3) at 10x magnification

Figure 5: Optical micrograph of single track specimen at scanning speed of 150mm/s (S4) at 5x magnification

0

0.5

1

1.5

2

2.5

50 60 70 80 90 100 110 120 130 140 150 160

Scanning Speed, mm/s

Mel

t P

ool D

imen

sion

, mm

Melt Width Melt Depth

Figure 6: Melting pool dimensions as a function of the scanning speed

Figure 7: Optical micrograph of as-received AISI 304SS [2]

Figure 8: Optical micrograph of LSM-treated AISI 304SS at 20x magnificaiton

Figure 9: Optical micrograph of overlapping region

Figure10: SEM micrograph of overlapping region [2]

Figure11: Polarisation curves of the 304SS before and after laser treatment [2]

As-received LSM at 120 mm/s LSM at 60 mm/s