The Effect of Laser Shock Peening on Intergranular Corrosion of Sensitized 316SS

Ryan Trice Author

07/29/2021 Added

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A study on the effect of LSP on the intergranular corrosion of 316 stainless steel using EPR testing and micro-structural analysis.

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[00:00:01.590]Hello. My name is Ryan Trice.
[00:00:03.420]And over the summer I researched the effect of laser shock peening
[00:00:07.310]on intergranular corrosion of sensitized 316 stainless steel.
[00:00:12.830]Laser shock peening
[00:00:14.120]is a novel surface modification technique used to induce compressive residual
[00:00:18.650]stresses on the surface of a
[00:00:20.150]material. This works through the
[00:00:22.370]explosive creation of a plasma producing a high energy shock wave,
[00:00:26.960]which then travels through a sacrificial material and it induces plastic
[00:00:31.040]deformation and compressive residual stresses on the surface of the material.
[00:00:37.700]Okay. It's been shown that compressive residual stress can greatly decrease the
[00:00:41.990]intergranular corrosion susceptibility of austenitic stainless steels.
[00:00:46.940]Intergranular corrosion is a attack in the vicinity of the grain
[00:00:51.530]boundaries of a stainless steel.
[00:00:53.570]We can see an example of this below in the diagram indicated by the red arrows,
[00:00:58.700]the samples were cut from commercially purchased 316 stainless steel
[00:01:03.530]flat bar stock.
[00:01:05.210]They were cut into 12 by 12 millimeter sample sizes.
[00:01:09.920]From here, the samples were put in a furnace to perform heat
[00:01:13.460]treating this took place at 700 degrees for 48 hours.
[00:01:18.080]Along with this,
[00:01:19.010]they were mounted in epoxy and electrical connections were made at the back
[00:01:23.390]One of the reasons for this mounting
[00:01:25.310]was to isolate a single surface of the material for analysis
[00:01:29.570]And corrosion susceptibility. Mechanical polishing
[00:01:33.030]Was then performed on this isolated surface with 600, 800, 1200 grit
[00:01:37.700]Silicon carbide sand paper.
[00:01:40.340]This was used to get a mirror polish on the surface along this electro-
[00:01:45.140]polishing was performed to get a more consistent surface layer.
[00:01:49.640]Following this electro- chemical reactivation
[00:01:52.460]testing (EPR testing) was performed on the materials to
[00:01:57.230]test their
[00:01:57.650]susceptibility, to intergranular corrosion.
[00:02:01.400]Following EPR testing
[00:02:03.110]The surface was analyzed using microscopes.
[00:02:05.870]To further characterize the depth and extent of grain boundary attack
[00:02:11.000]This cycle of mechanical
[00:02:12.200]polishing, electropolishing,
[00:02:13.410]EPR testing and surface profiling was repeated multiple times.
[00:02:18.020]On the samples to get a better idea of
[00:02:21.530]it's before laser shock peening corrosion susceptibility.
[00:02:28.250]Following this testing,
[00:02:30.470]the sample was then processed with laser shot peening at 10 scans
[00:02:35.750]from there
[00:02:36.170]it underwent EPR testing and surface profiling.
[00:02:40.130]These results were then compared to the previous conditions.
[00:02:44.600]If you would like to
[00:02:45.570]know more about the methods used,
[00:02:47.390]I would suggest pausing the video here and reading through this more in depth description
[00:02:52.820]Now I will discuss the results of our experimentation.
[00:02:58.070]First, we looked at effect of Laser shock peening on an
[00:03:02.550]un-heat treated sample of 316 stainless steel. from the graph
[00:03:07.380]on the right.
[00:03:08.010]We can see that laser shock peening produces a compressive stress on the surface of
[00:03:12.450]the sample.
[00:03:14.010]We can also see that this producing mass compressive stress at negative
[00:03:18.570]216 MPa
[00:03:20.490]plus or minus 30 MPA
[00:03:25.230]This took place at 10 laser scans or shooting the surface of the
[00:03:30.000]sample 10 times with the laser. From this point 10 scan,
[00:03:34.560]laser shock peening processing was then used for corrosion evaluation of
[00:03:39.210]the sample. In these sample too the left,
[00:03:42.030]we can see the results of EPR testing for sample 4 the lower purple
[00:03:46.920]line is the test result of sample four after 10 scan, laser shock peening,
[00:03:53.190]and the in figure. We can see a clear decrease in the reactivation peak.
[00:03:56.970]of the laser shock peening process sample the reactivation.
[00:04:00.570]Peak is the highest current density experienced by the sample.
[00:04:03.860]Compared to the base trials performed on sample.
[00:04:07.470]4, we see a 55.6% decrease.
[00:04:10.380]in the reactivation peak. Compared to the lowest observed reactivation,
[00:04:14.700]activation peak of all the samples.
[00:04:17.220]We see that the 10 scan laser.
[00:04:19.410]shock processed sample had a 10.5% decrease
[00:04:24.300]the reactivation peak, looking at it.
[00:04:26.910]The image to the right. We can see some intergranular corrosion after.
[00:04:30.750]EPR testing, here we see a grain.
[00:04:33.180]structure with some ditching at the grain boundaries, along with step features.
[00:04:38.280]Due too, no grain being fully surrounded by ditching.
[00:04:40.830]This would be considered a dual structure.
[00:04:43.200]etching sample. We.
[00:04:44.970]Also see some pitting appear in the sample.
[00:04:47.850]As seen in the red circles. This picture.
[00:04:50.340]Is taken up a 316 sample before laser shock.
[00:04:53.940]peening. The LSP processed.
[00:04:56.460]Sample showed no considerable difference in the etching structure when compared
[00:05:00.930]to the 316 unprocessed
[00:05:03.020]sample. Continuing.
[00:05:05.310]The micro-structural analysis of the samples.
[00:05:08.580]The LSP processed
[00:05:11.190]sample's grain size was found to have no considerable difference between.
[00:05:15.690]That and the unprocessed samples.
[00:05:18.810]This was important when calculating the normalized charge of the sample
[00:05:23.010]due to the normalized charge,
[00:05:24.540]taking into account the grain boundary area of the sample. .
[00:05:28.950]Looking to the graph to the left.
[00:05:30.900]We can see,
[00:05:31.380]the normalized charge of sample 4 along with the mean normalized charge
[00:05:36.090]of samples, two to six,
[00:05:38.610]and the normalized charge of the sample after 10 scans of laser
[00:05:43.140]shock peening. When comparing inter sample trials for sample four,
[00:05:47.700]you see that LSP had a 51% plus or minus 4%
[00:05:52.590]decrease in the normalized charge.
[00:05:55.860]And compared to all other sample trials, we see that the laser.
[00:06:02.450]shock peening processed sample had a 48% plus minus 18% decrease in normalized
[00:06:07.410]charge.
[00:06:08.750]The reason normalized.
[00:06:09.920]Charge is helpful in defining.
[00:06:12.680]The intergranular corrosion susceptibility of a sample is that.
[00:06:17.360]It helps to characterize the extent of attacks at the grain boundaries.
[00:06:22.550]The key results we can take away from this data is that the laser shock peening
[00:06:26.720]processed sample,
[00:06:28.190]shows a clear decrease in the reactivation peak and the normalized
[00:06:33.170]charge when compared to the unprocessed samples,
[00:06:36.510]along with this the average grain size did not appear to change following ten scan
[00:06:40.610]laser shock peening.
[00:06:42.680]The etching structure of their sample also appeared consistent between laser shock,
[00:06:46.400]peening and unprocessed samples.
[00:06:49.640]However more work needs to be done in evaluating the depth of the grain
[00:06:53.000]boundary attack in both cases. Overall,
[00:06:55.940]these results point towards a clear decrease in the intergranular corrosion
[00:07:00.150]susceptibility of 316 stainless steel following laser shot,
[00:07:04.580]peening of the sample. However more work should be done in evaluating.
[00:07:08.840]this decrease. Suggestions for further work include: further data
[00:07:13.760]collection of EPR tests of laser shock peening processed,
[00:07:17.870]samples. Use of other standard intergranular corrosion tests,
[00:07:22.400]such as bottling nitric acid test. More research into the role of molybdenum
[00:07:27.350]and its effects on intergranular corrosion.
[00:07:31.070]Furthermore depth profiling of the residual stress would be helpful in
[00:07:34.880]characterizing the effects of laser shock peening below the surface of the
[00:07:38.960]sample. I would like to thank Yongchul Yoo and Dr. Cui
[00:07:42.710]for their support and mentorship over the summer
[00:07:45.260]I'd also like to thank the National Nanotechnology
[00:07:49.460]Coordinated Infrastructure and the National Science
[00:07:53.240]Foundation for their funding and support of this research.
[00:07:58.000]Thank you and have a nice day.

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The Effect of Laser Shock Peening on Intergranular Corrosion of Sensitized 316SS

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