Grain direction is most commonly heard in phrases such as “going against the grain”, which results in what is called tear-out, when the wood fibers have been lifted up causing a rough surface (Gochnour 2004). I want to close with giving you some references that you can look for. That’s a typical morphology of a starch grain; the black lines are known as isogyres. If we take a look at it with darkfield illumination, we don’t see the scratches quite as much, but it’s really highlighting all the little inclusions that are on the surface or within the surface of that metal. This is an outstanding book because the author goes through some preparation techniques, metallography preparation techniques, optical microscopy stuff, different alloys, different cross sections from metal forming, deep drawing operations, welding operations, other joining type systems, and so forth. Note also in the NDIC image that all the little scratch marks that we saw earlier with the NDIC unetched are gone. This is a longitudinal section, so we can see the elongation of the grains that occurred during the cold-working process to thin the metal sheet down to the final thickness. Here is one of my favorite cross section images. There is a dark kind of webbing type net in the structures that is interwoven in there amongst the flatter grains in the sample. We’re seeing the scattering of light due to angles and topography differences, and so forth. In this particular case, we can see that there are lots of little scratch marks on the metal, and that’s from the polishing operation that was done on this one, and for the metallographers out there, if you look at this sample just visually after it has been polished, you’ll say “Wow, that’s a nice smooth mirror finish on that metal.” Put it under NDIC sometime and you’ll see that maybe it’s not quite as smooth as you thought. JavaScript is disabled. Here is a brightfield examination of a polished cross section of stainless steel that has very severe stress corrosion cracks. is often the result of. Now we can do some other things. If you are uncertain of the grain direction, there are techniques you can use to determine the direction in papers. WN: There are accessories for the NDIC. I am going to finish up a little bit here to give you four different images of the same region—the same field of view—using each of these four techniques to show you what they can really tell you. Now the brightfield, darkfield, and crossed polarizers are probably the most common illumination methods you’ll see in various publications and books, and so forth. As far as can you order steel with a certain grain direction in relation to it's length, I have no idea why you would want to do this but if you wanted to pay enough money, as in thousands of dollars for a large run, you could probably get it done. Tin, again, is an anisotropic metal; the iron base steel is not. Remember that you are dealing with a metal not wood; the grain is totally different in the two. four techniques to show you what they can really tell you, Nomarski differential interference contrast (NDIC). Then we have the book Images Of Materials. That is why when used in applications requireing max strength, forging is used for the primary shaping. In reflected light, there is a polarizing filter inserted into the light path, and then the analyzer is placed elsewhere there, and you rotate the analyzer. Wayne Niemeyer (WN): Thank you, Chuck and welcome everybody. If your tools are sharp and you’re cutting with the grain, your woodworking experience will be an enjoyable one. In other words, in the electrochemical sense, the weld becomes anodic, the base steel metal becomes cathodic, and now when you have a corrosion reaction process starting, the corrosion reacts very rapidly within the anodic region; in this case, the weld. The test slices should be at a 90 degree angle to each other. Tin, being an anisotropic metal, will show up because of the crossed polarizers, and this will help highlight it. Eight main factors affect deformation during sheet metal forming, and many relate to m… It’s actually better to look at it by scanning electron microscopy. On the left we have our plane polarized light, and on the right is the darkfield illumination of the weld zone. |. Cast steel parts and tools have a random grain direction and are in some applications, such as springs or pry bars, inferior to forged steel, even if only slightly. Look inside the red box here, you might not be able to see it too well, but there’s a grain in there that has parallel structures spaced at about at a 45° angle, and spaced only a micrometer or two apart from each other. Those reflections coming directly back up are actually blocked in the objective system, so that we wouldn’t see that. Our webinars provide insightful information and innovative approaches to microanalysis that apply to a wide range of industries. It’s actually pretty good for fiber use, looking especially at the natural fibers, to look at the surface structure of natural fibers. That’s the tin coating; the tin plating showing up now that we can’t see any other way by the light microscopy techniques. Wayne is a Senior Research Scientist with McCrone Associates who has over 40 years of experience in both electron microscopy and light microscopy. I am going to spend some time talking about this type of sample: it is a side seam joint weld from a tin plated steel food can. Controlled heat through thermal cycling determines the size of the grains, and properly done makes them of a uniform size, whether that be large or small, depending at which step the process is at. Some of these dark specks have what we call “comet tails” coming off of them like they are smeared a little bit; a smearing action. We can manually control the grain direction of the metal in the flat pattern, and subsequently how the bounding box is oriented to address these concerns. Of course there are other shapes for the basic crystals in different crystalline substances but a cube is the basic shape of an iron crystal and iron crystals form the matrix for steel. groups of grains make up the steel. But, in metals or surface imaging like this, we are actually seeing topography differences, and that can become very useful. Here we can see the grain structure outlines very easily—the nice light outlines, and then the dark regions are the flat regions of the grains that are reflecting directly back up to the objective and to the eyepiece. Directional grain does impart a tad bit more load resistance in high stressed parts and tools. However, wrong grain direction makes book less usable and even leads to structural damage and early deterioration: When you turn the pages, they tend to bend a bit. Actually, a good way to monitor the quality of your polishing operations is to look at it with NDIC. This particular book has all kinds of different alloys and is just loaded with pictures of microstructures, surfaces, and cross sections, and so forth. If you are a generalist working with different types of metals, alloys, and so forth, and want to compare what they look like under light microscopes, this is an outstanding resource for that. It’s normally used in transmitted light mode for biological samples, for cell structures where you have very little contrast; the transmitted light Normarski differential interference contrast gives you much better contrast on low-contrast-type materials. Another outstanding reference book. Let’s take a look at some contamination on tin plated steel. It’s final polished with 1 µm diamond paste slurry, and the brightfield illumination is what most metallographers would be looking at to look at their polishing system. Pages may also resist turning if the grain direction is wrong. The amount of these in current tool steel is close to non existent. More often than not however, novice woodworkers end up frustrated by ugly … you will have less problems if metal is annealed or … It has tons of different materials: refractory metals, stainless steels, nickel alloys, copper alloys, titanium alloys, electrical contact materials, carbon steels, steel castings, and tool materials—everything all in one book. The black material is now showing up as orangish-brown material and there’s a lot more of it—smaller features kind of scattered around that image—that we really wouldn’t even notice with the brightfield illumination. I always specify a grit (typical sandpaper grit) and specify the direction when it matters on the drawing. Notice the NDIC is giving you some three dimensionality perspective of those weld grains from the etching process. After finding the direction, it is recommended that one cleans their appliance in the same direction, along and not across the grain. Again, on the aluminum end, we can cut through the score area of the flat— that’s what opens up the hole so that you can pour the liquid out when you pull that tab up. Keep track of … While it may sound simple, it can be much more difficult than you can imagine. Now, you can take something like this image and take it offline into an image processing software and do particle sizing, particle distribution, particle frequency, you can do particles per unit area, and things like that to help characterize the sample. WN: There certainly are, because I have them on mine. If you want to do some reading on it, I would recommend. Grain lines occur while the fabric is being made, with fibers being woven together. This is the only way that I know of that you can really see tin coating that thin. You will be able to tell at a glance what direction the grain likely runs in almost any board, even if that board is rough-sawn and the grain lines are almost impossible to see. George is a world-renowned expert metallurgist, metallographer, fracture analyst, and so forth, and he does a lot of webinars also for these types of things. 36 minutes. In other words, it can be perpendicular to the length or parallel. Very, very fine detail that you can see with NDIC that you’re just not going to see in any way other than light microscopy. WN: Stress corrosion is caused by, obviously, a corrosion reaction on metals that have been stressed in some way or another. The first 50 pages are his chapter, and then after that it has all kinds of images from different techniques: scanning electron microscopy, transmission electron microscopy, and X-ray diffraction, and so forth; lots of different material images by various other instrumental techniques. At an angle of 45 degrees from the selvage is the bias of the fabric. The center part of it (just to the left of center)—now we see that that lighter gray material is now kind of a bright white material under the crossed polarizers, so we can do one more thing: we can insert a first order red compensator into the light path, and that will also now allow us to see that these bright materials that are laying there on the surface are actually particulates that are crystalline materials; they are quite birefringent. Like • Show 0 Likes 0. Now you can take an image like this—a digital image like this, and take it into some image analysis software and do some, perhaps, grain sizing or frequency distributions, percent coverage of specific size of the grains, and things like that. Our presenter is Wayne Niemeyer of McCrone Associates. Forging modern steel that has been rolled will not improve its grain. Now if we cross the polarizers—and these are unetched again—now we can just highlight the interior coating if we want to by adjusting brightness and contrast, and take the metal out of the image, if we’d like. In this stage, the microstructure of the metal becomes coarse and makes the workpiece lose some of its strength. The grain direction in any board can be read using at least two different methods. Or, we can go ahead and insert that first-order red compensator and just highlight the base metal itself, separate from the coating, and have the two side by side. The surface of tinplate steel is quite smooth and bright, and in brightfield episcopic illumination it appears quite bright and you sometimes have to reduce the intensity of the illumination to see the structures a little bit better. It’s just a matter what you want to try to show as to the types of accessories and filters, and so forth, that you want to put into the system. Grain growth is the third stage and this happens when annealing is allowed to continue after recrystallization is completed. Speak with a scientist. Sometimes it’s a thermal set epoxy, other times it can be a two component epoxy that cures at room temperature. If the grain direction is strong enough, you can identify the direction by “feel.” Take the sheet of paper in your hands and gently curl/bow the paper towards the middle, first in one direction and then in the other … We can also go to colorizing the features in here with the etch. Acually, the grain direction is not just cosmetic, but adds only a little more strength than cross grain does, and does have a slight advantage over cross grain. I have never been concerned with the grain direction just grain size. Here is an examination of some polished beryllium metal. Stage 3 – Grain Growth. There is no difference, all things being equal in steel and HT. You’ll notice at the bottom of the image there is a slight kink that has formed here, at the very bottom, and we can see that there’s some strain pattern going on, and this is a possibility now for some residual stresses that could be in this area that might eventually lead to a crack. Notice they’re circular; they have black lines going through them, and when I initially saw this, I thought for sure that these were starch grains. Light microscopy imaging techniques, such as brightfield, darkfield, and Nomarski differential interference contrast (NDIC), are used to examine metal surfaces and polished/etched metallographic cross sections. Those stress areas are going to be much more susceptible to corrosion attack. This is unetched; we don’t want to see the grain structure so much—we’re more interested now in actually seeing the corrosion cracks rather than the grain structure of the metal. Only in size. In cases where there are multiple bends in different directions, causing the grain line to run parallel with the bend, it is always worthwhile considering your material choice and thickness and also the size of the radius to limit cracking. Grain direction is set by the rolling, and is set in a directional pattern. So, here you can take these images, digital images, and put them into some image analysis software, and perhaps do some analysis for percent area coverage of the phase that is represented by the web-like material, something like that. Strength can typically be regained through a … In addition to the new "bounding box" sketch, there is a new option in the property manager for the Flat-Pattern feature: "Grain Direction".You can define a grain direction to use when calculating the bounding box for sheet metal parts. I do PLM fiber identification and I am new to NDIC. As far as cosmetic, you cannot see grain direction in steel with the naked eye anyway, unless you see alloy banding, or some similar flaw that might follow grain direction, or after using acid on it. A metal’s strength is directly related to its grain size; reducing grain size is a common way to improve material strength which often has little to no effect on the material’s … We want to get past that cutting damage into the more pristine base metal, then we can final polish these cross sections, and perhaps even etch them like I have shown on the right-hand side here with the aluminum from this beverage can end. Grain direction and its relationship to cutting tools is one of those mysteries most beginner woodworkers struggle with. Here’s an example of an electrolytic tin plated steel surface with crossed polarizers. It is an absolutely outstanding book. Now I will hand the program over to Wayne. Parallel to the selvage are long fibers called the warp, while perpendicular to the selvage are shorter fibers called the weft. Now the weld, in this case, is covered with an internal coating—that’s called the side seam stripe—to protect the weld from corrosion by the product that’s in the can. But, we can certainly see it in this imaging technique with NDIC. In today’s webinar, Wayne is going to talk about Ways to Examine Metals by Light Microscopy. On the left-hand side we have the plane polarized light illumination. Q: “What kind of configuration is needed for NDIC?”. You can also use this as sort of a quality test on your metallography to find out if you’ve actually removed the scratches and gotten away from the surface, and gone down more into the base metal. I can't tell a lick of difference which way a blade came of the sheet. Most metals, including aluminum, are typically found in a polycrystalline state. As a general rule, the grain line must run perpendicular to the bend to avoid the potential for cracking or fracture. It is atomically arranged crystals that form grains. Thanks. There might be books just for zinc alloys, and other books for copper alloys, and other books for steel alloys, and so forth, which is fine, and they are more up-to-date of course, but this particular book has them all. This is copyright 1991 by Oxford University Press. This particular can does not have a coating around all of the surfaces of the tin; only around the weld, and that coating is about a half inch or so in width to protect the weld. I suspect that the recrystallized material that we’re seeing in there—from the birefringence with the crossed polarizers—is probably from the tin that melted during the welding process and diffused into that fusion zone from both of those plates as they were welded together. First of all, that internal protective coating is now very bright white. 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