From: kelvin 15/02/2000 1:12:00 Subject: Metals Manufacture post id: 38493 Hi there When a metal such as iron is poured from the melt does it have a crystal structure and how important is the nature of the pouring and the structure of the metal influence the end use of the metal? Kelvin From: col 15/02/2000 6:34:00 Subject: re: Metals Manufacture post id: 38503 As far as I know the metal only starts to form crystals as it cools,just like water freezing.It is the manipulation of these crystals as the metal cools(by rolling,hammer forgeing etc)that gives different properties, eg.strength,hardness,workability etc.These properties are also influenced by various other things like addatives, heat treatment and other metals present as alloys. From: Alan™ 15/02/2000 8:11:00 Subject: re: Metals Manufacture post id: 38505 Kelvin, I'll give a complete answer when I get home from work. But douring pouring the metal is cooling and as a result some nucleation and crystallisation is occurring. The mechanical properties are effected by the pour. From: Brendan 15/02/2000 10:22:00 Subject: re: Metals Manufacture post id: 38518 Hi Kelvin, The crystallisation of iron is different for the different types of iron, pig iron, cast iron etc, will have different impurities in them. Not much of a memory on this but as far as I can remember the strongest steel is grown as one crystal therefore increasing it's strength. So I assume it also has something to do with the cooling rate. From: Alan™ 15/02/2000 20:22:00 Subject: re: Metals Manufacture post id: 38723 And now for the more complete answer. When a metal such as iron is poured from the melt does it have a crystal structure? I assume you remember some of your phase diagrams from your course and the did show you a temperature vs composition for a binary system. I know you hated them from an earlier post. The solidification for a pure metal will be higher than a final temperature of solidification for an alloy. But the starting point for the solidifaction will start of the alloy will be the same temperature as the pure metal as the first material to solidify will be pure. As the temperature decreases the composition will change until the eutectic composition is reached. The other thing to look at is what causes the crystallisation. Within the liquid metal you'll often find solid precititates that are sites for nucleation. Sometimes these sites will be the walls of the container holding the metal. Once the temperature of the liquid reaches the solidification temperature. Crystallisation will begin on the nucleation sites. So to answer to answer your question, more than likely unless the material is poured at a temperature much higher than the solidification temperature and the atmosphere is also very hot, so that the metal doesn't begin to crystallise until in the mould. and how important is the nature of the pouring and the structure of the metal influence the end use of the metal? This area can become critical. Probably not so much with continuous casting, but definitely when pouring into moulds, but probably not the way your thinking. The problem really lies with the introduction of trapped gas bubbles, impurities and surface oxidation. This oxidation occurs when ever the liquid metal is in contact oxygen (or air). THus top pouring into a mould is going to give a lot of surface contact. Amongst the better casters, they tend to pour the melt into a reservoir, which allows the oxides, bubble and impurities to rise and then pour through the bottom of the mould using an argon gas shroud. These types of impurites, oxides and gas bubbles, result in similar faults which can cause the cracks to initiate and hence premature failure. Continuous casting also uses very deep reservoirs which allow the above problems to float to the top. The speed of cooling also significantly effects the crystal structure of the material and the final structure of the solid crystal, as you can form meta stable crystal structures, because there wasn't time to form the stable equilibrium structure. This can also be achieved by reheating the material up to a significant temperature (this depends on the metal). Each structure will have different mechanical properties (plus's and minus's). Its the mechanical properties that really control the end use. You have to choose the right material for the right application. In addition faster cooling will result in smaller crystals. These actually tend have higher strength and hardness. Slowly grown single crystals have very good creep resistance. From: kelvin 16/02/2000 0:56:00 Subject: re: Metals Manufacture post id: 38860 Hi there thanks for the answer. it enlightened me immensley. I am not so anti phase diagram but they are not my favourite things in life. So how are the metals used in high temperture and precision environments such as the space shuttle different from those in my back shed -assuming we use the same metals in the construction? Ta kelvin From: Alan™ 16/02/2000 8:16:00 Subject: re: Metals Manufacture post id: 38892 Again, I'll answer your question tonight. It all depends on the alloying elements you use. Using oxides to protect the metal substrate, is the principle way. From: Alan™ 17/02/2000 21:53:00 Subject: re: Metals Manufacture post id: 39592 OK let’s try this again. The general run of the mill metals that you find around your house etc, generally come from about two dozen different alloys. These alloys are mass produced and have generally good properties all round. This includes all your steels, aluminiums, copper/brass/ bronzes, and a few other common ones. As you move into harsher environments, be that from a wear, corrosion, temperature etc, you require more specialized alloys, to handle these environments. These materials are generally available, but sometimes it will involve a special run at the manufactures. The consequence is that these materials will have some very good properties that are required but other non-essential properties are sacrificed. When you get into extreme conditions, you then have to tailor the materials for the specific purpose. A material that is suitable for pumping high-pressure steam may not be suitable is that in another case is the stream at the same pressure and temperature, if that steam was to have a Ph of 6 because of a high concentration of sulfuric acid. Yet if the steam was caustic neither of the other two alloys may be suitable. So how do they come up with these materials? Well if we look at just a simple binary carbon steel (just iron and carbon). This material may have reasonable properties, nothing to fancy. (Note that almost all steel alloys are more complex but ....) An addition of chromium to this base material will have several effects. The chromium reacts with the carbon and effectively reduces the carbon content. At the crystal level the body centered cubic (BCC) structure known as ferrite, field is enlarged, while the face centered cubic (FCC) structure known as austenite, field is reduced. Further there is a chemical reaction between the chromium and atmospheric oxygen that forms a stable oxide on the surface. This oxide layer protects the steel from forming "rust" and if the oxide layer is damaged the oxide layer will reform usually. By the way we just formed a ferric stainless steel (430, etc). Or the martensitic group (similar composition, but due to the heat treatment are a different crystal structure) (403, 410 etc) The addition of nickel into the alloy, will improve the strength, but it will permit the further addition of chromium. The nickel forces the carbon back into the iron, which means that the austenite structure is formed. You still get the chromium oxide layer and all the pluses there, with some better mechanical properties. This is an austenitic stainless steel (304. etc). If you were to increase the chromium content you would move into a region where both austenite and ferrite would co-exist. This is a special range of alloys known as duplex stainless steels (Sandvic’s 2304, 2507 etc) these alloys have much better ductility because of the duplex nature and corrosion resistance because of the extra chromium. Going back to the austenic stainless steel, if you were to add small amounts of molybdenum into the mix, the resistance to high temperature creep resistance is improved. Creep is a process where over a period of time the material elongates permanently below the yield point, ie still in the elastic region. (316 grade austenitic stainless). Aluminium can also be added in higher temperature alloys for creep resistance, these begin to have specific trade names. Alternatively titanium, tantalum or can be added to improve the materials weldability. I've delibrately kept within the stainless steel group. As it is one of the moderate temperature alloys. So now you have an alloy containing 5 - 6 different elements. If you now perceive that the material still a weakness, that may effect the usage of the material, you might add some more of something or a different element. Some elements ma have a very strong influence others a weak influence, some may have a limitation in how much you can add before a new crystal structure is formed that is detrimental, or its addition causes the loss of another property. In the super alloys for high temperature applications, there can be 30 or more different alloying elements. To see the actual composition, you are left dumbfounded how anybody could have come up with something so complex. It’s a bit like a witches brew, "2 bats wings, 3 toes of frogs, eye of newt". They are designed in such a way that the metal initially corrodes forming an oxide layer that is designed not to lift off but remains firmly attached. The actual oxide is probably described as being more like a ceramic than a metal. But supposedly* each element is there for a specific purpose to give or counteract a certain property. I should mention that the high temerature alloys tend to eliminate iron from the compostion, instead the base of the material is nickel and cobalt. * Actually, I personally think the say, "well nobody else has ever u From: Alan™ 17/02/2000 21:56:00 Subject: re: Metals Manufacture post id: 39594 * Actually, I personally think the say, "well nobody else has ever used ‘Xx’ before, lets throw a bit in and say it does something wonderful so we can then charge 20% more. Even some of the fairly common exotic alloys can cost $US1000 per kilogram. How does that all sound? From: kelvin 18/02/2000 0:40:00 Subject: re: Metals Manufacture post id: 39640 Hi Allen Thanks for that great answer. I therefore gather that metals will crystallize in the cubic system rather than in any of the other 6 crystal systems? With respect to alloys are they strict crystal structures in the same way minerals are ie in terms of where each atom sits in the crystal structure or are they more random in structure ie can you make a ball and stick model of say a chrome, molly, nickel, vanadium iron alloy? Kelvin From: Alan™ 19/02/2000 0:18:00 Subject: re: Metals Manufacture post id: 39925 Sorry for being abit slack in replying. We've just moved offices and my area is the largest of the lot (actually the same size as all the others combined and I have all the stock. And everybody's been posting too much in the last few days. I've just read 2 1/2 days worth of posts. I therefore gather that metals will crystallize in the cubic system rather than in any of the other 6 crystal systems? They do occur in the other 6 crystal systems. But fortunately for metallurgists almost all the useful ones come in three different structures, FCC, BCC and Close packed hexagonal. Gee I hated sterographic projections With respect to alloys are they strict crystal structures in the same way minerals are ie in terms of where each atom sits in the crystal structure or are they more random in structure ie can you make a ball and stick model of say a chrome, molly, nickel, vanadium iron alloy? Generally yes, they are strict crystal structures and yes we did use the ball and stick models, but that was more when we were starting to learn about dislocations. I assume in geology / mineralogy you'd use a similar term for crystal faults which appear as line faults. The reason why I said generally is a seem to remember that carbon in low concentrations can appear in intersticial sites. They disturb the crystal structure but don't go into the latice sites. If I'm right on this (I'll check up tomorrow), it will also be occuring to some extent in many alloys. But the vanadium, chromium, molly and nickel would all be substituted into the lattice structure. I'm fallingasleep here. I'll post this and then make any technical corrections tomorrow. From: Alan™ 19/02/2000 13:01:00 Subject: re: Metals Manufacture post id: 40014 OK, I'm awake and ready to go zzzzzzzzzzz. I'm happy with the first part of my answer. But a bit more detail could be included in the second part. With respect to alloys are they strict crystal structures in the same way minerals are ie in terms of where each atom sits in the crystal structure or are they more random in structure ie can you make a ball and stick model of say a chrome, molly, nickel, vanadium iron alloy? Generally yes, they are strict crystal structures and yes we did use the ball and stick models, but that was more when we were starting to learn about dislocations. I assume in geology / mineralogy you'd use a similar term for crystal faults which appear as line faults. The reason why I said generally is a seem to remember that carbon in low concentrations can appear in intersticial sites. They disturb the crystal structure but don't go into the latice sites. If I'm right on this (I'll check up tomorrow), it will also be occuring to some extent in many alloys. But the vanadium, chromium, molly and nickel would all be substituted into the lattice structure. After a quick glance few a few text. There are two groups of impurities. One group are the substitional which would tend to be atoms of similar size, ie Ni, Mo, Cr, V. The other group are the intersticial. These would be the smaller atoms as they disrupt the latticel structure less, carbon in low concentrations, in an iron crystal structure. But if that carbon concenraction was higher the carbon would occur in the lattice positions in a new crystal structure based upon Fe3C or cementite (but I can't locate the actual crystal structure). The other thing that should be metions is distortion in the lattice structure, not just by individual atoms but by precipitates. These may be a completely different crystal structure to the rest of the crystal, and may only fit in by distorting the lattice structure around it. From: kelvin 20/02/2000 1:45:00 Subject: re: Metals Manufacture post id: 40294 Thanks again Alan I therfore guess that the main role of non -ferrous metals is to change the properties of the iron by changing the crystal properties? What happens in the case of electroplating? How do the crystals of say the zinc compound 'stick" to the existing ferrous compound.? I guess we are entering the heady world of electochemistry? kelvin From: Alan™ 20/02/2000 20:10:00 Subject: re: Metals Manufacture post id: 40396 I therfore guess that the main role of non -ferrous metals is to change the properties of the iron by changing the crystal properties? I hope you mean alloying elements and not non ferrous. You'll get into trouble when dealing with aluminium alloys or any other non ferrous alloys. But yes, the main purpose of alloying elements, is to change the properties of the base metal, by changing the crystal structure. What happens in the case of electroplating? How do the crystals of say the zinc compound 'stick" to the existing ferrous compound.? I guess we are entering the heady world of electochemistry? I think Di & Dr Ed might be able to give you a better answer. But from memory there is some substitution of the atoms within the surface layer of the substrate and the plating. It's more than just a physical bond, it's also a chemical bond. My course was really weak in this area, even though my speciality is anodising. From: kelvin 21/02/2000 0:59:00 Subject: re: Metals Manufacture post id: 40521 Hi there If this is starting to become monotenous please tell me so. I guess the physical properties of teh metal eg malleability and melting temperature are a direct function of the crystal structures involved? In geology we use the idea of differentiation during cooling of a melt ie higher temperature minerals crystallize before lower temperature phases eg pyroxenex will crystallise before quartz. Does a similar thing happen in a metals melt ie do nickel compounds crystallise before say chromium or tungsten compounds? kelvin From: Alan™ 22/02/2000 22:11:00 Subject: re: Metals Manufacture post id: 41155 I guess the physical properties of teh metal eg malleability and melting temperature are a direct function of the crystal structures involved? Well the answer is yes. But it seems to simple to just say yes. Crystal structures can be manipulated in so many ways, heat treatment, mechanical working, cooling cycle, ageing, particle precipitation, grain size etc. In geology we use the idea of differentiation during cooling of a melt ie higher temperature minerals crystallize before lower temperature phases eg pyroxenex will crystallise before quartz. Does a similar thing happen in a metals melt ie do nickel compounds crystallise before say chromium or tungsten compounds? I'm actually having to think about this. OK, Yes. Generally you want small percipitates in metallurgy (gross generalization but accurate enough) to obtain these, they generally design the material so that these percipitates can form by reheating the material, without melting then. This occurs by diffusion. A few years ago, I was working with a material which in theory could be manufactured (it definately could be cast) but the manufacturer was using a special technique, for solidify the metal, while this was happening they were keeping the liquid homogenous by stirring it. If they had attempted to cast it, the various alloying elements would have come out of solution and solidified. From: kelvin 23/02/2000 0:44:00 Subject: re: Metals Manufacture post id: 41204 Hi Alan Thanks for that info. Is it posible to make a lightweight metal by bubbling air through the melt during solidification from the melt? kelvin Ps I habe no idea on why I have this sudden fascination with metals From: Alan™ 23/02/2000 9:42:00 Subject: re: Metals Manufacture post id: 41222 Is it posible to make a lightweight metal by bubbling air through the melt during solidification from the melt? Yes it is possible to do this, but if any oxygen was in the air (pretty usual one would hope :-) ) this would result in the metal oxidizing at high temperature causing inclusions etc, which would weaken the material. You could attempt the same thing using nitrogen. My thoughts here really is why. It would be very difficult to control where the bubbles ended up in the metal. You would end up structures that did not have consistant properties, say all the bubbles occurred at one end of a sheet or in a particular section. The result would be the material would be very weak in that region. There are however alternatives. Metal honeycombes exist, a 25 mm thick honeycombe structure with sheet material over the top will give you a material which is all most as strong as 25 mm thick plate, but with only a small percentage of the mass. These materials are used extensively in the aircraft industry. Engineering design will often think about ways to make a structure lighter. Think of the classic structural "I" beam. Or just using thinner sections The other aspect to this is that their are a huge range of materials out there. Titanium has a strength to weight ratio of steel that is 3 to 1. In other words you can achieve the same strength for a third of the weight. I think the figures are the same for magnesuim alloys and titanium alloys. Ps I habe no idea on why I have this sudden fascination with metals I've had it for years. When you've scene the grain structures under a microscope, you start seeing the same structures as patterns on cloth etc. Gee, she's wearing a widmanstatten plate stucture. Isn any one else following this thread other than Kelvin and myself? From: kelvin 23/02/2000 18:39:00 Subject: re: Metals Manufacture post id: 41376 Hi there you have once again answered the question beautifully. I was thinking of the bubbles as a way of making aircraft and race car structures. Whay does Niobium and the other exotic meatls do to a ferrous melt and to the properties of the final material? kelvin From: Alan™ 24/02/2000 23:42:00 Subject: re: Metals Manufacture post id: 41830 Whay does Niobium and the other exotic meatls do to a ferrous melt and to the properties of the final material? Big question this one, since 2/3 of all the elements are metals and all but half a dozen or are exotic. Also I don't have my hands on that much information. To the melt they virtually make no difference. They may or may not effect the solidification temperature to a small extent. The will also cause shifts in the various solidification lines. WRT the physical properties of the material to some extent I've covered most of this question already. Because of the shape of the iron carbon diagram, you have a couple of different situations. You have some elements which cause the ferrite sregion to be larger or the austenite region to be larger. Some elements react with the carbon causing carbides, graphite being precipitated or to assist the carbon to remain in the interstitial sites. Other things happening, are the size of the atoms in the interstitial and substitutional atoms and the effect of how dislocation move through the structure. Niobium, when comined with carbon (NbC4) or nitrogen 3 results in precipitates in the austenite phase. During hot working (ie rolling the material at high temperature) the precipitates inhibit the grains of austenite recrystallising. Thus the resulting crystals are smaller and the material with the percipitates still present, when subsequently cold rolled they produce an extremely high strength material. I'd hate to think I'm burning you off, I just don't have that much info in this area on specific elements. As I'm enjoying this thread. :-) I have found an additional bit of information. Microalloying of carbon steels, form a group of materials known as High strength, low alloy steels (HSLA). The strengthening mechanisms used in this group of alloys are grain refinement, precipitation of carbides, nitrides etc, effects to dislocation structure, solid solution strengthening and strain aging. With careful heat treatment you can control the final crystal structure and form meta-stable crystal structures. From: kelvin 25/02/2000 1:02:00 Subject: re: Metals Manufacture post id: 41839 Hi Alan Thanks for that answer. Here are 2 more questions to think about. I assume that the crystal size in most metals is small. I cannot see any crystals with my hand lens when I look at the kitchen sink. however when I look at a bit of thin tinplate behind the shed I see a regular "graininess' to the texture. i had assumed this graininess reflected the crystal structure. Is this true/ What happens when you cool a metal melts melt really fast? do you get a glass? having fun Kelvin From: Alan™ 25/02/2000 1:49:00 Subject: re: Metals Manufacture post id: 41846 I assume that the crystal size in most metals is small. I cannot see any crystals with my hand lens when I look at the kitchen sink. however when I look at a bit of thin tinplate behind the shed I see a regular "graininess' to the texture. i had assumed this graininess reflected the crystal structure. Is this true? Often the grains are bigger than you realise. At one stage (I'm not sure how popular it is currently) jet turbine blades were single crystals. Galvanised (rather than tin plate) metal are often fairly visible because of the slow cooling rates. Most crystals are small, less than half a mm. But why you can't generally see them is because of the reflection / smoothness on the surface. We often use combinations of acids or oxidising agents to etch the surface of the metal. Etching of the surface allows the different phases to be identified. For aluminium I used HF acid during my thesis. Nitric and Picric acid are both used for steel. What happens when you cool a metal melts melt really fast? do you get a glass? Yes, you can end up with metal glasses. Think about your cassette tapes, the metal ones are actually an iron silicon alloy that is actually a glass.. The rate of cooling has a significant effect on the final structure observed. The actual rate will depend upon the alloy involved. There are also often several other crystal stures possible before you reach the amorphous glass material. In a carbon steel, it goes something like ferrite, bainite, martensite and then the amorphous material. Bainite and martensite have been meta-stable crystals, I've mentioned earlier. From: kelvin 25/02/2000 2:04:00 Subject: re: Metals Manufacture post id: 41855 Ok Does adding non ferric components change the magnetic properties of the final metal product? kelvin From: Alan™ 25/02/2000 7:53:00 Subject: re: Metals Manufacture post id: 41879 Does adding non ferric components change the magnetic properties of the final metal product? Potentially, as only the BCC ferrite crystalline product is magnetic. Thus is you add alloyinging elements that cause the production of austenite, then the material will be non magnetic. From: kelvin fox 24/03/2000 8:53:00 Subject: re: Metals Manufacture post id: 49739 Hi Alan With respect to crystal structure, how does making a wire from metal effect the crystal structure of the metal? Kelvin From: David Brennan 24/03/2000 9:46:00 Subject: re: Metals Manufacture post id: 49745 Generally speaking it streches the grains of the metal in the longitundinal direction of the wire ie along it length. The leads to increased strength in this direction and depending on the material work hardening of the material. This is sometimes called mechanical fibreing. David From: Alan™ 24/03/2000 11:20:00 Subject: re: Metals Manufacture post id: 49776 The crystal structure becomes really elongated in the direction of the length of the wire. Depending upon the type of material bring drawn (pulled) into wire, this can become a real problem because of work hardening (remember the dislocations I mentioned earlier). If the wire work hardens to much, the wire will break during latter drawing processes. With most materials and depending on how fine the wire is, they reheat (anneal) the wire, to allow the material to recrystallize, this eliminates the work hardening. Depending on how hard their want the wire, they will anneal the wire at various stages during the process ie a soft wire will be annealled when the wire has been drawn to the final thickness, but a hard wire will be annealled much earlier in the drawing process. This forum is un-moderated. The views and opinions expressed are those of the individual poster and not the ABC. The ABC reserves the right to remove offensive or inappropriate messages.