I am doing an assignment for my materials class, im not asking for the answer but just some help. I have the assignment finished and handed in but the teacher gave it back with one comment, he asked me to explain the grain growth in steel during hydraulic press bending. Im completely stuck, please help. Thanks.
when you press bend metal , its gets a little hotter at the point of the bend , its like bending a piece of metal back and fourth , the area of the bending gets hot .the distance between the atoms at that point expands thus, a small growth in the size of the metal
There is such a thing as critical strain which will lead to extraordinary grain growth on annealing;but not without annealing.It would be to do with the density of grain growing sources and clearly the density of these nuclei woul increase with increasing severity of strain so I just of a guess that that those parts of a pressing with least strain might grow most.But I must tell you that in the case of Aluminium alloy extrusions severe and expensive coarse grain occurs near the surface and particularly toward the back end of the bar;so this would be where most severe deformation takes place.However in a pressed steel part you have all possible circumstances,some of which on ironed walls will resemble extrusion,others would be bending strains,tensile and compressive .I have to cop out of this now because as old wasname above,said it depends.But bear in mind that it is unlikely that the material you mention would be hot pressed,further arse grain might only be a surface problem and is likely to be cured by normalising so that new ferrite grows from austenite.A similar solution does't exisat for Al alloy extrusions,it has to be machined off.In a cold steel pressig you would only expect to see a cold worked sructure because the recrystallisation temperature of pressable steel would not be less than,say 550 degrees C .
Well, I can't answer the question for you because, if the press bending is done cold, the grains will not grow (at least not with normal steel alloys). One wonderful thing about metallurgy is that the real answer is it depends because there are all sorts of weird and unexpected things that can happen. Cold work (which is what press bending is unless it is done hot or the amount of deformation is extraordinary high and fast so the metal heats up a lot locally) adds stored energy (strain energy) which provides one of the driving forces for recrystallization. Recrystallization is not a yes/no process, it is a process that happens as a function of time and temperature. Recrysallization of a cold worked structure will refine the grain size if done properly but the final stage of recrystallization is grain growth and you can end up with a larger grain size than you started with. The other thing that can affect grain structure is the deformation itself. There are all sorts of transformations that can happen due to the deformation process (twinning/martensitic is one) but, again, these don't usually qualify as grain growth. It may be that the question was incomplete, maybe the question is how does the press bending affect the final grain size after a subsequent anneal? The metallurgy of steels is fascinating because there are so many different microstructures that can be produced, hundreds (or thousand) different alloys, equilibrium and non-equilibrium phases based on composition and thermo-mechanical processing. Hope this helps
