E XAMPLE Assume that a metal has a yield stress of 20 ksi if the grain size is 10 -4 mm, and 32 ksi if the grain size is 10 -6 mm. What will be the yield stress if the grain size was 10 -5 mm, all other things being equal? Solving, we find that k=0.01333 and 0 = 18.67
A NOTHER B LOCKER : O THER D ISLOCATIONS Recall that as plastic deformation proceeds the density of dislocations increases by several orders of magnitude. So dislocations block each other. This accounts for the strengthening that occurs during plastic deformation. (Done on purpose, we call it cold work.
W HAT ABOUT D UCTILITY ? A trade off is taking place. As we block dislocations, and the material gets stronger, we lose the capacity for plastic deformation. In other words, the ductility is decreased. AS WE BLOCK DISLOCATIONS, STRENGTH INCREASES AND DUCTILITY DECREASES. Exception : Fine grain size gives strength without significant decrease in ductility.
F OR C OLD W ORK AND A NNEALING B E ABLE TO : Calculate %cold work from change in cross- sectional geometry Describe the microstructural and property changes during Recovery, Recrystallization and Grain Growth, and the relationship between microstructure and properties
C OLD WORK In cold work, metals are strengthened at the expense of ductility. Cold refers to the fact that the material is plastically deformed at a temperature below it “recrystallization” temperature. (More on this later.) Also called strain hardening. Rolling: very common CW A0A0 AfAf
The grain structure of a low carbon steel produced by cold working: (a) 10% cold work, (b) 30% cold work, (c) 60% cold work, and (d) 90% cold work (250). (Source: From ASM Handbook Vol. 9, Metallography and Microstructure, (1985) ASM International, Materials Park, OH 44073. okasatria.blogspot.com/
H OW C OLD W ORK IS MEASURED It is measured as the percentage in area reduction during the deformation process. This is calculated in the same way that %RA ductility is calculated. BUT, the CW produced in some manufacturing process, not the tension test.
11 4 S TRATEGIES FOR S TRENGTHENING : 4: C OLD W ORK (% CW ) Room temperature deformation. Common forming operations change the cross sectional area: Adapted from Fig. 11.8, Callister 7e. -Forging A o A d force die blank force -Drawing tensile force A o A d die -Extrusion ram billet container force die holder die A o A d extrusion -Rolling roll A o A d
12 Ti alloy after cold working: Dislocations entangle with one another during cold work. Dislocation motion becomes more difficult. Adapted from Fig. 4.6, Callister 7e. (Fig. 4.6 is courtesy of M.R. Plichta, Michigan Technological University.) D ISLOCATIONS D URING C OLD W ORK 0.9 m
13 R ESULT OF C OLD W ORK Dislocation density = Carefully grown single crystal ca. 10 3 mm -2 Deforming sample increases density 10 9 -10 10 mm -2 Heat treatment reduces density 10 5 -10 6 mm -2 Yield stress increases as d increases: total dislocation length unit volume large hardening small hardening y0y0 y1y1
14 I MPACT OF C OLD W ORK Adapted from Fig. 7.20, Callister 7e. Yield strength ( y ) increases. Tensile strength (TS) increases. Ductility (%EL or %AR) decreases. As cold work is increased
W HY D O C OLD WORK … It’s about several issues… 1. Strengthening the manufactured part 2. Shaping the manufactured part 3. Cold Work can be used to impart a nice surface finish Often we can’t complete the shaping process with just one step of cold work. There just isn’t enough ductility in the metal. Plus, we need to get the material back to a state of 0% CW. (While keeping the new shape, of course!) How might we do that? First, think of what we have.
W HAT C OLD WORKED M ETAL IS LIKE Dislocation density very high. Residual stresses are very commonly encountered. The original grain structure is still in existence. But the grains have been stretched in the direction of the deformation. Electrical conductivity and thermal conductivity may be reduced. The state of internal energy is high. So what can be done to diminish these effects?
A NNEALING Annealing – a thermal process – we heat the cold worked metal. BUT WE DO NOT MELT IT Three phenomena are observed. Here is the order in which they are known to happen. 1. Recovery. Enough energy is supplied so that dislocations can spontaneously move to lower residual stresses. 2. Recrystallization. In the middle of the old, elongated grains, new small equi-axed grains begin to form, until we have a completely new grain structure. 3. Grain growth. If more heat is supplied over time the grains grow, smaller ones eaten by bigger ones.
F IGURES FROM T EXT SHOWING THE R ECRYSTALLIZATION S EQUENCE. (B RASS – 33% CW) H EAT TO 580C. startAfter 3 sAfter 4 s After 8 s After 15 minAfter 10 min at 700C
H ERE ’ S H OW THE P ROPERTIES C HANGE AS C HANGE A NNEALING T EMPERATURE Metal is Brass. This is based on an annealing time of 1 hour. Similar looking plots could be produced for a constant temperature with time as the independent variable.
“L AWS ” OF RECRYSTALLIZATION Thermally activated. Critical temperature. Critical deformation. Deformation affects the critical temperature. Initial grain size affects the critical temperature. Grain boundaries are good sites for nuclei to form.
R ECRYSTALLIZATION T EMPEPATURE Depends on Alloy Content. Lower for pure metals. Depends on the amount of previous CW. Metal is iron (Fe). Note that for less than about 5% CW, there will be no recrystallization. Final note: Recrystallization is very useful in grain size control.
R EVIEW OF THREE S TRENGTHENING M ECHANISMS 1. Solute Atoms. (Alloying) 2. Grain boundaries. (Grain boundary refinement) 3. Dislocations. (Cold Work, i.e. plastic deformation done on purpose. THERE ARE OTHER STRENGTHENING MECHANISMS WHICH WE WILL ALSO TALK ABOUT.