MATERIALS ENGINEERING – DAY 7

Complete Strengthening Mechanisms Cold Work Annealing

SOLID SOLUTION STRENGTHENINGAND GRAIN SIZE REFINEMENT

Why are alloys stronger than the base metal alone?

What is the advantage of having a fine-grained crystalline structure?

THE HALL-PETCH RELATIONSHIP

E f fe c t o f G rain S ize R e d uc tio n

s ys =s o+kyd

Yie ld S treng th

-1/2

d -1/2

EXAMPLE

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?

1000020 0 k

100000032 0 k

Solving, we find that k=0.01333 and s0 = 18.67

ksiy 9.2210000001333.067.18

ANOTHER BLOCKER: OTHER DISLOCATIONS

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.

E f fe c t o f P las tic D e fo rm atio n

Degree of strengtheningdepends onmaterial

Yie ld S treng th

%area reduction

WHAT ABOUT DUCTILITY?

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.

FOR COLD WORK AND ANNEALINGBE 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

COLD 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 CWA0

Af

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/

HOW COLD WORK IS MEASURED

It is measured as the percentage in area reduction during the deformation process.

%100%0

0

A

AACW f

This is calculated in the same way that %RA ductility is calculated. BUT, the CW produced in some manufacturing process, not the tension test.

WHY DO COLD WORK…

It’s about several issues…1. Strengthening the manufactured part2. Shaping the manufactured part3. 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.

WHAT COLD WORKED METAL 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?

ANNEALING

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.

FIGURES FROM TEXT SHOWING THE RECRYSTALLIZATION SEQUENCE. (BRASS – 33% CW) HEAT TO 580C.

start After 3 s After 4 s

After 8 s After 15 min After 10 min at 700C

HERE’S HOW THE PROPERTIES CHANGE AS CHANGE ANNEALING TEMPERATURE

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.

http://info.lu.farmingdale.edu/depts/met/met205/annealingstages.html

“LAWS” 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.

RECRYSTALLIZATION TEMPEPATURE

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.

REVIEW OF THREE STRENGTHENING MECHANISMS

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.