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Day 14: Heat treatments of Steel. Quenching and Tempering Steel (Basic, more complete discussion later) Spheroidizing Full Annealing Normalizing Hardenability (More on the quenching and tempering process.). Steels. High Alloy. Low Alloy. low carbon . Med carbon. high carbon .

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day 14 heat treatments of steel
Day 14: Heat treatments of Steel
  • Quenching and Tempering Steel (Basic, more complete discussion later)
  • Spheroidizing
  • Full Annealing
  • Normalizing
  • Hardenability (More on the quenching and tempering process.)
steels
Steels

High Alloy

Low Alloy

low carbon

Med carbon

high carbon

<0.25wt%C

0.25-0.6wt%C

0.6-1.4wt%C

heat

austenitic

Name

plain

HSLA

plain

plain

tool

treatable

stainless

Cr,V

Cr, Ni

Cr, V,

Additions

none

none

none

Cr, Ni, Mo

Ni, Mo

Mo

Mo, W

Example

1010

4310

1040

43

40

1095

4190

304

0

+

+

++

++

+++

0

Hardenability

-

0

+

++

+

++

0

TS

increasing strength, cost, decreasing ductility

+

+

0

-

-

--

++

EL

high T

pistons

Uses

auto

bridges

crank

wear

drills

applic.

gears

struc.

towers

shafts

applic.

saws

turbines

wear

sheet

press.

bolts

dies

furnaces

applic.

vessels

hammers

V. corros.

blades

resistant

Based on data provided in Tables 11.1(b), 11.2(b), 11.3, and 11.4, Callister 7e.

quenching and tempering
Quenching and Tempering

We have found that Martensite (M) is very hard, strong, and brittle. It sees little use as an end product.

But, if we reheat to below 727, (250-650) and leave the M there for a while, it changes into something with real usefulness. Tempered Martensite. (TM)

TM is ferrite, with an extremely fine dispersion of roughly spherical particles of Fe3C. Extremely strong, but has ductility and toughness. This is the primo stuff, as far as steel goes.

a typical q t steel
A Typical Q&T Steel

We will use Matweb to look over a couple of steels.

Note the differences in strength and ductility in a 1060 Steel. The annealed steel with have very coarse pearlite. The Q&T steel will have very finely distributed cementite.

spheroidizing
Spheroidizing

Strangely, sometimes we would like the steel to be just as soft and ductile as absolutely possible.

Why, do you think?

Pearlite is not the lowest energy arrangement possible between ferrite and cementite. If heated to just below the eutectoid temperature, and left for an extended time, the pearlite layers break down, and spherical clumps of cementite are found.

These spherical clumps are hundreds or even thousands of times larger that those in TM, and spaced much further apart.  Softest, most duct.

slide6

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

more on spheroidite
More on Spheroidite

You have to spend a lot of energy cooking steel. Spheroidizing is not really used with low carbon steels, since they are already soft and ductile enough.

Spheroidizing is done with the higher carbon steels, so they will be as ductile as possible for shaping.

Spheroidizing is done to improve the machineability of high carbon steels. Having the massive cementite regions enhances chip formation.

full anneal
Full Anneal

The idea is to get the soft metal and relieve stresses. We contrast this anneal with the “process anneal” associated with CW.

In the full anneal, we must fully austenitize the steel. This is followed by a furnace cool, the slowest cooling rate possible.

The result is coarse Pearlite mixed with primary phase. This steel will be close to spheroidite in its softness and ductility.

normalizing steel
Normalizing Steel

Here we austenitize the steel and then air cool as opposed to furnace cooling.

The result is a uniform microstructure, very uniformly spaced pearlite in equal sized grains throughout.

It is stronger and somewhat less ductile than full anneal.

Often done after forging to normalize the grain structure.

thermal processing of metals
Thermal Processing of Metals

Stress Relief: Reduce

Spheroidize

(steels):

stress caused by:

Make very soft steels for

-plastic deformation

good machining. Heat just

-nonuniform cooling

below TE & hold for

-phase transform.

15-25h.

Full Anneal

(steels):

Types of

Make soft steels for

Annealing

good forming by heating

g

to get

, then cool in

furnace to get coarse P.

Process Anneal:

Negate effect of

Normalize

(steels):

cold working by

Deform steel with large

(recovery/

grains, then normalize

recrystallization)

to make grains small.

Annealing: Heat to Tanneal, then cool slowly.

Based on discussion in Section 11.7, Callister 7e.

heat treatments

a)

b)

c)

Heat Treatments

800

Austenite (stable)

  • Annealing

TE

T(°C)

A

  • Quenching

P

600

  • Tempered Martensite

B

A

400

100%

Adapted from Fig. 10.22, Callister 7e.

50%

0%

0%

M + A

200

50%

M + A

90%

-1

3

5

10

10

10

10

time (s)

hardenability
Hardenability

We have seen the advantage of getting martensite, M. We can temper it, getting TM with the best combination of ductility and strength.

But the problem is this: getting M in depth, instead of just on the surface. We want a steel where Pearlite formation is relatively sluggish so we can get it to the cooler regions where M forms.

The ability to get M in depth for low cooling rates is called hardenability.

Plain carbon steels have poor hardenability.

jominy test for hardenability
Jominy Test for Hardenability

Hardenability not the same as hardness!

hardenability steels

flat ground

specimen

(heated to g

Rockwell Chardness tests

phase field)

24°C water

Hardness, HRC

Distance from quenched end

Hardenability--Steels

• Ability to form martensite

• Jominy end quench test to measure hardenability.

Adapted from Fig. 11.11, Callister 7e. (Fig. 11.11 adapted from A.G. Guy, Essentials of Materials Science, McGraw-Hill Book Company, New York, 1978.)

• Hardness versus distance from the quenched end.

Adapted from Fig. 11.12, Callister 7e.

why hardness changes w position
Why Hardness Changes W/Position

T(°C)

0%

100%

P

600

®

A

400

M(start)

200

®

A

M

M(finish)

Pearlite

0

Martensite

Fine Pearlite

Martensite + Pearlite

0.1

1

10

100

1000

Time (s)

• The cooling rate varies with position.

60

40

Hardness, HRC

20

distance from quenched end (in)

0

1

2

3

Adapted from Fig. 11.13, Callister 7e.

(Fig. 11.13 adapted from H. Boyer (Ed.) Atlas of Isothermal Transformation and Cooling Transformation Diagrams, American Society for Metals, 1977, p. 376.)

the result is presented in a curve
The Result is Presented in a Curve
  • Note:
  • Distance from quenched end corresponds to a cooling rate, and a bar diameter
  • Notice that some steels drop off more than others at low cooling rates. Less hardenability!

Rank steels in order of hardenability.

factors which improve hardenability
Factors Which Improve Hardenability

TTT diagram of a molybdenum steel 0.4C 0.2Mo

After Adding 2.0% Mo

1. Austenitic Grain size. The Pearlite will have an easier time forming if there is a lot of g.b. area. Hence, having a large austenitic grain size improves hardenability.

2. Adding alloys of various kinds. This impedes the g P reaction.

hardenability vs alloy composition

100

10

3

2

Cooling rate (°C/s)

60

100

%M

80

4340

Hardness, HRC

50

40

4140

8640

1040

5140

20

0

10

20

30

40

50

Distance from quenched end (mm)

800

T(°C)

TE

600

shift from

A

B

A to B due

400

to alloying

M(start)

200

M(90%)

0

-1

3

5

Time (s)

10

10

10

10

Hardenability vs Alloy Composition

• Jominy end quench

results, C = 0.4 wt% C

Adapted from Fig. 11.14, Callister 7e.

(Fig. 11.14 adapted from figure furnished courtesy Republic Steel Corporation.)

• "Alloy Steels"

(4140, 4340, 5140, 8640)

--contain Ni, Cr, Mo

(0.2 to 2wt%)

--these elements shift

the "nose".

--martensite is easier

to form.

quenching medium geometry
Quenching Medium & Geometry

• Effect of geometry:

When surface-to-volume ratio increases:

--cooling rate increases

--hardness increases

Position

center

surface

Cooling rate

low

high

Hardness

low

high

• Effect of quenching medium:

Medium

air

oil

water

Hardness

low

moderate

high

Severity of Quench

low

moderate

high

ferrous alloys
Ferrous Alloys

Iron containing – Steels - cast irons

Nomenclature AISI & SAE

10xx Plain Carbon Steels

11xx Plain Carbon Steels (resulfurized for machinability)

15xx Mn (10 ~ 20%)

40xx Mo (0.20 ~ 0.30%)

43xx Ni (1.65 - 2.00%), Cr (0.4 - 0.90%), Mo (0.2 - 0.3%)

44xx Mo (0.5%)

where xx is wt% C x 100

example: 1060 steel – plain carbon steel with 0.60 wt% C

Stainless Steel -- >11% Cr