aging and machinability interactions in cast iron

Aging and Machinability Interactions in Cast Iron

S.N. Lekakh, V.L. Richards

Missouri University of Science and Technology

Copyright 2012 American Foundry Society

ABSTRACT

This paper summarizes the study of the effect of natural

aging of cast iron on casting machinability. In the first

part, cast iron natural aging kinetics are discussed, taking

into consideration the effects of alloying elements, which

are nitride-forming (Ti), carbide-forming (Cr) and

nitrogen mobility modifiers (Mn). The study has

quantitatively established the aging process as a function

of temperature and cast iron chemistry. For practical

application, an aging time diagram was developed. A

second part of the review includes measurement of

machinability with respect to aging conditions, using such

parameters as a cutting force, tool wear, surface quality

and dimensional accuracy. Relationships between cast

iron aging and casting machinability were verified in

multiple laboratory and industrial tests. In this paper,

different possible scenarios for improving casting

machinability are discussed and optimal operating

parameter windows are suggested. Finally, as a

demonstration case, the optimal aging time for improving

casting machinability was calculated based on cast iron

chemistry and was experimentally verified.

Keywords: cast iron, aging, machinability, structure

CAST IRON NATURAL AGING

As a final step in this project, there is an opportunity to

retrospectively map and codify the understanding of how

age strengthening affects machinability, so that foundries

may be able to utilize the strength improvement by aging

and to schedule the optimal operation parameter window

for improving casting machinability. The mechanism by

which age strengthening changes the machinability of

graphitic cast irons is also discussed in this paper.

AGING KINETICS Room-temperature aging phenomena in different types of

ferrous alloys, including cast irons and steels, has been

documented in literature.1,2

In gray cast iron, tensile

strength increased by 5-15% after 5-30 days of room-

temperature aging.1 Different mechanisms are involved

during age strengthening of iron-base alloys. Edmonds

and Honeycombe3 provided a review of aging studies in

quenched Fe-N alloys that indicated a three stage

precipitation process beginning with the formation of

interstitial-atom clusters, followed by nucleation of

α”- Fe16N2 and ending with equilibrium α’-Fe4N.

Precipitation of α”-Fe16N2 can be nucleated

homogeneously at low temperatures and high nitrogen

super-saturations or heterogeneously on dislocations at

higher temperatures and low nitrogen super-saturations.

In some cases,4 a dip in strength is observed during the

start of the aging process.

Elevated temperature aging kinetics in the cast iron was

studied1, 5

based on Johnson-Mehl-Avrami and Arrhenius

kinetics:

Vf 1 exp kt n Equation 1

RT

Qkk exp0 Equation 2

where: Vf is the fraction transformed, k is the reaction

rate constant, n is time exponent, R is the gas constant, T

is the absolute temperature, k0 is an attempt frequency and

Q is the activation energy.

The typical age strengthening curves which are obtained1

at different temperatures are shown in Fig. 1. An

Arrhenius plot was constructed using the rate constants

versus the reciprocal of the absolute temperature (Fig. 2).

In the 182C - 285C (360F - 545F) temperature interval,

the activation energy Q was 16.8 kJ/mole and Q increased

to 35.7 kJ/mole at below 150C (302F). The measured

value near room temperature was close to the value of

activation energy of 44 kJ/mol for precipitation growth of

nitrides in the Fe-N system.7 Different activation energies

in different temperature ranges are due to changes of the

aging mechanism. At temperatures above 200C (392)F,

the metastable α”- Fe16N2 is replaced by the ordered

γ’-Fe4N. In this regard, the neutron scattering studies8

showed evidence of interstitial atom clusters that are

spherical in shape. The low temperature kinetic results

support the conclusion that the time exponent suggests an

equiaxially-shaped precipitate.

Paper 12-026.pdf, of 11AFS Proceedings 2012 © American Foundry Society, Schaumburg, IL USA

a)

b)

c)

d)

Fig. 1. These graphs show strengthening of cast iron during aging

(a) at room temperature, (b), at 182C, (c)

at 285C and(d) the Arrhenius plot of cast iron aging kinetics.

1

For practical application, the following relationships

between peak aging time (tpeak, h) and aging temperatures

(T,K up to 573K) and (T,F up to 600F) were suggested:1

KTtpeak

,

2020exp171.0

FTtpeak

,460

3636exp171.0

Equation 3

EFFECT OF ALLOYING ELEMENTS While the elevated temperature aging process is less

dependent on alloy composition, room temperature aging

kinetics is strongly affected by cast iron chemistry. From

a practical perspective, the effect of variations in Mn and

S on cast iron aging rate is important. Table 1 gives a

comparison of aging time for two cast irons which have

been studied.1, 6

In cast iron with 0.8-0.83%Mn, aging was

completed at 25 days while this process needed only 15

days for cast iron with 0.51%Mn at similar 0.04-0.06%

sulfur levels.

Table 1. Comparison of Iron Chemistry and Aging Time at Room Temperature

Parameters Test6 Test

1

Manganese, wt.% 0.51 0.80-0.83

Nitrogen, wt.% 0.0094 0.007-0.008

Natural aging time, days 15 28

To study the effect of alloying elements, aging kinetics of

cast irons from six heats with variations in Mn, N and S

were evaluated.9 Strength change curves typically had a

pre-strengthening peak, a “relaxation valley” and finally

achieved a full age strengthening. Alloying with Mn

affected both the time for achievement of pre-

strengthening and full strengthening peak. Cast iron from

a heat with 0.53% Mn had the highest reaction rate. Iron

with lower Mn and especially higher Mn contents had a

longer time of aging reaction.

Thermodynamic data and first principle calculations of

the interaction energy between interstitial N-atom and

substitutional Mn atoms in Fe-BCC lattice were used to

enhance the understanding of the aging mechanism. The

data and calculations suggest the hypothesis that Mn-Mn

clusters are likely to serve as the nucleation sites for the

formation of nitrides in Fe-Mn-N system. During this

initial period, presumably α”- Fe16N2 precipitates were

formed. At the same time, increasing thermodynamic

stability of this phase in cast iron with high Mn content

can delay sequential growth of the stable γ’-Fe4N

responsible for full age strengthening. As a result, a

nonlinear effect of Mn-alloying on aging kinetics was

observed. The effect of manganese on aging kinetics also

depends on sulfur concentration because these elements

have a strong tendency to MnS formation. In this case, the

full aging time illustrated in Fig. 3 will depend on

concentration of free Mn (Mnfree):

Mnfree %Mn-1.7%S Equation 4


a)

b) Fig. 2. These graphs show (a) a typical aging curve and (b) the effect of Mn on aging time estimated from maximum increase in tensile strength and electrical resistivity (b).

9

EFFECT OF CARBIDE/NITRIDE FORMING ELEMENTS Natural age strengthening of cast iron occurs in Fe-BCC

(ferrite) by iron nitride precipitation. Carbide forming

elements such as chromium promote decreasing the

amount of free ferrite in cast iron and reduce the total

possible strengthening effect.14

A representative

microstructure from the as cast machinability test article

produced from cast iron with 0.2% Cr is given in Fig. 4.

The metal matrix had a fully pearlitic structure with some

white spots identified as the Cr-alloyed steadite eutectic

(P-(FenCr1-n)3C). This phase has a significantly higher

microhardness than ferrite or even pearlite and is

considered detrimental to machinability. Statistics of

tensile test data before and after 21 days of natural

aging indicated a detectable but a low strengthening.

Machinability did not improve with aging in this case.

a)

b)

c)

d) Fig. 3. These graphs shoe the effect of “free Mn” on (a, b) pre-strengthening time and(c, d) full aging time (days).

220

230

240

250

260

0 10 20 30 40

Te

ns

le s

tre

ng

th, M

pa

Aging time, days

Heat D

0

10

20

30

40

0.2 0.4 0.6 0.8 1 1.2 1.4

Ag

ing

tim

e,

da

ys

Mn, weight %

Pre-strengthening

Full aging (strength)

Full aging (resistivity)

y = -20x3 + 59x2 - 47x + 16

0

5

10

15

0 0.5 1 1.5

Pre

-str

en

gth

en

ing

ti

me

, d

ays

"Free" Mn =%Mn-1.7%S

0

0.02

0.04

0.06

0.08

0.1

0.12

0.2 0.4 0.6 0.8 1 1.2

S, w

t.%

Mn, wt. %

4-5

6-7

6-7

8-10

8-10

y = -61x3 + 194x2 - 161x + 57

10

15

20

25

30

35

40

0 0.5 1 1.5

Op

tim

al a

gin

g t

ime

, d

ays

"Free" Mn =%Mn-1.7%S

0

0.02

0.04

0.06

0.08

0.1

0.12

0.2 0.4 0.6 0.8 1 1.2

S, w

t.%

Mn, wt. %

15-1715-17

15-17

18-21

22-29

30-40

30-40

22-29


a)

b)

c)

Fig. 4 These illustrate the (a) microstructure of gray iron with 0.2 %Cr, (b) Scanning Electron Microscopy/Energy Dispersive Spectroscopy (SEM/EDS) analysis of steadite phase and (c) box and whisker plot of Ultimate Tensile Strength (UTS) for un-aged/aged conditions.

14

Nitride forming elements such as Ti, Al and B can fully

suppress iron nitride precipitate strengthening.10, 11

For

example, Ti has a strong influence on arresting the age

strengthening behavior by forming TiN and drastically

reducing concentration of soluble nitrogen (Ns) in

Fe-BCC. If this were a stoichiometric extent, the free

nitrogen in the ferrite would be:

%Ns = %NTotal - 0.33%Ti Equation 5

However, since Ti forms a carbonitride (Ti(C,N)) the

actual Ti coefficient shown in Equation 5 based on the

thermodynamic calculations and experimental data is

more likely 0.2. Figure 5 illustrates the effect that

nitrogen, available at solidification to form metastable

solid solution in ferrite, has on age strengthening of cast

iron. Elevated nitrogen (0.009 %N) was enhanced by

ladle FeMn5N additions while iron with regular nitrogen

had 0.006 %N. Low soluble nitrogen left after TiN

formation does not allow for the production of detectible

age strengthening of cast iron.

The thermodynamic software, FACTSAGE, was used11

for a model calculation of the condition when cast iron

natural aging will be suppressed by Ti impurity (Fig. 6).

The equilibrium volume of iron nitride significantly

depends on the nitrogen content in iron as well as the

concentration of nitride-forming elements, in particular

titanium, which can form stable nitrides even during

solidification when atom species are very mobile. When

the cast iron alloy contains titanium and nitrogen

simultaneously, these elements react to form titanium

nitride during solidification resulting in lower “free”

nitrogen in the ferrite solid solution. This limits the

formation of iron nitride at room temperature and

consequently suppresses the iron aging effect. The

temperature range of super-saturation of ferrite lies from

room temperature to 300C (572F) and beyond this range

the possibility of aging is limited according to

thermodynamics.

a)

b) Fig. 5. These graphs show (a)the change in tensile strength during aging with Ti variations at two levels of N and (b) the effect of calculated “free” soluble nitrogen on age strengthening of gray iron.

11

Box-and-Whisker Plot

UTS, MPa

210 215 220 225 230 235

Un-aged

Aged

-1

0

1

2

3

4

5

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07

Ti (weight %)

Ten

sil

e s

tren

gth

ch

an

ge (

MP

a)

Elevated nitrogen

Regular nitrogen

y = 324.6x + 2.9708

R2 = 0.8737

-1.00

0.00

1.00

2.00

3.00

4.00

5.00

6.00

-0.012 -0.01 -0.008 -0.006 -0.004 -0.002 0 0.002 0.004 0.006 0.008

Free N = N - 0.33 * Ti

UT

S c

han

ge


Fig. 6. These graphs show the combined influence of temperature, Ti and N on the potential percentage of iron nitride formation during natural aging.

11

The experimental data10, 11

confirmed these

thermodynamic predictions. The particular percentage of

equilibrium iron nitride is important from a practical point

of view because this allows prediction of the age-

strengthening effect. The suppression of natural aging

took place when the concentration of titanium exceeded

0.04% in heats with typical 0.006% nitrogen levels while

in heats with elevated (0.009%) nitrogen; aging was

suppressed when titanium was higher than 0.06%. Under

these conditions, there are no possibilities for iron nitride

formation according to the thermodynamic predictions.

The thermodynamic modeling can also be used for more

complex industrial irons containing other impurities

which have a potential for reacting with nitrogen.

MACHINABILITY OF AGED CAST IRONS CUTTING TOOL FORCES The machinability test articles recommended by AFS

were used for facing cuts on a Computer Numeric Control

(CNC) lathe (Fig. 7a). These test articles were produced

in a laboratory foundry using no-bake molds and in

industrial foundries using green sand molds. Tool force

data was collected on a HAAS CNC lathe using a TeLC

DKM 2010 Turning Dynamometer.12-14

Figure 7b shows

a schematic of the tool force system, where: Fc is the main

cutting force, Fp is the passive force and Ff is the feed

force.

a)

b)

Fig. 7. (a) The machinability test of AFS 5J 10" diameter test article and (b)cutting forces measurement system are illustrated.

A series of laboratory tests produced pearlite/ferrite cast

irons with variations in carbon equivalent from 3.9% to

4.3%. These irons were tested13

in as cast condition and

after 25 days of natural aging (Fig. 8). In as cast or in un-

aged condition, the cutting forces increased with

increasing hardness in irons having less carbon equivalent

which is typical and expected. At the same time, there

was a reverse type of dependency in which the cutting

force decreased when the increasing hardness was due

only to natural aging in each iron. This unusual behavior

could be explained by the energy requirement for chip

formation. In un-aged cast iron, soft ferrite absorbs energy

for significant plastic deformation. This effect results in

edge build-up on the tool tip which could also promote

increasing cutting force by enlarging the deformation

region (similar to tool wear). In contrast, when iron aging

occurs as a result of Fe4N precipitation in ferrite, it

increases the iron’s strength and hardness and allows for

chip formation with a smaller amount of plastic

deformation, which could decrease the cutting force.

Similar results were achieved also in other cast irons

having ferrite in metal matrix and different graphite

shapes. For example, aging decreased cutting forces after

aging ductile iron with spherical graphite and significant

free ferrite.16

400300

200100

20

0.0

001

0.0

01

0.0

1

0.0

2

0.0

40.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

Fe

4N

(w

eig

ht

%)

Temperature ( 0C)

Ti (weight %

)

Low nitrogen (0.004%)

400300

200100

20

0.0

001

0.0

01

0.0

1

0.0

2

0.0

4

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

Fe

4N

(w

eig

ht

%)

Temperature ( 0C)

Ti (weight %

)

High nitrogen (0.009%)


Fig.8. This graph shows the cutting forces versus hardness for un-aged and aged cast irons.

13

However, aging does not always improve cast iron

machinability.14

For example, aging of cast iron

containing carbide forming elements (Fig. 4) produced a

completely opposite effect on casting machinability

(Fig. 9). There was a visible and statistically significant

increase of the average normal cutting forces for the aged

samples compared to un-aged samples. The ratio of

passive to normal cutting forces is used as an indicator of

tool wear because as a tool loses sharpness it has an

increasing passive reaction force. It was visible that this

ratio increased more significantly during cutting of aged

gray iron with carbide-promoting element content. The

microstructure in this case was pearlitic with some

steadite and free carbide but no free ferrite (Fig. 4).

a)

b) Fig. 9. During sequential cuts of high chromium as cast and aged gray iron,

14(a) averaged and standard

deviation of normal cutting force and (b) passive to normal cutting force ratio are graphed.

14

To verify the effect of microstructure on cast iron

machinability, castings from the same heat were tested

additionally after ferritizing/re-solutionizing heat

treatment.13, 14

This treatment transformed pearlite to

ferrite and produced a re-solutionizing effect which

allowed repeating the natural aging. The effect observed

was opposite to the previously discussed test of cast iron

with pearlite matrix and steadite phase, in that aging of

ferritized/re-solitionized gray iron improved

machinability. The cutting forces were decreased at all

cutting speeds which were studied (Fig. 10). It can be

concluded from these tests that all gray iron showing

improved machinability in the aged condition contained

some amount of free ferrite, while gray iron showing

increased cutting forces after aging had no free ferrite but

was entirely pearlitic with cementite/steadite phases.

a)

b)

Fig. 10. These graphs show (a) the effects of cutting speed and aging on cutting force of ferritized/resolutionized gray iron and (b) the effect of cutting speed and aging on an average distance between crack formed in chips.

14

This differing behavior of aged cast irons depending upon

metal matrix is related to energy of chip formation.13

Although gray cast iron is a brittle material in tension,

chips can experience significant plastic deformation

because the stress state during machining is dominated by

compression and shear. If chip formation is assumed to be

a plastic strain to fracture event, then changes in fracture

toughness would logically affect machining behavior.

Fracture work during tensile testing was estimated from

the stress-displacement curve (Fig. 11). In the pearlitic

340

360

380

400

420

440

460

480

500

160 180 200 220 240

Brinell Hardness

Av

era

ge

Cu

ttin

g F

orc

es

(N

)

Unaged

Aged

Heat 4

Heat 3

Heat 2

280

290

300

310

320

330

340

350

0 1 2 3 4 5 6

Cut #

No

rmal

Fo

rces (

N)

Day 0 Fn Averages

Day 30 Fn Averages

0.250

0.300

0.350

0.400

0.450

0.500

0 1 2 3 4 5 6

Cut #

Passiv

e/N

orm

al

Fo

rce R

ati

o (

N/N

) Day 0 Fp/Fn Averages

Day 30 Fp/Fn Averages

100

120

140

160

180

0 100 200 300

Cu

ttin

g f

orc

e, N

Cutting speed, sfpm

Ferritized

Ferritized + aged

0

0.5

1

1.5

2

0 100 200 300

Dis

tan

ce b

etw

een

cra

cks,

mm

Cutting speed, sfpm

Ferritized

Ferritized + aged


iron the work of fracture increased after aging and cutting

forces increased at the same time. On the contrary, iron

which was ferritized by heat treatment showed decreased

work of fracture due to aging as well as decreased cutting

forces.14

a)

b)

Fig. 11. These graphs show the work of tensile fracture

for un-aged (blue markers) and aged (red

markers) gray irons: (a) with essentially no free ferrite in the matrix and (b) with significant free ferrite in the matrix.

14

TOOL WEAR AND INDUSTRIAL MACHINING MEASUREMENTS Based on both the current work and that of previous

researchers, tool wear is lower when machining gray cast

iron aged at room temperature because aged iron requires

less work input from the machining center to form and

break off chips. The decrease in required work has been

demonstrated in the present study12-14

by tool force

measurements and in previous work by testing amperage

drawn while machining un-aged and aged iron.8 It was

shown that the least power was required to machine

castings aged for 3-6 days when compared to iron aged

for 1, 9 and 20 days (Fig. 12). At that optimal aging time,

machined castings had better surface quality (less

roughness), but all aged iron had better surface finish than

the un-aged iron.

a)

b) Fig. 12. These graphs show the (a) roughing amperage drawn to machine clutch disks as a function of the number of days the castings were aged and the number of castings machined and (b) the surface finish value.

8

Other tests were performed with industrial face machining

of brake discs for a passenger car.11, 12

Excessive tool

wear produced changing tool geometry and increased

cutting forces, which promoted elastic deformation of

casting with increasing tilt and destroying required

tolerance on perpendicularity (“tilt”). Tilt data from the

machining of industrial castings were compared in two

ways. First, Fig. 13a compares the tilt during machining

of 50 un-aged (day 1) castings with 50 aged (day 10)

castings. Second, Fig. 13b compares the tilt for the same

50 un-aged castings to 200 aged (day 10) castings. The

machining of the 50 un-aged castings required two tool

position changes as indicated by the bold arrows. No tool

position changes were required during machining of aged

castings after 50 or 200 castings, indicating more

consistent dimensions and reduced downtime for tool

position corrections. Fig. 13c gives a comparison of

measured tool wear for different operations. Aging

significantly decreased tool wear in most of the

operations.

7.4

7.9

8.4

8.9

9.4

9.9

0 5 10 15 20 25 30 35

Days Aged at Room Temperature

Un

iaxia

l T

en

sil

e W

ork

of

Fra

ctu

re (

J/m

m3)

4.5

4.6

4.7

4.8

4.9

5

5.1

5.2

5.3

0 5 10 15 20 25

Days Aged at Room Temperature

Un

iaxia

l T

en

sil

e W

ork

of

Fra

ctu

re (

J/m

m3)

90

100

110

120

130

140

150

0 100 200 300 400

Number of Castings

Ro

ug

hin

g A

mp

s

Day 1

Day 3

Day 6

Day 9

Day 20

20

30

40

50

60

70

80

90

0 100 200 300 400

Number of Castings

Su

rfa

ce

Fin

sh

Va

lue

(u

m)

Day 1

Day 3

Day 6

Day 9

Day 20


a)

b)

c)

Fig. 13. These graphs show the comparison of tilt data from machining: a) 50 un-aged castings with 50 aged castings and b) 200 aged castings and c) tool wear (flank area).

12,13

INDUSTRIAL RECCOMENDATION FOR IMPROVING CAST IRON MACHINABILITY BY AGING

Summarizing the previous and recent experimental

studies, it can be concluded that there are three different

possible scenarios for changes in machinability of gray

iron during natural aging (Table 2).

First scenario: aging does not occur and therefore,

has no influence on machinability. Lack of aging

effects in the iron can be caused by elevated nitride

forming elements (particularly Ti) relative to

nitrogen. Additions of nitrogen to iron are possible

and can enhance aging. Thermodynamic data can be

applied to determine if there is enough “free

nitrogen” for aging a cast iron. Figures 5, 6 and

Equation 5 can be used for the prediction of aging

effect in a particular iron. A simplified criterion

might be, if %N< (.15-0.20)%Ti, aging will not

occur.

Second scenario: If cast iron exhibits aging, this

phenomenon can be used for improving casting

machinability. Aging is accompanied by decreasing

cutting forces and tool wear. These irons have

enough “free” nitrogen to promote age strengthening.

Decreased cutting forces and increased mechanical

properties were proven in laboratory castings (Fig. 8)

having different carbon equivalents. These irons had

some amount of free ferrite and no free cementite or

steadite. Optimal aging time depends upon particular

“free Mn” content and could be evaluated using

Fig. 3. Acceleration and decreasing aging time for

improving machinability could be done by warm

(slightly elevated) temperature “aging” (Equation 3).

Third scenario: Gray iron has elevated

concentrations of carbide forming elements such as

Cr in addition to large %P. These combinations of

chemistry with a particular cooling rate could

promote steadite/cementite formation in fully

pearlitic matrix. If this iron has negligible free ferrite,

aging will increase cutting forces in this iron.

Effective inoculation and chemistry control will

affect casting machinability interaction with aging in

these cast irons. However, in this scenario the

foundry may find that “fresh” castings are more

machinable.

CONFIRMATION TEST

Five AFS 5J, 10 in. diameter test articles were poured into

no-bake molds from one 200 lb induction furnace heat.

Nitrogen content was enhanced by ladle treatment with

0.2% of Fe70Mn5N addition together with 0.3%

Fe75Si2Ba inoculants. Cast iron chemistry is shown in

Table 3. Microstructure of this iron was mostly pearlitic

with approximately 5-10% ferrite (Fig. 14). Measured

hardness in the middle section of the test article was 200-

210HB in the as cast condition (un-aged). The as cast

surface layer (1/8 in.) was removed in preliminary

machining to avoid the effects of cast surface structure,

mold-metal interaction and geometry variance on test

results. Test. articles were face CNC machined at

day 0, day 5, day 9, day 15 and day 22 with measurement

of cutting forces.11-13

-0.1

-0.08

-0.06

-0.04

-0.02

0

0.02

0.04

0.06

0.08

0.1

0 5 10 15 20 25 30 35 40 45 50

Casting number

Til

t (m

m)

Day 1

Day 10

-0.1

-0.08

-0.06

-0.04

-0.02

0

0.02

0.04

0.06

0.08

0.1

0 20 40 60 80 100 120 140 160 180 200

Casting number

Til

t (m

m)

50 Unaged Castins (Day 1)

200 Aged Castings (Day 10)

0

0.5

1

1.5

2

Op1 Op2 Op3 Op4 Op5 Op6

Cutting Operation Designation

To

ol

Wear A

rea (

mm

2)

Day 1: 50 castings machined

Day10: 50 castings machined

Day 10: 200 castings machined


Table 2. Different Aging/Machinability Scenario Observed in Gray Cast Iron

12-15

Scenario Gray Iron Aging effect confirmation

Phases, % area Aging effect on machinability Ferrite Steadite/

carbides

1 Lab (4.1% CE with Ti) No 5-15 - No effect

2 Lab (4.3% CE) Yes 25-27 - Improved

Lab (3.9% CE) Yes 5-7 - Improved

Industrial brake disks

Yes ~1 - Improved

Industrial test articles (ferritized)

Yes 40-60 1.8–2.0 Improved

3 Industrial test articles with elevated Cr and P Yes < 0.2 1.8–2.0 Increased tool forces, lower machinability

Table 3. Chemistry (wt. %) of Test Articles

C Si Mn S Cr Cu Al Ti N

3.26 2.03 0.61 0.08 0.15 0.2 0.01 0.008 0.010

Fig. 14. These are photomicrographs of the microstructure of gray iron for the machinability confirmation test.

Eight cuts (30 minutes total machining time) were

performed from each disc, using a new tool insert each

time. The thickness of the test article produced eight

duplicate cuts and each test was repeated twice. The test

results are shown in Fig. 15.

a)

b)

Fig. 15. a) Expected age strengthening kinetics for tested composition

9 and (b) effect of aging time on

cutting force (black markers) and tool wear (red markers) are graphed.

190

200

210

220

230

240

250

0 10 20 30 40 50

Ten

sle

stre

ng

th, M

pa

Aging time, days

Heat C

120

140

160

180

200

240

244

248

252

256

260

264

0 5 10 15 20 25

To

ol w

ea

r, μ

m

Ma

in c

utt

ing

fo

rce

, N

Aging time, days


Now, these test results are compared with the predictions

according to the suggested methodology.

Step 1—evaluation of the possible age strengthening:

Nfree =N-0.20Ti = 0.01-0. 2*0.008 = 0.0084 wt.% or

84 ppm; according to Fig. 6, total %N and %Ti leads

one to expect approximately 0.14 wt. % Fe4N.

Figure 5 indicates age strengthening will occur.

Step 2 – control microstructure: Fig. 14 shows a

matrix without free carbide/steadite having a small

amount of free ferrite around flake graphite. Age

strengthening of this type structure can improve

casting machinability according to the second

scenario (Table 2).

Step 3 - Aging time: according to Fig. 3, full aging

time is 15-17 days and according to Fig. 2 pre-

strengthening time is 7-9 days. The tool force

dropped significantly during the first five days and

was also low at 15 days, roughly corresponding to the

expected times for room temperature age

strengthening.

The predictions based on the previous studies were

confirmed in the last machinability test. A significant

decrease in cutting force and standard variation were

observed after 9-15 days of natural aging which is

between predicted pre-strengthening and full aging time.

Regarding other machinability parameters, tool wear not

only depends the average value of cutting force but also

the stability of cutting process and tool wear continued to

decrease up to the full aging time.

CONCLUSIONS

This paper summarizes the past study of the effect of cast

iron natural aging on casting machinability. Based on

thermodynamic and first principle atomistic calculations,

a variety of experimental techniques and industrial

studies, the mechanism was discussed and aging process

kinetics was quantified. More important, the aging

process was linked to alloy chemistry and simple rules

were suggested to predict aging and forecast peak age

strengthening.

In the second part of this paper, the different scenarios of

improving casting machinability by aging were

considered. Cutting forces measured in the laboratory,

tool wear observed during industrial machining and

machining surface quality were all linked to aging

phenomena. Finally, a conformation test was performed

for verification of the suggested optimal aging time for a

specific composition to improve gray iron machinability.

A set of rules was suggested for optimization of the

process window. Briefly, this set of rules for assessment

of an optimal machinability window of aged cast iron

includes:

Estimate free nitrogen based on total nitrogen and

concentration of titanium as % N >0.2 %Ti, but not

high enough to form gas porosity in order to have age

strengthening.

Check microstructure concerning ferrite/pearlite

content without steadite/carbides. If no free ferrite is

present, particularly with all pearlite and some

carbide or steadite, the foundry may experience better

machinability with fresh castings given a

composition which will age strengthen. If free ferrite

is present, age strengthening will provide a

corresponding improvement in machinability.

Finally, estimate room temperature aging time based

on free manganese left after sulfide formation.

Acceleration of aging with a low temperature bake is

possible.

ACKNOWLEDGMENT

This paper is based on work supported by the U.S.

Department of Energy under award number DE-FC36-

04GO/4230 as subcontracted through the Advanced

Technology Institute. Any findings, opinions,

conclusions, or recommendations in this report are those

of the authors and do not necessarily reflect the views of

the Department of Energy.

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aging and machinability interactions in cast iron

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