Role of Additives in P/M Machining
Robert J. Causton
Hoeganaes Corporation
Cinnaminson, NJ 08077
Presented at
PM2TEC 2002 World Congress
On Powder Metallurgy and Particulate Materials
June 16-21, Orlando, Florida
ABSTRACT
Many P/M parts are machined before final assembly. The increasing use of dedicated
lines, or cells, in which many sequential operations are conducted on an assembly
means that the time required to machine a P/M component will need to be reduced if it is
the bottleneck in the assembly process. This paper examines the effects of three freemachining additives: boron nitride, manganese sulfide and pre-alloyed, or resulfurized,
sulfur, upon the machining response of an FC-0205 P/M steel in a turning operation. The
effects of the additives on tool wear, cutting forces, chip form and surface finish are
compared.
INTRODUCTION
In the conversion of wrought and cast components to P/M processes, an increasing
number of P/M steels need to be machined prior to final assembly. In several cases the
machining operations are conducted as part of the production of a system, rather than
discrete component. In these cases, the different machining characteristics of P/M
steels, cast irons and wrought steels may cause machining of the P/M component to be
the bottleneck in an entire assembly operation. In such cases there is considerable
reluctance to consider further conversions to P/M components. This paper examines the
role of free-machining additives: manganese sulfide, hexagonal boron nitride and prealloyed, or resulfurized powders, in improving the machining response of a standard
MPIF FC-0205 P/M steel under defined turning conditions.
Free machining agents are added to P/M steels to improve their machinability including
factors such as tool life, metal removal rate and surface finish. Machining additives
achieve this in several ways (Fig. 1) including: initiating micro-cracking of the work piece,
reducing adhesion of the tool chip to the tool, built up edge formation and reducing
adhesive wear that lead to crater wear (Ref. 1).
Chip
Tool
Chip Separation
Lubricant
Crater
Wear
Work
Microcracking
Built-up
Edge
SST-95
Fig. 1: Potential Functions of a Free-machining Additive
TEST PROGRAM
The test program consisted of measuring the effect of the free-machining additives upon
the sintered properties, microstructure and machining response in turning of an MPIF
FC-0205 composition.
Premix Compositions
500 pound Pilot Premixes were made with compositions shown in table 1 below. Three
free machining additives were chosen for investigation: manganese sulfide (MnS), a prealloyed resulfurized iron powder (RS) and hexagonal boron nitride (BN). Manganese
sulfide is a recognized free machining additive for P/M steels. Pre-alloying an iron
powder with sulfur provides a finer more uniform dispersion of manganese sulfide than
admixed manganese sulfide. Hexagonal boron nitride is a recognized solid lubricant that
has potential as a free machining agent. The free machining additives were included at a
Table 1: Premix Compositions
Premix
1
2
3
4
5
6
7
Iron
(%)
96.65
96.30
96.05
96.65
96.65
96.55
96.45
Copper
(%)
2
2
2
2
2
2
2
Acrawax C
(%)
0.75
0.75
0.75
0.75
0.75
0.75
0.75
Graphite
(%)
0.60
0.60
0.60
0.60
0.60
0.60
0.60
Additive
None
MnS
MnS
RS
RS
BN
BN
Additive
(%)
0
0.35
0.60
0.14
0.30
0.10
0.20
“low” and “high” level based upon previous experience. The compositions of the
resulfurized powders were intended to produce similar manganese and sulfur contents
to those attained by the use of admixed manganese sulfide.
The resulfurized iron powders were produced as Pilot Plant heats. All other premix
ingredients were standard additives used for Production premixes except for the boron
nitride that was a high purity grade with similar particle size distribution as the
manganese sulfide powder.
Sintered Properties
The sintered properties of the premixes were measured using TRS test pieces
compacted to a green density of 6.8g/cm³. The test pieces were sintered at 2050ºF
(1120ºC), under 90%nitrogen/hydrogen, for 30min at temperature using an Abbot belt
furnace. The dimensional change from die size and TRS are shown in table lll below.
The sintered carbon, nitrogen and sulfur contents of sections cut from the sintered test
pieces are shown in table below.
Machinability Testing
The machining test was conducted using a hollow cylindrical test piece of 1.75” diameter
by 1” bore and 1” length ( 44.5mm x 25.4mm x25.4mm) . The test pieces were sintered
using the same conditions as the TRS test pieces. The apparent hardness of the
sintered test pieces was measure using a Mitutoyo hardness tester with results shown in
table below. Sections of the sintered machining test pieces were taken for
Metallographic examination. The turning test was conducted using the conditions shown
in table ll below.
Table ll: Turning Conditions
Insert Geometry
Grade
Speed
Depth of Cut
Feed
Coolant
SNG433
C2
300fpm
0.045”
0.0114”/rev
None
SNG433
C2
91.44m/min
1.14mm
0.290/rev
None
The three orthogonal components of the machining force were measured using a Kistler
three-axis dynamometer. The flank wear was measured with a measuring optical
micrometer at intervals of five cuts. The cutting test was halted after 100 cuts or tool
failure. Samples of the chips generated during cutting were taken for comparison.
Examples of the chips, and work piece surface after 75cuts are shown below.
Photomicrographs of the tool face at the end of the test, 100 cuts are also shown.
RESULTS
The sintered properties of the test premixes are shown in table lll. There was little
difference in the compaction pressure required for a green density of 6.8g/cm³.
Table lll: Sintered Properties of FC-0205 Premixes
Premix
1
2
3
4
5
6
7
Addition
(%)
None
0.35MnS
0.50MnS
0.14RS
0.30RS
0.1BN
0.2BN
S.D.
(g/cm³)
6.69
6.68
6.67
6.64
6.63
6.66
6.65
D.C.
(%)
+0.46
+0.47
+0.48
+0.49
+0.44
+0.49
+0.51
TRS
(ksi)
122.4
122.0
116.4
102.8
112.9
108.8
110.8
Hard.
(HRB)
69.6
69.5
68.7
64.0
66.3
65.2
65.3
Note
Compacted to 6.8g/cm³, 2050ºF, 90%nitrogen/10%hydrogen, 30min at
temperature
It appears possible that adding boron nitride increases dimensional change from die
size. There is no consistent effect of admixed manganese sulfide or use of resulfurized
iron powders upon dimensional change in the compositions tested, although it is
possible that the resulfurized powder of 0.14% sulfur content increases growth slightly.
The results indicate that increasing manganese sulfide content to 0.5% may reduce TRS
and apparent hardness slightly. The resulfurized powders have lower TRS and hardness
than the FC-0205 reference. Introducing the boron nitride powder also reduces TRS and
hardness compared to the FC-0205.
The basic chemical analysis of the sintered TRS test pieces is shown in table lV.
Table lV: Chemical Analysis of FC-0205 Test Pieces
1
2
3
4
5
6
7
Addition
None
0.35MnS
0.50MnS
0.14S
0.3S
0.1BN
0.2BN
C (%)
0.58
0.58
0.57
0.50
0.57
0.55
0.58
S %)
0.007
0.11
0.18
0.13
0.30
0.012
0.008
N (%)
0.013
0.016
0.016
0.012
0.015
0.067
0.12
The content of all the test pieces except the 0.14% sulfur resulfurized powder comply
with the FC-0205 specification, although the sintered carbon content of the 0.14%
resulfurized test pieces are slightly lower than anticipated. This probably accounts for
part of the difference in hardness and strength shown in table lll above. The other
analyses show that the test premixes comply with the target compositions.
Apparent Hardness of Test Pieces
The apparent hardness of the turning test pieces is lower than that of the TRS test
pieces, but the free machining additives have similar relative effects.
Table V: Apparent Hardness of Machining Test Pieces
Premix
Addition (%)
Hardness (HRB)
1
2
3
4
5
6
7
None
0.35 MnS
0.50 MnS
0.14 RS
0.3 RS
0.1 BN
0.2 BN
55.3
53.7
52.1
47.7
54.7
49.2
50.8
Hardness
(TRS)
69.6
69.5
68.7
64.0
66.3
65.2
65.3
It is possible that the larger cross section of the tuning test pieces combined with the
higher mass loading produced lower cooling rated from sintering temperature resulting in
a coarser microstructure.
Microstructure of Machining Test Pieces.
The microstructure of the FC-0205, in the as polished condition, with no machining
additives is shown in Fig. 2a. The P/M steel possesses a good degree of sinter with very
few oxides present at prior particle boundaries and slight pore rounding. When etched,
Fig. 2b, the FC-0205 P/M steel possesses a microstructure of relatively coarse ferrite
and pearlite.
a) as polished
b) etched nital/picral
Fig. 2: Microstructure of FC-0205 Mix 16 no machining additives.
The effects of the machining additives upon the microstructure of the FC-0205 in the as
polished condition are shown in Figs 3 below. Introducing MnS (Fig. 3a and 3b)
introduces light gray sulfide visible in the pores and at some prior particle boundaries.
a) 0.35% MnS
b) 0.50% MnS
c) 0.14% S resulfurized
d) Mix 4 0.3% sulfur resulfurized
Fig. 3: Microstructure of FC-0205 plus free machining additives
When the sulfur is pre-alloyed, the sulfide particles are more dispersed more uniformly
through the microstructure and are present both at particle surfaces and within prior
particles. Increasing the sulfur content, whether as MnS or by resulfurizing, increases
the volume fraction of sulfide. Unfortunately there was insufficient contrast between the
boron nitride particles and matrix to define the distribution of boron nitride particles.
When etched the microstructures consist of ferrite plus pearlite (Fig. 4), but there may be
slight changes in the amount and distribution of ferrite. The 0.14% resulfurized steel
possesses slightly more ferrite than the other test pieces, possibly due to its lower
carbon content. In both resulfurized powders, there are fewer coarse MnS particles at
the prior particle boundaries than in the test pieces with admixed MnS
a) 0.35%MnS
b) 0.50%MnS
c) 0.14%S Resulfurized
d) 0.3%S Resulfurized
e) 0.1% BN
f) 0.2% BN
Fig. 4: Microstructures of FC-0205 plus machining additives
Etched with a mixture of nital and picral.
MACHINING RESULTS
The machining tests showed that introducing the free machining additives improved tool
performance. They reduced tool wear, cutting forces and generally improved chip
formation. Although the machining conditions promoted flank, the chief wear mechanism
in all cases was crater wear. In many cases there was evidence of built up edge
formation, that was reduced significantly when cutting test pieces with the 0.5% MnS
addition or the 0.3%S resulfurized steel. These results are discussed separately below
to illustrate the effects of the free machining additives more clearly.
Cutting Test Results
The cutting forces and tool wear measured during turning of the FC-0205 with no
additives are shown in Fig. 5. They show that flank increased with number of cuts
throughout the test. In the case of the FC-0205, the cutting tool failed after 94 cuts.
Wear (0.0001")
Force (lbf)
250
50
Cut
Radial
Feed
Wear
200
40
150
30
100
20
50
10
0
0
0
20
40
60
80
100
120
Number of Cuts
Fig. 5: Cutting Test Results for FC-0205 Mix 16 no machining additives
Cutting forces showed considerable variation, with no consistent pattern. For FC-0205,
the cutting force varied about a mean of about 170lbf, but increased rapidly just before
tool failure. Radial force was lower than cutting force, a mean of about 130lbf, and in the
case of the FC-0205 appeared to increase towards the end of the test before tool failure.
Feed force was the lowest of the three forces, a mean of about 75lbf.
Flank Wear
Introducing a free machining additive increased tool life and reduced tool wear
significantly. When cutting FC-0205 with no additive, flank wear increased progressively
throughout the test until the tool failed after 94cuts. All free machining additive extended
tool life to the end of the tests and reduced tool wear significantly (Fig. 6). The presence
of 0.1% BN produced lowest flank wear after 100 cuts.
50
Wear (0.0001")
45
40
35
30
25
20
15
10
5
0
None
0.35% MnS 0.50% MnS
0.1%S
RS
0.35%S
RS
0.10%
BN
0.20%
BN
Free Machining Additive
Fig. 6: Flank Wear at end of Cutting Test
In the current test increasing boron nitride content from 0.1 to 0.2% increased flank wear
from 0.002 to 0.004”. There was little difference between the different sulfide additions
in flank wear at the end of the test. Increasing MnS content from 0.35 to 0.5% reduced
tool wear. However, increasing the resulfurized sulfur content from 0.14 to 0.30% had no
significant influence on flank wear at the end of the test.
There are considerable differences in the form of wear during the test. Introducing
0.50%MnS reduces the gradient, flank wear per cut, significantly (Fig. 7). In contrast
0.1%BN appears to produce rapid initial wear followed by a period of almost no wear.
It is possible to compare the effects of the free machining agents by the coefficients of
the regression equations (Ref. 2) fitted to the wear plots as shown in table Vl.
Table Vl: Summary of Tool Wear Data for FC-0205
Add.
(%)
C
M
R2
None
-2.74
0.481
0.977
MnS
0.35
+1.52
+0.303
0.987
Note: Assumes y= mx + c
MnS
0.50
0.95
0.255
0.995
RS
0.14
0.77
0.263
0.996
RS
0.30
1.989
0.263
0.989
BN
0.1
7.47
+0.137
0.685
BN
0.2
--1.810
+0.485
-0.938
Wear (0.0001")
50
45
y = 0.4806x - 2.7374
40
R = 0.9771
2
FC-0205
35
30
25
0.50%MnS
20
0.1%BN
15
10
5
0
0
20
40
60
80
100
120
Number of Cuts
Fig. 7: Effect of Additives upon Flank Wear of FC-0205 at 300fpm.
The data show considerable variation in zero intercept and gradient. They show that all
additives except 0.2%BN reduced the gradient of the wear plot significantly from the
0.481 measured for the FC-0205. Surprisingly the 0.1% boron nitride addition produced
the lowest rate of wear, but has the lowest value of R2 , indicating that the wear rate or
wear processes varied considerably during the test. The data indicate that there was
little difference in wear rate between admixed or pre-alloyed sulfides.
The values for intercept vary considerably. The data suggest that the presence of a free
machining agent reduces the initial tool wear. The extreme value of 0.747 for the 0.1%
boron nitride addition may reflect that the tool edge was damaged slightly, perhaps by
impact with the test piece, during the initial cuts before the first measurement of tool
wear. The negative values may indicate formation of a built up edge at an early stage in
testing.
The values for R2 indicating that the assumption of a linear model is good except for the
0.2%BN addition.
Cutting Forces
The three cutting forces varied considerably throughout the test. The cutting force was
larger than the radial and feed forces. There appears to be no consistent increase in the
forces through the test. Although it appears than in some tests the cutting force
increases rapidly just before tool failure. There may be a correlation between the cutting
and feed forces, although further analysis is necessary to confirm this.
Given the variation in the data for cutting force in the current tests, it appears that they
can be compared in terms of means and standard deviation. The summary of the cutting
data is shown in table Vll.
Table Vll: Summary of Cutting Force (lbf) for FC-0205
Add.
(%)
Cut.
s.d
None
176.7
25.1
MnS
0.35
147.4
27.0
MnS
0.50
159.6
23.1
RS
0.14
173.1
21.0
RS
0.30
132.9
33.3
BN
0.1
173.1
16.7
BN
0.2
139.2
35.5
Radial
s.d.
122.2
2.2
106.9
27.1
105.2
30.6
104.5
17.9
88.6
21.5
113.8
15.5
56.2
30.6
Feed
s.d.
78.9
15.2
73.3
13.0
63.4
11.9
75.8
11.2
56.6
12.2
74.9
9.75
79.7
10.8
The results suggest that introducing a free machining additive tends to reduce the
cutting and radial forces but has less effect upon feed force. For the resulfurized powder
and boron nitride, increasing the level of addition reduces cutting forces further.
Increasing the addition of MnS from 0.35 to 0.5% does not change cutting force
appreciably and there may be a slight increase. The presence of 0.14% sulfur in the
resulfurized powder did not reduce forces consistently compared to the FC-0205.
The cutting component of the machining forces was lowest when machining the 0.3%S
resulfurized test pieces. It is possible that the data for the 0.3% sulfur content indicates a
change in the cutting edge during the test (Fig. 8). It appears that the cutting force
decreased during the test with the exception of some scatter that may be due to one or
two test pieces. Turning FC-0205 with 0.1%BN required low a cutting force that were
significantly lower than those required with 0.2%BN. The cutting force was reduced by
additions of MNS and was less sensitive to the level of addition than the resulfurized or
with boron nitride.
All of the additives reduce the radial component of the machining force. Addition of 0.2%
boron nitride produces the lowest radial force. The radial force produced in turning the
0.35%S resulfurized steel is somewhat higher. There is little difference between the
radial l forces measured for the remaining materials. For boron nitride and resulfurized
powders, increasing the level of addition reduces the mean radial force. Increasing the
level of admixed MnS does not change the mean radial force.
The presence of the machining additive had little effect upon the mean feed force that
was only reduced by when turning the 0.3% S resulfurized test pieces or those with
0.5%MnS.
Cutting Force (lbf)
250
y = -0.4335x + 153.22
2
R = 0.127
200
150
100
50
0
0
20
40
60
80
100
120
Number of Cuts
Fig. 8: Cutting Force for 0.3%S resulfurized powder as free machining agent.
CUTTING INSERT
The cutting insert used for the FC-0205 shows both flank wear and significant crater
wear (Fig. 9). It appears that tool failure occurred when the so much tool material was
Fig. 10: Cutting Insert for FC-0205 at end of test (94 cuts)
removed by the wear processes that the cutting edge could not withstand the cutting
forces. There is also evidence of some adhering material from the work piece indicating
that a built up edge may have formed at some point during the cutting test.
In comparison the machining additives reduced the severity of the wear (Fig. 11).
a) 0.35%MnS
a) 0.35%MnS
b) 0.50%MnS
c) 0.14%S resulfurized
d) 0.3%S resulfurized
e) 0.1%BN
f) 0.2%BN
Fig. 11: Cutting Inserts at End of Test (100cuts)
It appears that sulfides, whether admixed as manganese sulfide or resulfurized (Fig. 11a11d), produce less wear than boron nitride and have similar wear patterns. Introducing
sulfides reduces flank wear and built up edge formation. Increasing the amount of sulfide
appears to reduce the amount of crater wear and reduces adhesion of removed work
piece to the cutting tool. The presence of sulfide also appears to reduce the size of a wear
land or scar that extends from the cutting edge along the edge of the wear crater.
Introducing boron nitride (Fig. 11e and 11f) allows the cutting tool to be covered with wear
debris and allows a built up edge to form. Increasing the level of boron nitride addition
reduces this effect significantly. It is not clear what the true flank wear is, since much of the
cutting area is covered in debris. There appears to be less flank wear and a less defined
crater when cutting with 0.2% boron nitride.
CUTTING CHIPS/SWARF
Machining the FC-0205 under the chosen cutting conditions produced very long,
continuous swarf (Fig. 12). Individual pieces varied from 0.5 to four inches in length.
1234
Fig. 12: FC- 0205 Machining Chips after 75 cuts.
At higher magnification the chips showed more evidence of heavy shear and partial
fracture into shorter segments. The edges of these segments were highly deformed and
fractured.
Chip form changed significantly with the presence, type and amount of addition tending to
change from continuous to segmented or serrated (Ref. 3). Generally, introducing the free
machining agents (Fig. 13) caused the machining swarf to adopt a more regular, highly
curved form. Individual pieces were helical in shape tended to be shorter in length.
Adding 0.35% MnS shows his effect clearly. The majority of the swarf consists of tightly
wound helical coils. Increasing the MnS content to 0.5% does not have a significant effect,
but may increase the number of short fragments.
a) a) 0.35% MnS
b) 0.50%MnS
c) 0.14%S resulfurized
d) 0.30%S resulfurized
e) 0.1%BN
f) 0.2%BN
Fig. 13: Effect of Machining Additives on Chip Form
The presence of 0.14%S in the resulfurized powder has a relatively small effect upon
chip form. The individual pieces are curved and vary considerably in length. Increasing
the sulfur content to 0.30% causes the chips to consist of very short curved pieces
generally less than 0.25” in length.
Boron nitride produces changes in chip form that fall between those of admixed MnS
and resulfurized powder. With an 0.1% addition, the chips are similar to those formed
with 0.35%MnS, being relatively long and either curved or slightly helical in shape.
Increasing the BN addition to 0.2% causes the chips to adopt a tightly wound helical
form, with sections varying from 0.25 to 1.5” in length. Changing the machining additive
from MnS to BN changes the surface texture of the chips particularly at the their edges.
With MnS the chips possess smooth edges, whereas with BN, the edges are much
rougher, appearing to carry short “needlelike” projections.”
SURFACE FINISH
The turning operation and the presence of a machining additive change the appearance
of the surface significantly. For the FC-0205 (Fig. 14) turning closes the surface porosity.
a) as sintered
b) machined 75 cuts
Fig. 14: Surface of FC-0205 Part
It also removes the scoring introduced by compaction and ejection. Turning introduces
parallel grooves defined by the nose radius of the tool and the feed per revolution. In the
FC-0205 with no free machining additives, the grooves lose definition with increasing
tool life, becoming shallower and somewhat “smeared” in appearance as the tool wears.
The surface also contains some dark debris that is parallel to and sometimes associated
with the “threads” of the machining grooves.
The surface appearance of the turned parts that possess a machining additive (Fig. 15)
is generally similar to that of the turned FC-0205. However, the grooves are better
defined and more regular in appearance except for the parts that contain boron nitride.
Introducing sulfide as admixed MnS or by pre-alloying (Fig. 15a-d) delays the effects of
tool wear significantly. The surface grooves are well defined. Small copper colored
particles
a) 0.35%MnS
b) 0.50%MnS
c) 0.14%S resulfurized
d) 0.30%S resulfurized
e) 0.1%BN
f) 0.2%BN
Fig. 16: Surface of Turning Test Pieces after 75 cuts
are visible that are elongated in the cutting direction. There appears to be little
difference between the effects of admixed MnS or resulfurized powder. Increasing the
sulfide addition appears to improve and preserve surface definition.
Use of boron nitride as a free machining additive produces a surface that is intermediate
between that with no sulfide additive and a low sulfide content. Increasing the BN
addition from 0.1 to 0.2% increases the definition of the turning grooves. The surface
lacks definition and appears either to be deformed by the turning operation, or to be
partially covered by a surface layer. At both levels of BN addition, the surface contains
black lines or fine cracks normal to the cutting direction. In the part that contains 0.1%BN
there is an irregular distribution of relatively coarse black “particles” across the machined
surface.
DISCUSSION
The turning test illustrated that the free machining additives performed many of the
functions anticipated. Tool wear was reduced, cutting forces reduced, tool life increased
and chip formation improved by the presence of the additives. They were less successful
in reducing crater wear entirely. Despite the overall similarities, there were major
differences in their effects of the additives particularly between the sulfides and boron
nitride.
When machining FC-0205 sulfides, both admixed and resulfurized, are more successful
free-machining additives than boron nitride. Boron nitride, particularly as an 0.1%
addition, produced somewhat higher cutting forces and was less successful in promoting
chip separation and reducing built up edge formation. Despite these problems boron
nitride produced low flank wear as an 0.1% addition, but not as an 0.2% boron addition.
However 0.2% boron nitride may have reduced crater wear.
This apparent contradiction may be due to intrinsic differences in the way chips are
formed by boron nitride and sulfides, or may reflect a beneficial change in local cutting
geometry caused by built up edge formation. The built up edge present on the tool
provides a local reduction in rake angle. The change in angle should reduce the forces
required to form and separate chips. In this respect the presence of the built up edge is
beneficial since it reduces cutting forces and flank wear. It is less beneficial in chip
formation and surface finish. The machined surface is less well defined, and particularly
in the case of 0.1%BN, possesses adhering particles that appear to be remnants of the
built up edge (Ref. 3).
The sulfides, particularly at the higher additions, reduce built up edge formation and
promote chip fragmentation. There also appeared to be more frequent micro-fracture
within the chips formed with sulfides leading to greater chip separation and
segmentation. It appears that the sulfides coat the tool face more effectively than the
boron nitride providing a surface lubricant that reduces friction and local heating due to
friction in addition to that from the cutting process. Examination of the chips indicates
that the sulfides initiate cracks within the chips and prevent the chips adhering to the tool
face. SEM examination detected high sulfur concentrations on many areas of the tool
faces. More detailed examination of the chips shows that chips containing boron nitride
were often blue or black in color suggesting that they reached higher temperatures than
the chips formed with sulfides. The higher temperatures appear to allow separated
fragments of the built up edge to form the “particles” observed on the surfaces of the
machined test pieces, and possibly to the chips to form the “needlelike” projections
observed on some of the larger chips.
Under the cutting conditions employed there were few major differences between the
admixed MnS and resulfurized powders. However, there was a much greater difference
between the high and low levels of sulfur content in the case of the resulfurized powder
than the MnS. The higher 0.35%S resulfurized powder was a more effective free
machining agent than the lower level. It also produced very short fragmented chips and
a very clean tool surface.
Under the machining conditions employed the free machining additives did not entirely
suppress crater wear. It is possible that other changes to the machining process, such
as a coated insert with chip breaker would improve tool life and chip formation further.
CONCLUSIONS
The free machining additives improved the machining response of the FC-0205
significantly.
The free machining additives reduced flank wear, and built up edge formation but had
less effect upon crater wear
Sulfides, both admixed MnS and in resulfurized powder, were more effective than boron
nitride.
In the resulfurized powder, the higher sulfur content was more effective particularly in
chip formation.
Free machining additives can change the dimensional change and mechanical
properties of the FC-0205 composition.
REFERENCES
1: Metal Cutting, E. M. Trent, Butterworth
2: Characterization of the Machining Operations of P/M Parts during Drilling Operations,
Carl Blais, MPIF Machinability Seminar, Oct. 99.
3: Fundamentals of Modern Manufacturing, M. Groover, Wiley