Machinability Additives for Improved Hard Turning of PM Steel Alloys
Bruce Lindsley and Chris Schade
Hoeganaes Corporation
Cinnaminson, NJ 08077, USA
ABSTRACT
The machining of ferrous PM alloys differs considerably from wrought materials. The role of porosity
and heterogeneous microstructures complicates the machining process, often making it more challenging.
In addition, the presence of martensite in the microstructure of more highly alloyed and/or sinterhardened PM components increases tool wear. One advantage of PM is that machinability additives can
be easily admixed into the powder and therefore into the final part. Manganese sulfide is a well known
additive for improving machinability. A new machining additive, designated MA, has been developed to
compliment MnS in PM steels. Hard turning tests were performed to evaluate the effect of both additives
on tool wear in different material systems. The MA additive was found to improve machinability beyond
that of MnS in sintered compacts containing martensite. It additionally reduced rusting on the part
surface. This paper discusses the improvement in machinability with these additives, with an emphasis
on sinter-hardenable PM steels.
INTRODUCTION
Ferrous PM is generally considered a net or near net shape process. Nevertheless, many pressed and
sintered parts are machined prior to final assembly, as certain features must be manufactured by
secondary machining operations [1]. These machining operations include transverse holes (drilling) and
undercuts (hard turning). PM parts are generally considered more difficult to machine than their wrought
counterparts. The machinability of PM steels differs from that of wrought steel due to the presence of
porosity and the often heterogeneous microstructure [2]. The porosity in PM steels makes lubrication
more difficult and reduces heat transfer from the cutting surface. Additionally, porosity produces an
interrupted cut with the tool, resulting in micro-impacts and a fatigue condition at the edge of the tool. To
achieve similar strengths as wrought products, PM alloys typically contain higher levels of carbon. As
the apparent hardness of a PM specimen approaches that of a wrought product, the microindentation
hardness must be significantly higher to offset the effect of porosity. The higher microindentation
hardness in combination with the porosity is most responsible for the different machining response of PM
compared to wrought steels.
PM, however, has the advantage of being able to admix materials into ferrous alloys, and both powder
producers and part makers have taken advantage of this ability by incorporating machinability additives,
such as MnS, into the steel. These free machining additives improve machinability by assisting in chip
formation, lubrication of the tool face and reduction of crater wear [2]. The presence of MnS in wrought
steels is used extensively in free-machining grades. During the cutting process, MnS deforms along the
shear plain, reduces tool contact time, and forms a lubricating layer on the tool [3]. Several studies have
shown the benefits of MnS additions to the machinability of PM steels [1,2,4-6].
While manganese sulfide has many beneficial attributes, it does have some limitations and potentially
negative effects. The use of MnS may damage the sintering furnace through the production of a sulfur
containing gas [4]. High humidity environments can quickly oxidize the MnS and deteriorate the
machinability enhancing properties [5]. In addition, MnS becomes less effective as the amount of
alloying increases [6].
With the advent of sinter-hardening alloys, the machinability of these materials in the as-sintered
condition is considerably more challenging. The hard, martensitic microstructure is more difficult to
machine, and requires advanced tooling that can withstand higher forces and increased wear tendencies.
In addition, traditional machining additives may not work effectively under these conditions. A new
machining additive (MA) has been developed to enhance the machinability of sinter-hardened PM steels.
The benefit of this additive will be discussed in sinter-hardened alloys, hybrid steels and in more
traditional as-sintered iron-copper-carbon and iron-nickel-carbon steels.
EXPERIMENTAL PROCEDURE
The evaluation of different machining additives was conducted with a range of alloys, including
prealloyed, hybrid, diffusion alloyed and premixed systems. The alloys studied were FLC2-4808
(Ancorsteel® 737 + 2% Cu + graphite), FLN4C-4005 (Ancorloy® 4 + graphite), FD-0405 (Distaloy
4800A, + graphite), FN-0205 (Ancorsteel A1000B + 2% Ni + graphite), and FC-0208 (Ancorsteel
A1000B + 2% Cu + graphite). The compositions are listed below in Table I. The chemical composition
of Alloy 2 is equivalent to FD-0405. To each of these alloys, three conditions were tested: no additive,
0.35% MnS, and 0.3% MA. The mixes were compacted into rings measuring 25.4 mm ID, 44.5 mm OD
and 28 mm high at a density of 7.0 g/cm3. The rings were then sintered in a 90% nitrogen – 10%
hydrogen atmosphere at 1120 °C for 15 minutes at temperature. A typical furnace cooling rate of
0.6 °C/sec was used; no accelerated cooling was employed. FLC2-4808, FLN4C-4005 and FD-0405
samples were tempered at 205 °C for 1 hour after sintering.
The machinability testing was conducted at Lehigh University, and in addition, hard turning customer
trials were run on Alloy 5. Turning samples were mounted on a lathe and cutting passes were made to the
outer diameter of the sample. The depth of cut was 1.25 mm (0.05 in.) per pass and the feed was 0.3 mm
(0.012 in) per revolution. A coated carbide tool (Seco-Carboloy TX-150) with a chip breaker design was
initially used for the FC-0208 composition. However, the chip breaker design was not tough enough for
the more heavily alloyed compositions, so the same coated carbide tool without the chip breaker (SNMA
433) was used for Alloys 1-4. In addition, a boron nitride (CBN 20) tool of similar geometry (SNGN
433S-LF) was also tested with Alloys 1-4 and a constant cutting speed of 2.5 surface meters per second
(500 sfm) was used. The tool wear data has been assigned a value of zero after the first
®
Ancorsteel and Ancorloy are registered trademarks of Hoeganaes Corporation
Table I. Nominal compositions (in wt%) of the alloys studied, balance Fe.
Alloy #
Designation
Ni
Mo
Mn
Cu
Gr
1
FLC2-4808
1.4
1.2
0.4
2
0.9
2
FLN4C-4005
4.0
0.5
trace
1.5
0.6
3
FD-0405
4.0
0.5
trace
1.5
0.6
4
FN-0205
2.0
-
trace
-
0.6
5
FC-0208
-
-
trace
2.0
0.9
5 cuts to eliminate the effects of the break-in period. Thereafter, tool wear was measured after every fifth
cut on the flank of the tool.
The effect of machining additives on surface rusting was studied on samples of FC-0208 and FD-0405
that contained no additive, 0.35%MnS or 0.3%MA. Several standard TRS samples were placed in a
controlled environment after sintering and exposed to an average relative humidity of 87% and
temperature of 24°C (75 °F) for one month. Gloves were used to avoid fingerprints on the samples prior
to the corrosion testing. The surface fraction covered by rust was measured for each condition using
quantitative image analysis.
RESULTS
Microstructure
The microstructures for Alloys 1 through 5 are shown in Figure 1. The cooling rate of 0.6 °C/sec
produced a nearly fully martensitic microstructure with some bainite in Alloy 1 (Figure 1a). The
composition of this alloy is such that slow cooling rates can be used to produce martensitic
microstructures. Alloys 2 and 3 have the same nominal composition, but a significant microstructural
difference can be seen between the two alloys. Both alloys contain martensite and Ni-Cu rich light
etching areas around the porosity in the structure. However, the centers of the base iron particles have
different microstructures. Alloy 2, which is made with a prealloyed Mo base, consists of a divorced
pearlite / bainite microstructure. Mo reduces the eutectoid carbon content in steels [7], and at 0.5% Mo,
the eutectoid composition is nominally 0.6% C; hence Alloy 2 has no hypoeutectoid ferrite. Alloy 3 is
made with a commercially pure iron base and consists of ferrite and lamellar pearlite. Much of the
pearlite is unresolved at these magnifications. The Mo in the diffusion alloyed Alloy 3 does not fully
diffuse to the center of these particles. Alloy 4 (FN-0205) is ferritic and pearlitic with nickel-rich regions
and alloy 5 is fully pearlitic. The alloys chosen produced a wide range of microstructures at this cooling
rate.
The sintered carbon and average apparent hardness is given in Table II. The sinter-hardening Alloy 1
produced a high hardness corresponding to the martensitic microstructure in the sintered and tempered
condition. The difference in hardness between Alloys 2 and 3 is a result of the slight different carbon
content and the difference in Mo distribution. It has been shown that a difference in 3 HRA between
FLN4C-4005 and FD-0405 at the same sintered carbon content can be expected due to this effect of Mo
[8]. The FN-0205 is the softest material of the group, given its ferritic and pearlitic microstructure.
(a)
(c)
(b)
(d)
(e)
Figure 1. Microstructures of the alloys at 0.6 °C/sec (a) Alloy 1 (FLC2-4808), (b) Alloy 2 (FLN4C4005), (c) Alloy 3 (FD-0405), (d) Alloy 4 (FN-0205) and (e) Alloy 5 (FC-0208).
Table II. Sintered carbon (wt%) and average apparent hardness.
Alloy #
Designation
Sintered C
Hardness
(HRA)
Hardness
1
FLC2-4808
0.80
65.5
30 HRC
2
FLN4C-4005
0.59
56.0
91 HRB
3
FD-0405
0.56
51.0
83 HRB
4
FN-0205
0.59
38.0
57 HRB
5
FC-0208
0.82
53.0
86 HRB
Hard-Turning
Machining of sinter-hardened materials is a difficult challenge for PM part makers and it was discovered
that utilizing MA additive is quite advantageous in these materials. When the martensitic Alloy 1 was cut
with the coated carbide tool (Figure 2), both MnS and MA reduced tool wear; the MA additive was,
however, much more effective. The tool wear rate of the MA containing alloy is similar to the wear rates
found with the much softer FN-0205 alloy (Figure 5) under the same cutting conditions. Considering that
the no additive FLC2-4808 alloy resulted in accelerated wear toward failure after 35 cuts, the MA
additive greatly reduced the wear rate in this alloy system. Interestingly, under these same cutting
conditions, when Alloy 1 was cut with the boron nitride tool, the tool wear rate was low for all three
conditions. No benefit of either machining additive was observed. It is thought that the CBN tool is hard
and strong enough to effectively cut the martensitic microstructure without additives. In addition,
martensitic microstructures are relatively hard and have low ductility, and therefore the benefit of chip
breaking that machining additives produce is not required.
Figure 2. Tool flank wear with number of cuts for different additive conditions. Alloy 1
(FLC2-4808) machined with coated carbide and boron nitride (CBN) tools.
The surface finish is also an important variable in machining. As shown in the Table III, the surface
finish of the workpiece machined with the carbide tool improves with machining additives. Again, the
MA additive exhibits improved machinability over the MnS additive. The surface finish is less rough
with the MA containing mix. With the CBN tool, the surface finish was worse with machining additives
compared with the no additive mix. The MnS mix was considerably worse. Overall, the surface finish is
superior with the CBN tool compared with the carbide tool under these conditions. The combination of
the carbide tool with the MA-containing mix also produced adequate surface results, and when combined
with the good wear results, may provide a cost-effective alternative to the more costly CBN tooling.
Table III. FLC2-4808 hard turning surface finish (in micrometers)
Tool \ Additive
No Additive
0.35% MnS
0.3% MA
Carbide
2.2
1.9
1.3
CBN
0.5
1.5
0.8
Figure 3. Tool flank wear with number of cuts for different additive conditions. Alloy 2 (FLN4C-4005)
machined with coated carbide and boron nitride (CBN) tools.
Figure 4. Tool flank wear with number of cuts for different additive conditions. Alloy 3
(FD-0405) machined with coated carbide and boron nitride (CBN) tools.
Tool wear for Alloys 2 and 3 is shown in Figures 3 and 4, respectively. Given the same nominal
compositions, it was expected that these materials would behave in a similar manner. With no machining
additives present (open, black circles in Figures 3 and 4), the machinability of FLN4C-4005 and FD-0405
cut with a carbide tool was considerably different. Tool wear after 50 cuts of the FLN4C-4405 was 70
µm whereas the tool wear with FD-0405 was 225 µm under the same conditions. It was unexpected that
tool wear would increase as the hardness decreased as the alloy changed from FLN4C-4405 to FD-0405.
With the CBN tool, the wear was similar for the two alloys, and unexpectedly, was much higher than the
CBN tool wear with the FLC2-4808 (Figure 2). Longer chips were formed in Alloys 2 and 3 compared
with Alloy 1, perhaps resulting in increased wear of the CBN tool. Within the nominal composition of
Alloys 2 and 3, the best machinability was found with the carbide insert and FLN4C-4405. The different
tool life between Alloys 2 and 3 is thought to be due to the Mo distribution in the material and its effect
on the resulting microstructure.
The introduction of MA into either alloy reduced tool wear for both cutting tools (solid, blue squares in
Figures 3 and 4). The MA provided consistently lower tool wear in all cases. The combination of the
CBN tool with alloy FD-0405 resulted in the lowest tool wear. It is interesting to note that for these
heterogeneous microstructures, MA provided the best improvement with the CBN tool, whereas when the
microstructure was martensitic (FLC2-4808), MA gave the biggest improvement with the carbide tool. It
is evident that machinability is dependent upon many variables and microstructure is an important
variable when choosing cutting conditions and tool materials.
The effect of manganese sulfide on tool life in these alloys was mixed (solid, red diamonds in Figures 3
and 4). No beneficial effect was observed in the FLN4C-4405 alloy for either tool material. In fact, tool
wear was slightly higher with MnS and the carbide tool. Although the effect of MA was significant, in
the FD-0405 diffusion alloy, the greatest reduction in tool wear was realized with the MnS additive. The
CBN tool wear was similar for the MA and MnS additions, but the MnS lowered carbide tool wear
considerably more than the MA. This difference in MnS behavior between the two alloys is surprising
and cannot be fully explained. Manganese sulfide may be less effective in harder materials [6], so the
increase in hardness of the FLN4C-4405 may reduce the effectiveness of the additive. The dramatic
change in behavior with MnS, especially with the CBN tool, is puzzling. It is possible that the condition
of the MnS changed, either before admixing or after sintering. The inherent oxidation problem with MnS
may be responsible for the different behaviors observed. Finally, surface finish was largely unchanged
between the two alloys or with machining additives (Table IV). Some roughening was found with the
addition of machining additives and the carbide tool. The surface roughness was not related to tool wear
with these alloys.
Table IV. FLN4C-4405 and FD-0405 hard turning surface finish (in micrometers)
Alloy
Tool \ Additive
No Additive
0.35% MnS
0.3% MA
Carbide
0.8
1
1
FLN4C-4405
CBN
0.8
0.8
0.8
Carbide
0.9
1.1
1.3
FD-0405
CBN
0.8
0.8
0.8
The tool wear for FN-0205 is shown in Figure 5 and little wear was observed with either additive. The
carbide tool wear was similar for MnS and MA, while some additional benefit of MnS was observed with
the CBN tool. The surface finish was good in all cases. Again, it is interesting that the carbide tool wear
with MA in the FN-0205 is similar to that of FLC2-4808. One would not expect the tool wear to be
similar with a hardness change of 57 HRB to 30 HRC for FN-0205 and FLC2-4808, respectively. This
highlights the benefit of MA additive with sinter-hardened materials.
Figure 5. Tool flank wear with number of cuts for different additive conditions. Alloy 4
(FN-0205) machined with coated carbide and boron nitride (CBN) tools.
The effect of MA additive on machinability in a non-sinterhardening system (FC-0208) was also
investigated, Figure 6. The effect of turning speed was studied in this alloy system. At slower speeds, the
MA additive resulted in significantly less wear than the MnS additive. The tool with the no machining
additive sample catastrophically failed after 10 cuts at this speed. Recall that a different tool geometry
was used with the FC-0208. The chip breaker design proved to be insufficiently robust to cut this
material. Also note the higher tool wear in Figure 6 compared to Figures 2-5. The tool wear of the MA
containing sample at 0.6 m/s was similar to that of the FN-0205 at 2.5 m/s, while the tool wear of the
MnS sample after 30 cuts is higher than tool wear axis of Figures 2-5. However, at higher speeds, the
behavior of the MnS and MA reversed, with the MnS resulting in virtually no wear. It is thought that at
this higher speed and higher energy condition, the MnS becomes lubricious and is able to coat the tool
face. The cutting speed is obviously an important variable on the effectiveness of each additive.
Customer trials on alloy FC-0208 with the three additive conditions showed similar tool wear results.
Hard turning tests were conducted over a range of speeds and a smaller depth of cut (DOC) was used. It
was again found that the cutting speed influenced the effectiveness of each machining additive. Under
the different test conditions used in the customer trials, most notably the smaller DOC, the MA additive
outperformed both the MnS and no additive samples at 2 m/s (400 sfm) and 3 m/s (600 sfm). The results
of these trials at 3 m/s (600 sfm) are shown in Figure 7. At a speed of 4 m/s (800 sfm), the MnS additive
was superior to the MA additive. The cutting speed at which the transition from superior MA
performance to superior MnS performance occurs changes with different cutting conditions. The cutting
conditions for FC-0208 should be optimized dependent upon machining additive.
Figure 6. Tool wear during machining of Alloy 5 (FC-0208) at two surface speeds and the three additive
conditions.
3
Tool Wear (x 0.001")
2.5
2
1.5
FC-0208
Baseline
FC-0208
+ MnS
1
FC-0208
2O3
+ MA
CaAlAdditive
600 SFM
0.010"/rev
0.010" DOC
Carbide
KC9110 Insert
0.5
0
0
100
200
300
400
500
600
700
800
900
1000
1100
Number of Passes
Figure 7. Customer evaluation of machining additives in FC-0208. Flank tool wear was measured on the
coated carbide insert.
Overall, the MA additive provided consistently lower tool wear for all conditions compared to the
samples with no additive and is an effective machining additive. Manganese sulfide was also found to be
effective in select alloys and conditions. Given all of the possible machining conditions present in the
PM industry, it is recommended that both additives be evaluated to determine the best machining
response for each application. This assumes that all other factors outside of machinability are equivalent.
Surface Corrosion
The samples that were exposed to the warm, humid environment exhibited different levels of rusting.
Due to the different alloy content, the FC-0208 samples had more rust than the FD-0405 samples. Within
each alloy system, the MA containing samples were very similar to the samples that contained no
additive. The MnS containing samples showed a marked difference from the other two sets. In both alloy
systems, the surface of the MnS containing samples had considerably more rust. Typical surface
conditions are shown in Figures 8 and 9. The surface fraction of rust measured by image analysis is
shown in Table V. The amount of rust on the surface of the MnS samples is almost an order of
magnitude higher than either the no additive or the MA containing samples for each alloy system. More
than half of the 0.35% MnS FC-0208 sample surface was covered with rust, whereas the 0.35% MnS FD0405 had many isolated islands of rust on the surface. The presence of MnS in both alloy systems greatly
increased the amount of rust on the sample surface. The MA additive appeared to have no effect on
sample rusting. Since both additives have been shown to improve machinability over a range of alloys
and cutting conditions, the use of MA will be preferable in certain applications given the accelerated
surface corrosion of test samples containing MnS.
Alloy
Table V. Area percent of rust after humidity testing.
Area Fraction of Rust (%)
No Additive
0.35% MnS
0.3% MA
FC-0208
8
52
7
FD-0405
1.3
11.3
1.9
No Additive
0.35% MnS
Figure 8. Surface of alloy FC-0208 after humidity testing.
0.3% MA
No Additive
0.35% MnS
Figure 9. Surface of alloy FD-0405 after humidity testing.
0.3% MA
CONCLUSIONS
The benefits of a newly developed machining additive (MA) have been presented. The addition of the
MA additive resulted in consistently lower tool wear across the board compared with mixes with no
additive. This additive is particularly effective in PM alloy systems that contain mixed to fully
martensitic microstructures. In FLC2-4808 and FLN4C-4005 alloys, MA additive also generated less tool
wear than MnS. In FD-0405 and FN-0205, MnS was either equivalent to or better than the MA. The MA
additive was also effective at select speeds in the iron-copper-carbon system, and its usefulness in this
alloy system will be application dependent. The MA additive has the additional benefit of being
chemically inert and non-reactive under typical environmental and sintering conditions. This benefit was
realized in both the consistent machinability results and in the corrosion testing. The presence of MA
additive in sintered compacts did not cause the accelerated rusting found on the surface of samples
containing MnS.
ACKNOWLEDGEMENTS
The authors would like to thank Ed Force at Lehigh University for performing the machinability
experiments and Arthur Rawlings, Craig Gamble, Paul Kremus and Ron Fitzpatrick for their assistance in
sample preparation and testing.
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