Machining of High Strength PM Steels - Höganäs AB

Machining of High Strength PM Steels
Olof Andersson*, Hans Thordenberg**, Sigurd Berg*
*Höganäs AB, S-26383 Höganäs, Sweden
**AB Sandvik Coromant, S-811 81, Sandviken, Sweden
Abstract
P/M is a net or near net shape process. Machining operations are used for many applications
in order to reach final tolerances or shapes not possible to achieve in the compaction step. The
use of high strength PM steels have increased in recent time. Guidelines for insert choice and
effect from additives are needed to improve the performance in the machining operation for
these application areas.
This paper describes the machinability of a fully bainitic high strength PM material. An
optimum is found regarding settings of the insert for which properties like insert life time and
surface roughness are improved. Machinability additives performance and their influence on
physical and mechanical properties are shown for machining of such PM material.
Introduction
PM is a well-known method for producing large series of complex parts to very close
tolerances. However, today many PM components require some type of machining operation
to reach final shape, e.g. shapes impossible to create with the compaction exclusively or due
to demands for even higher tolerances.
PM materials are often considered to be difficult to machine, especially compared to
conventional standard steels. However, by choosing a proper combination of tool material and
insert geometry together with optimised cutting data, and if also necessary, additives, it is in
many cases possible to reach a productivity level as high as of standard steels [1][2].
In this study a turning operation has been performed on a relatively new PM material, the
chromium and molybdenum prealloyed AstaloyCrM [3], in a fully bainitic condition with
high strength properties. In this paper the influence of tool grades, insert geometries, additives
and cutting data, when machining this type of PM material will be presented.
Experimental
Three powder mixes [Table 1] were manufactured in order to compact turning test specimens,
with geometry Ø64/Ø35 x 45 mm, and test bars to evaluate mechanical properties. All
specimens were compacted at 700 MPa to a green density of 7.0 g/cm³.
Table 1. Powder mixes based on AstaloyCrM
PM material Composition
Material 1 Astaloy CrM + 0.37% Graphite + 0.8% Kenolube
Material 2 Astaloy CrM + 0.37% Graphite + 0.8% Kenolube + 0.5% MnS
Material 3 Astaloy CrM + 0.37% Graphite + 0.8% Kenolube + 0.3% MnX
MnX is a patented product of Höganäs AB that has been found effective as a pre-mix additive
in the machining of PM steels.
Presented at PM2004, in Vienna, Austria, October 2004
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Green specimens were sintered in a laboratory belt furnace at 1120ºC for 30 minutes in an
atmosphere of 90% nitrogen/10% hydrogen, and with additional 0.2% methane to maintain a
good carbon control. The cooling rate in the end of the sintering cycle was 0.8ºC/s. For all
three material compositions a fully bainitic structure [Fig. 1] was obtained.
Figure 1: Sintered AstaloyCrM + 0.37%C. Etched with nital/picral.
The influence by the additives can be observed in terms of minor reductions in strength
[Table 2]. However, the micro hardness of the bainitic matrix is independent whether the mix
consists of additives or not.
Table 2: Mechanical properties of sintered test specimens
PM material SD HV10 MHV0.05 DC TS YS MnS IE
Material 1 7.0 236 376 -0.20 833 625 0.6 14
Material 2 7.0 228 370 -0.21 787 571 0.9 13
Material 3 7.0 234 372 -0.17 780 572 0.8 15
SD:
HV10:
Sintered density [g/cm³]
Hardness measured on polished cross section of turning test specimen [MPa]
MHV0.05: Micro hardness measured on polished cross section of turning test specimen [MPa]
DC:
TS:
YS:
A:
IE:
Dimensional change from as-green to as-sintered [%]
Tensile strength [MPa]
Yield strength [MPa]
Elongation [%]
Impact energy [J]
A CNC lathe (Mazak) was used to face turn the cylinder specimens. The surface to a depth of
0.5 mm was removed in order to secure the performance of the different inserts and
geometries. Influence of surface variation, like carburisation, decarburisation or oxidation is
therefore eliminated in comparsion test.
Turning tests of AstaloyCrM without additive (Material 1) – Preparatory tests
One of the first attempts to machine AstaloyCrM was carried out with a CVD coated
cemented carbide insert, CNMG120408-WF [4]. With start-up cutting data (Vc = 150 m/min,
f = 0.1 mm/r, ap = 0.5 mm) and dry machining the insert was inspected every tenth pass, up to
50 passes. The dominating wear mechanism was found to be adhesion in terms of severe built
up edge (BUE) and a heavily smearing effect, i.e. pressure-welded specimen material, on
clearance flank [Fig. 2]
Presented at PM2004, in Vienna, Austria, October 2004
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Figure 2: BUE and smearing on insert CNMG120408-WF
In an attempt to reduce BUE and smearing effects, cutting speed was increased in order to
change cutting edge temperature and in this way soften the built up layer and replace it with a
flow zone. The BUE can be reduced with an increased cutting speed however, pressuredwelded
material is still heavily smeared on the clearance flank and close to secondary cutting
edge [Fig. 3] which would in the end jeopardize the machined surface quality.
Vc = 200 m/min Vc = 250 m/min Vc = 300 m/min
Figure 3: BUE and smearing on insert CNMG120408-WF. Turning with increased cutting speeds.
In a next step to reduce BUE and smeared material on tool insert, cutting fluid (emulsion of
water and 5% Castrol Syntilo XPS) was applied. The use of cutting fluid was found to be of
significant importance [Fig. 4] since BUE and a large amount of the smearing could be
eliminated.
Vc = 200 m/min Vc = 250 m/min Vc = 300 m/min
Figure 4: CNMG120408-WF. Turning with increased cutting speeds and applied cutting fluid.
Turning tests of AstaloyCrM without additive (Material 1) – Optimizations
Two measurements have proven to minimize or eliminate the smearing tendencies; cutting
speed in some remarks but especially the addition of cutting fluid with significant result. To
make further optimizations, with regards to an increased tool life, different insert grades [4],
i.e. substrates and coatings, applied on insert CCMT09T308 [4], a positive design compared
with the negative CNMG, were tested. All tests were performed until flank wear on tool insert
had reached the standardized length of 0.3 mm. The two most successful grades [Fig. 5] were
grade CT5015 (uncoated WC-Co cermet) and a new developed grade GC3215 (solid carbide,
coated with TiCN, Al2O3 and TiN).
Presented at PM2004, in Vienna, Austria, October 2004
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No of passes to Vb = 0.3 mm
Figure 5: Tool life with different insert grades and same tool geometry.
With the cermet grade CT5015 the influence of geometry was tested by choosing different
cutting edge designs (-WF, -PF and -PM) applied on CCMT09T308 [4]. In this test an insert
with a more improved tolerance by grinding, the CCGT09T308-UM [4], also was tested. The
result [Fig. 6] of this test indicates that a well-defined edge radius and sharpness affects the
tool life when machining the AstaloyCrM without additives, i.e. the G-tolerance geometry
CCGT09T308-UM exhibits the best performance.
No of passes to Vb = 0.3 mm
150
125
100
75
50
25
0
250
200
150
100
50
0
CCMT09T308 (-KM/PM/MM)
Vc = 300 m/min
f = 0.10 mm/r
ap = 0.5 mm
With cutting fluid
GC1025 GC1005 GC4025 GC2015 GC2025 GC4015 GC3215 CT5015
CCMT09T308 (-WF, -PF, PM)
CCGT09T308-UM
Vc = 250 m/min, f = 0.1 mm/r
ap = 0.5 mm
With cutting fluid
- WF - PF - PM - UM
Figure 6: Tool life with same insert grade and different tool geometries.
Turning tests of AstaloyCrM with addition of MnS or MnX (Material 2 & 3)
The influence of additives in AstaloyCrM was dramatically shown in the first test with tool
insert CNMG120408-WF [Fig. 7]. With the same type of tool insert and with the same cutting
data that was used when AstaloyCrM without additive was machined (Vc = 150 m/min, f =
0.1 mm/r, ap = 0.5 mm) the presence of BUE and smeared material on clearance flank was
non-existent, even when the machining took place without cutting fluid, i.e. dry.
Without additive With + 0.3% MnX With + 0.5% MnS
Figure 7: Edge integrity of CNMG120408-WF.
With the intention to optimize machinability the so far successful grades, CG3215 and
CT5015 were applied on the insert tool CCMT09T308-WF. All tests were done with cutting
fluid. The best machining performance was found for the coated solid carbide GC3215 and
Presented at PM2004, in Vienna, Austria, October 2004
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not, as previous was stated in the case AstaloyCrM without additives, the uncoated cermet
CT5015 [Fig. 8].
No of passes to Vb = 0.3 mm
800
700
600
500
400
300
200
100
0
+ MnS
Figure 8: Tool life when machining AstaloyCrM + additive with
different insert grades and same tool geometry.
The additives are beneficial to tool life, especially AstaloyCrM + 0.5% MnS, even at feed
rate 0.3 mm/r. The presence of MnX eliminates pressured-welded material but a higher wear
rate, in terms of flank and crater wear, can be observed compared to the wear rate with the
addition of MnS. Yet, addition of MnX will improve the machinability.
Tool life for AstaloyCrM with or without addition of MnS (Material 1 & 2)
The Vt-graph [Fig. 9] shows a divided behaviour when AstaloyCrM without additive is
machined, expressed with two alpha-values (curve slopes). At lower cutting speeds, i.e. less
than 250 m/min, the wear process is gradually more and more influenced by smearing and
BUE formation. With increased feed rate (0.3 mm/r) tool life will be heavily reduced.
However, with the addition of MnS higher cutting speed (>250 m/min) together with higher
feed rates (up to 0.3 mm/r) can be used with maintained tool life.
Tool life to Vb = 0.3 mm [min]
100
30
20
10
Increased BUE and
smearing
Figure 9: Vt-graph
Gains by optimization of inserts and addition of MnX or MnS
A comparison of gains in tool life (number of passes) and productivity (feed rate) shows the
influence of additive [Fig. 10]. The gains are relatively to the turning tests of AstaloyCrM
without additive, that was turned with insert CCGT09T308-UM (CT5015) and feed rate
0.1 mm/r. The MnS and MnX variants were turned with insert CCMT09T308-WF (GC3215)
and feed rate 0.3 mm/r. With addition of MnS the gain in tool life and productivity is
remarkable, three times as high. Even the addition of MnX is beneficial with respect to
productivity. Insert geometry and feed rate will by natural causes influence the surface
roughness of the machined part. With the wiper geometry CCMT09T308-WF a lasting
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Presented at PM2004, in Vienna, Austria, October 2004
+ MnS
CCMT09T308-WF
Vc = 250 m/min
f = 0.3 mm/r
ap = 0.5 mm
With cutting fluid
+ MnX
CT5015 GC3215 GC3215
f = 0.2mm/r
f = 0.3mm/r
Without additive, insert CCGT09T308-UM (CT5015), f = 0.1 mm/r
With 0.5% MnS, insert CCMT09T308-WF (GC3215)
Without additive, f = 0.3 mm/r
1
100 200 250 300 400
1000
Cutting Speed [m/min]

surface quality will be obtained on parts made of AstaloyCrM + 0.5% MnS, even with feed
0.3 mm/r [Fig. 11]
Gain (relatively)
4
3
2
1
0
Gain "Tool life" Gain "Productivity"
Without additive With MnX With MnS
Figure 10: Comparison of tool life and productivity.
Vc = 250 m/min, ap = 0.5 mm, with cutting fluid.
Presented at PM2004, in Vienna, Austria, October 2004
Surface roughness Ra [µm]
3,5
3,0
2,5
2,0
1,5
1,0
0,5
0,0
GC3215-KM
GC3215-WF
CCMT09T308 (-WF, KM)
Vc = 250 m/min,
f = 0.3 mm/r
ap = 0.5 mm
With cutting fluid
0 5 10 15 20 25
Tool contact time [min]
Figure 11: Surface roughness vs. cutting time.
Astaloy CrM + 0.5% MnS
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Discussion
The use of cutting fluid has in previous investigations not been recommended in turning
operations with PM steel material because of the observed wear effects [5]. However, with
respect to the bainitic AstaloyCrM without additive, when the choice is between a
predictable and controlled wear (with cutting fluid) or an unpredictable and uncontrolled (dry)
the choice whether cutting fluid should be used or not appears obvious.
For comparative reasons all tool life tests have been performed with cutting fluid. Yet, the
turning operation of the additive variants would most likely perform as well, or even better, in
dry condition.
Conclusions
A. Turning of bainitic Astloy CrM without additive:
• The dominating wear mechanism is adhesion in terms of BUE and pressure-welded
material on clearance flank.
• The adhesion wear is reduced by using as sharp cutting edges as possible, increasing the
cutting speed and the use of cutting fluid.
• Best machining performance is achieved by using cermet grade CT5015.
• It is not possible to use high feed rates and productive wiper inserts. A higher load will
increase the adhesion wear.
B. Turning of bainitic Astaloy CrM with additive:
• The machining exhibits a completely different behaviour compared to the non additive
variant. BUE and smearing tendencies are in principle eliminated.
• One of the general machining recommendations for PM-material is to use as sharp cutting
edges as possible. This recommendation is not so important when MnS and MnX are
added to the tested PM material.
• Best machining performance is achieved with hard metal grade GC3215.
• Three times higher productivity and tool life is achieved by MnS compared with variant
without additive.
• Three times higher productivity is achieved by additive MnX compared with variant
without additive.
• The best overall performance is obtained with AstaloyCrM + 0.5% MnS. Excellent
results have been achieved, both in tool life and surface roughness, with feed rates up to
0.3 mm/r together with the high productive wiper insert, CCMT09T308-WF.
References:
1. Berg, S. ”Cutting data Recommendations for Turning of Various P/M Materials Using
Different Tool Grade and Geometry Selections”. Paper presented at 1998 Powder
Metallurgy World Congress, Granada, Spain October 1998.
2. Berg, S., Måhrs, O. ”Investigating the Relationship Between Machinability Additives and
Machining Parameters”. Paper presented at PM 2 Tec2001, New Orleans, USA, May 2001.
3. Lindberg, C. ”Mechanical Properties of a Water Atomized Fe-Cr-Mo Powder and How to
Sinter it”. Paper presented at PM 2 Tec1999, Vancouver, Canada, June 1999.
4. AB Sandvik Coromant. Catalogue: “Metal Working Products – Turning Tools 2003”
5. Höganäs handbook for sintered components. ”5. Machining Guidelines”.
Presented at PM2004, in Vienna, Austria, October 2004
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