INDUSTRIAL MATERIALSNew Coated Carbide Grades for Cast Iron TurningSusumu OKUNO*, Hideaki KANAOKA, Satoshi ONO, Shinya IMAMURA, Kazuhiro HIROSE,and Haruyo FUKUI---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Automotive components have increasingly complex designs and thin walls for weight reduction, and accordingly, high-strength,
difficult-to-cut materials are used. Meanwhile, high-speed and high-efficiency machining is also required to reduce lead time.
Under these circumstances, cutting tools need to have long tool life and stable performance. To satisfy these demands, theauthors have developed the new coated carbide grades AC4010K and 4015K for cast iron turning. This paper describes theirfeatures and cutting performance.
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Keywords: CVD, cutting tool, high-speed, high-efficiency, cast iron

1. Introduction
Indexable inserts used for cutting tools, which are
made by coating the surface of cemented carbide with ahard ceramic film (coated grades), have better-balanced
wear resistance and chipping resistance than other inserts.
Because of such superior characteristics, coated grades
have an increasing use rate and they currently account for
70% of all base materials of indexable inserts.(1) Coatedgrades are used in the cutting of various materials such ascarbon steel, alloy steel, stainless steel, and cast iron. For
each of these materials, various measures have been taken
to reduce the environmental impact and improve resource
efficiency. In the machining of automotive components
made of cast iron, in particular, efforts have been made toreduce their weight and thereby minimize exhaust gas
emissions and enhance fuel economy. These efforts have
led to thinner walls and increasingly complicated component shapes. To give sufficient strength to these thin-walled
components, conventional easy-to-cut gray cast iron (FC)has been replaced with high-strength, difficult-to-cut
ductile cast iron (FCD). On the other hand, high-speed,
high-efficiency machining is increasingly required due tothe rising demand for cost reduction and improved performance of machine tools. Therefore, cast iron cutting tools
are required to have high stability and a long tool life evenunder such severe requirements.
To meet the market needs described above, Sumitomo
Electric Industries, Ltd. has developed and launched newcoated grades AC4010K and AC4015K. The former grade
ensures high-speed continuous turning of gray cast iron,while the latter enables stable turning with an extended tool
life in general-purpose machining of mainly ductile castiron.
This paper describes the details of the development of
the new coated grades and their performance.

2. Development Target for AC4010K andAC4015K
Damage to coated grades in cast iron turning isroughly classified into three forms as shown in Table 1.
Prior to the development of AC4010K and AC4015K,

44 · New Coated Carbide Grades for Cast Iron Turning

Table 1. Examples of Cutting Tool Damage in Cast Iron Turning
and Cause of Damage
Wear

Chipping

Adhesion
Adhesion

Exampleofdamage
ChippingCauses

Required
Properties

Wear begins to rapidly
progress immediately
after the coated film isthinned as a result of
rubbing against hardcomponents. In
particular, cutting tools
often suffer this type ofdamage whenmachining FC at highspeeds.

Cumulative finechipping of the cuttingedge ridge due tocontact of the edge withfine undulations on thesurface during
intermittent orcontinuous machining.

Increased hardness andthickness of coated film

Increased hardness and Increased adhesion
adhesion force of coated force of coated film andfilm
smoother film surface

Fine powder from softcomponents is pressedagainst the cutting tool’s
surface and sticks
tightly to the surface.
When separating fromthe tool’s surface, thepowder peels the coatedfilm off the tool. In
particular, cutting tools
often suffer this type ofdamage whenmachining FCD.

cutting tools that had been actually used in cast iron turning
were collected and their damage was investigated with theaim of clarifying the characteristics required of thesecoated grades. The investigation results revealed that morethan 70% of these cutting tools had reached their tool life
due to chipping damage or the combination of chipping
damage and wear damage that are shown in Table 1.
Cutting tools had been damaged due to chipping wear
regardless of the FC or FCD they machined. However,damage to the cutting tools used for turning FCD having
higher strength was more conspicuous. It was concluded
from the above investigation results that protecting cutting
tools from chipping during turning would be the mostimportant measure to extend their tool life. As a result,development target was set to increase the chipping resistance of new coated grades to 1.5 times or more that ofconventional grades.
The damage investigation results also revealed thatsome cutting tools, when used for turning FC at highspeeds of more than vc = 500 m/min, had rapidly reduced
the coated film thickness on the cutting face due toscraping by chips and had reached their tool life. To reduce3. Features of AC4010K and AC4015K
Improving the chipping resistance of both AC4010K
and AC4015K was crucial for accomplishing the development targets. Increasing the mechanical strength and adhesion strength of the coated films was essential to meetingthe above need. We successfully overcame the above challenge by developing the following three technologies:
3-1 Suppressing crack propagation in coated film
Ceramic films on the surfaces of AC4010K andAC4015K are formed by the chemical vapor deposition(CVD).*1 In this process, the cutting tools are placed in avacuum furnace maintained at approximately 1,000°C.
After ceramic films are formed, the tools are cooled toroom temperature. In this cooling process, residual tensilestress is occurred in coated ceramic film because of thedifference in the thermal expansion coefficient between theceramic film and cemented carbide. Once a fine crack isgenerated in the coated film due to machining impact force
or other cause, the residual tensile stress in the coated film
helps the crack to propagate and deteriorate the chipping
resistance of the cutting tool. Sumitomo Electric hadalready acquired a technology to reduce residual tensilestress in or impart compressive stress to a coated film by
subjecting its surface to a unique treatment (surface treatment) . This technology is used for conventional cast ironturning grades to impart compressive stress to them.
However, as imparting the conventional level of compressive stress to coated films would be insufficient to meetuser requirements, we reviewed and improved the conventional stress imparting process and equipment. The
improved process and equipment enable the application ofcompressive stress of 1 GPa, which is two times theconventionally imparted stress, to Al2O3 films (Fig. 1).
A cutting performance evaluation test was carried out
to verify the effectiveness of imparting compressive stressto a coated film. In the evaluation test, FCD450 was intermittently cut at a speed of vc = 450 m/min under the conditions of 1.5 mm in the depth of cut (ap) and 0.3 mm/rev infeed rate ( f ). The tool life was defined as the actual lengthof time until the tool became unusable as a result of inten-

sive chipping-off of the cutting edge. The damaged statesof the conventional and newly developed tools after cutting
the test materials for 240 seconds are shown in Fig. 1. After
240 seconds of cutting, the tool that had been treated by theconventional process was largely chipped off at its edge. In
contrast, the tool to which compressive stress of 1 GPa had
been imparted by the new process were able to be continuously used even after 240 seconds, though the cutting edge
was slightly chipped off, and finally reached its tool life
when used for 420 seconds. The new tool after use for 240
seconds was visually examined in detail. Many cracks,
which were estimated to have been generated during
turning, were found on the tool surface. However, the propagation of these cracks was prevented by the high compressive stress that had been imparted by the new process, verifying that the new process is more effective in reducing theseverity of tool damage than the conventional process.

TensilestressStressin alumina
film(GPa)the tool damage caused by the above mechanism, it would
be effective to increase the thickness of the alumina (Al2O3)
film, one of the films coated on tool surfaces to resist heat.
However, increasing the thickness of the alumina film wasexpected to deteriorate the chipping resistance of thecutting tool. After various discussions, we concluded that itis difficult to give mutually contradictory characteristics toa single coated grade and decided on two types of coatedgrades AC4010K and AC4015K. We aimed to provide
AC4010K with the capability of turning FC at a high speedof more than vc = 500 m/min with wear resistance twotimes or more that of the comparable conventional grade
while maintaining high chipping resistance. For AC4015K,
we aimed to provide it with the capability of general
purpose turning of FCD and other materials with chippingresistance 1.5 times or more that of the comparable
conventional grade while maintaining the same level ofwear resistance.

0.50-0.5

Compressive -1stress

Conventional
process

New process

Cutting
faceFlank
face

2/21
Cutting time: 240 s
0.5 mm

0.5 mmFig. 1. Compressive Stress in Alumina Film and Chipping Resistance

3-2 Improving the strength of coated film by controlling
the growth orientation of Al2O3 crystallite
Figure 2 shows the cross-sectional structure of aceramic film that was coated on a cemented carbide basematerial by the CVD process. The structure usually consistsof Al2O3 and TiCN layers. The Al2O3 layer is formed on theouter side of the ceramic film and insulates heat, while theTiCN layer is formed on the inner side of the ceramic film
and resists wear. It is already known that, when turning FC ata high speed of more than vc = 500 m/min, thermal damage
to the cutting tool can be suppressed by thickly forming
Al2O3 film, a heat insulation layer. However, the strengthof the film decreases as its thickness increases. We
prepared a cutting tool by subjecting it to a specific heat
treatment after coating with Al2O3 film more thickly than aconventional film, and evaluated the machinability of thistool by intermittently cutting FC250.
In the test, the coated film separated from the cutting
face and subsequently the flank face of this tool wasdamaged as shown in the lower part of Fig. 2. As a result ofSEI TECHNICAL REVIEW · NUMBER 85 · OCTOBER 2017 · 45

detailed visual examination of the cutting tool before it wasdamaged due to the separation of the coated film from thecutting face, it was confirmed that the Al2O3 film had
begun to locally break and that subsequent propagation ofthis breakage led to the tool becoming damaged due to theseparation of the coated film. The cause of the tool damage
due to the separation of coated film was analyzed asfollows. In an Al2O3 film that is formed by the conventional
coating process, the constituent particles grow in random
directions as schematically illustrated in Fig. 2. Therefore,
the constituent particles are separated by the shearing stressthat is produced when chips scrape the surface. Based on
the above analysis result, we developed an Al2O3 film with
the Al2O3 crystallites arranged in parallel with the crystal
face perpendicular to the chip shearing direction or in thec-axis direction perpendicular to the cross-section of thefilm. Various studies that were made on film formation
parameters to control crystalline orientation have made it
possible to orient more than 90% of Al2O3 crystallites in
the c-axis direction. When a cutting tool coated with anAl2O3 film having newly oriented crystallites was used forintermittent turning of FC, damage to the tool attributable
to the separation of film from the cutting face decreased
dramatically as shown in the lower part of Fig. 2.

cross-sectional structure of a coated film that was subjectedto the honing process newly introduced for AC4010K andAC4015K. This figure verifies that, when subjected to thenew honing process, AC4010K and AC4015K reduce thesurface roughness of the base material of the cutting edge
to half that of a conventionally treated base material. In
consequence, the smoothened base material surface reduces
the surface roughness of the coated ceramic film. The resultof an FC intermittent turning test is shown in the lower partof Fig. 3. This figure confirms that AC4010K and
AC4015K reduce the surface roughness of the base material, thereby dramatically reducing the chipping damage ofthe cutting edge attributable to machining impact force.

Conventional

AC4010K
AC4015K

5μm

5μm

TiCN
layer

intermittently machiningof FC250

Conventional
alumina

Al2O3
layer

Chipping of film
0.5 mm

0.5 mmFig. 3. Improvement of Film Adhesion Strength by Smoothening of Substrate

TiCN
layer

Cemented
carbideAlumina withcrystallites
oriented

Schematic illustration

5 μmCrystallites areoriented in random
directions.

Crystallites areoriented in the same
direction.
 Damage toalumina film isdramatically reduced.

0.5 mm

0.5 mmFig. 2. Cross-sectional Structure of Film Coated on Cutting Tool and the Effectof Alumina Crystallite Orientation Control

3-3 Improving adhesion strength of coated film
To successfully develop AC4010K and AC4015K, wedeveloped a technology for improving the adhesion
strength of the coated film in addition to the technologies
for improving the crack propagation resistance of thecoated film by imparting compressive stress and enhancingthe strength of the alumina film by controlling its Al2O3
crystallites. Described below is an example of adhesion
strength improvement of coated film by minimizing thesurface roughness of the base material. Figure 3 shows thecross-sectional structure of the edge of a base material thatwas subjected to a conventional honing process*2 and the46 · New Coated Carbide Grades for Cast Iron Turning

4. Cutting performance of AC4010K andAC4015K
Examples of practical turning of FC and FCD with
AC4010K and AC4015K are shown in Figs. 4 and 5.
Figure 4 compares a conventional insert and AC4010Kused in the turning of FC250 brake discs under high-efficiency conditions of a maximum speed of 960 m/min and amaximum feed rate of 0.75 mm/rev. The conventional

Workpiece: Brake disc (FC250)
Tool: CNMG120408
Cutting condition : vc = ~960 m/min, f = ~0.75 mm/rev, ap = ~2.0 mm, wet
Conventional tool: 50pcs/c

AC4010K: 70 pcs/c

Feeddirection
Vbmax = 0.28 mm

Vbmax = 0.27 mm

Schematic
illustrationof machining
Conspicuous exposure of coated TiCNlayer

Fig. 4. Examples of FC250 Turning with AC4010K and Conventional tool

insert used for the same purpose reached its tool life when
it machined 50 workpieces since it deteriorated the qualityof machined surfaces due to the progress of wear anddamage to the cut boundary portion by chipping. Incontrast, AC4010K was able to machine 70 workpieces,
1.4 times the quantity achieved by the conventional insert.
After both insets reached their tool lives, the damaged
portions were examined on a scanning electron microscope. The result showed that AC4010K had limited chipping on the coated film in the boundary portion, therebysuppressing the exposure of cemented carbide even after
machining more brake discs than the conventional insert.
In addition, the thickened Al2O3 film suppressed the progress of fretting wear of the coated film and the exposure of
the TiCN film used as the lower layer. As described above,
AC4010K has improved chipping resistance and wear
resistance in high-speed, high-efficiency machining of graycast iron, thereby extending the tool life of cutting tools.
Figure 5 shows examples of turning the end faces ofFCD500 gear casings. Since the surfaces of the workpieces
were as-cast and rectangular, extremely complicated andintermittent turning operations were often required. The
cutting edge of the conventional cutting tool was intensively chipped. Edges of some of the indexable inserts
were lost and made the cutting tool unusable when it had
machined 10 gear casings. In contrast, AC4015K did not
suffer such chipping or damage and was able to be continuously used even after machining 1.2 times the quantity ofgear casings. As discussed above, the superior chipping
resistance of AC4015K ensures stable intermittent turningof FCD, which would be difficult with conventional grades.

Workpiece: Gear casing (FCD500)
Tool: CNMG120408
Cutting condition: vc = 220 m/min, f = 0.35 mm/rev, ap = 1.5 mm, wet
Conventional tool: 10 pcs/c

Schematic
illustrationof machining

1mm

Vbmax = 0.54 mm

AC4010K: 12 pcs/c

1mm

Vbmax = 0.21 mm＊1＊2

Technical Terms

Chemical vapor deposition (CVD): A method fordepositing a ceramic film on a base material by thereaction of vapor phase chemicals.
Honing process: A process for rounding or slightly
chamfering the cutting edges of a cutting tool.

Reference(1)

Monthly report of Japan Cemented Carbide Tool Manufacuturers’
Association (2015)

Contributors The lead author is indicated by an asterisk (*).

S. OKUNO*• Assistant Manager, Hard Materials Development
Department, Sumitomo Electric Hardmetal Corp.

H. KANAOKA• Sumitomo Electric Carbide, Inc.

S. ONO• Hard Materials Development Department, Sumitomo
Electric Hardmetal Corp.

S. IMAMURA• Group Manager, Hard Materials Development
Department, Sumitomo Electric Hardmetal Corp.

K. HIROSE• Group Manager, Global Marketing Department,Hardmetal Division

Fig. 5. Examples of FCD500 turning with AC4010K and conventional tool

H. FUKUI
5. Conclusion
We have developed technologies that are essential for
improving the strength of coated films and succeeded increating the new coated grades AC4010K and AC4015K.
The former grade enables high-speed, high-efficiency
continuous turning of FC and other materials, while thelatter ensures stable truing with extended tool life in thegeneral purpose turning of FCD and other materials. These
two coated grades are expected to help users reducemachining costs and enhance productivity in a wide rangeof uses, from high-speed, high-efficiency continuous turning
to intermittent turning regardless of the FC and FCD.

• Ph. D.
Assistant General Manager, Hard Materials
Development Department, Sumitomo Electric
Hardmetal Corp.

SEI TECHNICAL REVIEW · NUMBER 85 · OCTOBER 2017 · 47