A Study of the Machinability of an ASTM
Grade 3 Austempered Ductile Iron

2002 World Conference on ADI

M. Goldberg*, J.T. Berry**, G. Littlefair* and G. Smith*
*Southampton Institute, Southampton, UK
**Mississippi State University, USA

ABSTRACT

The paper discusses a comprehensive investigation of the machinability of an ASTM Grade 3 Austempered Ductile Iron utilizing four high grade cutting tools. The machining process selected was a turning operation, conducted under conditions associated with both a roughing cut and a finishing cut. Tool forces, surface finish and tool wear were examined in the investigation. The paper concludes with a benchmarking comparison between the tools.

INTRODUCTION

Throughout the last three decades, Austempered Ductile Iron (ADI) has been rapidly adopted and exploited world wide for manufacturing primarily automotive products. ADI (Austempered Ductile Iron) was first commercially applied in 1972 and by the mid 1970’s was employed in Chinese military trucks and commercial truck applications among European industries. Towards the end of the 70’s ADI had been utilized for light cars and trucks and was favorably looked upon by the US automotive industries. At the turn of the last century, the world wide consumption of ADI was estimated at 150,000 tons per year, with a projection forecast of 20% annual growth. ^{[1, 2, 3,4]}

ADI exhibits remarkable properties, such as high toughness, relatively light weight, good heat conductivity and good vibration damping, as well as a high level of ductility, recyclability and wear resistance. These useful mechanical properties are arrived at via a unique process of heat treatment that provides designers with further manufacturing flexibility and effective cost reduction compared to comparative forged steel components. Austempered Ductile Iron is continuously being incorporated into the ferrous application market. ^[4,5,6] Further information on the ADI distribution of applications and production capacity is illustrated in Figure 1 and Figure 2.

This paper describes a study of the machinability of ADI using four individual cutting tools. It describes the experiments conducted and the results obtained. The paper concludes with a benchmarking comparison between the tools, which highlights the most suitable tools for machining ADI.

Figure 1 - Distribution of ADI application in the UK

Figure 2 - ADI production capacity in North America and UK

BACKGROUND

Austempered Ductile Iron is a specially heat-treated ductile cast iron. The iron is subjected to an isothermal heat treatment called “Austempering”. ^[3,7,8,9] In contrast to conventional “as cast” ductile irons, ADI gains its mechanical properties by the heat treatment process and not by a specific alloy combination. Thus, the only condition for obtaining a desirable ADI component is a good quality ductile iron material. ^[9] The typical microstructure of ADI consists of ferrite; austenite and graphite nodules as depicted in Figure 3. ^[2,6,7,8,10]
The ferrite in the microstructure exists as needle-like arrangement and the white pattern between the ferrite represents the retained austenite. Thus, the metallurgical composition of the two, forms an Ausferrite structure. Furthermore, the dark spheres are the graphite nodules. ^[5,7,8,9] The austempering procedure is conducted in two stages; namely the Austenitising process and the Austempering process. Details of such treatments appear elsewhere ^[3,5,7,9] as well as in other papers at this conference.

Figure 3 – microstructure of ADI presenting a common ausferritic matrix of ADI casting grade 1 (500 X photomicrograph).^[5]

Table 1 - test results obtained by American Society of Testing Material. ^[5]

Regarding mechanical properties, Kovacs ^[5] reported that among many beneficial features, ADI has the advantage of offering manufacturing flexibility presenting the designer the option to alter significantly the casting mechanical properties by applying various modes of heat treatment. The ASTM standard grades covering the various strength levels are shown in Table 1, above. The mechanical properties developed through the austempering process are considered broadly as a consequence of either low or high austempering temperature. With a high austempered temperature, features such as high ductility, high fatigue and impact strengths could be acquired. Thus, high austempered temperature would produce relatively low yield and tensile strengths. In contrast, a low austempered temperature will produce high yield and tensile strengths with remarkable wear resistance, though lower ductility and impact strength would be the trade off. ^[5,7,9]

MACHINABILITY OF ADI

There has been considerable debate concerning the machinability of ADI and whether rough machining should be undertaken before or after the austempering treatment ^[11,12]

Consequently, an investigation was conceived which would examine the various aspects of machinability as they pertain to a particular grade of ADI, utilizing cutting tools recommended by a number of leading manufacturers.

The term machinability itself may be interpreted in a variety of ways. It generally implies that for a given set of cutting conditions a particular workpiece material may affect:

1. tool life – in example, flank wear

2. HP required in cutting

3. tool forces experienced

4. workpiece surface finish.

TESTING PROCEDURE

The experimental objectives were to assess the machinability characteristic of grade 3 austempered ductile cast iron using four individual cutting tool media and consequently to compare the machining performances. To achieve a comprehensive overview of the cutting characteristics, the experiment incorporated the examination of both roughing and finishing turning operations. The turning operations were performed dry. These two machining conditions were established to assess the machinability of the ADI component under extreme and moderate conditions. The roughing condition comprised a predetermined cutting speed of 425 m/min coupled with 2mm depth of cut and the finishing condition consisted a chosen cutting speed of 700 m/min together with 0.5 mm depth of cut. In an attempt to formulate a thorough comparison between the four cutting tool materials, a diverse range of feed rate parameters were utilized, ranging from 0.1 to 0.4 mm/rev. The PCBN tools, associated with this experiment, approached the workpiece in a 6° negative rake angle, and the ceramic tools approach rake angle was neutral (0°).

The machinability assessment performed in this experiment incorporated three common industrial criteria, namely force analysis, tool wear evaluation, and surface texture assessment.

The force analysis and tool wear evaluation criteria are related to the machining process and reflect its qualities. The surface texture assessment, however, is related to the machined surface, hence such measurements were conducted at the post machining stage. Furthermore, the results are described for each individual tool. These results were used as reference data for comparison of these four tool material compositions.

Figure 4 illustrates the experimental design in a form of a flowchart. The flowchart indicates the finishing operation in green color and the flow of operation for the roughing in blue color.

Figure 4 – The experiment design flowchart

The machining trials were performed on a CNC machine programmed with finishing and roughing operations that comprised one continuous pass of “metal removal”. One pass of metal removal incorporated removing material from an ADI casting of 250mm diameter with 40mm material thickness. The metal removal process was repeated four times with various feed rates. This array of machining conditions was then applied four times, each time using a different cutting tool, producing in total sixteen discrete trials. The machining trials, were undertaken on a Cincinnati Turning Centre (Cincinnati 200/15), equipped with a Kistler Dynamometer (Model 9257B), as displayed in Figure 5, to provide data that relates to the force experienced on the tool edge while engaged in machining.

At the completion of each machining trial, the cutting inserts were examined under an optical microscope with a measuring device in order to evaluate and record the VBmax tool wear inflicted on the flank (i.e. lower) faces. The cutting tool insert was replaced each time in order to provide a fresh cut edge.

Figure 5 – turning ADI on a lathe, using PCBN cutting tool and a Kistler dynamometer for recording cutting forces.

The surface texture assessment was conducted using a cut off length of 0.8 mm at right angles to the lay, as it depicted in Fig. 6. The sampling length, also known as “Meter Cut Off”, constitutes a reliable reference line when comparing the results with other surface texture parameters which taken in the same fashion. The sampling length, therefore, is sufficiently long to include a reliable amount of roughness and yet short enough to exclude waviness from the measurement.^[13,14,15] The specimen used for the turning trials was mounted on a flat “V” fixture jig standing on top of a granite flat surface. The roughness value, known as the prime surface texture, can be arithmetically defined by Equation 1. ^[14,15]

Equation 1
Where:
L = sampling length
V = vertical length
Areas R + S = as illustrated in Figure 6

The parameter Ra refers to a numerical value. The higher the Ra values the rougher the surface texture. The Ra values obtained for the experiment were measured automatically by the Talysurf instrument (manufactured by Taylor – Hobson). The measurements were obtained via stylus probe type instruments which moves over the surface equipped with a skid. The probe direction motion is vertical relative to the skid with an aim to identify the roughness of the surface. The electronic surface texture instrument was calibrated before the assessment was performed, ensuring repeatability and confidence in the results. All stylus-based instruments, however, equipped with a finite radius at the tip of the stylus, which fail to produce a true trace of the surface texture. The stylus is physically unable to penetrate the deepest valleys of the profile, resulting in accumulated truncation of any narrow deep
valleys. ^[14,15]

Figure 6 - the fundamental parameters of surface texture ^[13]

WORKPIECE MATERIAL

The workpiece specimen used was a casting of ADI grade 3, as depicted in Figure 7. The cast shape was designed to cater for both plane and interrupted turning and facing operations. The chemical composition and the mechanical properties of the workpiece are detailed below. (note – this design had been used in previous studies on the machinability of grey irons, elsewhere).

Hardness : 388 HBN
Ultimate Tensile stress: 1221 N/mm2
Elongation : 7.2 %

Figure 7– Photo of the ADI specimen

CUTTING TOOLS

Table 2 - cutting tool used in the experiment

The roughing and finishing operations, associated with the experiment, were repeated several times. Each trial investigated a different grade of PCBN or Ceramic based button insert as defined in Table 2:

RESULTS

Tool wear results

Figure 8 Tool wear results for Roughing operation

Figure 9 Tool wear results for Finishing operation

Force analysis results

Figure 10 cutting forces results for Roughing operation

Figure 11 cutting forces results for Finishing operation

Surface Finish results

Figure 12 Surface Finish results for Roughing operation

Figure 13 Surface Finish results for Finishing operation

ANALYSIS OF RESULTS
Tool wear
The tool wear developed on the flank (underside) plane of the cutting tool during roughing machining can be classified as either natural low wear or accelerated tool deterioration. Both cutting tool ALSc and CBN 1 exhibited satisfying wear resistance during the roughing machining whilst the other two experienced excessive tool deterioration, as it depicted in Figure 8. In contrast, the analysis of tool wear developed during finishing machining condition indicates that ALSc and CBN 2 were most durable when compared to their counterparts, hence enduring least tool damage, as it exhibited in Figure 9.

The cutting tool grade of ALTc exhibited the poorest tool wear with comparison to the other three tools, as it displayed in Figure 8 and Figure 9. In general, the tool wear characteristic was exclusively flank wear which was a direct consequence of adhesive / abrasive wear mechanism. The tool wear tended to grow as a result of increasing the depth of cut parameters, and even more significantly, by elevating the feed rate. The tool wear, experienced by each cutting tool, reflected a much lower level of deterioration in finishing machining condition, compared to the level of damage inflicted upon the tool during the roughing condition.

Cutting forces
The cutting forces, captured by the dynamometer, indicated that the dominant cutting force was in the radial direction, hence the radial force was exclusively considered. This phenomenon is inherited from the geometry of the insert. The cutting forces were relatively greater in roughing operation when compared with those obtained at the finishing operation. The decrease in cutting forces that occurred during the finishing machining condition, is a consequence of the generated heat that tended to evolve and concentrate at the interface between the workpiece and the cutting tool. This generated heat appeared to be much more profound during machining at the finishing condition with high cutting speed, and thus a thermo-softening effect was created within the vicinity of the cutting point, assisting the shearing mechanism. Further, the lowest cutting forces, which are most desirable, were obtained generally at the lowest feed rate of 0.1 mm/rev. The cutting force analysis at the roughing operation indicated that the two best tool performances were achieved by ALSc and CBN 1, as it exhibited in Figure 10. Conversely, the tool that exhibited the least satisfactory performance in terms of cutting forces were the CBN 2. When considering the finishing machining condition, the cutting tools that excel in terms of producing lower cutting forces were the CBN 2 and ALSc, as it depicted from Figure 11. In contrast, CBN 1 and ALTc exhibited unsatisfactory performances.

Surface finish
Invariably, the surface finish characteristic, displayed in Figure 12 and Figure 13, deteriorated as the feed rate increased. The most adequate surface finish was associated with the combination of 700 m/min cutting speed coupled with 0.5 mm depth of cut and 0.1 mm/rev feed rate. This produced a satisfactory surface finish for ADI casting when engaged with either CBN 2 or ALSc. The cutting tool ALTc produced similar, and occasionally even improved, surface finish characteristic compared to ALSc. In contrast CBN 1 consistently produced the least desirable surface finish, due to its relatively coarse grains structure.

CONCLUSIONS
The paper has described a machinability study of Grade 3 Austempered Ductile Iron with two grades of PCBN cutting tools and two grades of ceramics. The emphasis was placed on the machinability evaluation of these four individual cutting tool materials based on three machining criteria and the comparison between their performances.

The machining of ADI dictates the use of a cutting tool with high toughness and good thermal conductivity, with an aim to assist in dissipating some of the heat generated during the machining operation, whilst retaining some heat to develop the thermo-softening effect on the material (more details are given in reference 18 ).

The experimental results indicated strong correlation between tool wear, cutting forces and subsequently, surface finish. That correlation is demonstrated throughout these machining trials where a condition of excessive tool wear coupled with poor surface finish, is tied in with high levels of cutting force. The results obtained indicated that cutting tools such as ALSc and CBN 2 are most suitable for light cut high speed machining operations, whilst ALSc and CBN 1 adequately sustained roughing machining operations that incorporated large metal removal volumes.

The cutting tool ALSc constantly offers the versatile solution, which accommodates both finishing and roughing operations, producing satisfying machining characteristic with regards to assessment criteria, such as tool wear, surface finish or cutting forces.

ACKNOWLEDGEMENTS
The authors would like to thank Applied Process Inc., and WmLee Co., who produced and heat-treated the castings, and also the suppliers of the cutting tools, used in this study.

REFERENCES:

[1] Keough J.R. and Hayrynen K.L., “Automotive Applications of Austempered Ductile Iron (ADI): A Critical Review”, SAE Technical Paper Series, Paper no. 2000-01-0764, SAE 2000 Congress, Detroit, Michigan, March 2000.

[2] Forrest R.D., “Challenges and Opportunity Presented to The SG Iron Industry”, Foundryman, Vol.81, Apr., 1989.

[3] Lincoln J.A., “Austempered Ductile Iron: The Material of The 80’s”, Heat Treatment, Dec 1984, pp. 30-34.

[4] Chang N.S., Grupke C.C., Tarajos J.M. and Dahl G.M., “Machinability Evaluation of Austempered Ductile Iron for Automotive Crankshaft Application”, World International Conference on Austempered Ductile Iron, 1991, pp. 271-287

[5] Kovacs B.V., “Austempered Ductile Iron: Fact and Fiction”, Modern Casting, 1990, pp. 38-41.

[6] Panasiewicz J., Grupke C. and Huth J., “Chrysler’s Experience With Austempered Ductile Iron Crankshafts”, World International Conference on Austempered Ductile Iron, 1991, pp. 176- 195.

[7] Dorazil E., “High Strength Austempered Ductile Cast Iron”, Ellis Horwood Publication, 1991, ISBN 0-13-388661-1.

[8] Keough J.R., “An ADI Primer”, The Foundry Management and Technology, Nov., 1995, pp. 27-31.

[9] Gundlach R. C., and Janowak J. F., “A Review of Austempered Ductile Iron Metallurgy”, 1st international Conference of Austempered Ductile Iron, Chicago, 1984 pp 1-12.

[10] Elliot R., “Cast Iron”, Butterworth, 1988, ISBN 0-4-8-01512-8.

[11] Seah K.H.W. and Sharma S.C., “Machinability of Alloyed Austempered Ductile Iron”, International Journal of Machine Tools Manufacture, Vol. 35, 10, 1995,pp. 1475-1479.

[12] Pashby I.R. and Wallbank J., “Ceramic Tool Wear when Machining Austempered Ductile Iron “, Wear, V. 162, 1993, pp 22-33.

[13] Smith G. T, “Advanced Machining”, IFS Publication/ Springer Verlag 1989 ISBN 0-387-50650-0

[14] Dagnall H., “Exploring Surface Texture”, Taylor and Hobson, 1997.

[15] Galyer J.F. and Shotbolt C.R., “Metrology for engineers”, Cassell publication, 1990.

[16] King 1966 A.G. and Wheildon W. M., “Ceramics in Machining Processes”, Academic Press Inc., 1966.

[17] Edwards R., “Cutting tools”, Institute of Materials, 1993 ISBN 0901716480.

[18] Goldberg M., Smith G.T., Berry J.T. and Littlefair G., “Machinability Assessment and Surface Integrity Characteristics of Austempered Ductile Iron (ADI) using Ultra-Hard Cutting Tools”, Third International Machining and Grinding Conference, SME, October 1999, Ohio USA, pp 825-846.

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