DUCTILE IRON DATA FOR DESIGN ENGINEERS

 

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SECTION 10.  DESIGNING WITH DUCTILE IRON

Introduction
Designing with Castings
Freedom of Design
Casting Soundness
Freedom from Defects
Consultation
References

Introduction
In the past 40 years the use of Ductile Iron has grown rapidly, mainly through conversions from Gray and Malleable Iron castings and steelcastings, forgings and fabrications but also through its use in new components designed with Ductile Iron. Ductile Iron has been successful because it has offered the design engineer a combination of versatility and properties not available in any of its rivals. Its castability, machinability, damping properties, and economy of production are almost equal to those for which Gray Iron is famous, but its mechanical properties - strength, wear resistance, fatigue strength, toughness and ductility are competitive with many cast, forged and fabricated steel components. The conversion of Gray Iron castings to higher strength Ductile Iron has given the designer two alternative routes to improved component value: significant weight reduction with improved performance through redesign, or lesser but still substantial improvements in performance while maintaining the significant production and commercial benefits of keeping the existing design. Conversions from steel have offered similar, methods of improving cost effectiveness: new designs to improve performance and manufacturability, or the use of existing designs to provide equivalent performance, improved manufacturability and a 10 per cent reduction in weight. In summary, Ductile Iron has been successful because it has offered the designer superior value - higher quality and performance at lower cost.

The driving force of superior product value is clearly evident in the examples in Table 10.1 of successful designs involving conversions to Ductile Iron castings. These examples, taken from Designs in Ductile Iron and Ductile Iron Castings show that, in addition to improvements in product quality, performance and reliability, the replacement of other materials by Ductile Iron castings is also driven by substantial cost savings gained through lower casting cost and superior manufacturability. The numerous design improvements in this Table, subdivided according to their roles in the product value equation, follow.                                                Back to Top

Product Performance/Quality

Increased strength & safety
Higher strength /weight
Improved wear resistance
Improved sound damping
Reduced weight
Improved fatigue life
Uprated performance
Improved shock resistance
Improved integrity/reliability
Improved appearance
Marketing advantages
Manufacturability

Often used as-cast
Reduced machining tolerances
Reduced machining costs
Reduced number of parts
Reduced/eliminated welds
Reduced inventory costs
Reduced mfg time/costs
Solved warpage problem
Increased productivity
Simplified assembly
Reduced material costs

Many of these advantages have been discussed in the preceding Sections, which have addressed both fitness for purpose and manufacturability issues related to the design of Ductile Iron castings. This Section briefly highlights some of the advantages of designing with castings and points out the additional benefits of making those castings in Ductile Iron. Detailed aspects of casting design, and further information on designing with Ductile Iron can be found in the Section references, which specialize in these subjects.

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Table 10.1  Examples of conversions to Ductile Iron.

COMPONENT CONVERTED
FROM
CONVERTED
TO
COST
SAVINGS
OTHER DESIGN
IMPROVEMENTS
Off-road Truck
Suspension Cylinder
Welded Steel
Fabrication
BS2789
420/12
>20% Reduced machining costs.  Reduced
inventory and stock control costs.
Backhoe Loader
Stabilizer Foot
Steel
Weldment
ASTM A-536
80-55-06
49% Used as-cast.  All machining and
fabricating costs eliminated.
Rope Clamp and
Eye Nut
Steel
Forging
ASTM A-536
80-55-06
82% Stronger. Improved appearance.
Crankshaft for
Supercharged Engine
Steel
Forging
ASTM A-897
ADI
39% Lighter/stronger/improved wear resistance.
Improved sound dampening.
Diesel Engine
Timing Gears
Carburized
Steel Forging
ASTM A-897
ADI
30% Increased machine shop productivity.
Reduced wt. & noise.  Rapid "break-in."
Aircraft Towbar
Head
Steel
Weldment
ASTM A-536
80-55-06
76% Improved mech. properties.  Reduced
machining.  Improved appearance.
Worm Gear and
Post Screw
Bronze & Steel
Fabrication
ASTM A-536
60-40-18
46% Improved performance.  Simplified
final assembly.
4WD ATV
Wheel Hub
Aluminum
Casting
ASTM A-536
65-45-12
50% Light weight.  Increased strength and
safety.  Improved aesthetics.
Fertilizer
Injection Knife
Steel Forging
and Weldment
ASTM A-897
ADI
44% Excellent wear resistance.
Eliminated all fabrication costs.
Stainless Steel
Banding Jig
Tool Steel
Inv. Casting
ASTM A-897
ADI
77% Significant reduction in machining
costs achieved with equal performance.
Wire Rope
Clamp
Steel
Forging
ASTM A-536
80-55-06
92% Close tolerance as-cast.  High strength.
Marketing advantages.
Aircraft Door
Fixture
Steel
Weldment
ASTM A-536
65-45-12
78% Solved warpage problem.  Increased
strength.  Reduced number of parts.
Gas Turbine
Casing
Steel
Castings
BS2789
420/12
>30% Additional savings in machining costs.
17% less wt.  Better vibration damping.
Truck Drive Shaft
U-Joint Slip Yoke
Steel
Forging
ASTM a536
100-70-03
47% Reduced material and machining costs
for equivalent reliability.
Tractor
Brake Anchor
Steel
Fabrication
ASTM A-536
80-55-06
44% Equivalent mechanical properties with
reduced machining costs.
Air Compressor
Block
Steel
Weldment
ASTM A-536
65-45-12
46% Improved sound damping and product
integrity.  Reduced mfg. operations.
Automobile
Steering Knuckle
Eleven-part
Assembly
ASTM A-536
60-40-18
large Reduced mfg. operations, parts
inventory.  Improved reliability.
Photometer
Housing
Steel
Fabrication
ASTM A-536
65-45-12
45% Weight reduction.  Improved
appearance.  Improved performance.
Truck Cab
Mount
Steel
Fabrication
ASTM A-536
80-55-06
31% Improved fatigue life.  2 castings
replaced 34 parts and 25 welds.
Cam for Cotton
Picker
Hardened
Tool Steel
SAE J-434C
D5506
68% Reduced surface loads.  Increased
picking speeds.  Improved efficiency.
Backhoe Loader
Swing Pivot
Steel
Weldment
ASTM A-536
65-45-12
31% Reduced mfg. time.  Better machining
Improved wear properties.
Tractor Transmission
Hydraulic Lift Case
Gray Iron
Casting
BS2789
420/12
40% vs
Steel
Uprated design req'd stronger material.
Steel casting 40% more + pattern change
Plug Valve SS, Monel and
Titanium
ASTM A-536
60-40-18
66% Close dimensional tolerances.  Enabled
installation of plastic liner.
Air Compressor
Crankcase
Steel
Weldment
ASTM A-536
60-40-18
82% Improved sound damping and shock
resistance.

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Designing with Castings
Designing with castings offers the design engineer numerous methods with which to develop a better product in shorter time at lower cost. In order to take full advantage of these opportunities, the design engineer must follow certain principles of Ductile Iron casting design. The most important of these principles are:

  • use the design freedom offered by the casting process to optimize component performance. and
  • design for casting soundness and freedom from defects.

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Freedom of Design
The freedom of design inherent in the casting process is the ideal complement to the electronic design tools - CADCAM, solid modelling and FEA - which enable "electronic prototyping" to rapidly determine the optimum component shape and convert that shape into patterns for the production of castings. This process not only reduces product development time but also minimizes the need for fabricated prototypes which often "compromise" designs and perpetuate the compromise by becoming the production method.

Figure 10.1 illustrates how freedom of design enables castings to provide superior component performance. In this example, the easily cast box and "U" sections of the lever provide lower outer fibre stresses than the oval and I-beam sections. When produced as castings this lever, and numerous similar components, have more efficient load-bearing capabilities, enabling them to be either up-rated in performance or reduced in weight without increasing tensile or fatigue design stresses.

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Casting Soundness
The economical production of castings free from harmful shrinkage is a prequisite of good design. Because most cast metals shrink during solidification, prevention of shrinkage defects involves the use of directional solidification to produce feeding paths from attached feeders (risers) to every part of the solidifying castings and the avoidance of casting geometry which impairs the ability of the mold to extract heat from the soldifying casting. One of the major feeding problems is isolated sections which, due to size and geometry, soldify more slowly and cannot be fed through attached sections. "L", "T" and "X" junctions, with their associated right - and acute - angled surface geometries, are common hot spots in castings which should be avoided or modified to reduce both shrinkage and stress concentrations.

Figure 10.2 illustrates common methods for correcting shrinkage problems in isolated heavy sections. The use of risers and padding increase metal consumption and casting cleaning costs while chills increase molding costs. The ideal solution is to used cored holes to induce cooling, reduce weight and eliminate machining operations. The unique solidification behaviour of Gray and Ductile Irons (see Section II) minimizes shrinkage and feeding problems, offering significant design advantages and cost savings in the production of complex castings.

The inability of mold corners, especially acute angles, to extract heat retards freezing and brings the local thermal centre near the casting surface. These problems, which may occur at any sufficiently sharp change in casting surface direction, and the reduced cooling surfaces of multilegged junctions, require the modification of these junctions to improve casting integrity. Figure 10.3 shows how increasing L-junction radii reduces the stress concentration factor and drives the thermal center of the junction away from the casting surface. Figure 10.4 shows a component was changed from a rectilinear, junction-filled design typical of fabrications, to a more castable and efficient curvelinear design of equivalent or superior strength.

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Freedom from Defects
Section XI describes the integration of design and ordering to provide superior value to the end user and profitability to both the manufacturer and foundry. One key aspect of this new concept is simultaneous design, in which component value is optimized through concurrent improvements in performance, quality, supply and manufacturability.  Designing for freedom from defects is a good example of simultaneous design involving the cooperation of the designer and foundry.

Table 10.2  Ductile Iron Cyclic Fatigue Properties. 
(Information furnished courtesy of Meritor Automotive Inc., Troy, Michigan 1997).

ASTM 65-45-12 As-Cast (1,000 to 10,000 micro strain)  
Fatigue Strength Coefficient (ksi) 118.64
Fatigue Strength Exponent -0.08939
Fatigue Ductility Coefficient (inch/inch) 0.2257
Fatigue Ductility Exponent -0.6718
ASTM 65-45-12 Annealed (60-40-18) (1,500 to 30,000 micro strain)  
Fatigue Strength Coefficient (ksi) 112.29
Fatigue Strength Exponent -0.07052
Fatigue Ductility Coefficient (inch/inch) 0.1249
Fatigue Ductility Exponent -0.6256
ASTM 80-55-06 As-Cast (1,380 to 30,000 micro strain)  
Fatigue Strength Coefficient (ksi) 147.84
Fatigue Strength Exponent -0.08205
Fatigue Ductility Coefficient (inch/inch) 0.2634
Fatigue Ductility Exponent -0.6477
ASTM 80-55-06 Normalized (100-70-03) (1,650 to 30,000 micro strain)  
Fatigue Strength Coefficient (ksi) 141.91
Fatigue Strength Exponent -0.07048
Fatigue Ductility Coefficient (inch/inch) 0.1235
Fatigue Ductility Exponent -0.5502

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Consultation
This Section is only a primer on casting design and the designer is urged to consult the references, or better still, contact a Ductile Iron foundry, the Ductile Iron Group or any of its member companies. Survival and profitability for both the users and suppliers of castings requires not only high quality castings, but increased consultations on all aspects of quality, performance and manufacturability.

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References
Designs in Ductile Iron, The Ductile Iron Group, 1467 N. Elston Ave., Suite 200, Chicago IL 60622.

Ductile Iron Castings, Produced by the British Cast Iron Research Association, Alvechurch, Birmingham B48 7QB, England, in cooperation with member companies of BRIDUC. J. B. Caine, Design of Ferrous Castings, American Foundrymen's Society, Des Plaines, IL, 1984.

J. C. Morrison and K.J. Smith, "Cost effective substitution of steel components by SG ductile iron.", The Foundryman, March 1989, pp 121-129.

S. I. Karsay, Ductile Iron II, Quebec Iron and Titanium Corporation, 1972,

A Design Engineer's Digest of Ductile Iron, 7th Edition, 1990, QIT-Fer et Titane Inc., Montreal, Quebec, Canada.

Casting Design as Influenced by Foundry Practice, Meehanite Worldwide, Chattanooga, TN.