248 FTJ October 2006

In the year 2000, MTU Friedrichshafen Ltd introduced the model range 8000 (fig. 1). This is a high-performance diesel engine in a 20-cylinder-V-construction based on the common-rail technology with a power rating up to 9000 kw. Its primary application is fast, commercial ships and is also used in marine navigation and yachts.

In commercial applications, the engine is connected by a total of eight engine cradles (four on each side) to the body of the ship. This arrangement transmits the reaction forces and momentum to the ship's foundation.

The engine cradle steel construction, steel basis S355J2G3 according to DIN EN 10025, comprises several single components which are welded or screwed together (fig. 2). The base plate is welded with two side-sections with two through-holes in each section. These are used to screw the engine cradle to the crankcase. A connecting link welded to both side sections and the base plate serves as reinforcement.

The base plate has several holes, of which the one in the centre is

intended for connection to the engine suspension. Two of the eight engine cradles always possess a shackle, which is screwed on the right or left side. The momentum generated by the rotating exhaust turbocharger, at the engine

Designing engine-cradles for a high-performance ship diesel engine with high-strength ADI (austempered ductile iron) cast-iron material, results in a weight advantage of 30% compared with previous series-solution results. Additionally, the manufacturing costs are clearly reduced. This case study illustrates the added value of the austempering process when carried out as a post casting heat treatment.

Part 2 of the article will appear in the November issue of Foundry Trade Journal.

clutch side, is braced by the shackle.The remaining six engine cradles

are identical in construction, except for a shorter width. The steel-engine cradles with shackle consist of six single components (four without shackle). In addition to the actual joining process, preparation and subsequent mechanical post-processing of the weld seams is required. Due to the high manufacturing costs and the variety of parts for a steel cradle, an alternative design in the form of an integrative cast component was conceived.

As component design change would lead to significant costs for the small and medium number of pieces, the new design needed to provide not only equivalent component-performance, but also considerable cost and weight advantages.

Material selectionIn order to reduce the number of single components in the steel engine cradle and with it the associated pre-processing, joining and post-processing operations, the engine cradle should be produced as an integrative cast component in the future. The shackle can then be cast as required on the right or left side of the engine cradle by using an interchangeable component at the set-up stage. Because of the required high strength and obvious weight reduction, the innovative cast-iron material ADI-800 (austempered ductile iron, EN-GJS-800-8 according to EN 1564) was chosen.

The application of high-strength cast irons (ADI - austempered ductile iron) in high-performance diesel engines – part 1

The authors are Cahit Demirel, Thomas Behr, Kar-L Weisskopf from Ulm, Germany; Reiner Böschen from Friedrichshafen, Germany; and Christian Gündisch from Bocholt, Germany.

Engine cradle for the high performance Diesel motor made from ADI (model MTU Friedrichshafen Ltd, series BR 8000)

Finishing

Fig. 1. High performance diesel engine (Model MTU 20V 8000) with four engine cradles each side. One engine cradle is shown with mounting supports for the overlaying transmission turbocharger and inter-cooler bracing

FTJ October 2006 249

Finishing

ADI is a heat-treated cast iron with nodular graphite. In comparison with pearlitic cast iron with nodular graphite and is characterised by significantly higher static and dynamic strength and a higher ductility at the same time. This is proven to be essential, particularly for the required zero damage rate. Therefore the material ADI-800 is preferred for this application and also with regard to lightweight-aspects.

Furthermore, this material offers better characteristics concerning noise- and vibration-damping than steel. This is an advantageous characteristic, particularly for components in the engine suspension region. In spite of the material heat treatment for lightweight variants, the manufacturing costs of ADI cast parts can compete with conventional materials. Compared with steel and aluminium, ADI has a lower price per kilogramme (based on the attainable yield point of the material).

Manufacturing and features of ADIPearlitic cast iron with nodular graphite forms the basis for the production of ADI materials. It may be necessary to use a small amount of nickel and/or molybdenum alloying elements (depending on the maximum wall-thickness of the component) during the heat-treatment process.

The central component of the manufacturing of ADI is a three-stage heat-treatment (fig. 3). Firstly the component is heated to the austenite region at 900°C and held there for at least two hours, in order to enrich the initially low-carbon austenite with carbon. Afterwards it is transferred immediately to a salt bath with an exact temperature between 240 and 390°C and held isothermally for at least 1.5 hours. During this period, ferrite needles separate from the austenite until equilibrium is reached.

The resulting composite structure is referred to as ausferrite. Depending on the selected temperature of the salt bath, the required mechanical features of the material ADI can be achieved. The result of using higher temperatures in the intermediate stages (salt bath with approx. 360 to 380°C) is the desired ADI-800 with high ductility (table 1). Using several salt-bath-temperatures, ADI-grades can be divided into five (ASTM A 897M-90) or four (DIN EN

1564) categories.At present, the ISO/WD 17804 is

aiming to achieve simplification of the standard. The most important factor for the factory production of ADI is the exact balance between the component related, chemical alloy-composition and the parameters of the heat-treatment. The quenching speed has to be so high that no pearlite is formed. For thick-walled components in particular, this is provided by an accurately controlled addition of alloying elements like copper and a small amount of nickel and molybdenum. This causes pearlite not to form even at lower cooling rates, so that the continuous heat-treatment can be ensured for thick-walled components.

The exact stop-periods and temperatures are dependent on the

Fig. 2. Engine cradle (machined) with base plate, side sections left/right, each with two through-holes as well as welded brace and right shackle which is screwed on

component geometry and the selected alloy composition. A continuous communication between founder and heat treater is crucial for process safety. Appropriate fully-automatic and computer-controlled heat-treatment facilities are also required for the setting of the desired ADI-structure. These can be operated with precision and give reproducible results for the austemper-heat-treatment process.

ADI-suitable component dimensioningIn order to keep the manufacturing costs low, a hollow construction is chosen. Fig. 4 shows the cast-geometry of the broad engine cradle with a shackle on the left side. In this broader variant, the shackle is cast with an interchangeable component

Table 1. ADI Standards

250 FTJ October 2006

A People’s Organisation for People like YOU

The Time is Nowto join a growing number oflike-minded individuals andwatch your career progress

Professional Recognition and AccreditationProof of Continuing Professional Development

Ability for Lifelong LearningLifelong Careers Advice

Industry Seal of ApprovalQuality Status amongst PeersCommitment to Your Industry

Unlimited Networking and Assistance

Institute of Cast Metals EngineersTel: +44 (0) 121 601 6979 Fax: +44 (0) 121 601 6981 Email: info@icme.org.uk

www.icme.org.uk

to the mould either on the right or the left side of the side section. The two interior openings of the base plate are predefined by the casting pattern.

Because of the forces acting between crankcase and bearing, webs in the direction of the bearing connection are located between the side sections and the central bearing connection. A definite weight reduction on the base plate could be achieved by a reduction of the wall-thickness. In order to simultaneously guarantee the conservation of rigidity, the lower side was provided with a rib structure. Pockets were inserted on both sides of the side-sections to provide further weight reduction and production optimisation.

Additionally, the pockets improve the solidification procedure and offer a better heat dissipation during the heat treatment because of the reduced wall-thickness.

ADI Treatments Ltd; tel: (+44) 121 525 0303; e-mail: arron.rimmer@aditreatments.com

In part 2 of the article, the authors provide details of the simulation of the casting, ADI properties and endurance testing.

Fig. 4. Single component motor cradle cast using ADI with left founded shackle; (left) view from above with webs between the side sections and the bearing connection. The side sections have pockets on both sides; (right) view from below with wall thickness reduced base plate and ridge structure for rigidity

Finishing

Fig. 3. Schematic of the temperature gradient of the isothermal interstitial transformation

286 FTJ November 2006

Finishing

In part 1 of the article, the authors provided details of the engine-cradles (model MTU Friedrichshafen Ltd, series BR 8000), materials selection, manufacturing and features of ADI and an ADI-suitable component dimensioning.

Simulation toolsFEM was used to calculate the possible static and dynamic loading and the component was redesigned with regard to the chosen casting method. Adjacent components like the elastic engine bearing and the crankcase, as well as

the resulting pre-stress force of the screwed joint, were included in the FEM calculation.

Fig. 5 shows the result of a loading case calculation (screwed joints are blanked here). The motor cradle bearing connection is subjected to a vertical force emanating from the motor bearing. In spite of this stress the highest tensions are produced during assembly pre-stressing. These are located at the side sections screw bearing surfaces in the form of compression stresses. Further highly stressed areas are situated at the interface of the ribs to the base plate and at the bearing connection. The height and distribution of the tension remain at a non-critical level for ADI-800, however, because of the design.

The component development has been supported by the use of casting and solidification simulation. For optimal quality of the component, a constant die filling

(position and geometry of feeder, etc) as well as an adjusted solidification in section and feeder have to be achieved in particular. The simulation can reveal potential problem areas like an isolated centre of heat, on which the formation of cavities and porosity can occur.

For optimal quality of the component, feeder and heat sinks are established whose size and position are determined with the aid of the simulation. A snapshot

of the solidification simulation is shown in fig. 6. Three feeders are used in total, of which one is located on each of the side sections and another on the bearing connection. Furthermore heats sink are placed on the base plate and the bearing surface as well as the engine mounting. The temperature gradient from the cooling elements facilitates solidification to the feeders. Possible micro-porosities are displaced to uncritical areas such as in the interior of the side parts. According to the FEM simulation, these areas experience a lower load, so that potential porosities are not critical to the performance of the component.

Production of prototypesThe prototypes were cast at Eisengießerei Hulvershorn GmbH & Co KG in Bocholt, Germany, which specialises

The application of high-strength cast irons (ADI - austempered ductile iron) in high-performance diesel engines – part 2

Designing engine cradles for a high-performance ship diesel engine with high strength ADI (austempered ductile iron) cast iron material, results in a weight advantage of 30% compared with previous series solution results. Additionally, the manufacturing costs are clearly reduced.

Part 1 of this article appeared in the October issue of Foundry Trade Journal.

The authors are Cahit Demirel, Thomas Behr, Kar-L Weisskopf from Ulm, Germany; Reiner Böschen from Friedrichshafen, Germany; and Christian Gündisch from Bocholt, Germany.

Fig. 5. Stress distribution (cross section) resulting from vertically applied load at the bearing connection and pre-stresses. The highest stress (compressive) occurs on the screw surfaces on the side sections as well as in the bearing connections and the webs. The strain level is not critical to the design condition

Fig. 6. Solidification simulation indicating temperature profile after 18 minutes. By appropriately positioning the feeders, and the use of chills, the desired solidification pattern is achieved

Fig. 7. Moulding plates for the broad engine cradle made from ADI with interchangeable shackle (right or left)

FTJ November 2006 287

Finishing

in large and sophisticated ADI-components. The heat treatment was carried out at ADI Treatments Ltd in Birmingham, England; the company is a subsidiary of the Bocholt foundry, ensuring co-ordination from the outset between founder and heat-treater. This is essential for successful production of ADI.

Fig. 7 shows the adjustable moulding plates with adjustable component for the broad variant of the engine cradle with a shackle. The tool for the shackle can be attached on the left or the right side on the mould according to requirements. The casting dies, in furan resin bound quartz sand, are produced on a mechanised moulding facility. Machining is performed on a five-axis CNC machining centre in two clampings. The completely machined ADI cast engine cradle with left shackle is shown in fig. 8.

Through the systematic application of CAX-tools and the utilisation of the material potential of ADI, a weight reduction to 85kg (30%) is achieved, compared with the conventional construction of some 126kg. The integrative cast construction with the ADI material also gives cost savings in the manufacturing of the component. This is achieved through a reduction of the amount of single components and therefore the reduced joining and treatment operations.

Component characterisationThe comparison of material variables, which were found by destructive material testing with the given standard value of DIN EN 1564, more than confirms the achieved quality of the material. Tensile tests according to DIN EN 10 002, which were extracted from several component areas (amongst others bearing connection, web structure etc) show an average value of 645N/mm² yield stress for 0.2 % elongation (standard specification EN-GJS-800-8: 500N/mm², see table 1) and for the tensile strength an average of 900N/mm² (standard specification EN-GJS-800-8: 800 N/mm²).

The standard specifications are exceeded on different parts of the component. The fracture point is also above the standard specifications of 8% elongation in all regions. The appropriate microstructure examinations confirm the results of the tensile tests, because all tested ranges, even the range of maximum wall thickness of approximately 80mm, show a well formed and persistent ausferrite structure.

Fig. 9 shows the ADI structure, consisting of ferrite needles in an austenite matrix as well as nodular graphite. The structure test was performed in the region close to

the surface of the middle bearing connection, adjacent to the drilled hole.

Endurance test on the componentThe endurance tests were conducted on a dynamic endurance test bench, on which two engine cradles were tested in parallel in double load. Fig. 10 shows the test bench with the clamped engine cradles (the engine cradle in the back is rotated 180°C relative to that at the front, so that it is almost obscured in this figure). The vertical cyclic load per engine cradle was 1.7 times the required specific load and was endured by the engine cradles without damage.

The subsequent crack test in lime water revealed no evidence of cracking. The ultimate number of load cycles,

Fig. 8. Cast engine cradle made from ADI-800 (machined) with left shackle (cast at Eisengießerei Hulvershorn, Bocholt, Germany; heat treated at ADI Treatments Ltd in England)

Fig. 9. ADI structure in the region of the middle bearing connection beside the drilled hole

Fig. 10. Test bed for fatigue test at MTU Friedrichshafen Ltd with two clamped engine cradles (the second engine cradle is largely obscured). The parallel mounting allows the concurrent testing of both engine cradles within excess of 1.7 times the power in the vertical direction

288 FTJ November 2006

Finishing

(10 million cycles), was achieved free of cracks. Therefore the endurance of the ADI engine cradle was proven.

For the final engine tests at MTU in Friedrichshafen, an engine at the engine test bench was loaded with a complete set of ADI engine cradles. Special acceleration sensors were used for a variety of structure borne ultrasonic measurements on the engine cradles among others. In spite of significant weight reduction, the natural frequency and noise amplitude emissions are comparable with the steel engine cradles and are consequently in the acceptable range.

Based on the component and bench tests, the ADI engine cradles for commercial applications in shipping (ferries, yachts etc) were released at MTU. In addition to considerable cost reduction, the total engine weight was reduced by approximately 300kg.

SummaryOperating stresses were simulated through detailed FEM calculations of the critical loading scenarios and the geometry was optimised accordingly. In addition to the calculation of loading scenarios, the casting, feeding and cooling processes, which are necessary for good component and material quality, were designed with the

help of casting and solidification simulation. As such, the ADI engine cradle contains all functions of the welded and screwed steel alternative.

Destructive testing confirmed the component and material quality of the engine cradle of EN-GJS-800-8 according to DIN EN 1564. The required endurance limit of the ADI engine cradles was demonstrated by subsequent vibration fatigue tests. On an engine loaded with ADI engine cradles, structure borne ultrasonic measurements yielded frequencies and emissions in acceptable ranges and so enabled the ADI engine cradle to be approved for series manufacture.

ADI is a cast iron material that offers a high potential for cost and weight reduction in many applications compared to the conventional use of steel and aluminium. In addition to the substitution of existing components, the application spectrum of ADI is set to expand for current and future developments for high performance diesel engines at MTU as a highly cost effective material alternative.

ADI Treatments Ltd; tel: (+44) 121 525 0303;e-mail: arron.rimmer@aditreatments.com

World Foundry Congress 2006

‘Casting the Future’

Conference ProceedingsA CD of the World Foundry Congress 2006 conference proceedings is available from the Institute of Cast Metals Engineers (ICME) at a cost of £63.75 (including VAT, postage and packaging).

This is an excellent way of taking advantage of the wealth of information presented at the Congress of those who were unable to attend – don’t miss out twice!

For more information and to order a copy contact:Mrs Yvonne Marriott at ICME head office.ICME, National Metalforming Centre, 47 Birmingham Road,West Bromwich, West Midlands B70 6PY. United Kingdom.Tel: +44 (0) 121 601 6979.Fax: +44 90) 121 601 6981.Email: yvonne@icme.org.uk