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Advanced High Strength Thin

Ductile Iron - A Breakthrough

C. van Eldijk

TDIvalueWeb – the Netherlands

ABSTRACT

Thin Ductile Iron, TDI, is a spheroidal graphite steel matrix composite with wall thickness from 4 down to 2 mm and a microstructure with a nodule count from 1000 up to 7000 nodules per mm2. The much finer graphite particles cause an increase in fatigue strength, some lowering of fracture toughness and a shift in the Tough-Brittle-Transition-Temperature to lower temperatures compared to standard ductile iron grades.

This positions TDI as a superior solution vs. steel weldments and aluminium castings in compact, complex geometries where fatigue strength and weight reduction are critical.

Exploitation of the potential for (fatigue) strength critical functions in TDI require new design concepts and can be realised with new procedures for integrated engineering techniques such as geometry optimisation, simulation of mould filling and solidification including (residual) stress analysis, rapid prototyping and thermographical stress analysis.

A market share boost for DI including TDI is expected.

For strength critical applications, TDI is a technical design material par excellence

INTRODUCTION

Thin Ductile Iron, TDI, can be defined as a graphite/steel matrix composite with 1000 to 7000 spheroidal graphite particles in wall thickness down from 4 to less than 2 mm. Thin Ductile Iron with various matrix microstructures –ferritic to austempered TW(A)DI– is part of the Fe-C family and has an enormous potential in added value creation and market volume expansion providing up to 50% cost and/or weight savings vs. aluminium, steel and regular ductile iron in applications where (fatigue) strength is critical and geometry is complex. Confronted with new scarcities, rocketing alloy material prices and tightening of environmental regulations, TDI is a best practice answer to these new challenges.

More than just weight savings and total cost savings, TDI provides improved functional performance (strength, volume, maintenance, life span, noise reduction, environmental aspects). Exploitation of this potential of TDI has just started and will be illustrated in this paper with a case study from the transportation sector.

The production of TDI castings with a wall thickness down to 1.5mm is possible with modern, standard foundry practices. However, a full realisation of economic, commercial, technical and environmental gains can be realized in newly developed, specialized TDI(TADI) casting lines. At this moment, all elements for the start of large scale TDI series production are available: controlled casting procedures, new production technology and equipment as well as appropriate engineering software and new non destructive inspection techniques for quality control. To develop optimal, quality controlled, strength critical TDI castings, a design procedure is warranted involving the different partners in the TDI value chain, ranging from the engineering office to the OEM. Exploitation of the full potential of TDI is more than just a simple change of the recipe. To materialise, the potential of TDI, investments need to be made in the actual organisation and orchestration of TDI production. At the end of this paper, a short organizational outlook will be given.

Background

Several development projects concerning TDI with grants from the European Community, the Flemish and Dutch governments and Corus steelworks in the Netherlands in the period of 1987-2000 have been coordinated by the author. The outcome of these projects was more knowledge about improved production procedures for TDI, fracture mechanics properties for TDI as well as computer simulation models for mould filling, solidification and residual stress analysis. Several pilot projects have validated newly developed technologies. At this moment, a value web for strength critical TDI castings is starting up in cooperation with a number of partner organisations.

PRODUCTION / METALLURGY

Serious attempts to produce thin wall Ductile Iron, TDI, started more than 20 years ago using the available inoculants, raising the pouring temperature to impractical high values and sometimes using insulating moulding materials. The results were not repeatable, the microstructure not much finer than regular ductile iron and carbides that required annealing had to be accepted.

In the EU, Dutch and Flemish subsidized research projects during 1987-2000 an extended test program was carried out with lab tests and several full scale production runs on ten different foundry production lines. [1] This resulted in optimized controllable procedures for the production of TDI with wall thickness from less than 2 up to 4 mm including 7 mm. To secure a TDI microstructure free of carbide under standard foundry conditions in SiO2-based moulding material a sufficient high nodule count is required as presented in Figure 1. To obtain this high nodule count a high %Si level is needed (compared to regular ductile iron) as well as a higher amount of inoculant with a sophisticated procedure for stepwise addition. The general conclusion can be that TDI procedures can be operational with standard modern foundry practices, but the allowable process windows for the various process steps are significantly smaller than for regular DI and even more critical than for CGI castings.

Critical conditions that apply for TDI production include:

- Inoculation potential (charge composition)

- Hypereutectic final chemical composition (CE>4.7)

- Nodulisation and pre-inoculation

- Late inoculation in mould or stream

- Low residual Magnesium content

- High pouring temperature

- High pouring rate

- Fading effects

- Controlled mould filling

- No burrs on critical places

For the best and reproducible inoculation situation during solidification, nodulisation material and inoculation materials are important as well as the basic charge components. A high pouring temperature is required to prevent cold run effects and to ensure full solution of the late inoculation. However, it should not be too high so as to spoil the inoculation effects. A high pouring rate is very effective in preventing cold run. A good fluid flow and heat flow simulation package for mould filling is a must for designing the optimal gating system. The residual magnesium content should be as low as possible to prevent graphite deterioration and porosity, but high enough to prevent the negative effects of mould metal reaction.

The best practice in general terms for TDI production to achieve sufficient high nodule count is a Mg-treatment without further alloying elements (FeSiMg, NiMg, Mg-coke), one or more inoculation (pre-conditioning) treatments containing and a late inoculation in the mould (insert in the mould or powder in the pouring stream).

The most effective inoculant for ladle inoculation as well as in the mould inoculation is a FeSi-based inoculant containing some Rare Earth elements and Bismuth. [pic]

Figure 1: Series A: Lower limit for carbides free castings produced with optimized and wall thickness specific inoculation; Series B: uniform inoculation.

The moment of addition, sequence, amount and ratio for Cerium-RareEarth and Bismuth is critical. The best conditions are to be established for each specific production situation. Narrow process windows for temperature-time history, charge composition and minimized fading effects are required comparable to compacted graphite iron.

Graphite morphology

Under lab conditions [1] TDI material was produced for further material characterization (micro structure, fracture toughness and fatigue properties) in vertically cast test plates with dimensions of 10 x 20 mm and wall thickness of 2, 4 and 7 mm. In all cases a SiO2 based moulding material was used. In those test plates a graphite distribution was found as shown in Table 1.

The composition of the iron was 3.7-3.9%C, 3.1-2.7%Si, ................
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