ADI's paradox: excellence in search of acceptance.

In a recently concluded international conference on austempered ductile iron (ADI), the many advantages of the material were listed glowingly and the metallurgy of the material precisely defined. It was the consensus of the foundry personnel, academics and researchers in attendance that ADI was worthy of much wider industrial use because of its unique combination of adaptive characteristics.

Speaker after speaker recounted experimental data and research results that described the techniques of casting, heat treating and using ADI. Users verified cost and physical advantages realized by substituting ADI for alternative metals and underlined what makes it an excellent material for many design applications. it was clear from the conference that enthusiasm for ADI is high due to the material's range of achievable properties, castability and relatively low cost for finished parts.

Why, then, isn't more austempered ductile iron in use? In the face of the promises the metal holds for better performance and competitive cost, what seems to retard its wider application?

ADI offers excellent ductility, strength and toughness property options. The same casting, for instance, can be heat treated to have broadly different properties to meet specific designer demands. Wear resistance superior to that of steel of the same hardness level is reported.

Granted, ADI requires uniform foundry controls to assure consistent ductile iron quality, and heat treating requires proper equipment, care and knowledge, but these parameters have been defined and are achievable.

One explanation for ADI's slow start is that there is always a lag between research and development and general industry acceptance, but the delay for wider application of ADI has persisted longer than expected. Some reasons include; * deficiencies in quality control caused

by uninformed early producers; * the lack of perception of the inherent

advantages by upgrading to ADI; * the communication gap that exists

between ADI advocates and the

ultimate customer.

Added to these problems is the somewhat surprising misconception that ADI is just austempered steel with graphite nodules. it decidedly is not. The carbon concentration in a steel part is fixed, heat treating notwithstanding. Conversely in ADI, due to the presence of the graphite nodules, the carbon content of the iron matrix is adjustable by the heat treating selection. Steel solidifies as a single phase solid; ductile iron solidifies principally as a two-phase eutectic, an entirely different mode of solidification. Further, silicon, so essential to ADI, is considered detrimental to steel in more than minimal levels. These factors make for much different responses to the austempering heat treatment.

ADI also competes with established heat treated alloy steels and forgings for heavy-duty applications. These historical applications are more than casually affected by safety and product liability concerns. When a material has a history of successful application, the decision to substitute any alternative material is made slowly and with caution. Sometimes other alternatives are never even explored.

In assessing the current and future applications of ADI, it is well to keep in mind that when any new material is developed for engineering applications, many unknowns temper initial trials. There may simply be (for some designers) too few successes to justify specifying ADI for the high-stress applications for which it is best suited. it is difficult to decide to use a new material with complete confidence without an adequate basis of design information.

Wear applications are a special case. There is no ideal way to communicate all of the relevant technical information that delineates the many advantages of ADI. It is difficult, for instance, to evaluate wear applications in general because standard tests don't always predict service life. Field trials are better, but are costly, take time and may be difficult to arrange; so for the design engineer, there is strong pressure to remain with traditional materials.

Another problem with materials selection is the interpretation of laboratory test data. Test specimens are not production parts and it is risky to predict performance characteristics from them.

Erring on the side of conservatism is an accepted norm in designing and engineering a metal part. As noted, most design engineers are reluctant to trust their products and reputations to a material for which demonstrated engineering data is not available or is incomplete, especially when conventional materials have existing performance histories and cost records. Thus, it was an important step when the American Society for Testing and Materials (ASTM) published last year the first National Standard Specifications for five strength grades of austempered ductile irons. They are ASTM A 897M - 90 (metric) and A 897 90 (in-lb units). What is ADI?

Austempered ductile iron is a ductile iron that has been heat treated by the austempering process to make it tougher than regular ductile iron of half the strength. ADI is comparable in strength to heat treated wrought steels, has exceptional wear and fatigue resistance and has the ability to be work hardened. Highly versatile, it is strong, light, a good conductor of heat and exceeds the vibration dampening characteristics of steel.

ADI is slightly more costly to produce because it requires a high-quality ductile iron base. Rigid quality control and melting practices are essential for producing a consistent ADI product. Ductile iron castings that have been properly constituted, have a high nodule count, exhibit minimal alloy segregation and have virtually no shrinkage porosity, carbides or inclusions and excellent post-heat treating characteristics that make ADI so practical for high wear/ stress applications.

Austempered ductile iron's practical discovery dates from ft mid-1970s, nearly 20 years ago. Ductile iron, by contrast, was first made commercially in the mid-1940s, some 30 years earlier. But in its first 25 years, ductile iron, a stronger, tougher derivative of gray iron, lived up to its promise of superior performance for many casting applications and reached an annual production of 3.2 million torts. Currently, it nearly equals gray iron production in the U.S. ADI, though showing some signs of application growth, has yet to attract the use its proponents had hoped it would. World production of ADI for 1991 is projected at 50,000 tons.

Ductile iron became available at the right time. Foundries worldwide were bAng for a material that offered the improved performance that the metalurgical advances of the immediate postWW II era required. The slight production premium for ductile iron was offset by its' ability to be manufactured using standard foundry gray iron melting and casting equipment.

ADI is in a somewhat different position in that user technology is riot so much the driver of applications as is ultimate product cost, a much more difficult factor to assess in choosing ADI versus alternative materials. The phenomenon of global competition has made product pricing the single most important constant in selecting metal parts manufacturing methods and materials. Competition is the factor that affects end product cost and which, thus far, has worked to tje detriment of wider ADI applications. Product and Process

ADI's chemical composition is similar to conventtional ductile iron: about 3.6% carbon, 2.5% silicon, 0.3% manganese and less gun 0.015% sulfur. Copper, nickel and molybdenum may be added to increase its heat treating characteristics. All other casting process variables such as molding, nodulizing, inoculation and pouring temperature are to same for ADI as for ductile iron. Good nodule count is an important factor because it makes heat treating easier and results in higher strength properties.

Heat treating is the center of ADI formation. Ductile iron castings are heat treated to a specific temperature range between 1550--1750F. and held for one to three hours to allow the castings to become fully austenitic and the matrix to be saturated with carbon. The resulting microstructure is one of acicular ferrite and stabilized austenite called ausferrite.

The ADI microstructure is often and erroneously called bainite, as in steel. it is not bainite. The bainite transformation in steel is different from that in ductile iron. The bainite reaction in steel is a one-step process: austenite decomposes directly to bainite (acicular ferrite and carbide), whereas, there are two stePs to the formation of bainite in ductile iron.

In the first step, austenite transforms into acicular ferrite and carbon-stabilized austenite. This is the structure that provides the remarkable properties of ADI.

In the second step, however, when a casting is austempered longer than the required time to form the desired structure noted in step one, the matrix transforms to bainite, as in steel.

Bainite in ADI is detrimental and undesirable. The high ductility associated with du high strength of ADI is a result of ADI's unique microstructure, which is riot bainitic, but austerritic, the unique mix of austenite and ferrite.

The austenitization time and temperature are critical because they combine to affect the degree of movement of the carbon from the nodules to the matrix. The nodules act as either carbon supplies or carbon sinks and to extent of the carbon diffusion depends on the chemistry of the iron and the time/temperature austempering equation.

Some metals (especially tin, antimony and copper) inhibit the austenitizing movement of the carbon by forming a thin shell-like membrane between the graphite nodules and the iron matrix, significantly extending the heat treating time. To obtain good properties in ADI, the casting must be fully austenitized and to quench temperature, the single most important parameter in determining the mechanical properties of ADI, must be carefully controlled. ADI's tensile strength ultimately depends upon quench temperature.

After the austenitizing is completed, w casting is quenched at a temperature range of 460-75OF and held up to four hours. This austempering temperature and soaking time determines the ADI microstructure and physical properties. Generally, at lower austempering temperatures, ADI has higher yield and tensile strengths, high wear resistance and lower ductility and impact strength. Higher austempering temperatures result in higher ductility, fatigue and impact strengths and relatively low yield and tensile strengths.

Since ADI is targeted for high-stress applications, reliability of the material is extremely critical. As rioted, consistent high-quality ductile iron castings are essential. Starting with the base iron, the austernpering metallurgy of ADI has three main objectives: * avoiding pearlite and ferrite

transformation during quench; * forming acicular ferrite and high

carbon austenite quickly at the

austempering temperature; * avoiding the formation of bainitic

carbide.

The key to obtaining du desired ADI properties involves controlling the form, distribution and ratio of the ausferritic structure. This requires a critical soaking time at the selected austempering temperature. Too short austempering time results in the formation of martensite. Too long austempering time results in destabilization of high carbon austenite to carbide. Both situations significantly reduce the toughness of the material. Application of ADI

ADI offers the design engineer a material that fits a large number of applications requiring higher strength and ductility than normally found in other grades of ductile iron and steel castings, forgings and welded steel fabrications. Among the current uses for the ADI are gears, crankshafts and heavy-duty wear parts for earth moving and mining equipment.

It is possible to break down the market potential for ADI into two broad categories: first, the market for new applications requiring heavy-duty materials, and, second, to use of ADI castings as a replacement for existing heavy duty applications. Though these market opportunities now are are limited, it is clear tat a significant segment of the current market share for steel castings and forgings is fair game for conversion to ADI. Competitive cost and ASTM recognition will increase ADI's marketability.

The demand for ADI will eventually come from design engineers selecting materials able to meet extreme physical and mechanical requirements. This process will be aided by du ability of more foundries capable of consistently producing specific grades of ADI.

Finally, these casting specificers need to be convinced of the advantages of ADI, an education process that heretofore has been missing but that is now being rectified. References 1. Keough, J., Third ADI World Cont., pp 638 - 658 (1991). 2. Kovaks. B., Third ADI World Conf., pp 241 - 270 1991)3. Laub. J, Third ADI World Conf-, pp 622 - 637 (1991). 4. S.-CLee.C.-C. Lee, "Effect of Heat Treating

and Alloying Elements on Fracture Toughness 

of Bainitic Ductile iron," AFS Transactions,

pp 8274= (1988) 5. Kovacks, B.,"Austempered Ductile Iron:

Fact and Fiction," Modern Casting pp 38 - 41 (Mar 1990). TABULAR DATA OMITTED