Researchers are developing ductile iron that features properties similar to ADI, without heat treatment.

Susana Mendez, U. de la Torre, and Pello Larranaga, IK4-Azterlan, Durango, Spain; Ramon Suarez, Veigalan Estudio 2010, Durango, Spain; and Doru M. Stefanescu, Ohio State Univ., Columbus, Ohio, and Univ. of Alabama, Tuscaloosa, Alabama

(Click here to see the story as it appears in the October issue of Modern Casting.)

Ductile iron has a wide range of mechanical properties, depending on its metallic matrix. The material can replace cast and forged steel in a large number of applications due to its combination of high strength and toughness, in addition to lower density.

Because of this, the search for new ductile iron alloys with improved mechanical properties and lowered production costs is an important research field. The ductile iron with the highest resistance/ductility rate is austempered ductile iron (ADI), which gets its superior properties from its microstructure. This microstructure, called “ausferritic,” is different than that of conventional irons. Fine grains of ferrite yield high strength, and the distribution of austenite and ferrite together make ADI more ductile and tougher than conventional irons.

ADI achieves its microstructure through a heat treatment process called austempering. This is a well-established method, and ADI parts have replaced hundreds of steel forgings and fabrications in production volume levels.

As a secondary operation, austempering adds cost and time to casting production. Researchers are investigating a new process to achieve castings with the same microstructure as ADI (ausferritic) without heat treatment. In this process, engineered cooling is used to coerce the metal to form the desirable ausferritic microstructure. For end-users, it could mean lower cost, high strength parts with shorter lead times.

The process variables that must be controlled to achieve the as-cast ausferritic microstructure include the chemical composition of the metal and the cooling rate of the different sections of the casting. The length of the solidification process and the parameters of the transformation needed to create the ausferritic microstructure must also be included in the analysis.

Researchers first developed a way to achieve the as-cast ausferritic microstructure for a single alloy for a specific casting (steering knuckle). However, many automotive castings that are candidates for this technology present geometries with significant thickness variations and consequently different cooling rates. These differences can complicate or make impossible the production of fully ausferritic as-cast parts by engineered cooling.

In order to utilize engineered cooling for a wider variety of parts in real-world applications, work was needed to develop an experimental model that defines the thickness window in which an ausferritic as-cast microstructure can be achieved without the use of conventional austempering heat treatment by chemical composition adjustments.

Question: Can a simple method be developed to determine the process parameters necessary to produce as-cast ausferritic parts with given mechanical properties?

1. Background

One of the key points to achieve as-cast ausferritic microstructure is to define the minimum cooling rate.

Continuous Cooling Transformation (CCT) diagrams were developed for three different alloys with chemistry in the range of 3-5% Ni, 0-0.2% Mo and 0.1-1% Cu by weight. The change of the minimum cooling rate to prevent the formation of pearlite (pearlite would keep the ausferritic microstructure from forming) was linked to the content of the main alloying elements (nckel, molybdenum and copper).

Shakeout and isothermal transformation temperatures have a major influence on the final microstructure. Isothermal transformation refers to the transformation of the iron’s microstructure at constant temperature. Different thicknesses in the same casting involve different processing temperatures. To be successful, engineered cooling must provide a fully ausferritic microstructure in all the sections of a casting or at least in the sections defined by the designing engineer. For this reason, the thickness window where completely as-cast ausferritic microstructures are obtainable must be clearly defined.

The goal of the research was to develop an experimental model able to define the thickness window where the as-cast ausferritic microstructures can be guaranteed. Additionally, the model was validated in a semi-industrial process for the chemical composition range. When the thermal moduli of a casting are in the range of the processing thickness window, the model defines the optimum processing parameters with the aim of obtaining mechanical properties (such as ultimate tensile strength and hardness) that meet the requirements of the ADI materials.

2. Procedure

To obtain different cooling rates, castings with different thermal moduli and several geometries were poured. The studied thermal moduli range was between 0.16 in. and 0.6 in. (0.4 cm and 1.5 cm). The samples produced included plates (3.9 x 2.4 in. [10 x 6 cm] and from 0.4 to 3.1 in. [10 to 80 mm] in thickness, varying each 0.4 in. [10 mm]), cylinders with the height equal to the diameter, and keel blocks Y2 (as per the standard EN 1563).

To develop the CCT diagrams, the castings were removed from the molds early and then air-cooled. The cooling curve of each casting was recorded with a thermocouple inserted in the thermal center. With this information, the cooling rate for the different thermal moduli was experimentally calculated for the temperature range of the eutectoid transformation. The specimens were visually inspected with an optical microscope. The goal of the metallographic analysis was to find the pearlite occurrence and thus the minimum cooling rate to avoid the formation of pearlite as a function of the alloy composition.

Second, the processing temperature to obtain as-cast ausferritic microstructures was defined and related to the different thermal moduli of the castings.

Once poured and solidified, the test castings followed a controlled cooling process. At the beginning, all samples were shaken free from their molds at the same time and then aircooled in the temperature range of ausferrite formation. At this time, the samples were introduced into an insulating medium with a low thermal conductivity. The aim of this step is to maintain a constant temperature to enable the ausferritic reaction to occur. The isothermal transformation was defined as 90 minutes for all the samples.

Finally, after the isothermal holding, the samples were air cooled to room temperature and the cooling curves calculated (Fig. 1). The experimental data were used to obtain the relationship between the shakeout temperature and the thermal modulus.

Tensile and hardness specimens were machined from the samples. The ultimate tensile strength (UTS), yield strength (YS) and elongation were measured as per the standard EN 1563:2011. In addition, Brinell hardness measurements were carried out per the standard ISO 6506-1:2005.

3. Results and Conclusions

Based on the results of the experiment, an Excel spreadsheet model was developed to establish if a specific casting, with specific thickness differences, can be produced through engineered cooling with fully ausferritic microstructures on all sections.

The inputs of the model are the minimum and maximum thermal modulus of the casting where an ausferritic microstructure must be guaranteed and the mechanical property requirements.

Taking into account these inputs, the model analyzes the required alloying elements in the first step. By means of an iterative method, the model calculates the minimum nickel, molybdenum and copper content to prevent the formation of pearlite (which would prohibit the formation of the desirable ausferrite). As several alloy combinations can be considered, different criteria, such as economical or qualitative, could be the decisive factor in selecting the proper alloy.

In the second step, the model deals with the shakeout process. Based on the relation between the shakeout temperature and the thermal modulus, the model determines if the process is feasible for the maximum and minimum thermal modulus of the component and, if it is, the optimum shakeout temperature.

The third step deals with the isothermal transformation temperature window. For the same maximum and minimum thermal modulus, the model determines if it is feasible to achieve the target microstructure and, if it is, defines their optimal isothermal transformation temperatures, based on the required mechanical properties in terms of ultimate tensile strength and Brinell hardness.

The two critical temperatures—shakeout and isothermal transformation temperatures—have to be inside defined ranges that permit the formation of an ausferritic microstructure that meets the requirements of the ADI materials. Depending on the different thermal moduli of the casting, these temperatures will change. Based on these changes, the model calculates the thickness window in which this methodology is feasible and by extension, if a given casting could be produced with the engineered cooling process.

Mechanical properties differ based on the thermal modulus and processing temperature, which result in different ADI grades obtained (Table 1). As an example, Figure 2 shows the microstructures obtained for the modulus 0.26 in. (0.65 cm) and 0.5 in. (1.28 cm). It was observed in the study that the lower thermal modulus is associated with a higher amount of lower ausferrite. This results in higher strength for the lower thermal moduli, but lower ductility.

The experimental model has been validated with different geometries in a defined range of thermal moduli (0.16 in. and 0.6 in. [0.4 cm and 1.5 cm]) and for specific range of chemical composition (3-5%Ni, 0-0.2%Mo, 0.1-1%Cu by weight).   

This article is based on paper 15-010 that was presented at the 2015 AFS Metalcasting Congress.