Testing 1-2-3: New As-Cast Ductile Iron Raises Bar for Properties

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. 

ncountering a scenario in which you are forced to suddenly and immediately suspend melting operations for an extended period can be a death sentence for many metalcasting facilities. Small to mid-size businesses are the backbone of the industry, but many do not survive when forced into extended downtime. One disaster-stricken metalcaster, however, found resilience through its own perseverance and a circle of support from peers, friends, suppliers, teams from installation and repair providers, an original equipment manufacturer and even competitors.
Tonkawa Foundry, a third-generation, family-owned operation in Tonkawa, Okla., was entering its 65th year of operation this year when a significant technical failure ravaged the power supply and melting furnaces on January 17. Thanks to the textbook evacuation directed by Operations Manager Carrie Haley, no one was physically harmed during the incident, but the extent of emotional and financial damage, and just how long the event would take Tonkawa offline, was unclear.
Tonkawa’s power supply and two steel-shell furnaces would have to be rebuilt. No part of the reconstruction process could begin until the insurance company approved removal of the equipment from the site. The potential loss of Tonkawa’s employees and customers to competing metalcasters seemed inevitable.
Within two days of the incident, repair, installation and equipment representatives were on site at Tonkawa to survey the damage. Once the insurance company issued approval to begin work, the installation team mobilized within 24 hours to remove the equipment and disassemble the melt deck.
Since the damaged equipment was installed in the 1980s and 1990s, Tonkawa and an equipment services and repair company quickly strategized a plan and identified ways to enhance the safety, efficiency and overall productivity of Tonkawa’s melt deck.
“The most critical issue was for our team to organize a response plan,” said Steve Otto, executive vice president for EMSCO’s New Jersey Installation Division. “We needed to arrive at Tonkawa ready to work as soon as possible and deliver quickly and thoroughly so they could get back to the business of melting and producing castings, and minimize their risk of closing.”
Several years after Tonkawa’s melt deck was originally installed, an elevation change was required to accommodate the use of a larger capacity ladle under the spout of the furnaces. Rather than raising the entire melt deck, only the area supporting the furnaces was elevated. As a result, the power supply and workstation were two steps down from the furnaces, creating a number of inconveniences and challenges that impacted overall work flow in the melt area. Additionally, the proximity of the power supply to the furnaces not only contributed to the limited workspace, but also increased the odds of the power supply facing damage.
The damage to the melt deck required it to be reconstructed. It was determined to be the ideal opportunity to raise the entire deck to the same elevation and arrange the power supply, workstation and furnaces onto one level. The furnace installation company provided the layout concepts, and with the aid of Rajesh Krishnamurthy, applications engineer, Oklahoma State Univ., Tonkawa used the concepts to generate blueprints for the new deck construction. The results yielded a modernized melt system with an even elevation, strategically placed power supply, enhanced worker safety and increased operator productivity.
“Eliminating the steps and relocating the power supply farther from the furnaces was a significant improvement to our melt deck,” Tonkawa Co-Owner Jim Salisbury said.
Within four days of insurance company approval, all damaged equipment had been removed and shipped for repair.
The insurance company required an autopsy on the damaged furnace before any repair work could begin. The forensic analysis was hosted by EMSCO in Anniston, Ala., in the presence of insurance company personnel, as well as an assembly of industry representatives from the companies who had received notices of potential subrogation from the insurance company.
Tonkawa’s furnace was completely disassembled while the insurance company’s forensic inspector directed, photographed, cataloged and analyzed every turn of every bolt on the furnace over a nine-hour workday. The coil was dissected, and lining samples were retained for future reference.
While the furnace sustained extensive damage, it did not have to be replaced entirely.
Structural reconstruction was performed to address run-out damage in the bottom of the furnace, a new coil was fabricated and the hydraulic cylinders were repacked and resealed. Fortunately, the major components were salvageable, and ultimately, the furnace was rebuilt for half the cost of a new furnace.
“The furnace experienced a significant technical failure,” said Jimmy Horton, vice president and general manager of southern operations, EMSCO. “However, not only was the unit rebuilt, it was rebuilt using minimal replacement parts.”
Though work was underway on the furnaces, Tonkawa was challenged with a projected lead time of 14 weeks on the power supply.
When accounting for the three weeks lost to insurance company holds and the time required for installation, Tonkawa was looking at a total production loss of 18-20 weeks. From the perspective of sibling co-owners Sandy Salisbury Linton and Jim Salisbury, Tonkawa could not survive such a long period of lost productivity. After putting their heads together with their furnace supplier, it was determined the reason for the long turnaround on the power supply could be traced to the manufacturer of the steel cabinet that housed the power supply.
The solution? The existing cabinet would be completely refurbished and Tonkawa would do the work rather than the initial manufacturer. This reduced the 14-week lead time to just five weeks.
Tonkawa is the single source for a number of its customers. Although lead-time had been significantly reduced, the Tonkawa team still needed a strategy to keep the single source customers in business as well as a plan to retain their larger customers.
Tonkawa pours many wear-resistant, high-chrome alloys for the agriculture and shot blast industries. Kansas Castings, Belle Plaine, Kan., which is a friendly competitor, is located 50 miles north of Tonkawa. Kansas Castings offered Tonkawa two to three heats every Friday for as long as it needed.
“We made molds, put them on a flatbed trailer, prayed it wasn’t going to rain in Oklahoma, and drove the molds to Kansas Castings. We were molding, shot blasting, cleaning, grinding and shipping every Friday,” Salisbury Linton said.
Others joined the circle of support that was quickly surrounding the Tonkawa Foundry family.
Modern Investment Casting Corporation (MICC) is located 12 miles east of Tonkawa in Ponca City, Okla. Though MICC is an investment shop and Tonkawa is a sand casting facility, MICC’s relationship with Tonkawa dates back years to when Sandy and Jim’s father, Gene Salisbury, was at the helm.
“Gene was always willing to help you out,” said MICC owner, Dave Cashon. “His advice was invaluable for us over the years, so when the opportunity arose to support Sandy and Jim, we volunteered our help.”
 MICC offered to pour anything Tonkawa needed every Friday in its furnace. Tonkawa brought its alloy, furnace hand and molds, while MICC provided its furnace and a furnace hand for three heats. Many of the specialty parts Tonkawa produces were completed with MICC’s support.
When Salisbury Linton approached Cashon and asked him to issue her an invoice to cover the overhead Tonkawa was consuming, Cashon told her if she brought in six-dozen donuts every Friday morning they’d call it even.
“We’re all kind of like family,” Cashon said. “We’re all part of the same industry and though we may be friendly competitors at times, you don’t want to see anybody go through what they’ve gone through and it could have just as easily been our furnace that failed. While we all take the appropriate measures and perform maintenance to prevent these scenarios from occurring, they unfortunately still occur from time to time in our industry.”
Tonkawa had recently added steel work to its menu of services and Central Machine & Tool, Enid, Okla., was able to take Tonkawa’s patterns and fulfill its steel orders so it would not fall behind with those customers, while CFM Corporation, Blackwell, Okla., took three of Tonkawa’s employees on a temporary basis and kept them working during the downtime. Additionally, a couple of Tonkawa’s major suppliers extended their payables terms.
Thanks to Tonkawa’s suppliers, friends and its personnel’s own passion, persistence and dedication, the business is up, running and recovering—placing it among the few shops of its size to overcome the odds and remain in business after facing calamity.
 Nearly eight months after that devastating Saturday evening in January, Salisbury Linton reflected on the people and events that helped Tonkawa rise from the ashes. “We certainly would not have the opportunity to see what the future holds for Tonkawa if it weren’t for all the kind-hearted people who cared about what happened to us. Everyone still checks in on us.”