|The purpose-built furnace for ADI and CADI treatment of castings. ADI Treatments Ltd. operates the largest installation of this kind in use commercially, for parts up to 18-meters in diameter.|
|Fig. 1: A controlled-atmosphere austenitizing batch furnace (A); an intermediate purge transfer chamber (B); (C) an enclosed vestibule to protect the work from oxidation; (D) a salt quenching station. |
Carbidic austempered ductile iron is a niche material that offers wear-resistance similar to steel and a strength-to-weight ratio comparable to aluminum — but at a lower production cost. Now, a customized heat-treating process is making it more available to metalcasters.
The heat-treating process that produces austempered ductile iron is familiar to many metalcasting engineers and operators, but according to Arran Rimmer of ADI Treatments Ltd., a similar, niche material has been developed to improve the wear durability of conventional ADI. It's called carbidic austempered ductile iron, or CADI.
"Austempered ductile iron" is the term for a family of heat-treated cast irons (ASTM 897M and EN1564) that offer good characteristics for strength and durability. ADI is stronger per unit weight than aluminum and has wear-resistance comparable to steel. Still, it can generally be produced for as much as 50% less than those materials.
CADI describes a family of cast irons produced with carbides, introduced thermally and/or mechanically, that are then austempered.
Ductile irons are treated with magnesium and/or rare earths to produce spheroidal graphite. The same irons can be induced to form a carbidic microstructure in a number of ways, including alloying with carbide stabilizers (e.g., chromium, molybdenum, titanium), controlling inoculation, or controlling the cooling during solidification. The carbides produced this way also can be ‘dissolved' in a subsequent heat treatment.
When austempering cast irons, careful control of parameters is essential to achieve optimal material properties and consistency. ADI Treatments (www.aditreatments.com) has designed a heattreating furnace for CADI, and established facilities to offer the service for metalcasters and their customers.
The ADI Treatments process (see Figure 1, p. 28) uses a controlled-atmosphere austenitizing batch furnace (A) with integral re-circulating roof fans and radiant tubes, to ensure rapid heat up of heavy loads and high uniformity. An intermediate purge transfer chamber (B) and an enclosed vestibule (C) continue to protect the work from oxidation as it is transferred to a salt quenching station (D). The vestibule also isolates the furnace from salt ingress.
The quench medium is a nitrate/nitrite mix that is held at temperatures in the 230-400°C range. To ensure process homogeneity and final material properties, the salt is forcibly circulated through the load.
By inoculating the salt bath with water, quenching rates are enhanced to allow treatment of larger-section parts without adjusting cast composition. The salt is managed and recycled to obviate environmental damage.
Both the furnace system and the process are driven via a state-of-the-art human machine interface (HMI) that also provides access to the heat-treatment model programs.
The first stage austenitization cycle is followed by a controlled quench and an austemper cycle, typically lasting 1-2 hours. The traditional quench media, oil and water, are not used so that the load does not reach the Ms temperature and brittle martensite cannot develop. Instead the microstructure is transformed to an ausferritic matrix with a controlled carbide dispersion, an arrangement that confers excellent wear-resistance with adequate toughness.
Flexibility in the furnace and controller configurations allows the austempering process to be tailored to the part. The controlled atmosphere features eliminate oxide-scale formation, while the austempering process introduces minimal distortion and stresses.
Parts may be designed to account for dimensional changes during the heat treatment, so little or no finishing is required.
One customer for ADI Treatments process is SIMBA International, a British manufacturer of cultivating and drilling machinery that worked with ADI Treatments to identify the properties and benefits of CADI for ground-engaging components. ADI Treatments worked with a German metalcaster Hulvershorn Eisengisserei GmbH to develop the ADI and CADI cast compositions.
The heat treatment programs were established with reference to Applied Process Inc. (www.appliedprocess.com), which first licensed the process to ADI Treatments, and who have built a substantial database in heat-treating technology.
SIMBA's engineers wanted to explore using ADI and CADI materials for applications involving high-wear parts on a soil aerator the company had recently developed, the Solo Subsoiler. ADI Treatments helped to design and supply the heat-treated ADI and CADI castings.
Initially, the Solo's tip parts were produced in ADI, CADI, and three other wear-resistant materials for laboratory soil-abrasion tests. The results of the trials carried out by SIMBA are compared in Table 1.
ABRO 500 is a steel plate with a minimum hardness of 500 HB. The Ni-hard abrasionresistance level was normalized to 1.0.
Live field trials also were performed on the CADI and 22% CrFe parts. Over the lifecycle the white iron proved to be more energy efficient and wear-resistant, and therefore was selected for the tip application.
Then, attention turned to the adjacent Solo component, the ‘shin.' Based on the results of the tip trials, the designers chose CADI for this part. The heat-treated casting is lower priced than alternative materials and fabrication methods.
Wear life is compatible with the tip, enabling convenient replacement at the same interval, and impact resistance is fully satisfactory.
ADI Treatments reports this was the first European application for CADI, and SIMBA reports the redesigned component has been successful for several years. (In a similar application, CADI was introduced in the North American market by John Deere for threshing elements on its rotary combines, and for ripper points used in deep tillage.) Now, manufacturers in Europe are developing similar applications and examining the potential for material containing crushed iron carbide, added mechanically during casting.
Following the success of the SIMBA application, ADI Treatments commissioned a further study on the abrasive wear characteristics of the same CADI material and a heat-treated 17% white iron. The work carried out with the University of Birmingham is summarized in Table 2. The table also lists comparative data from tests on a selection of other reference materials. For wider comparison, results previously reported on a range of ADI and wear resistant steels and irons are included, too.
The data indicates that CADI offers significantly greater wear resistance than standard ADI materials, martensitic ductile iron, and some wear-resistant steels. While similar to that of Ni-hard I, CADI is less wear-resistant than high-chromium irons.
|Ni-hard||ADI||ABRO 500||CADI||22% CrFe|
|Table 1. Relative soil abrasion resistance. (SIMBA International Ltd.)|
|Material||HB||HRC||Weight loss mg||Source|
|ADI Grade 2||321||32||76.7|| |
|ADI Grade 4||415||43.2||70.3||3|
|Quenched ductile iron||59||52.8||3|
|Q&T steel (0.8% C)||63||59||3|
|ASTM A514-T1A steel||269||139||2|
|Table 2: Compilation of abrasive wear data Sources: 1. This study; 2. Climax Research Services reference materials; 3. R. Gundlach and J Janowak, "2nd International Conference on Austempered Ductile Iron: Your Means to Improved Performance, Productivity and Cost," Ann Arbor, MI. 7-19 March 1986, pp. 23-30|
|Material||Impact energy (J)|
|Carburized 8620 Steel||17|
|Pearlitic malleable iron||17|
|7003 ductile iron||49|
|Grade 5 ADI||52|
|Grade 3 ADI||91|
|Grade 1 ADI||117|
|4512 ductile iron||123|
|Table 3. Typical unnotched Charpy impact values (Joules) tested at 22°C. Source Applied Process Inc.|
CADI's wear performance increases with increasing carbide volume. This is accompanied by a corresponding reduction in impact properties. Unnotched Charpy test data for a typical CADI alloy containing 30-45% carbide and a range of traditional materials are compared in Table 3.
The CADI figure of 13J exceeds that of as-cast white iron, which has been reported as low as 3J.1
CADl's properties present some intriguing market opportunities, with potential applications in vehicles including camshafts and cam followers.
Agricultural applications may include rippers, teeth, plough points, wear plates, and harvester, picker and baler components. In construction and mining, potential applications include digger teeth and scarifiers, cutters, mill hammers, flails, guards, covers, chutes, plates, housings, transport tubes, and elbows, rollers and crusher rollers.
General industrial applications may include pump components, wear housings and plates, conveyor wear parts, skids and skid rails, rollers, and blast parts. Market entry is eased because no capital investment is required for the ductile iron producer to add the material as a new product line.
Carbidic austempered ductile iron offers cost savings and a useful mix of properties that position the material along with established wear-resistant steels and irons, says ADI Treatments, and furnace capability is key to achieving component performance and batch-to-batch consistency.
1. K. Hayrenan, "Transactions of the American Foundry Society V 111 Paper No 03-088," pp. 845-850, 2003.