Should you add ADI, CGI, SiMO to your iron menu? ADI, CGI and high SiMo ductile iron offer unique benefits to end-users and the metalcasters who supply them. Will adding one (or all) work for your iron casting operation?

Metalcasters often are problem-solvers for their customers, finding ways to reduce cost, reduce weight and improve performance of their castings. But are there some problems you haven't been able to solve because the metal you pour lacked the suitable properties?

Conventional gray and ductile iron will meet the needs of most iron casting customers, but a small, yet growing percentage of end-users are requiring material with higher strength, higher thermal properties and higher wear resistance. Austempered ductile iron (ADI), compacted graphite iron (CGI) and high silicon molybdenum (SiMo) ductile iron carry with them unique properties that go beyond that of conventional irons, which also allows them to be competitive with other materials such as aluminum and steel. If you haven't already, adding them to your repertoire of casting materials can widen your range of potential jobs and enable you to solve even more of your customers' problems.

ADI's Growth Potential

If your iron casting facility is looking for a new metal that will have far reaching applications, ADI's versatility is hard to ignore. The material has been used in a wide range of industries and is capable of entering still more markets in the future. Through 2008, U.S. ADI casting shipments are predicted to grow 18.1% annually, taking up an 8% share of total ductile iron shipments, according Stratecasts Inc. (Table 1).

Farrar Corp., Manhattan, Kan., has been offering ADI castings for a decade. Currently, 130 out of 1,700 total jobs are ADI castings at the firm, but the material has been a useful answer to some customers' pesky requirements.

"We listen for comments like, 'we need to save weight,' or 'we need it to be stronger,' or 'this is for an abrasive, high-wear application,'" said Joe Farrar, president and CEO of Farrar Corp. "Then we make the customer aware of how ADI might be able to meet their needs. A lot of times, they aren't even aware of what ADI can do."

"The advantage of ADI from a, metalcasting facility standpoint is that you're offering a high strength material," said Kathy Hayrynen, technical director at Applied Process, Livonia, Mich. "You have the ability to thin down sections, which allows you to make lighter components out of a perceived heavy metal."

The ability to create lighter castings, combined with ADI's superior strength-to-weight ratio, has made the metal competitive for many jobs typically thought best for aluminum. With a 10% weight savings vs. steel, ADI also is a prime candidate for conversions from steel fabrications. Table 2 lists the mechanical properties for the five grades of ADI.

"ADI can save the customer a lot of money and increase the casting facility's profit margin," Farrar said. "It can turn components into castings that otherwise would not be possible, so it creates more casting jobs and more ductile iron jobs."

During the austempering of ductile iron, the casting first is austenitized (typically in a range of 1,600-1,660F [871-904C]). The section size and chemical composition of the casting will determine the time and temperature for austenitizing. The goal is to achieve an austenitic matrix with a uniform carbide content.

Then, the casting is quenched to the austempering temperature (typical range of 460-740F [238-393C]). The choice of temperature will determine the final properties of the casting. If higher strength or greater wear resistance is desired, a lower quench temperature should be used. The lower the temperature, the longer it will take for the ausferrite to form.

One of the most important things in offering ADI castings is to start with high quality ductile iron. Austempering will help good ductile iron achieve improved mechanical properties, but it can't cure bad ductile iron. Rather, the poor qualities of bad ductile iron likely will be magnified by the heattreating process.

To ensure a good ADI casting is achieved, the following rules of thumb can be used for quality ductile iron:

* minimum of 100/sq. mm nodule count;

* 90% or more nodularity;

* less than 1% porosity;

* maxi. of 0.5% carbides and inclusions.

Whether you decide to perform the austempering in-house or contract a heat-treating facility, incorporating that final step of heat-treating into the initial casting design is important.

"Other than producing good ductile iron, you also have to consider the cross-sections of the castings," Farrar said. "The thicker the casting, the more alloys you need to allow it to through-harden. Otherwise, the austempering will not go completely through the casting."

The metalcasting facility also must accommodate for the increased machining difficulty due to ADI's hardness. However, accommodations by the metalcasting facility can be made to alleviate the added machining labor. ADI expands during austempering, and this expansion is so consistent that it can be measured and predicted. Many facilities, like Farrar, take this expansion into account by machining the components as-cast with the knowledge that the casting will grow into its tolerances through the heat treating process.

CGI: Filling the Gap

CGI was developed in the late 1940s at about the same time as ductile iron, but while the use of ductile iron was propelled into high volume production over the next 25 years, CGI was left in the back of the closet behind the winter coats. Why? CGI was too difficult to reliably produce. Both ductile and compacted graphite iron have small windows of chemical balance to reach their proper microstructures and mechanical properties, but CGI's window is five times narrower than ductile iron and the technology to successfully measure and control CGI was not available until the late 1980s.

Now that the technology for process control is available and more casting facilities have proven that high volume production of CGI castings is possible, some customers are specifying the metal for new production ramp-ups.

Although the pure mechanical strength of CGI is not as high as that of ductile iron, the superior castability, thermal conductivity and machinability makes CGI ideally suited for complex components with simultaneous thermal and mechanical loading, such as automotive cylinder blocks and heads (Table 3). The higher strength of CGI relative to conventional gray iron has resulted in some car and truck engine components to be specified in CGI.

"As engine technology advances, conventional gray cast iron is not strong enough to meet new requirements," said Steve Dawson, president and CEO of SinterCast, Sweden. "The higher strength and stiffness of CGI allows engine designers to meet performance and emissions objectives without increasing the size or weight of the engine package. " Although CGI is used to produce castings such as bedplates, housings, brackets, flywheels, gear covers, brake drums and hydraulic valves for farm machinery, construction equipment and refrigeration equipment, nearly two-thirds of long-term CGI production is forecast for cylinder blocks, mostly for diesel engines. For this reason, the vast majority of CGI casting firms are located in Europe, where 50% of all cars on the road have diesel engines. Currently, only 3.2% of the new vehicles in the U.S. use diesel engines, but J.D. Power and Associates expect that percentage to increase to 10% by 2012. It's still a far cry from the market share in Europe, but in the larger U.S. automotive market, even a 2.5% increase could mean 50,000 tons of CGI cylinder block shipments. Plus, trends toward larger vehicles, longer driving distances and rising fuel prices are making diesel engines a more attractive choice for the American consumer. Table 4 shows the forecast of U.S. CGI casting shipments through 2015, according to Stratecasts Inc.

The preparation of CGI is similar to that of ductile iron. It involves using a low sulfur-based iron, treating the iron with magnesium and having a quality control procedure in place. The production of high quality CGI is only stable within a range of approximately +/-0.004% magnesium, and the loss of even 0.001% magnesium can cause the formation of flake-type graphite, resulting in a significant decrease in mechanical properties. However, modern process control technologies based on a sample of iron taken from the melt can allow casting engineers to check whether the proper magnesium and inoculant conditions are present.

"We have now proven that CGI can be produced in high volumes, and series production is growing," Dawson said. "In 1999, there was no CGI cylinder block production. Today, 30,000 are shipped each month. That growth will accelerate as new programs come on-stream in both the passenger vehicle and commercial vehicle sectors."

High SiMo Stands the Heat

For metalcasting facilities looking to make further progress into the automotive engine market or to take on new applications that are used at high temperatures, high SiMo is worth considering as an addition to your iron arsenal.

Wescast Industries, Brantford, Ontario, Canada, has been producing high SiMo castings since the early 1990s. Originally, the organization produced high silicon ductile iron, but gradually made the shift to high SiMo as the market's demands changed. Now, 90% of what Wescast produces is in high SiMo.

"Exhaust gases produced by modern engines are becoming hotter as they are designed to be as fuel efficient as possible while meeting stricter emission regulations," said Tony Thoma of Wescast. "These higher temperatures demand more capable alloys.

The addition of up to 1% molybdenum and 5% silicon to ductile iron, however, greatly increases high temperature tensile strength, stress-rupture strength and creep strength (Tables 5-7). High SiMo castings are cost effective when used in applications with exhaust gas temperatures between 1,200-1,600F (649-871C), which makes the material a popular choice for exhaust manifolds and turbocharger housings.

Care must be taken by the metalcasting facilty to ensure quality high SiMo ductile iron is produced. According to Thoma, this includes:

* tight chemistry control and good segregation of raw materials;

* the use of very clean materials with low-tramp elements;

* good treatment and inoculation practices

* a long cooling line to be sure the microstructure has been satisfied in the as-cast condition;

* attention to detail.

In particular, carbon levels should be kept in a tight range for a particular casting section thickness. Up to 5% silicon can be used, but increasing the silicon content provides improved oxidation resistance and increased strength at the expense of toughness and machinability. The addition of up to 1% molybdenum is normal. More molybdenum enhances high temperature strength and improves machinability, but reduces toughness and may segregate to form grainboundary carbides.

Currently, high SiMo manifolds are in high demand, but Wescast knows better than to assume it will always be that way. The casting company maintains a research and development facility that is constantly experimenting with new and improved alloys.

"We are seeking more and more exotic alloys to accommodate the higher temperatures, and not just in iron. Some of the market is being satisfied with cast steel," Thoma said. "Certain high temp applications will require cast steel. A portion of the market will be looking for an alloy that falls somewhere between high SiMo and cast stainless steel in performance and cost.

Having the flexibility of meeting different property demands with an array of varying irons can allow a metalcasting facility to meet the changing needs of its customers. Thoma said that many times the customer knows the exact material it wants used in the component, but in several cases, the product engineer sets forth design parameters without knowing which metal would be most appropriate.

"Many customers have different needs for different applications," Thoma said. "By providing the different iron alloys, we can be one-stop shopping for the customer."

In the meantime, Wescast keeps one eye on what future applications might require from its material. Utilizing a diverse lineup of alloys can make it easier for an ironcasting facility to respond to changes in market demands.

"As the automotive industry moves on, cars will need to be lighter," Thoma said. "And what is the future of the internal combustion engine? Engines are getting smaller, not bigger. We are seeing fewer V8s and V6s in Europe. It all depends on fuel cost. If fuel continues to increase, we are preparing to react to the same thing in North America.

For More Information

"CGI'S Growth Could Change Iron's Face," ECS Staff Report, Engineered Casting Solutions, Fail 2004, p. 33.

"ADI: Ideal for High-Strength, High-Wear Applications," J.R. Keough, Engineered Casting Solutions, Winter 2001, p. 42.

"High Si--Mo Ductile Iron: Views from Users and Producers," G. Weber, G. Faubert, M. Rothwell, A. Tagg and D.J. Wirth, MODERN CASTING, March 1998, p. 48.

Shannon Kruse, Associate Editor

Table 1. Forecast of Austempered Ductile
Iron Casting Shipments *

Industry                     2003   2004      2005   2006

Internal combustion engine      6      6         7      8
Farm and construction          14     20        26     33
Power transmission              2      2         3      4
Motor vehicle                  56     65        75     86
Other                          16     20        26     30
Total shipments                94    113       137    161

                                           '04-'08
Industry                     2007   2008   AGR (%)

Internal combustion engine      9     10      10.8
Farm and construction          39     44      25.7
Power transmission              5      6      24.5
Motor vehicle                 100    117      15.9
Other                          35     39      19.5
Total shipments               188    216      18.1

* ADI is forecast to reach 4.1 % of the total ductile
iron shipments, excluding pipe, in 2005 and grow to 8.1%
by 2008 to 216,000 tons, an 18.1 % annual growth rate.

Table 2. Mechanical Properties of the Five ASTM
Standard ADI Grades (ASTM 897-90)

                                              Impact
         Tensile      Yield                   Energy     Typical
        Strength    Strength    Elongation     (ft.-    Hardness
Grade    (ksi) *     (ksi) *       (%) *      lb.) **     (BHN)

1          125         80           19          75       269-321
2          150         100           7          60       302-363
3          175         125           4          45       341-444
4          200         155           1          25       388-477
5          230         185          N/A         N/A      444-555

* Minimum Values

** Un-notched Charpy bars tested at 72 +/- 7F

Table 3. Typical Mechanical Properties of CGI in
1.2-in. (30-mm) Test Bars

                                        Type

Property                       1          2         3

Brinell hardness            217-220    163-241   130-179

Min. tensile strength psi    65,000    50,000     40,000

MPa                           448        345       278

Min. yield strength          55,000    40,000     28,000
0.2% offset psi

MPa                           379        276       193

Min. elongation                1%        1%        3-5%

% pearlite                  90% min.   10-90%    10% max.

Table 4. Forecast of Compacted Graphite
Iron Casting Shipments (000 tons)

Industry                     2003   2004   2005   2006   2007

Internal combustion engine     30     34     38     42     46
Motor vehicle                  37     39     42     44     46
Others                         19     22     25     30     32
Total shipments                86     95    105    116    124
Total sales ($ million)       139    160    179    200    218

Industry                     2008   2009   2010   2011   2012

Internal combustion engine     50     54     60     54     60
Motor vehicle                  50     54     50     40     44
Others                         34     36     38     36     36
Total shipments               134    144    148    130    140
Total sales ($ million)       241    266    281    251    276

Industry                     2013   2014   2015

Internal combustion engine     60     64     66
Motor vehicle                  50     60     60
Others                         40     44     46
Total shipments               150    168    172
Total sales ($ million)       300    341    354

Table 5. Effect of Silicon and Molybdenum on the
High Temperature Tensile and Creep Rupture
Strengths of Ferritic Ductile Iron

                     Tensile Strength                 Stress Rupture
                     ksi (MPa)                        ksi (MPa)

Material                800F     1,000F     1,200F    1,000 h @
                                                         1,000F

Gray iron            37 (255)   25 (173)   12 (83)     5.9 (41)
60-40-18 D.I.        40 (276)   25 (173)   13 (90)     8.3 (57)
4% Si D.I.           56 (386)   36 (248)   13 (90)      10 (69)
4% Si - 1% Mo D.I.   61 (421)   44 (304)   19 (131)     14 (97)
4% Si - 2% Mo D.I.   65 (449)   46 (317)   20 (138)    17 (117)

Table 6. Effect of Silicon and Molybdenum on Stress-Rupture
Strength of Ferritic Ductile Irons

                             Stress to
Type          Temperature    Rupture
of Iron           (C)        MPa (ksi)

                               100 h     1,000 h

w2.2% Si           650        40 (5.8)    20 (2.9)
4% Si             650        28 (4.1)
4% Si 1% Mo       650        43 (6.2)
4% Si             705        19 (2.7)    12 (1.7)
4% Si 1% Mo       705        33 (4.8)    23 (3.3)
4% Si             815         7 (1.0)
4% Si 1% Mo       815         9 (1.3)

Table 7. Influence of Silicon and Molybdenum on the
Thermal Cycling Behavior of Ferritic Ductile Iron

Type of Iron      Temperature    Cycles to
                  Cycling (C)     Failure

2.1% Si             200-650          80
3.6% Si             200-650         173
3.6% Si 0.4% Mo     200-650         375
4.4% Si 0.2% Mo     200-650         209
4.4% Si 0.5% Mo     200-650         493