CASTINGS - SAND AND SHELL
Out with the old and in with the new: it's an old adage which one wouldn't normally associate with the foundry business. I am going to use it, however, to describe the process of rethinking material specifications-- because, in the simplest terms, that is exactly what occurs during this particular process. Currently, many casting customers are spending countless dollars needlessly on manufacturing castings from old specs, when these castings could be made, at a considerably lower cost, of newer, more efficient materials. We at Taylor & Fenn encourage casting users to rethink the material specifications of their older castings and consider updating these specs to take advantage of today's newer, more cost-efficient materials.
If a casting user were to examine the specs of castings designed before 1950, one material would likely appear over and over again: carbon steel. It was the undisputed material of choice for decades. What many customers do not know is that there is now a replacement material that offers considerable advantages: ductile iron. Developed in the 1950's it has proven to be a less costly replacement for carbon steel, offering a 40 percent cost reduction and improved casting structure. In a similar improvement, ductile Ni-resist (austenitic ductile iron) now offers casting customers further savings as a replacement for stainless steel and bronze.
Why, one may ask, haven't more casting customers considered respecing their old castings sooner? I blame it on the tendency to follow an "If it ain't broke, don't fix it" attitude. In less sensitive economic times, this attitude matters little. However, it's an attitude that many casting customers can no longer afford. Consequently, the cost savings resulting from a "respecing session" look more and more attractive.
Manufacturers should begin the process of rethinking material specs by contacting a foundry for a respecing session. A session attended by the customer, pattern and materials engineers, metallurgist, and foundry representatives would involve a detailed discussion of the job, an analysis of all components involved, and an exploration of alternative materials.
It is very important for the foundry metallurgist to work closely with the customer so the component functions and the variables that affect performance can be thoroughly analyzed to arrive at the most cost-efficient solution to material conversion. Taylor & Fenn usually suggests at least two materials which would be appropriate replacements so the customer can weigh the cost and performance factors of both to arrive at a decision.
From the point-of-view of value analysis, challenging existing casting requirements provides customers with a way to get the cost out of their products by converting to materials that deliver maximum cost and performance efficiency. Today's newer metals and alloys have performance and cost benefits over traditional materials such as carbon and stainless steel. Lower melting points, easier casting cleaning and finishing, and better corrosion resistance are just a few of those benefits.
Manufacturers are also encouraged to include weldments and steel forgings in the respecing process. When weldments are converted to castings, the manufacturer eliminates weld inspection costs and gains a stronger component. In addition, forgings can be replaced with castings that are equally strong and less expensive.
To illustrate the benefits that can be gained from rethinking material specs, I'll focus on a respecing project Taylor & Fenn performed for the J. M. Huber Company, a clay benification mining operation located in Macon, Georgia.
J. M. Huber was using a hardsurfaced carbon steel blade to mix clay. Operating in an extremely abrasive environment, the blade had to be replaced every 5 to 7 days. A Taylor & Fenn representative approached the company with the idea of respecing the blade. After extensive analysis of the job requirements, stress factors, and available materials, Taylor & Fenn engineers suggested a blade made of a high chrome-content iron (white iron). The resulting new blade, operating under the same conditions, lasts 6 times longer.
According to Bob Brown, an engineer with the company, the respecing project resulted in substantial savings for J. M. Huber, by eliminating the frequent maintenance and down time caused by the old blade.
As the J. M. Huber example shows, it is important for a foundry to be aware of the metallurgical and performance characteristics of new materials. However, manufacturers should also he familiar with the capabilities of these alternatives. To that end, I will discuss the three metals which are commonly used to replace old materials: ductile iron, ductile Ni-resist and austempered ductile iron.
This metal is a common replacement for carbon steel and offers both cost and performance advantages.
- Because ductile iron has a lower melting point, it is far cheaper to process than carbon steel. And because ductile iron is poured at a lower temperature, it requires less expensive molding material than steel, whose molding materials must endure higher temperatures.
- The cost of cleaning a ductile iron casting is less than the cost of cleaning a carbon steel casting.
- Carbon steel-risers have to be torched off; ductile iron risers can be knocked off with the blow of a hammer.
- In strength comparisons, ductile iron and carbon steel are comparable; ductile iron, however, offers greater corrosion resistance, and is generally more reliable and durable than carbon steel. There are still cases where carbon steel may be preferred. If a considerable amount of welding is going to be applied to the casting, or if two ductile iron components require welding, a manufacturer should stick with carbon steel because of its easier weld characteristics.
This metal is often used to replace both carbon and stainless steel and bronze. It offers the following advantages:
- Because of its high nickel and chromium content, ductile Ni-resist offers greater corrosion resistance than carbon and stainless steel, bronze, and equal if not better corrosion resistance than stainless steel and bronze in certain applications.
- Ductile Ni-resist is a very stable metal; it isn't affected by extreme changes in ambient or operating temperatures.
The newest member of the ductile iron family can be used to replace traditional steel forgings or high-strength iron castings, and offers the following benefits:
- Austempered ductile iron has a bainitic microstructure which provides it with extremely high strength and durability.
- It is less expensive to machine than steel.
- Austempered ductile iron is much stronger than regular ductile iron. Because it resists metal fatigue, it makes an excellent choice for components that must operate under harsh conditions.
The following project involved a situation in which a company, which had already determined the appropriate metal for respecing, still needed metallurgical assistance. The company was Olmsted products, a manufacturer of high-capacity and custom hydraulic valves located in Traverse City, Michigan.
Olmsted Products manufactures hydraulic valves for use in North Sea drilling operations, one of the world's most hostile working environments. To cope with the extremes of temperature at the work sites, the valves required high impact at sub-zero temperatures. To meet these requirements, steel castings were utilized. The inspection and weld repairs, which required difficult logistic arrangements, were costing Olmsted Products thousands of dollars each year and had to be eliminated.
Olmsted decided that ductile Ni-resist would be the perfect substitute for steel.
With its high manganese content, ductile Ni-resist is a very high-impact material at sub-zero temperatures. After evaluating Taylor & Fenn's personnel and facilities, Olmsted contracted for production of eight valve bodies.
After reviewing the application of the valve bodies, Taylor & Fenn agreed that ductile Ni-resist would be the best replacement material. Through extensive testing, the foundry helped Olmsted meet the rugged qualifications set by Det-Norske-Veritas Inspection Agency, which inspects procedures and equipment associated with North Sea drilling operations.
According to Tom Fink, Olmsted product sales manager, the company is pleased with the results of the respecing and has signed a contract for eight more valve bodies. "By simply eliminating the weld-repair and inspection process, we've saved close to 5 to 10 thousand dollars per project," he said.
When considering the job of rethinking material specifications, manufacturers should take care to deal with foundries that are technically and physically able to carry out the kinds of changes necessary. We have established a 7 Step Procedure for manufacturers to follow when evaluating individual foundries:
- Customer Orientation. Customers should deal with a foundry that is dedicated to meeting their needs without compromise.
- Professional Affiliations. These associations are excellent sources for information about industry developments. Is the foundry a member of the American Foundrymen's Society, the Ductile Iron Society, and the Steel Foundry Society of America?
- Emphasis on Statistical Process Control (SPC). A foundry employing SPC can produce castings of consistently better quality by regulating the processes of melting, molding, mold sand preparation, core sand preparation, and heat treatment of all metals and alloys.
- Competent Professionals--in Metallurgy, Engineering and Design. A manufacturer should first visit the foundry to meet the personnel and review their credentials. A qualified, degreed metallurgist should be on site.
- Financial Stability. When selecting a foundry for a respecing project, manufacturers should be satisfied that they are dealing with a financially sound company. Request a full financial statement and annual report.
- Foundry Equipment. Does the foundry have a fully-equipped laboratory to ensure quality casting? Does it employ a variety of testing procedures including X-ray spectrography, LECO carbon determination, dimension checking, microstructure analysis, and calibration checks? Testing the materials for hardness, tensile and yield strengths assures adherence to customer specifications.
- Foundry Capabilities. A manufacturer should review a foundry's metal molding and casting capabilities to determine if it will be able to perform a respecing service.
This 7 step procedure should serve as a guide for manufacturers in choosing a foundry to perform respecing services. Not only should a foundry be able to deliver all required services of respecing--technical and engineering advice, metallurgical assistance, full lab facilities and total casting production capabilities--but the foundry should also be concerned that each casting customer gets the maximum value.
Manufacturers who respec old castings with today's new metals and alloys can look forward to significant cost savings and improved casting durability and performance. And with today's increased pressure for cost reduction, it's a change that few manufacturers can afford not to make.
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