Through a combination of filter framing and ceramic technology, Foseco is introducing Stelex ZR Ultra 10 and Stelex ZR Ultra 15 to complement their other steel filtration offerings, which include Stelex ZR, PrO, Kalpur and Hollotex.
Many steel castings are sophisticated, high technology components, and are often an important part of complex assemblies and systems for a huge range of applications. The castings are used in many industries, some examples are:
• Railway – for safety critical components including brake and engine parts
• Energy – for high integrity heavy-duty safety critical castings including high pressure pumps, valves and nuclear reactor parts
• Military – for high specification castings to be used as structural parts for military platforms and safety critical components in military aircraft
The complexity and integrity of steel castings that are produced is rapidly increasing and the market demand is not expected to decline. An example is the use of stainless steel castings in the high end automotive industry for turbo charger housings and exhaust manifolds that can operate at ever increasing temperatures on high efficiency engines.
Ceramic foam filters were first used in the steel casting industry towards the late 1980s and the technology associated with steel filtration systems and application has continually improved. Now, ceramic foam filters are applied in tiny components where the smallest of inclusions will result in the scrapping of the casting, through to castings weighing in excess of 40 tons.
The filters perform two major functions that generate a wide number of benefits for the casting producer:
• Filtration: Removing the majority of non-metallic inclusions in the metal stream at the filter face; smaller particles are then trapped by a variety of chemical and physical mechanisms within the filter structure. Various investigations show that filters are in the region of 80% efficient at removing inclusions
• Flow Control: Reducing the energy within the metal stream and facilitating low turbulence as the casting cavity fills
The benefits of filtration of steel castings are well documented and include higher integrity, less scrap, reduced rectification time and costs, improved surface finish and reduced inspection requirements.
The Modern Casting “48th Census of World Casting Production” shows that the global production of steel castings is 11 million tons. Market surveys estimate that less than 8% of the global steel casting tonnage is filtered, this relates to over 10 million tons of castings not being filtered.
Foseco undertook a market study to understand why the uptake of steel filtration technology has not been more rapid and reached a higher level. There are many reasons that can be cited, including casting size (some castings are currently too big to filter), casting type (some castings can be very low integrity), geography (low labour costs making high levels of repair cost effective). It became apparent that many foundries do use filtration technology but only apply the filters to a limited proportion of the castings they produce; the reason for this includes concern that the application of the filter will cause problems and inconsistencies during the casting process, particularly in terms of mould fill. It was also noted that there is a clear casting producer demand for higher efficiency filters.
The development of Stelex ZR Ultra
There was a clear market need for a steel casting filtration system that provided more consistent performance in demanding applications, together with a desire for enhanced filtration efficiency. The result is the development and introduction of an improved zirconia based ceramic foam filter to complement the Stelex product range.
Stelex ZR Ultra is based on improved zirconia ceramic technology and incorporates a ceramic frame around the sides or the filter. This combination allows the filters to be lower in weight, while maintaining the required strength to withstand the impact and passage of molten steel. The lower weight reduces the occurrence of pore blockage within the filter structure, and facilitates the ability to supply a product with finer porosity that performs consistently. In addition, the frame and robustness of the ceramic provide a filter that exhibits superior friability characteristics and therefore reduces the potential incidence of filter related defects in the casting.
Characterisation of Stelex ZR Ultra
Foseco uses a wide range of laboratory tests to characterise and benchmark filter products. In addition, tests and evaluations are performed with molten steel in laboratory testing environments and later at customer foundries.
The new filter is available in two porosities namely Stelex ZR Ultra 10 for coarse porosity and Stelex ZR Ultra 15 for fine porosity.
Some of the tests used in the development or Stelex ZR Ultra were air flow and pressure drop. The test is used to measure the resistance of flow as air is sucked through a filter at defined speeds. The air pressure is compared above and below the filter to generate a “Pressure Drop” result. These results provide an indication of the resistance to molten metal flow that a filter will introduce.
The results show that pressure drop associated with Stelex ZR Ultra 10 is 9% lower (better) than standard Stelex ZR 10ppi, despite the Stelex ZR Ultra 10 porosity being purposefully finer to deliver a higher filtration efficiency. The pressure drop associated with Stelex ZR Ultra 15 is only marginally higher than standard Stelex ZR 10ppi indicating the filters will be suitable for application where it is required to achieve high levels of filtration efficiency with little impact of metal flow. In addition, the spread of the results are lower for the Ultra products giving confidence that more consistent flow rate performance will be experienced when they are applied to the filtration of molten steel.
Market Research identified that there is a need to improve ceramic foam filter technology for application to steel castings that will provide a more robust solution with consistent performance in application particularly associated with mould fill.
It also showed there was a desire for ceramic foam filters that would provide enhanced filtration performance over the established 10ppi products, which cause difficulties in other aspects of the casting process.
This article was published in Foseco’s Foundry Practice magazine. For a full copy of the report and more information contact Foseco South Africa on TEL: 011 903 9500 or visit www.foseco.com