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Cast Steel Foam for Lightweight Strength

Hathibelagal Roshan
Click here to see this story as it appears in MODERN CASTING.

Steel plates and billets are manufactured as wrought products and have been used in armored and civilian applications for several decades due to their ease of manufacture and low cost. But Maynard Steel Casting Company is developing the innovation of steel plates and billets via a casting process that could break the monopoly of wrought metal and provide an opportunity for the casting industry to enter a market that was not previously accessible. 

Designers have a strong need for lightweight steel components in armored vehicles, naval structures, automobiles and civilian structures.  However, the availability of such a material on an industrial scale has been elusive for several decades.  Variable density lightweight steel using a sand casting process was recently developed at Maynard Steel after the facility looked at the latest developments in research and thought it could complete the puzzle, so to speak. 

The lightweight steel is available in the form of steel panels and billets in low-alloy steels with hardness ranging from 100-400 BHN.  Components made using this material (patent pending) could eventually be used in armored vehicles, naval ships and protective panels for architectural applications.  This material also has potential applications in the automotive industry in the form of lightweight pistons.

Metal foams are metals with pores deliberately incorporated in them. Introducing voids in metal reduces density and increases the apparent thickness.  Designing components with metallic foams result in higher plate bending stiffness and lower weight. Foam materials have been made from base materials such as polymers, ceramics and some metals such as aluminum and titanium. Aluminum and titanium foams are becoming popular in the aerospace and automotive industries.  Steel is one of the most widely used engineering materials by the designers, and yet there is no commercially available foam using steel as the base metal. As a result, applications of steel foams in civil structures have not yet been demonstrated.

This opens the opportunity for material with properties that are superior to solid steels in selected applications. Maynard Steel has been exploring cost effective steel foam and now can cast the material with consistent densities with predictable properties with hardness values ranging from 100-400 BHN. The properties could be attractive for ballistic-resistant applications not only in military structures but also in civil engineering structures such as buildings and bridges. Maynard Steel hopes the innovation opens a new area of research for material scientists to carry out material characterization and property determination with the availability of test material of any desired density and properties with a hardness range of 100-400 BHN.

Before Maynard Steel performs further testing to definitively determine the process’ feasibility, it is looking for a partner to focus its efforts. As of now, the product is not considered ready for the market.

Manufacture of Variable Density Lightweight Steel Foam
At Maynard Steel, steel foam is made using nobake sand molds and 3-D printed cores of proprietary design to incorporate the desired porous structure in the steel castings. The method includes the steps of: providing a 3-D printed sand core bonded with a furan nobake binder system, placing the 3-D printed sand core in a sand mold bonded with ester-cured nobake binder system, providing a suitable gating system for entry of molten steel into the mold, de-coring the casting after shakeout, and providing suitable heat treatment after the casting is separated from the gating system.

The manufacture of steel foam plates and components with consistent desired densities and predictable properties on an industrial scale with a low-cost conventional sand casting process is unique.  Maynard Steel envisions the new process could change how future designs of steel structures will be made giving the freedom of the important variable density. The metalcaster has successfully made steel plates and billets in plain carbon and low-alloy steels.  The manufacturing process is standardized to achieve desired porosity consistently.  The arrangement of cores in the molds is adjusted to achieve variable density steel plates with one side being solid and backed with porous steel foam.

Researching the Feasibility
Materials research is focused on fundamental research to achieve lasting strategic land power dominance through structural materials, ballistic materials, and manufacturing science, processing and sustainment. Structural materials focus on ballistic materials to enhance the performance and efficiency of army weapons and protection systems including lightweight, extreme performance materials, and energy absorbing materials.  Manufacturing science, processing and sustainment is focused on discovery, innovation, and maturation of manufacturing innovations and understanding material properties and degradation mechanisms to improve durability of army systems in extreme environments.

Porous metals made of aluminum and titanium are becoming increasingly popular as stiff but lightweight materials for use in structural components of automobiles and aircraft.  However, engineering applications require stronger and more economical materials than an aluminum or titanium foam can provide. Traditional structural steel has proven valuable as an engineering material, but the properties of structural steel have remained invariant for the past century.

Research conducted over approximately the last 10-15 years has shown that it is possible to fabricate steel foams at the laboratory scale and these foams can be made to have potentially desirable mechanical properties.  Despite these substantial advances in the materials science of steel foams, a commercially available product remains elusive, and therefore structural designers have not begun to explore the potential benefits of using steel foam in civil structural applications.

Perhaps because of the lack of commercial availability of steel foam, no applications have been developed or widely implemented. In the past few years, interest in metal foams, especially in steel foams, has increased remarkably. The reason for this is the wide range of applicability such as lightweight panels, crash energy absorption, exhaust mufflers, vibration and noise control in the automotive industry, filters, heat exchangers, high strength wall panels for sound insulation and bio-medical implants.  Steel foams exhibit excellent stiffness-to-weight ratios when loaded in flexure. In particular, steel foam panels have higher bending stiffness than solid steel sheets of the same weight.

The basic objective of the development process of metallic foams is the combination of physical and mechanical properties expected from metallic foams such as high stiffness, low specific weight, high gas permeability, low thermal conductivity, unusual acoustic properties, high impact absorption capacity and good electrical insulating properties.  

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