Fire-Resistant Steel Made to Order Using Thermodynamic Simulation

On November 3, One World Trade Center opened for business. At 104 stories, it's the tallest building in New York City, and the third tallest in the US. (If the height of the antenna on top is counted, it could actually be considered the tallest). Under construction since 2006 at a cost of $3.9 billion, the building's completion has helped to bring closure to New Yorkers after the destruction of the original World Trade Center in the terrorist attacks of September 11, 2001.

According to a study by the National Institute of Standards and Technology (NIST), one of the factors in the collapse of the original World Trade Center towers was the reduction in the yield strength of the steel reinforcement as a result of the high temperatures of the fire and the loss of thermal insulation. According to most US building codes, steel used in construction must maintain at least 50% of its room-temperature yield strength at 1,000F. For example, if the room-temperature yield strength is 36,000 psi, then the yield strength at 1,000F must be at least 18,000 psi. Based on the NIST study, the steel used in the original World Trade Center met this requirement, but did not greatly exceed it.

Although the reduction in the yield strength of the steel was only one of several factors leading to the buildings' collapse, in the aftermath of the attacks, NIST and the American Society for Testing and Materials (ASTM) began to discuss a higher standard for fire-resistant steels. Could a cost-effective steel, with a room-temperature yield strength of at least 50,000 psi, be developed that would be able to maintain two-thirds of that strength for a minimum of 20 minutes at 1,100F?

Researchers at Northwestern University have been working on this topic. Dr. Yip-Wah Chung presented the team's latest findings at the October 31 meeting of Northwestern's Center for Surface Engineering and Technology.

Heat-resistant steels are nothing new. For example, some austenitic stainless steels can withstand continuous use temperatures as high as 1,650F. However, these highly alloyed steels are far too expensive to use as a structural material in building construction. The group decided from the outset of the project that, in order to keep the new fire-resistant steel affordable, the level of costly alloying elements such as chromium, niobium, and vanadium needed to be kept below 0.55%. Furthermore, in order to ensure that the new material would be weldable, they decided to keep the carbon content below 0.1%. In addition, they decided that the material must not require heat treatment (such as quenching and tempering) in order to achieve its properties.

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In the old days, developing a new steel would require producing sample runs of a large number of candidate chemistries, then testing them, and refining the formula based on the results. Instead, the Northwestern team used Thermo-Calc software to predict the phase diagrams of possible steel compositions. They knew that the high-temperature strength would depend on microscopic alloy carbides with crystal structures closely resembling that of iron. In order to achieve this, they needed the alloying elements (chromium, molybdenum, niobium, and vanadium) to combine with the carbon in the proper way to form these carbides. At the same time, they needed to prevent the formation of other carbides with crystal structures that don't match iron very well, which could reduce the strength of the steel.

Using Thermo-Calc, the researchers identified a composition that would maximize the formation of desirable carbides, while eliminating the undesirable carbides. They then asked specialty alloy manufacturer Sophisticated Alloys to produce a sample. The new alloy, called FR-8, exceeded the project's original goal. Not only was it able to maintain 70% of its room-temperature yield strength after 20 minutes at 1,100F, but longer exposure actually made it stronger. After two hours at 1,100F, the yield strength increased to 80% of the room-temperature value.

The Northwestern University group is continuing to work to further develop this alloy. They also hope to use the same approach to develop improved high-temperature steels for turbine blades. If turbines could operate at higher temperatures, they would be more efficient. This could reduce emissions and save energy resources on a global scale.

This work is just one example of integrated computational materials engineering (ICME), an approach in which computational tools that operate at different length scales are used to design better materials. Earlier this year, NIST provided a $25 million grant to establish the new Center for Hierarchical Materials Design. This center -- a partnership between Northwestern, the University of Chicago, Fayetteville State University, Argonne National Laboratory, ASM International, and QuesTek Innovations -- will combine the latest computational methods with high-tech experimental techniques. It is part of President Obama's ambitious Materials Genome Initiative, which seeks to dramatically increase the pace of materials innovation. New materials have the potential to improve our quality of life, whether by preventing buildings from collapsing or by increasing the efficiency of our power plants. Here's hoping that NIST's $25 million investment will lead to many such material advances.