Science Buddies: "Ask an Expert"

Typically steel is heat treated in a multi-step process. First it is heated to create a solid solution of iron and carbon in a process called austenizing. Austenizing is followed by quenching to produce a martensitic microstructure. The steel is then tempered by heating between the ranges of 150°C-260°C (300°F-500°F) and 370°C-650°C (700°F-1200°F). Tempering in the range of 260°C-370°C (500°F-700°F) is sometimes avoided to reduce temper brittling. The steel is held at that temperature until the carbon trapped in the martensite diffuses to produce a chemical composition with the potential to create either bainite or pearlite (a crystal structure formed from a mixture of ferrite and cementite). It should be noted that when producing a truly bainitic or pearlitic steel the steel must be once again taken up to the austenite region (austenizing) and cooled slowly to a controlled temperature before being fully quenched to a low temperature. In bainitic steels, upper bainite or lower bainite may form depending on the length and temperature of the tempering process. It is thermodynamically improbable that the martensite will be totally converted during tempering, so a mixture of martensite, bainite, ferrite and cementite is often formed

http://en.wikipedia.org/wiki/Martensite Martensite is not shown in the equilibrium phase diagram of the iron-carbon system because it is a metastable phase, the kinetic product of rapid cooling of steel containing sufficient carbon. Since chemical processes (the attainment of equilibrium) accelerate at higher temperature, martensite is easily destroyed by the application of heat. This process is called tempering. In some alloys, the effect is reduced by adding elements such as tungsten that interfere with cementite nucleation, but, more often than not, the phenomenon is exploited instead. Since quenching can be difficult to control, many steels are quenched to produce an overabundance of martensite, then tempered to gradually reduce its concentration until the right structure for the intended application is achieved. Too much martensite leaves steel brittle, too little leaves it soft.

Hardness is a function of the Carbon content of the steel. Hardening of a steel requires a change in structure from the body-centered cubic structure found at room temperature to the face-centered cubic structure found in the Austenitic region. The steel is heated to Autenitic region. When suddenly quenched, the Martensite is formed. This is a very strong and brittle structure. When slowly quenched it would form Austenite and Pearlite which is a partly hard and partly soft structure. When the cooling rate is extremely slow then it would be mostly Pearlite which is extremely soft. Hardenability, which is a measure of the depth of full hardness achieved, is related to the type and amount of alloying elements. Different alloys, which have the same amount of Carbon content, will achieve the same amount of maximum hardness; however, the depth of full hardness will vary with the different alloys. The reason to alloy steels is not to increase their strength, but increase their hardenability — the ease with which full hardness can be achieved throughout the material.
Usually when hot steel is quenched, most of the cooling happens at the surface, as does the hardening. This propagates into the depth of the material. Alloying helps in the hardening and by determining the right alloy one can achieve the desired properties for the particular application.
Such alloying also helps in reducing the need for a rapid quench cooling — thereby eliminate distortions and potential cracking. In addition, thick sections can be hardened fully.

the cross section of the beam affects its strength in a manner depending upon the moment of inertia and the depth of the cross section, both measured from the neutral axis. For any given cross section these quantities are constant, and hence do no enter into the diagrams presented in this book since each standard shape is represented by a separate diagram, or, in the case of spandrel or grillage beams, each shape is represented by a line on the diagram

the failure of the steel was due to two factors: loss of strength due to the temperature of the fire, and loss of structural integrity due to distortion of the steel from the non-uniform temperatures in the fire.

It is known that structural steel begins to soften around 425°C and loses about half of its strength at 650°C.4 This is why steel is stress relieved in this temperature range. But even a 50% loss of strength is still insufficient, by itself, to explain the WTC collapse. It was noted above that the wind load controlled the design allowables. The WTC, on this low-wind day, was likely not stressed more than a third of the design allowable, which is roughly one-fifth of the yield strength of the steel. Even with its strength halved, the steel could still support two to three times the stresses imposed by a 650°C fire

The perimeter tube design of the WTC was highly redundant. It survived the loss of several exterior columns due to aircraft impact, but the ensuing fire led to other steel failures. Many structural engineers believe that the weak points—the limiting factors on design allowables—were the angle clips that held the floor joists between the columns on the perimeter wall and the core structure (see Figure 5). With a 700 Pa floor design allowable, each floor should have been able to support approximately 1,300 t beyond its own weight. The total weight of each tower was about 500,000 t.

As the joists on one or two of the most heavily burned floors gave way and the outer box columns began to bow outward, the floors above them also fell.

Prestressed steel reinfoced concrete beams rely primarily on the concrete for strength. The rebar (steel) embedded in concrete is normally only used to prevent fractures in the concrete from causing catestrophic failures. You might want to think about how cylindrical shaped wooden beads strung on a taut string through their middle will resist bending.

How will I know what category/phase of steel the steel wires will be (pearlmite, cementite, martensite)? Do they sell #9 steel wire at places like Lowe's or Home Depot?

How would I exert a controlled force on the steel, and how long should I wait after exposing it to heat? How do i measure the force? What is the maxium temperature of a bunsen burner?

then I guess my experiment will have no purpose?

Also why would the beads bend if it is on a straight string (sorry if i misinterpreted that)?

does that mean they abosorb heat, or does this have nothing to do with heat?

What if you put that metal thing with mesh on top of a bunsen burner; would that give the same effect as a Meeker?

I'm also beginning to think that since this project no longer has a purpose (Prestressed steel reinfoced concrete beams rely primarily on the concrete for strength. The rebar (steel) embedded in concrete is normally only used to prevent fractures).
if this project is not doable, I am sorry for wasting your time.

After the World Trade Center collapsed, I started to think, "Why did a building made of steel, the strongest alloy, collapse?" I heard rumors that the steel of the twin towers melted, causing the collapse. Of course, I'll find out later why this is just not true. I have also found out that in the process of my research that the World Trade Center towers were designed as new high-rise towers (after WWII). This meant that these towers used more steel and shaped lightweight steel into tubes, curves, and angles to increase its load bearing capability. However, high rise designs reduce the number of concrete used significantly (compared pre WWII buildings such as the empire state building). Many people believe that since the steel columns supporting the towers weren't encased in concrete, the fire resistance was not as good as it could have been, causing the distortion and loss of strength, ultimately resulting in the building falling in just one to two hours. From this came my research question; is concrete encased steel more fire resistant than steel by itself?

First of all, I had to find out more on steel and what it was. Steel is heat treated in a multi-step process. The first phase of steel, austenite (created by heating iron and carbon), is quenched and in the process transforms into martensite. Martensite is strong but is very brittle which is why it is tempered (heated to diffuse out the carbon) to transform the alloy into having a different structure. Basically the concentration of martensite is reduced until it leaves the right structure (which depends on what the steel is to be used for). Too much martensite leaves steel too brittle, too little leaves it soft.

Now that I understand more about how steel is created, I'll need to find out more on its strength. Apparently, the tensile strength of steel can withstand 40,000 pounds of force per square inch, or PSI. Concrete on the other hand, can withstand between 3,000 to 10,000 PSI, depending on the mixture.

Structural steel doesn't melt until about 1510°C. Thus, this makes the rumor of the steel melting false. But structural steel does begins to distort and soften around 425°C and loses about half its strength at 650°C. However, Leslie Robertson, a structural engineer who had helped built the WTC towers, has said that steel and concrete at various temperatures showed that at the heat levels experienced by the towers, the two materials become similarly weak. Many people however think that the more mass there is, the more fire resistance there will be.

It is said that the failure of the steel in the World Trade Center towers was due to two factors. These were the loss of structural integrity due to the distortion of the steel from the non-uniform temperatures in the fire, and the loss of strength due to the temperatures of fire. If there was only one factor, the building would have stayed up much longer. Thus, I will test whether or not concrete encased steel will have more strength when exposed to heat than compared to steel alone. This experiment will be conducted on a much smaller scale than the world trade center of course. I am planning on using steel wires, a Bunsen burner, and weights (to act as forces on the heated steel). This topic is important because if it is found out that concrete encased steel is more fire resistant, then building architects should be advised to encase their steel columns supporting the buildings with steel to increase the chances of the survival of the people inside.

Wouldn't the concrete, regardless of whether or not it is exposed to heat, increase the strength of the steel wire anyways?