Since “The Big Twist” failure that killed multiple people, an investigation of the wheel and rail deviations from the derailed car has been carried out in order to determine what course of action should be taken to prevent similar catastrophes from occurring. In order to achieve safety, the rails of the newly designed car were supposed to be composed of 4130 steel. A heat treatment process involving a water quench and tempering was also required to modify the rail to a desired balance between ductility and hardness.
The final Rockwell C-scale hardness was supposed to be 35-40.
The failed rail was found to have a Rockwell C hardness of only 4. 8. In our own reference experiment, we were able to produce specimens similar to the failed specimen, with a Rockwell C hardness of 3. 7. Our similar sample was austenized and air cooled, which leads to the conclusion that the failed rail was not quenched or tempered at all, but merely annealed and left to cool.
The resulting pearlite crystal structure of the material was soft and ductile, bending easily under stress and causing the car to derail and plummet to the ground.
A new roller coaster in Florida called “The Big Twist” killed four people when one of the cars separated from the track and fell to the ground 120 feet below. It was discovered that the outer left wheel assembly was bent more than 30 degrees from its normal vertical orientation due to twisting of the support beam that connected the wheels to the car.
Our research was performed to discover why the rail was weak enough to bend. The characteristics of steel can be greatly modified by changing the phase (or crystal structure).
The first step in heat treating steel is the process of annealing and austenizing. When the steel is heated to a high enough temperature (annealing), carbon is dissolved into the metal solution (austenizing) by fitting into the interstitial vacancies in the high temperature-induced face-centered cubic structure of iron (austenite). The next step in heat treating is to quench the steel in water or oil, quickly cooling the steel to room temperature in order to trap the carbon in the crystal structure (even at the reduced temperature).
This new phase is called martensite, intermediate to the face-centered and body-centered cubic structures. The internal stress caused by the distorted crystal structure causes the metal to be extremely hard and brittle. Finally, the heat treating is completed by heating the material once again (tempering) to gain a balance between strength and ductility. During tempering, nucleation of cementite occurs along with a growth in grain size, both reducing internal stress and restoring ductility. The resulting metal can be both harder and stronger than it can be without treatment.
Essentially, if the metal contains too much pearlite (it is only annealed or tempered too long or hot), it will be too ductile and soft to withstand the forces on the rail of the roller-coaster car, for example. Conversely, if the metal contains too much untempered martensite (it is only annealed and quenched without a tempering), it will be extremely hard but utterly brittle. Experimental Procedures and Results Using seven samples of 4130 steel as references, it was possible to determine what was wrong with the received sample of roller coaster rail with comparative analysis.
Seven reference specimens were placed in an austenitizing furnace at 844°C for one hour. After austenitizing, six of the samples were immediately quenched in water at room temperature and four of them were placed in tempering furnaces at 205°C, 370°C, 482°C, and 677°C for one hour. The other two were left at room temperature. The samples were next sanded with abrasive paper to remove surface discoloration and tested for Rockwell C-scale hardness, with three tests each to be averaged. After the hardness tests were completed, Charpy Impact Tests were performed on each specimen. A table of results can be seen below.