Current Conducting Electrode Arms (CCEA) are what conduce current into a furnace. These arms create the arc that melts the steel scrap in a furnace and may be considered the most vital aspect of any running EAF mill. CCEA are used in groups of 3 electrodes that are triangulated very specific distances apart in order to create the arc to melt scrap. This actual process can be a topic of its own blog, but here we want to focus on the importance of the bolting on the arms of the CCEA. Specifically, each arm is held by a total of three bolts (nine for the whole CCEA).
Any sway or movement in the electrodes left or right during this melting process will cause an irregular arc and lead to undesirable results. If the bolting snaps or pops out, results could be devastating. This is why such a large emphasis on the mathematics and specifics of the material, yield strength, and tensile strength of these bolts.
The biggest threats to these bolts are the dynamic pressures that get placed on these lynchpins of the CCEA. One example to simply explain dynamic pressure would be the handle of a bicycle. The most often cause of a bicycle falling apart, or malfunctioning, is with the handle of the bike. From constant extrinsic pressure (turning the handle, putting pressure forward or backward when riding in the hills), the bolts holding the handle steady to the base can either loosen or become stressed to the point of failure. This is also the case with items such as chairs or household stools.
For an EAF, the arcs taking place in the furnace causing the electrodes to wiggle up and down, pushing this dynamic pressure upwards towards the bolts. Static bolts have no reason to have a structural failure but when there are ever-changing pressures placed on the bolt, it can become stressed past elastic deformation and into plastic deformation.
Elastic deformation is the point of pressure where the material receiving this force can withstand the amount of force being applied and return to its original form. Plastic deformation is the point at which the material being dealt the force is stressed to the point where it does not return to its original form, causing a break/fracture/failure.
A couple of key terms relating to these deformations are Yield Strength (YS) and Ultimate Tensile Strength (UTS). Yield strength is the point right before the deformation of an item goes from elastic deformation to plastic deformation. UTS is the maximum stress that a material can withstand while being stretched. Taking these numbers into account when deciding on the bolting for CCEA is very important. Detailed below is a list of some common materials with their YS and UTS levels
This information is useless without any meaning or context provided to it. The goal of these studies is to determine how much stress needs to be put on a bolt to reach the desired yield strength to where the dynamic pressure of the arcs does not rattle the electrodes to cause any failures in the bolts or irregular flashes. Determining this pressure requires a little bit of math.
First, you must determine to take the surface area of the bolt at the weakest point of the bolt. At the threads, the diameter of the point is the smallest, causing this to be the point of failure during any stress past its yield point. For this reason, the surface area of this point must be figured into the equation. For this example, a 2-1/2” bolt will be used. Diameter of a 2-1/2” bolt at the threads is 2.1752”.
Surface Area (SA) = πr2
SA = π(1.08762)
SA = 3.7161075145401362256028835983189 Square Inches
We try to use at a minimum of 5 decimal places but will go to as many decimals as are provided in the calculation. Next, we must determine the amount of pressure (pounds) that a given material can hold until reaching its yield strength. This is found by simply multiplying the Surface area by the Yield Strength. We will use the 4140 material for this.
Pressure to Reach YS = 3.7161075145401362256028835983189(60,200)
Pressure to Reach YS = 230,398.6659014884459873787830 Pounds
This is the total amount of pounds of pressure that a 2-1/2” bolt made of 4140 material can hold before reaching a plastic deformation. When working on a CCEA job, the percentage of yield strength desired must be determined. In most cases, we like to stay at or around the 70% yield strength point. The number that is provided from this formula gives us the amount of pressure that the CCEA bolts must be treated to prior to installation.
Req. Treatment Pressure = 230,398.6659014884459873787830 (.70)
Req. Treatment Pressure = 161,279.06613104191219116514816704 Pounds
Bolts must be stressed with 161,280 (we will round-up for this one) pounds prior to installation on the CCEA. Since three bolts are used on each arm, 483,840 pounds of force must be sent upwards from the electrodes to make any significant movement.
Determining what each job requires in regard to these pressures takes time and discussions between engineers and the customer, but it is extremely important to the functioning of an EAF. If the wrong material is used or improper yield strength is chosen, there could be a massive failure in the CCEA.