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In the conversation about hot rolled vs. cold-finished products, all cold-finished products are hot rolled, but hot rolled products are not necessarily cold finished. Instead, it’s all a matter of how the bars are processed.
In a previous blog we explored the difference between MBQ (merchant bar quality) and SBQ (special bar quality). As stated, SBQ bars will go into an engineered application and most likely have multiple processes done to the metal before you get to the finished part. Let’s explore some of those processes.
Let’s start with hot rolled products, which are brought to approximate finished size by rolling at elevated temperatures, can be considered first-stage metal with minimal finishing and processing. Tolerances are wider, and the finish is not ideal for an aesthetic application.
Now, once you put that hot rolled product through a series of processes, it becomes a cold-finished product. But what makes it cold? Start with the obvious—the temperature. First, the bar is cooled at room temperature. Next, it is put through the process of annealing or temper rolling which provides the following benefits:
• Improving the dimensional tolerances
• Making it straighter
• Enhancing the surface appearance and physical properties
Cold-finished processes come in multiple flavors; let’s look at three in particular:
Cold drawing also increases torsional strength, surface hardness, wear resistance, and imparts significant improvement in machinability. In some cases, cold drawn bars may even be subjected to grinding and polishing operations.
Turning will remove the bar surface and, with it, all surface imperfections. The polishing produces a finer, brighter, smoother finish than cold drawing.
Let’s start with hot rolled products, which are brought to approximate finished size by rolling at elevated temperatures, can be considered first-stage metal with minimal finishing and processing. Tolerances are wider, and the finish is not ideal for an aesthetic application.
Now, once you put that hot rolled product through a series of processes, it becomes a cold-finished product. But what makes it cold? Start with the obvious—the temperature. First, the bar is cooled at room temperature. Next, it is put through the process of annealing or temper rolling which provides the following benefits:
• Improving the dimensional tolerances
• Making it straighter
• Enhancing the surface appearance and physical properties
Cold-finished processes come in multiple flavors; let’s look at three in particular:
1. Cold drawing
This refers to the process of pulling a hot rolled bar through a die of lesser size. Cold drawing gives a hot rolled bar a bright, clean, uniform surface and increases tensile and yield strength.Cold drawing also increases torsional strength, surface hardness, wear resistance, and imparts significant improvement in machinability. In some cases, cold drawn bars may even be subjected to grinding and polishing operations.
2. Turning and polishing
This is the process of machining the bar to the desired size on a lathe or turning machine, followed by a polishing process via rotating the bar through hardened steel rolls.Turning will remove the bar surface and, with it, all surface imperfections. The polishing produces a finer, brighter, smoother finish than cold drawing.
3. Grinding
Some bars will endure this process where they are ground to close tolerance in centerless grinders and kept very straight during the operation. They are then saw-cut on both ends to give a square, true cut, and burnished to a brilliant, bright finish. This results in bars with extremely close size tolerance, a high degree of straightness and a superior finish.
First, choose the carbon bar that is right for your application.
In the end, the differences between hot and cold products aren’t as cut and dry as the temperature at which they are rolled. It all comes down to how that bar is treated, as well as the toughness, normalization and the specific values you need for the application of the finished product.
