In the world of bar products, there are different ways to manufacture bars and some different things to consider when purchasing bar. Often lead time, quantity, and quality are what drive decisions when purchasing bar. The end-use application can also play a significant role and can range from a product that utilizes the full length of a 20-foot bar to portions of the bar cut and machined to a unique geometry. In this blog, we discuss the options that can provide both high and low quantity options for buyers - Hot Rolled Bar, Cold Rolled Bar, and Forged Bar. (Please note - There is another process which pushes molten bar through a die called hot-drawing or extruding. This method is not ideal for low quantity purchases.)
So What is Hot Rolled, Cold Rolled and Forged Bar?
Rolling and forging bar boils down to the same fundamental process of reducing the thickness or changing the cross-sectional area of metal by compressive forces. In other words, a large piece of metal known as a billet is pressed down to the size, shape, and length required for a project. This process not only gives the bar shape, but it also adds properties of strength to the bar.
What is Hot Rolled Bar?
This is the process of heating a billet to more than 1000 degrees Fahrenheit, above the recrystallizing phase of metal so that it’s workable, then rolling it through a planishing mill or rollers to give it its shape. Heating the metal is vital because changing the molecular structure of the metal form dendrites (thread-like sporadic structures) to grains (organized block-like structures), which only happens when metal is in a malleable state. The creation of stronger metal is not only due to the formation of the grains, but the directional grain flow achieved when the bar is rolled.
Pros & Cons Hot Rolled Bar
Hot rolled bar is quick and creates single or multiple pieces easily. Unfortunately, where hot rolled bar falls short is with dimensional tolerances. When the metal cools, it contracts and that leads to the likelihood of warping, areas of varied thickness, and a scaled finish, which means machining is required if dimensional precision or finish type is essential. Lastly, with hot rolled bar, timing is everything. If the rolling cycle is missed lead time is drawn out since additional bar wouldn’t be produced until the next mill progression.
What is Cold Rolled Bar?
This process takes everything we just discussed about Hot Rolled Bar and adds a step. When the bar reaches near-room temperature, it is drawn through dies or rerolled in a progressive rolling process. This additional step inherently adds strain hardening to the bar because it is worked after recrystallization. Strain hardening can only harden the metal so much, the bulk of hardening is achieved during the molecular restructuring provided by hot rolling, depending on the grade.
Pros & Cons Cold Rolled Bar
Cold rolled bar has better dimensions, straightness, and increased yield strength. It, again, is used for single or multiple piece requirements. But, with the extra step or rerolling, cold rolled bar is time-consuming and can extend lead times. Unlike a hot rolled bar, however, additional machining for precision isn't required, and the finish is not as scaled.
What is Forged Bar?
This process, like a hot rolled bar, takes a billet and heats it until it is malleable. This is where the processes differ, rather than rolling the bar to get it to size, forging either hammers or presses the bar to the rough dimensions required. The operation gives the bar a 3:1 reduction ratio minimum, which means that the grain sizes are much smaller and tighter in a forged bar, and it consolidates the centers if there are any piping issues. Then, depending on the diameter of the bar, the bar is put through a planishing mill giving it a smooth, rounded surface.
Pros & Cons Forged Bar
Forged bar process takes the best of hot rolled and cold rolled bar and achieves it quickly and efficiently. These bars maintain dimensions and straightness as they cool while having an even higher yield strength because of forgings' unique capability of producing sound-centers during the forging step. The 3:1 minimum reduction helps eliminate any centerline non-consolidation issues that affect bar quality. Unlike the additional step in cold rolling, the extra step of rolling after forging is done while the bar is hot, so there is no time wasted on additional processes after cooling. Forged bar can be used for single or multiple piece requirements and produced on demand. The other significant advantage is for large diameters. The rolled bar is produced to a specific maximum size where the forged bar has a broader span of diameter capabilities.
If you are new to forging or want to compare bar quality, the Scot Forge Bar Team is here to help you navigate your options. Contact our Scot Forge Bar Team for more information about forged bar. Forging may not always be the best option for your project, and if that is the case, we are happy to point you in the direction of vendors we know and trust.