What’s Cold Finished Steel Bar?
Types of Cold Finished Bars:
Many times we hear the term “cold finished” in the steel industry, but many times people will wonder, “What’s a Cold Finished Steel Bar, exactly?” There are three main types of cold finished (CF) categories. First, are cold drawn (CD) bars, which represent the largest in the way of tonnage. The second type is turned & polished (T&P) bars. The third type is turned, ground & polished (TG&P) bars. While there are other variations on these types, such as cold drawn + turned & polished (CD-T&P), turned & ground (T&G), and others, they are just variations on the main three types. Note that the term cold rolled (CR) is not used industry wide and has been replaced by (CD).
In summary, the cold finished processes are:
- Cold Drawing
- Shot blast (descale)
- Draw (pull) through carbide die
- Cut to length
- Turning & Polishing
- Rough turn (peel with revolving tool)
- Rotary straighten, which also polishes with rollers
- Cut to length
- Centerless grind with specific grinding wheel(s) to hold tight tolerances and improve surface finish quality.
What’s Cold Finished Steel Bar?
Differences Between Cold Finished and Hot Rolled Bars
Many people have heard of, seen, and used cold finished bars, but what are the actual differences between a hot rolled bar and a cold finished bar?
There are five very distinguishable and significant features of cold finished bars which set them apart from all others. Those features are size & roundness, straightness, surface finish, mechanical properties, and machinability. These features are the biggest reasons why people use cold finished bars, and sets the cornerstone for the establishment of the cold finished bar industry from back in the 19th century.
For that reason, it’s important to understand what end use application the product is intended for, and which of these five features are needed. This is especially true as cold finished bars are more expensive than hot rolled bars, and each extra cold finishing operation adds more costs.
Let’s review these five features in more detail.
Size and Roundness
The first feature is size. Comparing to a hot rolled bar, which is formed (rolled) at high temperatures, a hot rolled bar will vary greatly in size, with a larger variance the bigger the diameter. Hot rolled bars can also have an oval or eggy-shape to it. Roundness on a cold finished bar will coincide with size, being half the size tolerance.
The size tolerance on a 4” diameter 1018 HR bar is +.063” / -0”, compared to a 4” diameter 1018 cold drawn bar which has a tolerance of +0” / -.005”, or a 4” diameter 1018 TGP bar which Northlake holds to a .003” tolerance. With roundness tolerances of half the size tolerance, they are very small and accurate, giving you a nice round product.
The benefits of this feature are great, and many end-use applications can use these tolerances as-is. From air movement shafts, to threaded parts, to studs and fasteners.
Straightness is one of the most important features, but can be one of the most underappreciated. We define straightness as the bar extending continuously in the same direction without curving or bending, not deviating from a straight line. Straightness is critical for applications where the bar will be machined, especially relevant on a lathe, grinder, screw machine, or similar operation. Bent bars that go into end-use shafting or hydraulic cylinder applications will inevitably fail.
The normal straightness tolerance of hot rolled bars is 1/4” in any 5 ft, with special straightness upon request of 1/8” in any 5ft. Compare that to cold drawn straightness of 1/16” in any 5 ft for a 1018 bar over 15’ long. For even greater straightness precision, we hold our TGP to .001” per ft max (or 0.010″ TIR over the entire bar length), which is serious accuracy.
Of course straightness is perishable, and it’s very important to take proper care in handling, supporting, and manufacturing cold finished bars as they can be bent after straightening.
Probably the easiest of the features to see and verify is the third on our list, surface finish. One can easily recognize the difference in finish when comparing a cold finished bar to a hot rolled bar.
A hot rolled bar comes out of the mill with a rough, abrasive, discolored, and dirty surface. This is because the steel at the mill is finished at temperatures upwards of 1600° F to 2200° F, and at these high temperatures an iron scale forms on the surface of the bars. You’ll see in the areas where the scale flakes off, that there can also be pits and other marks in the bar. All in all it’s not very practical in that condition for many applications or end-uses.
Cold finished bars will be free from scale and an abrasive surface, whether it is from shot blasting & cold drawing, or rough turning & centerless grinding, CF bars will be suitable for thousands of end use applications.
Another important feature is mechanical properties. The increases in mechanicals will apply specifically to cold drawn bars, as other cold finished bars such as T&P and TGP will have the same mechanical properties as their hot rolled counterpart.
You may be asking yourself, what increases in mechanical properties might there be? Let’s use a 1045 grade of steel for our example, and let’s say the hot rolled bar has a yield strength of 50,000 PSI and a tensile strength of 75,000 PSI. If we cold draw this same bar, we can increase the yield to 75,000 or 100,000 PSI, and increase the tensile to 100,000+ PSI.
These numbers are only for demonstration purposes, and can vary depending on the grade, draft reduction, quality, producing mill, etc. As you can see, it’s possible to raise the cold drawn yield by 50% or more, and in turn the tensile by 20%. This increases the strength and makes the cold drawn stronger than hot rolled or mild steel. This can reduce or eliminate heat treating, handling, shipping, and save overall costs.
The final feature of cold finished bars is machinability. If you’re a machinist or parts manufacturer, this is probably your most important factor.
Cold drawing, or cold working the steel, gives it a crisper structure, and makes the tooling cut much better and more efficiently. The chip will be springier and break much better, ahead of the tool, while reducing friction and heat. The finish of the part will be improved, which will eliminate further operations.
Compare this to machining hot rolled bars. The machinist will have a more difficult time and find the HR steel is “too hard to cut”… which likely means it’s too soft or gummy.
Faster machining means more parts produced, more efficiently. This means maximizing cost savings from using cold drawn bars.
In Summary – Cold Finished Bar Benefits
To recap, steel buyers and manufacturers use cold finished bars for their main five features.
The first benefit is size and roundness. Cold finished bars have much tighter size and roundness tolerances.
The second benefit is straightness. The straightness of cold finished bars is much more precise than hot rolled bars.
The third benefit is surface finish. Cold finished bars are free of scale and can be ordered decarb free upon request. The bars are bright and smooth, usable as-is on many applications.
The fourth benefit is cold drawn mechanical properties. Cold drawn bars will have increased yield and tensile strength, often eliminating the need for heat treatment operations.
The fifth benefit is machinability. Machinability is improved with cold working, especially in lower carbon grades, which leads to better tool life.
Hopefully this will help you the next time someone’s asks “What’s Cold Finished Steel Bar?”
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