Not a stupid question at all. In fact many makers have been sorely disappointed in steel because of not knowing the difference. 416 stainless for instance can be had both ways. The surface looks better in cold rolled. I use it because it cleans up a little better in the milling vice. You have to be really careful with 440-c for instance. Hot rolled bar, is uglier on the surface. But is good stuff. There is a lot of sheared 440-c out there from cold rolled plate. It can have a very strong tractor tire tread grain when trying to finish. Seems that hot rolled is just a bit more dense. In steel going to be used forging, it probably doesn't matter. You can improve on the grain in cold rolled by forging, but hot rolled is damed good just as it comes form the mill. There is a lot of talk about grain refinement. It is mostly BS. You can get a shape that you want by forging with less metal that stock removal, But the grain refinement comes from the heat treating. Not from forging. Such things as edge packing, and breaking up the molecules is pure smoke and mirrors. Sells well to the UN educated. But is nothing more than Flem-flam at it's best. Mike
"How it is made", prolly the best show ever made for gearheads,
had a segment on how they make steel. Wuz most excellent indeed.
Oftentimes you see a blacksmith forging a bar (making, say,
damascus), working with something like 1"x1"x10" bar.
Now, imagine that bar is 15 feet x 15 feet x 20 feet, being forged
by a gian press, while being held in giant tongs. That's what it looks
like.
Before that happens, they smelter iron and additives in a giant
smelter, with 3 giant electrodes. Must be megaAmps of current
doing the job. As the metal melts, they add alloying elements and
frequently check the resulting "mix" for proper percentages, adjusting
as needed. As they pour, filters will catch larger debri. Of course,
to qualify for NASA, this process needs to be pretty darn clean. It might
have to be repeated few times, to get progressively cleaner, better steel.
Some metals, like brass, can be hardened by "working" it . Steel doesn't
exhibit this feature. Still, when they roll, a lots of stress ends up in the
metal . Also, when hot rolling, exterior layers loose carbon and other alloying
elements. Nasty, hard scale forms. It has to be removed - this makes steel
"decarb" free and it also brings it down to exact dimensions (precision-ground) or just slightly oversized (called just that). Another way to remove the scale is to pickle the steel in acid, but of course it is not precise at all.
When using hot roll for removal-method knifes, one has to be careful not to end up with a warped blade, again due to the stress of hot-rolling being hidden in the metal bar. At best, you will have to spend lots of time orrecting it.
Ok this was just on history channel and a while back how its made.
When steel is heated up the molecules align in to what they called a matrix. This becomes a stronger steel. In cold rolling they start with a huge billet of steel and run it throguh rollers compressing the steel. In cold roll steel youll have the same matrix as you did in the original billet (for the most part its really less change to the matrix). Cold rolling also uses many more rollers and or passes to get the stock to the final size and shape this is why it will tend to have a nicer looking finish.
With hot rolled steel the matrix will change become more dense and well aligned. In stock removal this is a good thing as it will grind and file etc smoother for us (more consitancy) than cold rolled. In the end though if your heat treating is good other than the ease of finishing both will end up the same or very close to the same in hardness grain structure etc.
Remember on raw hot rolled youll need to remove a portion of the mettal from top bottom and both esdges to get past the scale layer.
OT sort of but intresting. You know that alluminum foil you use to wrap your lunch in? Well it is a multi layer or damascus alloy. To get foil they use hot roller to thin out a very huge billet of aluminum. Once its at a given thickness it is folded in half and ran through the rollers again to increase its strength. Depending on how heavy a duty its to be used for it may be folded again and again ran through the rollers. So you heavy duty foil is folded twice giving a 4 layer damscus aluminum foil
Just a fun little fact i picked up watchign how its made heheh
Cold-finished steel bars offer improved straightness, tolerance, and concentricity compared to other types of steel bars. These bars are popular in the manufacturing of bearings, shafts, fasteners, oil pump shafts, concrete accessories, hand tools, vehicle axles, and wheel spindles.
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Advantages of Cold Finished Steel Bars
A cold finished steel bar is produced through a process called cold finishing, which involves cold drawing, grinding, and polishing. This process results in a higher yield point for the steel bars. Some advantages of cold-finished bars include:
- Bright new surface finish: Cold-finished bars have a visually appealing surface finish.
- Improved concentricity, tolerance, and straightness: These bars exhibit better dimensional accuracy and roundness.
- Enable cold-drawing: Cold rolled steel bars can undergo cold-drawing, which increases their tensile and yield strength while eliminating the need for expensive thermal treatments.
- Improved machinability: Cold rolled steel barhave improved machinability, leading to longer tool life, faster cutting speeds, and better surface finish.
Manufacturing and Appearance of Cold Finished Steel Bars
Cold rolled steel bar typically have a smooth grey finish. The final rolling of the bars is done when the steel is cold or at room temperature. The process is conducted with the bars immersed in oil to prevent oxidation and maintain the grey color. This ensures a smooth operation by the rollers.
Forming Limitations and Heat Treatment
When steel is cold-finished, only a limited number of shapes can be formed. Steel is a very tough material to machine when it is cold. Bending, hammering, and deforming it at room temperature can cause hardening, distortion, and potential cracking or weakening. To achieve the desired shape, the steel needs to be heated above a specific temperature, allowing for easy bending, forming, and shaping without damaging the material.
Additional Features and Applications of Cold Finished Steel Bars
Cold rolled steel bar can also undergo chemical coloring and patination. Since cold-finished steels don’t have oxidation on their surface, they are ideal for base metal fabrication, followed by a secondary coloring process. Additionally, the sharp and accurate tolerance and profile of cold-finished bars contribute to their aesthetic craftsmanship.
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Cold-finished steel bars have a wide range of applications in construction, automobile manufacturing, and other industries. They provide strength at a lower cost and find use in both structural and non-structural applications. Cold-drawn round bars are specifically useful in the automotive industry for making wheel rims.
Benefits of Cold-Rolled Bars
Cold-rolled steel bars have a high tensile strength, making them suitable for various industries, including construction, automotive, and aerospace. Cold-rolling involves drawing the metal through cold dies and rolling it into a bar shape. Cold finished steel bars, on the other hand, undergo full annealing at room temperature before rolling, resulting in lower carbon content compared to other types of cold-rolled steel bars.
There are several reasons to consider buying cold rolled steel bar types. They offer an economical alternative to hot-rolled steel bars since they do not require expensive, high-temperature furnaces. Cold-finished bars are less prone to warping and cracking due to the absence of heat during production. They can be produced in larger quantities at a lower cost per unit weight.
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