When Hot Rolled Steel? – When Cold Finished Steel?

Whoever said; “Cuts like butter, or, Machines like butter”, may not have meant that was a good thing. Visualize butter; not cryogenic butter. The thing is, you need a little hardness (read as strength) if you are expecting decent cutting and machining.

Generally, cold finished steel; which takes into consideration both Cold Rolled and Cold Drawn, provides higher hardness and higher strength than Hot Rolled Steel. Hot Rolled Steel here, means Hot Rolled Annealed (softened) Steel. Hot Rolled Q&T (Hardened) Steel, on the other hand, is most often harder than Cold Finished Steel. A ball park assessment of the comparable hardness would be something like this;

Cold Drawn/Cold Rolled = 20RC          Hot Rolled Annealed = 18RC     Hot Rolled Q&T = 30RC

If you want a rough idea of the relative strength of each, convert RC roughly to Brinell (BHN) hardness (RC times 10), and divide by two. So, 20RC = 200bhn = 100,000psi Tensile. 30RC = 300 bhn = 150,000psi Tensile, and so on. Now, don’t be using this for engineering. It is simply a rough comparison for reference sake.

Cold finished steels are worked at room temp. Hot finished steels are worked hot, almost 2000°F. “Cold Rolled” typically refers to flat bar, sheet, and plate. “Cold drawn” generally refers to bar shapes. Cold working steel strain or “work” hardens it. The higher hardness elevates the strength. But, since it is not a thermally hardened material, the cold working process also imparts stress. That stress is retained in the steel. It may cause movement during subsequent machining or grinding. Q&T Hot Rolled Steels respond to thermal treatment, so, they may be stress relieved at some point of their production. That process relieves stress that may have been retained from processing.

With all of the changes that are happening in the world of steel availability, the question may soon be, which one is available in the size you need. More and more hot rolled shapes, that were once considered staples of the industry, are disappearing. There is a far greater variety of bar shapes available in cold finished product that in hot rolled product; especially hot rolled Q&T.

Also, cold finished bars are provided in much more accurate cross sections and closer tolerances than hot rolled. So, Cold Finished bars are more available, better surface condition, greater aesthetics, stronger, and offer greater opportunity to obtain the size and shape you need. That is why Cold rolled squares and rectangles (flats) are ubiquitously used as rails for automation. They are often machined to accept custom wheels, cams, and mounting hardware. Cold finished alloy bars may be surface hardened to minimize greatly extend wear resistance and service life.

Hot Rolled Annealed steels are more malleable. They tend to retain less stress which translates to less movement. Hot Rolled Q&T steels, especially Alloy steels, offer much higher strength, but, they are somewhat less available than the great variety of sizes available in Cold Finished products, have less cross sectional accuracy, and generally are used where accuracy is not critical and/or a portion of the surface will be removed.

High grade cold finished alloy bars are clean and strong. Better service centers will employ subsequent straightening, even with bars from their general stock. They will also provide fabrication services to prep the mill bars for specific automation rail applications. This may involve; further straightening, drilling and countersinking, surface hardening (multiple or single surface), and custom end preparation.

-Howard Thomas, August 19th 2019

Hardening Steel With Heat

We have touched on the topic of the effects of heat on steel in the past. These little reminders are directed at those of us who are not accomplished thermal treaters of steel. And, that is based on the assumption that you wouldn’t be reading this article if you are already proficient in hardening steel. NOT INSTRUCTIONAL! “Random thoughts on hardening steel” in his article are strictly for information purposes only.

To a large part the carbon content of steel determines its hardenability. More carbon/higher hardness. Low carbon (below .26), virtually means no hardenability. Although some people will argue that, and that’s fine, it makes for a nice bet in a bar. A very general guide for a novice is; “through-hardening” alloy or carbon steel yields a maximum hardness equal to carbon times one hundred, i.e. .30 carbon = 30RC, .40 carbon = 40RC, etc. For general reference, most pre-hardened steels offered to the public, will be roughly 30RC. At that hardness, they have good “toughness”, which is a combination of hardness and strength. Surface hardness (skin hardness, flame hardness), is somewhere around ten points over the above shown numbers, so, you can get approximately a 50RC surface hardness on a piece of 4140. It’s actually a little harder but that will give you a ballpark idea.

Through hardened steel has many advantages. One potential disadvantage might be that it could be a bit brittle. Surface hardening is generally preferred when the application may have impact or flex. In that case you would want to retain some core “flex” (ductility) in the steel you are working with. In general terms; strength goes up when hardness goes up. Too hard and brittleness (potential for abrupt fracture) becomes a factor.

The speed at which you cool the steel from the hardening temperature will affect the resultant hardness. Like baking bread or deserts; think ingredients, time and temperature. Get them worked out correctly and you have a pleasant result.

Other elements added to steel (inside the steel) or added to the furnace as a gas, will also affect the overall hardenability. There is no substitute for knowing the grade of the steel and its current hardness.

ALL IS NOT LOST! If at first you don’t succeed, try, try, again. Steel is generally very forgiving. You can repeat the thermal treatment over and over without damaging the material provided you have not exceeded the hardening temperature. If you end up with a lot of surface decarb (the powdery grey skin and scale that forms on the surface of the heated steel due to the carbon burning off), you will have to remove that so that it doesn’t act as a “heat sink” and inhibit subsequent hardening attempts. You will also have to research the thermal treatment procedure of “annealing”, as that may be required prior to your subsequent hardening attempts.

It will be necessary to remove the decarb anyway if you intend to accurately determine the hardness of the steel. The old rule used to be; “Grind to bright metal.” Decarb can still be present in bright metal. So, grind to remove all decarb. It is a safe bet, with almost any carbon or alloy steel; if you are getting a Rockwell “C” hardness reading under 22RC, you are probably still in decarb, since most carbon and alloy steels will pick up some hardness following a thermal procedure unless you just happened to accidentally perform a  perfect anneal.

-Howard Thomas, August 6th 2019

Cut To Length Steel And Random Bar Lengths

Some time ago we posted an article about random bar lengths. The attention from that blog contributed to this follow-up about cut-to-length bars. More importantly, how that term differs from random (rdm) bar lengths.

Random bar lengths can be anything the distributor deems beneficial to his selling of the steel bars in  his, or her, inventory. It is most common in the general steel marketplace to see random bar lengths listed as 10/12ft rdm, stainless bars were often listed as 11 to 14ft rdm. You might say that is more less the default in the industry. However, even that category does not define exactly what might be shipped to you. You may get a 13ft bar, or even a 16ft bar; depending on what the distributor has on the floor at the time. In rare instances you may get an 8ft bar or 9ft bar, although shipping a shorter bar is not all that common. The practice of offering random bar lengths can be a very good thing. You should be able to save money on the purchase, less than the higher cost of requiring a bar to be cut to a specific length. Just make sure you and the seller are on the same page at the point of inquiry.

Cut-to-length bars are just that; bars are cut to the exact length you require. You just need to be clear in expressing your requirements; and include a length tolerance that works for both parties. For many decades “plus 1/8″, minus 0” was quite common, and the default if nothing was specified. It pays to be sure both parties understand the same requirement. Especially if it is a first time order or a new vendor or customer. Your expectations understood by one supplier may be lost on the new supplier.

There are other concerns that may affect your order regarding cut-to-length. You may want to specify “No Mill Ends”, or, “Trim Mill Ends”. Mill ends (the original ends of the bars as they were shipped to the distributor from the steel mill), may have been trimmed at the mill, and just fine. They may also be sheared, in which case there may be a slight taper to the end of the bar. The diameter at the bar end may be a bit undersize. It may not be a square cut. It may be slightly dished, it may have identification stamped into the end. If you have not allowed some trim stock to the length you ordered, this may be an issue. It is not a major issue on most orders, but it is worth mentioning if it could be.

If the type of cut required is to a very close tolerance you may want to specify “Square Cut Ends”, and then the tolerance you require. There may be additional charges depending on how specific your needs are.

Very small diameters (those under 5/8″ Dia.) may require special dialogue or instructions if your expectations are exacting. Can the vendor bundle and cut? Once again, there could be additional charges incurred. Very large diameters (over 10″Dia.) usually require a little extra stock be left on the cut. Depending on the size and the intended use, some people will leave 3/8″ or 1/2″ on the cut. Chromed bars often come with “masked areas” at the end of mill length bars. That is an area that has not been chromed. It may be several inches. If you typically purchase full mill random lengths of chrome bars from one supplier, you may want to insure both you and your new supplier understand the specific bar mults of the finished shafts you intend to make. Discuss “trimmed ends” at the point of inquiry.

Newer saws and better saw blades are able to cut phenomenally close these days. There seem to be blades for nearly every metal and hardness. But, just because the saw and the blade can cut close, does not mean that’s what you will be getting. Communication goes a long way to avoid a mishap. Remember; “When it goes bad, who’s wallet comes out?”

-Howard Thomas, June 21st 2019

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Howard Thomas

Howard Thomas

Experience

Sr. Acct. Mgr. (US Southwest) / Metallurgical Consultant
Associated Steel Corporation
Jan 2017 – Present

Past Vice President / General Manager
Associated Steel Corporation
Apr 1998 – Jan 2017

Past Vice President / General Manager
Baldwin International
Apr 1974 – Mar 1997

Education

Cleveland State University
Kent State University
University of Denver

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