by Michael Shade, IQS Editor
If you grew up speaking the English language, you’re likely to be familiar with this adage: “What doesn’t kill you makes you stronger.” I’ve always been a bit perplexed by this statement – not by its meaning, which is obvious, but by the fact that it’s persevered in the lexicon of expressions in our language despite its silliness. Think about it. If I was cleaning my gutters and lost my footing on the ladder, nothing about falling two stories is going to enhance my strength or improve my health. Or, if I accidentally slammed my hand in my car door, it’s unlikely that I’d gingerly pull it out and say, “Wow! Who’s ready for a thumb war?” The compression of the door would more likely have a deformative effect than an empowering one. But there is one context in which the adage is eminently appropriate: cold rolling.
Cold rolled steel image courtesy of Block Steel Corp.
Apparently, if you whack or squish certain kinds of metal hard enough (but not too hard), you can make them stronger. Just like the old saying from the first paragraph, that’s a bit of a counter intuitive concept. Before we get into what cold rolling is and does, it will be helpful to quickly go over the concept of strain hardening. Strain hardening, which is also sometimes referred to as work hardening, is the process of applying pressure to a metal in order to improve its strength. Here the “what doesn’t kill you…” part is important to remember. The over-application of force will, of course, damage or destroy its subject. But if it’s just right, it imparts favorable properties upon the workpiece. Strain hardening isn’t appropriate for all metals; usually, only metals with high melting points are candidates for this process. For the appropriate metals, strain hardening can be applied in any number of ways, including bending, shearing, drawing and rolling.
Cold rolling is when pre-strain-hardened metal is forced between rollers and deformed. A rolling process is said the be “cold” when it’s performed at or near room temperature, far below the melting point of whatever metal is being rolled. The illustration below is very helpful in understanding how cold rolling works and what it does:
Image credit (Wikipedia).
As you can see, on the left, a metal workpiece is forced between the rollers, emerging on the other side in compressed form. The metal on the right side of the rollers is characterized by a higher strength and better surface finish than the metal on the left side of the rollers. The metal on the right is also harder and less ductile than before.
A number of different metals are candidates for cold rolling, steel being among the most common examples. Cold rolled steel is used in a large variety of applications, from the automotive industry, to the consumer appliance industry. Steel service centers produce cold rolled steel for use in washing machines, in radiators, in bathtubs, in furniture and in a wide range of other domestic and commercial appliances. Cold rolled steel is applied in these contexts because it’s strong and won’t easily be deformed; this is owed to the cold rolling process, which increases the metal’s strength and reduces its susceptibility to further deformation.
Cold rolling isn’t the only process engineers use to change the physical properties of steel, but there’s one very important reason why cold rolling is an attractive option: it’s cold. Steel is highly susceptible to oxidization. Oxidation risks increase as temperatures increase, which can make heat treating less favorable than cold working when it comes to steel. Cold rolling all but eliminates the risk of oxidization while accomplishing a range of physical property alterations. It’s an all around ideal process. Its main drawback is that tooling costs can be quite high, making initial investment in rolling equipment quite expensive. For that reason, the process is usually performed on a scale large enough to justify the steep initial investments.
Cold rolling is good for a lot of purposes – strengthening metals, reducing their ductility, improving surface finishes and… oh, right – they’re one of the few examples where that old axiom applies.