The Finest Swedish Steel

Vanadium’s role in steel strength and flexibility.

Reheating and then rapidly cooling steel creates a fine-grained structure to increase strength, but with that strength comes brittleness. Enter vanadium.123artfotodi

How many times did I read that description as a child! And when I’d ask my parents what it meant, I couldn’t get a satisfying answer. Written sources seemed to imply that virtuous, clean-living Swedes produced virtuous steel.

But it turns out that Swedish iron ores naturally contained tiny percentages of the element vanadium, and that vanadium allows production of a tough, fine-grained steel whose high tensile strength is not crippled by brittleness—as is the case with ordinary carbon steel.

One of the methods of strengthening steel (and other metals as well) is by grain refinement. As a melt of steel slowly solidifies as an ingot, individual metal crystals grow from the cooler wall of the mold in the form of dendrites toward the still-molten center. The result is a very coarse-grained material of low strength.

But if that material is reheated to a temperature that destroys those long, needle-like metal crystals, and is then cooled very rapidly by plunging it into water, new crystals hardly have time to grow at all before they are solidified. This is quenching. If we apply stress to this fine-grained material, yielding within its tiny crystals cannot go far before it encounters a grain boundary, which stops further movement. This makes the steel stronger.

On the other hand, those who have worked with carbon steels know that with such increased strength comes brittleness. A brittle material, receiving a sharp impact, is more apt to break like glass or ceramic than it is to bend.

To temper such a material’s high strength with toughness, the quenched steel is next heated to a specific temperature, traditionally described visually by terms such as “pale straw” or “deep blue,” etc. You can look up these descriptions and corresponding temperatures in books or on the internet. The colors which appear on the material result from the formation of various thicknesses of transparent oxide on the metal. The different oxide thicknesses result in interference between incident and reflected light waves, acting as a filter to cancel some frequencies (colors) while transmitting others. Many of us have admired such interference colors on welded titanium. Materials engineers hate to see those colors because they mean that someone failed to adequately purge the glove box with inert gas.

In retrospect, the thing I should have done was buy some fresh bolts and put them in a hot oven for an hour.

Why is quenched-but-not-tempered carbon steel brittle? An iron-carbon compound called cementite appears at crystal grain boundaries in quenched steel. Cementite is officially a ceramic, so it can’t yield as metals do, by atoms giving up one interatomic bond only to form others just as strong with new neighbor atoms. When a bond in a ceramic fails, the strain that caused the failure is handed on to the next interatomic bond and the next—as a fracture wave.

What made Swedish steel stand out was that its grain size tended to be naturally small, but without the need for quenching that led to brittleness.

Hardened steel can be brittle because of the internal structure that makes it stronger.123artfotodi

When I was trying to race my Yamaha TD1-B roadracer in 1966, that engine’s side-to-side rocking broke engine bolts. It felt foolish to come in from practice to find just the nut, with part of the bolt thread still in it, dangling from my carefully applied twisted safety wire. Those bolts hadn’t been properly tempered to give them useful toughness.

In retrospect, the thing I should have done was buy some fresh bolts and put them in a hot oven for an hour. But I had to be a hero and make new engine bolts from bar stock, out of mild steel. Mild steel has next to no carbon in it, limiting its strength to 50–60,000 psi.

I installed my new trick bolts, torqued them in place, and lock-wired both nut and head. They looked great! Coming in from next practice I decided to check bolt torque, so I cut away the wire and applied the torque wrench. Nothing! Where did the installation torque I had applied 40 minutes before disappear to?

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I’d jumped out of the frying pan of too strong and brittle into the fire of not strong enough—my new bolts had just yielded to the hammering of normal engine vibration. My bolts, like cool chewing gum, would just go right on stretching until they broke.

I bought new stock engine bolts, reduced the installation torque a bit, and hoped for the best.

It appears that vanadium, when present in steel in a tiny amount such as one- or two-tenths of 1 percent, acts to nucleate the formation of a great many tiny crystals as the metal solidifies. This is called grain refinement. Their grain boundaries are not covered with cementite, so the material is not brittle. Yet the presence of so many grain boundaries places a zillion obstacles in the way of the yielding process, resulting in high strength. Additionally, vanadium also forms hard structures with carbon that act as keys to further impede the yielding process. Finest Swedish steel.

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