Tag: knives and steel

How can a spring “remember” its position? When I stretch a spring or compress a spring it returns to basically the same size. What is it about the atoms/molecules that make up a spring that allows it to return to its original state? — JH

Nearly all metals are crystalline, meaning that their atoms are arranged in neat and orderly stacks, like the piles of oranges or soup cans at the grocery store or the cannonballs at the courthouse square. When you bend a metal, its crystals can deform either by changing the spacings between atoms or by letting those atoms slide past one another as great moving sheets of atoms. When the atoms keep their relative orientations but change their relative spacings, the deformation is called elastic. When the atom sheets slide about and move, the deformation is called plastic.

Metals that bend permanently are experiencing plastic deformation. Their atoms change their relative orientations during the bend and they lose track of where they were. Once plastic deformation has occurred, the metal can’t remember how to get back to its original shape and stays bent.

Metals that bend only temporarily and return to their original shape when freed from stress are experiencing elastic deformation. Their sheets of atoms aren’t sliding about and they can easily spring back to normal when the stresses go away. Naturally, springs are made from materials that experience only elastic deformation in normal circumstances. Hardened metals such as spring steel are designed and heat-treated so that the atomic sliding processes, known technically as “slip,” are inhibited. When you bend them and let go, they bounce back to their original shapes. But if you bend them too far, they either experience plastic deformation or they break.

Non-crystalline materials such as glass also make good springs. But since these amorphous materials have no orderly rows of atoms, they can’t experience plastic deformation at all. They behave as wonderful springs right up until you bend them too far. Then, instead of experience plastic deformation and bending permanently, they simply crack in two.

One last detail: there are a few exotic materials that undergo complicated deformations that are neither temporary nor permanent. With changes in temperature, these shape memory materials can recover from plastic deformation and spring back to their original shapes.

What do some permanent-magnet generators have stainless-steel axles? — RC, Port-au-Prince, Haiti

Many forms of stainless steel, including those designated as “18-8 stainless,” are completely non-magnetic. In contrast to normal steel, which has a microscopic magnetic structure and is easily magnetized by a strong magnetic field, these non-magnetic stainless steels are entirely free of magnetic structure. They cannot be magnetized, even temporarily. In machinery that contains strong permanent magnets, using non-magnetic stainless steel for the mechanical parts avoids undesirable attractions between parts and distortions of the required magnetic fields. While copper, aluminum, or brass could also be used—they are non-magnetic as well—stainless steels are generally much tougher metals.

Do crystals “grow”? — JJ

Yes, crystals grow. They begin as tiny seed crystals when a few atoms or molecules manage to arrange themselves accidentally in an orderly fashion. From that point on, new atoms or molecules that join the seed continue the orderly pattern and the crystal grows. Some crystals grow from solutions that contain more of particular atoms or molecules than those solutions can handle. Other crystals grow from molten materials that are cooling off and beginning to freeze. Still other crystals form from atoms or molecules that are diffusing randomly through solids, liquids, or even gases and encounter the proper crystal on which to stick.

Why does a spring want to come back to the original position it started at? — JF, Hazen, ND

When you stretch or bend a spring, you are displacing atoms within the crystals that make up that spring. Each atom in those crystals moves a tiny bit nearer or farther from its neighbors and it begins to experience tiny forces that would push it back toward its original position if you let go of the spring. When you do let go of the spring, these tiny forces act together to return the spring to its original shape while returning the individual atoms in the crystals back to their original positions. However, if you bend a spring too far, the atoms begin to slide across one another and they can no longer find their way back to their original positions. In that case, the spring has become permanent bent and won’t return to its original shape when you let go. Good spring materials are those that can tolerate a substantial amount of stretching or bending without allowing their atoms to slide across one another. Many common metals don’t make good springs because this sliding occurs much too easily.

Why is stainless steel a sterile material? Why is it used for surgical tools and to pierce ears?

Stainless steel is not inherently sterile but it can be made sterile and its lack of corrosion provides no hidden cavities that might harbor germs. A stainless steel surface can be made relatively flat and it will remain that way indefinitely. In contrast, a rusting steel surface has a complicated surface that is constantly changing. That surface is harder to keep clean than a flat stainless surface. Although stainless steel seems ideal for medical purposes, it is not hypoallergenic. Many people react badly to nickel, which is present it high quantities in surgical stainless. It also turns some people’s skin green.

How do steak knives differ in structure from the “super” cut-through-anything non-damageable knives?

A good knife is distinguished both by its cutting edge and the backbone that supports that edge. The ideal knife has a very hard cutting edge (one that never undergoes plastic deformation and thus never becomes dull) and a very tough backbone (one that can absorb an enormous amount of energy before breaking). The backbone can experience plastic deformation when necessary, in order to absorb energy. Cheap steak knives are made of only one steel: a moderately hard and moderately tough material. They gradually dull because of plastic deformation in their edges but they never break because their backbone is flexible. A great knife is made of several steels, which can be formed by proper heat treatment of a single piece of metal: a very hard edge and a very tough backbone. It never gets dull because its cutting edge never yields and it never breaks because it bends before breaking.

How is the strength of a clipping device such as Caribeener, hook, or chain link calculated? I think it is measured in kilo-newtons. What elements are taken into consideration when that strength is measured?

One of the most critical measures of a clip-ping device is the maximum tension that it can tolerate without failing. I would expect a tester to measure that failure tension by putting the clipping device in a simulated working environment and exposing it to greater and greater tension until it fails. For example, a chain link would be put between two sturdy hooks and then the hooks would be pulled apart until the chain link broke or deformed permanently. Since tension is a force, it’s natural to measure it in newtons or kilo-newtons (1000 newtons). (There are 4.4482 newtons in 1 pound of force.) But what constitutes failure is complicated since anything that is exposed to tension deforms somewhat. However, if the tension is less than a certain threshold, the deformation will be purely elastic—meaning that the device will return to its original shape once the tension is released. But if the tension exceeds that threshold, the deformation will be plastic—meaning that it will be permanent and the device will not return to its original shape once the tension is released. I would expect the rated strength of a clipping device to be a reasonable fraction (probably about 50%) of the tension required to cause plastic de-formation of that device.

Is it true that striking two hammers together will release little splinters? If so, why?

The head of a hammer is made of very tough steel. Depending on the type of hammer, that head may even hardened tool steel. In that case, the head will not yield, except to the most incredible forces. It will instead deform elastically and then return to its original shape. However, if you smack two hardened hammerheads against one another, the forces that they exert on one another may become so great that the heads will shatter. The symptom will probably not be the release of a few tiny splinters but rather large chunks of hard steel flying off in all directions.

What makes stainless steel stainless?

Stainless steel resists corrosion because one of the metals (iron, nickel, and chromium) or one of their oxides is bound to be stable in almost any chemical environment. Corrosion stops at the grain boundaries around the stable materials so that they form a protective layer above the other materials beneath them. — Thanks to David Ingham for this answer

Why is silver used so often for tableware?

Silver is used in tableware because of its whitish luster and preciousness. It is not really the most practical metal for cutlery. It tarnishes slowly by reacting with sulfur pollutants, which are present in the air in trace amounts. Pure silver is also very soft because it allows slippage to occur easily. To harden tableware, silver is alloyed with about 5% copper. The resulting material is much harder than either of the pure metals. Jewelry silver has even more copper; up to about 20%.