Sausages And Steel

Sausages and steel

TDC

Kevin Cameron

ONE RECENT MORNING I MADE SOME fresh sausage meat and fried a plate of old-time patties. As they looked to be browning rapidly, my wife reached over and turned down the stove. (What husband is trusted alone in the kitchen?) When we sat down to breakfast we were both surprised to find that just inside a crisp exterior the sausage was tender and good. We both noted that commercial sausage that brown would have been hard all the way through. Why the difference?

“Ah,” said Gwyneth, “These haven’t been frozen. All the cell walls in this meat are intact and able to retain moisture. In commercial sausage, the freezing breaks the cell walls and then the moisture diffuses to the surface when it’s cooked. Then they’re hard all the way through.”

A light went on in an attic of my brain. One of my dreaded obscure analogies was taking shape. Over the past two winters, I had let my interest in exhaustvalve materials spread into other areas“buckets” for the turbine wheels of aircraft turbochargers, developed during the 1930s, then the hot-section parts of jet engines and back to the durability problems of racing motorcycle exhaust valves in the late 1940s and ’50s. I have a box full of 3x5 cards, each covered with crampy little printing, devoted to such fascinating subjects as coherent precipitates and script morphology. I have sat for hours, trolling various sources for tidbits that I write down. After a long period of this, the general outlines of the subject begin to make some sense to me.

When the railroads adopted carbon steel in place of iron, a disagreeable discovery was that it was nearly impossible to harden thick sections of steel satisfactorily. Upon rapid cooling (“quenching,” by thrusting the red-hot metal into a bucket of water), the skin of the part would be very hard, but deeper into the section that hardness disappeared and the material remained soft inside. Worse yet, the rapid cooling necessary to obtain hardness left such thick sections full of residual stress.

When a carbon steel is heated in preparation for hardening, the high temperature causes iron carbide (three iron atoms joined to one of carbon) to dissolve in the surrounding iron, which it does quickly. After quenching, the iron carbide rapidly precipitates out of solution to form a great multitude of near-molecular-sized parti-

cles. Upon slower cooling, the carbide not only comes out of solution, but also has time to cluster together, forming fewer, larger particles. Many tiny carbide particles act as keys to prevent layers of iron atoms from being forced across one another easily. This strong resistance to deformation is hardness and, taken to extremes, it becomes brittleness as well.

A slower rate of cooling resulted in softer, less strong resistance to deformation, as the resulting larger and fewer carbide particles were less effective in pinning atomic planes against sliding across one another.

Because it takes time for heat to flow through any substance, attempts to harden thick pieces of carbon steel failed because no matter how rapid the quench, the inside of the part took longer to cool than did the outside. The result was a part hard on the outside to the point of brittleness, but still soft on the inside.

Now for the sausage analogy.

Toward the end of the 19th century, metalworkers found that adding even a bit of nickel to carbon steel completely changed the way it responded to heat treatment. A nickel carbon steel could be hardened at a much slower cooling rate than was needed for plain carbon steel. The presence of the nickel was apparently slowing the rate of precipitation of the iron carbide from solution. This

made it possible to retain the desirable multitude of molecular-sized carbide particles through a longer cooling process, without the danger that the particles would clump together to form fewer, larger particles that result in reduced strength. And this new ability to create hardness at slower cooling rates meant that thicker parts could be successfully hardened all the way through.

What had triggered my dreaded analogy-maker was the idea that the presence of intact cell walls in the never-frozen sausage was acting like nickel in carbon steel-it was slowing down the diffusion of something. In this case the diffusion of moisture out of the frying meat, leaving its inside tender as a result, just as the nickel in steel slows the movement of carbides through the crystal lattice, thereby slowing the : growth of carbide particles as the metal cooled.

Far-fetched? Yes, but the brain is constantly playing with all the things it either knows or suspects, and ideas pop out. This can make my breakfast conversation rather strange, but for harder-working and more ambitious persons than myself, this same process creates industries and makes vast fortunes.

Winchester Anns adopted nickel-alloy steel for their gun barrels in 1895, and nickel steel also brought about a revolution in battleship armor. Railroading and shipbuilding-great consumers of highstrength materials-rapidly adopted the fruits of the new knowledge. Before 1900 almost all know-how about metals belonged to top foundry foremen and their trusted apprentices, and was largely of a cookbook nature. After 1900 metallurgy emerged as a formal subject, and both practical and theoretical studies went forward rapidly, hand in hand.

Nickel and other alloying elements also harden steel by having a different atomic diameter than iron. The presence of oddsized atoms in the iron crystal lattice creates local regions of strain that resist deformation. Nickel has a mild effect of this kind but vanadium’s effect is much stronger. Flarold Wills in Henry Ford’s modest research department pointed this out. Soon, a smaller total amount of vanadium steel was used to build as durable a Model T as a greater mass of ordinary carbon steel. More fortunes made.

I live modestly and cook sausage when my wife permits.