Much Ado about Carbon

Welcome back to Metallurgy Mondays! So far we’ve covered how iron is an amazing metal with a number of important jobs that make our world work - from the iron protein hemoglobin transporting oxygen through our bodies, to the iron in the food we eat strengthening our organs, to the iron in the steel in your car keeping you safe as you commute to work, to the iron forming the structure you probably work with - iron is crucial to our development as a species and a society.

Last time we touched on how carbon can alloy with iron in a special way that makes the metal we call “steel”; but what is the deal, really, with the carbon-iron relationship?

Back in the old days, iron was smelted in what is known as a bloomery furnace. Ore and charcoal would be layered into the furnace and fired over the course of a day - or sometimes many days. During that time part of the bloom, or metallic mass left over at the end, would contain a very small percentage of steel (usually on the outside edge of the bloom) compared to the main percentage of iron. This is because the carbon in the charcoal migrated into the iron naturally, causing more-or-less steel to form.

But how does that make sense?

It’s pretty complicated, but the easiest way to understand is to compare it to diffusion. If you take a glass of water and put a drop of red food coloring into it, you will see that drop distinctly for a while

Given time, though, eventually that drop will diffuse on its own into an equilibrium state where the water is a consistent color, with the red evenly distributed within the glass.

The key here is to remember: Diffusion takes time.

So what if we are in a hurry, and that drop takes four hours to diffuse on its own? We can wait and let it do it at the natural rate, or we can stir it. Stirring is the same as ADDING ENERGY. If we add energy we can get things done faster with pretty much the same end result as letting things happen naturally at a low energy state.

Carbon is much the same. At room temperature carbon will diffuse at a rate close to .001” per millennia. That’s REALLY slow. At forging temperature, with our energy added in the form of heat, it will diffuse at a rate closer to .001” per second in some conditions.

For our purposes, all we need to understand is that:

1. In a suitable environment that is carbon-rich, carbon will diffuse into iron naturally.

2. At room temperature, carbon diffuses VERY SLOWLY into iron like a drop of dye into still water

3. At high temperature, carbon diffuses VERY FAST into iron like vigorously stirring dye into water.

Bloomery furnaces dominated the iron making process for thousands of years until we discovered the direct correlation between iron and carbon. We sort of always knew, but it was considered magic or tradition. An Anglo-Saxon king would pack a billet of iron in a box with the bones of his ancestor and roast it over a fire for a long period of time, and the carbon from the bones would diffuse into the iron converting it to mostly steel, thereby allowing the forging of a superior blade from previously lower quality metal. The metallurgical success was attributed to a mystical gift from the ancestor, and it worked!

From the late-1700s until the mid-1800s we made leaps and bounds in iron smelting technology, culminating in the Bessemer process that was solidified in 1856. Before then the mainstream way of making steel was to use a furnace known as a Puddling furnace which was limited to a certain size of production and produced fumes that tended to make workers sick, with most puddlers dying in their 30s. It took extensive labor and long hours, and to increase production to meet demand a business owner’s only recourse would be to build more furnaces and hire more workers who were willing to shorten their life expectancy to make more steel.

These puddling furnaces used a VERY high carbon iron ingot called Pig Iron, which is actually a pretty cute name derived from when the ingots were poured because they were thought to look like piglets suckling off a mama pig. Awww. Pig iron was made by smelting iron ore in a blast furnace and contains roughly between 4-5% carbon, which makes it too brittle for any use other than refining to make steel.

There’s a bit of a spicy drama surrounding whether Henry Bessemer or William Kelly came up with the revolutionary process of turning pig iron into what we now call “machine” or “mild” steel, but Bessemer won the history books. The process that today carries his name REVOLUTIONALIZED steel production and quite literally changed the world. The process allowed impurities and carbon to be removed from the molten pig iron using air forced through the molten iron, and since the iron mass heats up as a result of the oxidation reaction, it was super easy to keep it molten during the process.

There was a whole pile of research that went into figuring out what it would take to dial in the process. I won’t cover that right now, but if you’re a metallurgy nerd like me it is absolutely fascinating and I encourage anyone interested to go read about it.

At the end of the research and development, it turned out that the Bessemer process was more accurate, producing cleaner steel with a predictable carbon content. It was less labor intensive and safer for laborers who were working in the mill, and best of all: IT WAS CHEAP

The beginning of modern metallurgy really got kickstarted by this process. In 1865 the American Civil war ended, and during that war people were shooting each other with black powder muzzleloaders and fighting on horseback, and amputating limbs to prevent gangrene from superficial wounds. Flash forward only 49 years later and WW1 kicked off with airplanes, tanks and automatic machine guns, along with the miracles we gained as far as medical science, safe food storage, better housing and cheaper automobiles - and eventually computers and the conveniences we all take for granted every day - because we understand how carbon and iron make steel.

That Anglo-Saxon king thanked his gods for the steel he magically made with the bones of his warrior ancestor, and his life as a warrior was trusted to that steel. Today, the steel that holds up the suspension on your car is better quality than anything that king could ever imagine. We forget how very magical this metal was and still is, and how lucky we are to be able to take it for granted every day. The next time you open your fridge, think about how lucky you are to have a sanitary storage for your food made of stainless steel. The next time you get in a car, think of how lucky we are to have such reliable steel to carry us miles and miles while propelled by miniature explosions over and over again in that motor made with steel parts. From exploring the bottom of the ocean, to the surface of Mars, and the rest of the universe, we have steel to thank for everything we have, and by extension, carbon.

Thanks for reading! We will continue next Monday, leave a comment to get a conversation going if there is something that needs clarity or if you feel something was missed!