Metallurgy for Cyclists II: Steel is Real
by Scot Nicol
"Once giants lived in the earth, Conan. And in the darkness of chaos, they fooled Crom, and they took from him the enigma of steel. Crom was angered, and the earth shook. Fire and Wind struck down these giants... but in their rage, the gods forgot the secret of steel and left it on the battlefield. And we who found it are just men -- not gods, not giants, just men. The secret of steel has always carried with it a mystery. You must learn its riddle, Conan. You must learn its discipline. For no one, no one in the world can you trust -- not men, not women, not beasts... this you can trust."
Conan's dad, from the film "Conan the Barbarian."
Bicycle framebuilders have known the secret of steel for a long time. In fact, steel has been used to build more bicycle frames than any other material. It has also been used about 50 years longer than any other framebuilding material.
In this second installment of our six-part series on frame materials, you'll learn something about where steel comes from, and more about its advantages and disadvantages in bicycle-frame fabrication. But first, I'd recommend a re-read of the first installment of the series to familiarize yourself with the terminology.
Steel is made mostly of iron, whose atomic symbol is Fe, from the Latin ferrum. That's where get the term 'ferrous', as in ferrous and non-ferrous materials. As you may have guessed, steel is a ferrous material, and aluminum and titanium are non-ferrous.
Iron is the fourth most abundant element in the earth's crust, so we won't be running out of it in the near future (chromium and molybdenum are different stories, however). Iron rarely occurs as a chemically pure metal, except in meteorites. On this planet, it's found in various forms, among them magnetite (Fe3O4), hematite (Fe2O3), siderite (FeCO3), pyrite (FeS2)... and many other forms that end in 'ite'.
How do we get from iron to steel? We add and subtract a couple of ingredients while it's molten, and voilà: steel (actually it's a very involved and evolved process involving exothermic reactions, but we'll save that for the extended-play version of this article).
4130 steel -- an alloy steel, commonly known in the bike industry as chromoly -- contains a long list of alloying agents. These are: 0.28- to 0.33% carbon, 0.4- to 0.6% manganese, 0.8- to 1.1% cromium, 0.15- to 0.25% molybdenum, 0.04% phosphorous, 0.04% sulfur, and 0.2- to 0.35% silicon. The other 95-plus percent is made up of good old-fashioned iron. Now, there are hundreds of kinds of steel, but 4130 finds its way into very many bike frames because of its weldability, formability, strength, ductility and toughness. (Many low-buck frames are made with 1020 steel, which is called plain carbon steel, and has significantly lower strength than the chromium-molybdenum steels.)
The numbers that I'm throwing out are assigned by the Society of Automotive Engineers and the American Iron and Steel Institute: 41XX designates a chromium-molybdenum steel, while 10XX designates a plain carbon steel. The first number specifies the type of steel: 1 = plain carbon, 2 = nickel, 3 = nickel chromium, 4 = nickel, chromium and molybdenum, 5 = chromium, etcetera, ad nauseam... The second number means different things for different alloys. In the case of 4130, it defines the percentage of chromium and molybdenum in the alloy. The last two numbers tell you the amount of carbon, expressed as hundredths of a percent. 4130 therefore has 0.3% carbon.
From now on, in the bicycle lexicon of this series, I'll be using the terms 4130 and CrMo interchangeably, even though not all CrMo's are 4130. CrMo remains by far the most common of all the steels used to build high-quality bicycle frames. And I'm making an assumption that the readers of VeloNews who ride steel frames aren't riding Muffys. (That's the generic name for the Murray-Huffy style of bike you can buy at those fine American institutions like K-Mart and Wal-Mart.)
Muffy-grade steel is barely above rebar on the steel "food chain". Rebar is essentially a blend of melted 1956 Chevys, washing machines and shopping carts.
The bicycle-frame designer must take many different factors into account when choosing materials for fabrication. Even after considering all the available options, no clear choice may emerge. But, regardless of the application, there are always good reasons to choose steel. Let's go over the physical characteristics that we defined last time, and look at how steel compares with titanium and aluminum, its most obvious competitors.
Unfortunately for steel, it is -- to use 1990s vernacular -- "density challenged". Weighing in at 0.283 pounds per cubic inch, it's almost twice as dense as titanium and pretty near three times the density of aluminum. Light weight is where it's at with bicycle frames these days, and high density makes it tough to push that weight envelope. Fortunately for steel, there are other important properties to consider.
This is where steel shines. Young's Modulus for steel is approximately 30 million pounds per square inch. The titanium alloy Ti3Al-2V is 15.5 million psi, and 6061 aluminum is approximately 10 million psi. Those ratios (three to two to one) are almost identical to the density ratios between these three materials. That means that the stiffness-to-weight ratios for the three materials are about the same, provided that you concentrate on the material's behavior in tension or compression.
In case you need a reminder: Young's Modulus is the ratio of stress-to-strain in the proportional region of the curve, that is, below the yield limit. Too complex? All you need to remember is: the bigger the number, the stiffer the material.
Now, I hear you say, wait just a minute. If steel has a modulus much greater than that of aluminum, why are those big-tubed aluminum bikes so incredibly stiff?
Young's Modulus expresses the stiffness of a unit size sample of a material. Build that same amount of material into tubes with different wall sizes and diameters, however, and you change its stiffness. In fact, as a tube's diameter increases, its stiffness increases to the third power of that number. Comparing a one-inch tube and a two-inch tube of equal wall thickness, the fatty is going to be eight times as stiff as the little weenie tube. And the weight will only double. Now does the ride of those Kleins and Cannondales start to make sense?
Another simple illustration of this principle is to compare two tubes of the same weight, and look at the increase in stiffness as you increase the diameter. Take a one-inch steel tube with a wall thickness of 0.049 inches. Compare that to a 1.5-inch tube with a wall thickness of 0.032 inches. They weigh the same, but the 1.5-inch tube is 1.6 times as stiff.
Your next question should be: "Why not increase the diameter of steel tubes like you do with aluminum, so that we get an even lighter bike?" This is where the "beer-can effect" comes into play. As a tube's diameter-to -wall thickness ratio gets above 60- or 70:1, the likeliohood of failure due to buckling increases. This is the phenomenon known in the trade as "beer canning." Aluminum and titanium, being lower-density materials, allow you to have thicker, buckle-resistant walls.
If you remember, elongation is an indicator of ductility -- how far a material will stretch before it breaks. It's usually expressed as a percentage. Steels used in bike tubing typically have elongations of 9 to 15%. As elongation decreases, the risk of brittle frame failure increases. If the elongation number ever dips below 10%, I consider it a reminder to take a good look at the overall properties of the material.
Tensile strengths vary tremendously across different steel alloys and different brands. Consider a pair of standard CrMo steels: True Temper OX3 and straight gauge American airframe tubing. Either of them will build a good bike.
True Temper has a yield strength of 169 KSI, while airframe tubing has a strength of only 90 KSI. Does that make the True Temper a better choice? Depends. If the True Temper OX3 tubing is twice as strong, can you use it to build a frame with half the wall thickness of the airframe tubing version? Yes. Will the second frame be as strong? No. Will it be as stiff? Heck no. Will it last as long? Doubt it.
Remember, you need more than one number to understand how a frameset will behave. It's easy for a metallurgist to convince an ad guy about the superiority of one material over another. Don't be taken in. If you're building -- or buying -- bikes, you need to see the big picture.
Steel is a wonderfully reliable material for building bikes. It's safe to say thatit's the most successful material ever used. It's easy to work with, can be welded or brazed, requires only simple tools for fabrication, fails in a predictable manner, and it's cheap! As a result, there have been few serious challenges to steel's superiority over the last century.
However, it's 1994 now, and steel is being challenged. The numbers of aluminum bikes have been increasing for two decades, titanium has been used successfully for 10 years, and now other promising materials are coming to our attention. To learn more about these, stay tuned... The next installment of this "Heady metal" series will cover aluminum.