Chromoly steel is represented by the number 4130. A lot of riders assume that higher numbers equate to better quality tubing. This is not true. The numbers associated with metals actually contain information about the metal. 4130 is a family of steels designated by the SAE (Society of Automotive Engineers). 4xxx steels are Molybdenum steels. 41xx steels are Molybdenum steels with one other material, Chromium. That's why it's called Chromoly. The last two numbers indicate the carbon content of the metal. In this case, the number 30 indicated a .30% carbon content, making Chromoly a high-carbon steel. The high carbon content allows the frame to be heat treated, but makes the material slightly harder to weld. Hi-Tensile steel, known as 1020 is a plain mild steel, indicated by the 1. The first 0 indicates no extra alloys. The 20 indicates a .20% carbon content making the steel a low carbon metal that cannot be heat treated effectively for use in bicycles. The reason 4130 is better than 1020 steel is that in bicycle applications, the greater strength to weight ratio makes the bikes easier to ride, the alloys make the material more hard wearing (rust resistance, etc.), and the vibration qualities are arguably more pleasant to ride. In applications outside bikes where weight isn't a factor but malleability is, 1020 steel may be the better choice.
Now that I've cleared up what 4130 is, I feel like I should clear up the difference between strength and stiffness. Imagine you walk up to a tree in the early summer. Imagine you pick up a dry stick from under the tree and you try to bend it. It won't bend. It will stay the same shape until it snaps. This stick is stiff but weak. Now imagine you rip a fresh stick the same size and width from the tree and you try to bend it. Because it's still full of moisture, it's soft but it will bend a fair amount before finally snapping. This stick is soft but strong. It takes a greater amount of force to make the stick break, or yield. 4130 is used in BMX frames instead of 1020 steel or 6061 aluminum, for example, because it strikes the best balance between strength and stiffness. An impact that may dent a 1020 frame or crack a 6065 frame or otherwise push a frame past its "yield point" would ideally put a tiny distortion in a 4130 frame and not much more. The same applies for longevity in terms of regular wear as well as impact damage. Aluminum frames don't rust, but are so stiff they fatigue noticeably with time. Mild steel frames will rust much more easily than a 4130 frame. 4130 is just right.
Continuing with the topic of metal fatigue, 4130 frames do experience fatigue but they are extremely long lasting if well maintained. No matter how bad it is, even if it's about to die, steel frames do not "go dead" or lose their ride quality with use. Any wear that may compromise the ride quality would be a big, visible crack or hole that allows the frame to flex beyond its normal range of motion (or past its "yield point") and would render the frame unusable anyways. No matter hold old your steel BMX frame is, it will always provide the same ride as it did when it was new because the elastic response, or elastic modulus, of the frame is an inherent quality of the metal that never changes. In fact, all kinds of steel have the same basic elastic modulus, regardless of age.
The differences between different brands of tubing and frames from different manufactures is not in the metal, but in the design and manufacturing technique. The inherent qualities of steel never change. For example, if you made two identical frames but made one from 4130 and one from 1020, they would be the same weight and have the same elastic modulus. The different alloys in 4130 steel actually allow manufactures to make frames differently (thinner tubes, butting, CNC, and other machining techniques) to dial weight and ride quality while maintaining the same strength. When someone tells you that a frame manufactured overseas won't be as good, they're usually wrong but even if they are right, the metal has nothing to do with it. If a Taiwanese company and and American company both made identical frames out of identical Reynolds steel, both frames would provide identical ride qualities and the same overall strength. The differences between the American frame and the Taiwanese frame would be that the Taiwanese frame would cost a little bit less due to lower production costs, and the American company would be able to provide better customer service (i.e. faster warranty) being based on the same continent. Country of origin does not affect the quality of the bike.
I'm done. Sorry for ranting like a crazy homeless guy. I might do a rant about frame geometry later this week.
