Why is Hi-Ten steel considered to be generally inferior Cro-Moly as a frame material? Some companies are making somewhat costly bikes out of Hi-Ten steel.

Not anything I would consider costly.

A new chromoly frame only (mtb) would be between say $200 (o.k) to $800 (awesome).

A complete bike under $300 and only a portion is in the frame = tensile steel.

Chromoly alloy steel is stronger (harder) and resists fatigue more.
Being stronger, frames are lighter as the tube wall thickness decreases.

I would, if this is you asking about a purchase? Suggest Alu frames.
Alu is superior to tensile steels, may require suspension to smooth the ride.

Chromoly has become more of an exotic metal in new frames.
Some riders swear by 4130 chromoly as THE metal for frameworks.

I never saw a costly Hi -ten frame.Huffys,roadmasters and other mart stuff certainly isn't.

It used to be common practice to label a mostly hi-ten(carbon steel) fram as cromoly if they had some cromo in it. Such as a seat tube or a downtube. It was sneaky. So far as I know that doesnt happen anymore fortunately.
I personally love 4130 cromo. It is just about the toughest frame material out there, so long as the rust is prevented(which isnt hard).
The difference is in what they CAN do with the two. Carbon steel is very soft as compared to cromoly or aluminum. The typical huffy frame(which is the prime carbon steel) the rear triangle can be bent open or closed with one hand(I had to junk a few for science in childhood). Carbon steel rusts relatively easily. Due to all these traits, they have to make the tubes thick. AKA heavy.
4130 Chrome-Molybdenum steel is second only to titanium in overal strength but is far cheaper. It has a much higher spring stregth as well, which makes a frame rugged. That gives the 'lively' feel people including myself rave about. Due to its strength, it can be thinned out greatly and still be useful. On the better frames, the "butted" frames, they can taper the tubing to some amazingly thin diameters and still not worry. I was totally amazed when I welded a part of mine, the stuff was paper! But it made it through a 20 mph hit on a sharp rock, then the remaining 7 miles of drops and shutes to get home without bending anymore.
Some of the new generation of Cro-mo frames are getting almost aluminum wieghts. One I saw was down to 4 lbs for a large sized mountain frame. You will not likely find any suspended designs on the shelves with ferrous tubes anymore, but the dirt jump and hardtail freeride guys chose 4130 for a reason.

You probably should check a materials handbook.

In a purist sense carbon steel can be the hardest. Not in the bike world. In the bike world it is more akin to butter. More importantly it lacks the resiliency that the other materials offer.

Tell me about the titanium/cromo rubbish.

Formulate a good question and I will reply.

I think you will find that Ti trails 4130 in stiffness,tensile strength and ultimate strength. And 4130 is relatively low rung steel in todays market.

The ONLY problem (rust really isn't a problem...) with steel for frame materials is that it is "density" challenged. For given tube dimensions (diameter, thickness, etc...) it is indeed extraordinarily strong, but very heavy. That is you can make a much thicker titanium tube that is as strong as a steel tube (which will be much much thinner) and weighs less, due to titanium's density advantage. Same with Carbon and aluminum. Having said that, modern steel frames are quite light. For example for my frame size, 52cm, I was quoted an approximate weight of 2.9lbs for a compact frame made from Columbus Spirit steel tubing, which is highly shaped and butted. The thinnest portions of these tubes are less than 0.5mm. A similar frame of titanium might be 2.6-2.8lbs and maybe 2.2-2.5 in carbon (just guesses). A highly durable, very stiff steel frame can easily be in the 3-3.5 pound range, with which one could have a 16-17lb total bike. You can get even lighter with Ti or carbon, though.

I don't think the answer is as simple as any of the replies I have seen. If you start with a cheap tubing set of straight gauge tubing, there is a lot less labor in building the frame. When you get into thin wall, multi-butted, heat treated tubing, you don't hire just anyone to weld (or in some cases fillet braze) your framesets. I believe there's also a heating process after the frame is welded to restore certain properties inherent in the tubeset. This would not be necessary with cheaper tubing. Obviously, you would expect better performance from a highly manipulated tubeset that was optimized for the specified application.

What I'm getting at is that the answer is two-fold. I'm also not aware of costly bikes made form cheap tubing. Do you have an example? I'll further point out that only one frame material will last virtually forever, and that's titanium. Contrary to what people might think, it's a relatively inexpensive material. Processing it is what drives up the cost. Machining requires very expensive tooling, and welding requires that the joint be absolutely contamination free, or the weld will quickly fail. If you were having a custom frame made to your specifications, and wanted it to last, you cannot find a better material than titanium. Argue that if you wish, but we weren't going to put up with the uncomfortable stiffness of aluminum, or the extra weight of steel for our tandem. This is a frame we will own for many years, and expect to rack up lots of saddle time. I've gotten totally off topic, but I'll sum it up by saying that I've seen people having heaps of fun out in the bush on rigid bikes that cost AU$300. It's not the tubing, or the bike, it's about how much fun you have. If you're my age, you probably had a Mongoose made from 4130, with 44x16 gears, and rode that all day everywhere. We're spoiled now.

Wrong. Both can be welded by robots,and there is no after welding heattreatment or processing.

Ideally there would be(there is heat treatment in aluminum out of necessity) but the cro-mo tubing is made specificly to not need it as much, allowing them to forego that step. Some of the newer alloys are what they call air-hardened, negating the need altogether.
This is also why the fillet brazed stuff is still regarded as the ideal construction means for steel. Everywhere there is a weld junction there will be different stresses which can create problems later in the way of fatigue. Thus in fillet brazing the temperature is much lower and the joint, being bronze, has a certain resiliency that reduces the fatigue factor. On my Giant frame the front triangle is tig welded and the rear and accesory mounts are brazed. The extent of the heat treatment usually used in steelwork that I have been taught at school and elsewhere is merely to normalize it by torching it evenly around the joint and letting it cool.

Well maybe, but only if it's done with silver,and there are damm few doing that any more. Air hardening stuff looses no strength in weldiig so has no issues in that regard. Welding or brazing isn't exactly new technology and frames of all kinds of steel have been made with all methods for years.It's not worth loosing sleep over the fine points,altho Grant Petersen might.

Bronze fillet brazing uses alot of heat,maybe even more than brazed lugs. Besides it's found essentially on custom frames and not many at that.It's not worth talking about especailly in the discussion of generic cro-mo vs hi-ten.

Do you ever not nit pick, Syd?

The stables always need sweeping.

Thanks for that. I was going to invite a frame builder to offer his opinions too. As a slight aside, I know of at least one builder that pins his lugs in prior to welding/brazing, just to avoid heating the tubing a 2nd time. Oh, we ride Ti, nuttin' better!

A fork made from Hi Ten can be just as strong as a fork made from 4130 Cro Mo. But, if they are equally strong, the Hi Ten fork might weigh half a pound more. Recently, I have seen a few aluminum frame bikes that use Hi Ten forks. (These are bikes in the $200 to $400 price range). But, it has been several years since I have seen a Hi Ten frame in a bike shop. Aluminum frames have gotten too cheap for the factories to fool with Hi Ten.

It's the shape of the tubing as well. This stuff is made by someone.
Some do it WAY better than others.
The forks shown have a curve, no lower bend.
This isn't 4130. Proprietary chromoly Ritchey Logic by Tange.
This is a very strong ATB racing fork.
Not heavy, strong.

I'm not sure how you would make a tensile steel fork very strong. 1 cm thick wall?
Honeycombed internal lattices?

Anyway. This is a HQ solid fork, cushy and strong.
The one on the bikes mine, maybe 14 yrs old. Gets run everyday.
Gets pounded on every other.

C'mon, tensile steel in bikes is for low quality, entry level, kids and ones that are so horrible to ride they sit in the garage till someone sells them.

Would you buy pedals with tensile steel axles instead of chromoly?
Not I.

As was stated before, there are many proprietary alloys of cromoly steel.

I want that tandem!

Thanks. We hear that a lot, followed by choking sounds after someone asks how much. If you're in the states and have connections, you can build one surprisingly cheap. No Hi-Ten here.

Does someone actually ride on that rear seat?