[saberience]

Every source says that titanium is as strong as the most commonly used steel. Sure if you're going for lesser used alloys of steel you may as well compare to lesser used alloys of titanium. Or just compare iron with titanium, as that's really comparing one element with another, and is the "fair" comparison.

And anyway, your original comment suggested someone was totally in the wrong for thinking a 4mm titanium plate was strong, which is obviously incorrect. 4mmm of titanium plate is clearly going to be really strong and resistant. They wouldn't make plane engines from it if it wasn't.

[petertodd]

> They wouldn't make plane engines from it if it wasn't.

...but they don't! Jet engines can only use titanium for certain low pressure, low temperature, sections. The high temperature parts are made from nickle/iron-based superalloys. And aluminum still gets significant usage, because for many geometries an aluminum part has a better strength/weight ratio.

Like I said, titanium is strong. But it's not magic. Stronger than any aluminum alloy, weaker than commonly used steel alloys. Hitting a 4mm plate of titanium with a hammer just isn't a very special experience. I've done it.

Hitting a 4mm tool steel plate definitely can be a special experience. Because it's so strong and hard that you could easily cause the thing to shatter, sending sharp shards in unpredictable directions...

[MeImCounting]

No the parent is correct. Steel is by and large stronger than titanium of the same size. Pray tell what is this "most commonly used alloy of steel"? Because just fyi different steel alloys are used for different applications just like different titanium alloys are also used for different applications.

Titanium has excellent strength to weight properties compared to steel. A 4mm titanium plate would absolutely be dented by common shop hammers. This doesnt mean that "titanium isnt strong" it just means they have different material properties.

[scythe]

Steel has a range of strengths. The "most commonly used steel" is probably just mild steel and yeah Ti-6Al-4V is going to be the rough equal of mild steel on strength broadly assessed. But a high strength steel alloy will be three times that strong, and titanium can only be pushed so far.

[SV_BubbleTime]

I would use “cold roll” instead of “mild steel”.

But otherwise, yea irrc… Grade5 Ti (6AL4V) is approx equal to ultimate tensile of 303 stainless (extremely common) at 50% the weight.

BUT… Ti doesn’t even get close to 600 steels (like Inconel) or even common 17-4PH, etc.

[petertodd]

Exactly.

Indeed, if your design goal is strictly "don't get dented when hit by a hammer", the "strongest" material could easily be a good synthetic rubber!

[MeImCounting]

For most non-architectural design goals striking the right balance of toughness strength and hardness is generally what you want correct? I would imagine for building a bridge you care much more about elasticity and creep strength.

[petertodd]

Also fatigue resistance.

Bicycle design is a good example of where this matters: steel has a significant fatigue limit, and can endure cyclic stresses below that limit indefinitely. Aluminum has no fatigue limit, so any flexing is inevitably eating away at fatigue life. Thus aluminum bike frames have to be made much stronger and stiffer than otherwise necessary, to avoid bikes breaking unexpectedly due to fatigue. And that in turn means that aluminum bike frames don't have as much of a weight advantage over steel as you'd expect.

[johnwalkr]

For a rigid road bike aluminum can definitely be made stronger and lighter, even though what you wrote about fatigue limit is technically true. People like steel because they feel it’s more comfortable to ride. For mountain bikes, you will find almost zero steel bikes. Here the stiffness and lightness of aluminum (and carbon fiber at the high end) is almost universally preferred.

One advantage that adds to the potential lightness of aluminum and carbon fiber bike frames is manufacturing method. Aluminum is cheap to machine and hydroform into efficient shapes, and carbon fiber can also be layed up into efficient shapes.

[justinator]

> For mountain bikes, you will find almost zero steel bikes

Surly makes (only) steel mountain bikes, and I think there are approx. a... there's a lot of them out there. One reason is that they are inexpensive (relatively) and take a lot of abuse.

https://surlybikes.com/bikes/trail

[petertodd]

> For a rigid road bike aluminum can definitely be made stronger and lighter,

Absolutely. What I meant was that while an aluminum bike frame can be lighter than steel, it's not as much lighter than steel than you'd expect. Steel bike frames tend to be only ~15% heavier than aluminum, not 50%.

Personally I bought a steel road because the difference in weight vs the aluminum alternative was small enough that I decided to go with the bike that looked nicer, and would last longer. Besides, I could use to lose a lot more weight than any bike ever could...

[janc_]

Right now, top quality steel bike frames at the minimum bike weight allowed by the UCI are stronger than top quality carbon fibre bike frames of the same weight. Aluminum frames of the same weight would not be considered usable probably... (Pro cyclists would still use carbon fibre bikes because they can be made more aerodynamic).

[johnwalkr]

That’s with a minimum weight imposed. Doesn’t change the fact that aluminum and titanium alloys generally have better strength-to-weight ratios than steel.

[Gare]

And fork is made out of steel (or carbon) even on aluminium bikes

[nullc]

Titanium is quite expensive. I don't know if it makes sense to compare it to mild steel and not compare it to fancier steel selections which are still much less expensive than titanium and also much easier to work.

AR500 has a HRC of 47, modulus of 220 GPa, and tensile strength of 1740 MPa. Ti-6Al-4V is 37, 113.8 GPa, and 880 MPa respectively. The AR500 costs less than half as much as the Ti, and is much easier to work (though obviously working will degrade the properties).

The titanium is super really light, however... so the choice of material will depend on how relatively important weight is vs size and how simple your geometry is such that the added difficulty in working with Ti doesn't add problems.

Obviously there are also other grades of Ti too, but I think the comparison generally holds: If you don't care about weight/mass there is a steel selection which will be stronger, cheaper, and easier to form.

If you do care a lot about weight, an aluminum alloy often comes out the winner unless you just don't care much about costs or have fatigue concerns.