T-50 vs. T-45 steel

I was reading a book about the restoration of a Hawker Hurricane. It said that Hawker used T-50 steel tubing in their aircraft, and that only T-45 was available today; and that they had to ‘re-certify’ the T-45 to ensure it was up to T-50 standards.

  1. What’s the difference between T-45 and T-50 steel?

  2. Why is T-50 steel unavailable?

  3. How can they ‘re-certify’ T-45 to T-50? Isn’t it one or the other? Or is it like when you buy ‘Choice’ meat at Costco, which may actually meet ‘Prime’ standards?

  4. Generally, what do the ratings numbers mean?

Nothing in my materials references talks about T-45 or T-50 steel. There is a T-100 (ASTM A514) high strength low alloy steel that is commonly used for civil construction and heavy/offroad construction equipment. I’m guessing that these T-45 and T-50 are either British Steel specifications, which like BS are now deprecated and no longer in use, or are references to a tube specification.

As for why a particular steel formulation may not be available: it may seem kind of stupid, because most common steel formulations are based on ASTM, AISI, or ASM specs (or the equivalent in Europe and Asia) but many steel formulations, especially high alloy or exotic materials, are proprietary and patented to the steel company, and may not be available if the company goes out of business. For instance, there are a bunch of Bethlehem Steel alloys that are no longer available at any price, which means a like replacement has to be equivalenced, i.e. demonstrated to meet or exceed all strength, toughness, thermal, and corrosion properties. (The irony is that the most common heavy crane rail configurations are all Bethlehem profiles; even though the company doesn’t exist, its name is all over the industries that manufacture things out of steel.) The Ladish D6AC steel, used (in among other things) the STS Solid Rocket Motor casings, is perpetually on the edge of being unavailable, as Ladish keeps threatening not to make it unless manufacturers can order a full mill run.

It is very expensive and time consuming to produce a new steel formulation, so mills prefer to manufacture their particular inventory of steels rather than made to order specification unless the customer is willing to purchase by the mill run. And steel formulations often go ‘out of style’ if a better (i.e. tougher, more corrosion resistant, more fatigue resistant, et cetera) steel becomes available for the same application. So taking a design from thirty or forty years ago that uses anything more exotic than A36 or A572 (common structural grades) often requires resourcing and equivalencing new steel.

But that’s nothing compared to having to build or modify a structure using thoriated magnesium. I hate that stuff.


That may well be, since Hawkers were built with tubular steel frames.

(As I mentioned in another thread, the structures were fascinating. Riveted and bolted together instead of being welded, round tubes made square or rectangular at the ends, streamlined bracing wires inside the fuselage…)

T45 is a carbon manganese cold drawn seamless tube. The full spec is BS 4T45:1972.

Weight %:
C: 0.17 - 0.25
Si: 0.10 - 0.35
Mn: 1.30 - 1.70
Cr: 0.25 max
Ni: 0.40 max
Mo: 0.10
P+S: 0.040 max

Typical Mechanical Properties:
Treatment: Hardened and Tempered
Tensile Strength (Rm, N/MM2): 700 / 900
Proof Stress (Rp 0.2, N/MM2): 620
Hardness HB: 201 / 262

T50, I got nuthin’. T45 is apparently obscure, and T50 must have been too rarely called for to stay on the books, or else (as Stranger says) whoever made it went out if business. But it’s not at all surprising that the replacement needed to be tested and certified to meet the requirements of that application.

This is in contrast to the Spitfire which was very complex and expensive to build. The Hurricane was the airplane equivalent of the Russian T-34 tank.

I don’t think the Spitfire is as complex as people say it is. It had a monocoque structure that, in the end, is just flat bits strengthened with stamped bits and riveted together. The plane was a mass of compound curves, and the elliptical wings are said to have been difficult. But assembling the parts doesn’t seem to be that much more difficult than, say, a P-51. (I have not built any airplanes.)

Look at this fuselage detail and this one. (Website.) The second photo shows a wing spar. The spars were 12-sided and had one tube inside of another. Two spars were joined by an aluminum web. (The Spitfire had five tubes per spar, of varying lengths to provide taper, but they were square tubes – not 12-sided.) In my non-expert opinion, I think the Hurricane was the more complex aircraft structurally. But I believe it was easier to repair, since damaged parts could simply be unbolted and replaced.

Back to the original question…

If I’m interpreting this correctly, the T-number doesn’t refer to the composition of the metal per se, but instead refers to a certain seamless tube made of that alloy. If that’s correct, might one assume that T-45 tubes and T-50 tubes had the same composition, but slightly different dimensions?

How much is a mill run of steel?

Since Stranger mentioned it, he probably can give you a better answer than I can. I’m guessing a ‘mill run’ depends on the size of the mill, but you’re talking about many tons at least. Only a guess, though.

Are you going to start building Hurricanes? If you are, save me one. :smiley:

Nope; the various T-specs are quite different, composition-wise, and do not include dimensions. The tubing as a whole must demonstrate certain mechanical properties, and there are various ways to achieve that.

A mill run is one production run of steel; in essence, how much the mill can produce from a single charge (quantity of iron). For open hearth and LD/BOS process this is usually somewhere between 100 and 300 tons, although there are micro-mills that produce speciality mills using the electric arc furnace process that will produce runs as small as a few tons. The problem with making small mill runs is that there is a lot of energy involved in heating up the oven that is basically a fixed cost. There is also a lot of effort and quality control in processing the raw materials to make certain the constituents are in the correct proportions. As a result, foundries would prefer to do large continuous or sequential batches rather than changing compositions.

It is also the case that the steel manufacturing industry in the United States is all but dead, except for a handful of specialty alloy producers, and so most steel has to be sources from Asia or Europe, where market restrictions and politics can limit the ability of a purchaser to buy materials JIT. As a result, middlemen like Ryerson have to hold a significant inventory to cover demand, so their warehousing costs are higher.


Okay, so I know this is a dumb question, but how many Hurricanes in a mill run? Fleets of 'em, I assume?

Empty weight of a Mk. IIc is 5,745 lb (2,605 kg). Subtract the weight of the engine (1,640 lb (744 kg)) and cannons (378 lb (172 kg)), and you’re left with 3,727 lb (1,694 kg). Of course you have aluminum structures and skins, hydraulic and pneumatic plumbing, fluids, instruments, paint, plexiglass, the wooden formers and longerons, etc. Maybe a ton of structural steel per plane? That seems a bit high to me. I don’t know.

Good luck finding enough Merlin engines for your fleet. (You could use the Packard V-1650, but good luck finding those too. You might make do with Allison V-1710s just to get them in the air.)

One day in 1979, a large pickup truck pulled in with a Merlin engine pulled into the gas station at worked at. I asked the drivers whether they were going to install it in a P-51. They just stared at me wondering how some dumb-ass teenager would know anything about a Merlin. They said no, it was for something else. They avoided talking to me after that. Strange. Why not just cover it with a tarp?

But even naked, siting on that wooden stand, the Merlin looked fast!

Those pics remind me of a friends comment about British design. More parts doing less.

tut, tut, this gentleman would disagree

OK, I decided to sign-up to respond to this thread on Sunday, but for some reason, my registration didn’t go through correctly and I couldn’t post until now.

I’ve been lurking for years (I first started reading the dope when 15 k of g in a f p d was going). Interesting board, to say the least.

Anyway, while I don’t have direct information on T45 or T50 steel, I do know steel and I also am very familiar with steel standards.

I suspect the “50” or “45” in the grade refers to the strength; being a British Standard, I’d guess it would be Tonnes per square inch. Per nametag’s post, it would seem that’s correct; that is, 620 N/mm^2 = 620 MPa = 89923 PSI = 44.96 Tonnes/in^2. Close enough for metric conversions.

So, if T45 has a yield strength (Proof Stress) of 45 Tones/in^2, then T50 would be expected to have a yield strength of 50 Tonnes/in^2 or 100,000 PSI. Nametag’s post says the processing is “hardened and tempered”, so the increase in strength would probably result from a slightly higher carbon content, say instead of 0.17-0.25 it could be 0.24-0.30, but that is just speculation. Generally, carbon contents below 0.30 are considered hardenable by water quenching without cracking. That is just a rule of thumb, but it does indicate there could well be some overlap in the chemical compositions of the grades.

I’d guess that T50 is no longer available because AISI 4130 (about 0.30 C with Chromium and Molybdenum alloying, instead of just Manganese) tubing can be processed to meet the same strength levels , but can be easily welded without losing significant strength and has higher toughness. 4130 wasn’t used much in WWII, particularly in England, because Chrome and Moly were in short supply (strategic materials).

It could be possible to re-classify the T45 as T50 if tests indicated that the strength levels met the T50 requirements. It is not unusual for material to test out 10% greater than the minimum requirements. Personally, I feel there are problems trying to do this, since if it is hard to tell if the test you are taking is the lowest or the highest of the lot, but with enough samples, I guess it would work. At worst, you might be 10% below the design strength. If that would be OK, then it really wouldn’t matter.

The numbers refer to the yield strength of the material, or 0.2% proof stress. That is the stress required to produce a permanent strain of 0.2%.

A suitable alternative would have be 4130 tubing heat treated to 100,000 PSI minimum yield. That is what I would have chosen over trying to re-classify material that was produced to a lower strength level.

As far as Stranger’s comment that the steel industry is all but dead in the US (except for specialty steels), well, that isn’t true, unless you want to classify nearly every type of steel produced as being a specialty grade (you could make that claim, since all steel produced is made to specifically special requirements). Nearly all the steel in a Chevy, Dodge, Ford, Honda, or Toyota that is built in the US is made in a domestic steel mill. Yes, some is imported, but the vast majority is made in the USA. Do a little googling on Steel Dynamics, Inc, or Nucor Corp, if you want to learn about some of the newer names, or US Steel, Timken, or Bethlehem Steel if you like the older, more well-established companies. I don’t think any of these are about to shut down anytime soon.

Excavating (for a mind)

Thanks for the detailed answer, Excavating (for a mind).

I’ll read or see something, and questions will occur to me. In this case, it was an airplane restoration. I suspect that the restorers wanted to use materials that are as close as possible to the originals, and that’s why they used the T45. If I were doing it, I’d probably use the modern material on the assumption that it would last longer.

Since the aircraft is not going to be flown as originally intended (i.e., in combat conditions), T45 is probably sufficient. I read in a Spitfire book that the ones now flying are up to 25% lighter than when they were in military service, so the stresses would be less.

Welcome to the Dope! :slight_smile: