Material with the worst heat transmission?

[MechaniX_034]

Hi again, another stupid question from a non-engineer. Might anyone be able to tell me which material of the following has the worst heat transmission and is the best for a thermal barrier between parts in contact? The material will be of thickness no less than 3mm, and needs to insulate against some pretty high temps.

• Stainless steel
• Mild steel
• Aluminium
• Titanium

Flex is a major concern since it will be used in an extreme high pressure design scenario.

Thanks again!

[Zico]

See http://www.engineersedge.com/properties_of_metals.htm

You will be pleased to know that both Stainless Steel 304 (followed by closely S/S 430) are the least conductive of the materials you mention.

[Zico]

What sort of temps are you talking about.. and will thermal expansion figures also be relevant to your application?

[AU N EGL]

Wrap your item in gold foil. Great heat insulator.

Worst is anything Chromed.

[Tim Falce]

AU N EGL, wrapping the part in anything isn't going to stop transmitted heat. I think the OP wants a good insulating type of gasket or barrier, or have I read it wrong.

[AU N EGL]

Gold Foil is the best heat insulator



Notice the tubes of the roll cage and the firewall. all wrapped in gold foil for insulation against high heat.

Maybe if the OP would tell us WHAT is the inteded part? Exhaust header, or what? would be help full. and what temps does he expect?

[zac510]

He has asked for an insulator between parts in contact.

[dtype38]

I believe the answer to the original question was answered by the the original response by Zico.

For those who couldn't be bothered to follow his link, the thermal conductivities of the requested materials are as follows:

Stainless steel : 8.09-8.11 Btu/hr.ft.degf
Mild Steel : 26-37.5 Btu/hr.ft.degf
Aluminium : 136 Btu/hr.ft.degf
Titanium : 12.65 Btu/hr.ft.degf

Obviously the material with the lowest conductivity provides the best thermal insulation against conducted (in contact) heat. That leaves the limiting factor of flex... which could mean he wants resistance to it or ability to withstand it... so anyone got a link to a table of modulus of elasticity for these materials?

[Zico]

Sure.. Modulus of Elasticity in graph form.. http://www.engineeringtoolbox.com/yo...lus-d_773.html

And data.. http://www.postdiluvian.org/~mason/m...lasticity.html

Everthing you need to know about S/S http://www.stainless-steel-world.net/pdf/11021.pdf

and in Laymens terms..

Stainless steel grades

• 100 Series—austenitic chromium-nickel-manganese alloys
  • Type 101—austenitic that is hardenable through cold working for furniture
  • Type 102—austenitic general purpose stainless steel working for furniture
• 200 Series—austenitic chromium-nickel-manganese alloys
  • Type 201—austenitic that is hardenable through cold working
  • Type 202—austenitic general purpose stainless steel
• 300 Series—austenitic chromium-nickel alloys
  • Type 301—highly ductile, for formed products. Also hardens rapidly during mechanical working. Good weldability. Better wear resistance and fatigue strength than 304.
  • Type 302—same corrosion resistance as 304, with slightly higher strength due to additional carbon.
  • Type 303—free machining version of 304 via addition of sulfur and phosphorus. Also referred to as "A1" in accordance with ISO 3506.[5]
  • Type 304—the most common grade; the classic 18/8 stainless steel. Also referred to as "A2" in accordance with ISO 3506.[5]
  • Type 304L—the 304 grade but specially modified for welding.
  • Type 308—used as the filler metal when welding 304
  • Type 309—better temperature resistance than 304, also sometimes used as filler metal when welding dissimilar steels, along with inconel.
  • Type 316—the second most common grade (after 304); for food and surgical stainless steel uses; alloy addition of molybdenum prevents specific forms of corrosion. 316 steel is used in the manufacture and handling of food and pharmaceutical products where it is often required in order to minimize metallic contamination. It is also known as marine grade stainless steel due to its increased resistance to chloride corrosion compared to type 304. SS316 is often used for building nuclear reprocessing plants. Most watches that are made of stainless steel are made of Type 316L; Rolex is an exception in that they use Type 904L. Also referred to as "A4" in accordance with ISO 3506.[5] 316Ti (which includes titanium for heat resistance) is used in flexible chimney liners, and is able to withstand temperatures up to 2000 degrees Fahrenheit, the hottest possible temperature of a chimney fire.
  • Type 321—similar to 304 but lower risk of weld decay due to addition of titanium. See also 347 with addition of niobium for desensitization during welding.
• 400 Series—ferritic and martensitic chromium alloys
  • Type 405—a ferritic especially made for welding applications
  • Type 408—heat-resistant; poor corrosion resistance; 11% chromium, 8% nickel.
  • Type 409—cheapest type; used for automobile exhausts; ferritic (iron/chromium only).
  • Type 410—martensitic (high-strength iron/chromium). Wear-resistant, but less corrosion-resistant.
  • Type 416—easy to machine due to additional sulfur
  • Type 420—Cutlery Grade martensitic; similar to the Brearley's original rustless steel. Excellent polishability.
  • Type 430—decorative, e.g., for automotive trim; ferritic. Good formability, but with reduced temperature and corrosion resistance.
  • Type 440—a higher grade of cutlery steel, with more carbon in it, which allows for much better edge retention when the steel is heat-treated properly. It can be hardened to around Rockwell 58 hardness, making it one of the hardest stainless steels. Due to its toughness and relatively low cost, most display-only and replica swords or knives are made of 440 stainless. Also known as razor blade steel. Available in four grades: 440A, 440B, 440C, and the uncommon 440F (free machinable). 440A, having the least amount of carbon in it, is the most stain-resistant; 440C, having the most, is the strongest and is usually considered a more desirable choice in knifemaking than 440A except for diving or other salt-water applications.
  • Type 446—For elevated temperature service
• 500 Series—heat-resisting chromium alloys

• 600 Series—martensitic precipitation hardening alloys
  • 601 through 604: Martensitic low-alloy steels.
  • 610 through 613: Martensitic secondary hardening steels.
  • 614 through 619: Martensitic chromium steels.
  • 630 through 635: Semiaustenitic and martensitic precipitation-hardening stainless steels.
    • Type 630 is most common PH stainless, better known as 17-4; 17% chromium, 4% nickel.
  • 650 through 653: Austenitic steels strengthened by hot/cold work.
  • 660 through 665: Austenitic superalloys; all grades except alloy 661 are strengthened by second-phase precipitation.

• • Type 2205— 2205 is the most widely used duplex (ferritic/austenitic) stainless steel grade. It finds applications due to both excellent corrosion resistance and high strength.

Too much info?

[JimW]

Originally Posted by AU N EGL

Wrap your item in gold foil. Great heat insulator.

Au contraire. Gold is an excellent conductor of heat, possibly the best. It reflects thermal radiation well but that's not the issue here.

Regards

Jim

[MechaniX_034]

Thanks for all the replies gents!

Originally Posted by Zico

What sort of temps are you talking about.. and will thermal expansion figures also be relevant to your application?

Temperatures at time of operation will range from ambient to approximately 940.C Thermal expansion won't be an issue as adequate clearance in the area would do the trick(combined with directed airflow)

[Notso Swift]

Are Ceramics definatly out of the question?
THe best insulators are ceramic, but they are brittle. That said if it is in a sandwich you may be better served.

[dtype38]

Or make the item in Stainless Steel and have it ceramic coated like an exhaust manifold?

[JohnD]

"Flex is a major concern since it will be used in an extreme high pressure design scenario."

The best insulator in general is - nothing!
A vacuum, or a gas.
Would a honeycomb composite be out of contention?

JOhn

[MechaniX_034]

Well, what I intend to do is to sandwich a thermal barrier between the pad and the caliper in my brakes. The Brembo's on my setup have gone from red to black, and the caliper is showing signs of increased heat on the near side. not good when that's the same side that the brake line's attached to. Thus far they've gotten hot enough to fade 900-degree pads in just three laps(sprints) because of that I've very limited flying laps out of five at the very best, with the rest being used to cool the brakes down.

Thus, a honeycomb is definitely out as it will probably collapse the moment (if) any of the walls fail.

And yes, I have brake ducts(they're the crappy Japanese ones though, am building up new stuff as we speak)

[tristancliffe]

Perhaps you should seriously consider CFRC brake discs and pads with large ducts, ala F1?

If you have 'faded' 900°C pads, then insulating them won't make matters better because the pads will fade earlier. Ultimately you need to increase the thermal capacity of your brake system (bigger, wider discs), so that over the course of a sprint they either absorb the heat or can get rid of more of it on the straights.

What do the really quick people on the sprints do with brakes? Might it not be best to copy their setup a bit?

[JohnD]

Ah, So!
Even I would have discounted a honeycomb composite there.

My solutions in order of expence:
1/ better air flow to brakes. Air is cheap!
2/ water cooling. A water jet into the inner air intake of your vented disk. Not a big one, a windscreen washer jet is enough, and a small electric pump to drive them. This is ideal for a sprint car as the water supply will be limited. You can arrange a switch to turn on the pump when you apply the brakes - a feed from the brake lights would do, as such a small pump doesn't need much current. You don't need a lot of water as the latent heat of evaporation of water is enormous, much greater than that of exotic liquids - see the Wiki http://en.wikipedia.org/wiki/Latent_heat - and you don't want to squirt so much as to rapidly cool and crack the disks, just remove a lot of heat as steam.
3/ Recirculated brake fluid. This uses the whole volume of the brake fluid in pipes and master cylinder as a heat sink - again useful for sprints, but less so for endurance racing, where it can only radiate from the pipes, unless there is a seperate heat exchanger.
John

[ian_w]

Originally Posted by MechaniX_034

Well, what I intend to do is to sandwich a thermal barrier between the pad and the caliper in my brakes. The Brembo's on my setup have gone from red to black, and the caliper is showing signs of increased heat on the near side. not good when that's the same side that the brake line's attached to. Thus far they've gotten hot enough to fade 900-degree pads in just three laps(sprints) because of that I've very limited flying laps out of five at the very best, with the rest being used to cool the brakes down.

Thus, a honeycomb is definitely out as it will probably collapse the moment (if) any of the walls fail.

And yes, I have brake ducts(they're the crappy Japanese ones though, am building up new stuff as we speak)

Are you actually talking about pad fade or brake fluid boiling? A thermal barrier between the pad and the piston will only help the latter ( and make the former worse ).

If it is a fluid boiling issue then make sure your caliper pistons are steel / stainless or titanium under no circumsances should you use aluminium as it just conducts your pad heat directly into the brake fluid. Another thing to do is remove the pistons from the caliper and machine the ends with a 'castle' shape. This does two things - firstly reduces the thermal 'path' to the fluid and secondly lets some air circulate in this critical area.

You should obviously also use a high quality racing brake fluid ( e.g. AP 600 ) rather than normal road car fluid.

Another thing about insulators, no matter how good they are they sill need to have a finite thickness to make any difference. Simply spraying on a super whizz-band cermaic coating that is only a few microns thick only works in the world of snake oil salesman not in a world where normal physics applies.

[MechaniX_034]

Originally Posted by ian_w

Are you actually talking about pad fade or brake fluid boiling? A thermal barrier between the pad and the piston will only help the latter ( and make the former worse ).

If it is a fluid boiling issue then make sure your caliper pistons are steel / stainless or titanium under no circumsances should you use aluminium as it just conducts your pad heat directly into the brake fluid. Another thing to do is remove the pistons from the caliper and machine the ends with a 'castle' shape. This does two things - firstly reduces the thermal 'path' to the fluid and secondly lets some air circulate in this critical area.

You should obviously also use a high quality racing brake fluid ( e.g. AP 600 ) rather than normal road car fluid.

Another thing about insulators, no matter how good they are they sill need to have a finite thickness to make any difference. Simply spraying on a super whizz-band cermaic coating that is only a few microns thick only works in the world of snake oil salesman not in a world where normal physics applies.

Hi Ian, sorry for being so long. I'm using Castrol's SRF at the moment, and have used AP600 previously. Did a sprint over the weekend(NSW VS VIC) and VIC walked away with both the top place(I came in 3rd) as well as the majority in the top ten and top three positions.

I have checked the caliper pistons and can confirm that they are steel. On the weekend I had to manage the brakes to get at least two flying laps in a session. Neither fluid boil or pad fade were apparent(thanks to sessions being longer, and I could have two cooling laps in addition to the cool down)

I'm beginning to think that the problem here lies with me and not the mechanical parts(done a couple of events since the OP)

However, concerning machining the piston, I've managed to have a close look at a Superkart this week and noticed that the calipers in it are drilled out. Would you gentlemen recommend the "castle" idea or simply having reliefs put in as in image 2:

Image 1: Castle



Image 2: Relief

[tristancliffe]

Fading brakes in two laps? When I sprinted out little F3 car with my brother the brakes were barely coming into the zone by the end of our back-to-back runs (with driving change cooling period in between admittedly).

The castle reduces the materal available for conduction. The relief idea won't make any difference other than lightening the pistons by about a gram. Castles or nothing (with radii on the base of the castellations [spelling]).