Cryogenics wr2 Aluminum Bronze & Nickel Aluminum Bronze

[metman]

In concert with this thread, I will be conducting independant research on the metallurgy of the subject alloys because of my limited knowledge about Bronze alloys in general. It has been 40 years since studying it in school and I have only touched on the comparative strengths of some of these alloys in the last couple of years.

I am considerring making use of LN2 (-320F as I recall) for shrink fitting bushings into Aluminum machined housings. This is purely a mechanical convenience for ease of assembly so I am not attempting to enhance metallurgical/mechanical properties in any way. Therefore I would appreciate no reference to the possibility of improving properties of these alloys by cryotreatment. It is not part of my agenda so don't have time for it.

On the other hand I do need to know if there are any known deleterious effects of subjecting Al Bronze or Ni-Al Bronze alloys to -320F.

I have been invloved in some of the threads on this forum about cryotreatment so I know there are some bonified experts here and I will be re-reading some of those threads so I can identify who you are but I don't recall off the top of my head any specific refernce to these alloys.

 

[EdStainless]

As I recall some of the NAB grades can have problems with low temp ductility. I believe that it has to dowith poor heat treatment resulting in nonuniform microstructures. I'll look into when I get back to the office.

My only other comment is to double check the thermal expansion curves. Many alloys have CTE that decreases at lower temperatures. You might get 80% of the desired shrinkage by only goin g to -150F and it would make your life easier.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[mewhg]

Metman,

I have done a lot of work shrinking bronze bushing in LN2. There has never been a problem with the bushings working properly after installation.

The largest bushing I shrunk to fit into a machined bore was 52" ID. However, this one was packed with dry ice.

 

[unclesyd]

LN2 took over as the standard in our machine and compressor shop from dry ice and acetone or freon.
Both the machine shop and compressor shop have both the transport and open top dewers for LN2. The compressor shop also has a 24"x24" cold box.

As I recall there was only one incident where there was any problem. It was cracking of the housing when someone in their infinite wisdom decided to speed up the heat recovery process with a torch, the housing cracked.

Several of the compressor repair companies use our LN2 if working in the area.

 

[metman]

Thanks for your quick responses. This is very helpfull. I just uncovered a printout that I had from Oct 2002 off the i-net of ASM info on heat treatment of Copper Alloys.

The subject alloys act much like Steel in heat treatment so now I am curious about the Mf temperature for UNS C63000 (AMS 4640)which is one of the primary alloys I am working with. If the Mf is below ambient, then I believe I should retemper at 1200F after cold treatment to relieve brittleness. These parts might go onto an airplane and I cannot take ANY chances on brittleness. Maybe this is what EdStainless is referring to. In any event, can someone tell me what is the Mf for 63000? This would mean an intermediate cold-treat/temper before cold shrink for installation if Mf is below ambient.

Also Ed -- can you give me a source or link for the CTE of NAB grades at sub zero temps?

I do not currently have accesss to AMS on-line but intend to in the near future.

 

[metengr]

metman;
The ductility problem with C63000 is based on an improper quench and temper heat treatment ( ASM Handbook, Volume 4). See below…
{...Unlike steels, aluminum bronzes are tempered above the normal transformation temperature, typically in the range from 565 to 675 °C (1050 to 1250 °F). In the selection of tempering temperatures, consideration must be given to both required properties and the hardness obtained upon quenching. Normal tempering time is 2 h at temperature. Moreover, heavy or complex sections should be heated slowly to avoid cracking. After the tempering cycle has been completed, it is important that aluminum bronzes be cooled rapidly using water quenching, spray cooling, or fan cooling. Slow cooling through the range from 565 to 275 °C (1050 to 530 °F) can cause the residual tempered martensitic phase to decompose, forming the embrittling - eutectoid. The presence of appreciable amounts of this eutectoid structure can result in low tensile elongation, low energy of rupture, severely reduced impact values, and reduced corrosion resistance in some media. For adequate protection against detrimental eutectoid transformation, cooling after tempering should bring the alloy to a temperature below 370 °C (700 °F) within about 5 min, and to a temperature below 275 °C (530 °F) within 15 min. Normally, the danger of eutectoid transformation is much lower in nickel-aluminum bronzes, and these alloys can be air cooled after tempering.}

What is the heat treatment condition of this material for your application? If the material is in an annealed condition or as-forged or extruded, you should have no handling problems with this material at low temperatures.

 

[metman]

Thank you metengr. This is the same printout that I referred to above. I think the q&t version is what I have so that is the reason for my concern given EdStainless' comments and not knowing the Mf temperature even though the statement "the danger of eutectoid transformation is much lower in nickel-aluminum bronzes" relieves concern of improper heat treatment. I need to verify what AMS 4640 calls out but I'm fairly certain it is in the YS range of 60ksi which would not be the annealed version.

It is curious that the ASM desk edition in one place refers to the 1200F treatment for C63000 as annealed and in another place refers to the 1200F treatment as tempering. I will have to get the page numbers when I get back to my workplace and post them in case someone else is interested in this conumdrum.

Another twist is that the bushings are Cad plated which precludes another 1200F tempering cycle.

I am probably overworking this whole thing but cannot let it rest until I know the Mf temperature because of the possibility of transforming fresh, untempered Martensite by cold treatment.

 

[metengr]

metman;
I believe your clue toward avoiding the harmful martensitic transformation is embedded in the ASM excerpt;

"For adequate protection against detrimental eutectoid transformation, cooling after tempering should bring the alloy to a temperature below 370 °C (700 °F) within about 5 min, and to a temperature below 275 °C (530 °F) within 15 min."

If you need further assurance, I would consider running a mock-up of the NiAl bronze material in your possession, and compare hardness before and after immersion in liquid nitrogen.

 

[metman]

Thanks metengr;
Interesting thought about the excerpt.

I will do the test you suggest.

To clear up my previous comment about anneal vs tempering:
It is not the desk edition rather ASM METALS Reference Book Third Edition. Page 473 is: Annealing temperatures for cold-worked coppers and copper alloys and gives 1200-1300F for UNS C63000 whereas on page 477 it refers to Solution treated 1575F, q&t 2hr at 1200F.

Previously while hurrying, I missed the clause "cold-worked." So in that case it is not referring to annealing of q&t product.