Metal plating knowledge request - PROS ONLY PLEASE

[thedeatons]

Okay, so I'm having all the rear axle components re-zinc plated, to match factory specs... The local plating shop in San Diego said I should have the axle's hardness checked, on a Rockwell hardness scale. The engineer mentioned that if it's hardness was a 35 or higher on the "C" scale (as in C35 and up) then the part would need to be baked afterward for stress relief, due to the hydrogen embrittlement that is caused during the zinc plating process. He said if the axle was heat treated before, then it needs to be stress relieved basically...
So, I got the Rockwell hardness checked today and it came out to a 20 on the "C" scale (C20). This means that it is softer than what the plating shop recommends for needing stress relief after plating, but the testing shop said that baking it for stress relief won't hurt anything... I have had recommendations from different local shops about baking these pieces, and the zinc plater recommends baking within four hours of the zinc plating at 400 degrees for two hours time. Another shop mentioned 375 degrees within one hour of plating for a time of two hours... Does anyone have any experience with stress relieving parts? The zinc plating shop was going to charge me $90 to bake the parts at 400 degrees for two hours, but everything fits in my oven at home, so I will do it there, I just need to know the "correct" baking temperature and time. I plan to zinc plate the axle, hubs, axle nuts, and rear brake cable holder (that actuates the brake shoes). Any good opinions on this are welcome. There are no other options for me, so please don't mention "that you would just paint it" or anything like that... this is basically a concours resto, so these parts must be zinc plated.... Thanks guys.... James

[atctim]

I can't find anything on the net as far as a hardness to stess relief chart, but I can tell you this from Foundry experience - I never knew of any time limitations on heat treatment for stress relief. If I were you I would simply heat treat in your kitchen oven. Something else very useful would be the Rockwell hardness specs from the original axel specs from Honda. That way you can see where you are at and where you need to be. I have no idea where you might find that info, other than have a non-tampered with axel rockwell tested. Not sure if you know anything about Rockwell testing, but it is a simple procedure, and if you know anyone in a similiar field they could do this test for you. When you Rockwell test, all you do put an impression on what you are testing for harness. ie - it is like a round ball bearing that "X" amount of force is pushed on the material being tested. The softer the material, the larger imprint the "ball" leaves on it. Then there is a special scope you look through that measures the width of the "ball print", thuis giving you your rockwell hardness.

Advice: find someone you know in either the foundry field or any metalergical field and have them test an original axel, then yours. The test takes like 2 minutes to do.

Hope that helped a little.

[thedeatons]

Already had the test done, which is how I know the current hardness of C20.... I mentioned it in the second paragraph

[TeamGeek6]

Yep, had metallurgy. "Embrittlement" in this case is a surface phenomenon, theres nothing in the plating process that would "soak through" to the core of the axle (the zinc is a surface treatment only). I assume lack of heat treat afterward would mess up the plating and cause warranty problems. This is a specialty and best left to the advice of the plater. As to heat soak temperature, whatever they tell you, 300-400 is common for some steels, but again, this is different - its zinc. Yes, any oven will reach that temp, but Im not sure how to get it in as an axle is pretty long.

Hydrogen embrittlement is a chemical thing that causes small, local explosions in the material. It happens in copper. Hydrogen reacts with oxygen and blows the the surface material off when the steam escapes.

I serously doubt their processes are hot enough to affect the axle through its cross section, so I dont know why anneal or other treat is needed for that.

Rockwell scales are for surface characteristics, so thats the correct scale to use:

http://www.calce.umd.edu/general/Fac...ss_ad_.htm#3.1

Just a thought - if it wont fit in an oven, its not too hard to make a simple oven with a thermostat, if it costs less than $90. Something like a long rectangle box with high temp insulation and a dryer element on 220 VAC.

No, I dont know more than they about zinc plating, but I do know enough about metallurgy to say theres nothing in their advice that contradicts.

[thedeatons]

Hey Guys... I appreciate the comments so far, but please make sure you read my original post in it's entirity.... The baking afterward has nothing to do with warranty or the cosmetics of the plating. The baking is to stress relieve the material by baking out the hydrogen, because the hydrogen turns the metal brittle, and will cause the axle to break suddenly under load, instead of bending first as the manufacturer intended.
I am specifically asking about baking temps and times, and once again, I already mentioned that the axle will fit in my oven at home...
Keep 'em coming... James

[bigredhead]

By process of elimination.

Heat won't hurt it.. but lack of it might..... go with the longer time !

" This is a non-pro reply, and should be taken as such " lol...

[KASEY]

so is there no possibility of finding a new oem axle?
seems like alot of worries involved for something that might be setting on a shelf somewhere,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,, ,,,,,,,,,,,,

[TeamGeek6]

The baking is to stress relieve the material by baking out the hydrogen, because the hydrogen turns the metal brittle, and will cause the axle to break suddenly under load, instead of bending first as the manufacturer intended.

How will some microscopic deformations in a layer of metal a couple of thousandths of an inch thick on the surface cause axle breakage when the surface, unless it is polished, is already loaded with cracks?

[thedeatons]

New rear axle from bikebandit.com=$244.41

I know they're not the cheapest parts store on the planet, but unless I can find one about $150-175 cheaper, it's not worth it... Good thought though, it made me think... James

[dunehopper027]

I plate helicopter parts for the government. my opinion is that I would get anything in the oven withing two hours of plating. 400 degrees does sound about right for 2 hours. I wouldn't suggest doing it in your own oven due to the chemicals that are being baked and released.

[Red Rider]

I wouldn't suggest doing it in your own oven due to the chemicals that are being baked and released.

Well hell, if it'll save the guy $90, what's wrong with taking a chance on putting it in the kitchen oven. People have lived next to chemical & toxic waste dumps for years without too many deformations in future generations.

[thedeatons]

I talked with the plater AGAIN today... He said that everything he told me was according to the Mil-Spec (military specification) guide to zinc plating, which adheres to stringent military requirements for their parts, which have to be tested in a stressful environment (shock and vibe testing) before reaching the field/ship. I work in the military assembly field currently, and deal with military specifications on a daily basis. So... I can understand his process being more stringent than the civilian sectors. I will talk with him more about this later in the week when I take the parts by. Most of them are in the sandblasters right now... Oh, by the way, the baking process must happen after the zinc is coated but before the yellow chromate is applied, or discoloration will occur. The yellow chromate is just a coloring for cosmetics, the zinc coating is actually a dull gray. I am going to talk with the plater regarding baking the part in my own oven vs. his and also the benefits/disadvantages of stress relieving regardless of hardness. Although it's only $90, my project is already over $1000 so far, with more to go, so any trimming of the fat is nice, especially when I go over this stuff with my wife... I'm sure everyone understands... James