cs1

Does anyone know why I can't have a 753 frame chrome plated? I was told that it wasn't safe on that alloy. I can't figure out why. Any help would be appreciated.


Thanks


Tim

LWaB

High strength steels and hydrogen embrittlement don't go together. Detailed explanations would take a while.

Falanx

Then why does the US Navy have 300M and AF1410 landing gear cadmium plated?

753 isn't very susceptible to static fatigue. As heat-treated and supplied (753 is tempered at 870K, so most of the martensite has been lost and replaced with recrystallised ferrite and alloy carbides), it contains fine, disperse dispersed alloy carbides which act as hydrogen sinks, and it doesn't contain a substantial volume fraction of martensite. Hydrogen embrittlement tends to be an issue in much, much higher strength steels with low fracture toughnesses or subject to much higher loadings (as a function of component strength) than any part of a bicycle frame.

Can you tell me who recommended you don't get it plated, cs1?

Dave Moulton

Reynolds recommended that you not chrome 753; it’s called covering their @$$. I chromed dropout faces only on 753 frames I built. This meant that about 2 inches of the seatstays, chainstays, and the fork blades were chromed under the paint.

I felt it was okay as these were heavier gauge than the main tubes. I never had a problem. Also it was my understanding that the temerature of my paint curing oven removed most of any hydrogen embrittlement. However I would not chrome the main tubes, I had to cover my @$$.

cs1

Thanks for the replies. I can't remember exactly who recomended against chrome plating. I do know they mentioned hydrogen embrittlement. I've always had a soft spot for chrome plated bicycles. Nothing looks better than a chrome plated Paramount. Seeing as the frame was a Waterford, I figured I was half way there.

Tim

Falanx

I know that one

However, Reynolds also recommend you use a SPECTACULARLY wrong filler wire for 953. I've taken to a pinch of salt with everything they recommend.

DannoXYZ 

Chrome-plating requires three layers, copper, tin and chrome for best adhesion. To prepare the bare-metal for the first layer, commercial-strength hydrochloric or sulphuric acid is typically used. The strongly dissociated hydrogen causes embrittlement in the metal; you'll want to have thick enough tubing to account for some loss of strength.

The final layer is added using chromic acid H2CrO4 - "hexavalent chromium". Spill a single beaker of this stuff and you'll need to evaluate an entire city block around the place.

Basically, the strongly acidic and toxic chemicals used in chrome-plating weakens the metal. Cad-plating is a quick, fast & cheap way to prevent rust on untreated steel. It's done through an electrolytic dip in a cadmium-salt solution, no acids to worry about...

Falanx

(For the TL;DR version, skip to asterisked bit)


Actually a single underlayer of nickel works best on ferrous alloys. I'm not quite sure what you're trying to say with the bit about strongly dissociated hydrogen, because hydrogen per se doesn't cause embrittlement. As I pointed out above, it only really affects untempered hardened conventional alloy steels, of which 753 isn't. And, you are actually better off with thinner tubing, not thicker, as the diffusion distance of hydrogen under the concentration gradient experienced across the steel at normal plating temperatures is much greater than any wall thickness, but the rate of diffusion back out of the steel is much lower...

Probably the most important to remember with Cr plating.


Solutions also contain H+ ions... from their water content. From the point of the steel, it doesn't really matter what the source of hydrogen, the partial pressure outside of the steel is almost infinitely higher than within at the onset of preparation. It's going to absorb it whether or not it's come from an acid, water or custard.

The reason Cd plating doesn't cause any issues is exactly the same as why CR doesn't cause you any issues from static fatigue: Because no-one is crazy enough to put an untempered high strength steel into service in any application anymore. This recommendation is based on Second World War engineering practice - a time so long ago that the Basic Oxygen Steelmaking process had not yet been invented.



******
The TLD;DR version of this is, as Dave pointed out, Reynolds are covering their donkeys on the recommendation, but with a recommendation that was sensible only when the alloy was actually invented - The late 1940s.

TBART

Excellent info. as always!

cs1

Outrageously technical answers so far. Am I better off selling the frame and buying one in 4130 or 531 then having it chrome plated?


Tim

Falanx

I'm not sure where you're coming from? They seem a little non sequiter?

Thylacine

What ever happened to Electrodeless Nickel Plating?

NoReg

"Because no-one is crazy enough to put an untempered high strength steel into service in any application anymore"

Can you expand on that? You mean nobody puts a high strength hardened but not tempered steel into use? At what point did they do that? Just expecting an interesting story, not questioning the point.

Falanx

Yeah. No probs. It'll be long though....;

Up until the mid-to-late 1940s, some applications - often military ones where components were subject to very high loading were through-hardened and stress relieved (a process that is often referred to as tempering, but demonstrates no martensite decomposition, so is really a lie). The components were, in other words untempered, meaning the martensite that was responsible for their hardness (and also the component most suceptible to the effects of trapped hydrogen) had not begun to decompose and liberate carbon as fine carbide particles and ferrite.

Now, as long as your component is redundant and never going to see contaminated water, or and corrosive environments this is acceptable. How many applications can you think of that fit that bill? Not many, I'll wager.

Then something wonderful happened...

In the fifties, Inco laboratories made some groundbreaking discoveries. First they developed the maraging steels, which suffer little from hydrogen issues due to their very soft high-nickel, carbon-free lath martensites and their enormous, ultrafine and profuse intermetallic precipitate strengthening (which is on a scale that conincides with the most efficient hydrogen sinks).

Alongside them came the nickel-cobalt secondary hardening steels, of which things like Aermet is a family member. These steels use the martensite of a sonventional maraging steel and intead of intermetallics, they strengthen it with profuse, incredibly stable ultrafine mixed carbides. This which Aermet was such a ***** to work with for cycle frame tubing manufacturers. It just eats tooling, unlike conventional maraging steels.

From this point forward, the approach to use of steel changed. In 1948, when the Basic Oxygen process for producing high quality clean, low carbon and nitrogen steel was developed, we had a way to ensure clean, and then with AOD/VIMVAR/etc electric vacuum secondary steelmaking techniques, ultra, ultra, just plain silly-clean steels, it became very easy to produce these ultra high strength, ultra high toughness, ultra clean steels just the way we wanted them.

Suddenly super-high mechanical properties didn't require us to quench high-carbon steels and leave them full of quenched in stresses to ensure they were hard/strong enough. We could get 2/3/4GPa UTS from a steel containing NO carbon. We could make a steel, harden it in still air and then temper it to death and it would only get stronger while sacrificing no toughness.

These days you'd have to be either nuts or just plain tight to go back to quenching the living daylights out of a 0.6% carbon steel just so you could have 55+Rockwell C hardness.


What's most important about this is that our inventory steels, our national standard grades - all the ones we use to make bicycle frames - now all contain either molybdenum or vanadium or both (with a couple of really stupid exceptions), both of which produce fine scale carbides when you temper a steel at about 550-650C. These carbides are perfect trapping sites for any hydrogen that has been absorbed by the steel. And they will trap a hell of a lot, trap it stably, trap it safely.

Falanx

There's always one.