Why is one 340 connecting rod bronze color?

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Well, you know, oxy-acetylene and TiG steel welding rods are copper coated to keep them from rusting.... and that works just fine; I have had some hanging around for a decade or more and are still fine to use. So the idea of corrosion resistance has some basis to it. Maybe these are for marine use??
 
so the mystery continues.....well..if I am wrong as noted above,..apparently no own offers any other facts...LOL



Tony, I think you are right on the money. Best explanation I have heard yet.
 
Well, you know, oxy-acetylene and TiG steel welding rods are copper coated to keep them from rusting.... and that works just fine; I have had some hanging around for a decade or more and are still fine to use. So the idea of corrosion resistance has some basis to it. Maybe these are for marine use??

But copper does oxidize rapidly in the right environment. They're probably copper clad for several reasons, including conducting electricity and heat shielding to some degree.

A con rod is bathed in oil constantly, not much reason to try to save it from oxidation, and it wouldn't explain the presence of just one rod at a time in some motors.

The rework idea seems legit. I've seen chrome used in some applications, but it's not easy to grind or bring to a good size after plating, versus copper or bronze which is.
 
I have worked some metal to size and contour in my life.
A thin layer of copper may not "hammer out" when used to make finishing dimension.
A thick layer, even backed by steel, would muuush out quickly.
 
I have worked some metal to size and contour in my life.
A thin layer of copper may not "hammer out" when used to make finishing dimension.
A thick layer, even backed by steel, would muuush out quickly.

and when it comes to a press fit on a wrist pin, they're probably aiming for tenths (.0001) rather than thousandths. It's not like trying to bush a hole..
 
Well, you know, oxy-acetylene and TiG steel welding rods are copper coated to keep them from rusting.... and that works just fine; I have had some hanging around for a decade or more and are still fine to use. So the idea of corrosion resistance has some basis to it. Maybe these are for marine use??

This is exactly the reason.

If it is coated to add material and to "tighten up" the pin or rod journal end then why is there no copper on those surfaces? Because it's coated then machined. If it was added after machining to tighten it up then those bores would still have copper colouring. Even the rod in the video posted was machined steel on both ends.

I believe they were coated for preventing corrosion BEFORE installation while they waited in some bin somewhere in a warehouse or factory. J.Rob
 
To be precise, connecting rods are not "cast". They are forged, and forgings are often much rougher looking than castings, thus the misshapen rods. As long as the two bores are parallel and properly spaced, I don't think the factory cared much about how the rod looked.
 
Here you go gents, has nothing to do with corrosion resistance, it revolves around the hardening process. Or to be more precise, preventing the hardening process. It allows you to have hardend bearing surfaces, while preventing the hardening of the full rod body, so she will bend, rather than fracture. What did we ever do without the Google machine?? Grab a cup of coffee and enjoy the read...

...A connecting rod in an internal combustion engine provides linkage between the piston and the crankshaft. Particularly in two stroke cycle internal combustion engines, this linkage is done in a low lubricant environment. To accomplish this linkage, the connecting rod generally uses rolling element bearings between the crankshaft and the connecting rod at a first, large end of the connecting rod and also between the piston pin and the connecting rod at a second, small end of the connecting rod. The first, large end of the connecting rod includes a large end hole. Conventionally, after a rod is forged or cast, the rod is cracked in a brittle manner to form a two component connecting rod that is assembled on to the crankshaft through the large end hole. Thus, after cracking, the connecting rod may be reassembled with bolts in a manner that gives optimal registry. Cracking of connecting rods is known in the industry.

It is also known that the connecting rod may be forged from a low carbon steel alloy having, at the very least, less than 0.70% carbon by weight, and usually having less than 0.20% by weight. However, in these conventional connecting rods, the rolling element bearing cannot successfully run on the forged surface because the low carbon steel is not strong enough to resist mechanical deformation caused by the load transmitted from the piston through the roller bearings during the combustion process. To resolve this problem it is known to carburize low carbon steel connecting rods to increase the carbon level near the surface area. The higher carbon level material can then be heat treated to form a specific microstructural constituent known as martensite.

Carburizing is the addition of carbon to the surface of low carbon steels at temperatures generally between 850° and 950° C. (1560-1740° F.) at which austenite is the stable crystal structure. It is known that austenite has a high solubility for carbon and therefore it is ideal to carburize at the austenite temperature. Hardening of the carburized surface is accomplished when the high carbon surface layers are quenched to form martensite. Thus, the carburization process allows for a high carbon martensitic case with good wear and fatigue resistance to be superimposed on a tough, low-carbon steel core.

During the martensitic heat treatment process after carburization, the low-carbon steel connecting rod undergoes a solid state transformation. Initially, the part is in a body centered cubic (BCC) structure at room temperature. The BCC structure is a fairly soft metallic state and is only able to dissolve a limited amount of carbon. During the heat treatment, the part is heated until it reaches a temperature where the low energy condition of the material is preferable to transform into a face centered cubic (FCC) structure. In the FCC structure, many more carbon atoms are able to fit into the interstitial portions of structure as compared to the BCC structure. After the carbon molecules have diffused to the interstitial positions, the part is rapidly cooled or quenched. During the quenching process, the part is transformed at a temperature where the structure is generally of the BCC type. However, if the cooling is sufficiently fast enough, then the carbon atoms do not have enough time to diffuse from the interstitial positions of the FCC structure, and the carbon atoms remain packed in the interstitial positions. At room temperature, the diffusion coefficient of carbon is very low and carbon will essentially be trapped in the position it is in. Since the BCC structure cannot contain this much carbon at room temperature, a third structure, martensite, with a body centered tetragonal (BCT) structure is formed. This crystalline structure has a very high amount of internal stress. Due to this internal stress, the product is extremely hard but brittle, usually too brittle for practical purposes. This internal stress may also cause stress cracks on the surface of the product. From the quenched condition, the part is tempered to increase the toughness, but only slightly, as a surface hardness of 60-63 hardness Rockwell C is desired. The tempering process is well known to those in the art.

Forging is a manufacturing process where metal is shaped by plastic deformation under great pressure into high strength parts. There is no melting and consequent solidification involved. Forging's plastic deformation produces an increase in the number of dislocations resulting in a higher state of internal stress. This strain hardening is attributed to the interaction of dislocations with other dislocations and other barriers, such as grain boundaries. Simultaneously, the shape of primary crystals (dendrites) changes after this plastic working of the metal. Dendrites are stretched in the direction of metal flow and thus form fibers of increased strength along the direction of flow.

Conversely, the manufacturing process of casting consists of pouring or injecting molten metal into a mold containing a cavity with the desired shape of the casting. Metal casting processes can be classified either by the type of mold or by the pressure used to fill the mold with liquid metal. Since casting is a solidification process, the microstructure can be finely tuned, such as grain structure, phase transformations and precipitation. However, defects such as shrinkage porosity, cracks and segregation are also linked to casting's solidification process. These defects may lead to lower mechanical properties in some castings. Subsequent heat treatment is often required to reduce residual stresses and optimize mechanical properties in cast products.

The connecting rod is machined to a near-net shape before the heat treating process. This is done because the cost to machine large amounts of hard material is costly and it is also difficult to create the hardness profile that is required for the part. Unfortunately, the solid state phase transformation distorts the connecting rod. Particularly, this distortion occurs during the quenching operation of the current process when the crystal structure changes from FCC to BCT. Such connecting rods are measured for distortion and straightened by bending. Accordingly, it is desired to create as little distortion as possible of the connecting rod from heat treatment through the phase changes. Ideally, the distortion is limited to less than the final grinding allowance. Thus, in conventional connecting rod manufacturing, the connecting rod has to be straightened before it can be used. Such straightening is a non-value added operation during the manufacturing process of a connecting rod.

Problems also arise if the connecting rod is completely carburized. The section of the connecting rod between the ends usually has a configuration of a thin-walled I-beam. Carburization of the thin-walled I-beam results in an unfavorable through thickness hardness condition. While this condition has good strength properties, it has poor fracture toughness. Thus, the connecting rod is subject to brittle fracture from any impact type loading, and an engine misfire event could cause a fractured connecting rod.

To address the through thickness hardness condition, connecting rods are normally copper plated. Since the diffusion of carbon in copper is very low, the copper effectively masks the carbon and prevents diffusion. After the copper plating process, the connecting rod may be selectively machined to remove the copper in areas where the part is to be carburized. As aforementioned, these areas are typically where the bearing rollers contact the rod at the thrust faces of the first and second ends of the connecting rod. Since these areas have to ultimately be machined, the copper plating is not considered to be a value added operation.
 
That satisfied my Recommended Daily Allowance of very specific metallurgical considerations of carbon molecular behavior in steel forgings during heat treatments.

Next let's hear about nitriding our cranks.

'snk..bthf..

Beavis...... He said "crank".
 
I just copy/paste from the inter webs, If it was written by me, it would have been on Big Chief paper with a red crayon..
 
Just remembered something from my steel melt shop days.
Copper IN steel (too high) is about the only thing that cannot be fixed while the heat is in the furnace.
Not many times, I saw heats of steel downgraded for copper, but it happened.
If it were too high for any grade, it would have to be scrapped and recharged in smaller parts.
Copper must be diluted out of steel,
no other process or addition will subtract it.
 
I stand corrected and am grateful for it. Not a value added operation--which is why it is not done anymore. J.Rob
 
While it makes sense, why isn't it on all the rods then? Did some just not have the copper washed off?
 
Crank Works - CWI Pro Rods

Heat Treatment
Our rods start with a copper plating of the non-critical areas. They are then heat treated to stringent specifications by a Nadcap, ISO, and AMS certified heat treating facility. This process substantially hardens the rods for increased strength. From there, the rods are stripped of their copper plating so that they can be properly shot-peened.
 
the only thing I can think of was that there were multiple parts suppliers feeding rods into the MOPAR assembly line. Some went back to strip the copper off, while others just pushed them out as is since it has no effect on their performance other than to boggle the minds of owners 50yrs later.

I mean think about it, the line was manned by people just like you and me, if it really didn't matter, I would be mixing them as well out of boredom
 
the only thing I can think of was that there were multiple parts suppliers feeding rods into the MOPAR assembly line. Some went back to strip the copper off, while others just pushed them out as is since it has no effect on their performance other than to boggle the minds of owners 50yrs later.

I mean think about it, the line was manned by people just like you and me, if it really didn't matter, I would be mixing them as well out of boredom
Good point. That probably makes the most sense.
 
then again, shot peening would remove it, so maybe some suppliers shot peened, some didn't
 
With the price of copper, you'd want to chemically strip it so that it can be reclaimed.
Shot peening wouldn't be a good way to remove the plating. Doubtful if peening would do much except make it look pretty.
 
then again, shot peening would remove it, so maybe some suppliers shot peened, some didn't

Or just a difference in the order of operations. Shot peen or blasting after carburize wouldn't always be needed. Shot peening as a process also probably wasn't in the cards for most engines.

I could just see some suit finally walking the assembly line and seeing mismatched rods and losing his marbles as he went to ream certain suppliers.
 
With the price of copper, you'd want to chemically strip it so that it can be reclaimed.
Shot peening wouldn't be a good way to remove the plating. Doubtful if peening would do much except make it look pretty.

Depends on the machine and it's setup. I had a couple 10'x10'x14' shot peen blasters for stainless and aluminum parts at an old job. They're effective. I wouldn't doubt that the copper would be long-gone, but unless care was taken to protect the big and little ends, you wouldn't have much left to grind/hone/finish after re-machining the bores.

Copper prices have also fluctuated over the years. Some years it's definitely worth reclaiming, others not so much. Most shops I knew that dealt with it would sit on scrap until the spot came back into favor for recycling. Chemical stripping also isn't cheap these days with EPA water discharge regs and the like for manufacturers - so I could see blasting it off if the price was way down.
 
if you shot peened it, would it be done before or after the carburize process? that might be it right there if different companies had different preferred methods

I also think some shops recovered the copper, some shops didn't. material value vs labor/equip/operation cost of the recovery process might make it more cost effective to just deliver them all coppery and what not..
 
Can't believe with all the small block rods I've had over the years that I didn't run across one of these..... of course I couldn't ever get one of those orange cans in the Keystone 30 pack either during the hunting season.. :) good read!
 
if you shot peened it, would it be done before or after the carburize process? that might be it right there if different companies had different preferred methods

Shot peen for strength would be done after carburize. The carb temperature is too high to retain the mechanical benefits if shot peened before. Shot peen isn't always applied though, and may or may not remove the copper. I know the machines I worked with would strip that copper off in a heartbeat - which is why we wouldn't have put them in our equipment, the contamination would relegate that work station to use ONLY with parts that don't get any subsequent finishing operations (platings and the like).
 
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