Metallurgy and Strength by Color?

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Not sure this is the right place to ask but I've been surprised more than once by the knowledge base we have here about metals so I'll give it a shot.Can you tell a materials relative makeup and strength by color?  In the Toyota Supra community, it has been generally accepted that a certain tranny variant has higher power holding capacity by a hundred or so ft lbs or torque.  The W55 (~350 torque) , which comes out of the Lexus IS300, vs the W58 (~450 torque)that comes out of a NA Supra.Both trannies are from the same family, same specs, with prob every part swappable (including bearings), save a slight variation in gear ratios (W58 slightly taller).  What accounts for the strength difference?Only obvious thing that I can see is the slightly different shade of metal in the gearset with the W58 being a darker hue.  Does that imply a different/stronger steel?W55 top.  W58 bottom. W55W58.This is all conjecture but maybe you guys can lend some insight?Last edited by TeCKis300; 01-28-2008 at 04:15 PM.
Reply:no your not going to be able to tell how strong the parts are by colorChuckASME Pressure Vessel welder
Reply:Hey TeCKis,chopper5 is correct regarding the different hues of metal. The only way to truly determine the difference in steel grades is with a hardness tester. I have seen & experienced many visual variations of steel that was the same grade and even from the same lot, so a visual exam will shed nothing regarding hardness/strength. Hope that helps a bit. Good luck...  DennyComplete Welding/Machine/Fab. ShopMobile UnitFinally retired*Moderator*"A man's word is his honor...without honor there is nothing.""Words are like bullets.... Once they leave your muzzle, you cannot get them back."
Reply:Of course we cannot determine the strength of a material just by visually observing it.  However, in this case I believe a few things can be deduced.  From the scenario that you have put forth (same gearboxes...each in a different car....one can tolerate more torque than the other) it seems very obvious to me that there are two things that may be different among the two gear sets.  Based only on the difference in color of the two sets I am concluding that the "stronger" of the two has either been exposed to some type of heat treatment process or it was manufactured from another grade of alloy.  Metals will typically, but not always take on a much different hue after being heat treated.I am not disagreeing with the other two previous posts....it is not possible to determine visually the strength of most materials, but in this case I believe the conclusion I have drawn are more than acceptable.-Graham-Mechanical EngineerAutosport Mechanic/Fabricator
Reply:Originally Posted by grahamtheengineerOf course we cannot determine the strength of a material just by visually observing it.  Based only on the difference in color of the two sets I am concluding that the "stronger" of the two has either been exposed to some type of heat treatment process or it was manufactured from another grade of alloy.  Metals will typically, but not always take on a much different hue after being heat treated..
Reply:A hundred pounds or two hundred more resilience to torque wouldn't necessarily need to be a housing materials grade or type issue. Heck it could be subtly incorperated at various points throughout the whole dern tranny. Slightly beafed up around a bearing retainer area in the housing. Maybe a particular shaft equals the same dimensions but different grade. Size of the pilot pins, mounting ears, on-and on.
Reply:After heat treating the gears need to be cleaned from the carbon deposits burned on the the material.A Rockwell will tell you how hard the outside casing is, it wont tell you how tough the gear material is.  9310 at 48c  vs. 8620 at 48c are 2 diffrent things altogether.s.r.
Reply:Short answer to the OP's question - No, you can't use the color of metal to determine all that much about its strength.As most of the other folks have already said.  Longer answer - The devil is in the details.  The 'strength' difference between the two transmissions could be due to a whole slew of things.  It could be a materials difference.  It could be a heat-treatment or other 'treatment' (cryogenic treatment, shot-peening, etc) difference.  It could be a slight, almost unnoticable, design difference between two parts (fillet radius diameter difference on shoulders, an extra mm or two on a shaft diameter even if the bearing diameters are the same, etc, etc).  It could be different grade or type bearings, even if they are the same size (angular contact versus plain radial contact ball bearings, etc, etc).  It could be that the factory lied, or changed their mind on how they rated them.  It could just be the slight difference in gear ratios.  It could be that one trans is rated for 100,000 miles before fatigue or bearing failure, and the other is rated for 75,000 miles before failure.  It could be more than one of the above.
Reply:Thanks for the discussion guys.  I wasn't expecting definitive analysis...just any potential "trends" that could be at play here.The casings are more than likely identical.  It's the strength of the gearset that seems at play here.  The common failure mode for these trannies is stripped teeth in any gear.  Which can be attributed directly to teeth strength and/or shaft flex placing more load on the ends of the teeth?I do believe that the differences are due to treatment/alloy, as grahamtheengineer  pointed out, rather than physical minute dimensional differences as that would help in cost ammortization of a mass produced component.Heat treatment is likely.  Shot-peening?  Cryo, prob not as that process is not commonly used for mass production.These trannies were being used in ~225 ft lbs applications stock.  400+ is a bit much to ask.  I wonder how much more strength can be have via cryo?
Reply:Most likely the case has more ribbing in some spots, thicker casting in some critical areas and maybe a different grade for the gears. It amazes me how small and lightweight the cases are nowadays yet they are so strong!HTP Invertig 201Lincoln Idealarc SP250Miller 180 AC StickBy farmall:They should have held the seagull closer to the work, squeezing evenly for best deposition.
Reply:The color differences you're seeing are very likely do to some kind of surface hardening heat treatment (gas carburizing, nitriding, or some hybrid of both).  Color could, and I emphasize could, indicate a deeper(thicker) case hardened layer which might also have a higher surface hardness.  You could also be dealing with high carbon steel that's been quenched and then tempered.  Variations in the tempering process could produce variations in color of the finished parts.  So, color will not tell you anything definitive about these parts.The color differences you're noticing could also be due to  the amount of thermal exposure and transmission fluid degradation you're seeing.  Without inside knowledge of the part manufacturing process, or a destructive metallurgical evaluation, there's no easy way to find out for certain.Simplest thing I can think of would be to take a file and try to scribe a non-contact surface on the same part from both transmissions.  If one is noticably harder to scratch, then you've likely found that there is some difference.  A note of caution, scratching parts, especially shafts, is a good way to cause a fatigue failure.  So don't do this if you can't afford to have this risk.If you have some failed gears that you can sacrifice, use a bandsaw with lots of coolant flood and cut each part in half.  Take some 100, 240, and 400 grit sand paper, and wet sand each gear smooth.  Use a piece of glass as a flat surface for polishing on.  Then take some dilute acid and etch each part. 2% Nitric acid diluted in alcohol is best, but 1% - 2% Muratic acid, diluted with water, will work also.  Nitric acid is tough to get, but muratic is available at the hardware store or pool supply store.   The carburized surface layer will tint darker than the base material on the polished face of each part.  If the two gears have different heat treatments, you should be able to see a difference in how thick the surface layer is.  This is about all you can tell with the naked eye.  A microscope and a better(finer) polish would let you determine what kind of heat treatment was done(carburize, nitride, hybrid, or simple quench & temper).