materials scientists - please come in for a metalurgy question

JHZR2

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Hi, As I have indicated in a few general brake pad/rotor questions that I have posted on here, I am considering swapping all of my pads/rotors for a few reasons I won't get into here. My question is this: besides corrosion resistance, what effect would the removal of chromium have on the properties of a brake rotor? I was under the impression that the GM rotrs were darker in color due to a change that made them harder - to combat the change to ceramic pads. owever, i am doubting that now that Ive read that GM took chromium out to cut costs. Im in a bind as to whether or not to buy GM OE rotors, rather than aftermarket stuff that can be great or be terrible - very hit or miss. The hardness of the metal and the result on ceramic (hard) pads, and the effect that one has on another is my main concern, so any insight would be most appreciated. I hope to optimize braking performance with your help. Thanks! JMH
 
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The rotors would probably be somewhat softer without the Cr and would wear faster. The deal with these parts is you can design the pads to wear out faster or the rotors to wear out faster, but you always have to dissipate the same amount of kinetic energy when you're braking, unless you can shed a few hundred lbs of weight from the vehicle or add re-generative braking. I'd talk to folks who race 'Vettes and see what aftermarket parts they run.... Tooslick
 

Kestas

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From where is GM removing the chromium?.... the base cast alloy, or are they removing a chromate conversion coating? The levels of Cr used in cast iron rotors has little effect on corrosion resistance and only a mild effect on hardness. Cr is not the main variable in controlling rotor hardness during production. Sn or Sb levels have a profound effect as does timing of the shakeout after casting, which affects the cooling rate through the critical temperatures. Cr (and Mo) is desirable since it increases thermal conductivity and has better stress-rupture properties.
 
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http://www.mitsubishi-motors.com/corporate/about_us/technology/review/e/pdf/2003/15E_06.pdf Today, disc brakes are the most popular brakes for passenger cars, light-duty trucks and other small-sized vehicles, while mainly drum brakes are used in heavyand medium-duty trucks. Brakes on heavy- and medium-duty trucks sustain thermal and mechanical loads that are much greater than on small-sized vehicles, which discourages manufacturers from employing disc brakes in heavy- and medium-duty trucks for durability reasons. However, the recent trend toward greater engine output and higher performance in heavy- and medium-duty trucks compels the manufacturers to use brakes with high braking performance and superior safety features such as anti-fade characteristics for which disc brakes are more promising than drum brakes(1)(2). Graphite flake cast iron has long been used as the material for disc brake rotors (hereinafter referred to as “rotors”) because of its overall excellence in thermal fatigue strength, anti-squeak and anti-vibration characteristics, and wear resistance. In the case of small-sized vehicles, high carbon graphite flake cast iron(3) with high thermal conductivity is widely used to reduce the thermal load on rotors. As rotors for heavy- and medium-duty trucks are subjected to a larger thermal load than rotors for smallsized vehicles, they require higher thermal fatigue strength. For this reason, graphite flake cast iron with nickel (Ni), molybdenum (Mo) and chromium (Cr) additions is widely used as the material for truck’s rotors, although some rotors use cast steel(4). Our company uses a material equivalent to JIS FC250 for the rotors of light-duty trucks and FC250- based graphite flake cast iron with 1.2 % Ni plus Mo and Cr for the rotors of large- and medium-duty trucks for increased thermal fatigue strength (for simplicity, the currently used material for rotors of light-duty trucks is referred to as “FC250” and that for rotors of large- and medium-duty trucks as “current material” hereinafter). The current material is excellent in thermal fatigue strength as it has been proven to be more than four times stronger than FC250 in high-speed braking tests using actual rotors, but the substantial amount of Ni that it contains boosts the material cost
 

JHZR2

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wow, great info. Thanks all! As for where theyre removing Cr... I believe it is from the base alloy itself. Thanks again. JMH
 
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