Residual stress is a structural weakness that occurs in the casting process of iron, the main material used in brake discs.
During normal street-use, residual stress does not cause any problems with the discs. When the discs are used at race circuits at high temperature conditions for an extensive of time, the residual stresses within the disc can lead to thermal cracking and deformation. The heat treatment process relieves the residual stresses to prevent thermal cracking and deformation from occurring.
|non-heat treated disc||heat treated disc|
|Crack resistance||When a one-make series Honda DC5 Integra is driven at 2min 10sec/lap on the Motegi race circuit||Hairline cracks occur on the braking surface after 1 hour of use.||Hairline cracks occur on the braking surface after 2 hours of use.|
|When an street legal Honda DC5 Integra is driven at 2min 20sec/lap on Motegi race circuit||Hairline cracks occur on the braking surface after 2 hours of use.||Hairline cracks occur on the braking surface after 5 hours of use.|
|Judder resistance||When a one-make series Honda DC5 Integra is driven at 2min 10sec/lap on the Motegi race circuit||Minor juddering (vibrations) occur after 1 hours of use.||Minor juddering (vibrations) occur after 2 hours of use.|
|When an street legal Honda DC5 Integra is driven at 2min 20sec/lap on Motegi race circuit||Minor juddering (vibrations) occur after 2 hours of use.||No juddering (vibrations) occur after 4 hours of use.|
At DIXCEL, a strict temperature control is implicated at each step of the heat treatment process.
The details of the time and temperatures of the heat treatment process cannot be disclosed, so hypothetical values will be used to explain the heat treatment process.
First, the temperature is increased by 5 degrees centigrade every 10 minutes. When the temperature reaches 300 degrees centigrade, the temperature is kept the same for 8 hours.
Next, the discs are cooled by temperature being lowered by 5 degrees centigrade every 10 minutes. The graph on the left shows that the temperature control is very ideal.
The whole heat treatment process is completed over a period of 24 hours. This allows for slow and gradual process under perfect humidity control.
This helps prevent deformation, strengthens the bonds between the molecules, and allows for an increase in heat resistance.
※The temperatures and times listed in this explanation are hypothetical values. The actual temperatures and times that DIXCEL uses are different.
The advantage of having using the slotted disc is an increase in stopping power. The advantage of the heat treated disc is an increase in durability.
For the slotted discs, our testing results showed an average braking power increase of 15-20 percent. The heat treated discs have better protection against thermal cracking, juddering, and distortion.
They also increase the life of both the pad and disc. For users who want best of both worlds, DIXCEL recommends the FS or HS series discs.
Cast iron and carbon are the two main materials used to make a brake disc. The benefit of Carbon is that it has a high thermal resistance and is lightweight. The downside is that Carbon is expensive, so it is used mainly by high budget racing teams. Cast iron is the more commonly used base material. There are three types of cast iron, each type has different graphite composition; grey cast iron, CV cast iron and ductile cast iron. Grey cast iron (flakes graphite cast iron) has excellent processibility and anti-abrasion capability. Grey cast iron has the advantage of being easily mass-produced, making it the most commonly used by discs manufacturers. The drawback is that it could be deformed or cracked under repeated sharp changes in temperature in the high temperature range (about 800℃). Ductile cast iron is an excellent material. The tensile strength of ductile cast iron is equal to that of steel. Ductile cast iron also has an high anti-heat capacity (stability against expansion and contraction). Unfortunately, it has low surface hardness, which can cause abnormal wear and/or abnormal heating due to its high exothermicity, if the material is used for brake discs. CV cast iron (Compact Vermicular cast iron) has an intermediate character between grey cast iron and ductile cast iron. The quality control of CV cast iron during the manufacturing process is extremely difficult, so the quality varies. Sometimes its closer to grey cast iron but other times is closer to ductile cast iron. After extensive testing, grey cast iron with special additives are being used in DIXCEL brake discs. OEM Products often use grey cast iron with FC150〜200 (FC is numeric representation of the strength of cast iron). DIXCEL uses grey cast iron with FC200〜250 for higher durability. After extensive research and development, DIXCEL developed a disc which has special additives to strengthen the disc's vulnerability to sharp temperature changes in the high temperature range. The superior precision and balance of the disc goes without saying.
The two most popular types of brake discs are solid and ventilated discs. Ventilated discs have cooling vanes between the braking surfaces which allows air to flow through, and has a cooling effect on the disc. More cars are becoming equipped with ventilated disc on the front brakes, and high-performance cars have ventilated discs on the front & rear.
Brake disc engineering involves the precision procedures Disc Thickness Variation (DTV), run-out, Mounting Surface Flatness (MSF), friction surface parallelism, and balance.
f any of the precision machining standards are not met, the risk of a disc developing juddering will be high. On top of the five precision machining standards, the friction surface of the disc is machined to improve the bedding process of the new brake pads, and provides more stable braking from initial use. The groove between the mounting surface and the friction surface of the disc is designed to optimize the cooling effect, which will prevent thermal cracking and distortion. At DIXCEL, we put all discs through a thorough final product inspection. Be rest assured our products are of the highest quality.
Generally more slots the higher operating friction level, but increases air noise (noise from the discs rotating) and faster pad wear.
Unlike brake pads, it is not precise to indicate disc heat resistance temperatures in the form of 'up to ℃'. All brake discs are made of generally the same material, so they all run the risk of possible thermal cracking and distortion when temperatures reach 600℃ or higher.
Many different factors cause these problems, so we do not specify the heat resistance temperatures of brake discs.
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