Making carbon fiber is a multi-step chemistry process:

• Start with a precursor fiber, usually polyacrylonitrile (PAN) — a synthetic polymer
• Stabilization: heat the PAN to 200-300°C in air, oxidizing it and linking the polymer chains
• Carbonization: heat to 1,000-1,500°C in an oxygen-free atmosphere, driving off non-carbon atoms as gases until the fiber is over 90% carbon

The peak temperature sets the final properties. Higher heat produces stiffer, more graphite-like fibers. Lower heat yields fibers that are stronger but more flexible, so engineers pick the right grade for each job.