Carbide volume fraction is a way to measure how much carbide is in a steel. So if you have 100 cubic inches of steel and a volume fraction of 5%, then 5 cubic inches is carbide all mixed up in little tiny bits.
These carbides are the same as those in hardened steel. This is where the phase diagrams come in handy. In a simple steel like 1095, the carbide is Fe3C, iron carbide, or cementite, all are the same. It is present at room temperature in annealed 1095 steel, at about 15% volume fraction. When heated for hardening, it will typically have 2% to 5% volume fraction. These are the same carbides as at room temperature, and when quenched they are still the same. The phase diagram can be used to calculate volume fraction. Carbides can be bigger or smaller, it depends on temperatures used.
These carbides are the same as those in hardened steel. This is where the phase diagrams come in handy. In a simple steel like 1095, the carbide is Fe3C, iron carbide, or cementite, all are the same. It is present at room temperature in annealed 1095 steel, at about 15% volume fraction. When heated for hardening, it will typically have 2% to 5% volume fraction. These are the same carbides as at room temperature, and when quenched they are still the same. The phase diagram can be used to calculate volume fraction. Carbides can be bigger or smaller, it depends on temperatures used.