Linked from
The 12 pages that link to Martensite, each with the reason it gives.
QuenchingBroader topic: It is the principal structure sought when steel is quenched for hardness.
TemperingRelated: Tempering modifies the martensite produced by hardening, reducing its brittleness.
AusteniteRelated: Rapid cooling can transform austenite directly into this hard phase.
Tool steelRelated: Quenching can form martensite, giving tool steel high hardness before tempering.