Mass effect is the variation in hardness across of the components having higher thickness through heat treatment.
Hardness of plain carbon steel depends upon its carbon content and the rate of cooling from the hardening temperature. A part having less thickness will cool more quickly than a part having higher thickness if both are cooled in the same quenching bath. In a thicker component, outer layer will cool faster than the core and heat will get trapped at the centre. This leads to a variation in hardness across a section of the component. The result can be another layer of martensite and inner core of article, this variation in hardness is referred to as mass effect.
Owining to "mass effect" plain carbon steels having large sections are said to have a poor hardenability, as they cannot be fully hardened throughout.
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