What method is NOT effective in increasing the hardness of steel?

Stretching steel is not an effective method for increasing its hardness. Steel hardness is usually increased through processes that alter its microstructure, which includes phases like martensite, or through work hardening. Quenching, which involves rapidly cooling the steel from a high temperature, transforms the microstructure and greatly increases hardness by creating a harder phase. Tempering, on the other hand, is a subsequent process that reduces brittleness while maintaining some level of hardness achieved through quenching. Heat treating encompasses both quenching and tempering, optimizing the steel's physical properties.

Stretching, as a method, does not introduce the necessary changes in the microstructure to achieve increased hardness. Instead, it may lead to elongation or slight deformation, which does not contribute effectively to the hardness of the material. Therefore, it is not regarded as a viable method for enhancing the hardness of steel in the context of material processing.