Answer:

Quench-hardening is a heat treatment process used to increase the hardness and strength of steel. During this process, the steel is heated to a high temperature and then rapidly cooled by quenching in a liquid medium, such as water or oil. The rapid cooling rate transforms the microstructure of the steel, resulting in increased hardness. However, it also leads to certain changes in the mechanical properties of the steel.

1. Brittleness
After quench-hardening, the steel generally becomes more brittle. This is because the rapid cooling rate causes the formation of a hard and brittle phase called martensite. Martensite has a highly strained crystal structure, which makes it prone to cracking and fracture. As a result, the steel loses its ductility and becomes more brittle.

2. Percentage elongation
Percentage elongation is a measure of the ability of a material to deform plastically before fracture. It is a measure of ductility. After quench-hardening, the percentage elongation of steel decreases significantly. This is due to the formation of martensite, which has a low ductility. As the steel becomes more brittle, it loses its ability to deform plastically and undergoes brittle fracture at lower levels of strain.

3. Impact strength
Impact strength is a measure of a material's ability to withstand impact loading without breaking. It is a combination of toughness and strength. After quench-hardening, the impact strength of steel generally decreases. This is because the formation of martensite leads to a decrease in toughness. Martensite is a hard and brittle phase, which makes the steel more susceptible to cracking and fracture under impact loading.

In conclusion, after quench-hardening, the steel becomes more brittle, loses its percentage elongation, and experiences a decrease in impact strength. This is mainly due to the formation of martensite, which has a highly strained crystal structure and low ductility.