What happens to a machine part that was not tempered when it is subjected shock loads or bending stress?

A machine part that was not tempered when subjected to shock loads or bending stress is more susceptible to failure due to the presence of internal stresses and a lack of ductility. Tempering is a heat treatment process that follows hardening and involves heating the metal to a specific temperature below the critical point and then cooling it at a controlled rate. The primary purpose of tempering is to reduce the hardness of the metal while increasing its toughness, ductility, and resistance to shock loads.

When a machine part is not tempered properly or not tempered at all, it remains in a hardened state with high hardness but low toughness. Under shock loads or bending stress, the part may experience sudden impacts, vibrations, or forces that exceed its load-bearing capacity. The high hardness, combined with the internal stresses present in the metal due to the rapid cooling during hardening, makes the part more prone to brittle fracture.

The lack of tempering also reduces the ductility of the material, which is the ability to undergo plastic deformation without fracturing. As a result, the part exhibits reduced resistance to crack initiation and propagation, increasing the likelihood of sudden and catastrophic failure under shock or bending stress.

In contrast, a properly tempered machine part has a more balanced combination of hardness and toughness, achieved by tempering at an appropriate temperature and cooling rate. This allows the metal to absorb and dissipate energy more effectively, reducing the risk of brittle failure and improving the overall performance and durability of the machine part under shock loads or bending stress.