Question 70 QMED05 - Machinist-Pump Technician

The Correct Answer is B **Explanation for why Option B (tempering) is correct:** Tempering is a heat treatment process specifically designed to reduce the brittleness and internal stresses introduced during the hardening (quenching) process. Hardening typically leaves the component very hard but brittle due to the formation of martensite. Tempering involves reheating the hardened component to a specific temperature *below* the critical hardening temperature (A1 line) and then cooling it slowly or quickly. This reheating allows some transformation of the unstable martensite microstructure, enhancing ductility, toughness, and impact resistance while slightly reducing hardness to a desired level. The description provided perfectly matches the definition and purpose of tempering. **Explanation for why the other options are incorrect:** * **A) Annealing:** Annealing is a heat treatment process used primarily to soften a material, relieve internal stresses, improve machinability, and refine the grain structure. It usually involves heating the component above the critical temperature range (sometimes close to the melting point) followed by very slow cooling (often in the furnace). This is a different process and temperature regime than reheating a *hardened* component to a lower temperature. * **C) Low temperature hardening:** This term is not a standard, recognized heat treatment process for general metallic components. Hardening processes generally require heating the component *above* the critical temperature (austenite range) before rapid quenching. The term described (reheating *after* hardening) is tempering. * **D) Case hardening:** Case hardening refers to a group of processes (like carburizing or nitriding) used to create a hard, wear-resistant surface layer (the "case") while leaving the core relatively soft and tough. This involves changing the chemical composition of the surface layer (e.g., adding carbon) and is an initial hardening process, not the subsequent reheating step described after the component is already hardened throughout.