SSRAO

Explanation:
Nitriding is carried out in the ferritic region, typically at temperatures around 500-590 °C, which is well below the lower critical temperature (A1 = 727 °C) of steel. Therefore, no phase transformation to austenite occurs during nitriding. In contrast, carburizing, cyaniding, and flame hardening require heating into the austenitic range (above A1 or A3).

Explanation:
For eutectoid steel, progressively lower isothermal transformation temperatures generally produce finer pearlite, then upper bainite, then lower bainite. If austenite remains untransformed and the steel is cooled below the martensite-start (Ms) temperature, martensite begins to form. The exact products depend on the time-temperature path and alloy composition.

Explanation:
The Jominy end-quench test is a standard laboratory test used to determine the hardenability of a steel specimen. It measures the depth to which a specific steel alloy can be hardened upon quenching from the austenitic temperature range by spraying water on one end of a standard cylindrical specimen.

Explanation:
Tempering involves reheating a quenched martensitic steel to a temperature below the lower critical temperature (A1). This gives thermal activation for carbon to diffuse out of the metastable, highly stressed body-centered tetragonal (BCT) martensite structure, leading to the formation of a more stable structure consisting of fine, dispersed carbides (like epsilon carbide or cementite) in a ferrite matrix, which significantly boosts toughness.

Explanation:
Stress-relief annealing is performed below the lower critical temperature (A1) to reduce residual stresses introduced by cold working, machining, or welding. The extent of recovery and any other microstructural changes depends on the steel, its prior condition, temperature, and holding time; it should not be described as guaranteeing no structural change.

Explanation:
Martempering involves quenching an austenitized steel component into a hot salt bath maintained slightly above the Martensite start (Ms) temperature, holding it until the temperature across the cross-section becomes uniform, and then cooling it in air through the martensitic range. This uniform cooling minimizes the thermal gradients and differential transformations that cause warping, distortion, and thermal cracking.

Explanation:
Austempering is an isothermal heat-treatment process in which austenitized steel is quenched to a temperature above Ms and held until the desired bainitic transformation has occurred. The resulting structure is bainitic; whether the transformation is complete depends on the steel composition and process conditions.

Explanation:
Recrystallization involves nucleation and growth of new, relatively strain-free grains in the cold-worked material. Stored deformation energy provides the driving force. Dislocation annihilation and rearrangement are characteristic of recovery, while reduction in grain-boundary area through grain growth occurs at a later stage.