In your own words, describe the following heat-treatment procedures for steels and, for each, the intended final microstructure: (a) full annealing (b) normalizing (c) quenching (d) tempering.

When we talk about full annealing, we're discussing a crucial heat treatment process designed to ensure steel has the perfect microstructure for certain applications. This involves heating the steel to a temperature slightly above its upper critical limit — around 30-50 degrees Celsius higher — and maintaining this temperature to allow for even heat distribution throughout the material.

Once the steel is uniformly heated, it undergoes a slow cooling process within the furnace itself. Slow cooling is key to achieving a microstructure known as coarse pearlite, characterized by its softness and enhanced ductility. The resulting fully annealed steel is then not only easier to work with but is also more malleable, making it an ideal candidate for cutting, shaping, and forging practices used in creating various steel components.

Normalizing steel is a step distinct from full annealing, primarily due to the cooling method employed. After heating the steel above its upper critical temperature, the material is allowed to cool in the open air.

What does this achieve? The faster cooling rate leads to the formation of fine pearlite instead of coarse pearlite — this subtle shift has a significant impact on the steel's properties. Fine pearlite is stronger and harder than the microstructure achieved through full annealing. Therefore, normalized steel strikes a balance, offering increased hardness, strength, and toughness. These qualities make normalized steel well-suited for a variety of structural applications and manufacturing jobs where such attributes are desired.

Quenching is all about speed. In this heat treatment procedure, steel is taken to high temperatures — beyond its upper critical limit — and then rapidly cooled using liquids like water, oil, or molten salt. Why this abrupt drop in temperature? It prevents the formation of pearlite, instead producing a microstructure called martensite.

Martensite is valued for its immense hardness and wear resistance — qualities that make quenched steel highly suitable for cutting tools and components that require a durable edge. However, with high hardness comes a downside; martensite is also brittle and can be prone to cracking if not managed properly. Subsequent heat treatments like tempering are often used to ameliorate this brittleness and improve the steel's overall toughness.

Following the hardening effects of quenching, tempering is a heat treatment process that dials back some of that hardness to reduce brittleness. Steel that has been quenched and is in a martensitic state is reheated to below its lower critical limit — usually within the range of 150-700 degrees Celsius. The temperature and duration depend on the desired final properties of the steel.

The goal here is to convert some of the brittle martensite into tempered martensite, a more balanced microstructure that retains the hardness necessary for various industrial applications, but with increased ductility and toughness. Tempered steel is an integral part of tools, springs, and automotive parts, where it's essential to strike that perfect harmony between strength and flexibility.