Material Science, Heat Treatment of Steel, Part 2

Summary

This video, presented by Professor Bonair, delves into the essential processes of steel heat treatment, focusing on annealing, hardening, and tempering. It explores various types of annealing, such as soft annealing for improved machinability, stress relief for reducing internal stresses, and recrystallization for restoring material properties after cold forming. The video also covers the science behind hardening and tempering, highlighting the transformation of steel structures and the impact of alloying elements on hardness and toughness. Key concepts such as hardenability, quenching, and surface hardening are also discussed, providing a comprehensive understanding of steel treatment techniques.

Takeaways

• 😀 Steel contains cementite, which plays a role in hardness and strength but can make machining and cold forming challenging.
• 😀 Soft annealing reduces hardness and increases machinability, which is crucial for processes that require low forces and long tool life.
• 😀 Soft annealing aims to transform cementite lamellae in pearlite into small grains, which decreases hardness but increases toughness.
• 😀 Stress relief annealing helps reduce internal stresses caused by uneven cooling or mechanical processes, preventing distortion in the workpiece.
• 😀 Stress relief annealing is performed at temperatures between 550°C and 650°C and is crucial for materials subjected to significant mechanical processing.
• 😀 Recrystallization annealing restores formability and reduces dislocation density, typically occurring at temperatures between 600°C and 700°C.
• 😀 Recrystallization annealing produces a fine-grained microstructure, improving both strength and toughness, but the process must be carefully controlled to avoid coarse grain formation.
• 😀 Hardening and tempering are separate annealing processes used to modify steel’s hardness and toughness, typically involving accelerated cooling (quenching) followed by tempering at different temperatures.
• 😀 Hardening involves rapid cooling to form martensite, a microstructure that is very hard but brittle, while tempering adjusts the hardness-toughness balance by reheating to a lower temperature.
• 😀 Hardening depth (the ability of a material to be hardened from surface to core) and hardenability (how deep a material can be hardened) are influenced by carbon content and alloying elements, such as manganese.
• 😀 Unalloyed steels require strong quenching mediums (like water) for effective hardening, whereas alloyed steels can be hardened in milder quenching mediums (like oil or air).

Q & A

What is the main purpose of soft annealing in steel?

Soft annealing reduces the hardness of steel to facilitate mechanical processing, improve tool life, and produce higher surface quality on workpieces.

How does cementite influence the properties of steel?

Cementite contributes to the hardness and strength of steel, but it is unfavorable for machining and cold forming because it makes the material less workable.

What is stress relief annealing, and why is it performed?

Stress relief annealing is done to reduce internal stresses in steel caused by uneven cooling, casting, welding, or forging, thereby minimizing distortion during subsequent processing.

Why should stainless steels with niobium or tantalum not undergo stress relief annealing?

These stainless steels must not be stress relief annealed because the process can negatively affect their microstructure; detailed reasoning is covered in high-alloy steel studies.

What is recrystallization annealing and when is it necessary?

Recrystallization annealing restores ductility and formability lost due to cold work by forming a new, fine-grained microstructure, and is necessary for processes like deep drawing.

How does the annealing temperature affect the outcome of recrystallization annealing?

The annealing temperature, along with the degree of deformation and annealing time, must be precisely controlled; incorrect parameters can result in coarse grain formation instead of the desired fine-grained structure.

What is the main difference between hardening and tempering of steel?

Hardening aims to achieve maximum hardness by forming martensite through rapid cooling, whereas tempering adjusts the balance of hardness and toughness by reheating at controlled temperatures.

What is martensite, and why is it both hard and brittle?

Martensite is a microstructure formed when austenite is rapidly quenched, trapping carbon atoms in the lattice, which distorts it and increases hardness but also creates brittleness due to internal stresses.

What is retained austenite and how does it affect hardness?

Retained austenite is a metastable phase of austenite that remains after quenching; it is relatively soft and lowers the overall hardness of the steel if present in significant amounts.

What factors determine the critical cooling rate during hardening?

The critical cooling rate depends on the steel's carbon content and alloying elements like manganese. Higher carbon increases the rate, while some alloying elements can reduce it.

What is the difference between hardenability and hardening capacity in steel?

Hardenability refers to the depth of hardness achieved in a steel workpiece, while hardening capacity is the maximum hardness achievable at the edge, primarily influenced by carbon content.

Why might low-alloy steels allow milder quenching media like oil instead of water?

Alloying elements reduce the critical cooling rate, allowing the steel to harden sufficiently in milder quenching media like oil, avoiding risks of cracking from rapid cooling in water.