3: Hot vs Cold Finishing of Steels

Hot Finishing

Definition: Hot finishing involves processes such as rolling, extrusion, forging, and other plastic deformation of steels at high temperatures (800 to 1,300 degrees Celsius).

High temperatures correspond to the austenite phase as per the iron-carbon phase diagram.
Above recrystallization temperature allows continual rearrangement of the steel's grain structure during deformation.

Reduced Dislocation Movement: The FCC (Face-Centered Cubic) structure facilitates easier dislocation movement, aiding in plastic deformation.
Continuous Annealing: As the material is deformed, it continuously undergoes heat treatment, which helps control grain structure, preventing high residual stress from forming.

Better Weldability: Due to reduced residual stress.
Stability in Machining: Parts remain stable during machining processes, with minor dimensional changes.
Dimensional Control: Generally less precise than cold finishing due to thermal contraction during cooling.
Surface Finish: Tends to be poorer and develops oxide scale due to exposure to air at high temperatures.
Mechanical Strength: Lower than cold finished parts due to removal of strain hardening benefits and larger final grain size.
Cost-Effective Production: Fewer steps are required, making hot finishing cheaper and more suitable for large structural components.

Definition: Cold finishing tends to be more expensive due to requiring additional manufacturing steps post-hot finishing.

Cold finished parts typically start as hot finished stock that must undergo acid pickling to remove oxide scales.

Required to manage ductility loss since cold working leads to work hardening which can lead to brittleness if not controlled properly.

Higher Load Requirements: More energy is needed to achieve shape changes due to the cold working process.

Dimensional Stability: Better dimensional control and surface finish than hot finished parts. Cold finished components can have precise dimensions and surface smoothness, often requiring no further processing.
Mechanical Strength: Higher than hot finished parts due to retained work hardening and activation of multiple slip systems during processing.

Temper Grades:
- Number 1 (Fully Hard): Minimal ductility, prone to cracking under shape changes.
- Number 2 (Half Hard): Limited shape manipulation available.
- Number 4 (Skin Rolled): Capable of significant changes without cracking.
- Number 5 (Dead Soft): High ductility but prone to yield point issues, leading to surface bands without careful manipulation.

Longitudinal Direction: Elongation direction during rolling/extrusion.
Transverse Directions: Directions perpendicular to the longitudinal direction, further categorized as short and long transverse directions.

As cold work percentage increases:
- Yield Strength and Tensile Strength: Both increase due to work hardening.
- Elongation Before Failure: Decreases as ductility is transformed into strength.
- Reduction in Area: Cold worked materials can show an increased resistance to further shape changes due to stress accumulation.