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).

Temperature and Phase

• 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.

Benefits

• 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.

Characteristics of Hot Finished Components

• 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.

Cold Finishing

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

Initial Steps

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

Intermediate Anneals

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

Cost Factors

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

Characteristics of Cold Finished Components

• 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.

Classification of Cold Work Steel Products

• 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.

Nomenclature in Cold Working

• Longitudinal Direction: Elongation direction during rolling/extrusion.

• Transverse Directions: Directions perpendicular to the longitudinal direction, further categorized as short and long transverse directions.

Mechanical Properties Changes

• 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.