4140 Annealed Hot Rolled Bars (ASTM A29, ASTM A322)

The term 4140 Annealed Hot Rolled steel refers to a medium-carbon alloy steel delivered in its softest, most ductile, and most machinable condition.

• AISI 4140 is a medium-carbon, low-alloy steel containing Chromium (Cr) and Molybdenum (Mo).
• The medium carbon content (approx. 0.40%) makes it highly hardenable, while Cr and Mo provide improved strength, toughness, and wear resistance.
• It's designed for applications requiring high strength and good wear resistance after subsequent heat treatment.

Annealing is a critical heat treatment process that alters the steel's microstructure to achieve its lowest hardness and maximum ductility.

• Process: The steel is heated to a high temperature (above the critical temperature), held there to allow the structure to become uniform, and then cooled very slowly (often inside the furnace).
• Purpose:
  1. Maximum Softness: To reduce the hardness of the steel, making it the easiest condition for machining.
  2. Stress Relief: To remove internal stresses induced during the rolling process.
  3. Refine Grain Structure: To prepare the steel for optimum response to subsequent heat treatments (like quenching and tempering).

Hot Rolled (HR)

• Process: The bars are formed by passing the steel through rollers while the material is at a high temperature.
• Characteristics:
  • Surface Finish: The bars have a relatively rough surface and often retain a blue-black mill scale.
  • Dimensional Tolerance: They have wider dimensional tolerances and are less straight than cold-drawn bars.
  • Size Availability: This is the most common process for large diameter bars.