Question:
I have two questions for you. First, what is the big difference between 4130 and 4140 steels? What is the heat-treat range for these? I need to wind up with a hardness of 25-32 HRC.

Answer:
I’m glad to be of assistance. Here’s some information about these steels that you might find useful. Also, you should consider reviewing the references shown before for more information on these materials.

4130 is a medium-carbon (nominal 0.30%C) chromium-molybdenum alloy steel. Due to its carbon content, it is most often water quenched to attain a maximum as-quenched hardness of approximately 48 HRC.

By contrast, 4140 is a medium-carbon (nominal 0.40%C) chromium-molybdenum alloy steel that is most often oil quenched to attain an as-quenched hardness ranging from about 54-59 HRC (as quenched), depending on the precise carbon content (which affects the hardenability of the material).

Heat-treatment results (hardness, microstructure, properties) are dependent on such factors as the exact material chemistry, part section thickness and the method of heat treatment.

For 4130 steel, the recommended heat treatment [1] consists of heating to austenitizing temperature, typically 1600°F (870°C), followed by water quenching. Water quenching will achieve a higher as-quenched hardness than oil quenching. Tempering (reheating after quenching) will achieve the desired hardness range. From data available to me for a water-quenched, ½-inch-diameter round part, you will need to temper at (approximately) 1025-1200°F (550-650°C) to achieve 25-32 HRC. I would recommend tempering at the lower temperature, checking as-tempered hardness and retempering at a higher temperature if needed.

For 4140 steel, the recommended heat treatment [1] consists of heating to austenitizing temperature, typically 1570°F (855°C), followed by oil quenching. Tempering (reheating after quenching) will achieve the desired hardness range. From available data for an oil quenched, ½-inch-diameter round part, you will need to temper at (approximately) 950-1100°F (480-595°C) to achieve 25-32 HRC. Again, I would recommend tempering at the lower temperature, checking as-tempered hardness and retempering at a higher temperature if needed.