We heat treat a great deal of AISI 4140 for harden and temper. Our austenitize temperature is 1570°F (855°C) and temper at 450°F (235°C). We always check the “as-quenched” hardness, and more often than not, we have hardness variances from heat to heat. Why is this?

Not all AISI 4140 is created equal. If you consider the specified analysis of AISI 4140, you will see the potential variances in the spec.
• Carbon = 0.38 - 0.43%
• Manganese = 0.75 - 1.00%
• Chromium = 0.80 - 1.10%
• Molybdenum = 0.15 - 0.25%
If you do the math on the individual potential gross variances you will see:
• Carbon = 12% potential variance
• Manganese = 25%
• Chromium = 27%
• Molybdenum = 40%
As can be seen, although the carbon potential is within the specification, it does not take a great deal of carbon to influence the as-quenched hardness condition. It is because of these potential variances that the as-quenched hardness values will change. In order to “tighten” the analysis range, it would be necessary to purchase the “H” band steel grade. This simply means that the analysis specification is much closer on the allowable span of each element.