Questions:
We are thinking of making a part out of SAE 1050 steel and austempering it to produce a hardness of 42–48 HRC. However, the part is quite thick, being approximately 0.130” (3.3 mm) in the thickest section. We were wondering if it is too thick for austempering? Also, what should our salt temperature be?

Answers:
Desired microstructure, section thickness and material composition are the most important factors in determining whether you can austemper a part. I note that you do not mention if a 100% bainitic structure is desired. Producing the proper microstructure is a key aspect of getting the mechanical properties you require (besides just hardness). For example, the optimum hardness range for high toughness in a 0.50% carbon steel is 40-52 HRC.[1]

The concern with thick parts is the ability to achieve proper transformation into the center of the component. The danger is that you will form (fine) pearlite rather than bainite in the core (although under certain circumstances, in certain applications, this has been reported to be advantageous).

Having the correct time at austenitizing temperature (e.g., belt speed in a conveyor furnace) is critical to ensure transformation as well. It should be possible to austemper 1050 steel (0.48-0.55%C, 0.60-0.90% Mn) up to a maximum section thickness of 0.125-0.150". Your part at 0.130" thick falls within this range.

The Ms (martensite-start temperature) for this steel is approximately 595ºF (315ºC) - note that this is a calculated value. For a complete bainitic transformation, you typically want to hold at 25-50ºF (10-25ºC) above the Ms. Doing so should produce a part within your stated hardness range (see below).

Here's what to expect at various salt temperatures (minimum time under the salt, in seconds, is shown in parentheses). Specifically:
  • 610-635ºF (320-335ºC) °° 48 - 50 HRC (600–900 s)
  • 625-635ºF (330-335ºC) °° 45 - 50 HRC (600–710 s)
  • 675-685ºF (355-360ºC) °° 42 - 45 HRC (315–380 s)
  • 685-695ºF (360–370ºC) °° 42 - 44 HRC (320–380 s)
  • 750-765ºF (400-410ºC) °° 38 - 41 HRC (120–140 s)