Decarburization is a natural part of processing steel - typically, there are limits for the allowable depth of decarb built into specifications, especially, ASTM and SAE specifications.
You'll decarburize steel when you reheat it for shape processing, such as rolling from a billet to a bar, or during heat treating. Typically the only method to ensure that you’ve eliminated it after shape processing is surface removal of some sort such as surface grinding.
With heat treating, you have options to insure that you don’t add to any preexisting decarb. Options include processing in salt baths, the use of protective atmospheres such as endothermic, nitrogen/methanol, nitrogen/natural gas, or vacuum will either prevent or minimize generation of additional decarburization. Depending on specifications, sometimes you can get away with just diluting the furnace atmosphere with nitrogen. They’re all more expensive options then just processing in an open air furnace heated either electrically, with radiant tubes, or with open fired burners.
Induction hardening and due to the decreased time at temperature will also decrease the amount of decarburization produced. In general, the higher the processing temperature and the longer time at temperature, the more decarb is generated. Also, it’s not a linear function.
To learn about Rockwell hardness testing, get a copy of ASTM E18, the Standard Test Methods for Rockwell Hardness and Rockwell Superficial Hardness of Metallic Materials. The Rockwell C test is a macro material test – typical indentation diameter at a 32 Rockwell C hardness is going to be around .050inches (I’m extrapolating from a couple of measurements taken on indentations on 26 C and 45 C test blocks) Rockwell tests are supposed to be a minimum of 3 indentation diameters away from each other, otherwise you do not get a valid reading. You also need to be a minimum of 2 diameters away from an edge or you don’t get an accurate, valid reading. For round test pieces, you should be using a “V-grooved anvil that will support the specimen with the axis of the V-groove directly under the indenter or on hard, parallel, twin cylinders properly positioned and clamped in their base.”
I’d also recommend getting a copy of ASTM A370, Standard Test Methods and Definitions for Mechanical Testing of Steel Products.
Note also, that ASTM A370, for testing bar products a minimum surface removal of 0.015” is called for to provide for accurate hardness penetration and for tubes, ASTM A370 notes that the hardness test is typically taken on the inside surface, a flat on the outside surface, or on the wall cross-section depending on the product limitation.
I suspect that you are encountering decarb, and that it is affecting your surface readings – The question is whether the specification you’re working to permits a level of decarb, and if so how much. Personally, a level of 0.025” on a 2 “ tube doesn’t sound excessive to me, but it may be depending on the specifications you’re working to.
Great job!
