Material Testing/Verification

Material Testing/Verification

Review the basics, like the chemistry meeting requirements, but look further. Calculate the DI or hardenability to be sure the material can harden up to engineering requirements. I recommend ASM International’s articles on hardenability and ASTM A255 Standard for DI Calculations for core hardness. With ASTM A255I, just look up the key element percentages and receive values to multiply together to arrive at a direct DI number. These translate into the thickness of material that will successfully harden properly without extra effort by the heat treater.

The other option would be to use the Jominy test results with a heat and, again, work backward to calculate hardenability similar to the DI calculation above also described in ASTM A255. One tip: The thickest section of the bearing is the diameter of the largest inscribed circle you can make inside a cross section of the bearing.

Look at the JK ratings. This is a system of measuring the cleanliness of the material and is defined by ASTM E45. Bearings are typically governed by an ASTM (ASTM A295 or A485 for high-carbon or A534 for lower-carbon steels) or ISO standard (ISO 683-17), which will include maximum values on residual elements and JK results. Staying inside these values helps ensure the heat treater and customer will not have any issues. Excessive quantities or large inclusions can lead to cracks during heat treatment, especially for heavy-section bearings or rolling elements. The cleanness of the steel is directly correlated with bearing performance in terms of rolling contact fatigue (RCF) life and is of high importance to the customer. An out-of-tolerance value could lead to rejection of an entire order.

I always recommend to a heat treater to verify the correct chemistry or at least rough grade testing using an XRF analyzer or gun. These units are expensive but very useful to verify basic chemistry results and avoid a heat-treating problem due to incorrect material.