Metallographic Verification of Surface Hardening Heat Treatment

Surface hardening heat treatments are popular in the manufacture of steel products as a means of significantly improving strength and fatigue resistance and mitigating wear. In the production of bearing / gear / others components, manufacturers know it is important to control the case depth of the hardened surface layer to ensure safety and reliability. Most current state-of-the-art solutions for hardening depth measurement are based on a statistical sampling approach using the inspection of certain selected characteristics, followed by various destructive testing methods.

TYPICAL STEPS FOR CASE DEPTH ANALYSIS

Case depth examination is typically required to verify each carburizing or carbonitriding heat treat batch or lot. A gaseous atmosphere is generated within the furnace which diffuses into the component by the laws of equilibrium. The amount of time and the carbon potential of the atmosphere dictate the resulting total case depth and effective case hardness. Measure the total case depth as the depth below the surface where no change in microstructure can be observed resulting from heat treatment diffusion. Typical steps for case depth analysis include.

• Section & prepare the sample for metallographic examination
• Etch the prepared metallographic specimen with a suitable etchant to reveal the microstructure
• Measure the total case depth as the depth below the surface where no change in microstructure can be observed resulting from heat treatment diffusion.
• Examine the case microstructure for any detrimental microstructural conditions.
• Measure the chemical composition of case (if required).

EILAB CAPABILITY FOR INCLUSION RATING

Induction hardening is a type of surface hardening in which a metal part is induction-heated and then quenched. The quenched metal undergoes a martensitic transformation, increasing the hardness and brittleness of the part. Induction hardening is used to selectively harden areas of a part or assembly without affecting the properties of the part as a whole.

Common case depth test method & specification are:

• ASTM E407  – Standard Practice for Micro-etching Metals and Alloys.
• SAE J423 – Methods of Measuring Case Depth.
• SAE J422 – Recommended Practice for Determination of Inclusions is Steel.