Case Depth Determination

What is Case Depth Determination?

Strength, wear resistance, and fatigue resistance can all be greatly increased in wrought, cast, and sintered ferrous components with surface hardening heat treatments. As a result, surface-hardened parts are frequently put to use in demanding situations that call for further inspection to verify design specifications including surface hardness, case depth, and case microstructure. Since stresses tend to be greatest close to the surface, the designer is interested in case depth as a characterization of the gradient in material properties near the surface. 

Although there is no universally accepted definition of “case depth,” it is common practice to measure the component’s total and effective case depths using physical testing, chemical testing, or microstructural analysis. There is consensus on what constitutes “total case depth,” but there is less agreement on how to define “effective case depth,” which may not even be essential in some contexts. Although several methods of surface hardening are used in industry, we will only be discussing the case depths that occur from carburizing and induction hardening of wrought steel for the sake of this discussion.

Case-Depth Measuring

Therefore, hardness testing is the most frequently used means of defining the surface hardened layer; consequently, metallographic specimens are typically provided to confirm a surface hardening procedure. We see three popular methods for hardness testing to determine case depth profiles. To guarantee accurate results, all three methods necessitate meticulous sample preparation and reliable hardness testing methodology. The cross-section procedure is typically used because of its ease of use, but the taper and step methods have their uses as well. For example, when characterizing shallow case depths or very coarse microstructures, the higher loads (larger indentations) provided by these approaches can help reduce data variability.

Depth profiles of carburized and induction-hardened cases are one application of hardness testing. Carburization is the process of quench hardening steel after carbon has been diffused into its surface at austenitizing temperatures. In induction hardening, a high-frequency alternating magnetic field is used to rapidly heat the steel’s surface, and then quench hardening is performed. Because of the distinct approaches taken to creating the case, the hardness profiles generated by the two surface hardening methods have quite different forms. The carbon diffusion gradient largely determines the shape of the hardness profile produced by carburising. Because induction hardening relies on heat conduction to shape the hardness profile, it produces a more prominent profile. The case depth is defined by the hardness depth reference, which can be used to characterize both profiles despite their obvious visual differences.

Defining Case Complexity

Case depth validation and other features can be correctly defined once the design intent of the component is understood. The depth profiles of surface-hardened cases are typically described using the following values:

• Surface hardness is typically given as a range (lower and upper bounds).

• Maximum allowable surface hardness: This is the start of the core case territory.

• Total case depth is the vertical distance from the surface to the point where the chemical or physical differences between the case and the core are no longer discernible.

•  Effective case depth: around 66%-75% of the entire case depth, but typically defined as the depth to a given hardness.

Effective case depth is the most vague of the four concepts used to describe case depth. Since the case-core transition in induction hardening is quite sharp, it may be argued that effective case depth is not necessary. To define the hardness profile of the vast majority of induction-hardened components, simply determining the depth to the minimum specified surface hardness and total case depth should suffice.

Video 01: Case Depth Hardness Check (TRUEMET Micro Vickers)