Some new Results in the Field of Non-Destructive Case Depth Measuring

Winfried MORGNER and Fritz MICHEL,

MDZ Werkstoffprüfung, Magdeburg, Germany

Abstract. A magnetic method to measure the case depth of inductive hardened automotive parts will be discribed. The method consists in magnetizing the parts and measuring the re-sidual tangential magnetic field strength. Calibration is necessary by parts with well known case depth. Demagnetization is required too.

The ultrasound backscattering is the well known base for measuring the thickness of hard-ened surface layers ( case depth measuring ) [1, 2]. However, there are some areas, where the application of the ultrasound method is difficult or even impossible. This is the matter with parts having a thickness of the hardened layer smaller than 1.5 mm and a core strength higher than 1000 MPa. This is the matter too with parts having a complex geometry, such as crankshafts, racks and gear components, especially in the teeth area, where the distance be-tween the teeth is to narrow for the ultrasonic sensor. Difficult too is the determination of the point, where the hardened layer is running into the unhardened part. Another problem is the unexpected appearance of unhardened areas.

Fig. 1 Residual magnetic field strength measurement between the teeth of an automotive rack to evaluate the thickness of the hardened suface layer

ECNDT 2006 - Poster 118

1

To solve these problems a new magnetic method was developed, which, employed to ( i ) detect local hardness defects on parts having a complex geometry and ( ii ) monitor the hard-ness depth gradient pattern, utilizes field/ pattern variations in the residual tangential field after constant field magnetization [3]. The small measuring sensor gives the possibility to measure the hardness depth even between the teeth of automotive racks (fig.1). The measuring uncertainty after calibrating the device is about ± 0.15 mm ( see fig. 2) .

Fig.2 Correlation and linear regression analysis between the thickness of hardened layers of racks and the residual magnetic field strength

Fig. 3 Residual magnetic field strength scan over a gear component with defects of hardening. Scanning the residual tangential field over the whole surface of the parts offers the possibil-ity to find unhardened areas ( fig. 3). In the end it is required, to demagnetize the parts.

2

References: [1] Koppelmann, J.:

Materialprüfung 9(1967) pp.401-405 and 14 (1972) 156-160 [2] Neumann, R., H. Willems:

Automatisierte Messung der Einhärtungstiefe an induktionsgehärteten Komponenten mittels Ultraschall. DACH-Tagung, Luzern1991. Sammelband DGZFP pp.637-644

[3] Morgner, W., K.-O. Prietzel, F. Michel:

Non-Destructive Case Depth Measuring and Monitoring. Proceedings of the XI. Int. Symposium of Nondestructive Characterization of Mate-rials, Berlin 2002. pp.759-765

3