November AMP_Digital

FEATURE A D V A N C E D M A T E R I A L S & P R O C E S S E S | N O V E M B E R / D E C E M B E R 2 0 1 9 6 0 MEASUREMENT ACCURACY The main source of measurement error is evaluation of surface position: The shape of the surface signal depends on accurate coupling and operator skill. Another error source is setting the marker that provides the time of flight when the pulse reaches the interface. The steeper the signal rises, the lower the error. Thus, a shear wave angle as low as reason- able is used and scanning into the direction of decreasing SHD is recommended. Achievable accuracy of better than ±0.1 mm is possible for standard parts with high quality sur- faces. However, the operator must control the “good” shape of the A-scan during data acquisition. Accuracy based on mi- croindentation hardness profiles compared with the back- scatter method is slightly lower, estimated as ±0.2 mm on average, depending on the material microstructure. CORRELATION OF SHD TO THD SHD is the point where the hardness depth profile reaches the limiting hardness. ISO 18203 [1] defines the limit- ing hardness as 80% of the specified minimum surface hard- ness. In many cases, the hardness depth gradient shows a sudden decrease when reaching the core material (Fig. 3a). The hardened case is characterized by martensitic micro- structure above a short transition zone of mixed phases, fol- lowed by the core microstructure unaffected by the harden- ing process. Total hardening depth (THD) is the point where the scattering core material begins a well-defined interface (Fig. 3a) [1] . For this type of interface, a straightforward cor- relation can be established between the SHD and the THD by taking off a small offset as shown in Fig. 3a. The correlation is less clear whenmaterial hardness val- ues are scattered as shown in Fig. 3b for a normalized steel. The interface at the unaffected core is still well defined and measurement of THD is accurate. However, scattered hard- ness datamakes the transition fromSHD to THD less defined. A straightforward correlation can be established by taking off a constant offset value, which is usually slightly larger than for the case-hardenedpart and shouldbe determinedby cut- ting representative samples. Parts with sections of different geometries pose prob- lems for optimizing the heating and cooling regime during the hardening process. The transition from the martensitic case to the unaffected core is extended and changes locally. The microstructure of the transition zone could contain fine grains and coarse grains that increase with depth. Heating and cooling could cause grain refinement of the complete core material [6] . The SHD can still be measured accurately when the transition zone does not contain scattering structures. The offset value required for a good correlation could depend on the measured SHD and on part geometry, circumstances 10 that require a full calibration procedure. SHD and THD are measured on a representative sample for each position of interest. Calibration can be performed and supported by the instrument in the expert mode. THDmeasurement becomes increasingly uncertain when the transition zone contains scattering structures. The authors recommend a procedure that helps to explain the results of hardening. Usually, there are positions (often close to the runout) where good results can always be achieved. By scanning toward critical areas, an experienced engineer can gain reliable insight into ma- terial microstructure. When the core is refined completely, SHD is no longer assessable except through the material microstructure. The difference in THD and SHD can be reasoned based on the microstructure. The extended seam of mixed phases depends on local heating and cooling conditions and might Fig. 3 — (a) Hardness depth profile of quenched and tempered steel 42CrMo4, and (b) hardness depth profile of a normalized steel 42CrMo4 [5] . (a) (b) (continued on page 12)