May/June_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 | M A Y / J U N E 2 0 2 0 6 5 12 9 and is applicable for a large range of uses. It has been suc- cessfully introduced worldwide to test all kinds of material for determining microstructure and hardening characteris- tics. There are numerous applications for such a system. HOW DOES IT WORK? The use of several test frequencies to induce different field strengths in the coil means that all these differences are detectable or viewable. Different structures from different heat treat methods (and by heat treat process faults) create different permeability curves which are detected. Thus, the eddy-current testing becomes reliable; expected, and unex- pectedwrong structures are detected, with awider view. The test takes place at different field strengths, so to say “preven- tively” over a larger area. If the vector tips are enclosed with an elliptic tolerance zone, the test can be reduced to a comparison of the vectors to inside (OK) or outside (NOK) of the tolerance zone. Testing current production with such a frequency band (eight frequencies), as shown in Fig. 1, and the comparison with the previously created tolerance zones made with good parts is known in professional circles as PMFT. A wide fre- quency band (at least 1:1000) detects all abnormal structures detectable by eddy current as faulty and sorts them out. Ten to twenty OK parts only are needed to setup the instrument and to formthe tolerance zones (calibration). A challenge test with NOK parts (e.g., not-hardened, incorrectly quenched, austenitizing temperature not reached, too short or too long tempering, annealing temperature too high or too low, etc.) can be done, but is not needed. The test system will reliably detect faulty parts with both known and unknown defects. The multi-frequency test method works reliably for all kinds of defects which may happen during heat treatment of steel. It works like a fingerprint comparison, only when all param- TABLE 2 – POSSIBLE ERRORS DURING INDUCTION HARDENING Which parameter was incorrect? How was it incorrect? What are the effects? Austenitizing temperature Too high Overhardening, incorrect structure martensite + residual austenite Too low Underhardening, incorrect structure martensite + bainite + ferrite Austenitizing time Too long Overhardening, case too high, incorrect structure martensite + residual austenite Too short Underhardening, shallow case, incorrect structure martensite + bainite + ferrite Quenching Too fast Incorrect structure martensite + residual austenite Too slow Incorrect structure martensite + bainite + ferrite Formation of vapor bubbles Soft spots not defined Annealing temperature Too high Hardness too low Too low Hardness too high Annealing time Too short Hardness too low Too long Hardness too high Rate of feed Too slow Shallow case, misplaced case, austenitizing time too short Too fast Case too high, misplaced case, austenitizing time too long Damaged inductor Undefined Undefined Malpositioning Undefined Unsymmetrical hardening pattern, overheating, melting Fig. 1 — Test range with eight test frequencies and tolerance fields, as shown in this fingerprint system.