Steel structures depend on alloy and heat treatment for durability and strength. Ferritic and martensitic structures behave ferromagnetically, and austenitic structures behave nonmagnetically. Therefore, steel alloys, at certain heat treatments, will have certain magnetic properties.
The characterization of magnetic properties, including its reaction to magnetic fields, is made by its permeability. Behavior of ferromagnetic material is characterized by its hysteresis. There is also temperature dependence evident in the magnetic properties of steel structures. Influences on the permeability of a steel structure are Ni-equivalent, Cr-equivalent, austenitisized temperature and time, quenching speed and media, and annealing time and temperature.
From the physics side, the influences include magnetizing frequency, field strength, and temperature. Figure 1
Properties valued in automotive components include stiffness, surface hardness, case depth, core hardness, wear resistance, and corrosion behavior. Evaluation of these properties is typically made by destructive methods, such as hardness tests, metallography, and micro hardness tests. Unfortunately, you also destroy the part that is tested, and, by doing so, you only test for a single flaw criteria.
The historical approach for non-destructive structure test that was and still is in use is the hysteresis effect to separate different structures on geometric identical components. The principle is to evaluate the harmonic content of a distorted sinusoidal voltage of a certain test frequency in an electric circuitry and test coil arrangement whereas the component under test influences with its hysteresis behavior the magnetic field of the measuring circuitry. The basic disadvantage of this method is that you need test components with the correct structure and “bad” parts to select the necessary test frequency, field strength and choice of evaluated harmonics. Also, in order to create harmonics you need strong magnetic fields to enter the non linearity of the permeability, which causes high electric energy with thermal drift of the measuring circuitry. The created bad parts also never reflect all of the realistic possibilities of process failure and/or material mix.
This modern, non-destructive electromagnetic structure test is the low-energy preventative multi-frequency test. It was developed in the 80’s. The basic requirements for this method are a stable, driftless system and no bad parts for the setup of system. Figure 2
Based on the dependence of the magnetic properties of steel structures to frequency and field strength of alternating magnetic fields, data can be collected based on structure variations caused by alloy and heat treatment variations.
Materials with different alloys and heat treatments will react differently to alternating magnetic fields. This system is realized simply by making the component under test the core of a transformer, whereas the function of exciter and receiver coils are effected by the magnetic coupling of the coils and eddy current effects. Low energy used causes low field strength with the benefits of low permeability, no signal distortion and no thermal drift of analog systems. Wide range of the used frequency commonly multiplexed eight frequencies in the ratio lowest to highest of 1:1000 or more benefits the possibility that a frequency is included in the test where a difference in collected data is seen caused by any known or unknown changes in structural properties beyond the tolerance of the required structural property.
Practical Implementation for Quality Control in Heat-Treatment Shops
Standard procedure for quality evaluation in the metallographic lab of a heat-treating manufacturer is cutting of random samples of a heat treated batch of a certain component with micro and macro hardness tests and microscopic structure analysis. This is time consuming and costly. The alternative is a non-destructive test accompanied by destructive tests with substantially less number of destroyed good components. The benefits include:
• reduced costs due to substantial less cutting
• fast result – within seconds versus possibly hours in destructive test
• simple understandable result – component is in tolerance of the properties or something is different
How it is done:
See Figure 3: Instrument configuration for eight frequencies and one channel
Up-to-date instruments for a non-destructive structure test can serve a certain number of test channels with dedicated instrument setups. A test channel coil arrangement is set F.e. for a certain part of a gearbox, and setups are created for the various ratios instrument. Each setup is fed with acquired data from pieces of the normal production. Than the pieces with extreme data are chosen and properties with destructive methods are evaluated. This shows the tolerance of the running production. If this does not show the specified tolerance of relevant properties, additional data with more pieces is possible. Additional benefits include that it will be possible to recognize trends in stored test data when the process is drifting and, further, the possibility of increasing the number of tested random samples without additional will cost up to 100% test on doubtful batches. Figure 4
This method was developed in Austria for the heat treatment shop of a gearbox manufacturing facility of a worldwide acting car manufacturer and has been used since with multiple installations. Figure 5
General applications of a non-destructive structure test, not only for steel material, besides the stated use and mostly with vacuum heat treatment technology heat-treatment shop supervision, are:
• 100 percent inspection in mostly automated systems with induction hardening to recognize trends or sudden process changes Figure 6
• “Optimized random test” of samples to fulfill the requirement to evaluate the tolerance of a process by selecting the parts with extreme data
• Inspection of long products (bars, tubes, wire, etc.) for material mix/batch cleanness with additional statistic evaluation to eliminate scattered data caused by structure changes due to straightening
• Homogeneity test for extruded/sintered and continuous heat-treated long products Figure 7