The project focused on three tasks:
Task 1: Investigate the current nondestructive testing technologies for case depth and hardness evaluation. Magnetic Barkhausen Noise Testing is used in the research.
Task 2: Design and prepare carburized samples that has a series of hardness and case depth conditions.
Task 3: Conduct the NDT testing and correlate the measurement with the testing results. Method of hardness and case depth prediction is provided.
Tempered 4140 steel rods was used for the hardness testing. Diameter of the rod is 0.5 inch. They were quenched from 850℃ then tempered. The rods are measured with Magnetic Parameter (MP) which is presented in Figure 1. For tempering temperature lower than 450℃, the hardness is higher than 35HRC. It is hard to correlate the hardness with MP. For tempering temperature that is higher than 550℃, there is linear correlation between the MP with hardness.
The case depth has been studied with the Magnetizing Voltage Sweep (MVS) method. The testing method was initially proposed by Suvi Santa-aho. Two frequencies were used for the testing. Each frequency will lead to a measurement depth and obtain the information corresponding to the depth. At each frequency, magnetizing voltage changes from 0 to 16 vpp and a profile as a function of magnetizing voltage was measured. There is a max slope that can be used to correlation the properties of the steel at certain depth. In the figure, 8Hz and 125Hz was used for measurement. There are three group of samples with case depth of 0.8, 1.2, and 1.8mm. The large case depth will lead to a larger slope ratio. In this way the case depth can be evaluated as Figure 2 presented.