Evaluation of induction hardened case depth through microstructural characterisation using magnetic Barkhausen emission technique

The influence on the hysteresis loop and the magnetic Barkhausen emission (MBE) of microstructure within the case of different induction hardened carbon steel shafts has been studied. The hysteresis loop shows a distortion with a sudden reduction in the rate of magnetisation (dB/dH ) before approaching the maximum magnetic flux density indicating surface hardening. The systematic changes in the MBE profile for different voltages applied during induction heating indicate the microstructural variations within the case. A single peak MBE profile for a fully martensitic structure gradually changes into two peaks on reducing the induction hardening voltage indicating the formation of an additional soft ferrite phase within the case. The systematic changes in the two MBE peak heights indicate the synergistic decrease in the volume fraction of martensite and the increase in the volume fraction of ferrite phase within the case due to reduction in the induction hardening voltage. The changes in the MBE profile for different case depth specimens are more prominent than the hysteresis loop. This study shows that the MBE alone gives better insight in evaluating the induction hardened components (having case depth ≲1·5 mm), since the height and position of the two MBE peaks are directly influenced by the volume fraction and composition of hard and soft phases within the case. In general, this study reveals the high sensitivity of the MBE technique to the finer microstructural changes due to surface heat treatment in ferritic steels

Evaluation of induction hardened case depth through microstructural characterisation using magnetic Barkhausen emission technique

The influence on the hysteresis loop and the magnetic Barkhausen emission (MBE) of microstructure within the case of different induction hardened carbon steel shafts has been studied. The hysteresis loop shows a distortion with a sudden reduction in the rate of magnetisation (dB/dH ) before approaching the maximum magnetic flux density indicating surface hardening. The systematic changes in the MBE profile for different voltages applied during induction heating indicate the microstructural variations within the case. A single peak MBE profile for a fully martensitic structure gradually changes into two peaks on reducing the induction hardening voltage indicating the formation of an additional soft ferrite phase within the case. The systematic changes in the two MBE peak heights indicate the synergistic decrease in the volume fraction of martensite and the increase in the volume fraction of ferrite phase within the case due to reduction in the induction hardening voltage. The changes in the MBE profile for different case depth specimens are more prominent than the hysteresis loop. This study shows that the MBE alone gives better insight in evaluating the induction hardened components (having case depth 1.5 mm), since the height and position of the two MBE peaks are directly influenced by the volume fraction and composition of hard and soft phases within the case. In general, this study reveals the high sensitivity of the MBE technique to the finer microstructural changes due to surface heat treatment in ferritic steels

Insight into the microstructural characterization of ferritic steels using micromagnetic parameters

The influence of tempering-induced microstructural changes on the micromagnetic parameters such as magnetic Barkhausen emission (MBE), coercive force (H-c), residual induction (B-r), and maximum induction (B-max) has been studied in 0.2 pct carbon steel, 2.25Cr-1Mo steel, and 9Cr-1Mo steel. It is observed that, after short tempering, the micromagnetic parameters show more or less linear correlation with hardness, which is attributed to the reduction in dislocation density, but long-term tempering produces nonlinear behavior. The variation in each of these parameters with tempering time has been explained based on the changes in the size and distribution of ferrite laths/grains and precipitates. It has been shown that the individual variation in the microstructural features such as size and distribution of laths/grains and precipitates during tempering can be clearly identified by the MBE parameters, which is not possible from the hysteresis loop parameters (H-c and B-r). It is also shown that the MBE parameters can not only be used to identify different stages of tempering but also to quantify the average size of laths/grains and second-phase precipitates

Effect of tensile deformation on micromagnetic parameters in 0.2% carbon steel and 2.25Cr-1Mo steel

The influence of prior tensile deformation on the magnetic Barkhausen emission (MBE) and the hysteresis (B-H) curve has been studied in 0.2% carbon steel and 2.25Cr-1Mo steel under different tempered conditions. This study shows that the micromagnetic parameters can be used to identify the four stages of deformation, namely (i) perfectly elastic, (ii) microplastic yielding, (iii) macroyielding and (iv) progressive plastic deformation. However, it is observed that the MBE profile shows more distinct changes at different stages of tensile deformation than the hysteresis curve. It has been established that the beginning of microplastic yielding and macroyielding can be identified from the MBE profile which is not possible from the stress-strain plot. The onset of microplastic yielding can be identified from the decrease in the MBE peak height. The macroyielding can be identified from the merging of the initially present two-peak MBE profile into a single central peak with relatively higher peak height and narrow profile width. The difference between the Variation of MBE and hysteresis curve parameters with strain beyond macroyielding indicates the difference in the deformation state of the surface and bulk of the sample. (C) 1999 Acta Metallurgica Inc. .

Identification of different stages of low cycle fatigue damage using magnetic Barkhausen emission in 9Cr-1Mo ferritic steel

Magnetic Barkhausen emission (MBE) technique has been used to assess the low cycle fatigue (LCF) damage in 9Cr-1Mo ferritic steel. LCF tests have been carried out at ambient temperature at total strain amplitudes of plus/minus 0.25, plus/minus0.50 and plus /minus 0.75%. MBE measurements have been performed on LCF tested specimens interrupted at various life fractions. The various stages of LCF such as cyclic hardening, cyclic softening, saturation and crack initiation and propagation have been examined and identified using peak height value of the root mean square (RMS) voltage of MBE. The cyclic hardening, which occurred in the early stage of LCF cycling, decreases the MBE peak height value. The progressive cyclic softening stage displayed reversal in MBE response i.e., the MBE peak high value increased following softening due to rearrangement of dislocations into cells. Further, the saturation stage, where the stress value remained constant for a large number of cycles due to formation of stable dislocation substructure, results in a constant value of MBE. Finally, the onset of rapid stress drop and cusp formation in the stress-strain hysteresis loop, which indicate surface crack initiation and propagation, exhibited a rapid increase in the MBE peak values. This is also confirmed by MBE measurements on the crack and away from the crack using surface MBE probe. The increase in MBE is ascribed to the movement of additional reverse domains produced at the crack surfaces

An assessment of low cycle fatigue damage using magnetic Barkhausen emission in 9Cr-1Mo ferritic steel

A non-destructive, magnetic Barkhausen emission (MBE) technique has been used to assess various stages of low cycle fatigue (LCF) damage in 9Cr-1Mo ferritic steel. The initial decrease in the MBE peak height in the early stage of LCF cycling indicates the cyclic hardening stage, in which the formation of dislocation tangles reduces the mean free path of the domain wall movement. The increase in the MBE level again on further cycling indicates the progressive cyclic softening stage where the rearrangement of dislocation tangles into cells enhances the domain wall movement. The unaltered behaviour of MBE on continued cycling shows the saturation stage where the stabilization of dislocation substructure maintains the MBE level. Finally, a sharp increase in the MBE peak value identifies surface crack initiation and propagation, which is ascribed to the movement of additional reverse domains produced at the crack surfaces. This study establishes that the MBE technique can be used to assess the progressive degradation in the fatigue life of the ferritic steel components