Interference Microscopy on Laser Hardened Samples of Steel

H. K. D. H. Bhadeshia

Martensitic transformations involve a coordinated movement of atoms. If a sample of austenite is polished flat and then allowed to transform into martensite then the flat surface acquires relief. The relief caused by an individual plate is an invariant-plane strain with a large shear component. By contrast, reconstructive transformations such as the formation of austenite during heating (or of allotriomorphic ferrite during cooling) do not cause such surface displacements.

Surface relief studies are often conducted using interference microscopy.

Henrikki Pantsar has developed a technique in which a polished steel sample with an arbitrary starting microstructure is treated with a high power diode laser in an inert atmosphere (Pantsar, H. Use of DIC Imaging in Examining Phase Transformations in Diode Laser Transformation Hardening of Steels, 23rd International Congress on Applications of Lasers and Electro-optics, 2004, San Francisco, Ca, USA, 8 pages.)

He has discovered that the surface relief due to martensitic transformation is nicely preserved following the passage of a laser (with the sample immersed in inert gas), in spite of the very high temperatures reached. The relief can subsequently be observed using differential interference microscopy, as shown below.

All the micrographs have been provided by Henrikki.

Fig6 Surface relief in Fe-0.16C-0.26Si-0.96Mn-0.11Cr-0.12Ni-0.02Mo wt% sample, following laser surface-treatment at 10 mm/s, which resulted in partial austenitisation. The sample here is away from the centerline of the laser track. The regions of pearlite which transformed into austenite in the heating cycle, changed into martensite during cooling, resulting in surface displacements. The remainder of the microstructure is ferrite. Hardness 220 HV	Fig7 Surface relief in Fe-0.16C-0.26Si-0.96Mn-0.11Cr-0.12Ni-0.02Mo wt% sample, following laser surface-treatment at 10 mm/s which resulted in partial austenitisation. The sample is located at the centerline of the laser track. The regions of pearlite which transformed into austenite in the heating cycle, changed into martensite during cooling, resulting in surface displacements. The remainder of the microstructure is ferrite. Hardness 296 HV	Fig8 Surface relief in Fe-0.16C-0.26Si-0.96Mn-0.11Cr-0.12Ni-0.02Mo wt% sample, following laser surface-treatment at 5 mm/s which resulted in a much greater degree of austenitisation. The sample is located at the centerline of the laser track. The regions of pearlite which transformed into austenite in the heating cycle, changed into martensite during cooling, resulting in surface displacements. There is only a small quantity of ferrite. Hardness 410 HV
Fig9 Surface relief in Fe-0.46C-0.21Si-0.74Mn-0.10Cr-0.08Ni-0.01Mo wt% sample, following laser surface-treatment at 10 mm/s which resulted in partial austenitisation. The sample is located away from the centerline of the laser track. The regions of pearlite which transformed into austenite in the heating cycle, changed into martensite during cooling, resulting in surface displacements. The smooth parts of the relief correspond to ferrite from the original microstructure. Hardness 440 HV	Fig10 Surface relief in Fe-0.46C-0.21Si-0.74Mn-0.10Cr-0.08Ni-0.01Mo wt% sample, following laser surface-treatment at 10 mm/s which resulted in complete austenitisation. The sample is located at the centerline of the laser track. The austenite then transformed into martensite during cooling, resulting in characteristic surface displacements. Hardness 715 HV	Fig11 Surface relief in Fe-0.46C-0.21Si-0.74Mn-0.10Cr-0.08Ni-0.01Mo wt% sample, following laser surface-treatment at 5 mm/s which resulted in complete austenitisation. The sample is located at the centerline of the laser track. The austenite then transformed into martensite during cooling, resulting in characteristic surface displacements. Hardness 744 HV
Fig12 Surface relief in Fe-0.44C-0.34Si-0.70Mn-01.10Cr-0.16Ni-0.18Mo wt% sample, following laser surface-treatment at 5 mm/s which resulted in complete austenitisation. The sample is located at the centerline of the laser track. The austenite then transformed into martensite during cooling, resulting in characteristic surface displacements. Hardness 748 HV	Fig13 Surface relief in Fe-0.44C-0.34Si-0.70Mn-01.10Cr-0.16Ni-0.18Mo wt% sample, following laser surface-treatment at 10 mm/s which resulted in complete austenitisation. The sample is located at the centerline of the laser track. The austenite then transformed into martensite during cooling, resulting in characteristic surface displacements. Hardness 767 HV	Fig14 Surface relief in Fe-0.44C-0.34Si-0.70Mn-01.10Cr-0.16Ni-0.18Mo wt% sample, following laser surface-treatment at 5 mm/s which resulted in complete austenitisation. The sample is located at the centerline of the laser track. The austenite then transformed into martensite during cooling, resulting in characteristic surface displacements. Hardness 748 HV
Fig15 Surface relief in Fe-0.44C-0.34Si-0.70Mn-01.10Cr-0.16Ni-0.18Mo wt% sample, following laser surface-treatment at 5 mm/s which resulted in complete austenitisation. The sample is located at the centerline of the laser track. The austenite then transformed into martensite during cooling, resulting in characteristic surface displacements. Hardness 748 HV	Chromium carbides This is a sample from an 11.5Cr 1.1C wt% steel which has martensite but it also illustrated undissolved chromium carbides.

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