Cross-section micrography of martensitic stainless steel AISI 420. The specimen has a carburized layer on the surface, but this picture only show the inside by trimming the original picture.
Cristiano José Scheuera, Rodrigo Perito Cardosoa, and Silvio Francisco Brunattoa
CC BY 4.0
<p><cite style="font-style:normal">Cristiano José Scheuera, Rodrigo Perito Cardosoa, and Silvio Francisco Brunattoa (2015). "Low-temperature Plasma Assisted Thermochemical Treatments of AISI 420 Steel: Comparative Study of Obtained Layers". <i>Materials Research</i> <b>18</b> (6). <a href="https://en.wikipedia.org/wiki/Digital_object_identifier" class="extiw" title="w:Digital object identifier">DOI</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1590/1516-1439.013815">10.1590/1516-1439.013815</a>. <a href="https://en.wikipedia.org/wiki/International_Standard_Serial_Number" class="extiw" title="en:International Standard Serial Number">ISSN</a> <a rel="nofollow" class="external text" href="https://www.worldcat.org/issn/1980-5373">1980-5373</a>. Retrieved on <span class="mw-formatted-date" title="2020-09-29">2020-09-29</span>.</cite>
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From  <a rel="nofollow" class="external text" href="https://www.scielo.br/img/revistas/mr/v18n6//1516-1439-mr-1516-1439013815-gf01.jpg">Figure 1 (a1)</a>.