Abstract

This study has aimed to illustrate the effect of one surface hardening method(
Nitriding) upon fatigue life time of medium carbon steel alloys . Therefore all
tests. concerning round bending of fatigue ,have been conducted at ordinary
laboratory circumstances , within amean stress , [ value = 0&stress ratio ,R = -1],
as stable and changing stress capacities , were forced in order to know the metal
resistance to these stresses under the different dynamic loading .There by to give
the safety life time of this metal , nitriding process (liquid & Gas ) was carried out
to the metal ,which contributed in improving the fatigue resistance , as the
improvement amount was based on the hardened depth , and the fatigue boundary ,
increases with nitriding layer depth . strength tests, micro and hardening were
carried out for the applied heat treatment circumstances ,because these tests are
considered to be as criteria and connected with revelant correlation , concerning
fatigue tests results .
Actually , micro structure examination has carried often before treatment ,
where as the micro structure was found before treatment [ ferrit & perlite ] while
nitrogen phases were observed to be high with a layer of crome iron nitrides on the
treated service with nitriding , that confirm the nitriding role in structural barriers
formation on the service , that acts on stopping towards the crack..
Consequently, the images of (SEM) , regarding the fracture service , have
declared a follow up images towards fatigue cracks . This crack is originated under
the thick hardened serface.Layer and developes , is a lease plastic region , as it was
detected that fatigue life time , increases when loading from (high – low ) at the
changing capacity of fatigue tests rather than( Low- High).