PROCESSING CONDITIONS AND HARDNESS DEVELOPMENT IN NITRIDED COMMERCIAL STEELS

ABSTRACT
The effects of pre-nitriding heat treatments and the influence of alloying e1ements on
the development of surface hardness have been investigated using Enl9 and En4lB
steels, gas-nitrided at different temperatures and nitriding potentials for 24 to 96 h. The
surface hardness is lower at 5700C than those nitrided at 4700 and 5200C. The
hardness is reduced with increasing the tempering temperature and this effect is
pronounced in En19 steel. The hardness profile is independent of tempering
temperatures on nitriding at 5200C. The profiles are shallower with increasing
tempering temperature in Enl9 steel when nitrided at 5700C. The prior austenitizing
temperatures appeared to have no influence on hardness or hardness profile of the
nitrided case. The build up of hardness up to a certain depth is established rapidly and
remains constant with nitriding time. Below that depth the initial hardness is low and it
increases steadily with nitriding time. The combined hardness curves after different
treatment times have the appearance of a hysteresis loop.
The results show a constant proportionality of hardness increment to square root of
alloy content when nitrided in 80% NH3 at low temperatures. The hardness increment
per percentage of alloying element is sensitive to the choice of base hardness of iron.
The low hardness development at 5700C is related to reduction in the volume fraction of
the fine CrN precipitates due to a coarsening effect and also due to precipitation of
coarse nitrides on lath boundaries. This arises since more chromium segregates as
(FeCr)3C at high temperatures and in-situ nucleation of CrN occurs at the carbides
(mainly at lath boundaries), giving non-hardening coarser CrN. This phenomenon was
found to be less effective in aluminium containing steel (En41B). Since aluminium does
not segregate to carbide as does chromium in En19 steel, fewer large nitride particles
will form and the volume fraction of fine precipitate will be comparatively high in En41B
steel.