PROCESSING CONDITIONS AND HARDNESS DEVELOPMENT IN NITRIDED COMMERCIAL STEELS | ||||
| The International Conference on Applied Mechanics and Mechanical Engineering | ||||
| Article 96, Volume 13, 13th International Conference on Applied Mechanics and Mechanical Engineering., May 2008, Page 115-129 PDF (562.74 K) | ||||
| Document Type: Original Article | ||||
| DOI: 10.21608/amme.2008.39678 | ||||
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| Author | ||||
| MRIDHA S. | ||||
| Professor, Dept. of Manufacturing and Materials Engineering, International Islamic University Malaysia, P. O. Box 10, 50728 Kuala Lumpur, Malaysia. | ||||
| Abstract | ||||
| 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. | ||||
| Keywords | ||||
| Gas nitriding; Hardness profile; Tempering temperature; Nitriding potential; Hardness loop | ||||
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