wct nitriding 2012.ppt
TRANSCRIPT
Nitriding
Atul S.C.
WEAR COTE TECHNOLOGIES
Principal Reasons for Nitriding
Nitriding vs Carburizing
Carburizing Nitriding• Treatment temperature Low
is high (austenitic) (ferritic)• Carbon added Nitrogen• Quenching reqd. Not reqd.• Distortion high Low• Heavy case depth Low• Not suitable for high temp Suitable
service• Scuffing resistance poor Very good
Dissociation of Ammonia and pick-up of Nitrogen in steel during gas nitriding
Iron-Nitrogen Phase Diagram
Nitriding requires alloying elements (Al, Cr, V etc.) that form hard nitrides to ensure high hardness
(EN40B-BS970 Specification: Carbon 0.25%,Silicon 0.30%,Manganese 0.60%, Chromium 3.25%, Molybdenum 0.55%)
Gas Nitriding Furnace
Gas dissociation- the amount of decomposition of theammonia gas. The exhaust gas is a mixture of 2NH3 + H2 +N2 and ammonia is the only one soluble in water.
Control of Nitriding Process
Microstructure of Nitralloy 135M (0.4C-0.6Mn-0.3Si-1.6Cr-0.35Mo-1.2Al), quenched and tempered, Nital etched
RED COMPOUND YELLOW NITRIDIE NEEDLES IN FERRITE PHASEBLUE PEARLITE
SAE 1144 (0.45C-1.5Mn-0.25Si): Throttle Valves
SAE 1006 (0.08C-0.3Mn): Automotive Seat Rails
H13 Hot-work Tool Steel (0.4C-5Cr-1.3Mo-1V),Quenched and Tempered,Nital Etched: Forging Diesapplication
SAE 4140 (0.40C- 1.0Cr-0.85Mn-0.25Si-0.2Mo):Automotive Shaftsapplication
White Layer Characteristics
• The white layer consists of a mixture of 2 phases viz., Fe4N and Fe2-3N in gas nitriding
• The mismatch in the growth of these phases, the white layer has a tendency to be friable especially when thick
• For fatigue applications, the white layer is removed by blasting
• If a mono-phase only develops, it can be left as it is; this provides very good sliding characteristic.
Liquid Nitriding
• Also called molten salt bath nitriding or soft nitriding
• A mixture of cyanate and carbonate salts is melted and used as a medium for nitriding
• Unlike gas and plasma processes , this process can be used for all classes of steels
Liquid Nitriding
• Treatment times are short
• Case depth consists of compound layer of a few microns and a diffusion zone beneath, that can extend to about 0.5mm depending steel
• Plain carbon and lean alloy steels develop deeper cases than alloy steels
• Most economical of the three processes
Liquid Nitriding
Properties:
• Compound layer hardness depends on the base composition
• Hardness increases with carbon content
• Hardness increases with alloying elements that form nitride, e.g., Cr, aluminium, vanadium, molybdenum increase
• Nickel, copper do not affect hardness
Liquid Nitriding
• The compound layer consists predominantly of epsilon nitride ( Fe2-3 N)
• This is harder than gamma prime (Fe4N)
• Has low friction co-efficient, so smooth sliding characteristics
• Enhances the scuffing resistance, i.e., prevents adhesive wear resistance
• Controlled pores on the surface helps to retain oil
Liquid nitriding with post oxidation
The corrosion resistance of nitrided layer can be enhanced by a post oxidative treatment
Components from molten salt bath are quenched in another molten salt bath for developing an oxide layer
This gives a black color due to the spinel oxides of FeO-Fe3O4
Liquid nitriding with post oxidation
The corrosion resistance can further be improved by a polishing operation or by polishing followed by re-oxidation
For example
This is necessary for the smooth sliding of valve stem in the guide
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