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

For further details please write to us at

atul@wearcotetechnologies.inspc@wearcotetechnologies.inwearcote@gmail.com