Page 10 - Vol_1_Chapter_VI

instruments are used in the manufacturing.

6.3 Nitriding

Nitriding is a process of thermal treatment of metals in order to harden the surface. The most common
use of this process is for low alloy steel. They are also used on titanium, aluminum and molybdenum.
Typical applications include gears, camshafts, crankshafts, cam followers, extruder screws, valve parts,
die-casting tools, extrusion dies, firearm components, forging dies, injectors and plastic-mold tools. 8
The processes are named after the medium used to donate. The three main methods used are:
− Gas nitriding
− Salt bath nitriding
− Plasma nitriding

6.3.1 Gas Nitriding

Process description
Gas nitriding is typically performed in convection furnaces, either of pit
type or a box furnace, at a temperature in the range of 500–520 °C (932–968 °F) and in an ammonia
atmosphere. The ammonia may be diluted with nitrogen or hydrogen. The parts to be nitrided are
placed on fixtures or in “baskets”, which are transferred to and loaded into the furnace.
The furnace cover or door is then closed. To ensure precision with regard to compound layer and
diffusion zone thickness, it is important to havegood enough furnace temperature uniformity, typically
±5 °C (±41 °F) (Fig. 6. 6).
Nitriding furnaces have to be tight for safety reasons and also because of the odour resulting from
possible leakage of ammonia gas. 9

Benefits
Favored for components that are subjected to heavy loading, nitriding imparts a high surface hardness
which promotes high resistance to wear, scuffing, galling and seizure. Fatigue strength is increased
mainly by the development of surface compressive stresses. The wide range of possible temperatures
and case depths, which allow adjustment of different properties of the treated parts, give gas nitriding
a broad field of applications.

Application
A low temperature application is nitriding of spring steels to prolong the fatigue life of springs for
automotive use. In general, all ferrous materials can be gas nitrided up to 5% chromium. For higher
contents of alloying elements and for gas nitriding of stainless steel, plasma nitriding might be
considered. Gas nitriding of sintered steels with low density is not recommended. For optimum results,
the material should be in a hardened and tempered condition prior to gas nitriding.

Copyright © by DIGIGREEN All Rights Reserved Volume I - Chapter VI: Digital and Green Surface Conditioning 10