Heat Treating Technologies


HEAT TREATMENTS:

NITRIDING
Brief Description of Nitriding Technologies

A good technical “dictionary” definition of nitriding would run more or less as follows:

NITRIDING – a surface (or case) hardening process whereby nitrogen is diffused into the surface of components made of a ferrous alloy, by exposing the metal to a nitrogen carrying medium, such as gaseous ammonia, cyanide containing salts, or ionized nitrogen gas (plasma), while maintaining a suitable temperature, usually in the 450 °C to 600 °C (850 °F to 1100 °F) range. The hardening effect is produced by nitrogen forming hard nitrides with iron, chromium, aluminum, and certain other elements in the alloy.

Since the early days of nitriding, in the 1920’s, various techniques of utilizing the aforementioned media have evolved. These three distinct groups are generally referred to as gas, salt and ion (or plasma) nitriding. The proponents of their selected favorites have promoted their respective advantages without adequately explaining the differences or similarities between them. This is what we shall endeavor to do here.

Reasons to Use a Nitriding Process

Components are nitrided for a variety of reasons, but primarily to:

  • increase the surface hardness,
  • thus enhancing their wear and fatigue resistance,
  • corrosion protection,
  • appearance
Brief Description of Nitriding Technologies

From a purely theoretical point of view all three methods are capable of producing a desirable hardened case. With proper process control a whole spectrum of case properties may be achieved. However, difficulties develop very quickly when we look at a practical side of nitriding. Generation of the required nascent nitrogen, maintaining its proper supply near the surface of the metal, proper control of process parameters in steady state, as well as between the various stages of the process, in dynamically changing conditions, are challenges that few have been able to master. In addition one must deal with economic and environmental aspects of running a nitriding installation. In general terms the three methods compare as follows.

Salt Bath Nitriding

Salt bath is a quick and easy process, suitable for crude components, but expensive to operate and environmentally hazardous; additional post-processing cleaning is required. Melted cyanide and cyanate containing salts are the source of nascent nitrogen. The main advantage is short heating times as cold components are lowered into a molten salt, thus heating the work piece rapidly.

Plasma (Ion) Nitriding

Plasma nitriding is versatile and capable of producing good results on simple components, but requires very skilled operators; significant risk of local over-temperature effects; deep cavities and bores virtually impossible to nitride; easy masking; environmentally friendly; suitable for specialized applications. Nascent nitrogen is obtained by ionizing nitrogen gas in an electrically charged field.

Gas Nitriding

Gas nitriding is the most versatile and economically viable if controlled properly; old or poorly designed equipment subject to ammonia leaks, lack of uniformity or simply plagued by the absence of knowledge how to operate such equipment correctly, aspects which have occasionally given all of nitriding an undeserved, poor reputation; components will nitride properly in any configuration even when lightly touching, thus the capacity of a gas nitriding vessel of a certain size is much greater than that of a plasma (ion) furnace. The source of nascent nitrogen is ammonia gas.
NITROCARBURIZING
General Description

Nitrocarburizing is a technique derived from nitriding and it is frequently referred to as “FNC” (ferritic nitro-carburizing). It is a surface or case hardening process whereby nitrogen is diffused into the surface of components made of ferrous alloys, by exposing the metal to a liquid or gaseous medium carrying nitrogen and carbon. This could be molten salts, gaseous ammonia or ionized nitrogen gas, with an addition of carbon compounds. The process may be conducted within a temperature range of 450-650°C (approximately 840-1200°F), although nitrocarburizing is usually carried out at around 580°C (approximately 1075°F). The hardening effect is produced by nitrogen forming hard nitrides with iron, chromium, aluminum, and certain other elements in the alloy, with the presence of carbon in the processing medium ensuring good stability of various nitride phases and promoting the highest possible hardness. Steels with a relatively low carbon content benefit from this process most, while the effects on steels containing more than 0.4% C are similar to those obtained in nitriding. Nitrocarburizing should not be confused with carbonitriding, a completely different high temperature process derived from carburizing.

Reasons to Use a Nitrocarburizing Process

Primarily low carbon steels are nitrocarburized to dramatically increase their resistance to wear. In some cases corrosion resistance and appearance of the steel surface may be improved at the same time. In most cases the treatment is applied as a final manufacturing operation, or as one of the last few steps.
CARBURIZING
General Description

Carburizing is a process of controlled diffusion of carbon into the surface of a component, followed by quenching and tempering, with the objective of increasing the component’s surface hardness. The process is generally applicable to low carbon and low alloy steels. There are two carburizing process types available commercially – vacuum carburizing and conventional carburizing.

Conventional Carburizing

In this thermal process ferrous alloys are heated to above their transformation temperature and exposed to carbon rich atmosphere. Processing temperatures in conventional carburizing typically are in the 1450°F - 1900°F (790°C - 1040°C) range. The diffusion of carbon into the part and the subsequent quench leads to a part with a hard, wear resistant surface and a tough, shock resistant core.

Reasons to Use a Carburizing Process

The reasons to use this process in preference of any other heat treating method are as follows:

  • high surface hardness with a tough, durable core,
  • case depths up to 0.250” are achievable,
  • ability to use inexpensive steels and still produce components with hard surface properties,
  • generally used for parts subjected to cyclical loading
  • cost effective for parts where some distortion is acceptable
Vacuum Carburizing

Vacuum carburizing is a state-of-the-art thermal process where carburizing is effected under very low pressures. First the parts are heated in vacuum to above the transformation temperature of the alloy. Then they are exposed to carbon-carrying gas, or gas mixtures, under partial pressure.

Relative to conventional carburizing, the main advantages of the method are:

  • repeatable results to within ±0.001" (±25µm)
  • significantly reduced size changes and distortion
  • improved fatigue strength
  • better control of the surface layer chemistry
  • the process is environmentally friendly
CARBONITRIDING
Conventional Carbonitriding

Carbonitriding is a process similar to carburizing except that ammonia is added to the carburizing atmosphere, which produces an effect of supplementary nitrogen diffusion into the component and results in a higher surface hardness.



Reasons to Use a Carbonitriding Process

The reasons to use this process in preference of any other heat treating method are as follows:

  • high surface hardness with a tough, durable core,
  • case depths up to 0.250” are achievable,
  • ability to use inexpensive steels and still produce components with hard surface properties,
  • generally used for parts subjected to cyclical loading,
  • cost effective for parts where some distortion is acceptable
  • carbonitriding is used for applications where slightly higher surface hardness levels are required
Vacuum Carbonitriding

It is a thermal process of simultaneously diffusing both carbon and nitrogen into ferrous alloys under partial pressure. This leads to an extremely hard and wear resistant surface. Vacuum carbonitriding is a significant improvement over conventional gas carbonitriding. The process contains all of the inherent benefits of vacuum carburizing, but also has the additional benefit of precise computer control of surface ammonia content. Furthermore, this process does not require any additional refractory burn-outs so not only is the end product of higher quality, but it is often less expensive than with competing conventional gas processes.