NITREX HEAT TREATING SERVICES

Carbon Restoration
In the event that in a heat treating operation the components suffered from decarburization (loss of carbon content) of the surface, we are capable of salvaging them using the Carbon Restoration technique.

Note: carbonitriding is sometimes confused with nitrocarburizing. Please read the descriptions of both processes to avoid misunderstandings.

Carburizing – Conventional
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 steels. When conducted in a “conventional”, rather than in a vacuum furnace, we can refer to the process as conventional carburizing.

In this thermal process ferrous alloys are heated to above their transformation temperature and exposed to carbon rich medium. Processing temperatures fall 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.

Solid, liquid and gaseous carbon-carrying medium may be employed, however, the first two are rarely used. Nitrex-offered carburizing is conducted in computer controlled integral quench and pit gas carburizing furnaces. A full range of case depths if feasible with an economically derived limit of approximately 0.250″ (6.4 mm).

In addition, Nitrex is capable of selective carburizing where only specific areas of the part are to be treated.

Carburizing – Vacuum

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. Nitrex has developed a revolutionary process called “Pulse- Pressure”, a method quickly becoming the industry standard.

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

The basic aspects or carburizing in general are described in the Conventional Carburizing section.

Summary of Benefits:

• Conversion of retained austenite (soft) into martensite (hard)
• Increased strength, toughness, stability and durability
• Increased density of the steel structure
• Lower coefficient of friction
• Decreased residual stresses and brittleness
• Significantly improved abrasive wear resistance

Ferritic Nitrocarburizing – FNC / Nitreg®-C: Potential-Controlled Nitrocarburizing

A shorter cycle time of the Nitreg® process carried out predominantly on carbon and low alloy materials.

Nitreg®-–C is a controlled version of nitrocarburizing (nitrocarburising or FNC process). It allows for precise KN control during the process. The additions of carbon bearing gases to the nitriding atmosphere help to increase the relative content of the epsilon phase.

The advantage of a KN controlled technology is best evidenced when increased wear and/or corrosion resistance is sought. Such properties of the nitrided case are not only influenced by the thickness and relative phase composition of the WL. They also strongly depend on the relative level of porosity developed in the WL.

The KN control is essential in producing the desired WL configuration

Example of various porosity levels achieved through KN control are shown below.

Benefits:

• Increased wear/corrosion resistance on selected alloys
• No distortion
• No brittleness
• Excellent process reliability

• Conversion of retained austenite (soft) into martensite (hard)
• Increased strength, toughness, stability and durability
• Increased density of the steel structure
• Lower coefficient of friction
• Decreased residual stresses and brittleness
• Significantly improved abrasive wear resistance

Hardening / Quenching – Conventional
Hardening is one of the oldest metallurgical processes known to man, originally in the form of heating a sword in the fire and then throwing it into the lake to make it harder. The more modern approach is to heat components in an atmosphere furnace followed by quenching, generally in heated oil.

The expression “conventional hardening” is used here to differentiate the process from vacuum hardening. It should also be noted that “hardening” is usually referred to as “quenching”.

A more refined version of this process is Vacuum Hardening.

Hardening – Vacuum 
Vacuum Hardening is an improvement over Conventional Hardening in that the components’ surfaces are protected from possible negative effects of exposure to a gaseous atmosphere. Vacuum treated material is quenched in gas or liquid, depending on the specification requirements.

The expression “vacuum hardening” is used here to differentiate the process from conventional hardening. It should also be noted that “hardening” is usually referred to as “quenching”.

Ion Nitriding / Plasma Nitriding
As an alternative to gas nitriding, nitriding plasma (ion) nitriding process has been developed to overcome the shortcomings of the earlier traditional uncontrolled gas nitriding processes and to offer certain operational advantages that gas nitriding does not have.

Plasma is essentially a gas nitriding treatment in which the method of delivering nitrogen atoms to the surface of nitrided components is quite different from the standard gas nitriding processes. It occurs at a very low pressure and under high voltage.

From the metallurgical, tribological and mechanical properties standpoint the properties of nitrided case obtained with Nitreg® and well controlled plasma technology are comparable.

Plasma offers certain distinct advantages such as:

• The ease of masking the component surface where nitriding is to be avoided.
• Ability to nitride low density powder metallurgy parts

Ask us for an advice about which nitriding method is better suited to your situation.

If your drawing calls for “ion” or “plasma” nitriding we are at your service.

Summary of Benefits:

• control of the thickness of the compound (white) layer and its properties
• elimination of closed nitride networks within the diffusion zone
• control of case depth
• control of surface hardness
• no distortion
• family of derivative and related processes

Nitreg®-C: Potential-Controlled Nitrocarburizing (FNC process)
A shorter cycle time of the Nitreg® process carried out predominantly on carbon and low alloy materials. Nitreg®-–C is a controlled version of nitrocarburizing (nitrocarburising or FNC process). It allows for precise Kn control during the process. The additions of carbon bearing gases to the nitriding atmosphere help to increase the relative content of the epsilon phase.

The advantage of a KN controlled technology is best evidenced when increased wear and/or corrosion resistance is sought. Such properties of the nitrided case are not only influenced by the thickness and relative phase composition of the WL. They also strongly depend on the relative level of porosity developed in the WL.

The KN control is essential in producing the desired WL configuration

Example of various porosity levels achieved through KN control are shown below.

Benefits:

• Increased wear/corrosion resistance on selected alloys
• No distortion
• No brittleness
• Excellent process reliability

Nitreg®-S: Potential-Controlled Nitriding of Stainless Steel
The rules applying to nitriding of stainless steel or refractory alloys are no different than those for the other groups of steels, with one exception.

The exception is the proprietary de-passivation stage that allows for a removal of oxides of alloying elements such as Cr, Ni and others which, if not removed, will effectively block the nitriding process. The same way they block the rust formation on the stainless steel surface.

All types of stainless steel can be nitrided. The martensitic, austenitic or PH materials inclusive.

Example of a nitrided stainless steel part

Nano-S^TM: Potential-Controlled Nitriding of Stainless Steel
NANO-S^TM is a surface hardening process that improves the wear and galling resistance of stainless steel components without affecting the inherent corrosion resistance.

Benefits:

• Attains excellent wear resistance
• Improves fatigue strength
• Retains intrinsic corrosion properties
• Prevents galling
• Does not alter chemical composition of alloy
• Has no effect on the steel’’s non-magnetic nature
• No change in the color, shape or size
• Uniformly hardened even small bores, tight grooves and sharp edges
• Green technology, no waste pollution

Treatable Materials:

• Austenitic Stainless Steels
• Martensitic Stainless Steels
• A286
• Custom 465
• Duplex Stainless Steel
• Hastelloy C22 and C276
• Inconel 625 and 718
• Inquire about other materials

Nitreg®-Ti: Potential-Controlled Nitriding of Titanium Alloys
Titanium alloys, used mostly in the aerospace and defense industries can also be successfully gas nitrided leading to an increased wear resistance and providing an attractive golden finish.

The technology is not widely known since the applications involved are also very specific. For best results consult with our engineers who will advise you on certain unique aspects of the manufacturing sequence of operations.

Examples of a nitrided titanium alloy parts.

Normalizing
Normalizing is a relatively simple process whose parameters, however, depend greatly on the type of steel and the desired result. The main purpose will usually be an improvement and homogenization of the grain structure.

ONC®: In-process Post-Nitriding/Nitrocarburizing Oxidation
When resistance to atmospheric corrosion and an attractive black finish are the predominant requirements, ONC® is the appropriate process.

Its objective is to transform the very top portion of the WL obtained with either Nitreg® or Nitreg®-C technologies into a complex spinel type structure consisting mostly of Fe3O4 type of iron oxide.

Such a post-nitriding oxidation treatment has a net effect of enhancing the corrosion resistance of an already nitrided component. This integrated process (i.e. Nitreg®+ ONC® or Nitreg®-C + ONC®) simultaneously enhances corrosion and wear resistance of steel, while giving the surface an attractive dark or black appearance, expressly desired by many customers.

ONC®, applied in combination with the Nitreg® potential-controlled nitriding process or the Nitreg®-C potential-controlled nitrocarburizing process, is a clean technology that in many instances can replace chrome plating and salt bath nitriding with their inherent problems of pollution and cost.

Depending on the type of steel, parts treated in the Nitreg®-ONC process can easily pass well over 200 hours of salt-spray test per ASTM B117 before the first corrosion spot appears. Fig. 2 shows a comparison of metallurgical and corrosion test results obtained on three materials treated by the Nitreg®-ONC® process.

Benefits:

• Improved corrosion resistance
• Attractive black surface finish
• Inherent wear resistance

Plasma (Ion) Nitriding
As an alternative to gas nitriding, nitriding plasma (ion) nitriding process has been developed to overcome the shortcomings of the earlier traditional uncontrolled gas nitriding processes and to offer certain operational advantages that gas nitriding does not have.

Plasma is essentially a gas nitriding treatment in which the method of delivering nitrogen atoms to the surface of nitrided components is quite different from the standard gas nitriding processes. It occurs at a very low pressure and under high voltage.

From the metallurgical, tribological and mechanical properties standpoint the properties of nitrided case obtained with Nitreg® and well controlled plasma technology are comparable.

Plasma offers certain distinct advantages such as:

• The ease of masking the component surface where nitriding is to be avoided.
• Ability to nitride low density powder metallurgy parts

Ask us for an advice about which nitriding method is better suited to your situation.

If your drawing calls for “ion” or “plasma” nitriding we are at your service.

Precipitation Hardening 
Age or Precipitation Hardening (also Aging) is a change in material properties (generally hardening) effected by holding parts at moderately elevated temperatures, without any change in the chemistry of the alloy.

Quenching and tempering are described separately in this section.

Quenching / Hardening – Conventional
Hardening is one of the oldest metallurgical processes known to man, originally in the form of heating a sword in the fire and then throwing it into the lake to make it harder. The more modern approach is to heat components in an atmosphere furnace followed by quenching, generally in heated oil.

The expression “conventional hardening” is used here to differentiate the process from vacuum hardening. It should also be noted that “hardening” is usually referred to as “quenching”.

A more refined version of this process is Vacuum Hardening.

Sand Blasting
Sand blasting is used primarily to remove rust, paint, scale or significant blemishes on the surface of components. Depending on the need, the technique is used either before heat treatment (preparation) or after (e.g., removal of oxidation).

Shot Blasting or Peening
When tiny steel balls are used the process of blasting parts using this medium will be called shot blasting or peening. Jets of shot are aimed at components while they tumble in a rotating chamber. Some differentiate between “blasting” or “peening”, but the process is the same nevertheless. The former expression is used when the objective is to clean the parts or remove sharp edges, while the latter indicates a desire to mechanically compress the surface layer of the material. In this manner compressive stresses are introduced which in most cases will improve the fatigue resistance of the component.

Straightening
Certain types of components, particularly longer objects such as shafts, tend to distort in high temperature treatment. In most cases they can be successfully straightened using special hydraulic presses.

Stress Relieving
As the name of the process clearly indicates, Stress Relieving is used to reduce the residual stresses in the microstructure after machining or certain heat treating operations, to prevent distortion from occurring later.

Clamp Tempering
Heat treatment is inherently distorting to the parts. To reduce distortion some components must be restrained during tempering by being “clamped”, and process is then called clamp tempering.

Tempering – Conventional
Tempering is almost always required after hardening (both in an atmosphere furnace as well as in vacuum, as described further), to reduce the hardness (and brittleness) to a desirable level.

The expression “conventional tempering” is used here to differentiate the process from “vacuum tempering”.

Tempering – Vacuum
Vacuum Tempering is almost always required after hardening (both in an atmosphere furnace as well as in vacuum), to reduce the hardness (and brittleness) of the treated material to a desirable level.

Generally it is not necessary to use vacuum tempering after vacuum hardening, i.e., conventional tempering is most of the time used. Vacuum tempering may be used on high value products, when totally clean surface appearance is required.

The expression “vacuum tempering” is used here to differentiate the process from “conventional tempering”.

Vacuum Annealing
The process is virtually identical to conventional annealing, except, as is usually the case, this premium quality Vacuum Annealing method protects the components’ surfaces from chemical reactions with gases present in the atmospheres present in the conventional process.

Vacuum Brazing
Vacuum Brazing uses a vacuum furnace but it is not, strictly speaking, a heat treating process. Brazing, in general, is a process of joining two components with a metallic bond by briefly liquefying the latter while the assembly is under vacuum conditions.

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.

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. Nitrex has developed a revolutionary process called “Pulse- Pressure”, a method quickly becoming the industry standard.

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.

The basic aspects or carburizing in general are described in the Conventional Carburizing section.

Vacuum Hardening
Vacuum Hardening is an improvement over Conventional Hardening in that the components’ surfaces are protected from possible negative effects of exposure to a gaseous atmosphere. Vacuum treated material is quenched in gas or liquid, depending on the specification requirements.

The expression “vacuum hardening” is used here to differentiate the process from conventional hardening. It should also be noted that “hardening” is usually referred to as “quenching”.

Vacuum Tempering
Vacuum Tempering is almost always required after hardening (both in an atmosphere furnace as well as in vacuum), to reduce the hardness (and brittleness) of the treated material to a desirable level.

Generally it is not necessary to use vacuum tempering after vacuum hardening, i.e., conventional tempering is most of the time used. Vacuum tempering may be used on high value products, when totally clean surface appearance is required.

The expression “vacuum tempering” is used here to differentiate the process from “conventional tempering”.