Heat & surface treatment

Case hardening is one of the thermochemical processes. In this process, the surface layer of components and tools is carburized with a carbon-emitting medium and then quenched, improving the mechanical properties (e.g. wear) of the component's surface layer.
Carburization is usually at temperatures of 1030 to 950° C. After the hardening of the carburized parts, tempering is generally required to reduce the stresses resulting from hardening and to achieve the required strength.

A range of different systems such as chamber furnaces and continuous furnaces are available for case hardening. Oils are generally the media used for quenching.

The conventional way to increase hardness or strength is by tempering. A second, somewhat more specific method is austempering, or bainitic hardening.

In this form of heat treatment, the component is austenitized in the same way as for other forms of hardening, i.e. it is heat-treated at temperatures of 800 to 900° C, depending on the material.

Quenching then takes place in a hot salt bath. The temperature of the salt bath depends on the material, and can range from 220 to 450° C. The part remains in the salt bath at a constant temperature (isothermal) until the structural transformation of austenite to bainite is complete (intermediate stage). No martensite is formed. Depending on the material, the transformation can be completed in a few minutes; but can sometimes also take several hours.

Bainite structures have very specific properties, characterized by high strength (hardnesses), maximum toughness and, generally speaking, relatively little distortion. Very low residual austenite contents that can otherwise only be achieved by deep-freezing are also possible.

Heat treatment is the process where a number of heating and cooling operations are performed below atmospheric pressure for the specific purpose of altering the physical properties of an alloy.

Use of conventional heat treatment processes for hardening of work pieces and components had been widely used over the decades. The vacuum heat treatment of work pieces and components with overpressure gas quenching is today’s standard.

Applications for furnace include bright annealing, vacuum brazing, hardening of moulds and dies, general heat treating, tempering and sintering.

Nitriding is a case hardening heat treatment process which runs at temperatures between 450 – 600 °C for steel. The aim of the process is to diffuse nitrogen into the substrate surface to form nitrides in combination with different alloying elements of the substrate and to form compound layers that have high wear resistance properties. A normal nitriding depth goes from 0,01 mm up to 0,7mm for which the nitriding time can be up to 100 hours, and can rise the hardness of the steel up to 1200 HV

Since nitriding changes the chemical composition of the surface of the substrate and the process is carried out at medium temperature, it is classified as a thermo chemical process. Although many different materials can be nitrided with the aid of new nitriding technologies, this paper will focus on nitriding of steel substrates.

At the temperature that the process is held, nitrogen diffusion occurs in the ferritic phase of the steel, thus, no phase transformation occurs during cooling of the substrate.

Avoiding a phase transformation of the bulk during the nitriding process offers a big advantage because it minimizes the distortions that arise from the normal carburizing and quenching heat treatment processes.

• High surface hardness and wear strength, together with reduced risk of galling.
• High resistance to tempering and high-temperature hardness.
• High fatigue strength and low fatigue notch sensitivity.
• Improved corrosion resistance for non stainless steel.
• High dimensional stability compared to other heat treatment processes.

Gas Nitriding

Gas nitriding is a thermochemical surface treatment in which nitrogen is transferred from an ammonia atmosphere into the surface of steels at temperatures within the ferrite and carbide phase region. After nitriding, a compound layer and an underlying diffusion zone (i.e. case) are formed near the surface of the steel. The compound layer, also known as the white layer, consists predominantly of ε - Fe2-3- Fe4N phases and can greatly improve the wear and corrosion resistances. The hardened diffusion zone, which is composed of interstitial solid solution of nitrogen dissolved in the ferrite lattice and nitride and/or carbonitride precipitation for the alloy steels containing the nitrides forming elements, is responsible for a considerable enhancement of the fatigue endurance. Furthermore, being a low temperature process, nitriding minimizes the distortion and deformation of the heat treated parts. Therefore, nitriding is an important surface treatment for steels.

NITREG®-S

Stainless steels respond to nitriding differently than other ferrous alloys, and there are also significant differences within the stainless group as well. The primary reason for this is that depending on the chemistry of the steel it will behave differently with respect to the kinetics of layer formation, and it is a rather difficult process to control. In other words, unless you know what you are doing you may end up with nothing, or too little, or too much case depth or white layer, or even damaged parts.

NITREG®-S is a process in which any stainless steel may be nitrided, with complete control over the formation of nitrided layers.

NITREG®-S ADVANTAGE

Attains excellent wear resistance

• Improves fatigue strength
• 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

Plasma nitriding.

Plasma nitriding (Ion nitriding) is a plasma supported thermochemical case hardening process used to increase wear

In plasma nitriding, the reactivity of the nitriding media is not due to the temperature but to the gas ionized state. In this technique intense electric fields are used to generate ionized molecules of the gas around the surface to be nitrided. Such highly active gas with ionized molecules is called plasma, naming the technique.

Advantages of ion nitriding

Advantages of ion nitriding over ammonia nitriding are as follows:

• Shorter (by 20-50%) treatment cycle.
• Better process control and automation. The process parameters (pressure, voltage, temperature, gas flow, DC current) are easily controlled.
• Higher surface hardness may be achieved due to lower process temperature (up to 1200 HV).
• Better dimensional stability (lower distortions) due to lower process temperature and uniform heating.
• Lower energy consumption due to lower temperature and shorter treatment cycle.
• Reduced gas consumption.
• Safer operation.
• Lower environment pollution.

Ion nitriding is used for Case hardening of Alloy steels, Stainless steels, Titanium alloys.

The term annealing refers to the treatment of a workpiece at a given temperature for a given time period, with the subsequent cooling adapted to achieve the required material properties.

The major annealing processes are as follows

• Normalizing
• Stress-relief annealing
• Soft annealing
• Spheroidized annealing
• Coarse-grain annealing
• Diffusion annealing
• Recrystallization annealing
• Solution annealing
• Induction annealing

In the process of inductive heating, medium or high frequency alternating electrical current is used to generate an induction current in the workpiece through an inductor adapted to the contour to be hardened, producing heat.

The increase in hardness is caused by a transformation of the heated layer into martensite (during quenching), and the hardness that can be achieved depends on the carbon content and composition of the alloy.

Quenching is controlled, with the time window depending on the material, and is usually done as a supplementary quench with a synthetic polymer solution.