Heat treatments of tools: nitriding.

In this article we talked about the 3 parameters which determine a high quality tool:

• strength and fatigue resistance
• efficiency
• precision.

The tools, indeed, are required to:

• bear the heavy loads applied in sheet metal bending
• resist the repetitive cycles of metal processing
• resist the abrasive wear caused by sheet metal sliding.

In order to provide the tools with these technical properties, it is vital that heat treatments are carried out. Heat treatments are processes used to create protective layers or alter the physical and chemical properties of the tools, through one or more heating cycles below melting temperature.

These processes provide the tools - punches and dies - with really precise qualities, which enhance their strength, hardness and durability.

Eurostamp mainly applies two types of heat treating processes: nitriding and induction hardening. The latter is a particular type of "localized" treatment, which allows to obtain a higher surface hardness compared to the results of "core" hardening, and to determine exactly the surface area involved and the depth of hardening.

This article is going to focus on the first process.

Nitriding: what is it?

Nitriding is a thermo-chemical treatment resulting in a chemical change in the metal surface. It is a process aimed at lower depths surface hardening. Nitriding is one of the industrial treatments carried out on dies, as they are the tools that are more subject to abrasive wear.

How does it happen?

This heat treatment takes place in a furnace in a controlled atmosphere, where nitrogen is diffused (nitrogen might come from thermal dissociation of ammonia or from salt baths).

Atomic nitrogen, bonding with the ferrite on the steel surface, creates nitrides. Nitrides change the crystal structure of steel and cause the surface hardening of the die. To be precise, the furnace temperature is usually between 520° and 545 °C. Being a relatively low temperature, it keeps surface deformation or changes in the surface to the lowest. According to the predetermined thickness of the nitride layer (lower than 0,3 - 0,5 mm), once the required temperature is reached, the tools are held at this temperature for a variable amount of time, which is always longer than in other heat treatments.

This is followed by a "controlled" cooling phase (controlled in terms of temperature and time), which ends the nitriding treatment.

Advantages.

Nitriding provides the die with a high surface hardness which is otherwise hard to achieve with other heat treatments.

In addition, nitriding:

• provides the tools with a longer durability
• ensures scratch protection and fatigue resistance
• ensures corrosion protection (nitrides enhance surface corrosion protection)
• reduces cleaning frequency; (above all when working on galvanized steel) sheet metal debris is a usual part of the operations. However, a nitrided die prevents the steel chips from adhering to the surface, thus requiring less frequent cleaning.

Finally, being a highly effective treatment, no further treatments are required to improve the die performance.

Nitriding and hardening.

As we said at the beginning, another heat treatment carried out on the dies is case-hardening. This process does not change the chemical structure of steel and the hardening is limited to the working areas, that is, the contact points.

If it is possible to predict that the die will be subject to high abrasive wear or scratches, even in the "internal" V groove that is not usually involved in induction hardening, nitriding is recommended rather than induction hardening.

This is the reason why nitriding is a highly performing process: because it results in harder, more resistant and more durable dies.

The next article is going to focus on a common surface treatment for the tools: phosphating.

You might be interested in: Toughness of tools according to heat treatment.

For performing and resistant tools, you can count on Eurostamp.

Do you need advice for your bending needs? Since 1970, Eurostamp has been designing and manufacturing top quality press brake tooling, punches and dies.

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