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Plasma nitriding to enhance component performance

The demand for lighter and safer cars has driven the use of higher-strength steel grades for car bodies. Aerospace components have many of the same needs, but with more stringent control requirements for a wider variety of materials.
Steel is now far stronger than it was in the past. Some automotive parts, such as floor panels, are even made from press-hardened steel (PHS) with a tensile strength of 1,500 – 1,800 MPa. This places very high demands on the stamping dies used in these applications.
In recent years, the plasma/plasma nitriding process has achieved great success in the surface hardening treatment of these molds. This is especially true when the mold is made of cast material with a high content of chromium, molybdenum or other elements that produce very stable and hard nitrides. These alloys produce extremely hard nitrided layers with depths exceeding 0.25 mm.
Stamping dies have traditionally been chrome plated. It produces a very smooth surface with a low coefficient of friction. As high-strength steel becomes harder than the base material beneath the surface chromium plating, the limitations of chromium plating become apparent.
Due to the increasing hardness of automotive panel materials, the surface requires more protection than a 10–20 micron chrome layer. In addition, the electrolytic deposition of the chromium plating process means that the most chromium is deposited on the protruding parts, while the chromium layer on the recessed parts is very thin or even non-existent. This will also produce dimensional deviations.
In contrast, nitriding is not a coating, so there are no issues with chipping or peeling. It can make a certain depth of the matrix reach a high enough hardness required to withstand greater impact, thereby reducing impact damage. It also does not produce dimensional changes because the process proceeds evenly across the raised and recessed areas.
Plasma nitriding process
This process is widely used for the hardening of large deep-drawing die components such as punches and blank holders. The materials of such parts are usually gray cast iron or ductile iron (such as G2500, G3500, D5506, etc.) and cast steel (including S0050A and D-2). Progressive dies and various inserts are made from Caldie, M-2, S-7 and other high-quality tool steels, often using a combination of plasma/plasma nitriding and PVD (physical vapor deposition) coating.
Ion/plasma nitriding is the best solution for processing large single-piece mold components. The process takes place in a large vacuum chamber equipped with resistance heaters and plasma generators. The component is covered with a glow discharge plasma, which generates reactive nitrogen ions that act as nitriding. The process inside the vacuum chamber can be observed.

The nitriding layer consists of a compound layer and a diffusion layer. It can be clearly seen that the hardness distribution of the plasma/plasma nitrided sample is significantly better than that of the chromium plated sample. The hardness gradient of the latter is quite large, which may be the reason why the chromium plating layer in high-stress contact areas (such as rounded corners) is prone to cracking and falling off.

In contrast, the surface hardness of ion/plasma nitrided molds changes smoothly, which is advantageous for all metal forming molds subjected to extremely high Hertzian stresses. It should also be noted that the nitrided surface produces compressive stress.

Stamping die after plasma nitriding

The new generation of advanced high-strength steels (AHSS) used in the automotive industry poses challenges to stamping dies because the stresses required to produce sufficient plastic deformation in them exceed 400 MPa.
Small molds or inserts can be made of the highest strength tool steel, and then nitrided and PVD or CVD (chemical vapor deposition) coating treatments are performed to produce a surface layer with high hardness and wear resistance (such as CrN, TiAlN, etc.), which can Meet the requirements to withstand the contact stress required for the forming of steel parts.
Large stamping dies are usually made of cast iron or cast steel. These materials are often not hard enough to withstand the extremely high Hertzian contact stresses required for forming operations. Such molds can be induction or flame hardened, but due to their size they are not suitable for PVD or CVD processing. In this case, chromium plating has been successfully used for many years, but it has reached its limit with the tooling required to form AHSS steel grades.
Ion nitriding creates a hardened layer on the surface of any iron- or titanium-based alloy component. The nitrided layer produced by precipitation hardening high alloy steel (such as 15-5 PH, 17-4 PH, etc.) has no intergranular network structure (IGN), and the thickness of the white bright layer is limited, which can make aviation parts resistant to wear and bending. Excellent performance in fatigue and rolling contact fatigue (RCF).