Duplex Coatings - Lesson 1: The correct heat treatment of tool steels
Welcome to the Ionbond Summer School! In a few short lessons, we will be teaching you the basic concepts behind our duplex coatings: PVD coatings on top of a plasma nitriding layer. These coatings are especially suited for forming and molding tools. Let's have a look at how your tool has been treated before it arrives at Ionbond – whether it has had the correct heat treatment.
The right prerequisites for duplex coating
Plasma nitriding creates a hard surface with a high resistance to abrasive wear and thermo-mechanical fatigue. It enhances the load-carrying capability of the tool and increases its durability in production. In a duplex coating, the plasma nitriding step is followed by a PVD coating, to offer additional hardness, wear resistance and friction reduction. Both plasma nitriding and PVD coating are usually done at temperatures low enough to not require heat treatment of the tools afterwards.
However, it is still important to know the heat treatment history of a tool for the success of duplex processes. Exceeding the tempering temperature during plasma nitriding or the PVD process results in distortion of the tools. When a segmented die is assembled from individual details, distortion and twisting can make it impossible use the die in production, resulting in exceptionally high costs.
Take note of your D2 cold work steel tools
The full heat treatment history of a tool is especially important when it is made of cold work steel AISI D2 (DIN 1.2379 or X153CrMoV12). This D2 steel is a highly alloyed, ledeburitic steel with 12% chromium content. Due to its toughness, dimensional stability, compressive strength and abrasive and adhesive wear resistance, it is used as tool steel in a broad variety of forming and molding applications. D2 steel is commercially available worldwide under various product names.
Two heat treatment approaches - only one suitable for duplex coating
The problem with D2 steel is that there are two heat treatment approaches, and one is unsuitable for subsequent duplex coating.
1. Heat treatment with low-temperature tempering
One approach to heat treatment of D2 includes hardening at temperatures between 1,020°C and 1,080°C with subsequent cooling in oil, in a hot bath or in air. Tempering is typically conducted at temperatures between 170°C and 250°C. This produces a hardness of approximately 62 HRC. …
Although this hardness provides sufficient support for a subsequent PVD coating, the tempering temperature is far below the process temperatures of plasma nitriding and PVD coating. Performing a duplex coating on a tool in this condition would to lead to distortion and loss of dimensional accuracy and tolerances.
2. Heat treatment with secondary hardening
Secondary hardening of D2 is essential for tools that will be duplex coated. This includes hardening at temperatures between 1,060°C and 1,080°C, followed by three tempering steps at the secondary hardness maximum, around 550°C. At minimum, the tempering temperature must be as high as the nitriding temperature to make sure the dimensional accuracy of the tool remains intact. Deep-freezing directly after tempering minimizes residual austenite and further increases dimensional accuracy.
Share your tool’s history with your coating service provider
Now you know why it is so important to make sure that your service provider for plasma nitriding and PVD coating is informed about the exact heat treatment history of your tool. This not only ensures customized surface engineering solutions for your tool, but also guarantees superior performance of the duplex-treated tool in your production.
That was it for today! See you in the next lesson, which is all about plasma nitriding.
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Discuss your challenges with Tobias Brögelmann
Dr. Ing. Tobias Brögelmann, Global Segment Manager Forming and Molding Tools, will be glad to support you.
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Dr. Ing. Tobias Brögelmann
Global Segment Manager Forming and Molding