Martensitic hardened and tempered steel strip with lead-free surfaces
The conventional martensitic hardening and tempering process involves quenching the previously heated steel strip in a molten metal bath. Even after subsequent cleaning, trace residues of the molten metal can remain on the surface of the steel strip. While these residues are well below the limits defined by the REACH and RoHS regulations, certain applications require surfaces that are completely free of heavy metals. For these applications, we have equipped one of our most powerful hardening and tempering furnaces at our Hagen site in Germany with a hydrogen quenching system. Thorsten Köhler, Head of Materials Technology at the Hagen site, explains:
What makes our furnace unique is a specially designed nozzle system that directs hydrogen onto the strip surface. This technology allows us to dissipate heat extremely quickly and in a highly controlled manner. In addition to achieving a precisely defined microstructure, we retain the familiar bright or gray-blue surface – completely free of heavy metals.
Another advantage is the range of available sizes. Depending on the design and alloy, the furnace can be used to harden and temper steel strip with widths of up to 825 mm or thicknesses of up to 4 mm. In addition, the achievable range of cross sections is nearly unmatched across the industry.
Thanks to our specialized hardening and tempering processes, we have also been able to offer our profiles and flat wire products with lead-free surfaces in a gray-blue finish for over 20 years.
Precisely controllable quenching – for optimally customized material properties and an extended range of grades
Waelzholz’s extensive expertise was incorporated into the development of the furnace in collaboration with several specialized suppliers. Köhler explains: “Our team of heat treatment experts defined the key process steps and worked closely with component suppliers to align specifications. The result is a furnace precisely customized to our requirements.” One benefit of this development work is the ability to control the nozzles in a flexible and highly targeted manner, allowing cooling curves to be fine-tuned with pinpoint accuracy. This gives us significantly greater freedom in defining material properties. For example, we can precisely adapt the quenching behavior to the strip geometry. The nozzle pressure can also be adjusted to suit the specific needs of the order.
Compared with conventional processes, hydrogen quenching gives us far more options. It allows us to heat-treat new material variants, and we are already working with several customers on related development projects.
Full surface inspection thanks to state-of-the-art testing technology
In addition to being free of heavy metals, surface finish often plays a critical role. In applications such as the food industry or medical technology, even the slightest scratches or surface defects can be grounds for rejection. For this reason, Waelzholz has integrated eddy current testing – a proven inspection method used in our production processes for many years – directly into the hardening and tempering furnace.
During eddy current testing, an electromagnetic field is induced into the steel strip. Disturbances in this field provide reliable indications of potential defects. Compared with conventional camera-based inspections, a key advantage is that cameras capture only visible surface changes, whereas eddy current testing can also detect irregularities that are not visible to the eye.
“We’ve been using this testing technology to the benefit of our customers for over 20 years. At the end of the day, what matters most is that our customers receive steel materials with precisely the properties they need to ensure the reliability of their products – consistently and across multiple production batches.”
Daniel Kemper, Vice President Sales, Waelzholz Germany