High-speed hardening and tempering: flat wire and profiles with homogeneous microstructures and residue-free surfaces

When homogeneous materials with a residue-free surface and unparalleled forming properties are required, Waelzholz’s patented high-speed hardening and tempering method is the solution of choice. The process of heating the steel strip to well over 800 °C in a few fractions of a second and cooling it just as quickly produces an extremely fine-grained and homogeneous martensite microstructure. This is how we manufacture flat wire products and profiles made of wire with above-average bending uniformity and residue-free surfaces, such as those required for a variety of applications in the automotive industry.
 

When manufacturing flat wire and profiles made of wire, Waelzholz implements its patented high-speed hardening and tempering process on specially developed production lines using two different methods. In both inductive and conductive hardening and tempering, current is applied to the flat rolled wire, which heats it due to a phenomenon known as resistance heating. In the inductive method, current is applied without any contact by means of a magnetic field; in the conductive method, on the other hand, the electrical charge is applied via rollers that come into contact with the surface of the material. Since the energy is applied directly to the material in both methods without any transition losses, they not only effectively and uniformly harden and temper the materials, but also do so in a particularly energy-efficient manner.

Matthias Schmitt, materials technology specialist, explains: “With the help of these special hardening and tempering methods, we are able to heat flat wire products and profiles made of wire to well over 800 °C within fractions of a second and then cool them down again in an instant. Due to these incredibly rapid temperature changes, the hardening and tempering process is already complete before larger grains can form in the microstructure. This is how we produce a uniformly fine-grained, extremely homogeneous martensite microstructure that is impossible to achieve using conventional hardening and tempering methods.”

High-speed hardening and tempering prevents cracks and residues on the material surface

The two high-speed hardening and tempering methods make it possible to heat flat wire products and profiles at production speeds of several hundred meters per minute. Thanks to these high speeds and temperatures, the quenching process can be carried out with water instead of oil. In conventional hardening and tempering processes, quenching with water can cause quench cracks when the strip is run through slowly. There is also a certain risk of metal bath residues forming on the strip around its edges and inner radii, especially in the case of complex profile shapes.

“The high speed of our water-based cooling processes prevents both cracking and residues on the material surface,” explains materials engineer Schmitt. “This represents a key benefit for many customers, because optimal component coating and galvanizing processes require surfaces that are completely residue-free. This is the case, for example, with our U-shaped profile used as a connecting element for ball bearings. Customers receive a material from us that is free of heavy metals and is also easy to form.”

Outstanding homogeneity and bending uniformity of the martensitic microstructure for flat wire products and profiles

Depending on the design, the length of the flat rolled or profiled wires can exceed 20 kilometers. Thanks to high-speed hardening and tempering, the material not only exhibits extremely homogeneous properties over the entire length of the steel strip, but also across entire production batches. In addition, the extremely fine and uniform martensitic microstructure significantly improves the toughness of the material. 

“Our customers can implement downstream bending processes, for example, with completely uniform parameters and without having to make any adjustments, even across batches of 25 coils. This not only helps ensure that our customers’ products have extremely homogeneous and therefore reliable material properties, but is also particularly cost-effective,” says Stefan Bauernfeind, responsible for flat wire products and profiles in Sales, describing the benefit for his customers.

Time and cost savings thanks to eliminating piece hardening

Another advantage of high-speed hardened and tempered materials is the savings potential with respect to piece hardening – our high-speed hardened and tempered flat wires and profiles made of wire are extremely hard, yet still well suited for bending tight radii. This eliminates the need for time-consuming and cost-intensive piece hardening in downstream processes for our customers who manufacture components for demanding applications.

Citing an example of an application in the automotive industry, Stefan Bauernfeind says: “Our spring rails made of high-speed hardened and tempered narrow strip are used, for example, inside the rubber profiles of high-quality windshield wipers. Thanks to its uniform structure, the flat wire we supply exhibits an exact and long-lasting level of spring tension that ensures the rubber profile remains permanently pressed against the windshield.” As a result, the product helps ensure that windshield wipers perform effectively over a long period of time, thereby making driving safer.