Higher hardness cutting knives thanks to laser hardening

Integrated laser hardening offers a solution for hardening slim, elongated objects which suffer deformations with traditional hardening methods. This process also offers a number other advantages. 

In a modern tool shop, most steel components are hardened for life. These include moulds, injection openings, mould punchings, but also guide rails and machetes or knives. The latter two place special demands on the production process. Due to their often slim and elongated shapes they deform considerably during the hardening process and require a lot of hard finishing (grinding). This increases turnaround time and costs considerably.

Laser hardening offers a solution here: these components can be milled to size and, in the same clamping setup, immediately hardened by means of a laser. Because laser hardening only hardens certain areas, geometrical deformations will be very limited, far below the desired tolerances, which means that hard finishing such as grinding is no longer necessary. Laser curing is also expected to allow higher hardnesses due to the very high cooling rates achieved in this process.

Preliminary study

In this study 45° knives made of C45 steel are hardened. The hardnesses have been measured in function of the laser parameters (power supply, power) and the length of the hardened area on the blade surface has also been further investigated.

Figure 1

For laser hardening a spot of 4 mm diameter was used. Hardness measurements were carried out at a distance of 0.5 mm from the edge using the HR15N measurement (15 kgf, diamond indentation). The tests were carried out in function of the laser power and the speed (feed), both of which have an important impact on the heat development in the area to be hardened. In ideal conditions, the piece is heated to just below the melting point and then cooled rapidly by means of thermal conductivity or radiation.

Figure 2

The quality of the knives is initially checked visually to see if there is any melt. Although molten surfaces also have an increased hardness, their surface roughness and therefore functionality is severely affected. At 250 W, for example, we see that melt occurs at a feed of 600 mm/min, while at 800 mm/min it is completely absent. The light grey area in these photos is caused by the change in microstructure (martensite instead of ferrite and perlite). Melting in a surface is also disadvantageous because (undesirable) tensile stresses occur which negatively influence the wear behaviour. Correct laser hardening, on the other hand, introduces compressive stresses.

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