Optimization of the temperature field for laser hardening of cold-work tool steel
Cold-work tool steels are a class of steels with a high amount of carbon and carbides. This property gives them a high resistance against abrasive wear. For conventional local heat treatments, hardening of these steels can be challenging due to the high amount of carbon narrowing the process window. In these cases, laser hardening is state of the art. With the laser, the peak temperature can be controlled precisely. The time, which the carbon has to diffuse, is determined by the size of the laser spot and the process speed. The process avoids damages to the material and allows to locally harden tool steels. New beam shaping technologies allow furthermore the manipulation of the intensity distribution according to the thermal field. With these it is possible to not only adapt the peak temperature and interaction time but to to freely design the whole temperature over time, which the material is imposed to. With this new dimension, the thermal field can be optimized to harden even challenging tool steels. The following study investigates the optimization of the temperature field for hardening the cold working tool steelX153CrMoV12 (1.2379). This steel gets its high wear resistance by large chromium carbides, which are embedded in a martensitic matrix. The optimization will try to solute just enough carbon from the carbides to maximize the hardness in the matrix but without creating residual austenite. The temperature function over time is studied by an adapted hardenability test by end quenching. Based on these results a temperature-time-curve with two constant temperatures levels is proposed. The proposed temperature field is validated using a freeform mirror, which produces the intensity distribution according to a solution of an inverse temperature field problem.