Hardness prediction in multi-pass direct diode laser heat treatment by on-line surface temperature monitoring

This study was attempted to develop a process model, using the temperature values measured by the coupled infrared temperature measurement system (pyrometer and camera) correlated with the measured values of case depth hardness of the tool steel AISI S7 (hypo-eutectoid steel) for the specified multi-pass laser heat treatment conditions (1000–2500 J). A number of heat treatment experiments by changing the laser power (1400–1800 W), scanning speed (15–25 mm/s), size of overlap (1–6 mm), and the length of scan (10–30 mm) on the tool steel AISI S7 were carried out by using a 2-kW direct diode laser. The results indicated that the loss of case depth hardness uniformity was highly influenced by the tempering temperature and the change of cooling rate. The combination of higher laser power, lower scanning speed, smaller size of overlap, and the longer length of scan were influenced the formation of mixture of phases (martensite, bainite, ferrite, and pearlite) which in turn affected the case depth hardness uniformity. The process model can be used as a handy tool for the process engineers to configure the processing conditions in order to obtain the desired case depth hardness for the hypo-eutectoid steels.