Fig 1: Radiation profiles of a gas laser (left) and a solid state laser (right).
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The hardening of steel is done by heating up a workpiece to above its critial temperature (curie point) then cooling (quenching) it. This heat treatment process causes significant changes in the microstructure of the steel which directly corresponds to its metallic properties. The martensite structure that is formed is responsible for the increased material hardness. The main methods of hardening are flame, induction, and laser hardening. Laser hardening is especially attractive for surface hardening of complicated shapes or large objects because it allows for absolute control on the surface properties.

How it Works[edit | edit source]

Light Amplification by Stimulated Emission of Radiation (Laser) can be used to provide the heat necessary for the treatment process. There are two main laser types used in heat treating which are gas-lasers and solid state lasers. Gas lasers provide a relatively simple Gaussian profile of radiation while a solid state laser is much more complicated. These profiles can clearly be seen in Figure 1. Gas lasers are usually used as their simplified profile allows better feedback and prediction models. The absorbed radiation from these lasers is what heats up the surface layer causing austenite to form. As the laser continues on its path the previous point is rapidly cooled by the surrounding part. This is what causes the phase change from austenite to martensite. Martensite is formed because during the quenching process dissolved carbon atoms are trapped within the austenite face centered cubic structure. As the temperature is dropped the austenite becomes mechanical unstable and rearranges to form the body centered tetragonal crystal structure. The austenite phase can be seen in the Steel phase diagram Figure 2.

Control Systems[edit | edit source]

The most important and difficult part about laser surface hardening is the control of the laser. It is extremely difficult to predict the affect the laser has on the surface because so many factors are involved. These include: material composition, type of laser, speed of laser, power of laser, thickness of material, and material geometry. Proportional integral differential (PID) equations are used based on the material properties, velocity, power, and temperature to determine the effect on the material. The surface temperature is measured and the output level is varied in order to keep this temperature at a specific temperature just below the melting temperature. This ensures that the thickness of the layer remains relatively constant. However, significant research is being done in control systems that take into account the subsurface properties of the material to better accommodate for changes in material thickness or geometry.

About PID
PID control systems use real time data to instantaneously calculate the desired power output level for the laser. The system uses a pyrometer to measure the thermal radiation in the workpiece. This can be calibrated based on the material type to determine the surface temperature at the center of the laser beam. This temperature can be monitored and fluctuations that are caused by a change in material geometry can be compensated for by varying the laser output power.

PID Mathematics


References[edit | edit source]

[1] Benedict, Gary F. (Chandler, AZ), 1985, " Method and Apparatus for Laser Hardening of Steel," 06/509530(4533400).

[2] Ganeev, R. A., 2002, "Low-Power Laser Hardening of Steels," Journal of Materials Processing Technology, 121(2-3) pp. 414-419.

[3] Ganeev, R. A., 2002, "Low-Power Laser Hardening of Steels," Journal of Materials Processing Technology, 121(2-3) pp. 414-419.

[4] Homberg, D., and Weiss, W., 2006, "PID Control of Laser Surface Hardening of Steel," Control Systems Technology, IEEE Transactions on, 14(5) pp. 896-904.

[5] Komanduri, R., and Hou, Z. B., 2004, "Thermal Analysis of Laser Surface Transformation hardening—optimization of Process Parameters," International Journal of Machine Tools and Manufacture, 44(9) pp. 991-1008.

[6] Obergfell, K., Schulze, V., and Vöhringer, O., 2003, "Simulation of Phase Transformations and Temperature Profiles by Temperature Controlled Laser Hardening: Influence of Properties of Base Material," Surface Engineering, 19(5) pp. 359-363.

[7] Xu, Z., Leong, K. H., and Reed, C. B., 2008, "Nondestructive Evaluation and Real-Time Monitoring of Laser Surface Hardening," Journal of Materials Processing Technology, 206(1-3) pp. 120-125.

[8] Zhang, H., Shi, Y., Xu, C. Y., 2004, "Comparison of Contact Fatigue Strength of Carbon Case Hardening and Laser Hardening of Gears," Surface Engineering, 20(2) pp. 117-120.

[9] Zhang, H., Shi, Y., Xu, C. Y., 2003, "Surface Hardening of Gears by Laser Beam Processing," Surface Engineering, 19(2) pp. 134.

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Keywords materials processing, hardening, steel, heating
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Created November 14, 2008 by Anonymous1
Modified April 21, 2023 by Felipe Schenone
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