In laser heat treating, energy is transmitted to the material*s surface in order to create a hardened layer by metallurgical transformation. The laser is used as a heat source, and rapidly raises the surface temperature of the material. Heat sinking of the surrounding area provides rapid self-quenching, thus producing a hardened transformation layer.
Since the laser can be precisely controlled, dimensionally as well as directionally, it is most effective when it is used to selectively harden a specific area, rather than bulk heating of an entire part.
The Nd:YAG laser*s 1.06 µm wavelength is strongly absorbed by most heat treatable metals, surface coating is not required, and generally less power is required than with longer wavelength lasers (such as CO2).
Recently, U.S. Laser Corporation has developed reliable high power continuous Nd:YAG lasers of power levels in the 500 - 2000 watt range suitable for heat treating. Nd:YAG lasers generally are cheaper to operate, smaller in size and for most metals, work more efficiently than CO2 lasers. The laser head is smaller in size and can be moved, and used with robotics, machine vision and fiber optic beam delivery systems.
Unique Characteristics of Laser Heat Treating
Because a source of light can be concentrated to produce a small spot of intense heat energy, there are numerous advantages when considering the use of a laser.
Minimal Heat Input - Since the source temperature is so high, transformation occurs quickly and heat input to the part is low. This reduces distortion in the heat affected zones.
Precise Control - Since the light energy is concentrated, the area of heat treating can be located with great precision. As for its flexibility, the heat treat area can be projected within a small diameter bore through the use of directing mirrors.
Non-Contact, Open Air Processing - Since the energy comes from light, nothing physically touches the workpiece. There is no force exerted on the part. In addition, magnetism and air do not affect the laser*s beam.
Both CO2 and Nd:YAG continuous wave lasers currently have the power capabilities to heat treat metals at reasonable rates. The CO2 laser, however, has poorer surface absorption in most metals, and thus requires the surface to be coated to improve its absorption characteristics. Since the surface absorption of the Nd:YAG laser wavelength is significantly higher, generally less power is required.
Suitable Materials for Laser Heat Treating
Except for single phase stainless steels and certain types of cast iron, most common steels, stainless steels and cast irons can be surface heat treated (hardened) by the laser process. Each kind of steel has special characteristics which need to be considered.
Low Carbon Steel (0.08% to 0.30% carbon) - Very rapid quenching is required to form martensite in low carbon steel. A shallow case depth of up to 0.5 mm can be achieved. That maximum hardness which can be reached is dependent upon the percent carbon content in the steel.
Medium and High Carbon Steel (0.35% to 0.80% carbon) - These material are better choices than low carbon steel because the higher carbon content allows a longer period for quenching in order to reach high hardness. The maximum case depth without use of a water quench is around 1.0 mm.
Alloy Steel - This is the most desirable type of steel to use with the laser process. The alloy elements, specifically manganese, molybdenum, boron and chrome, aid in hardenability. These steels can be heat treated up to a 3 mm case depth without concern for back tempering. The maximum hardness which can be achieved is dependent upon the carbon content.
Tool Steels - These also can be treated easily by the laser process. Results are similar to those achieved with alloy steels.
Martensitic Stainless Steel - Martensitic stainless steel can also be treated by the laser process. Though its microstructure does not readily display the hardened region, microhardness measurements clearly indicate high hardness.
Pearlitic Cast Iron - All cast irons with pearlitic structures can be hardened by the laser. Because of the uneven distribution of carbon along the graphite flakes, some finger melting can be occasionally found close to the hardened surface.
Special Considerations for Laser Surface Heat Treating
The nature of the laser surface heat treating process is rapid heating and self-quenching to obtain the desired microstructure. As a result there are several items that must be considered in advance in order to fully take advantage of this process.
Microstructure of Parts - The most desirable types of microstructures for the laser process are quenched and tempered or austenitized and tempered conditions. Fully annealed and spherodized structures are not recommended for this process.
Microstructural Homogeneity of the Parts - Laser surface heat treating requires a homogeneous structure because there is little time in which to diffuse and redistribute the alloy elements throughout the material. Parts with heavy segregation will not respond uniformly to the laser process.
Fine Microstructure or Small Grain Size - The smaller the grain size in the part, the faster the response to the laser process. Grain size is one of the major factors in determining the hardenability of parts.
Hardness of Core - Core hardness is important if the part will see service at high pressure conditions after heat treating. If the background material is dead soft, the hardened layer will peel off very quickly in service.
Parts Cleaning - The surface of the parts which will be laser heat treated should be thoroughly cleaned. Heavy dirt, rust, and grease on the surface will cause uneven case depth.
Surface Coating - When using CO2 lasers, a thin layer of coating is commonly applied to the metal surface to enhance the absorptivity of the metal by the laser beam. Phosphate and black paint are the most common coatings due to their low susceptibility to moisture, but oxide and graphite can also be used. The ideal thickness for the coating is around 0.02 mm to 0.05 mm. This coating is generally not required when Nd: YAG lasers are used.
Major Laser Heat Treating Parameters
Major laser heat treating parameters consist of the following:
Power Density - Generally speaking, the higher power density, the deeper the case depth. However, if all other variables are fixed, there is a maximum depth that can be achieved. When that limit is exceeded, surface melting will occur. If some finish machining is a requirement as a final step after hardening, then some surface melt (i.e. several thousandths) can be tolerated.
Travel Speed - If, after maximizing all variables, travel speed is increased, case depth will be decreased until there is no reaction with the material. Decreased travel speed will cause significant surface melting and/or a lower hardness.
Hardness Requirement - The maximum hardness that can be achieved on a given material is governed by the carbon content in the material. When a maximum hardness is required for a certain carbon content, then the case depth is controlled by the cooling condition of the part. If the hardness requirement is lower, then we can lower the power density and slow the travel speed to allow more time to drive the heat down deeper and create a deeper case depth.
Cooling Condition - As a general rule of thumb, at least six or seven times the case depth thickness of material is needed beneath the surface to insure self-quenching and to insure reaching the required case depth and hardness. This requirement can sometimes be circumvented by using various methods to assist in quenching. Air jets, water mist, or, if the part geometry allows, water or oil can be utilized. These processes can aid in obtaining maximum surface hardness.
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