The present disclosure generally relates to heat treatment processes for forming martensitic steels, and more particularly, to a uniform flame hardening process for heat treatment of complex-shaped alloys such as may be employed in power generation systems.
Flame hardening and induction hardening are heat treatment processes in which a thin surface shell of a metal part is heated rapidly to a temperature above the metal's critical point. As the austenite cools, it often transforms into a mixture of ferrite and cementite as the dissolved carbon falls out of solution. If the rate of cooling is very fast, the alloy may experience a slight lattice distortion known as martensitic transformation, instead of transforming into a mixture. The rate of cooling determines the relative proportions of these materials and therefore the mechanical properties (e.g. hardness, tensile strength) of the steel. Using steel as an example, after the grain structure has become austenitic (austenitized), the part is quickly quenched, transforming the steel from the austenite phase to a martensite phase For example, slow cooling can cause transformation to pearlite, bainite, and martensite, with the final structure being a combination of the three. Of these different phases, the martensite phase provides desirable properties such as hardness, wear resistance, and the like. In contrast, other phases such as pearlite and bainite can result in relatively soft and ductile steel. To achieve hardness, therefore, the steel must be cooled rapidly so that it bypasses the first transformation phases and transforms directly from austenite to martensite. Hardening results in high wear resistance.
Flame hardening typically employs direct impingement of a high-temperature flame or high-velocity combustion product gases onto the part. The part is then cooled at a rate that will produce the desired levels of hardness and other properties. The high temperature flame is obtained by combustion of a mixture of fuel gas with oxygen or air; flame heads are used for burning the mixture. Depths of hardening typically range from about 0.8 to 25.4 mm or more depending on the fuels used, the design of the flame head, the duration of heating, the hardenability of the work material, the quenching medium and method of quenching used, and the shape of the part.
Induction hardening generally includes heating a metal part by electromagnetic induction, where eddy currents are generated within the metal and resistance leads to Joule heating of the metal. An induction heater generally includes an electromagnet, through which a high-frequency Alternating current (AC) is passed. Heat may also be generated by magnetic hysteresis losses. The depth of induction hardened patterns can be controlled through choice of induction-frequency, power-density and interaction time.
The use of the above noted heat treatment processes permits a part to be machined when in the softened state greatly reducing time and equipment wear as well as allowing the part to be made from a less costly material, thereby effecting an overall cost saving in comparison with other technically acceptable methods. For example, the process gives inexpensive steels the wear properties of alloyed steels, and parts can be hardened without scaling or decarburization, thereby eliminating costly cleaning operations.
Turbine blades can be fabricated from hardened steels as well as other alloys and are often used in harsh environments. The blades, particularly on their leading edge and their tip, typically require a material hardness capable of withstanding the extreme conditions to prevent erosion wear during operation. Uniform hardness and optimized grain sizes across the various surfaces is desirable to maximize performance. However, uniform hardening of complex shapes such as turbine blades is oftentimes difficult to achieve. Significant variations in microstructure across the surface can occur as a result of current hardening processes. The flame hardening process typically exposes the entire structure to heat, which can become excessive in low thickness areas and insufficient in areas of greater thicknesses. As an example, prior art FIG. 1 is a photographic image of a blade tip illustrating the problem associated with existing hardening processes. The magnified images show a close-up of both the blade tip (FIG. 2) and the base airfoil structure (FIG. 3) for comparison. As can be seen there is a significant difference between the grain size of the blade tip and the grain size of the base airfoil. The blade tip has been overheated, leading to a larger than desired grain size, which can lead to erosion and performance problems for the turbine blade.
Accordingly, a need exists for a heat treatment process that can provide a uniform microstructure for complex-shaped components.