Patent Application
20100170079
This invention relates generally to turbine combustor hardware and, more specifically, to the detection and determination of an appropriate course of action to be taken with respect to a nitride layer formed on the surface of an austenitic steel alloy.
Nimonic® alloy 263, a precipitation-hardenable nickel-chromium-cobalt alloy (referred to herein as “N263”), is a proprietary alloy of International Nickel Corp. often used in the manufacture of combustor hardware for turbomachinery. One of the deterioration mechanisms associated with N-263 is nitriding that occurs when the alloy is exposed to temperatures above 2000° F. Nitriding is an elevated-temperature, irreversiable process by which the alloy absorbs nitrogen atoms into the surface when exposed to a high-nitrogen environment. The nitrogen atoms are diffused into the alloy to form a high-nitrogen surface layer which must be removed. The current repair procedure is to use a simple magnet to check for the presence of a nitride layer on the surface of the alloy. Once a magnetic response is detected, the surface is ground in a trial-and-error method until the magnetic response is gone. The current method is crude, particularly since the detection technique does not determine the amount of nitride to be removed. In other words, the current methodology does not give shop personnel any information regarding the amount of distress present in the alloy material, nor the extent of any machining required to remove the undesirable nitride layer.
Thus, the present techniques are time consuming and imprecise, and accordingly, there remains a need for a more effective detection/removal procedure.
In one aspect, the invention relates to a method of treating a component made of a non-magnetic alloy on which a magnetic surface layer has formed comprising: (a) engaging a magnet of a calibrated magnet gauge with a surface of the component; (b) disengaging the magnet from the surface of the component and measuring a force required to disengage the magnet; (c) correlating the force with a thickness of the magnetic surface layer; and (d) if the thickness of the surface layer is greater than a predetermined minimum thickness, removing the surface layer.
In another aspect, the invention relates to a method of detecting a nitride layer on a surface of a nickel-chromium-cobalt alloy component comprising: (a) engaging a calibrated magnet gauge with the surface of the alloy component; (b) detecting the presence, if any, of a nitride layer on the surface of the alloy component; and if present; (c) measuring the force necessary to remove the magnet gauge from the surface of the alloy component and using that Information to determine a thickness of the nitride layer.
The invention will now be described in connection with the above-identified drawing figures in greater detail below.
At the other extreme, however, if the detected thickness layer is above a maximum threshold, i.e., the nitride layer is so thick that the part must be declared unusable, then here again, no action need be taken and the part will be either disposed of or destroyed.
For thickness determinations that are within a range between the minimum and maximum threshold values, the nitride layer will be removed by, for example, a grinding or machining (or any other suitable process or operation) that may itself be calibrated to remove only the detected thickness of the nitride layer.
Thus, by using a calibrated magnetic gauge to determine the thickness of any nitride layer that may have formed on the component part, it is possible to more easily and quickly determine what action is to be taken, particularly when combined with process steps that eliminate certain components above or below predetermined threshold thickness measurements from any further processing.
It will be appreciated that a similar process may be used to detect undesirable surface deterioration on other alloys, so long as the underlying substrate is non-magnetic and the surface deterioration layer magnetic.
While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.
