Heat treated articles, for example articles that are formed of metal alloys such as steel, are typically quenched from elevated temperatures by coolants, such as water, water-based liquids, oil, or other liquid media, to effect metallurgical hardening. Often certain areas of the workpiece need to be protected from the coolant to preserve low hardness and high ductility in those areas. A quench fixture is often employed to direct the coolant to some areas and partially exclude it from other areas.
Examples of cooling selected areas of a workpiece are described in U.S. Pat. No. 5,000,798 issued Mar. 19, 1991 to Murray A. Nott, et al and titled Method for Shape Control of Rail During Accelerated Cooling. In the Nott apparatus, coolant is sprayed onto selected areas of the workpiece to control the shape or straightness of the article. Much earlier, U.S. Pat. No. 1,828,325 was issued to Heinrich Kurz on Oct. 20, 1931, for Process for the Manufacture of Rails with Hardened Heads. The Kurz patent describes a quench fixture, and method of using the fixture, in which different coolants are directed to separate preselected areas of the workpiece to develop different metallurgical properties in different portions of the workpiece. For example, air or steam is directly against the upper side of the web of a rail while a liquid coolant is directed to the head of the rail.
In an attempt to prevent the quench media from effecting areas of the workpiece that are not to be hardened, shrouds intended to contain the quench media have been used, but with only limited success. For example, U.S. Pat. No. 4,486,248 issued to Robert J. Ackert, et al on Dec. 4, 1984 for Method for the Production of Improved Railway Rails by Accelerated Cooling In Line with the Production Rolling Mill describes quench media spray heads disposed within a shroud structure. As mentioned above, such shrouds have been only partially successful in preventing the quench media from effecting areas of the workpiece in which hardening is not desired. Leaks of the coolant past the shroud results in quenching and transformation hardening in areas intended to remain soft, and can cause breakage of the workpiece during future service. Attempts to pressurize the unquenched areas with air can result in localized cooling and unintended hardening. Vacuum devices to siphon coolant flows are not always effective.
The present invention is directed to overcoming the problems set forth above. It is desirable to have a heat treat fixture and a method of heat treating in which areas of the workpiece that are not to be cooled by the quench media are protected by pressurized steam which disburses any quench coolant that may inadvertently pass through a barrier between the quenched and unquenched areas of the workpiece. It is also desirable to have such a fixture and method where the steam pressure is maintained at a value sufficient to exclude quench coolant from contacting preselected nonquenched areas of the workpiece. Furthermore, it is desirable to have a pressurized steam chamber wherein the steam temperature can be held at a temperature sufficient to maintain the preselected nonquenched areas at or above the transition temperature, i.e., the Martensite start temperature (M.sub.s in the case of steel), to allow slow cooling of the nonquenched areas after quenching the workpiece.