Metal treatment – Process of modifying or maintaining internal physical... – Heating or cooling of solid metal
Reexamination Certificate
2000-12-15
2003-01-28
Ip, Sikyin (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Heating or cooling of solid metal
C148S525000, C148S526000, C148S902000
Reexamination Certificate
active
06511559
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
The present application is directed to subject matter of German Patent Application No. 199 28 773.2, filed on Jun. 23, 1999, the disclosure of which is expressly incorporated by reference herein in its entirety.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to the hardening of edge layers of machine components. Objects for which utilization is practical and useful are components that are heavily exposed to wear and fatigue that are produced from precipitation-hardenable materials because of the high demands in material strength with, at the same time, high toughness. The invention may be used particularly advantageously for increasing the wear resistance of components made from stainless, precipitation-hardenable martensic steel, such as, e.g., turbine buckets, pump shafts, highly stressed bolts in aeronautics, components in shipbuilding, or special tools. An additional area of use is components exposed to wear made from high duty martensite hardening (Maraging) steel that cannot be used in a fully hardened state when high demands of toughness are present.
2. Discussion of Background Information
During use, edge zones of components placed under wear and fatigue stresses are exposed to significantly different stress than the core of the component. This fact is known to be considered in producing in the edge zone a harder, more wear or fatigue resistant structure using thermal, physical, chemical, mechanical, thermodynamical, or thermomechanical procedures as compared to the core, whose structure is adjusted such that it primarily meets the present demands in hardening and toughness.
This background of the invention shall be explained in greater detail using a characteristic component, prototypically chosen, without limiting its general purpose. Rotating blades of low-pressure stages in steam turbines are exposed during their use to extremely high pseudostatic (centrifugal forces, blade torsion), cyclic stresses (periodic exposure to steam pressure, blade oscillations), and tribologic (impingement) loading. In particular, the constant impact of condensed water droplets leads to an eroding wear in the area of the leading edge of the blade. Martensite-hardened 13% steels are able to meet these complex demands. Here, the blade material is used in the hardened, tempered state (meeting the requirements in toughness, stress corrosion resistance, corrosion fatigue resistance, sufficient static and cyclic stress resistance; hardness about 250-350 Vickers hardness numbers (VHN)) and the area of the leading edge of the blade is short-time hardened, e.g., via flame, induction, or laser hardening (very high resistance to wear by impingement, 390-680 HV). Increasing requirements in static and cyclic working stress as well as resistance to stress corrosion or corrosion fatigue have lately lead to the use of non-corroding precipitation-hardenable martensitic steel. In relation to tempered steel, they do not receive the biggest part of the increase in hardening and toughness by the formation of martensite but by a controlled precipitation hardening.
For this purpose, the steel contains 10-20 wt-% chrome and 2-11 wt-% nickel, usually copper (1-5 wt-%), and aluminum, titanium, or niobium as a precipitation former. In turbine constructions, a typical representative of this type of steel is the steel X5CrNiCuNb16-4. The heat treatment usually contains at least one solution annealing at 1030-1080° C. (duration of approximately 1 h.) and the precipitation treatment per se in the temperature range between 480° C. and 620° C. (duration 1-4 hs.) The achievable mechanical characteristics, hardening, yielding stress R
p0.2
and tensile strength R
m
, reach their maximum at the lower limit of conventionally possible tempering temperature of 480° C. and diminish drastically in increasing aging temperature (see Drawing 1). For instance, in the temperature range of 480-620° C., the hardening drops from 425 HV to 285 HV, the yielding level from 1170 to 750 MPa, and the tensile strength from 1310 to 930 MPa. Due to the required toughness levels, resistance against stress corrosion, and corrosion fatigue, the tempering temperature must be chosen that high that the 0.2% yielding level remains below 1040 and the tensile strength below 1000 MPa. This means that the low range of the possible tempering temperature producing the high velocities cannot be used (see Drawing 1).
Therefore, the shortcoming of this conventional heat treatment process lies in the resistance to wear from impingement being too low. This is based on the hardening of 340-370 HV being too low near the surface.
It is known that the surface hardening of precipitation-hardenable steel can be increased by plasma-nitriding up to about 1000 HV [e.g., brochure of the company Böhler Edelstahl GmbH (Kapfenberg, Austria) about the steel N700.] The shortcoming of this process includes that no improved resistance to impingement is achieved here, either. This shortcoming is based, e.g., on the fact that the achievable depth of nitriding of about 0.15 mm is much too low.
Other processes for tempering edge layers are not suitable either, since they affect the necessary aging treatment impermissibly or the achievable increase in hardness or the depth of the hardness is too low.
For improving the condition of the material itself, a process has become known in which a structure with a higher 0.2% yielding tension and tensile strength is achieved by coupling a short-time tempering with a conventional tempering treatment [see E. E. Denhard, Jr.: “Precipitation-hardenable stainless steel method and product,” U.S. Pat. No. 3,660,176.] For this purpose, the entire partially-finished product is exposed to a thorough short-time heating in the temperature range between 816° C. and 1149° C. of the solution annealing treatment within a time frame of 1 to 15 seconds by a direct flow of current and rapidly cooled. Then a conventional tempering treatment occurs in the conventionally used temperature range. It is thus possible, with a solutions annealing temperature of 1149° C., a solution annealing time of 2 seconds, a tempering temperature of 482° C., and a tempering time of 1 hour, to raise the 0.2% yielding level from 1328 MPa to 1695 MPa and the tensile strength from 1378 MPa to 1700 MPa. The achieved hardness is not listed.
The shortcoming of this process is that it is not suitable for being used in components of complex forms such as turbine buckets. This shortcoming is caused by the heating measures being used, such as conductive or inductive heating, depending on geometric relationships.
Another essential shortcoming is the fact that the tougness and endurance range and, in particular, the resistance against stress corrosion and corrosion fatigue of a turbine bucket treated in this manner would be too low. The reason for this lies in the hardness of the interior of the blade being much too strong. If the turbine bucket were to be tempered at higher temperatures, however, the hardness in the area of the leading edge of the bucket would be too low. With this process for improving the state of the material itself, it is not possible to simultaneously accommodate the different requirements that are placed upon the edge layer and the core of the component.
Another shortcoming lies in the fact that a conventional performance of tempering hardening cannot utilize the capacity for hardening in the state of the short-time solution annealing completely. This is caused by two facts: first, higher hardening states of structure that contain the entire cross-section of the component cannot be used due to low toughness, and second, new metal-physical degrees of freedom that offer short-time solution annealing for subsequent tempering hardening are not known.
SUMMARY OF THE INVENTION
The object of the invention is to provide a new and effective heat treatment process that allows components of precipitation-hardenable materials to be provided with considerably better wear-resistant edge layers w
Brenner Berndt
Tietz Frank
Fraunhofer-Geselleschaft zur Foerderung der angewandten Forschun
Greenblum & Bernstein P.L.C.
Ip Sikyin
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