Patent Application
20060192347
The present invention relates to multi-layer steel (MLS) gaskets and in particular to processes that alter the physical characteristics of MLS gasket active layers to improve desirable gasket features.
In recent years, MLS cylinder head gaskets have become a preferred design choice, wherein all (typically at least two) gasket layers have been formed of steel. Beaded layers, also called “active” layers, have generally been fabricated of 301 stainless steel, a relatively robust metal with a commensurately high spring rate, for meeting requisite performance requirements over the useful life of the gaskets.
The trends to reduce fuel consumption and emissions have placed increased demands on the performance on these gaskets. Reducing fuel consumption by using lighter materials in engine cylinder blocks and head assemblies has proven successful, although the lighter alloys used typically experience greater deflection with equivalent cylinder compression ratios. This reduced stiffness may result in additional deflection within the head assembly and cylinder block, resulting in greater motion between the head assembly and cylinder block, and thus, increased demand on a cylinder head gasket to accommodate relative deflection.
Reducing emissions by increasing the engine compression ratio has also proven successful. However, this increase in cylinder pressure typically results in increased motion between the mating surfaces of the head assembly and cylinder block. These contributing factors, and others have resulted in the technology of MLS gaskets becoming an area of constant innovation.
The gasket areas immediately adjacent the circumference of engine cylinder bore apertures are subject to considerably greater stresses for assuring proper sealing than areas of the gasket radially remote from the apertures. These gasket areas immediately adjacent the circumference of engine cylinder bore apertures also experience greater dynamic displacement between the mating surfaces than areas of the gasket radially remote from the apertures.
This displacement between the mating surfaces results in axial motion within the active layers and creates a micro-motion between the active layer and any adjoining surface. This motion typically results in wear of the surfaces at regions of relative motion, commonly called fretting. When the adjoining surface is another layer of the gasket, wear on this layer may result in splitting or cracking of the gasket. When the adjoining surface is one of the mating components, surface wear may result in an ineffective seal. Typically, an elastomeric coating is applied to MLS gasket layers to improve sealability and permit the beaded layer to slide along the mating surface.
Processes that increase the surface strength in order to decrease fretting may undesirably decrease the capacity of a bead portion to accommodate relative displacement between mating surfaces. A bead portion of 301 stainless steel may be heat treated to increase the hardness to a desirable range of between ¾ hard and extra hard (350 to 500 Hv). Typical heat treating processes involve heating steel into a range of 400-450° C. to change the contents and structure of martensite. These heat treating processes may not be compatible with other processes that desirably increase the strength at the surface of the bead portions. What is needed, therefore, is an active layer for a metal gasket that is processed in a manner that affords desirable hardness, surface strength, and spring rate.
In one embodiment, the present invention provides a metal layer for a MLS gasket having at least one bead region and a surface subjected to a nitriding process. The metal layer is not subjected to a heat treatment process that heats the metal layer above the critical temperature range.
In another embodiment, a method of producing a MLS gasket with an active layer includes forming at least a half bead in an active layer and nitriding at least a portion of the active layer. The nitriding does not involve heating the metal layer above the critical temperature range.
In a further embodiment, a method of producing at least a portion of a MLS gasket includes cold forming a metal layer, and nitriding at least a portion of the metal layer. The nitriding does not involve heating the metal layer above the critical temperature range.
The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:
During installation, of the gasket of
To reduce fretting, cracking, and other types of undesirable wear within a MLS gasket, active layers must have sufficient hardness and strength. To insure that a bead region continues to seal during operations with increased axial displacement, the durability of the bead region may be improved.
To obtain a desired hardness for metal layers 32, 132, 134, a cold forming process is used. Preferably a cold rolling process is performed on sheet steel that is later formed into metal layers 32, 132, 134. In using a cold forming process, the hardness of metal layers 32, 132, 134 is increased (due to cold work hardening) to a desirable range of between ¾ hard and extra hard (350 to 500 Hv).
To increase the strength of metal layers 24, 32, 124, 132, 134, nitriding is performed. While liquid and plasma nitriding may be utilized, gas nitriding is preferably performed. Gas nitriding is a conventional process that exposes a heated metal component to a nitrogen rich media, such as anhydrous ammonia. During nitriding, nitrogen atoms are stripped from the media and combine with iron atoms to produce a diffusion layer within the metal. The metal component is heated (typically below 540° C.) to keep the steel in the current condition and encourage the nitrogen-iron reaction. Therefore, nitriding is accomplished below the critical temperature range for steels such as 301 stainless steel, and does not involve drastic phase changes of the steel.
Nitriding of at least the bead regions 46, 146, 166 forms a diffusion layer of FexN that increases the strength of metal layers 32, 132, 134 and thereby reduces fretting. While nitriding is most beneficial within bead regions 46, 146, 166, all portions of metal layers 32, 132, 134 are preferably nitrided.
Nitriding, pursuant to the invention, obtains high surface hardness, increases wear resistance, and improves fatigue life. Nitriding also increases the material's durability and resistance to cracking. While the metal layers 32, 132, 134 are preferably coated with known elastomer compounds to improve sealability, the nitrided active layers reduce the reliance on the elastomer to maintain an effective seal throughout the life of the gasket. Improved gaskets produced in accordance with the present invention can also be used to seal between cylinder head assemblies and exhaust manifolds, since the hardened, nitrided layer would experience reduced fretting in other applications as well.
Plasma, or ion, nitriding does not involve a separate heating of the components to be nitrided, but rather is performed by placing a metal component in a vacuum and using high-voltage electrical energy to form a plasma through which nitrogen atoms are accelerated to impinge on the component. While this impingement of nitrogen atoms on the surface of the metal component will heat the component, plasma nitriding offers a low temperature option for nitriding that may result in less distortion. Masking techniques may be employed to selectively nitride portions of a metal component.
The nitriding process may be performed before or after the cold forming process. The cold forming process may also be employed to form at least a portion of bead regions 46, 146, 166. Bead regions 46, 146, 166 may be half beads, full beads, or other distortions formed within a planar gasket that experience a deflection and provide a sealing contact as the gasket 20 is compressed between the mating surfaces. Metal layers 32, 132, 134 are typically referred to as active layers due to the movement experienced by the bead regions 46, 146, 166.
While the invention has been described with respect to specific examples including preferred modes of carrying out the invention, those skilled in the art will appreciate that there are numerous variations and permutations of the above described systems and techniques that fall within the spirit and scope of the invention as set forth in the appended claims.
