Abstract:
A process that combines tubular body pressure induced shaping at elevated temperatures with a controlled rapid quenching operation using a gaseous quenching medium in a common unit. The achievable cooling rate permits the in-die shaping and quenching of tubular structural components of martensitic steels without requiring the use of a separate discrete quenching.

Description:
TECHNICAL FIELD 
     The present invention relates generally to the field of structural metal body fabrication and heat treatment and more particularly to methods for gas induced formation and quenching of heat treatable steel tubular body structures to achieve desired shape and compositional characteristic with the formation and retention of substantial percentages of martensite within the final part. 
     BACKGROUND OF THE INVENTION 
     Tubular structural components for use in applications such as automotive production, aircraft manufacture and the like are generally known. Currently, such tubular structural components are often shaped using a hydroforming process operated at room temperature. Such hydroforming processes have found particular application in the fabrication of structural components made from lightweight alloys and mild steels. Shaping of advanced high strength steels (AHSS) such as martensitic steels has typically utilized an initial thermal forming process followed by separate quenching by a liquid phase quench solution and annealing treatments applied to achieve the desired martensitic steel microstructure. Although the use of such discrete thermal forming and quench treatments has been used successfully, such practices are relatively complex and may require substantial skill to avoid variation and distortion in the final product. 
     SUMMARY OF THE INVENTION 
     The present invention provides advantages and alternatives over the prior art by providing a process that successfully combines tubular body shaping at elevated temperatures with a controlled rapid quenching operation using a gaseous quenching medium in a common unit so as to improve efficiency while simultaneously providing improved control of the quenching parameters. The achievable cooling rate permits the in-die shaping and quenching of tubular structural components of martensitic steels without requiring the use of a separate discrete quenching unit. 
     According to one potentially preferred aspect, a process is provided wherein a tubular member of heat treatable steel is formed to a desired shape in a heated mold cavity by application of internal pressure using a gaseous fluid. Following the development of a desired shape, the structure is thereafter subjected to a rapid introduction of cooling gas while being held within the mold. The cooling gas is delivered at a rate and temperature such that the steel alloy undergoes at least a partial martensitic transformation. The cooling gas may be held at an elevated pressure during the quenching process to promote heat transfer. Thereafter, heat may be reintroduced to the mold cavity to provide any desired tempering. Accordingly, a substantially simplified and streamlined process is provided for the formation and heat treatment of tubular martensitic steel structures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will now be described by way of example only, with reference to the accompanying drawings which constitute a part of the specification herein and, together with the general description above and the detailed description set forth below serve to explain concepts of the invention wherein: 
         FIG. 1  illustrates a metallic tubular blank; and 
         FIG. 2  illustrates schematically a system for the gas pressure shaping and quenching of the tubular blank in  FIG. 1 . 
     
    
    
     While embodiments and practices according to the invention have been illustrated and generally described above and will hereinafter be described in connection with certain potentially preferred procedures and practices, it is to be understood that in no event is the invention to be limited to such illustrated and described embodiments procedures and practices. On the contrary, it is intended that the present invention shall extend to all alternatives and modifications as may embrace the principles of this invention within the true spirit and scope thereof. 
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Reference will now be made to the drawings, wherein like reference numerals are utilized to designate like components in the various views. In  FIG. 1 , a tubular blank  10  of a heat treatable steel alloy is illustrated. As will be appreciated, the heat treatable steel alloy undergoes at least partial transformation from austenite to martensite when the alloy is heated and then rapidly cooled. Specifically, when the heated alloy is cooled at a rate above a critical level, equilibrium changes are suppressed and the austenite fcc lattice structure present at the elevated temperature changes rapidly to a martensite body-centered tetragonal microstructure. Such maternsitic materials have substantially improved strength characteristics relative to a corresponding non-heat quenched material that cools under equilibrium conditions. 
     A system of thermal shaping and quenching a steel alloy tubular blank  10  to achieve martensitic transformation is illustrated schematically in  FIG. 2 . As shown, the system includes a heated mold  12  of ceramic, graphite or the like incorporating a heated and insulated interior cavity  14 . The cavity  14  is sized to accommodate the tubular blank  10  and may be shaped to correspond to the final desired shape of the structure formed from the tubular blank  10  after pressure induced thermal shaping as will be described further hereinafter. 
     By way of example only, and not limitation, it is contemplated that the mold  12  may be in the form of a ceramic die incorporating an embedded induction coil  16  or other heating element as may be desired. The heat applied by the induction coil  16  or other heating element causes the temperature of the tubular blank to be raised above its softening point and into the austenitic phase such that the blank  10  may be pressurized at its interior and shaped into conformance with the contours of the cavity  14 . In order to facilitate this pressure induced molding, the cavity  14  is preferably sealed at both ends by seals  20 . In this regard, the seals  20  at either end of the cavity  14  are preferably provided with controlled gas flow openings to permit the introduction and withdrawal of gas at rates as may be desired. 
     As illustrated, it is contemplated that the system may utilize a singular gas supply  22  of a substantially non-reactive gaseous fluid such as helium, argon or nitrogen. It is contemplated that the gaseous fluid may be stored in either a gaseous or liquid state although a liquid state may be preferred for large volume requirements. The gas supply  22  may be operatively connected to a control valve  26  to permit the flow of gas into the system. In practice, the control valve  26  may be operated either manually or remotely to direct gas flow from the gas supply and into the mold cavity  14  along a predefined circuit. 
     As previously indicated, pressurizing gas from the gas supply  22  may be transported into the heated cavity  14  so as to occupy space at the interior of the tubular blank  10 . During this pressurizing step the control valve  26  is adjusted to transmit pressurizing gas through a first supply leg  30  so as to build pressure at the interior of the heated and softened tubular blank  10 . Under this pressurized condition, the tubular blank  10  is caused to expand outwardly and substantially conform to the contours of the cavity  14  as illustrated. As will be appreciated, the introduction of pressurizing gas during this shaping process is preferably carried out at a relatively low volumetric flow rate while maintaining the cavity in a substantially plugged condition with the tubular blank  10  held at a temperature above its softening temperature. Thus, relatively little thermal energy is transmitted to the gas during the shaping process. 
     Once the tubular blank  10  has been shaped to the desired profile, it is contemplated that the gas supply  22  previously used for shaping may thereafter be used to provide a gaseous quenching medium to effect a rapid quench of the heated and shaped tubular blank  10  such that a martensitic reaction is introduced within the alloy forming the tubular blank  10 . According to the illustrated practice, the quenching of the tubular blank  10  may be commenced by adjusting the control valve  26  so as to direct flow from the gas supply  22  and through a second supply leg  40 . Unlike the pressure shaping step, during the quenching operation the inlet and outlet to the cavity  14  are set to allow some degree of flow of quenching gas through the cavity and across the interior surface of the tubular blank  10 . 
     According to one contemplated practice, the flow rate through the tubular blank may be set so as to maintain a positive pressure of quenching gas at the interior of the tubular blank during at least a portion of the quenching process. Such higher pressures improve heat transfer characteristics. In practice it is contemplated that such gas pressure may be established at levels up to about 20 bar or more during the quenching step although the actual level will depend on factors such as the material forming the tubular blank, the dimensions of the part being formed and the desired final microstructure. 
     As shown, the second supply leg  40  may include an in-line heat exchanger of chilling unit  42  used to substantially cool the gas before it is introduced into the cavity  14 . By way of example only, for nitrogen the temperature is preferably reduced to about 15 degrees C. prior to introduction into the cavity although higher or lower temperatures may be used if desired. As will be appreciated, by recirculation of the quenching gas through the mold cavity  14  and across the heat exchanger  42 , a substantial rate of quenching may be achieved while avoiding the use of excessive volumes of gas. 
     Importantly, it has been found that the introduction of the low temperature gas into the mold cavity such that it flows across the interior surface of the shaped tubular member provides sufficient heat transfer to establish formation of martensite within the previously heated and formed tubular blank despite the fact that the exterior surface is held in contacting relation with mold walls. This is particularly true when the quenching gas is maintained under pressure. In this regard, it has been found that by adjusting the flow rate and temperature of the quenching gas that cooling rates sufficient to establish martensitic transformation can be achieved even with relatively thick walled structures. In fact, cooling rates approaching the critical cooling rate for formation of a fully martensitic structure may be approached. 
     It is to be understood that while the present invention has been illustrated and described in relation to potentially preferred embodiments, constructions, and procedures, that such embodiments, constructions, and procedures are illustrative only and that the present invention is in no event to be limited thereto. Rather, it is contemplated that modifications and variations embodying the principles of the present invention will no doubt occur to those of skill in the art.