Technical Field of the Invention

The present invention relates to an arrangement for treatment of articles by hot pressing, and preferably hot isostatic pressing, and to treatment of articles by hot isostatic pressing.

Background of the Invention

Hot isostatic pressing (HIP) is a technology that finds more and more widespread use. Hot isostatic pressing is for instance used in achieving elimi- nation of porosity in castings, such as for instance turbine blades, in order to substantially increase their service life and strength, in particular the fatigue strength. Another field of application is the manufacture of products, which are required to be fully dense and to have pore-free surfaces, by means of compressing powder.

In hot isostatic pressing, an article to be subjected to treatment by pressing is positioned in a load compartment of an insulated pressure vessel. A cycle, or treatment cycle, comprises the steps of: loading, treatment and unloading of articles, and the overall duration of the cycle is herein referred to as the cycle time. The treatment may, in turn, be divided into several portions, or phases, such as a pressing phase, a heating phase, and a cooling phase.

After loading, the vessel is sealed off and a pressure medium is introduced into the pressure vessel and the load compartment thereof. The pressure and temperature of the pressure medium is then increased, such that the article is subjected to an increased pressure and an increased temperature during a selected period of time. The temperature increase of the pressure medium, and thereby of the articles, is provided by means of a heating element or furnace arranged in a furnace chamber of the pressure vessel. The pressures, temperatures and treatment times are of course dependent on many factors, such as the material properties of the treated article, the field of application, and required quality of the treated article. The pressures and temperatures in hot isostatic pressing may typically range from 200 to 5000 bars, and preferably from 800 to 2000 bars and from 300 to 3000 °C, and pre- ferably from 800°C to 2000°C, respectively.

However, the supporting structure of the insulating portion of a prior art furnace suffers from problems related to the high temperature gradients (i.e. a significant temperature difference between warm and cold pressure medium over a short vertical distance) created inside the furnace. One such severe problem is so called beads or bulges, which arises in the portion of the supporting structure exposed to the highest temperature gradient. This typically occurs in the lower part of the supporting structure and the bead appears as a ridge-like protrusion on the inner surface (i.e. on the surface facing the furnace chamber). The beads often gradually grow over time depending on the number of cycles and may eventually require maintenance or may require that the deformed part of the supporting structure is changed or repaired. Hence, the bead may lead to a reduced life of the furnace. Further, it may also lead to undesired production interruptions.

Accordingly, there is need within the art for an improved pressing ar- rangement for hot isostatic pressing and an improved supporting structure for such a pressing arrangement.

Summary of the Invention

A general object of the present invention is to provide an improved pressing arrangement for hot isostatic pressing and a supporting structure for such a pressing arrangements, which eliminates or at least reduces the problems with beads.

A further object of the present invention is to provide an improved pressing arrangement for hot isostatic pressing having a furnace and insulat- ing portion with a significantly improved durability and longer operating life compared to prior art hot isostatic pressing arrangements.

These and other objects of the present invention are achieved by means of a pressure vessel and method for such vessel having the features defined in the independent claims. Embodiments of the present invention are characterized in the dependent claims.

In the context of the present invention, the terms "cold" and "hot" or "warm" (e.g. cold and warm or hot pressure medium or cold and warm or hot temperature) should be interpreted in a sense of average temperature within the pressure vessel. Similarly, the term "low" and high" temperature should also be interpreted in a sense of average temperature within the pressure vessel.

According to a first aspect of the invention, there is provided a pressing arrangement for treatment of articles by hot pressing comprising a pressure vessel including a furnace chamber at least partly encompassed by at least one substantially cylindrical heat insulating portion. The substantially cylindrical heat insulating portion comprises a supporting structure and an insulating layer provided on the supporting structure. A number of slits having a vertical extension over at least a part of the supporting structure where the temperature changes from warm to cold is arranged in the supporting structure.

The present invention is based on the insight that beads are created in the supporting structure when the thermal stress in the supporting structure (which often is made of stainless steel) exceeds the yield limit of the material in the supporting structure. Further, circumferential stresses in the material caused by the thermal stress are also believed to contribute to the creating of the beads. The thermal stress is, in turn, caused by the high temperature gradients caused in the furnace due to the abrupt temperature difference between cold and ward pressure medium and the high heat conduction number of the pressure medium. The bead often appears as an annular ridge on the inner surface of the supporting structure in a part or portion of the supporting structure where the temperature gradient is the highest.

Today, the deformed part of the supporting structure may need to be changed when the bead has grown too big, which may occur already after a few hundred cycles. The present invention on the other hand reduces the growth of beads dramatically and thus prolongs the operating life of the supporting structure and hence the furnace. Accordingly, the present invention provides advantages over the prior art pressing arrangements. For example, by significantly reducing the growth of the bead, the life length of the insulating portion can be extended and maintenance costs can be reduced.

Thus, according to an aspect, the present invention achieves this reduction of the growth of beads by providing longitudinal slits in the supporting structure, which reduces the circumferential stresses in the material and, in turn, reduces the growth of the bead significantly.

In a prior art pressing arrangement for hot isostatic pressing, the part or portion of the supporting structure exposed for the highest temperature gradient, i.e. where the temperature rapidly changes from warm to cold, and thus the part or portion where the bead may occur is typically the part or portion being at about the same height as the bottom heat insulating portion. That is, the part or portion of the supporting structure above an opening to a second guiding passage formed between a housing of the furnace chamber and the heat insulating portion including the supporting structure is likely to develop beads.

According to the present invention, the longitudinal slits are preferably arranged to extend at least over this part or portion of the supporting struc- ture.

According to an embodiment of the present invention, the supporting structure comprises a substantially cylindrical upper portion having a curved inner surface having a first curvature and a number of longitudinal segments having a vertical extension of at least a part of the heat insulating portion, each segment being attached to the cylindrical upper portion at a first end. Adjacent longitudinal segments may be separated by a slit. The segments have different shapes than the upper portion and has a lower curvature radius, for example, the segments may have a rectangular cross-section in a radial direction.

The inventor has realized that the circumferential stresses can be even further reduced, i.e. that the size of the beads or growth of beads can be even further reduced, by arranging longitudinal segments having a different curvature than the substantially cylindrical upper portion in the part or portion of the supporting structure being exposed for the highest temperature gradient. Preferably, the longitudinal segments are arranged to extend at least over a part or portion of the supporting structure being at about the same vertical position as the bottom heat insulating portion. In embodiments of the present inven- tion, the segments extends from a vertical position below the inlets to a second guiding channel to a position above the inlets a distance from a few decimeters to about one meter.

In embodiments of the present invention, the longitudinal segments may have a flat inner surface, i.e. a very small curvature, or may have a con- cave curvature seen from the inside of the furnace. Hence, it has surprisingly turned out that a supporting structure for the insulating portion of a pressing arrangement for hot isostatic pressing with significantly reduced circumferential stresses can be created by designing it with longitudinal segments arranged particular in the portion of the supporting structure where the tempera- ture gradients are the highest.

The function of a pressing arrangement according to the present invention is the same as in a prior art pressing arrangement. Generally, to achieve cooling within the pressure vessel and of the articles being treated within the pressure vessel, pressure medium is circulated through the furnace chamber and a cooler region of the pressure vessel, such as the intermediate space outside the furnace chamber. Thus, while the amount of pressure medium contained in the furnace chamber is approximately constant, there is a positive net flow of heat away from the articles in the furnace chamber.

In order for the walls of the pressure vessel to sustain the high temper- atures and pressures of the hot isostatic pressing process, the hot isostatic press is preferably provided with means for cooling the pressure vessel. For instance, the means for cooling may be a coolant, such as water. The coolant may be arranged to flow along the outer wall of the pressure vessel in a pipe system, or cooling channels, in order to keep the wall temperature at a suita- ble level.

When the pressure medium is brought into contact with the pressure vessel wall, thermal energy is exchanged between the pressure medium and the wall, which may be cooled by a coolant from the outside of the pressure vessel. In this manner, the pressing arrangement is, advantageously, arranged to circulate the pressure medium within the pressure vessel, thereby creating an outer, passive convection loop. The purpose of the outer convection loop is to enable cooling of the pressure medium during cooling of the articles and to enable cooling of the heat exchanger unit during heating of the articles. This makes it possible to cool the heat exchanger unit during pressing and heating of the articles. That is, thermal heat is transferred from the pressure medium to the heat exchanger unit during cooling of articles and from the heat exchanger unit to the pressure medium during pressing and heating of articles. In this manner, the cycle time may be reduced, since after cooling of the articles the press may be immediately operated to press and heat a new set of articles.

The hot isostatic pressing arrangement may also comprise a flow generator, located beneath the furnace chamber in the vicinity of the heat ex- changer unit. The flow generator enhances circulation of the pressure medium within the pressure vessel, i.e. in the outer convection loop. The flow generator may, for example, be in the form of a fan, a pump, an ejector, or the like.

The furnace chamber comprises a guiding passage formed between the heat insulated casing of the furnace chamber and the load compartment. There may be located a further flow generator within the furnace chamber for enhancing the circulation of the pressure medium therein, thereby creating an even temperature distribution. The flow generator will force the pressure medium upwards through the load compartment and downwards through the further guiding passage. As a result, an inner, active convection loop is created. The further flow generator, such as a fan, a pump, an ejector, or the like, may be used for controlling the inner, active convention loop.

In the outer convection loop, the pressure medium is cooled at the outer walls of the pressure vessel, i.e. at the inner surface of the pressure ves- sel, where the pressure medium flows towards the bottom of the pressing arrangement. At the bottom of the pressing arrangement, a portion of the pressure medium may be forced back into the furnace chamber, in which it is heated by the articles (or load) during rapid cooling. According to an embodiment of the present invention, a second end of each segment is arranged to be freely movable in a radial direction.

In embodiments of the present invention, the second end is attached to a hinge allowing the radial movement.

According to embodiments of the present invention, each of the segments has a substantially rectangular cross-section, a substantially rhombic cross-section, a substantially triangular cross-section, a substantially circular cross-section, a substantially ellipsoidal cross-section, or a substantially trapezoidal cross-section.

In embodiments of the present invention, the segments are rigidly attached to a substantially cylindrical lower portion at a second end.

According to embodiments of the present invention, a sheet is provided on the inside or outside of the supporting structure such that a substantially cylindrical sheet portion is formed on the inside or the outside of the support- ing structure.

Other objectives, features and advantages of the present invention will appear from the following detailed description, the attached dependent claims, and from the appended drawings. Brief Description of the Drawings

The various aspects of the invention, including its particular features and advantages, will be readily understood from the following detailed description and the accompanying drawings. In the following Figures, which not necessarily are drawn to scale, like reference numerals denote like elements or features of embodiments of the present invention throughout. Further, reference numerals for symmetrically located items, elements or feature indicators are only denoted once in the Figures. On the drawings:

Fig. 1 is a side view of a pressing arrangement according to an embodiment of the invention;

Figs. 2a-2c show cross-sectional views of an embodiment of the heat insulating portion according to the present invention;

Fig. 3a shows a perspective view of the embodiment of the supporting structure shown in Fig. 2a-2c; Fig. 3b shows a perspective view of an embodiment of the supporting structure shown in Fig. 2a-2b;

Fig. 4a-4c show cross-sectional views of an embodiment of the heat insulating portion according to the present invention;

Fig. 5 shows a perspective view of the embodiment of the supporting structure shown in Fig. 4a-4c;

Figs. 6a-6j show cross-sectional views of the longitudinal segments;

Fig. 7 shows a perspective view of a further embodiment of the supporting structure according to the present invention;

Fig. 8 shows a perspective view of yet another embodiment of the supporting structure according to the present invention;

Fig. 9 shows a perspective view of another embodiment of the supporting structure according to the present invention; and

Fig. 10 shows a perspective view of a further embodiment of the supporting structure according to the present invention.

Detailed description of embodiments

The following is a description of exemplifying embodiments of the present invention. This description is intended for the purpose of explanation only and is not to be taken in a limiting sense. It should be noted that the drawings are schematic and that the pressing arrangements of the described embodiments may comprise features and elements that are, for the sake of simplicity, not indicated in the drawings.

Embodiments of the pressing arrangement according to the present in- vention may be used to treat articles made from a number of different possible materials by pressing, in particular by hot isostatic pressing.

Figure 1 shows a pressing arrangement according to an embodiment of the invention. The pressing arrangement 100, which is intended to be used for pressing of articles, comprises a pressure vessel 1 with means (not shown), such as one or more ports, inlets and outlets, for supplying and discharging a pressure medium. The pressure medium may be a liquid or gaseous medium with low chemical affinity in relation to the articles to be treated. The pressure vessel 1 includes a furnace chamber 18, which comprises a furnace (or heater) 36, or heating elements, for heating of the pressure medium during the pressing phase of the treatment cycle. The furnace 36 may, as shown in for example figure 1 , be located at the lower portion of the furnace chamber 18, or may be located at the sides of the furnace chamber 18. The person skilled in the art realises that it is also possible to combine heating elements at the sides with heating elements at the bottom so as to achieve a furnace which is located at the sides and at the bottom of the furnace chamber. Clearly, any implementation of the furnace regarding place- ment of heating elements, known in the art, may be applied to the embodiments shown herein. It is to be noted that the term "furnace" refers to the means for heating, while the term "furnace chamber" refers to the volume in which load and furnace are located. The furnace chamber 18 does not occupy the entire pressure vessel 1 , but leaves an intermediate space 10 around it. During normal operation of the pressing arrangement 100, the intermediate space 10 is typically cooler than the furnace chamber 18 but is at equal pressure.

The furnace chamber 18 further includes a load compartment 19 for receiving and holding articles 5 to be treated. The furnace chamber 18 is sur- rounded by a heat insulated casing 3, which is likely to save energy during the heating phase. It may also ensure that convection takes place in a more ordered manner. In particular, because of the vertically elongated shape of the furnace chamber 18, the heat-insulated casing 3 may prevent forming of horizontal temperature gradients, which are difficult to monitor and control.

In the furnace chamber 18, there may also be located a fan 30 for circulating the pressure medium within the furnace chamber 18 and enhance an inner convection loop, in which pressure medium has an upward flow through the load compartment and a downward flow along a peripheral portion 12 of the furnace chamber.

An upper lid 8 and a lower lid 9 are removable arranged to allow the pressure vessel 1 to be opened and closed. The pressure vessel 1 may further comprise a fan 31 , which is located beneath the furnace chamber 18, for guiding pressure medium into the furnace chamber.

Moreover, the outer wall of the pressure vessel 1 may be provided with channels or tubes (not shown), in which a coolant for cooling may be provided. In this manner, the vessel wall may be cooled in order to protect it from detrimental heat. The coolant is preferably water, but other coolants are also contemplated. The flow of coolant is indicated in Fig. 1 by the arrows on the outside of the pressure vessel.

Even though it is not shown in the figures, the pressure vessel 1 may be opened, such that the articles within the pressure vessel 1 can be removed. This may be realized in a number of different manners, all of which being apparent to a man skilled in the art.

A first guiding passage 10 is formed between the inside of the outer walls of the pressure vessel and the casing 3. The first guiding passage 10 is used to guide the pressure medium from the top of the pressure vessel 1 to the bottom thereof.

Further, the heat insulated casing 3 comprises at least one heat insulating portion 7 and a housing 2 arranged to surround the heat insulating por- tion 7, which thermally seals off the interior of the pressure vessel 1 in order to reduce heat loss. A bottom heat insulating portion 7b provides insulation in the lower end of the pressing arrangement 100.

The heat insulating portion 7 comprises a supporting structure 71 and an insulating layer 72 arranged on the supporting structure 71 , for example, on an outside surface as shown in Fig. 1 .

Moreover, a second guiding passage 1 1 is formed between the housing 2 of the furnace chamber 18 and the heat insulating portion 7 of the furnace chamber 18. The second guiding passage 1 1 is used to guide the pressure medium towards the top of the pressure vessel.

The second guiding passage 1 1 is provided with inlets 24 for supplying pressure medium thereto, as well as at least one opening 13 at an upper part of the pressure vessel 1 for allowing flow of the pressure medium into the first guiding passage 10. Typically, the temperature changes from warm to cold in a region close to the inlets 24 and a bead often appears as a ridge-like protrusion in the inner surface of a part or portion of the supporting structure 71 above the inlets 24 at about the same vertical position as the bottom heat insulating portion 7b.

With reference now to Fig. 2a-2c and 3a, an embodiment of the present invention will be described. Figs. 2a-c show cross-sectionals views of the heat insulating portion 7 along the dashed lines B-B, C-C, and D-D, respectively, in Fig. 1 seen from the direction of the arrow A in Fig. 1 . Fig. 3 shows a perspective view of the supporting structure shown in Fig. 2a-2c. In Fig. 3, the insulating layer 72 has been removed for clarifying purposes.

According to this embodiment of the present invention, the supporting structure 71 comprises an annular upper portion 73, an annular lower portion 80 and a number of longitudinal slits 74, in the illustrated embodiment eight slits, each extending over at least a part of the supporting structure 71 between the portions 73 and 80. Thus, a number of elongated portions 85 of the supporting structure 71 are formed between the slits 74. In this embodiment, the upper and lower segments 73 and 80 and the elongated portions 85 have curved inner surfaces 77 with the same curvature.

Preferably, the slits 74 are evenly distributed in a circumferential direction around the supporting structure 71 . The slits 74 preferably have the same vertical length and the same width in circumferential direction.

In embodiments of the present invention, the elongated portions 85 have a different curvature compared to the upper and lower segments and, in particular, a smaller curvature, or a substantially flat inner and/or outer surface, or a concave curvature seen from the inside of the furnace 18.

Preferably, the slits 74 are arranged in a vertical location of the supporting structure 71 such that when the supporting structure 71 is placed inside a pressing arrangement 100, the slits 74 at least extends over a part or portion of the supporting structure at about the same height as the bottom heat insulating portion 7b. For example, the slits 74 may extend from a point above the inlets 24 at approximately the same vertical height as the bottom heat insulating portion 7b or from a point below the inlets 24 and vertically upwards a distance of about a few decimeters to about a meter.

In Fig. 3b, an embodiment of a supporting structure 91 where the slits 74' extends over the lower part of the supporting structure is shown. That is, this embodiment does not comprise a lower portion 180 as the embodiment shown in Fig. 3a. Accordingly, the elongated portions 85' of the supporting structure 91 formed between the slits 74' also extends over the lower part of the supporting structure 74'.

With reference to Fig. 4a-4c and 5, another embodiment of the present invention will be described. Figs. 4a-4c show cross-sectional views of a heat insulating portion 7 along the dashed line B-B, C-C, and D-D, respectively, in Fig. 1 seen from the direction of the arrow A in Fig. 1 . Fig. 5 shows a perspective view of the supporting structure shown in Fig. 4a-4c. In Fig. 5, the insulating layer 72 has been removed for clarifying purposes.

The supporting structure 71 ' comprises an annular upper portion 173 and an annular lower portion 180 having a curved inner surface 177 having a first curvature. Further, a number of longitudinal segments 75 are arranged between the upper and lower portions 173 and 180. In this embodiment of the present invention, the segments 75 are attached to the upper and lower por- tion 173 and 180 in upper and lower ends 78 and 79, respectively.

In embodiments of the present invention, the segments 75 are rigidly attached to the upper portion 173, for example, by means of welding or screwed on the upper portion 173. The segments 75 may also be rigidly attached to the lower portion 180, for example, by means of welding or screwed on the upper portion 180.

In alternative embodiments, the segments 75 are movable attached to the lower portion 180 such that a radial movement of the lower end 79 is allowed.

The segments 75 have a vertical extension over at least a part of the heat insulating portion 7 and adjacent segments 75 may be separated by a slit 174. In this embodiment, the segments 75 have a substantially rectangular cross-section as illustrated in Fig. 4a. However, other cross-sectional geometries are also conceivable as shown in Fig. 6b-6j. In this illustrated embodi- ment, 12 vertically elongated segments are arranged but, however, as the skilled person realizes, a higher number of lower number of elongated segment is also conceivable.

Preferably, the segments 75 are evenly distributed in a circumferential direction. The segments 75 preferably have the same vertical length and the same cross-sectional geometry and cross-sectional dimensions. However, as the skilled person realizes, it may be possible to have differently designed segments in a supporting structure, for example, segments with rectangular and circular cross-sectional geometries.

In embodiments of the present invention, the segments 75 have a different curvature compared to the upper and lower segments 173 and 180 and, in particular, a smaller curvature, or a substantially flat inner surface, or a concave curvature seen from the inside of the furnace 18.

Preferably, the segments 75 are arranged in a vertical location of the supporting structure 71 ' such that when the supporting structure 71 ' is placed inside a pressing arrangement 100, the segments 75 extends at least over a part or portion of the supporting structure 71 ' at about the same height as the bottom heat insulating portion 7b. For example, the segments 75 may extend from a point above the inlets 24 at approximately the same vertical height as the bottom heat insulating portion 7b or a point below the inlets 24 and vertically upwards a distance of about a few decimeters to a about a meter.

In Fig. 6a - 6j, different conceivable cross-sections of the longitudinal segments are shown.

In Fig. 6a, a segment 75 having a substantially rectangular cross- section is shown. The segments 75 having a rectangular cross-section have substantially flat inner surfaces 76 and substantially flat outer surfaces 88.

In Fig. 6b, a segment 1 15 having a smaller curvature of the inner surface 1 16 than the upper and lower circular portions 173 or 180 is shown. The outer surface 1 17 also has a smaller curvature than the upper and lower cir- cular portions 73 and 80. A cross-sectional view of a part of the upper circular portion 173 is also shown as comparison.

In Fig. 6c, a segment 125 having a different curvature of the inner surface 126 than the upper and lower circular portions 173 or 180 is shown. The segment 125 has a concave curvature of the inner surface 126 seen from inside the furnace 18. A cross-sectional view of a part of the upper portion 173 is also shown as comparison.

In Fig. 6d, a segment 135 having a concave curvature of the inner sur- face 136 and a concave curvature of the outer surface 137 seen from inside the furnace 18 is shown. A cross-sectional view of a part of the upper portion 173 is also shown as comparison.

In Fig. 6e, a segment 145 having a triangular cross-section is shown. Preferably, the triangular segment 145 is arranged such that a flat portion 146 constitutes a substantially flat inner surface of a supporting structure.

In Fig. 6f, a segment 155 having a rhombic cross-section is shown. The inner surface 156 is substantially flat also in this embodiment.

In Fig. 6g, a segment 165 having a trapezoidal cross-section is shown. The inner surface 166 is substantially flat also in this embodiment.

In Fig. 6h, a segment 175 having a circular cross-section is shown.

In Fig. 6i, a segment 185 having an ellipsoidal cross-section is shown. In Fig. 6j, a segment 195 having a cubic cross-sectional is shown. The segments 195 having a cubic cross-section have substantially flat inner surfaces 196 and substantially flat outer surfaces 198.

In Fig. 7, another embodiment of the present invention is schematically shown. According to this embodiment, the supporting structure 200 comprises vertically elongated segments 275 attached to, for example, an upper ring-like part 281 . Adjacent segments 275 may be separated by a slit 274. In this illustrated embodiment, the segments 275 have a rectangular cross-section. However, other cross-sectional geometries are conceivable, for example, segments having geometries as shown in any one of Fig. 6b - 6j.

In Figs. 8 and 9, further embodiments of the present invention are schematically shown. According to this embodiment, the supporting structure 300 comprises vertically elongated segments 375 attached to an upper sub- stantially circular portion 373. Adjacent segments 375 may be separated by a slit 374. In this illustrated embodiment, the segments 375 have a rectangular cross-section. However, other cross-sectional geometries are conceivable, for example, segments having geometries as shown in any one of Fig. 6b - 6j. In Fig. 10, another embodiment of the present invention is schematically shown. According to this embodiment, the supporting structure 400 comprises vertically elongated segments 475 attached to an upper substantially circular portion 473. Adjacent segments 475 abut on each other. In this illu- strated embodiment, the segments 475 have a rectangular cross-section. Other cross-sectional geometries are also conceivable, for example, segments having geometries as shown in any one of Fig. 6b - 6j. The segments 475 are provided with circular inlets 424, which however may have other geometrical shapes, for example, ellipsoidal. Further, each segment 475 may be provided with more than one inlet 424. Moreover, it is not necessary to provide all segments with inlets. For example, the segments may alternately be provided with inlets.

Even though the present description and drawings disclose embodiments and examples, including selections of components, materials, tempera- ture ranges, pressure ranges, etc., the invention is not restricted to these specific examples. Numerous modifications and variations can be made without departing from the scope of the present invention, which is defined by the accompanying claims.