FIELD OF THE INVENTION

The present invention relates to a method for enhanc ^¬ ing fatigue durability of a conveyor belt of a strand sintering furnace. Further, the present invention relates to a conveyor belt of a strand sintering fur ^¬ nace .

BACKGROUND OF THE INVENTION

Continuous strand sintering is used for agglomerizing pellets after pelletizing a concentrate powder, improving the strength and the reactivity of the pel- lets.

As an example of the strand sintering technique, a strand sintering furnace could be mentioned, which is used in the production of ferro-chromium and divided into several sequential zones, different temperature conditions prevailing in each one of them. The strand sintering equipment includes a conveyor belt, which is a perforated steel belt. It is conveyed as an endless loop around two deflector rolls. At the forward end of the furnace, wet fresh pellets are fed onto the steel belt to form a pellet bed. The steel belt conveys the bed of pellets through the drying, heating, and sintering zones of the furnace and to a stabilizing or equalizing zone, after which the bed of pellets fur- ther travels through sequential cooling zones. After travelling through the cooling zones, the sintered pellets exit the strand sintering equipment at its tail . As disclosed e.g. in documents WO 01/55659 Al and WO 2009/022059 Al, the conveyor belt of a strand sinter ^¬ ing furnace is formed from a number of rectangular steel plate elements that are sequentially welded to each other by weld seams. Each plate element includes a plurality of holes arranged into a plurality of groups of perforations to enable the flow-through of the gas used in the sintering process.

During operation, the conveyor belts are subjected to static and dynamical loads, corrosive environment and elevated temperature. Dynamical loads, i.e. fatigue loads, cause damage that commonly limits the lifetime of the belt. Cyclic loads (fatigue loads) are generat ^¬ ed, when the belt rotates around the d.eflector rolls. Because the perforations act as stress raisers, fa ^¬ tigue cracks are typically initiated, and. start to grow. This leads to damage, especially in the perfo ^¬ rated regions. Fatigue occurs when a material is sub ^¬ jected to repeated loading and unloading and at least part of the loading cycle is tensile. If the loads are above a certain threshold,- microscopic cracks will begin to form at the surface . Eventually a crack will reach a critical size, and. the structure will suddenly fracture . Load related factors that influence the fa ^¬ tigue life are for example stress amplitude and mean stress. The tensile part of the load cycle will cause fatigue as crack surfaces are torn open and the crack is able to proceed.

The conveyor belt is made by welding. Weld seams are problematic in cyclically loaded structures because a weld seam changes the geometry locally and as a conse ^¬ quence act as a stress raiser. Additionally, tensile residual stresses are generated and the weld seam mi- crostructure may not attain the properties of the base material .

The current repair method is to weld patches onto the cracked area, but it helps only temporarily as repair welding impairs the properties of the surrounding ma ^¬ teria.! and causes distortions due to non-uniform heat ^¬ ing, Consequently, the belt must be discarded and re ^¬ placed, which limits the lifetime of the belt.

OBJECT OF THE INVENTION

The object of the invention is to eliminate the disad ^¬ vantages mentioned above. In particular, it is an object of the invention to provide a method by which the fatigue durability of a conveyor belt can be enhanced.

Further, it is an object of the invention to provide a conveyor belt having a prolonged fatigue life and a longer lifetime.

Further, it is an object of the invention to provide a conveyor belt in which the improved fatigue life can be achieved with leaner (and cheaper) stainless steel alloys .

Further, it is an object of the invention to provide a method which may improve yield stress of the material of the conveyor belt, which may alleviate problems with local yielding.

Further, it is an object of the invention to provide a method which can be used for a new conveyor belt while it is manufactured and also for existing conveyor belts which are already in use.

SUMMARY OF THE INVENTION

According to an aspect of the invention, the present invention provides a method for enhancing fatigue du ^¬ rability of a conveyor belt of a strand sintering fur ^¬ nace. The conveyor belt is formed from a number of rectangular steel plate elements that are sequentially welded to each other by weld seams, each plate element including a plurality of holes arranged into a plural ^¬ ity of groups of perforations to enable the flow- through of the gas used in the sintering process. Ac ^¬ cording to the invention, the conveyor belt is treated to create compressive residual stresses at a surface of the conveyor belt at least in critical regions which are susceptible to fatigue breakage.

According to another aspect of the invention, the present invention provides a conveyor belt of a strand sintering furnace. The conveyor belt is formed from a number of rectangular steel plate elements that are sequentially welded to each other by weld seams, each plate element including a plurality of holes arranged into a plurality of groups of perforations to enable the flow-through of the gas used in the sintering process. The conveyor belt includes compressive residual stresses at a surface of the conveyor belt at least in critical regions which are susceptible to fatigue breakage .

The invention makes it possible to prevent fatigue failures in the conveyor belt, thus prolonging its fa ^¬ tigue life. Further, the invention makes it possible to use leaner (and cheaper) stainless steel alloys for the material of the belt and still gain a long fatigue life. Further, an advantage of the invention is that it may improve yield stress of the material of the conveyor belt, which may alleviate problems with local yielding. The method of the invention can be imple ^¬ mented to newly manufactured conveyor belts in connec ^¬ tion to their manufacturing process. The invention can as well be implemented to conveyor belts already in use to prolong the fatigue life of such conveyor belts . In one embodiment of the invention, in the method, the conveyor belt is treated to create compressive residual stresses at the surface of the conveyor belt at regions of the groups of perforations.

In one embodiment of the invention, in the method, the conveyor belt is treated to create compressive residual stresses at the surface of the conveyor belt in regions of the weld seams.

In one embodiment of the invention, in the method, the surface on the outer side of the conveyor belt, which outer surface, in operation, is repeatedly subjected to tensile stress, is treated to create the compressive residual stresses on the outer side of the conveyor belt .

In one embodiment of the invention, in the method, the surfaces of both sides of the conveyor belt are treated to create the compressive residual stresses on both sides of the conveyor belt.

In one embodiment of the invention, in the method, the conveyor belt is treated to create compressive residual stresses substantially only in the regions of groups of perforations and in the regions of the weld seams while other areas of the conveyor are left untreated. In one embodiment of the invention, in the method, the treatment to create compressive residual stresses is chosen from a group of treatment processes including shot peening, ultrasonic hammering, laser shock peen- ing. Shot peening is a well-known cold working process used to produce a compressive residual stress layer and modify mechanical properties of metals. It entails impacting a surface with shot (round metallic, glass, or ceramic particles) with a force sufficient to cre ^¬ ate plastic deformation. The plastic deformation induces a residual compressive stress in a peened sur ^¬ face, along with tensile stress of smaller magnitude in the interior. Surface compressive stresses confer resistance to metal fatigue and to some forms of stress corrosion cracking. The tensile stresses on the surface are problematic because cracks tend to start on the surface. Ultrasonic hammering- is a well-known metallurgical processing technique, similar to work hardening, in which ultrasonic energy is applied to a metal object. The ultrasonic treatment can result in controlled residual compressive stress, grain refine ^¬ ment and grain size reduction. Low and high cycle fa- tigue resistance are enhanced. Further, laser shock peening is the process of hardening or peening metal using a powerful laser. Laser peening can impart on a surface a layer of residual compressive stress that is four times deeper than that attainable from conven- tional shot peening treatments.

In one embodiment of the invention, in the method, the conveyor belt is treated in a flat form to create said compressive residual stresses. After treatment, the free ends of the conveyor belt are welded together by an installation weld seam to form the conveyor belt into an endless loop form. Thereafter, the installation weld seam is treated to create compressive residual stresses in the region of the installation weld seam.

In one embodiment of the invention, the conveyor belt includes compressive residual stresses at the surface of the conveyor belt in the regions of the groups of the perforations. In one embodiment of the invention, the conveyor belt includes compressive residual stresses at the surface of the conveyor belt in the regions of the weld seams. In one embodiment of the invention, the material of the conveyor belt is chosen from stainless steel grades including: ferritic chromium-alloyed stainless steel, austenitic-martensitic precipitation hardened stainless steel, austenitic stainless steel, austenitic-ferritic duplex stainless steel.

In one embodiment of the invention, each plate element comprises two long edges in the lateral direction of the conveyor belt, which are parallel to and spaced from each other, the long edge of a similar adjacent second plate element being connected to each long edge, and two short edges in the longitudinal direc ^¬ tion of the conveyor belt, which are spaced from each other by a distance corresponding to the width of the conveyor belt.

In one embodiment of the invention, the groups of per ^¬ forations are rectangular and elongated, extending in the direction of the conveyor belt, the groups of per- forations being parallel to each other and spaced from each other by a first imperforated area.

In one embodiment of the invention, the groups of per ^¬ forations are subdivided into a number of subgroups of perforations spaced from each other by a second imper ^¬ forated area.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to pro ^¬ vide a further understanding of the invention and constitute a part of this specification, illustrate embodiments of the invention and together with the de ^¬ scription help to explain the principles of the inven ^¬ tion. In the drawings: Figure 1 is a plan view of a part of an embodiment of the conveyor belt according to the invention,

Figure 2 shows the detail P of Figure 1, Figure 3 schematically shows a cross-section of the conveyor belt treated to include compressive residual stresses on its surfaces and the distribution of ten ^¬ sile and compressive stresses in a situation when no load is exerted, and

Figure 4 shows the cross section of Figure 3 in a sit ^¬ uation when a bending load is exerted on the belt.

DETAILED DESCRIPTION OF THE INVENTION

Figure 1 shows part of the conveyor belt 1 of a strand sintering furnace (not shown) . The conveyor belt 1 consists of a number of rectangular steel plate ele ^¬ ments 2 that are sequentially welded to each other by weld seams 3.

Each plate element 2 includes a plurality of holes 4 arranged into a plurality of groups 5 of perforations to enable the flow-through of the gas used in the sin ^¬ tering process. The arrangement of the holes 4 into groups 5 and subgroups 5' substantially corresponds to that disclosed in WO 2009/022059 Al .

Each plate element 2 comprises two long edges 6, 7 in the lateral direction y of the conveyor belt, which are parallel with and spaced from each other. The long edge of a similar adjacent second plate element is connected to each long edge 6, 7 by a weld seam 3. The two short edges 8, 9 are in the longitudinal di ^¬ rection x of the conveyor belt 1. The short edges are parallel and spaced from each other by a distance de ^¬ fining the width L of the conveyor belt. The groups 5 of perforations are rectangular and elongated, extend ^¬ ing in the direction of the conveyor belt. The groups 5 of perforations are parallel to each other and spaced from each other by a first imperforated area 10. The groups 5 of perforations are further subdivid- ed into a number of subgroups 5' of perforations spaced from each other by a second imperforated area 11.

The outer side is of the conveyor belt 1 in its end- less loop form, in operation, is repeatedly subjected to tensile stresses as it turns around the deflector rolls. Therefore at least the outer surface of the conveyor belt 1 has been treated to induce compressive residual stresses at the outer side surface of the con- veyor belt at least in critical regions A and B which are susceptible to fatigue breakage.

The critical regions are the regions A of the groups 5 of the perforations and the regions B of the weld seams 3. These regions A and B are schematically indicated in Figures 1 and 2 as areas enclosed by dot-and-dash lines. The compressive residual stresses at the sur ^¬ faces of the belt can be achieved by subjecting the regions A and B to shot peening, ultrasonic hammering or laser shock peening etc..

The material of the conveyor belt 1 may preferably be ferritic chromium-alloyed stainless steel (e.g. 3Cr 12) , austenitic-martensitic precipitation hardened stainless steel, austenitic stainless steel or austen- itic-ferritic duplex stainless steel. In the perforated region 5, 5', the holes 4 act as stress raisers, meaning that the local stress is sig ^¬ nificantly higher than the applied stress. Weld seams 3, on the other hand, always have high tensile residu- al stresses that are caused by restricted thermal ex ^¬ pansion of the weld seam during weld deposition and cooling. By implementing the method, of the invention, the harmful tensile residual stresses in the weld can be alleviated, or beneficial compressive residual stresses can be created in the perforated area, and the fatigue life can be prolonged significantly. When the belt is treated with a. suitable process {e.g. with shot peening, ultrasonic hammering, laser shock peen- ing) , the underlying metal contracts the free movement of the surface, and compressive residual stresses are created on the surface.

Figure 3 illustrates a cross-section of the conveyor belt 1 in which both opposite surfaces of the regions of the groups 5 of perforations have been treated so as to create compressive residual stresses. Figure 3 shows a stress distribution inside the steel material of the belt in the unloaded condition wherein the surfaces are in compression while the core is in tension to balance out the forces as shown in Figure 3.

Figure 4 shows the belt structure and the stress dis ^¬ tribution of Figure 3 under a load resulting from the additive combination of stresses due to bend loading and the initial compressive residual stresses . As shown in Figure 4, the surface of the belt structure is still in compression eve on the convex side (upper side in Figure 4) and fatigue failures do not occur . The center is under a tensile stress and the concave side (lower sid.e in the Figure 4) is in compression. The belt structure can be loaded until the convex side is i tension, but this tensile stress would be far ?hat i t ;ld be i f the be l l s truct i

It is obvious to a person skilled in the art that with the advancement of technology, the basic idea of the invention may be implemented in various ways. The in ^¬ vention and its embodiments are thus not limited to the examples described above; instead, they may vary within the scope of the claims.