FI124911B - Method for raising the fatigue strength of a conveyor belt of a belt sintering furnace, and conveyor belt - Google Patents

Description

METHOD FOR ENHANCING FATIGUE DURABILITY OF A CONVEYOR BELT OF A STRAND SINTERING FURNACE, AND CONVEYOR BELT

FIELD OF THE INVENTION

The present invention relates to a method for enhancing the fatigue durability of a conveyor belt of a Strand sintering furnace. Further, the present invention relates to a conveyor belt of a Strand sintering furnace.

BACKGROUND OF THE INVENTION

Continuous Strand sintering is used for agglomerating pellets after pelletizing a Concentrate powder, improving strength and reactivity of the pellets.

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 the stabilizing or equalizing zone, after which the bed of pellets further travels through the sequential cooling zones. After traveling through the cooling zones, the sintered pellets exit the Strand sintering equipment at its tail.

As disclosed e.g. in documents WO 01/55659 A1 and WO 2009/022059 A1, the conveyor belt of a Strand sintering 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 dynamic loads, corrosive environment and elevated temperature. Dynamical loads, i.e. fatigue loads, cause damage that usually limits the lifetime of the belt. Cyclic loads (fatigue loads) are generated when the belt rotates around the deflector rolls. Because the perforations act as stress raisers, fatigue cracks are typically initiated and start to grow. This leads to damage, especially in the perforated regions. Fatigue occurs when a material is subjected 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 fatigue life are for example stress amplitude and mean stress. The tensile part of the load cycle will cause fatigue as the 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 of a weld seam changes in geometry locally and as a consequence act as a stress raiser. Additionally, tensile residual stresses are generated and the microstructure of the weld may not reflect the properties of the base material.

The current repair method is to weld patches onto the cracked area, but it only helps temporarily as the repair welds the properties of the surrounding material and causes distortions due to non-uniform heating. The belt must be discarded and replaced, which limits the lifetime of the belt.

OBJECT OF THE INVENTION

The object of the invention is to eliminate the disadvantages 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 can improve yield stress on 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 an existing conveyor belt which is already in use.

SUMMARY OF THE INVENTION

According to an aspect of the invention, the present invention provides a method for enhancing the fatigue durability of 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 plurality of holes arranged into a plurality of groups of perforations to enable the flow of gas used in the sintering process. According to the invention, the conveyor belt is treated to create compressive residual stresses at the 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 plurality of holes arranged into a plurality of groups of perforations to enable flow-through of the gas used in the sintering process. The conveyor belt includes compressive residual stresses at the 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 fatigue 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 can improve yield stress on the material of the conveyor belt, which may alleviate problems with local yielding. The method of invention can be implemented to newly manufactured conveyor belts in connection 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 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 only in regions of groups of perforations and in regions of weld seams while other areas of the conveyor are left untreated.

In one embodiment of the invention, in the method, the treatment to create a compressive residual stress is selected from a group of treatment processes including shot peening, Ultrasonic hammering, laser shock peering. 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 a shot (round metallic, glass, or ceramic particles) with a force sufficient to create plastic deformation. The plastic deformation induces a residual compressive stress in a fine surface along with a tensile stress of smaller magnitude in the interior. Surface compressive stress conferencing 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. Ultrasonic treatment can result in controlled residual compressive stress, grain refinement and grain size reduction. Low and high cycle fatigue 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 conventional 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 regions of groups of perforations.

In one embodiment of the invention, the conveyor belt includes compressive residual stresses at the surface of the conveyor belt in regions of the weld seams.

In one embodiment of the invention, the material of the conveyor belt is selected from stainless steel grades including: ferritic chromium-alloyed stainless steel, austenitic-martensitic Precipitation hardened 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, a long edge of a similar adjacent second plate element being connected to each long edge, and two short edges in the longitudinal direction 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 perforations are rectangular and elongated, extending in the direction of the conveyor belt, the groups of perforations being parallel to each other and spaced from each other by a first Imperforated area.

In one embodiment of the invention, the groups of perforations are subdivided into a number of subgroups of perforations spaced from each other by a second Imperforated area.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings that are included to provide further understanding of the invention and to form part of this Specification illustrate the embodiments of the invention and together with the description help to explain the principles of the invention. In the drawings:

Figure 1 is a plan view of a section of an assembly belt according to the invention,

Figure 2 shows the detail P of Figure 1,

Figure 3 schematically shows a cross-section of a conveyor belt treated to include compressive residual stresses on its surfaces and distribution of tensile and compressive stresses in a situation where no load is exerted, and

Figure 4 shows the cross section of Figure 3 in a situation where the 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 elements 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 sintering process. The arrangement of holes 4 into groups 5 and subgroups 5 'substantially corresponds to that disclosed in WO 2009/022059 A1.

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 direction x of the conveyor belt 1. The short edges are parallel and spaced from each other by a distance Defining the width L of the conveyor belt. The groups 5 of perforations are rectangular and elongated, extending in the direction of the conveyor belt. The groups 5 of perforations are parallel to each other and spaced from each other by the first Imperforated area 10. The groups 5 of perforations are further subdivided into a number of subgroups 5 'of the perforations spaced from each other by a second Imperforated area 11.

The outer side is the conveyor belt 1 in its Endless 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 stress at the outer side of the conveyor 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 on the surfaces of the belt can be achieved by subjecting the regions to 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 austenitic-ferritic duplex stainless steel.

In the perforated region 5, 5 ', the holes 4 act as stress raisers, meaning that local stress is significantly higher than applied stress. Weld seams 3, on the other hand, always have high tensile residual stresses that are caused by restricted thermal expansion of the weld during weld deposition and cooling. By implementing the method of invention, the harmful tensile residual stresses in the weld can be alleviated, or the effective 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., shot peening, Ultrasonic hammering, laser shock peering), the underlying metal contracts, the free movement of the surface, and the 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 the perforations have been treated so as to create compressive residual stresses. Figure 3 shows the stress distribution inside the steel material of the belt in the unloaded condition that 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 distribution of Figure 3 under 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 even on the convex side (upper side in Figure 4) and the fatigue failures do not occur. The center is under tensile stress and the concave side (lower side in Figure 4) is in compression. The belt structure can be loaded until the convex side is in tension, but this tensile stress would be far less than what it would be if the belt structure was not treated to include the compressive residual stresses at the surfaces.

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

Claims (14)

A method for improving the fatigue resistance of a belt sintering conveyor belt (1), said conveyor belt consisting of a plurality of rectangular steel plate elements (2) welded successively to each other by welds (3), each plate element (2) having a plurality of holes (4) a plurality of perforation groups (5) to allow gas flow through the sintering process, characterized in that the material of the conveyor belt (1) is selected from stainless steel grades: ferritic chromium alloy stainless steel, austenitic-martensitic stainless steel, rust-free stainless steel, duplex steel, and the conveyor belt (1) is treated to provide compression residual stresses on the conveyor belt surface at least in critical areas susceptible to fatigue fracture. Method according to Claim 1, characterized in that the conveyor belt (1) is subjected to a compression residue to produce tension on the surface of the conveyor belt in the areas of the perforation groups (5). Method according to Claim 1 or 2, characterized in that the conveyor belt (1) is treated to create tension residual pressures on the conveyor belt surface in the areas of the weld seams (3). Method according to one of Claims 1 to 3, characterized in that the surface outside the conveyor belt (1), the outer surface of which is repeatedly subjected to tensile stress in operation, is subjected to compression residue to exert tension outside the conveyor belt. Method according to one of Claims 1 to 3, characterized in that the surfaces of both sides of the conveyor belt (1) are treated in order to create tension residuals on both sides of the conveyor belt. Method according to one of Claims 1 to 5, characterized in that the conveyor belt (1) is subjected to substantially only the areas of the perforation groups (5) and the weld seams (3) to obtain compressive residual stresses and the other areas of the conveyor belt are left untreated. Method according to one of Claims 1 to 6, characterized in that the treatment to achieve compressive residual stresses is carried out by ball blasting, ultrasonic hammering, laser blasting, etc. Method according to one of Claims 1 to 7, characterized in that the conveyor belt (1) is processed in a flat form to achieve said compressive residual stresses; the free ends of the conveyor belt are welded together by an installation weld to form a conveyor belt in the form of an endless loop, and thereafter the installation weld is treated to provide residual compression tension in the region of the installation weld. A belt sintering conveyor belt (1), the conveyor belt consisting of a plurality of rectangular steel plate elements (2) welded successively to each other by welds (3), each plate element (2) having a plurality of holes (4) arranged as a set of perforations. (5), to allow gas flow through the sintering process, characterized in that the material of the conveyor belt (1) is selected from stainless steel grades: ferritic chromium alloy stainless steel, austenitic-martensitic precipitated stainless steel, austenitic stainless steel, austenitic stainless steel , and the conveyor belt (1) contains residual compressive stresses on the surface of the conveyor belt at least in critical areas prone to fatigue fracture. Conveyor belt according to Claim 9, characterized in that the conveyor belt (1) contains tension residual stresses on the surface of the conveyor belt in the areas of the perforation groups (5). Conveyor belt according to claim 9 or 10, characterized in that the conveyor belt (1) contains tension residual stresses on the surface of the conveyor belt in the areas of the weld seams (3). Conveyor belt according to one of Claims 9 to 11, characterized in that each plate element (2) comprises two long edges (6, 7) in the lateral direction (y) of the conveyor belt, which are parallel to each other and spaced apart from each other. a long edge (6, 7) being joined by a long edge of a second adjacent plate element, and two short edges (8, 9) in the longitudinal direction (x) of the conveyor belt, which are spaced apart, corresponding to the width (L) of the conveyor belt. Conveyor belt according to one of Claims 9 to 12, characterized in that the perforation groups (5) are rectangular and elongated and extend in the direction of the conveyor belt, which perforation groups (5) are parallel to each other and at a first unpunctured area (10). Conveyor belt according to Claim 13, characterized in that the perforation groups (5) are divided into a number of perforation subgroups (5 ') which are spaced apart from one another in the unperforated area (11).