AU2013328502A1

AU2013328502A1 - Method for enhancing fatigue durability of a conveyor belt of a strand sintering furnace, and conveyor belt

Info

Publication number: AU2013328502A1
Application number: AU2013328502A
Authority: AU
Inventors: Paavo LAIHONEN; Mari Lindgren
Original assignee: Outotec Finland Oy
Current assignee: Outotec Finland Oy
Priority date: 2012-10-09
Filing date: 2013-10-07
Publication date: 2015-04-02
Anticipated expiration: 2033-10-07
Also published as: FI124911B; ZA201503137B; AU2013328502B2; CA2885307A1; CN104812685A; SE1550426A1; EA028479B1; CN104812685B; CA2885307C; EA201590513A1; BR112015007803A2; WO2014057170A1; FI20126061A

Abstract

The invention relates to a method for enhancing fatigue durability of a conveyor belt (1) of a strand sintering furnace, and a conveyor belt. The conveyor belt is formed from 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 conveyor belt (1) 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. The conveyor belt (1) includes compressive residual stresses at a surface of the conveyor belt at least in critical regions which are susceptible to fatigue breakage. Thereby the fatigue durability of the conveyor belt is improved.

Description

WO 2014/057170 PCT/F12013/050971 1 METHOD FOR ENHANCING FATIGUE DURABILITY OF A CONVEYOR BELT OF A STRAND SINTERING FURNACE, AND CONVEYOR BELT FIELD OF THE INVENTION 5 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 re lates to a conveyor belt of a strand sintering fur nace. 10 BACKGROUND OF THE INVENTION Continuous strand sintering is used for agglomerizing pellets after pelletizing a concentrate powder, im proving the strength and the reactivity of the pel 15 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 20 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 25 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 sin tering zones of the furnace and to a stabilizing or equalizing zone, after which the bed of pellets fur 30 ther travels through sequential cooling zones. After travelling through the cooling zones, the sintered pellets exit the strand sintering equipment at its tail. 35 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 WO 2014/057170 PCT/F12013/050971 2 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 5 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 10 loads, cause damage that commonly limits the lifetime of the belt. Cycl'iclds (fatigue loads) are generat ed wLen tLe belt rotates around. the deflector rol s. Because thie perforations act asi stress risers,~- _a tigue cracks are cally initiate and. start to 15 g row This iS'eads to damage, especially in he perfo rated regIons. Fatgige occu rs when aal is ected to repeated lading and unloading and at least part of the loading cycle is tenie te loads are above a certain t hold, ro p cracks will 20 begin to form at -he surface, Evntall a crack will each a r a se and. the str u -cture wi- sud dJ en I y f racture. Lad r el Ia ted factors that influence the -a t igue lif arE forx-ampie stress amlitde diI mean S i part of the load cyce wi cause 25 fatigue e as crack surfaces are torn open ad e crack s able to Proceed. Theconvyor be is made by welding. Weld are problemItic iLn c loaded str;Lutures becaUse a 30 weld seam chang _es g tr loc all a nd as a conse que1C act as a stres raiser. Additional', tensile sesidua] stcesses ae gejneaued and the seam R mi cr u re may not attain the poperties of the base ma ter ial. 35 The current r epair mew:thod is Lo weld patches onto the cracked area, but it helps oniy temporarily as repai-r WO 2014/057170 PCT/F2013/050971 3 we ding impairs the pro pe1tie.S of thne s1u rrouning ma ter:cia.1 and caus distortions d to non-niform heaL i ng. Consequently, the belt must be discarded and re placed, which limits the lietm of th belt 5 OBJECT OF THE INVENTION The object of the invention is to eliminate the disad vantages mentioned above. 10 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 15 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 20 be achieved with leaner (and cheaper) stainless steel alloys. Further, it is an object of the invention to provide a method which may improve vield stress of the matter 25 f he convyor belI t, 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 30 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 35 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 WO 2014/057170 PCT/F12013/050971 4 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 5 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. 10 According to another aspect of the invention, the pre sent 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 15 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 pro cess. The conveyor belt includes compressive residual 20 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 25 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 30 it may improve yIeld stress C tne material of the conveyor belt, which may alleviate problems with local yieldin.q. The method of the invention can be imple mented to newly manufactured conveyor belts in connec tion to their manufacturing process. The invention can 35 as well be implemented to conveyor belts already in use to prolong the fatigue life of such conveyor belts.

WO 2014/057170 PCT/F12013/050971 5 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 5 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 10 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 15 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 20 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 25 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. 30 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, lasershock peen ing, Shot peening is a well-known cold working process 35 used to produce a compressive residual stress layer and modify mechanical properties of metals. It entails impacting a surface with shot (round metallic, glass, WO 2014/057170 PCT/F12013/050971 6 or ceramic particles) with a force sufficient to cre ate plastic deformation. The plastic deformation in duces a residual compressive stress in a peened sur face, along with tensile stress of smaller magnitude 5 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. U1rasonic hammering is a well-known 10 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 15 tigue resistance are enhanced. Further, laser shock peenina 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 20 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 25 free ends of the conveyor belt are welded together by an installation weld seam to form the conveyor belt in to an endless loop form. Thereafter, the installation weld seam is treated to create compressive residual stresses in the region of the installation weld seam. 30 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. 35 WO 2014/057170 PCT/F12013/050971 7 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. 5 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 10 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 15 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 20 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 25 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 30 perforations spaced from each other by a second imper forated area. BRIEF DESCRIPTION OF THE DRAWINGS 35 The accompanying drawings, which are included to pro vide a further understanding of the invention and constitute a part of this specification, illustrate WO 2014/057170 PCT/F12013/050971 8 embodiments of the invention and together with the de scription help to explain the principles of the inven tion. In the drawings: 5 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, 10 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 15 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 20 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. 25 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 30 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 35 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.

WO 2014/057170 PCT/F12013/050971 9 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 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 10 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 15 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 20 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 25 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, uLtirason-ic ImrlmeriJng 30 or laser shock pe enig etc., The material of the conveyor belt 1 may preferably be ferritic chromium-alloyed stainless steel (e.g. 3Cr 12), austenitic-martensitic precipitation hardened 35 stainless steel, austenitic stainless steel or austen itic-ferritic duplex stainless steel.

WO 2014/057170 PCT/F12013/050971 10 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 5 al stresses that are caused by restricted thermal ex pansion of the wel-,d seam, during wveld 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 10 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 15 of the surface, and compressive residual stresses are created on the surface. Figure 3 illustrates a cross-section of the conveyor belt in which both opposite surfaces of the regions 20 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 25 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 30 amd 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 fatigue failures do not occur. The center une a tensile stress and the concave 35 side (lower side in the 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 WO 2014/057170 PCT/F12013/050971 11 less than what it wouid be if the belt structure was no rea ed to include the compressie residual stres the sur faces. 5 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 10 within the scope of the claims.

Claims

1. A method for enhancing fatigue durability of a con veyor belt (1) of a strand sintering furnace, said 5 conveyor belt being formed from a number of rectangu lar steel plate elements (2) that are sequentially welded to each other by weld seams (3), each plate el ement (2) including a plurality of holes (4) arranged into a plurality of groups (5) of perforations to ena 10 ble the flow-through of the gas used in the sintering process, characteri zed in that the conveyor belt (1) is treated to create compressive residual stresses at a surface of the conveyor belt at least in critical regions which are susceptible to fatigue 15 breakage.

2. The method according to claim 1, c h a r a c t e r i z e d in that the conveyor belt (1) is treated to create compressive residual stresses at the surface of 20 the conveyor belt in regions of the groups (5) of per forations.

3. The method according to claim 1 or 2, c h a r a c t e r i z e d in that the conveyor belt (1) is treated 25 to create compressive residual stresses at the surface of the conveyor belt in regions of the weld seams (3).

4. The method according to any of claims 1 to 3, c h a r a c t e r i z e d in that the surface on outer 30 side of the conveyor belt (1), 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. 35

5. The method according to any of claims 1 to 3, c h a r a c t e r i z e d in that the surfaces of both sides of the conveyor belt (1) are treated to create WO 2014/057170 PCT/F12013/050971 13 the compressive residual stresses on both sides of the conveyor belt.

6. The method according to any of claims 1 to 5, 5 c h a r a c t e r i z e d in that the conveyor belt (1) is treated to create compressive residual stresses substantially only in the regions of groups of perfora tions (5) and in the regions of the weld seams (3), while other areas of the conveyor belt are left un 10 treated.

7. The method according to any of claims 1 to 6, c h a r a c t e r i z e d in that the treatment to create compressive residual stresses is implemented by shot 15 peening, ultrasonic hamm (e-i rig, laser shock Peening, etc..

8. The method according to any of claims 1 to 7, c h a r a c t e r i z e d in that the conveyor belt (1) 20 is treated in a flat form to create said compressive residual stresses; 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, and thereafter the installation weld seam is treated to 25 create compressive residual stresses in the region of the installation weld seam.

9. A conveyor belt (1) of a strand sintering furnace, the conveyor belt being formed from a number of rec 30 tangular steel plate elements (2) that are sequential ly welded to each other by weld seams (3), each plate element (2) including a plurality of holes (4) ar ranged into a plurality of groups (5) of perforations to enable the flow-through of the gas used in the sin 35 tering process, c h a r a c t e r i z e d in that the conveyor belt (1) includes compressive residual stress- WO 2014/057170 PCT/F12013/050971 14 es at a surface of the conveyor belt at least in criti cal regions which are susceptible to fatigue breakage.

10. The conveyor belt according to claim 9, c h a r 5 a c t e r i z e d in that the conveyor belt (1) includes compressive residual stresses at the surface of the conveyor belt in the regions of the groups (5) of the perforations. 10

11. The conveyor belt according to claim 9 or 10, c h a r a c t e r i z e d in that the conveyor belt (1) includes compressive residual stresses at the surface of the conveyor belt in the regions of the weld seams 15 (3) .

12. The conveyor belt according to any of claims 9 to 11, c h a r a c t e r i z e d in that the material of the conveyor belt (1) is chosen from stainless steel grades 20 including: ferritic chromium-alloyed stainless steel, austenitic-martensitic precipitation hardened stainless steel, austenitic stainless steel, austenitic-ferritic duplex stainless steel. 25

13. The conveyor belt according to any of claims 9 to 12, c h a r a c t e r i z e d in that each plate element (2) comprises two long edges (6, 7) in the lateral di rection (y) of the conveyor belt, which are parallel to and spaced from each other, the long edge of a 30 similar adjacent second plate element being connected to each long edge (6, 7), and two short edges (8, 9) in the longitudinal direction (x) of the conveyor belt, which are spaced from each other by a distance corresponding to the width (L) of the conveyor belt. 35

14. The conveyor belt according to any of claims 9 to 13, c h a r a c t e r i z e d in that the groups (5) of WO 2014/057170 PCT/F12013/050971 15 perforations are rectangular and elongated, extending in the direction of the conveyor belt, the groups (5) of perforations being parallel to each other and spaced from each other by a first imperforated area 5 (10).

15. The conveyor belt according to claim 14, c h a r a c t e r i z e d in that the groups (5) of perfora tions are subdivided into a number of subgroups (5') 10 of perforations spaced from each other by a second im perforated area (11).