Strip steel for flapper valves

Transcription

1 Strip steel for flpper vlves C268121_Sndvik_S-343-ENG_214x300.indd

2 Our customers benefit from our experience nd production Strip steel for fl pper vlves is one of the most dvnced products in our progrm. To fulfi l the tough demnds plced on this steel, we utilise the experience we hve gined from more thn 100 yers of strip production. We lso hve our own metllurgy nd cn control the whole production chin from steel melt to fi nished product. This enbles us to produce steels with consistent nd relible properties, which gives more effi cient vlves with longer lifetime. In close co-opertion with our customers nd end users, we develop our steels continuously for the fl pper vlves of tody, nd tomorrow. Engineered steels for chllenging pplictions Flpper vlves must be cpble of working continuously for long periods without filure. This plces stringent requirements on mterils, especilly on bending nd impct ftigue strength. We cn supply strip steel for flpper vlves, which meet the most demnding customer specifictions. Sndvik offers progrm of crbon steel, Sndvik 20C, nd two mrtensitic, stinless chromium grdes, Sndvik 7C27Mo2 nd Sndvik Hiflex. All grdes hve good fltness nd surfce finish, excellent thickness tolernces, high purity levels nd good blnking properties. The stinless steels re recommended where corrosive conditions require superior performnce. The stinless steels, nd especilly Sndvik Hiflex, hve improved ftigue properties, compred with the crbon grde. These properties cn be utilised by the vlve designer. The vlve lift, for exmple, cn be incresed, resulting in more efficient compressor becuse of lower flow losses cross the vlve. Alterntively, the tongues cn be mde shorter with the sme vlve lift, which reduces the size of the compressor. This gives the end user mny dvntges nd leds to svings on both mteril nd spce. Strip steel cn be slit to the required width nd the edges cn be shved to very high stndrd of smoothness, if specified. Close width tolernce requirements cn lso be met in the slitting line. Stndrd thicknesses in ll grdes re vilble from stock for immedite delivery. Brief descriptions re given below. For more detiled informtion, plese refer to the enclosed dt sheets. Sndvik 20C Crbon steel with 1% C offering high ftigue strength. Sndvik 7C27Mo2 Mrtensitic, stinless chromium steel with 0.38% C, 13.5% Cr nd 1.0% Mo. This grde is the first choice when higher ftigue strength is required. It is lso suitble when the environment, e.g. moist ir, clls for stinless steel. Sndvik Hiflex This is the new genertion of Sndvik 7C27Mo2 nd the optimum choice for flpper vlve steel. Fine tuning of the micrometllurgy nd strip production hs resulted in mteril with even higher bending ftigue strength nd 25% increse in impct ftigue strength, compred with the norml Sndvik 7C27Mo2. Sndvik Hiflex is well suited to use in crbon dioxide compressors for ir conditioning, where higher pressures nd tempertures plce incresing demnds on the vlve mteril. Sndvik s flpper vlve steels re used in: Refrigertors nd freezers Air conditioning equipment Het pumps Brke systems Industril ir compressors Vcuum pumps Recommendtions re for guidnce only, nd the suitbility of mteril for specifi c ppliction cn be confi rmed only when the ctul service conditions re known. Continuous development my necessitte chnges in the technicl dt without notice. Sndvik nd Hifl ex re trdemrks owned by Sndvik Intellectul Property AB. 2 3 C268121_Sndvik_S-343-ENG_214x300.indd

3 The ftigue strength decides the compressor s service life A flpper vlve is subjected to bending stresses when it opens, nd impct stresses when it closes nd hits the vlve set. This lod pttern is repeted billions of times during compressor's lifetime nd, therefore, the ftigue properties re of utmost importnce. The illustrtion to the right shows wht hppens during one lod cycle. Ftigue strength is ffected by rnge of fctors: Microstructure Sttic strength Non-metllic inclusions Surfces Edge finish Residul stresses nd relxtion Sndvik s flpper vlve steel is produced with these fctors in mind so tht our customers cn rely on excellent nd consistent mteril properties, from delivery to delivery. See lso pge 6. In ddition to the bove fctors, the design nd mnufcture of the flpper vlves gretly influence ftigue properties. Stress conditions Impct Impct Mchine for impct ftigue testing, developed by Sndvik. Sndvik Hifl ex Sndvik 7C27Mo2 Sndvik 20C Impct intensity, index Lod cycles Impct ftigue strength for Sndvik s fl pper vlve steels, mesured s impct velocity, t filure rte of 50%. The fi gures cn be used for comprison of the grdes only. Stress, MP, 1 MP = 145 psi Impct ftigue Every time flpper vlve hits the stop when opening nd the vlve set when closing, the vlve mteril is ffected by the impct. This my eventully led to ftigue filure, which ppers round the top of the tongue s smll chippings, from which further crcks my propgte. The mteril s resistnce to impct ftigue is tested in mchine designed by Sndvik, which simultes compressor. The impct velocity is mesured with lser sensor, nd the test result is presented in our dt sheets s the mximum llowble impct intensity. As cn be seen in the digrm to the left there is considerble difference in impct ftigue strength between the grdes. ftigue Reversed bending ftigue strength (men stress = 0) is tested in our mechnicl testing lbortory. In the enclosed dt sheets, the ftigue limit t filure rte of 5% is given. In prctice, flpper vlves re often subjected to fluctuting bending stress, with the minimum stress = 0. In our dt sheets, we give vlues lso for fluctuting bending stress, which hve been clculted from our test vlues with the id of Goodmn digrm nd Gerber s formul. The ftigue strength vlues tht we present in our dt sheets re vlid t 20 C (68 F). However, tests hve shown tht the grde Sndvik 20C cn be used up to 150 C (300 F) nd Sndvik 7C27Mo2 nd Sndvik Hiflex up to 300 C (570 F) with retined ftigue strength properties. The Wöhler digrm on the next pge shows the reversed bending ftigue strength for Sndvik s flpper vlve steels. As cn be seen, the ftigue properties re high for ll three mterils, but the stinless grdes nd especilly Sndvik Hiflex exhibit the highest vlues. These grdes will, therefore, offer design dvntges in terms of higher llowble stress levels. Stress conditions in compressor vlves during one lod cycle Testing of reversed bending ftigue Ftigue limit for Sndvik s fl pper vlve steels in reversed bending t filure rte of 5%. Lod cycles 0 Reversed stress, men stress = 0 One lod cycle Stress Fluctuting stress, minimum stress = 0 0 One lod cycle =Amplitude 4 5 C268121_Sndvik_S-343-ENG_214x300.indd

4 Fctors influencing the ftigue strength Microstructure of crbon (left) nd stinless fl pper vlve steel times mgnifi ction. Microstructure After hrdening, the steel consists of very finegrined mrtensitic mtrix with evenly distributed crbides. The production process ensures tht no decrburistion tkes plce s this would diminish the strength properties. The microphotos show the structure of crbon steel nd stinless, mrtensitic chromium steel. Sttic strength Sttic strength cn be mesured by tensile testing, which reltes to the hrdness of mteril. A high tensile strength is required to enble the mteril to withstnd the ftigue forces flpper vlve is subjected to throughout its service life. If, however, the sttic strength of the mteril is too high this will mke it brittle, cusing crcks to form. Sndvik flpper vlve steels hve consistent sttic strength optimized to provide mximum ftigue strength. Predelivery testing nd fi nl inspection. Ftigue strength Tensile strength The ftigue strength s function of the tensile strength. Hrdening nd tempering of fl pper vlve steel is crried out on specilly built, modern production lines with in-line polishing equipment. The process results in: Optimum tensile nd ftigue strength A high degree of fl tness Excellent surfce fi nish. The picture shows one of the two decoilers feeding strip to verticl furnce. Non-metllic inclusions There is no such thing s n inclusion-free steel. However, creful selection of the rw mteril nd n optimum steel melting process enble us to produce flpper vlve steels with low contents of very smll nd well distributed non-metllic inclusions. The number of detrimentl inclusions is kept to minimum. Sndvik constntly monitors the level of nonmetllic inclusions, in order to ensure tht the ongoing process development will result in further improvement in the metllurgicl qulity. Surfces Ftigue frctures initite esily t surfce imperfections. Sndvik s flpper vlve steel is, therefore, delivered with very smooth, bright-polished surfces, virtully free from scrtches nd other detrimentl surfce defects. This reduces the risk of ctstrophic filures. Our internl product specifictions specify the mximum llowed vlues for scrtches nd other surfce defects, e.g. pits nd roll mrks. The surfce of mteril is, however, never bsolutely smooth, but hs some degree of roughness. The surfce roughness stndrd for our flpper vlve mteril is very high. In our dt sheets we present vlues for the roughness criteri R nd R mx. Edge finish The edge finish is s importnt for ftigue properties s the surfce finish. Initition of ftigue frctures cn be voided only if the edges re smooth. Strip for flpper vlves cn, therefore, be ordered with shved edges. See lso the heding Tumbling on pge 9. Residul stresses nd relxtion Residul stresses in the strip surfce nd edges hve strong influence on the ftigue resistnce of the flpper vlve. Residul compressive stresses re fvourble, s they counterct the pulsted tensile stresses in flpper vlve, when it deflects. Sndvik s flpper vlve steel hs such beneficil residul, compressive stresses in the surfces in the s delivered condition. Their intensity is further incresed during the tumbling opertion. The residul compressive stresses in the surfce my decrese somewht during service. This is referred to s relxtion nd it leds to decresed bending nd impct ftigue strength. Tests with Sndvik s two stinless flpper vlve steels hve shown tht they hve little tendency for relxtion. Sndvik Hiflex displys considerbly less thn Sndvik 7C27Mo C268121_Sndvik_S-343-ENG_214x300.indd

5 Tool mterils High-speed steel (AISI M2) nd cemented crbide re used mostly for the mnufcture of punches nd dies. In the mjority of cses, we recommend M2. Sndvik Hrd Mterils supplies suitble cemented crbide grdes. For the choice of grde, we recommend reference to specilist mnufcturer of qulity tooling. Fltness Our flpper vlve steel is supplied with mximum out-of-fltness of 0.20% of the nominl strip width, both cross nd long the strip. Blnking involves plstic deformtion of the lyer next to the blnked edge surfce. This my cuse outof-fltness in long nd nrrow vlves. However, the influence diminishes with incresing hrdness nd with decresing mteril thickness. Shrp tool edges, the recommended blnking clernce nd high pressure on ejectors nd strippers re the best mens of reducing this type of out-offltness. It cn be further reduced by het tretment nd tumbling. See below. Het tretment Out-of-fltness of the flpper vlves fter tumbling of the edges cn be sign of n unstisfctory tumbling opertion. In such cses, stress relieving of the vlves, before tumbling, my solve the problem. Tumbling cn then be expected to give the beneficil compressive stresses s described bove. Stress relieving should be crried out in fixture nd with holding time of minutes. The temperture should not exceed 250 C (480 F) for Sndvik 20C. For Sndvik 7C27Mo2 nd Sndvik Hiflex the mximum temperture is 350 C (660 F). Rotry method Vibrtory method Strt 3h 22h Strt 3h 6h Vlve design nd mnufcture A well designed component nd creful mnufcturing re essentils for the function nd service life of fl pper vlve. Only then, the excellent properties of the vlve steel cn be fully utilised. Vlve design When designing flpper vlves, it is importnt to void configurtions tht give rise to notch effects, such s smll rdii nd sudden chnges in crosssectionl re. Also, nrrow slots should be voided if possible, s it is difficult to chieve stisfctory rounding of the edges during tumbling. See lso below under Tool design. Blnking Hrdened nd tempered steel increses the lod, by up to 2.5 times, on the edge of blnking tool compred to n nneled steel. Becuse the mteril thickness in flpper vlves is usully thin, the lod on the tool edge will still be reltively smll. Stinless steel generlly cuses greter tool wer nd higher burrs thn crbon steel, but otherwise the blnking properties of the different steel types re similr. Tool design The design of vlves is closely linked to the design of punches nd dies. Co-opertion between the designers of the product nd the tool is, therefore, extremely importnt. The following points should then be tken into ccount: 1. Corner rdii should be mde s lrge s possible. 2. Slender tool sections should be voided. 3. Light-guge punches must be designed with cylindricl hed. 4. Hole dimeters should be lrger thn 3 x the strip thickness. For smller hole sizes, the punch should be ccurtely guided in spring-loded stripper. 5. The blnking clernce should be chosen so tht the punch does not brek nd the slug does not ride up. Clernce between punch nd die Usully shered edge, with nrrow sher zone nd wide, even brek zone with low burr height, is preferred. Also, low burr height must be combined with the longest possible tool life. Therefore, we recommend blnking clernce of 2 10% of the strip thickness, depending on the ctul strip thickness. A smller clernce is only justified when the hole slug tends to ride up with the punch. Exmples of edge profi les fter diff erent tumbling times. Tumbling After blnking, the cold-worked edges re rough nd need to be well rounded. This cn be chieved by tumbling, which removes burrs, smll blnking defects nd the cold-worked zone produced during blnking. It lso increses the intensity of compressive residul stresses in the surfces of the vlve. If performed for long enough, it gives the edges smooth, rounded contour. Not lest importnt, it converts the tensile stresses in the edge into compressive stresses. All this gretly enhnces the ftigue strength, nd improves the fltness of the vlve. Sndvik Hiflex responds well to tumbling nd this llows shorter process time thn for the other grdes. Reversed bending ftigue strength, MP The two digrms show the effect of surfce tretment on the reversed bending nd impct ftigue strength of Sndvik 7C27Mo2 nd Sndvik Hiflex. In both cses Sndvik Hiflex exhibits the most positive effect. During nd fter the tumbling opertion, components, especilly those mde of crbon steel, must be dequtely protected ginst rust, which would otherwise considerbly reduce their ftigue strength. A ph regultor contining rust inhibitor cn be used, or else rust inhibitor cn be dded seprtely. The components must be thoroughly clened nd rinsed fter tumb ling, nd then oiled. 0 Sndvik 7C27Mo2 Sndvik Hiflex Sndvik 7C27Mo2 Sndvik Hiflex Sndvik 7C27Mo2 nd Sndvik Hifl ex eff ect of surfce tretment on the ftigue strength in reversed bending. 5% filure rte. 1 MP=145 psi Impct intensity +25% +25% Sndvik 7C27Mo2 nd Sndvik Hifl ex reltive eff ect of surfce tretment on the impct ftigue strength. +64% 8 9 C268121_Sndvik_S-343-ENG_214x300.indd

6 Sndvik Group The Sndvik Group is globl high technology enterprise with 47,000 employees in 130 countries. Sndvik s opertions re concentrted on three core businesses: Sndvik Tooling, Sndvik Mining nd Construction nd Sndvik Mterils Technology res in which the group holds leding globl positions in selected niches. Sndvik Mterils Technology Sndvik Mterils Technology is world-leding mnufcturer of high vlue-dded products in dvnced stinless steels, specil lloys, metllic nd cermic resistnce mterils, s well s medicl implnts nd process plnts. Integrted production for consistent properties With our own metllurgy nd strip production, we control the whole production chin from steel melt to the fi nished product, using the ltest, most relible technology. Qulity Assurnce Sndvik Mterils Technology hs Qulity Mngement Systems pproved by interntionlly recognized orgniztions. We hold for exmple: the ASME Qulity Systems Certifi cte s Mterils Orgniztion, pprovl to ISO 9001, ISO/TS 16949, ISO 17025, nd PED 97/23/EC, s well s product pprovls from TÜV, JIS nd Lloyd s Register. Environment, Helth & Sfety Environmentl wreness, helth nd sfety re integrl prts of our business nd re t the forefront of ll ctivities within our opertion. We hold ISO nd OHSAS pprovls. Steel melting is high-tech process The melting of flpper vlve steel is precise opertion, where Sndvik s leding metllurgy, inclusion nd trce-element technology comes into its own. By creful selection of the rw mteril nd blncing the production process, very clen steel cn be chieved. This know-how leds to mterils, which offer the best possible performnce. The rt of strip mking We hve been in the flpper vlve business since they were first used, which mens tht we hve lerned the rt of rolling thin strip with excellent surfces nd perfect shpe. Our strip steel lso hs close dimensionl tolernces, prerequisite for sfe nd efficient functioning of vlves. A hrdening process for optimum properties Hrdening nd tempering of flpper vlve steel is crried out on specilly built, modern production lines. The hrdening process cn be closely controlled throughout in order to give suitble microstructure, resulting in optimum tensile nd ftigue strength. The strip lso receives high degree of fltness gurntee of tight vlves. An excellent surfce finish is lso chieved by in-line polishing. Our integrted production, combined with our own R&D, ensure ongoing product development nd our bility to meet customers individul requirements. The picture below shows cold rolling. We re where you need us to be An effi cient communictions nd logisticl network mkes Sndvik Mterils Technology n esy prtner to do business with. Around the clock, dvnced IT systems link our slesmen, order clerks, stock controllers, mrketing support personnel nd distributors to provide globl vilbility of our products. The whole world is our home mrket. And we cn shre technicl nd commercil informtion with our customers, using the ltest web techniques. Plese visit us on our website: C268121_Sndvik_S-343-ENG_214x300.indd

7 Sndvik Mterils Technology SE Sndviken, Sweden, Phone , Fx S-343-ENG Printed in Sweden: AB Sndvikens Tryckeri. Lyout: Ason&Co. Text: comotec. C268121_Sndvik_S-343-ENG_214x300.indd