Flap-X: Development of a New Compressor Reed Material

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1 Purdue University Purdue e-pubs Internatinal Cmpressr Engineering Cnference Schl f Mechanical Engineering 2016 Flap-X: Develpment f a New Cmpressr Reed Material Alexander Lf Research & Develpment, vestalpine Precisin Strip AB, Sweden, alexander.lf@vestalpine.cm Chris Millward Research & Develpment, vestalpine Precisin Strip AB, Sweden, chris.millward@vestalpine.cm Azhar Nawaz azharnawaz@vestalpine.cm Fllw this and additinal wrks at: Lf, Alexander; Millward, Chris; and Nawaz, Azhar, "Flap-X: Develpment f a New Cmpressr Reed Material" (2016). Internatinal Cmpressr Engineering Cnference. Paper This dcument has been made available thrugh Purdue e-pubs, a service f the Purdue University Libraries. Please cntact epubs@purdue.edu fr additinal infrmatin. Cmplete prceedings may be acquired in print and n CD-ROM directly frm the Ray W. Herrick Labratries at Herrick/Events/rderlit.html

2 1266, Page 1 Flap-X: Develpment f a new cmpressr reed valve material Alexander LOF 1, Chris MILLWARD 1, Azhar NAWAZ 1,2 * 1,2 vestalpine Precisin Strip AB, Research & Develpment, Munkfrs, Sweden 1 (Phne: , alexander.lf@vestalpine.cm) 1 (Phne: , chris.millward@vestalpine.cm) 1,2 (Phne: , azhar.nawaz@vestalpine.cm) * Crrespnding Authr ABSTRACT Flap-X is a new reed valve steel develped fr cmpressr applicatins. The develpment wrk was undertaken after cnsistent feedback frm custmers requiring higher fatigue strength and imprved damping prperties. The renewed interest in imprved valve strip is a direct result f the industry s latest trends twards energy efficient cmpressrs. Different steel grades were investigated during prcess develpment and Flap-X material, which is an ptimized versin f the well-knwn cmpressr steel UHB SS716, shwed the mst prmising results in terms f prductin feasibility and desired enhancements in ally design, metallurgy and mechanical prperties. Finished valve strip f each candidate material was used t quantify micrstructure, tensile strength and fatigue life. Fracture investigatins n the fatigue test samples were undertaken t detect any cntributing nn-metallic inclusins. Vibratin damping was measured at temperatures and frequencies crrespnding t perating cnditins f reed valves in a cmpressr. Impact fatigue strength was measured in a recently develped test rig. Bth fatigue life and damping prperties are imprved at increased tensile strength in all f the investigatins. 1. INTRODUCTION A majrity f cmpressr reed r flapper valves fr varius refrigeratin applicatins are made f UHB 20C (20C) r UHB SS716 (716) steel grades. Reed valves are als prduced using ther grades; hwever, mst f the prductin vlumes delivered by vestalpine Precisin Strip AB is f these tw steel grades. 20C is a high carbn steel similar t AISI 1095 whereas 716 is a mdified versin f AISI 420, which a martensitic stainless steel (Jhanssn and Nrdberg, 1984) The renewed interest in imprved valve strip is a direct result f the industry s latest trends twards energy efficient cmpressrs. This is reflected in recent market and ur direct custmer s feedback. Cmpressr OEMs are als required t develp better slutins as the result f Natinal and internatinal legislatin fr reducing energy cnsumptin. Fr this reasn, the manufacturers f refrigeratrs and air cnditins are priritizing the surcing f high strength materials frm multiple suppliers t cntinue t be cmpetitive whilst meeting the designer s expectatins. The designers f the cmpressrs want a flapper valve strip with imprved impact and bending fatigue prperties in additin t material s nise reductin at perating frequencies. Flap-X is a new flapper valve material that fulfils designer s requirements f higher fatigue and damping prperties. The material perfrmance has als been verified in varius cmpressr OEMs wn tests. 2. FIRST PAHSE The main fcus f the wrk was t enhance the functinal perfrmance f the stainless valve ally. Table 1 lists the type f materials investigated in the first phase f the wrk. A minimum f a pilt heat (apprx. 60 tns) f each versin was prduced. In certain instances multiple heats f ne specific versin were prduced t quantify the

1266, Page 2 reprducibility f the prcess rute. Standard 716 chemical cmpsitins were mdified by micr-allying with certain allying elements fr imprved hardening respnse in 716-A versin.

3 1266, Page 2 reprducibility f the prcess rute. Standard 716 chemical cmpsitins were mdified by micr-allying with certain allying elements fr imprved hardening respnse in 716-A versin. Secndary metallurgy treatment was fine tuned t change the chemistry f nn-metallic inclusins frm hard-brittle Al 2 O 3 t sft-defrmable SiO 2 in 716-B versin. 716-C was ESR prcessed (electr slag re-melting) after ingt casting t achieve a hmgenus micrstructure with reduced nn-metallic inclusins and segregatin. Hardening trials were undertaken in prductin. Finished valve strip samples were used fr all f the fatigue tests. Table 1: List f investigated materials Material Prcess-Rute Standard 716-A Mdified chemistry 716-B Mdified Inclusins 716-C Mdified-ESR 716-D Mdified A+B Lab hardening trials were undertaken fr prducing samples fr the damping tests as per a standard (ASTM-E756), which required thick samples t guarantee the reprducibility f the thermal lss factr measurements. Tensile and bending fatigue tests3 fr each specific material and tensile strength were aimed t be reprducible with ± 50 MPa tlerance t be able t spt the heat t heat variatins. Steel s functinal prperties fr a specific applicatin are dependent n large number f prcessing factrs. Flapper valve steel in its finished frm is essentially advanced high strength steel (AHSS), i-e: multiphase micrstructure cntaining martensite, retained austenite and retained secndary carbides with tensile strength and elngatin in the range f >1800 MPa & < 10 % respectively. Multiple strengthening mechanisms are used t achieve this range f strength and ductility resulting in ptimal fatigue prperties. Figure 1 in the fllwing schematic illustrates the basic steps in valve strip prcessing and shws the level f cmplexity that is invlved in achieving the desired set f flapper valve functinal prperties after strip prductin. Figure 1: Basic metallurgical steps and desired micrstructure during valve strip prcessing In rder t imprve the functinal prperties f the new valve strip, Flap-X, different appraches were tried. Micr-allying Mdifying the chemistry f nn-metallic inclusins Using advanced melting rutes after secndary metallurgy prcessing Optimal hardening f the strip

1266, Page 3 The main cnsideratin in trying different appraches t develping the new ally was nt t negatively affect the basic stainless flapper valve strip attributes.

4 1266, Page 3 The main cnsideratin in trying different appraches t develping the new ally was nt t negatively affect the basic stainless flapper valve strip attributes. These attributes are described in the fllwing list. Crrsin resistance Hardening respnse f each batch fr vlume prductin Precisin rlling capability Flatness after hardening A fine surface finish Fllwing metallurgical parameters were investigated after each prductin trial f a specific thickness f the valve strip. 1. Matrix Micrstructure 2. Secndary Carbides 3. Retained Austenite 4. Hardness and Tensile Strength 5. Flatness 6. Residual stresses A martensitic matrix with secndary carbides and retained austenite was the typical micrstructure in all f the trials as shwn in the fllwing figure 2. Figure 2: FEG-SEM EBSD image shwing a martensitic matrix (red), secndary carbides (yellw) and retained austenite (blue). Theretically, after the final martempering r s called hardening treatment, the micrstructure shuld be fully martensitic, hwever, due t the nn-ideal cling; it is always the case that sme f austenite is nt transfrmed int martensite. In flapper valve applicatins this retained austenite in the micrstructure is beneficial fr imprving impact fatigue strength. The effrt in the prductin trials has been t understand hw t vary and cntrl the retained austenite. As part f this wrk, strip with a range f tensile strength r hardness levels was prduced. The retained austenite was measured in all f the strip samples by using X-Ray diffractin technique (XRD). A rati, R, in the fllwing equatin 1 is defined t express the relatinship f micrstructure, hardening temperature and the resulting mechanical prperties. It s imprtant t nte that actual martempering f the valve strip carried ut in a cntinuus hardening line is far mre cmplex prcess with many mre variables than just austenitizing temperatures. Hardening speed, cntrlled cling in the quenching znes (emplying highly specialized il bath), tempering temperatures and the furnace s nline flatness mnitring cnstitute a few mre f these variables. Furthermre, different sets f parameters are used fr hardening different thicknesses in this cntinuus prcess. R = UTS / T (1) Where UTS is ultimate tensile strength T is austenitizing temperature f the prductin furnace ( C)

$T crss check the XRD measurements, the vlume fractin f retained austenite was als validated by using FEG-SEM EBSD image analysis studies (Research & Develpment, 2013).$

5 1266, Page 4 Generally, retained austenite increased with an increase f hardening temperature and tensile strength as shwn in the figure 3. The abslute value f retained austenite in a fully martensitic micrstructure is difficult t establish due the lw levels present and the resulting influence n measurement uncertainty (Gerge F. Vander Vrt, 2009). The average standard deviatin f all the retained austenite measurements using XRD was ±2. T crss check the XRD measurements, the vlume fractin f retained austenite was als validated by using FEG-SEM EBSD image analysis studies (Research & Develpment, 2013). Retained austenite in material with a 1810MPa tensile strength was cnsistently lwer than the higher tensile ptimized versins. Furthermre, a lab simulatin f the actual hardening prcess was undertaken using dilatmetry (Research & Develpment, 2015) t verify the high tensile and retained austenite bservatin using the 716-A material. Here again, the samples frm the study were measured fr retained austenite using the SEM EBSD vlume fractin methd. Figure 3: The effect f retained austenite n tensile strength Figures 4 & 5, shw a cmparisn f micrstructure frm tw simulatins where the retained austenite was varied frm 6 % t 14% respectively. The increment f retained austenite in the martensitic matrix in this case was the functin f higher hardening temperatures. Figure 4 & 5: FEG-SEM EBSD image shwing a martensitic matrix (red), secndary carbides (yellw) and retained austenite (blue). A lab simulatin f the ptimized material als shwed that increasing retained austenite in the matrix is mre r less unaffected by the medium and lwer hardening speeds, as shwn in the figure 6. The maximum retained austenite in the lab and prductin trials differs due t the different maximum temperatures emplyed in lab simulatin and prductin trials.

6 1266, Page 5 Figure 6: The effect f furnace speed n UTS/T rati at different tempering and lead bath temperatures. Secndary r retained carbides after martempering can be shwn t be directly related the hardening respnse and mechanical prperties f martensitic stainless steels fr valves. It is f critical imprtance t cntrl the density and the size f carbides in varius prcessing stages after ingt casting. The density r the percentage area fractin, measured using LOM image analysis, was inversely prprtinal t the tensile strength fr all materials after hardening. A minimum level f secndary carbides is aimed in the final micrstructure fr achieving a cmbinatin f higher tensile & yield strength in additin t increased retained austenite. Figure 7: The effect f tensile n secndary carbides The flatness f the valve strip after hardening is negatively influenced by an increase the tensile strength, mainly due t the higher thermal shck effect as shwn in the fllwing figure 8. The un-flatness f flapper valve is extremely undesirable as it may lead t refrigerant leakages in the end applicatin. In thery (Piruznia, 2014), the main factr influencing the flatness during martempering is quenching stage where it s critical t cntrl and reduce uneven temperature gradients that lead t strip distrtins. Fr the new material, Flap-X, bth cld rlling and martempering were carefully ptimized t prduce strip within internal and custmer specificatins. Figure 8: The crrelatin between flatness measured acrss the rlling directin and tensile strength

7 1266, Page 6 The fatigue strength f a steel ally is influenced by a great many factrs. Ally type, micrstructure, tensile strength, nn-metallic inclusin cntent and the presence f ther un-desired phases in the micrstructure are a few material related factrs that can influence fatigue strength. These factrs are generally influenced psitively by imprvements in prcess and quality cntrl f the steelmaking. Fr stainless flapper valve steels, the main the factrs are steel cleanness, tensile strength, ptimal micrstructure, damping and cmpressive residual stresses. Three different types f fatigue regimes, namely: axial, bending and impact fatigue influence the material s failure. These different fatigue mechanisms are assciated with specific mdes f applied stresses and frequencies. Fr the internal prduct develpment investigatins, the quantificatin f fatigue life in thin martensitic strip is a detailed and painstaking prcess (Research & Develpment, 2014). There are s many factrs that influence the results. The sample preparatin, test cnditins, analysis methd and the sample thickness are as critical as material3 metallurgy and mechanical prperties. Tensile r axial fatigue tests are a gd starting pint in determining the fatigue life imprvement. The critical vlume under stress, fr hur-glass shape sample in axial fatigue tests is much larger than impact and bending fatigue tests. Therefre the prbability f a nn-metallic inclusin assciated fracture becmes higher in this test. That leads t a quicker and definitive quantificatin f pilt r prttype materials fatigue strength which is helpful in early phase f the fatigue related ally develpment. On average, tensile fatigue life increased with tensile strength as shwn in the fllwing figure 9 fr all f the tested versins. Intensive fracture studies were carried ut in rder t crrectly identify the causes fr fracture initiatin. These investigatins shwed that mst f the fractures started frm sme type f inclusin. Fr example, it was fund that the batch with lw fatigue results (sectin 1 in the figure 9) had mre than twice the inclusin size cmpared t the batch with higher fatigue results (sectin 4 in the figure 9). Figure 9: Tensile Vs Fatigue Limit fr (each batch) measured in tensile fatigue tests at R- 0. The size f the bubble (Bubble diagram) shws the average inclusins size in the fracture samples accrding t Murakami area. (Murakami, 2002) 3. SECOND PAHSE The reverse bending fatigue tests were dne using a Kruse type machine. It cnsists f a variable speed electric mtr equipped with a variable pitch crank, cnnecting rd, specimen clamping device, mtr cntrls, specimen break detectr and a cycle cunter (see figure 10)

8 1266, Page 7 Figure 10: Reverse bending fatigue test setup (R=-1) As the thickness f the tested samples were <1.5 mm, light weight nyln cranks were used t minimize any dynamic influence f a heavy cnnecting rd. Finished strip samples frm each material were tested using a staircase apprach. Evaluatin f mean fatigue life was dne accrding t the SS IS (E) standard. The methd attempts t identify the mean fatigue limit, upper and lwer limit f the mean, at a prescribed cnfidence levels. The results are summarized in the fllwing figure 11. Flap-X with the mdified inclusins and chemistry shwed the highest imprvement cmpared t SS716 and the reference spring steel available in the market. Only the high ally stainless grade AEB-L and S-Cat L+ tl steel had a higher mean fatigue than Flap-X. Figure 11: Mean reverse bending fatigue f different versins (heats) f Flap-X and the cmparisn with ther materials. It s als seen that bending fatigue increases with an increase f tensile strength as shwn in the fllwing figure 12. Figure 12: Mean Fatigue and Tensile Strength f flap-x and reference materials.

9 1266, Page 8 Material s thermal vibratin damping and thermal stiffness prperties are als a critical factr in chsing the ptimal material fr a specific applicatin and in the design f the cmpressrs. Fr cmpressr and reed designers, it s imprtant t knw the damping prperties f the flapper valve material. The standard ASTM E-756 test methd was used t measure the vibratin-damping prperties f the hardened strip fr the lss factr, η, and Yung s mdulus, E, at different temperatures. Each test strip f specific thickness was munted in the cantilever beam test apparatus lcated inside an envirnmental chamber. It was then excited via randm nise and the respnse was measured via the piezelectric crystal. Frequency respnse was in the range f 70 Hz t 3000 Hz whereas the bending mdes 2 t 6 were measured fr each sample. Materials used fr damping measurements were 716-A & 716-D. Yung s mdulus, E, measured fr Flap-X decreases with an increase f the strip temperature as shwn in the fllwing figure 13. Figure 13: Yung s Mdulus (E) at different temperatures The damping lss factr f different materials als increases with an increase f temperature shwn in the fllwing figures 14. Figure 14: Damping Lss factr (η) f SS716 and Flap-X at different frequencies. The thermal damping lss factr in the figure 15 is defined as the average lss factr (different temperatures) measured fr each specific material at different frequencies. It was als cmpared with reference steel. Fr all the samples, average thermal damping value decreased with an increase f the applied frequency. Flap-X exhibited an increased thermal damping factr cmpared t the reference steel.

1266, Page 9 Figure 15: Thermal Damping Lss factr (η) f Flap-X Impact fatigue tests were cnducted in a newly develped test rig specifically designed fr impact fatigue testing, in a similar manner t

The new rig als uses better cntrl valves and regulatrs fr precisin cntrl f the cmpressed air pulses in terms f duratin, frequency and pressure.

10 1266, Page 9 Figure 15: Thermal Damping Lss factr (η) f Flap-X Impact fatigue tests were cnducted in a newly develped test rig specifically designed fr impact fatigue testing, in a similar manner t that utlined in experiments by previus wrk (Dusil & Jhanssn 1978). Reed mvement was measured in terms f lift, impact velcity and frce f each impact (Altunlu & Lazglu 2012). The new rig als uses better cntrl valves and regulatrs fr precisin cntrl f the cmpressed air pulses in terms f duratin, frequency and pressure. All this allws fr the pssibility f ptimizing the impact testing fr each material and reed design fr repeatable tests and accurate cmparisns. Figure 16: Flap-X valve in new test rig. The new material Flap-X exhibited a higher impact fatigue life cmpared t SS716 as shwn in the fllwing figure 17. This higher impact fatigue life perfrmance is primarily assciated with ptimized chemical cmpsitin f the ally and the nn-metallic inclusin cntent in additin t higher tensile strength. Cmpressive residual stresses are als an imprtant factr affecting the impact fatigue strength f flapper valve materials. Figure 17: Impact fatigue strength f Flap-X and SS716 measured n finished valves f mm thickness

11 1266, Page DISCUSSION The Flap-X develpment cnsisted f an exhaustive and thrugh cycle f theretical material design fllwed by full scale practical validatin f the theries. While this apprach is time cnsuming and expensive, in sme ways it can be cnsidered t be a superir apprach that can reduce the time t market fr new material. The reasn fr this is that the use f full scale trials, rather than small pilt plant scale trials, reduced the final industrializatin stage f the develpment. In the case f Flap-X, the design f the functinal specificatin fr the new prduct was defined by talking t cmpressr manufacturers abut the future demands f the material that they wuld like t be used fr their future cmpressr designs. The main feedback frm these cmpressr manufacturers was that fatigue life was the key imprvement area. This requirement was related t bth the pssibility f reducing the prbability f premature valve failure but als t give them the ptin t prduce valves frm thinner strip. Thinner reed valves are mre energy efficient and als quieter. These are bth prperties f cmpressrs that are bth desirable and are required frm a legislative perspective. Anther requirement was an imprvement in the damping capacity f the material. This is als related t the suppressin f nise (as well as being beneficial frm a fatigue life perspective). These functinal requirements were set against the cnflicting desire t minimize the fundamental level f changes frm the established and trusted materials they were currently using. In additin, there was a requirement t minimize material changes that might negatively influence the pssibility t prduce it in precisin strip frm. The final cnstraint n the degree f changes that culd be applied relates t the requirement t minimize changes that wuld have a detrimental effect n the reed valve manufacturer s prcess i.e. the final material shuld still be manufactured using the same basic tling and tumbling prcess. Against this backdrp, the develpment f Flap-X has managed t achieve significant imprvements in the desired functinal behavir f a material fr reed valves withut cmprmising either the prcessing cnstraints f the material during cld rlling t precisin strip r during the prductin f the reed valves. This material shuld allw cmpressr designers and manufacturers t achieve their gals fr mre energy efficient and quieter cmpressrs. This psitive utcme shuld cmplement ther appraches t achieving the recently agreed significant envirnmental imprvements. 5. CONCLUSION The fatigue life f reed valves is influenced by a number f metallurgical factrs during strip prcessing and it is essential t have a gd understanding and prcess cntrl f the strip prductin. The new ally, Flap-X, has an ptimal chemical cmpsitin, micrstructure and mechanical prperties, making it significantly superir t SS716 fr cmpressr applicatin. The imprved perfrmance f Flap-X has been verified in bth internal and external R&D tests, and als in cmpressr manufacturer s tests. Flap-X is being recmmended by cmpressr manufacturers fr demanding cmpressrs reeds with lng lifecycle requirements and lw nise requirements. ACKNOWLEDGEMENT This paper is published by the permissin f vestalpine Precisin Strip AB, Munkfrs, Sweden. The supprt f Stellan Erikssn in this wrk is greatly appreciated. REFERENCESS 1. B. Jhanssn, B. and Nrdberg, H. (1984) Prperties f high strength steels, Internatinal Cmpressr Engineering at Purdue, Paper ASTM- E756, Standard Test Methd fr Measuring Vibratin-Damping Prperties f Materials (2010), 3. Fatigue Tests, Internal Reprts (2014), Internal Reprts, Research & Develpment, vestalpine Precisin Strip AB 4. Gerge F. Vander Vrt (2009), Retained Austenite and Martensite, Industrial Heating Magazine September SEM-EBSD Investigatins (2013), Internal Reprts, Research & Develpment, vestalpine Precisin Strip AB

12 1266, Page SEM-EBSD Investigatins (2015), Internal Reprts, Research & Develpment, vestalpine Precisin Strip AB 7. Piruznia, P. and Jänssn P (2015) A Mathematical Mdel f Martempering f thin Martensitic Stainless Steel. The 6th Internatinal Cngress n the Science and Technlgy f Steelmaking (ICS2015). 8. Y. Murakami, Y. Effects f small defects and nn-metallic inclusins (2002), Metal Fatigue, 1 st Editin. 9. Dusil, R. and Jhanssn, B. (1978). Material Aspects f Impact Fatigue f Valve Steels. Internatinal Cmpressr Engineering Cnference at Purdue Paper A. C. Altunlu, I. Lazglu ; 2012

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