Alaşımlı beyaz dökme demirlerin mikroyapı analizi ve aşınma özellikleri

Abstract

Bu çalışmada çeşitli ısıl işlem koşullarının beyaz dökme demirlerin mikroyapısı, sertliği ve abrasiv aşınma özellikleri üzerindeki etkisi incelenmiştir. Bu amaçla beyaz dökme demirler 9kQ°Ç ile 11GODC arasında değişen sıcaklıklarda ostenitleme işlemine tabi tutulup çeşitli ortamlarda soğutulmuştur. Ayrıca Cr/C oranı k.2k olan beyaz dökme demirler 10DGaC de k saat ostenitlenip havada su verildikten sonra 20GaC ile 50QaC sıcaklıklar arasında temperlenmiştir. Isıl işlem uygulanan beyaz dökme demirlerde sertliğin artan ostenitleme sıcaklığı ile azaldığı, artan ostenitleme süresi ile de arttığı belirlenmiştir. Beklenildiği gibi temperleme ile sertlik azalmaktadır. Aşınma deneyleri yüksek krom-molibdenli beyaz dökme demirler ile Ni-Hard IV alaşımına uygulanmıştır. Aşınma deneyleri sonucunda Cr/C oranı 5.4 olan be¬ yaz dökme demirin en iyi aşınma direncine sahip olduğu saptanmıştır. Cr/C oranı k.2k olan beyaz dökme demirlerde uygulanan ısıl işlemler sertliği artırmasına rağmen aşınma direncini düşürmüştür. Söz konusu malzeme grubunda sertliği daha düşük olmasına rağmen döküm yapısındaki beyaz dökme demirin en iyi aşınma direncine sahip olduğu belirlenmiştir. Sertliği düşük olan Mi-Hârd IV alaşımının (42Rc) aşınma direncinin de beklenildiği gibi yüksek krom-mo- libdenli beyaz dökme demirlerden daha düşük olduğu saptanmıştır.

Cast irans, like steels, are basically alloys of iran and carbon. in relatian to the iron-iron carbide diagram, cast irans cantain a greater amount af carban than that necessary ta saturate austenite at -the eutectic temperature. Therefare cast irons cantain betueen 2 and 6.67 percent carban. Since high carban cantent tends ta make the cast iran very brittle, raast commercially manufactured types are in the range of 2.5 ta k percent carban. The best methad af classifying cast iran is accarding ta tnetallagr.aphic structure. There are faur variables ta be cansidered uhich lead ta the different types af cast iran,namely the carban cantent the allay and impurity cantent, the caaling rate during and after freezing and the heat treatment after casting. These variables contral the canditian af the carban and alsa its pysical form. The carban maybe cambined as iran carbide in cementite, ar it may exist as free carban in graphite. The shape and distributian af the free carban particles uill greatly influence the physical praparties af the cast iran. "The types af cast irans as fallous: üJhite cast irans in uhich ali the carban is in the co.mbined farm as cementite. Malleable east irans in uıhich mast ar ali af the carban is ün;cambined in the farm af irregular raund particles knaum as temper carban. This is abtained by heat treatment af uhite cast iran. Gray cast irans, in uhich mast ar ali af the carban is ûncambined in the farm af graphite flakes. vı Chilled cast irans in uhich a uhite cast-iran layer at the surface is combined uith a1 gray iran interiar. Madular cast irans in uhich, by special allay additians, the carban is largely ncombined in the form af campact spheraids. This structure differs fram mallHable iran in that its abtained directly fram sali- dificatian and the raund carban particles are more regular in shape. Allay cast irans, in uhich the praperties ar the structure at any af the abave types are modified by the additian.af allaying elements. The high allay uhite cast irans ara primarily used far abrasian resistant applicatians such as slurry pumps, classifier uear shces, brick malds impeller blades and liners far shat blasting equipment, refine disks in pulp refiners impact hammers, railer segment and ring segment in caal grinding milis, feed and lifter bars and mili liners in ball milis far hard-rack mining, pulveri"2ar, ralls tillage taals, bucket teeth, scraper blades,screu canveyors and grain handling equipmant. In~ these applicatians, uear af materials causes acanamical uaste and production losses. Far this reasan,the materials uhich are much harder and uıear resistant must be impraved. High Cr-Ma uhite cast irans are öne af the suitable graup af the malterials far this aim. Generally,in metallic materials, uear resistance is prapartianal ta hardness. Hauever uhile the hardness increases, the ductitily and the taughess decrease. The hardness af a material is related ta presence af carbides and ar hard matrix phase in the micrastructure. Since uıhite cast irans cantain carbides in hard matrix, they have superiar bulk hardness and uear resistance. There- fore they are suitable far uear related applicatians. High allay uhite cast irans are mast aften melted in electric furnaces, specifically electric are furnaces, and induction furnaces in uhich the precise cantral af compasitian and temperature can be achieved. vıı The high-chramium uhite cast irans possess a com- bination of excellent abrasion resistance tagethsr uith a reasonable degree of taughneas and the passibility af annealing a facilitate machining operations. For this reasan they are finding increasing use in many fields öf applicatians. Ali high allay uıhite irans cantain chramium ta prevent the farmatian af graphite upon saldificatian and ta ensure stability af the carbide phasses. Most alsa cantain nickel,, malybdsnum, capper ar cambinatians af these allaying-elements ta prevent the farmation af pearlite in the microstructure. The large valume fractian af primary and eutectic carbides in the microstructure pravidea the high hardness needed far crushing and grinding other materials. The metallic matrix supparting the carbide phase in these irans can be adjusted by allay cantent and heat treatment ta develap the praper balance betueen the resistance ta abrasian and the toughness needed ta ujithsta.nd repeated impact. The high allay uhite cast irans fail into tuıa majör groups; 1 - Nickel-Chromium uhite cast irons uhich are tua- chramium allays containing 3 to 5 % Ni and 1 ta k % Cr uith ane allay madification that cantains 7 ta 11 % Cr. 2- * The chramium-malybdenum uıhite irans cantaining 11 to 23 % Cr up ta 3 to Ma and often addi- tianally alloyed uıith nickel ar capper. * This graup camprises the 25 % ar 28 % Cr uhite irans, mhich may cantain other allaying canditians af malybdenum and/ar nickel up ta 1 R °/n ı ı -j /o. Surface deterioratian is impartant in engineering practice. it is often the majör factor limiting the life and the performance of machine components. üJear maybe defined as unintentioal deteriaration resulting fram use ar enviranment. it maybe considered essentially a surface phenamenan. The displacement and detachment af metallic particles fram a metallic surface maybe vııı caused by contact uıith anather metali ( adhesive ar metallic uıear), a metallic ör a nan-metallic abrasive (abraaian) ar maving liquids ar gases(era3İan). lılear is affected by a variaty af conditians, such as the type af lubricatian laading, spead, temperature materials, surfaca finish, hardness, campasition and campability of the mating parts involved. Idear involving a single type is rare and in mast cases bath abrasive and adhesive ujear accur. in adhesive uear tiny prajectians praduce friction by mechanical interference uıith the relative mation af cantacting surfanes increasing resistance ta further mavement. Abrasive uıear accurs uıhen hard particles slide ar rall under pressure accrass a surface, ar uıhen a hard surface rubs accrass anather surface. The üjear resistance af a material is generally calculated by measuring the ueight lass due ta uıear. Other methads are thickness lass, size changeaf impressian. in this study^ the effects af heat treatment an the micrastructure, hardness ana1 abrasive uear resistance af Cr-Ma cast irans uere investigated. in additian commerCial Ni-Hard IV allay alsa been used. The chemical campasitions af the materials used in this study are given in Table 1. Heat treatmants applied ta uıhite cast irans uıere as; quenching in variaus medium after austenitizing betüjeen 94aaC-11QO C far 20 minutes and k haurs after quenching the uıhite "cast irans uith Cr/C ratia af 4.24 uıere tempered betujeen 20D°C and 500°C the effect of austenitizing temperature and time Cr/C ratia af the uhite cast irans uere investigated, by micrastructural analysis and hardness measurements (Table 2). Dptical micrascapy and scanning electran micrascapy uere utilizied far micrastructural analysis. Hardness of the materials measured uıith Re hardness tester and micro hardness tester. ıx Hardness measurements performed on the heat treated white cast irons showed that the bulk hardness decreased with increasing austenitizing temperature, decreasing austenitizing time. The maximum hardness was obtained at austenitizing temperature of 940 C with the white cast iron which has Cr/C ratio of 4.50. Among the investigated compositions, increasing of Cr/C ratio increased the bulk hardness of the white cast irons. Cooling rate from austenitizing temperature has a severe effect on the microstructure. The carbide coarsening occurs as the cooling rate decreases. On the other hand, cracking succeptibility of the white cast irons increased with increasing cooling rate... White cast irons having a Cr/C ratio of 5.40 have both high hardness and superior wear resistance. Heat treatments performed on white cast irons with a Cr/C ratio of 4.24 showed that wear resistance decreased while hardness increased. Thus the higher wear resis tance was obtained in the as-cast structure which has the lowest hardness. At a constant hardness level, lowest wear resistance was obtained from the white cast iron which has the low amount of carbides segregated along grain boundaries. Commerical Ni-Hard 11/ alloy showed both the lower hardness and wear resistance compared to white cast irons due to the low matrix hardness. xn TABLE: 2. Heat Treatments Applied to White Cast Irons Were Investigated in Experiments. Abrasive wear tests mere performed as-cast and heat treated white cast irons and as-cast Ni-Hard IV alloy. Wear resistance of these materials were performed by a two-body pin-on disc tester at rnom temperature. Wear specimens were slide on grinding wheels, under the compression stress of 1.62 MPa. Sliding distance was selected as 50m and 1DGm. XI Hardness measurements performed on the heat treated white cast irons showed that the bulk hardness decreased with increasing austenitizing temperature, decreasing austenitizing time. The maximum hardness was obtained at austenitizing temperature of 940 C with the white cast iron which has Cr/C ratio of 4.50. Among the investigated compositions, increasing of Cr/C ratio increased the bulk hardness of the white cast irons. Cooling rate from austenitizing temperature has a severe effect on the microstructure. The carbide coarsening occurs as the cooling rate decreases. On the other hand, cracking succeptibility of the white cast irons increased with increasing cooling rate... White cast irons having a Cr/C ratio of 5.40 have both high hardness and superior wear resistance. Heat treatments performed on white cast irons with a Cr/C ratio of 4.24 showed that wear resistance decreased while hardness increased. Thus the higher wear resis tance was obtained in the as-cast structure which has the lowest hardness. At a constant hardness level, lowest wear resistance was obtained from the white cast iron which has the low amount of carbides segregated along grain boundaries. Commerical Ni-Hard 11/ alloy showed both the lower hardness and wear resistance compared to white cast irons due to the low matrix hardness.