Isıl işlem koşullarının düşük alaşımlı Ni-Cr-Mo çeliğinin mekanik özelliklerine ve aşınma direncine etkisi

Erhazar, Cem
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Fen Bilimleri Enstitüsü
Bu çalışmada, % 0. 28 C, % 0.28 Si, % 0.69 Mn,%1.75IMi, % 0.54 Cr, %0.39 Ma, % 0.022 S ve % 0.022 P bileşimindeki düşük alaşımlı nikel-krom-molibden çeliğinin mekanik özelliklerine ve aşınma direncine ısıl işlem koşullarının etkisi incelenmiştir. Yukarıda bileşimi verilen çelik, 900aC'de ostenitlenip, yağda ve suda su verilerek ve 900 C'den fırında 750 C' ye kadar kademeli alarak soğutulup, bu sıcaklıkta 1/2 saat bekletildikten sonra, suda ve yağda su verilerek sertleştirilmiştir. Gerek doğrudan, gerekse kademeli su verilmiş numuneler, 300,400,500 C'lerde 1/2 saat temperlenip suda soğutulmuşlardır. Bu ısıl işlemlerin uygulandığı numunelerle oda sıcaklığında, sertlik, çekme, darbe ve aşınma deneyleri yapılmış ve aşağıdaki sonuçlar elde edilmiştir- Doğrudan ve kademeli su verme yöntemlerinin ve temperleme İşleminin uygulandığı numunelerin sertlikleri arasında belirgin bir fark bulunmamaktadır. ii- Su verme ve temperleme işlemlerinden sonra yapılan çekme deneylerinde, suda su verilmiş olan numunelerin, yağ da su verilmiş olanlara göre daha düşük mukavemete ve sunekliğe sahip oldukları görülmüştür. Bunun nedeni, soğutma ortamının su olması halinde, numunelerde su verme sırasında mikroçatlakların oluşmasıdır. ili- Aşınma deneyleri sonucunda ise, temperleme sıcaklığı arttıkça, sertliğin azalmasına bağlı olarak, aşınma diren cinde bir miktar azalma gözlenmiştir.
Steels are iron-carbon alloys that contain 2% carbon. Although steel ia allay of iron and carbon, term of alloy steel is used to make clear other alloying elements in steel. Carbon is principal allaying element in steel. It improves hardness and strength. But ductility and weldability decrease with increasing carbon. Manganese is beneficial to surface quality in all carbon ranges. It contributes to strength and hardness, but to a lesser degree than does carbon. Increasing the manganese content decreases ductility and weldability. Nickel is used as an alloying element in constructional steels. In combination with chromium, nickel produces alloy steels with greater hardenability, higher impact strength than are possible with carbon steels chromium is added to steel to increase resistance to corrosion, oxidation and hardenability. Molybdenum increases hardness and wear resistance, decreases temper embrittlement., Molybdenum alloy steel castings have found extensive use in the mining industury as ball mill liners, grinding balls where the steel ia required to resist the erosive attack of materials. Sulphure. is very detrimental to surface quality, particularly in the lower carbon and lower manganese steels. Increasing phosphorus, increases strength and hardness and decreases ductility. Steels are classified into two groups according to their compositions. i - Plane carbon steels ii - Alloy steels Plane carbon steels, contain carbon as the principal alloying element. Other elements are present in small vx quantities, including those added far deoxidatian carbon steels can be classified according to their carbon content in to three boarding groups! i- Lou carbon steels 0.20^%C ii- Medium carbon steels 0.20 ^ % C < 0.50 iii- High carbon steels 0.50^,% C Alloy steels that are included this group can be classified two categories. i- Low alloy steels ii- High alloy steels Low alloy steels contain less than about 5% allaying elements. Aluminum, titanium and zirconium are used far the de oxidation of low allay steels. Low alloy steels are applied when strength requirements are higher than those obtainable with carbon steels, also have better toughness and hardenability than carbon steels. Types of low alloy steels are; carbon-manganese steels, manganese-molybdenum steels, manganese - nickel-rchromium-molybdenum steels, chromium-molybdenum steels and nickel-chromium-molybdenum steels, Nickel-chromium-molybdenum steels are represented by the ^300 series of steel in SAE. ^3k0 steel that belong to ^300 series is used automotive, aircraft crankshafts, connecting rods, gears. A lower carbon variety **330 is used heavy duty parts of rocfe drills and hollow propeller blade's... High alloy -steels contain more than about 5% alloying elements. These steels are widely used for their corrosion resistance in aqueous media and for service in hot gases and liquids at elevated temperatures ( > 650 C). These steels can be classified into two groups Î i- Corrosion resistant high alloy steels ii- Heat resistant high alloy steels Corrosion resistant high alloy steels are used as materials of construction for chemical processing equipment involving corrosion servise in aqueous at Vll temperatures normally below 315 C and aj^so used for special services at temperatures to 650. C. Heat resistant high alloy steel castings are extensively used for applications involving service temperature in excess of 650 C. Heat treatment that is used to improve mechanical properties of metal is the uidest method. In this procedure, steel is rapidly cooled from a suitable elevated temperature.^ This is called quenching. In this case, in spite of strength and hardness of steel increase, toughness and ductility of steel decrease. For this reason after quenching, steel is heated to a temperature below the transformation range and cooled at a suitable rate, primarily to increase ductility and toughness. The heating of steel to some temperature either in or above the transformation range, to put the iron in the gamma condition. Carbon and other elements are thus dissolved in the gamma iron, forming the solid solution austenite. The next step is to cool it at the proper rate to develop the desired structure. When during the cooling of a given steel, the austenite transform to an aggregate of ferrite and carbide or to martensite. With a slow rate of cooling, the transformation takes place at a temperature slightly lower than the lower transformation temperature (A^), and the resulting structure is characterized by coarse lamellar pearlite of low strength and hardness and high ductility. If the speed of cooling is increased as in quenching, the product of the austenite transformation at these low temperatures is not pearlite, but mantensite, the characteristic constituent of fully hardened steel. For quenching, many different media have been used. These are water, oil, brine solutions. As a quenching medium, plain. water approaches the maximum cooling rate attainable in a liquid. Its other advantages are that is inexpensive and readily available, is easily disposed of without attendant problem of pollution. Quenching oils can be divided into several groups. Based on their composition, quenching effect and use temperature, quenching oils are categorized as conventional, fast, martempering or hot quenching. The tgrm "brine" as applied to quenching, refers to aqueous solutions containing various percentages of salt. Brine offers the following advantages compared with plain water. Distorsion is less severe töan in water quenching. But brine viia quenching has some disadvantages. A hood may be needed to carry off the corrosive fumes emanate from brine baths, cost is higher than for water. There are five quenching methods that are used for quenching of steels. Direct quenching is the most midely used method of treating steel. Direct quenching practice is relatively simple and economical. Time quenching is used uihen the cooling rate of the part being quenched has to be changed abruptly at sometime during the cooling cycle. Selective quenching is used when areas of a part are selected to remain relatively unaffected by the queening medium. Streams of quenching liquid are directed at high pressure to local areas of the workpiece in spray quenching practice. Procedure of fog quenching utilizes a fine fog or mist of liquid droplets and the gas carrier as coaling agents. Tempering of steel is a process in which previously hardened steel is heated to a temperature below the transformation range and cooled a suitable rate, primarily to increase ductility and toughness. Steels are tempered by reheating after hardening to obtain specific values of mechanical properties and to relieve quenching stresses In alloy steels, the general effect of alloying elements on tempering is a retardation of the rate of softening especially at the higher tempering temperatures Alloying elements can be characterized as carbide-forming or non-carbide-f orming. Elements such as nickel, silicon, aluminum and manganese which have little or no tendency to occur in the carbide phase, remain in solution in the ferrite and have a minor effect on tempered hardness. The carbide-forming elements, chromium, molybdenum, tungsten, vanadyum, tantalum, niobium and titanium retard the softening process by formation of alloy carbides. Toughness and ductility of some alloy steels are affected by cooling rate after tempering with this at a certain tempering temperature interval a decrease in toughness and ductility happens. This is called temper embrittlement. Two varieties of embrittlement serve to reduce ductility during tempering. The first has been called 350 C embrittlement. It is a malady afflicting low allay steels that have been quenched to martensite, then tempered in the range 250-350 C. The problem can be avoided by adding sufficient silicon to the steel to inhibid formation of cementite. The second variety of embrittlement is community called temper embrittlement. Temper ebrittlernent IX occurs in allay steels having much lower yield strengths because of tempering at high temperatures 600-700 C n fallowed by slaw coaling through the range 600 to 350 C Molybdenum, titanium, zirconium delay the onset of embrittlement. Dear generally defined aa a progressive loss or displacement af material from a surface as a result of relative motion between that surface and another. Wear is closely related to friction and lubrication. There are many physical wear mechanisms. Adhesive wear occurs when one surface bonds to another with subsequent motion, rupture occurs in one of the materials. Adhesive wear takes place between two metals because of sliding friction and wear pieces breaks from soft metal. If two metals have the same hardness, wear can occur on two surfaces and if lubrication is perfect, adhesiv wear will decrease. Abrasive wear occurs when a hard pratube'rance on the surface of a material an a hard, loose particle trapped between surfaces plastically deforms or cuts as a surface as a result of motion. Fatique wear unlike abrasive and adhesive wear which explain loss or displacement af material resulting from a single interaction fatigue requires multiple interaction. In fatigue, a surface experiences repeated stress cycling. Leading to cracking.Fretting wear occurs as a result of corrosion- In this type of wear pits are seen an surface. The microstructure of a steel is a dominant factor influencing = its wear resistance. It is necessary to classify, the steels according to their microstructure before- the true effects of variations in composition, treatment or mechanical properties can be properly evaluated. The results are discussion on the effects af certain variable influencing wear will therefore be divided in to three microstructural classifications, i- Martensitic steels ii- Pearlitic steels lii- Austenitic steels Abrasion resistance of martensitic steel (0.7-1.5%Mn. 0.*»-0.8%Sif 0.a-1.5%Cr, 0.2-Q.5%Mo) improves at a fairly rapid rate up to about 0.7 percent carbon. Further increases in carbon content tend to became less effective. Usually the wear resistance Qf steels improves with increase in carbon content due to the formation of carbides. High austenitizing temperatures produce best abrasion resistane. For alloy additions of molybdenum, vanadiom, tungsten and chromium to steels, threshold alloy/carbon ratios exist where carbide, types other than M~G form (M7C,, MgC ve MC) The hardness of these carbides improves' wear resistance when tempered. The low alloy martensitic steels do not show any change in their hardnessvand abrasion resistance up to 2QÜ-°C However when the tempering temperature is above k30aC, there is a substantial improvement in toughness but, this is accompanied by a rather serious loss in abrasion resistance. Pearlitic structures tend to improve in abrasion resistance as their carbon content is increased up to about 1.0 percent and as their hardness is increased. Pearlitic chromium-molybdenum ' steels show no measurable loss in abrasion resistance when tempered up to about 530 C.üJhen tempered 650 C they show a loss. in both hardness and abrasion resistance. Abrasion resistance and other characteristics of several austenitic steel compositions are discussed. Adequate toughness and better abrasion resistance are obtained from 12 percent manganese steel. In this study, the effect of heat treatment on mechanical properties and abrasive wear resistance of a low alloy steel which has a chemical composition of;Q.28%C. 0.28% Si, 0.69%Mn, 0.39%Mo, 1.75%I\li, 0.45%Cr, 0.022%S and 0.022% P was investigated. Two quenching methods (Direct and step quenching) were applied to the steel. i- Direct Quenching: Quenched in oil and water from 900 C ii- Step Quenching: Cooled from 9D0aC to 750PC in furnace and quenched in oil and water. o After quenching steels were tempered between 200-600 C and cooled in water. To determine the effect of quenching methods and tempering temperature on mechanical properties and abrasive wear resistance, hardness, tension, Impact and wear tests were performed at room temperature. The XI following results were obtained from the expeniments. i- In as quenched condition, hardness of the steel is almost the same for all quenching methods. Tempering has similar effects on hardness for all quenching methods. ii- Direct quenching and step quenching in water, cause formation of microcracks on the tensile specimens. Thus oil and water quenched specimens have similar hardness after tempering, hut strength and ductility of the water quenched specimens are lower than those of oil quenched specimens. iii- Direct quenching in oil causea-higher toughness (tensile strength, x strain) than step quenching in oil, However step quenched samples in oil have higher impact resistance at room temperature. iv- Abrasive wear resistance of the heat treated steels (Direct quenching and step quenching) slightly decreases with increasing tempering temperature due to decrease of hardness.
Tez (Yüksek Lisans) -- İstanbul Teknik Üniversitesi, Fen Bilimleri Enstitüsü, 1992
Anahtar kelimeler
Metalurji Mühendisliği, Aşınma dayanımı, Isıl işlem, Mekanik özellikler, Çelik-metal, Metallurgical Engineering, Wear resistance, Heat treatment, Mechanical properties, Steel-metal