27 MnSi 5 ve 23 MnNiMoCr 5 4 kalite zincir çeliklerinin mekanik ve aşınma özellikleri
Mechanical and wear properties of 27 MnSi 5 and 23 MnNiMoCr 5 4 quality chain steels
- Tez No: 21966
- Danışmanlar: DOÇ. DR. HÜSEYİN ÇİMENOĞLU
- Tez Türü: Yüksek Lisans
- Konular: Metalurji Mühendisliği, Metallurgical Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1992
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 86
Özet
ÖZET Bu çalışmada maden konveyörlerinin hareket ettirilmesinde kullanılan zincirlerin yapım malzemesi olan 27 MnSi 5 ve 23 MnNiMoCr 5 4 kalite çelik çubukların sabit bir sıcaklıktan (900°C) su verme ve çeşitli sıcaklıklar da temperleme (300, 400, 500, 6008C) sonucu mekanik özelliklerin ve aşınma direncinin değişimi incelenmiştir. Bu amaçla, su verilen ve temperlenen bu iki farklı bileşimdeki çeliğin metalografik etütleri yapılmış, su verme ve temperlemeyle sertliklerinin değişimi ölçülmüş, çekme ve darbe deneyleriyle çeşitli mekanik özellikleri belirlenmiştir. Daha sonra, bu çeliklere iki elemanlı abrasiv aşınma deneyleri yapılmış ve aşınma oranları ve aşınma dirençleri hesaplanarak aşınma davranışları belirlenmiştir. Malzemelerin 900°C'da ostenitlenip su verilmesiyle 27 MnSi 5, 23 MnNiMoCr 5 4 ' den daha yüksek sertliğe sahip olmasına rağmen temperlemeyle bu sertliğini ve mukavemetini hızla kaybettiği ve »350°C'un üzerindeki temperlemelerde 23 MnNiMoCr 5 4 ' den daha düşük sertliğe ve mukavemete sahip olduğu tesbit edilmiştir.“Çekme mukavemeti x Uzama”gözönüne alındığında, genelde tüm temperleme sıcaklıklarında 23 MnNiMoCr 5 4 daha yüksek tokluğa sahip olduğu görülmüştür. 350 °C' a kadar temperlemelerde 27 MnSi 5 'in darbe enerjisi («15 J), 23 MnNiMoCr 5 4'ün darbe enerjisinden («50 J) daha düşük olduğu, 350°C'un üzerindeki temperle melerde her iki malzemenin darbe enerjisinde hızlı bir artış başladığı belirlenmiştir. Gerek 27 MnSi 5 ve gerekse 23 MnNiMoCr 5 4'ün aşın ma dirençleri temperleme ile azalmaktadır. Ancak, 27 MnSi 5 'de =20-40 HRc sertlik değerlerinde aşınmanın sertlik değişiminden pek etkilenmediği, sertliğin 40 HRc'nin üzerinde olduğu durumda artan sertlikle aşınma direncinin arttığı görülmüştür. 23 MnNiMoCr 5 4 ise temperleme sıcaklığının artmasıyla hem sertliğin hem de aşınma direncinin azaldığı tesbit edilmiştir.
Özet (Çeviri)
Round link chains are widely used in the anchors, mining conveyors and lifting machines. Besides carrying the loads safely, they may also provide the turning of cogwheels. Under service conditions they are generally subjected to tension and bending stresses. However, chains utilized in abrasive environment such as mining, should have high wear resistance, besides the properties required in the standards. Round link chains are produced from steel bars and wires. The first step in the manufacture of the round link chains is to form round links from the bars or wires with a rounding process. The tips of the round links is generally joined by flash welding. Another production method of round link chains is“casting”. Chain production by casting is carried out by known steel casting methods. Chains are generally normalized after welding process. If high strength is required, quenching and tempering heat treatments are applied. Under service conditions, wear is one of important factors that effects the failure of the chains. For example, wear often limits both the life and the performance of the chains used in mining conveyors. Wear is the loss of material from the surface by transfer to another surface or the creation of wear debris. Serious defects are created in the surface. These surface defects may act as foci for other types of damage, such as fatigue or stress-corrosion. There are four primary wear mechanisms: Adhesive Wear, Abrasive Wear, Spalling or Pitting and Chemical or Corrosive Action. The correct solution to a wear viproblem may depend strongly on the identification of a spesific basic mechanism. Applying a thin film boundary lubricant has little beneficial effect if the damage is caused by the presence of abrasive particles, but is extremely effective if the damage is caused by adhesion. Wear is affected by a variety of conditions, such as the type of lubrication, loading, speed, temperature, materials, surface finish, and hardness. In the wear cases, usually one type of damage is predominant. Adhesive wear is the most common type of wear. It contributes little to the occurrence of sudden failures. Adhesion results from the welding together and subsequent shearing of asperities which occurs when two metal surfaces are slid past each other. Adhesive wear failures are usually caused by the improper selection of materials or a malfunction of the lubrication system. Pitting is the result of the fatigue failure of the surface metal. Repeated application of relatively low stresses may result in numerous pit-like cavities in the metal surface. The characteristics of surface fatigue damage are different from those of ordinary fatigue. One primary difference of bulk and surface fatigue is that no apparent endurance limit exist; that is, there is no stress level below which the material remains unaffected by surface fatigue damage. Fretting is the most common form of corrosion- assisted wear. Fretting or f retting-corrosion, is due to a slight oscillatory motion between two mating surfaces under load. It manifests itself as pits in the surface surrounded by oxidation debris. Although adhesive wear is the most common form of wear damage, abrasive wear is more dangerous. It may occur suddenly with the introduction of a contaminant. It produces high wear rates and catastrophic failure of a system. Abrasive wear occurs when two surfaces are in sliding contact. One of them is harder and rougher than the other. Similar damage may also result, when hard, abrasive particles are embedded in a softer matrix. The damage occurs because of a plowing action: the harder particles or asperities create grooves or furrows in the softer material. viiThe material formerly contained in the furrows is transformed into wear particles that are usually loose and nonadhering. This loss of material from the surface explains the high wear rates of abrasive wear. There are three predominant types of abrasive wear: gouging, grinding and erosion. Gouging usually results in massive physical deformation of the surface. It is the result of an abrasive of relatively large diameter, which is driven in and along the surface under loading. Abrasive wear due to grinding occurs when two surfaces are in sliding contact and abrasive grains are present between them. The grains may be fixed to one surface or to an integral part of that surface (two-body wear), or loosely held between the surfaces {three-body wear). According to some scientists, two-body wear does not occur if the particle is small or softer than the sliding members of the couple. Abrasive wear occurs when the abrasive grains are fractured under load producing sharply faced fragments which remove material by plowing and scratching the surface. Erosive wear occurs as a result of the impingement of abrasive grains suspended in a fluid; gaseous or liquid. Bach contact produces a small scar at the metal surface. In some situations, the medium itself maybe corrosive and may contribute to the overall degradiation of the surface. Applications involving gouging are characterized by extremely high wear rates. The other mechanisms do not generate such high wear rates. To minimize the abrasion, the hardness of the surface must be greater than that of the abrasive. The microstructure of a steel is a dominant factor influencing its wear resistance. Therefore, it is necessary to classify steels according to their microstructure: martensitic steels, pearlitic steels, and austenitic steels. viiiIn martensitic steels, carbon content, austenitizing temperature, tempering temperature and alloying elements are important. Abrasion resistance of martensite improves at a fairy rapid rate up to about 0.7% C. Further increases in carbon content tend to become less effective. Steels with over 0.7% C will show best abrasion resistance when their pro-eutectoid carbides are in solution. The solution of these carbides is a function of the austenitizing temperature used during heat treatment. As the austenitizing temperature increases, the abrasion resistance of the martensitic steels increases. The use of tempering temperatures up to 200°C on the high carbon martensitic steels visibly improves their resistance to spalling and breakage without measurably lowering their wear resistance, eventhough a slight drop in hardness often is evident. When the high carbon martensitic steels are tempered at temperatures above about 430 °C, there is substantial improvement in their thoughness; but this is accompanied by a rather serious loss in abrasion resistance. To produce a martensitic structure in a heavy-section casting, the steel must have high hardenability, which in turn requires the use of alloying elements. The minimum alloy requirements depend principally on the austenitizing temperature and the rate of quenching during heat treatment. Further alloy additions over this minimum have some interesting effects on the abrasion resistance of the steel. These appear to be associated to the austenite-retaining characteristics of each alloying element. In the wear tests, the wear resistance of the material is generally calculated by measuring the weight loss due to wear. Other methods are thickness loss, trace (mark) change, radioisotop methods. In this study, the effect of heat treatment on the mechanical properties and abrasive wear resistance of 27 MnSi 5 and 23 MnNiMoCr 5 4 quality steel bars used in the manufacturing of round link chains were investigated. Samples taken from the steel bars were water-quenched from various austenitizing temperatures and tempered at temperatures between 300-600 °C. Metallographic examinations were performed on the quenched and tempered steels to examine the microstructures. Specimens used for optical metallography were prepared using standard polishing techniques. ixTo determine the mechanical properties of the heat treated steels, hardness, tensile and impact tests were performed. Hardness of the heat treated samples were measured as Rockwell C. Tensile testing was carried out in an Instron Universal testing machine with a cross head speed of 0.5 mm/s. Charpy impact resistance were determined by V-notch specimens at room temperature. Five tests were carried out on hardness test and three tests were carried out on tensile and impact tests. Mechanical properties were determined as the average of these results. To measure the wear resistance of the materials at room temperature under 32. 7N load, a two body pin-on-disc wear tester was used. Abrasion tests were performed on cylindirical specimens, impinging vertically on a rotating disc coated with the appropriate abrasives. The effects of two kinds of fixed abrasives were studied using the rotating coated disc test: Al,Si oxides based and Al,Pe,Ti oxides based. Abrasion wear test specimens were machined from the heads of ruptured tensile tests specimens with 5mm in diameter. From the experiments, it was observed that by water quenching from 900°C, 27 MnSi 5 had higher hardness and strength than 23 MnNiMoCr 5 4. However 27 MnSi 5 had lost these hardness and strength rapidly with increasing tempering temperature and it had lower hardness and strength than 23 MnNiMoCr 5 4 above about 350°C. When“tensile strength x elongation”was taken into account as toughness, quenched and tempered 23 MnNiMoCr 5 4 steel had higher toughness than 27 MnSi 5 at all tempering temperatures studied. It was found that tempering up to about 350 "C the impact energy of 27 MnSi 5 («15 J) was lower than that of 23 MnNiMoCr 5 4 («50 J). Tempering above 350*C, impact energies of the both steel increased rapidly. It was observed that the wear resistance of 27 MnSi 5 and 23 MnNiMoCr 5 4 decreased with decreasing tempering temperature due to decrease of hardness. However, such a relation was not clearly observed below 40 HRc in 27 MnSi 5.
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