Bazı sıcak iş takım çeliklerinin yüksek sıcaklık aşınma davranışları

Abstract

Bu çalışmada metal ve alaşımlarının sıcakta şekillendirme işlemlerinde kullanılan, DİN 1.2344 (% 3.5 C, % 5.1 Cr, % 1.2 Mo, %0.9 V), DİN 1.2365 (% 0.29 C, % 2.5 Cr, % 2.4 Mo, % 0.5 V) ve DİN 1.2367 (% 0.31 C, % 4.9 Cr, % 2.5 Mo, %0.5 V) kalite sıcak iş takım çeliklerinin, oda sıcaklığından 500 °C'a kadar değişen sıcaklıklardaki metal-abrasiv ve oda sıcaklığındaki metal-metal aşınma davranışları incelenmiştir. Her üç çeliğin oda sıcaklığından 500 °C'a kadar değişen deney sıcaklıklarında Al203 esaslı bir abrasiv aşındırıcı üzerinde, disk üzerinde pim aşınma deneyi esasına uygun olarak yapılan metal-abrasiv aşınma deneylerinde, tüm deney sıcaklıkları ve yükleme ağırlıklarında DİN 1.2367 kalite çeliğin en yüksek, DİN 1.2344 kalite çeliğin ise en düşük aşınma direncine sahip olduğu tespit edilmiştir. Oda sıcaklığından 500 °C'a kadar değişen sıcaklıklarda yapılan aşınma deneyleriyle, yüksek sıcaklık aşınma direncinin arttırılması açısından (özellikle 400 °C'ın üzerindeki sıcaklıklarda), Mo miktarının % 1.2'den % 2.5'e çıkartılmasının, Cr miktarının % 2.5'den % 5'e arttırılmasından daha etkili olduğu saptanmıştır. 64.5 HRC sertliğe sahip sementasyon çeliğinden yapılmış aşındırıcı disk üzerinde, üç farklı yükleme ağırlığında yapılan metal-metal aşınma deneylerinde, tüm yükleme ağırlıklarında DİN 1.2344 kalite sıcak iş takım çeliğinin en düşük, DİN 1.2367 kalite sıcak iş takım çeliğinin ise en yüksek aşınma direncine sahip olduğu tespit edilmiştir.

The movement of one solid surface over another is fundamentally important to the functioning of many kinds of mechanism, both artificial and natural. Surface deterioration is important in engineering practice. Its is often the major factor limiting the life and the performance of machine components. Wear maybe defined as unintensional deterioration resulting from use or environment. It maybe considered essentially a surface phenomenon. The displacement and detachment of metallic particles from a metallic surface maybe caused by contact with another metal (adhesive or metallic wear), a metallic or a non metallic abrasive (abrasion) or moving liquids or gases (erosion). Wear generally defined as a progressive loss or displacement of 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 mechanism. Generally several wear mechanism have been clasificated into four basic group. These are: Adhesive Wear, Abrasive Wear, Spalling or Pitting and Chemical or Corrosive Action. The correct solution to a wear problem may depend strongly on the identification of a specific basic mechanism. Applying a thin film boundary lubricant has little beneficial effect it the damage is caused by the presence of abrasive particles, but is extremely effective it 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. Adhesive wear occurs when one surface bonds to another with subsequent motion, rupture occurs in one of the materials. It takes place between two metals because of sliding friction and wear pieces breaks from soft metal. If two metals have the some hardness, wear can occur on two surfaces and if lubrication is perfect, adhesive wear will decrease. Although adhesive wear is the most common form of wear damage, abrasive wear is more dangerous. It may occur suddenly with the XIV introduction of a contaminant. It produces high wear rates and catastrophic failure of a system. Abrasive wear occurs when hard particles slide or roll under pressure across a surface, or when a hard surface rubs across another surface. In abrasive wear, material is removed or displaced from a surface by hard particles, or sometimes by hard protuberances on a counter face, forced against and sliding along the surface. Several qualifying terms can be used in describing abrasion. A distinction is often made between two-body abrasive wear and three-body abrasive wear. Two-body wear,is caused by hard protuberances on the counterface, while in three-body wear hard particles are free to roll and slide between two, perhaps dissimilar sliding surface. A drill bit cutting rock might experience two-body wear, while grit particles entrained between sliding surfaces, perhaps present as contaminant in a lubricating oil, would cause three-body wear. Wear rates due to three-body abrasion are generally lower than those due to two-body abrasion, although the various mechanisms of material removal in the two cases differ only in relative importance rather than in nature. In some situations, wear is caused by hard particles striking the surface, either carried by a gas stream or entrained in a following liquid. This type of wear is called erosion, often qualified as solid particle erosion or solid impingement erosion to distinguish it from the damage caused by the impact of liquid jets or drops. Mechanism of abrasive wear can involve both plastic flow and brittle fracture. Under some circumstances plastic flow may occur alone, but both often occur together, even in materials conventionally thought of as ideally brittle. Models for abrasive wear by each type of mechanism in isolation have been developed, but the models usually ignore the possibility of the other mechanism. In the first case the hardness of the counterface is an important factor in determining its wear resistance, whereas in the second the fracture toughness is more important, although hardness still place a role. 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 fretting-corrosion, is due to a slight oscillatory motion between two mating surfaces under load. It manifest itself as pits in the surface surrounded by oxidation debris. XV Under service conditions, wear is one the important factors that effects the failure of the tools. For example, wear often limits both the life and the performance of the tools used in hot work, cold work and high speed application. Tool steel is the name of the main producing materials of special tools used in production of the industrial parts and components by plastic deformation and cutting methods. In terms of steel classification principles, tool steels have been searched in a different category. Tools must resist to the faster and higher streses without deforming, breaking and wearing during the usage. Tools steels must keep their own properties at high temperatures. In order to get these properties all together in optimum conditions, special alloyed tool steels must be used. Tool steels were classified in different ways depending on classifying methods of countries on the earth. According to a very well known system in Turkey and used in Europe tool steel are divided into three major groups. These are,. Cold-work tool steels i. Hot-work tool steels ii. High-speed tool steels. In terms of properties, tool steels can also be classified into three groups.. Non-thermostable tool steels i. Semi-thermostable tool steels ii. High-thermostable tool steel. When choosing a tool steel performance of tool, the method of production, tolerance and the product should be considered as parameters. There are two wide categories for tool steels used in material shaping. These are cutting tolls and die materials. Many manufacturing operations involve punching, shearing, or forming of metals at high temperatures. Hot-work tool steels have been developed to withstand the combinations of heat, pressure and abrasion associated with such operations. Hot-work tool steels usually have medium carbon contents (0.35 to 0.45%) and chromium, tungsten, molybdenum, and vanadium contents of 6 to 25 %. These group of steels are divided into three subgroups. XVI . Chromium hot-work tool steels i. Tungsten hot-work tool steels ii Molybdenum hot-work tool steels. To obtain a well performance from tool steels in shaping processes the combination of strength, wear resistance and toughness should be maintained. The properties of hot-work tool steels can be shown as, i. A high thermostability to get enough a high strength against the plastic deformation which may occur on working surface of the die during the heating. ii A higher toughness than that of high speed tool steels. iii. A well thermal fatigue resistance. iv. A high stability and scale resistance against the affection between the containing surfaces with the shaped metal. Microstructure, hardness and thermostability are very effective factor in the abrasion resistance at high temperatures, in order to examine effect of microstructure and hardness on abrasion resistance, three type of hot-work steel were chosen. These steels are shown as, - DIN 1.2344 Hot-work tool steel - DIN 1.2365 Hot-work tool steel - DIN 1.2367 Hot-work tool steel Metal hardness is one of the most important characteristics for wear resistance. It is effected carbon content, alloying element and heat treating condition. Increasing the carbon content of steel, results in microstructural alteration that increases as-quenched hardness. For this reason wear resistance increases. In addition, increasing the austeniziting temperature, the abrasion resistance of steel increases. In a microstructure consisting of a carbides in a martensit matrix provide the resistance to abrasion. The amount, size and distribution of carbides in a steel microstructure have a distinct influence on wear resistance. For the most part, wear resistance increases as the amount or size of carbide particles at the wear surface increases. Also matrix hardness is important to wear resistance if hard microconstituents are widely dispersed in a matrix that is not hard enough to have good wear resistance of its own, the matrix may wear away rapidly leaving the hard particles projecting from the surface, where they can cut into a mating surface. In this study wear behaviors of DIN 1.2344, DIN 1.2365 and DIN 1.2367 quality tool steels which are used in forming metals and alloys at high XVll temperatures are investigated. The effect of high temperatures on those three hot-work tool steels are compared by tempering and pin-on disc wear tests (metal-abrasive and metal-metal) Finally the following results are obtained from tempering and wear tests. 1- Hardness measurements performed after double tempering of quenched steels at various temperatures showed that DIN 1.2344 quality steel has the lowest and DIN 1.2367 has the highest temper resistance. 2- DIN 1.2344 and DIN. 1.2365 quality steels had 52 HRC and DIN 1.2367 quality steel had 55 HRC hardness after tempering at 600 °C. 3- Mo and V based carbides are observed in microstructures of those three hot-work tool steels which are tempered at 600 °C. However, additionally Cr based carbides are present in the microstructure of DIN 1.2344 quality steel. 4- According to the metal-abrasive wear tests performed on AI2O3 coated abrasive paper at various temperatures and metal-metal wear tests performed on cemented steel discs (64.5 HRC) at room temperature, DIN 1.2344 quality steel has the lowest and DIN 1.2367 quality steel has the highest wear resistance. 5- Wear resistance of those three tool steel decreased with increasing temperature. However above 400 °C, increase of temperature has more detrimental effect on DIN 1.2344 quality steels Therefore it is concluded that at constant Cr content (5% Cr ) increase of Mo from 1.2% to 2.5% increases high temperature wear resistance of tool steels significantly. However, at a constant Mo content (2.5% Mo.) increase of Cr from 2.5% to 5% has no severe effect on high temperatures wear resistance.