Soğuk iş takım çeliklerinin sertlik ve mikroyapı özelliklerinin aşınma davranışlarına etkileri

Abstract

Bu çalışmada farklı bileşimlere sahip değişik amaçlar için kullanılan X210Cr12, 90MnCr\/8 soğuk iş takım çelikleri ve C35 alaşımsız makina yapım çeliğinin sertlik ve mikroyapı özelliklerinin aşınma dirençlerine etkileri incelenmiştir. Deneylerde kullanılan bu farklı bileşimdeki üç tip çelik östenitleme de su verme sonrasında çeşitli sıcaklıklarda temperlenmişlerdir. Su verilmiş ve temperlenmiş çeliklerin sertlik değerleri ölçülmüş ve temperleme sıcaklıklarıyla sertliklerindeki değişmeler belirlenmiştir. Bu çeliklerin optik mikroskop ile SEM'de metalografik incele meleri yapılmıştır. Bu çalışmalar sonucu sertlikleri ve mikrayapıları belirlenen çeliklerle aşınma deneyleri yapılarak çeliklerin aşınma dirençleri hesaplanmış, aşınma, davranışları belirlenmiştir. Östenitleme ve su verme sonrası en yüksek sertlik değerine(SGHRc) sahip olan x21DCr12 çeliği 300 G'a kadar temperleme ile sertlikte azalma gösterirken 400 C'daki temperlemede ikincil sertleşme göstermektedir, ve sertliğinde artış olmaktadır. 90mnCrV8 soğuk iş takım çeliği ve C35 alaşımsız makina yapım çeliğinde ise artan temperleme sıcaklığı ile sertliklerinde düşme olmaktadır. Bu sertlik düşüşü 300 C'a kadar temperleme sıcaklıklarında yavaş, bu sıcaklıktan daha yüksek temperleme sıcaklıklarında ise hızlıdır. Su verilmiş ve kullanım koşullarına uygun olarak temperlenmiş değişik bileşimlerdeki X210Cr12, 90mnCr\/8 soğuk iş takım çeliklerinin ve C35 alaşımsız makina yapım çeliğinin A1203 esaslı zım parada yapılan aşınma deneylerinde, sertliklerinin düşüşüne paralel olarak aşınma dirençleri de düşmüştür. X2lQCr12, 90MnCrV8 soğuk iş takım çeliklerinde 35-55HRc sertlik değerlerinde aşınmanın sertlik değişiminden pek etkilenmediği, sertliklerin 55 HRc'nin üzerindeki değerlerinde, artan sertlik değerine karşılık aşınma direncinin arttığı görülmüştür. C35 alaşımsız makina yapım çeliğinde, aşınma direncini ve sertliğin etkisi 50HRc değerinden sonra belirginleşmektedir. Aşınma-direncinde, malzemenin sertleşebirliğini etkileyen alaşım elementi miktarı ve ısıl işlem etkin rol oynamaktadır. Bu nedenle sert martensit ve çok miktarda çözünmemiş krom karbür den oluşan yapıya sahip X210Cr12 -soğuk iş takım çeliği en iyi aşınma direnci göstermiştir. Uygulanan yük miktarının artması aşınma miktarını arttırdığı için hesaplanan direncinde de pek önemli bir değişiklik olmamıştır. Farklı sertlik değerine sahip çelikler; kaynakla 62-64 HRc sertliğinde kromkarbür dolgusuyla hazırlanan diskte, birbirlerine yakın sertlikte olanlar, çok uzun olmayan çalışma süresince, yakın değerlerde aşınmışlardır. Metal diskteki bu metal-metal aşınması A1203 esaslı zımparadaki abrasiv aşınmaya kıyasla çok düşük değerlerde aşınma miktarları vermiştir.

Wear has gat a very big technological importance For this reason it has been examined very closely. If wear occurs; a system, this system's working per formance, and life are affetted. To elimnate all this. unwanted effect, lile have to control wear, at least, we have to know our metarial's wear behaviour and what kind of wear occurs our system. Wear, generally- defined as a progressive lossor displacement of material from a surcafe a result of relative motion between that surface and another. Serious defects are created in the surface. Several wear classifications have been developed. But I will mention about two most common wear classi fications. These are physical mechanisms and opera tional mechanisms.,. Physical Mechanisms are divided into four basic group. - Adhesive wear - Abrasive wear - Spalling and Pitting - Chemical or Corrosive Action Operational Mechanisms are related materials, environment, motions, and these mechanisms are divided into nine group. VI - Dry ar lubricated wear - Rolling wear - Impact wear - Filtering. - Metal/metal wear - Polymer/metal wear - Mild wear - Severe wear - High-temperature wear Eeach af these categories involves different combinations of basic wear mechanisms, thus illustrating the influence of the conditions of and around the wear contact. Wear is closely related to friction and lubrica tion. Also there are a lot of parameters which influenced in wear These are; - Material Parameters: Composition, grain size, modulus, thermal conductivite; degree of work hardeningj hardress. - Desing Parameters: Shape, loading, type of motion roughness, vibration, cycle time. - Environmental Parameters: Temperature, humidity, atmosphere, contamination. - Lubrication Parameters: Type of lubricant, lubricant stability, type of fluid lubrication. In the wear cases, usually one type of damage is predominant. The correct solution to a wear problem may depend strungly on the identification of a spesific basic mechanism. Applying a thin film boundary lubricant has a little beneficLall effect if the damage is caused by the presence of abrasive particles, but is extremely effective if the damage is caused by adhesion. Vll Adhesive wear is the most common type of wear. It occurs when one surface bands to another, and with subsuguent motion, rupture occurs in one of the materials. The presence of a lubricant reduces the coefficient of friction as well as the amount of transfered material. If the surfaces are not lubricated, the transfer of material increases linearly with the load applied normal to surface. Achived investigations show that the amount of Adhesive Wear is proportional, - To the normal load, - To the distance slid - To the hardness of the surface being warn, These conclusions suggest that the solutions to many wear problems may be found by altering these parameters. To reduce adhesive wear, - Lubrication and carrefuUy material chose, - Increased material hardness may be enough. The other important physical mechanism is abrasive wear. Abrasive wear is most dangerous wear shape and it may occure suddenly in the system. Abrasive wear occurs when two surfaces in sliding contact. The wear is most freguatly caused by nonmetallic materials, but metallic particles can also cause abrasion. Generally a substance is seriously abraded or scratched only by materials that are harder than it self. In simple mechanical abrasion, abrading particles first penetrate the metal, and the resulting deforma tion depends on local configuration and load as well as on the mechanical properties of the material. Abrasive wear is commonly divided into three types, vm - Gauging, - Grinding - Erosion Gouging abrasion involves removal of relatively coarse particles from the steels wearing surface and is similar to the removal of metal by machining or grinding with a coarse -gritgrinding whell. Grinding abrasion involves removal of relatively fine particles from the wearing surface. The pinching action of two metal surfaces causes the abrasive to fragment. Unit compressive or shear stresses are very high. The harder abrasives are capable of indenting or scratching steel. Metal may be removed from the wearing surface by microscopic gouging or by a combination of local plastic flow and micracracking, Ball milling is a good example of grinding abrasion. Erosion abrasion occur by very light rubbing contact from sharp abrasive particles. The stresses are due mainly to velocity and are normally insufficent to cause much fragmentation of the abrasive. Erosion also may occur when a flawing, liquid containing suspended abrasive particles flows over a surface. Liquids them selves may cause erosion when a stream of liquid impinges directly on a surface, when fluid flow conditions induce cavitation in the liquid adjacent to a surface, or when droplets carried by a flowing gas impact against a surface. Microstructure and hardness are very effective factor in the abrasion resistance, in order to examine effect of microstructure and hardness on abrasion resistance three type of steel were chaosen. These stells are shown DIN, as, - X210Cr12 cold work tna*l stell - 90 MnCr\/a cold work tool steel and - C35 plan carbon steel. Metal hardness is one of the most important characteristics far wear resistance. It is effected carbon, content, alloying element and heat treating IX condition. increasing the carbon content of steel, results in microstructural alteration that increases as-quenched hardness. For this reason wear resistance increas. Abrasion/resistance of steels improves at a fairy rapid rate up to about D.7 % C. Further increases in carbon content tend to became less effective stells with over D.7 %C will show best abrasion resistance when their pra-eutectoid carbides are in solution. The solution of these carbides is a function of the austenitizing temperature used during heat treatment. As the austenizing temperature increases, the abrasion resistance of steels 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 microconsti- tuents 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, the effect hardness and micro- structure on the wear resistance of x21 0Cr1 2, 90MnCr\/8 cold work tool steels and C35 plain carbon steel were investigated. All type steels were austenitized and oil-quenched then tempered different temperatures. Metallographic examinations were made on the quenched and tempered stells to examine the micro- structures. Then all specimens were taken photograp with optical microskope and SEM. at 4D0°c tempering temperature hardness does not decreas and secondary hardening occurs. Chromium carbides provide-this secondery hardening. Ta measure the wear resistance of thes type of metals at room temperature a two ba disc wear tester urns used. Firstly with Al abrasive and 8 kg lood mere used all type abrasion test. Then in order to examine lo on abrasion resistance, 5,8 and 10 kg load in abrasion test. Also in order to examine metal abrasion behavior, abrasion test were with chromecarbide. As the hardness values decreased their abrasion resistance decreas decreases of the abrasion resistance of X21 90MnCrU8 cold work steels are sharp uç to 5 this hardness they show a slaw change^ an th hand the decreases of the abrasion resistan plain carbon steel are sharp about 5QHRc, hardness it shows very show change. 2°3 e three dy pin-on- based metals ' s ad effect were used metal- performed of steels e. The ÜCr12, 8 HRc, under e other ce of C35 under this In general X21 QCr1 2 cold steel shows the best abrasion resistance in all type of steels worked, because of the martensit and chrome carbide in its microstructure.