Manganez bronzlarında ısıl işlemle sertleşme mekanizması ve faz dönüşümleri

Abstract

Bu çalışmada % 59.76Cu, % 36.75 Zn, % 1.kO Mn, % 1.50 Al, % 0.48 Fe bileşimindeki (alaşım 1) ve % 66.65 Cu, % 24.28 Zn, % 1.72 Mn, % 5.61 Al, % 1.73Fe bileşimindeki (alaşım 2) döküm manganez bronzlarının ısıl işlemle sertleşme mekanizması ve faz dönüşümünün incelenmesi amaçlanmıştır. Metalografik incelemeler B50 C ' ye kadar değişik sıcaklıklardan su verilmiş numuneler üzerinde yapılmıştır. Sertlik değerleri mikrosertlik cihazında Vickers ucuyla 500 gr. yük kullanılarak ölçülmüştür. Faz dönüşümlerini incelemek için yüksek sıcaklık kamarasında mikroskobik çalışmalar yapılmıştır. Su verilmiş numunelerin faz analizleri X-ışınları difraksiyon metoduyla yapılmıştır. Bu çalışmalardan elde edilen sonuçlar aşağıda verilmiştir. Çinko eşdeğeri % 46.1 olan alaşım 1: ve çinko ('.eşdeğeri % 52.7 olan alaşım 2'nin döküm yapısı o< ve p fazların dan ibarettir. Alaşım 2'de <=4 fazı Widmanstatten morfolojisindedir. Yüksek çinko eşdeğeri sebebiyle, p fa zı miktarının daha fazla olduğu alaşım 2 daha serttir. Su verme sıcaklığı arttıkça p fazının miktarı artmakta dır. °< fazının sertliği fi fazından düşüktür ve su verme sıcaklığından etkilenmemektedir. 500-550 C ' de her iki alaşımda da çinko buharlaşması olmaktadır. X-ışını difraksiyonu ile faz analizine göre e* fazı ASTM 8-349, P fazı ise ASTM 2-1 231 nolu kartlarla temsil edilmektedir.

Copper and copper-base alloys have a mide range of properties that account for their extensive use as engineering materials. Their high electrical and thermal conductivity, ease of formability and excellent corrosion resistance under certain conditions are three characteristics that make them attractive. In order to produce high conductiviy copper suitable for electrical purposes, it is necessary to use very effective refining treatments to remove the impurities. The properties of copper-base allays are largely dependent on alloying elements. The effects of alloying elements are as follows: Zinc is added to copper as a predominating alloying constituent, in amounts of 5-40%, to form allays known as brasses. These groups are called leaded red and semi-red, silicon, yellow, and high strength yellow brasses. Zinc imparts strenght. It is completely soluble in copper, farming solid solution except in such cases as in high strength yellow brasses in which a duplex type of structure is obtained. Smaller amounts af zinc up to 5% are used in tin bronzes to tighten up the structure. Zinc is not considered a very detrimental impurity in most allays. Tin is added to capper in amounts of 5-20% to form a series of alloys known as tin bronzes and leaded tin bronzes. Tin strengthens and hardens copper, making it tough and resistant to wear and increases its corrosion resistance. Tin is not generally harmful as an impurity except in high strenght manganese bronzes, where it is limited to 0.2%. It is generally felt that in this allay; tin lowers the strength and ductility. Aluminum is added to copper as a predominating alloying constituent to farm a series of high strength alloys knouin as aluminum bronzes. It is soluble in copper to the extent of about 9.5%. It is added to high strength yellow brasses in varying amounts, being a very necessary part of the high tensile alloy. Aluminum, when present as an impurity has very detrimental effects upon high leaded bronzes. Iron is added to copper alloys as a strengthening constituent for silicon, aluminum, and mangase bronzes, It combines with aluminum or manganese arboth to farm hard compounds. These compounds imbed themselves into the matrix to give the alloys wear resistance. Iron, when present as an impurity, is not desirable since it forms hard spots and is detrimental to machining. Manganese is used primarily as an alloying constituent for high strength alloy brasses, where it forms compounds with other alloying elements such as iron and aluminum. It is also used, to some extent, for deoxidizing. It is not considered very detrimental as an impurity. It is useful to examine the size difference between copper and solute atoms to predict the extent of solid solution strengthening. A large size difference should favor strengthening, but this usually causes limited solubility. Thus the more usable salutes far solid solution strengthening are those for which the size difference is modest and the solubility large. Zinc has the best combination of size difference and solubility. Copper-base alloys are divided into two main groups: Wrought copper-alloys and casting copper alloys. Factors influencing the uses of wrought copper and copper allays concern electrical conductivity, thermal conductivity, machinability, formability, fatigue characteristics, strength, corrosion resistance, the ease with which alloys can be joined, and fact that these materials are non-magnetic. Copper and its allays have a wide range of rich, pleasing colors, tdhen it is desired to improve one or more of the important properties of copper, alloying often solves the problem. A wide range of alloys.therefore, has been developed and commercially employed, such as the high copper alloys, brasses, tin bronzes, heat treatible VI allays, capper-nickel allays, nickel silvers and special bronzes. Capper is allayed with other elements, because pure copper is extremely difficult to cast as well as being prone to surface cracking, porosity problems, and to the formation of internal cavities. The casting characteristics of copper can be improved by the addition of small amounts of elements. Copper allays in cast form is designated in UNS (Unified Numbering System) numbering system as CB0000 ta C99999. Unlike pure metals, allays solidify aver a range of temperatures. Solidification (or freezing) begins when the temperature drops below the liquidus, it is completed when the temperature reaches salidus. The copper-base casting alloys can be subdivided into three groups according to solidification (freezing range). The three groups are as follows: Group 1 allays are alloys that have a narrow freezing range, that is a range of 5Q°C between the liquidus and salidus. Far example: Manganese bronzes, aluminum bronzes, yellow brasses. Group 2 alloys are those that have an intermediate freezing range, that is, a freezing range of 5G to 11 D C between the liquidus and the solidus. For example: Beryllium- coppers, silicon bronzes, copper-nickels. Group 3 allays have a freezing range of well aver 110 C, even up to 170 C. Far example: Tin bronzes, leaded red brasses, high-leaded tin bronzes. Copper alloy castings are used in applications that require superior corrosion resistance, good bearing-surface qualities, high thermal or electrical conductivity, and other special properties. These applications may be divided into six principal groups; i- Plumbing hardware, pump parts, and valves and fittings. ii- Bearings and bushings. iii- Gears. iv- Marine castings. v- Electrical components. vi- Architectural and ornamental parts. V3L1 Capper and copper alloys may be heat treated for any of these purposes: Homogenizing is applied to dissolve and absorb segregation and caring found in some cast and hot worked materials, chiefly, those containing tin and nickel. In homogenizing, high temperatures and relatively long times are employed to eliminate or decrease coring in cast metal that is to be hot or cold worked. Diffusion and homogeniza- tian are slower and more difficult in tin bronzes, silicon bronzes and copper nickels than in most other copper allays. The time and temperature required for the process varies with the allay, the cast grain size and the desired degree of homagenization. Temperatures are above the upper annealing range. Typical soak times vary from 3 to over 10h. Annealing is a heat treatment intended to soften, and increase the ductility and/or toughness of metals and allays. It is applied to wrought products and to castings. Heating rate, temperature, time at temperature, atmosphere, and cooling rate may affect the results} annealing of cold worked metal is accomplished by heating to a temperature that produces recrystallization and, if desirable, by heating beyond the recrystalliza tion temperature to produce grain growth. Annealing is also applied to castings of some duplex allays, such as manganese bronzes and aluminum bronzes. Stress relieving is a process intended to relieve internal. stress in materials or parts without appreciably affecting their properties. Stress relieving heat treatments are applied to copper allays as one means af accomplishing this abjective. Stress-relief heat treatments are carried out at temperatures below those normally used for annealing. If allowed to remain in sufficient magnitude, residual surface tensile stresses can result in stress-corrosian-cracking of materials. in storage- or service. High strength in most cappers and capper allays is achieved by cold working. Hut for certain allays containing small amounts af beryllium, chromium or phosphorus, unusually high strength and hardness, can be obtained by precipitation hardening. All precipita tion-hardening copper allays have similar metallurgical characteristics. They can be solution treated to a Vlll soft condition by quenching from a high temperature, and then subsequently precipitation hardened by aging at a moderate temperature for a time usually not exceeding 3h. The transformation hardening mechanism most often observed in copper alloys is associated w'.ith two phase aluminum bronzes. These allays are hardened by cooling rapidly from a high temperature to produce a martensitic type of structure, and then are tempered at a lower temperature to stabilize the structure and partly restore ductility and toughness. At temperatures of 815 to 1 01 D C, the two room-temperature phases transform to beta in the same manner that alpha plus Fe-C in steel transforms to austenite. Rapid quenching produces a hard, brittle structure due to formation of metastable, ordered, close-packed-hexagonal beta, which is referred to as martensitic beta. Tempering far 2h at 595 to 650 C causes reprecipitation of fine acicular alpha in a tempered beta-martensite structure, reducing hardness while increasing ductility and toughness. Copper-base alloys are mainly divided into two groups : i- Brasses (Cu-Zn allays) ii- Bronzes (Aluminum, manganese, tin bronzes.. etc) Copper-zinc solid solution allays are probably the most widely used copper-base allays. They retain the good corrosion resistance and formability of copper but are considerably stranger. Zinc is hexagonal close-packed, so the solubility in copper cannot be complete. However, copper is face-centered cubic, a close-packed structure, and the atom size difference is only about k%, so extensive solubility is expected. Maximum solubility of zinc in copper is about 38%, and at 20 C it is about 35 %. The thermal equilibrium diagram for copper and zinc is very complex especially where the zinc content is in excess of 40%. There are two main groups of casting alloys : i- Alpha brass (70% copper-30%zinc). ii- Alpha-beta brass (60% copper-40% zinc). IX The alpha solid solution has good ductility while beta has high strength, these two phases being present together give a goad combination of the two properties. Alpha-beta brass is more commonly used for castings and is the basis of high tensile brasses. A high tensile brass is a 60-40 alloy to which has been added varying amounts of tin, lead, iron, aluminum, manganese and silicon. As all of these elements act in some way similar to zinc their zinc equivalent value must be determined to ascertain if the alloy is alpha and beta or beta in composition. Aluminum bronzes have some characteristic properties. These are as fallows: i- High strength, ii- Good working properties, iii- Resistance to corrosion and wear, iv- High resistance to fatigue. v- Fine golden colour. vi- Possibility of heat-treatment in manner similar to steels. Aluminum bronzes contain approximately 10% aluminum together with varying amounts of other elements. Aluminum bronze has good corrosion resistance, excellent mechanical properties. The main casting alloys have a microstructure of alpha and beta but if the alloy contains more than B.5 % aluminum then beta would be the first phase to solidify. The effect of the eutectoid reaction at 565 C is to produce a new phase gamma which is very hard and brittle and its presence can be detrimental to the casting. By rapid coaling it is passible to suppress this reaction, so that an alpha-beta structure is present. It is not always possible to control the cooling rate within the mould so that two ways of avoiding the formation of gamma are used. The first is the addition of nickel and iron, which slow down the eutectoid reaction so that it does not take place at normal cooling rates and the second is the removal of gamma by heat treatment. A martensitic type of structure can be formed if the alloy is quenched from 900 C. m Tin is a solute that has a large size difference and reasonably high solubility, so that the capper-tin allays should have quite usable strength. Indeed, the Cu-Sn solid solutions are considerably stranger than the Cu-Zn alloys. The copper-tin phase diagram shows that the solubility of tin decreases markedly with decreasing temperature. However, below about 300°C the rate of precipitation of £ is low, so that alloys up to about 10% Sn will be single phase otafter proper homogenization and cooling to 25 C. It also appears that these allays would be precipitation hardenable, since S has a high hardness. The wide temperature range between the liquidus and solidus makes these alloys very susceptible to coring. Also, cast allays with tin contents as low as 8% Sn frequently contain S, a result of the fact that caring allows the outside of the «< dendrites to attain about 13.5% Sn. This composition of oC will react with the liquid to form some p by a peritectic reaction. This Ş then decomposes by an eutectoid reaction to oC and Tf ; the "5 then decomposes to form o< and S. This sequence can be quite complex and dependent upon the coaling rate. The other bronze allay used in as-cast condition is manganese bronze. Mangase bronzes are sometimes classified as high strength yellow brasses (or high tensile alloys), because of their higher zinc content. Excellent mechanical properties are combined with good corrosion resistance in sea water. Since all these properties are highly dependent on microstructure, an understanding of it is helpful in alloy selection far end use. Manganese bronze allays are five component systems Iron acts as a grain refiner and is usually present at about 1%, above this the corrosion resistance is reduced. Manganese forms a stable solid solution with any aluminum present and this increases strength while decreasing ductility. Aluminum produces beta and gamma alloys which are very brittle, because of its high zinc equivalent. Aluminum has a great influence an the tensile strength. Structurally, these alloys contain these phases: i- An alpha alloy, ii- An all beta alloy, iii- An alpha-beta allay XI In this study, the phase transformation and hardening mechanism by heat treatment of two cast manganese bronzes mere investigated. The chemical compositions of manganese bronzes investigated were; 59.76% Cu, 36.75% Zn, 1.40% Mn, 1.5D%A1, 0.48%Fe for alloy 1 and 66.65%Cu, 24.23% Zn, 1.72% Mn, 5.61% Al, 1.73% Fe for alloy 2 Zinc equivalents of the alloys were calculated from: o/ 7inn = ",.<" = ?!,," + Zn % +(%Additions x Coefficients).^nn M Zinc equivalent = xCUU Zn% + Cu%+(%Additions x Coefficients) as 46.1% Zn far allay 1 and 52.7% Zn for alloy 2. The metallographic examinations and hardness tests were performed on the samples of the alloys which were water quenched from various temperatures up to B5D C. For this purpose, as-quenched samples were etched by 5 gr. FeCl3+ 50ml HC1 +100 ml H-O after polishing. Microhardness measurements were performed with microhardness testing machine by applying 500 gr. load with Uickers indenter. High temperature microscopic studies were carried out on a hot stage camera to examine the phase transformations during heat treatment. The phase analysis of the as- quenched samples was made by utilizing X-ray diffraction method. The results of this study are as follows: i- Bath of the as-cast allays contain alpha and beta phases at room temperature in their microstructures However, in alloy 2, M phase has widmanstatten morphology. ii- The alloy 2 which has higher amount of p phase in its microstructure is harder than the allay 1, due to high zinc equivalent. Increasing of the quenching temperature, increases the amount of beta phase present in microstructures. This causes an increase in the bulk hardness-of the sample. xn iii- The hardness of alpha phase is lower than beta phase; and it does not change with increasing temperature. However, the hardness of beta phase increases with increasing temperature. iv- Zinc vaporization is observed at about 50D to 550 C in bath alloys. v- According to X-ray diffraction phase analysis, alpha phase is represented by ASTM 8-349 and beta phase is represented by ASTM 2-1231 cards.