Sıcak iş takım çeliklerinde üretim koşullarının yapı ve özellikler üzerindeki etkisi

Abstract

Bu tez çalışmasının konusu, farklı yöntemlerle üretilen AI5IH13 kalite sıcak iş takım çeliklerinin, üretim sonrası işlemler ile yapı ve özellikler bakımından kullanıcıya teslim edilebilir koşullara getirilmesini kapsamaktadır. Deneylerde kullanılan H1 3 kalite sıcak iş takım çeliklerinin bir kısmı kullanılmaya hazır bir şekilde MKE' de üretilmiş, ve bir kısmı da korkmaz çelik dağıtıcı firmasından temin edilmiştir. Ayrıca, üretim sonrası yapılan işlemler ile MKE ve Korkmaz Çelikten alınan örneklerin tavlanmış durumdaki yapıları ile mukayese ederek, teslim edilebilir koşullara getirmek amacıyla, özel olarak Haliç Tersanesi ' nde H1 3 kalite döküm yaptırılmıştır. Deneylerde öncelikle döküm yapıları incelenmiş ve bu amaçla küçük çapta olmak üzere nihai %5 krom olacak şekilde laboratuarda ergitme işlemi yapılmıştır. Daha sonra Haliç Tersanesi'nde dökülmüş halde alınan çelik örneği, döküm sonrası yapılan işlemler ile sıcak dövme için hazırlanmıştır.Döküm yapısını kırmak için yapılan dövme işleminden sonra, numunelere tavlama ve homojenizasyon+Tavlama işlemleri u\ Deneysel çalışmalardan elde edilen sonuçlar değerlendirildiğinde, dövme sonucu oluşan bantlaşmaların homojenizasyonla giderildiği gözlenmiştir. Bunun yanısıra, hama jenizasyondan hemen sonra yapılan tavlama işlemi ile elde edilen yapıların, MKE'den temin edilen H1 3 çeliğinin tavlanmış yapısı kadar ince ve homojen bir karbür dağılımına sahip olduğu görülmüştür. Yapılan temperleme işlemleri esnasında, homojenize edilmiş numunelerin en yüksek ikincil sertleşme gösterdiği, çizilen temper eğrilerin den anlaşılmıştır. Sonuç olarak, yapılan tüm çalışmalar bu konuda daha önce yapılan çalışmalar ile önemli ölçüde uygunluk göstermiş ve özellikle homojenizasyon işleminin çeliklerin yapı ve bunun sonucu olarak elde edilen özellikleri pozitif yönde etkilediği saptanmıştır.

Tool steel is the name of the main producing materials of special tools used in production of the industrial parts and components oy plastic deformation and cutting methods. In terms of steel classification principles tool steels have been searched in a different cathegory. Tools must have to resist to the faster and higher tensions without deforming, breaking and wearing during the usage. Tool steels must also continue their such properties at high temperatures. In order to get these properties all together in optimum conditions, there has been a necessity to the special alloyed tool steels. The main properties of allaying elements can be given as follows. a)- able to change the eutectoidal composition and temperature. b)- affect the transformation diagrams and change the curves towards the right-side. c)- change the martensitic transformation temperature d)- affect the mechanical properties. It's possible. to classify tool steels into 3 groups depending on such properties of alloying elements. 1- Hypereutectoidal steels 2- Ledeburitic steels 3- Eutectoidal steels. VI Tool stesls were classified in different ways depending an classifying methads of countries on the earth. According to a very well known system in Turkey, and used in Europe tool steels are divided into 3 groups. These are, a)- Cold work tool steels b)- Hot work tool steels c)- High speed tool steels In terms of properties tool steels can also be classified into 3 groups. a)- IMon-thermostable tool steels b)- Semi-thermostable tool steels c)- High-thermostable tool steels Uhen choosing a tool steel performence of tool, the method of production, tolerances and the product should be considered as parameters. There are two wide cathegaries for tool steels used in material shaping. These are cutting tools and die materials. Traditional production of tool steels is made by pouring the liquid metal into the metal ingots. Ingots have relatively small volumes with a liquid metal capacity of 3GG to 7GG kg. The results of using small volume ingots can be given as follows. a)- Thinner eutectic network is formed. b)- Inhamogenity of carbide can be minimized with less deformation ratios. c)- The amount of porosity is decreased. Vll d)- The metal flQu during the deformation process is increased. Tq obtain the optimum values it's necessary to consider the factors given as. - The conditions of deformation - The volume of product - The allaying method of steel - Steel making process The phases occured by the addition of the elements such as chromium, malibdenum, vanadium and tungsten can be shown with binary and ternary phase diagrams. Carbides taking place in tool steels can generally be classified into 2 groups as carbides dissolving in austenite and excess carbides. Carbides dissolving in austenite (The former group) are (is) determined as in 3 types. These are, 1- Eutectic carbides 2- Secondary carbides 3- Ternoary carbides Carbides in tool steels cause the secondary hardening by dispersing during the tempering treatment. For this reason tool steels can be used at high temperatures. Carbide inhomogenity in tool steels occurs in two different ways. 1- With carbide network on grain boundaries 2- With carbide banding The former problem can be salved by applying a high ratio of hat defqrmation. But in this case carbide banding occurs. Homogenizatian treatment is used for eliminating such a problem. Vlll The properties af tool steels can be given as, - Hardenability - Wear resistance - Hardness and strength an heat - Terpering resistance - Dimensional stability The methods of heat treatment for tool steels are generally similar to eachother. These are as follows: a)- iMormalisation b)- Annealing - Full annealing - Sub-critical annealing and stress removing - Spheroidizing annealing c)- Hardening - Austenitizing - Quenching d)- Tempering Tool steels used for hat forming are summarized in 3 groups. These are chromium, molibdenum and tungsten groups. 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, a)- A high thermostability to get enough a high strength against the plastic deformation which may occur on working surfaces of the die during the heating. b)-.-A higher toughness than that of high speed tool steals. IX c)_ a uell thermal fatigue resistance. d)- A high stability and scale resistance against the affection between the contacting surfaces with the shaped_metal. Considering these properties die steals are mentioned in 3 groups. These arE. 1- Medium thermal stable die steels 2- Elevated thermal stable die steels 3- High thermal stable die steels Forging of die steels should be carried out at a given temperature range. During the tempering to about 450 to 500 C tempera tures a secondary hardening generally occurs. The reasons for this are \1C, Mo"C ar MofiC type of carbides very finely dispersing at those temperatures. Subzero treatment is used to remove the retained austenite securing after the hardening treatment. The new methods were developed to increase the properties of tool steels and to remove impurities from the steel body. These methods are vacuum gas purging process, electro slag remetting process and etc. And, recently tool steels can also be produced by using powder metallurgy method. This thesis contains after-production treatments of AISI H13 grade hot work tool steels. In the experimental studies the properties of existing carbides have been searched and the hardness values have been measured. The experimental studies have been carried out by using four different types of steel samples. These are 1- 5% Cr steel produced in laboratory 2- Steel produced in Haliç Tersanesi 3- H13 quality steel from MKE 4- H13 quality steel from Korkmaz Çelik A.Ş. First af all the steel samples produced in laboratory have been analised in SEM. Since the sizes of these samples were too small the samples from Haliç Tersanesi have been used instead. It's been founded that the steel samples contained plenty amount of carbides when the original structures were examined. The forging treatment has been applied to eliminate the carbide network and a considerable amount of carbide banding has been observed. The banding structure could not be removed by annealing treatment. But, when 1: homogenization and later on annealing treatments were carried out, a homogenious structure has been carried out. After that a hardness value of 18 HRc has been measured. This structure was similar to MKE ' s steel structure. It's been found that after tempering homogenizated and annealed /steels and shown a secondary hardening at the temperatures of ^50 to 500DC-.After the SEM observa tion of the tempered steel samples from MKE finely dispersing vanadium carbide particles have been determined in the structure. Especially, those particles have largely been seen at the temperatures that the secondary hardening had occured.l As a result, the hornogenity of hot work tool steels predominates favourable effects on properties. Although the conditions of manifacture have great effects on the structures, it's possible to get desired properties and structures by applying treatments after the manifacture. The phases e.g. the carbides taking place in the original structure should also be in a very well pattern in order to get desired results after the hardening and the tempering treatments. A very well carbide orientation can be obtained and carbide banding accured after the forging can also be eliminated when homogenization treatment has been carried out at high temperatures e.g. 1200 C and long times at about 5 or 6 hours after the forging process.