Alloy Steels. Engineering Materials. Introduction : Msc. Shaymaa Mahmood

Transcription

1 Alloy Steels Introduction : Steels are, essentially, alloys of iron and carbon, containing up to 1.5 % of carbon. Steel is made by oxidizing away the impurities that are present in the iron produced in the blast furnace. The earliest attempt to produce an alloy steel was in 1822 and it has progressing in producing the alloy steel because of using alloy steel in those industries upon modern civilization largely depends. Pure metal objects are used where good electrical conductivity, good thermal conductivity, good corrosion resistance or a combination of these properties are required. Therefore alloys are mainly used for structural materials since they can be formulated to give superior mechanical properties. It is called as alloy steel because there are other elements added to the iron beside the carbon with specific amount for each element. These elements improve the properties of the alloying steel and make it used with applications more than the carbon steel. So the most used elements with the alloy steel and with their amount as a percentage of : - 2 % Manganese (Mn) % Chrome (Cr) or Nickel (Ni) % Tungsten (W) or Cobalt % Molybdenum (Mo) or Vanadium - different amount of Aluminum (Al), Copper (Cu) and Silicon (Si).

2 Alloys steels are generally classified into two major types depending on the structural classification : - Low alloy steels : It is one that possesses similar microstructure to, and requires similar heat treatment to, plain carbon steels. These generally contain up to 3 4 % of one or more alloying elements for purpose of increasing strength, toughness and hardenability. The applications of low alloy steels are similar to those of plain carbon steels of similar carbon contents. Low alloy steels containing nickel are particularly suitable for applications requiring resistance to fatigue. - High alloy steels : Those steels that possess structures, and require heat treatments, that differ considerably form those of plain carbon steels. A few examples of high alloy steels are given below: 1. High-speed tool steels Tungsten and chromium form very hard and stable carbides. Both elements also raise the critical temperatures and, also, cause an increase in softening temperatures. High carbon steels rich in these elements provide hard wearing metal-cutting tools, which retain their high hardness at temperature up to 600 C. a widely used high-speed tool steel composition is containing 18% of tungsten, 4% of chromium, 1% of vanadium and 0.8% of carbon. This high-alloy content martensite dose not soften appreciably unit it is heated at temperatures is excess of 600 C making them usable as cutting tools at high cutting speeds. 2. Stainless steels When chromium is present in amounts in excess of 12%, the steel becomes highly resistance to corrosion, owing to protective film of chromium oxide that forms on the metal surface. Chromium also raises the α to γ transformation temperature of iron, and tends to stabilize ferrite in the structure. 2

3 There are several types of stainless steels, and these are summarized below: a) Ferritic stainless steels contain between 12-25% of chromium and less than 0.1% of carbon. b) Martensitic stainless steels contain between % of chromium, together with carbon contents ranging from 0.1 to 1.5 % c) Austenitic steels contain both chromium and nickel. When nickel is present, the tendency of nickel to lower the critical temperatures overrides the opposite effect of chromium, and the structure may become wholly austenitic. 3. Maraging steels These are very high-strength materials that can be hardened to give tensile strengths up to 1900 MN/m 2. They contain 18% of nickel, 7% of cobalt and small amounts of other elements such as titanium, and the carbon content is low, generally less than 0.05%. A major advantage of marging steels is that after the solution treatment they are soft enough to be worked and machined with comparative ease. Alloy elements cab be classified depending on the using of the alloy (its application) or according to the basic influence of the element on the alloy steel properties as follow: 1. alloying elements tend to make carbides such as Cr, W, Ti, V and Mo it is used in the applications that needs to higher hardness. 2. alloying elements due to analyzing carbides such as Ni, Al, Co and Si. Si& Co Fe C 3Fe C 3 3. alloying elements stabilizing uestinet γ such as Ni, Co, Cu and Mn. 4. alloy elements stabilizing ferret α such as Cr, W, V, AL and Si. 3

4 The principal effects which these alloying elements have on the microstructure and properties of a steel can be classified as follows: 1. The Effect on the Allotropic Transformation Temperatures. Some elements, notably nickel, manganese, cobalt and copper, raise the A 4 temperature and lower the A 3 temperature, as shown in figure 1. In this way these elements, when added to a carbon steel, tend to stabilise austenite (γ) and increase the range of temperature over which austenite can exist as a stable phase. /o ALLOYING ELEMENT Figure 1. Relative Effects of the Addition of an Alloying Element on the Allotropic Transformation Temperatures at A 3 and A 4 which tending to stabilise y. Other elements, the most important of which include chromium, tungsten, vanadium, molybdenum, aluminum and silicon, have the reverse effect, in that they tend to stabilize ferrite (α) by raising the A 3 temperature and lowering the A 4, as indicated in figure 2. 4

5 /o ALLOYING ELEMENT Figure 2. Relative Effects of the Addition of an Alloying Element on the Allotropic Transformation Temperatures at A 3 and A 4 tending to stabilize α. 2. The Effect on the Stability of the Carbides. Some of the alloying elements form very stable carbides when added to a plain carbon steel. This generally has a hardening effect on the steel, particularly when the carbides formed are harder than iron carbide itself. Such elements include chromium, tungsten, vanadium, molybdenum, titanium and manganese [ these elements are called the carbide stabilizer]. When more than one of these elements are present, a structure containing complex carbides is often formed. 3. The effect on grain growth. The rate of crystal growth is accelerated, particularly at high temperatures, by the presence of some elements, notably chromium. Fortunately, grain growth is retarded by other elements, notably nickel and vanadium, whose presence-thus produce a steel which is less sensitive to the temperature conditions of heat-treatment. 5