Carbon steel alloying elements

Decarburization occurs in steels and cast irons in hydrogen gas by the reaction of H with C in the steel. The decarburization rate is primarily dependent on the diffusion rate of C in the steel, but is also affected by the carbon content of the steel, alloying elements in the steel, such as chromium, impurities in the hydrogen, and of course time and temperature. Carburization of steels, the reverse of decarburization, is usually conducted at temperatures of about 900°C, but decarburization can occur at temperatures as low as 800°C. " ... 

Soft microstructures can be obtained by using steel with low contents of carbon and alloying elements (including manganese) to reduce the hardenability of the HAZ. Additionally, the use of a large weld bead, thin plate, and preheat will reduce the quenching rate in the HAZ. After a bead has been deposited the HAZ can be softened by tempering either as a result of subsequent weld runs or by a postweld heat treatment (PWHT or stress relief). 

Unalloyed steels contain essentially only carbon as alloying element, and the carbon content has a major influence on the properties (Table 2.10-3). The carbon is present in the form of iron carbide FcjC which, together with the iron matrix, forms the structural material steel. 

For example,copper has relatively good corrosion resistance under non-oxidizing conditions. It can be alloyed with zinc to yield a stronger material (brass), but with lowered corrosion resistance. Flowever, by alloying copper with a passivating metal such as nickel, both mechanical and corrosion properties are improved. Another important alloy is steel, which is an alloy between iron (>50%) and other alloying elements such as carbon. 

Low Alloy Steels. These aHoys are carbon steels to which other elements have been deHberately added to impart a particular property. 

Plain Carbon and Low Alloy Steels. For the purposes herein plain carbon and low alloy steels include those containing up to 10% chromium and 1.5% molybdenum, plus small amounts of other alloying elements. These steels are generally cheaper and easier to fabricate than the more highly alloyed steels, and are the most widely used class of alloys within their serviceable temperature range. Figure 7 shows relaxation strengths of these steels and some nickel-base alloys at elevated temperatures (34). 

When a component at an austenitizing temperature is placed in a quenchant, eg, water or oil, the surface cools faster than the center. The formation of martensite is more favored for the surface. A main function of alloying elements, eg, Ni, Cr, and Mo, in steels is to retard the rate of decomposition of austenite to the relatively soft products. Whereas use of less expensive plain carbon steels is preferred, alloy steels may be requited for deep hardening. 

Heat Treatment of Steel. Steels are alloys having up to about 2% carbon in iron plus other alloying elements. The vast application of steels is mainly owing to their ability to be heat treated to produce a wide spectmm of properties. This occurs because of a crystallographic or aHotropic transformation which takes place upon quenching. This transformation and its role in heat treatment can be explained by the crystal stmcture of iron and by the appropriate phase diagram for steels (see Steel). 

Steels having adequate hardenabiHty develop martensitic stmctures in practical section sizes. Molybdenum is a potent contributor to hardenabiHty, and has been shown to be even more effective in the presence of carehiUy selected amounts of other alloying elements (26). The end-quench test has become the accepted method for measuring hardenabiHty, and the data can be correlated with section size. Technical societies worldwide have standardized hardenabiHty limits (bands) for a large number of carbon and alloy steels standards of the Society of Automotive Engineers are examples (27). 

The durabihty and versatility of steel are shown by its wide range of mechanical and physical properties. By the proper choice of carbon content and alloying elements, and by suitable heat treatment, steel can be made so soft and ductile that it can be cold-drawn into complex shapes such as automobile bodies. Conversely, steel can be made extremely hard for wear resistance, or tough enough to withstand enormous loads and shock without deforming or breaking. In addition, some steels are made to resist heat and corrosion by the atmosphere and by a wide variety of chemicals. 

The physical and mechanical properties of steel depend on its microstmcture, that is, the nature, distribution, and amounts of its metaHographic constituents as distinct from its chemical composition. The amount and distribution of iron and iron carbide determine most of the properties, although most plain carbon steels also contain manganese, siUcon, phosphoms, sulfur, oxygen, and traces of nitrogen, hydrogen, and other chemical elements such as aluminum and copper. These elements may modify, to a certain extent, the main effects of iron and iron carbide, but the influence of iron carbide always predominates. This is tme even of medium alloy steels, which may contain considerable amounts of nickel, chromium, and molybdenum. 

Alloying elements such as nickel, chromium, molybdenum, and copper, which may be introduced with scrap, can increase the hardenability, although only slightly, because the concentrations are ordinarily low. However, the heat-treating characteristics may change, and for appHcations in which ductihty is important, as in low carbon steels for deep drawing, the increased hardness and lower ductiHty imparted by these elements may be harmful. 

For slightly less than 10% of products, alloying elements are introduced to produce properties not available for carbon steels where the functional elements are usually considered to be carbon, siHcon (to 0.6%), and manganese (to 1.65%). Copper, which may be present up to 0.6 wt %, is relatively rare compared to the ubiquitous siHcon and manganese. 

An important item in this array of matenals is the class known as maraging steels. This group of high nickel martensitic steels contain so Htde carbon that they are often referred to as carbon-free iron—nickel martensites (54). Carbon-free iron—nickel martensite with certain alloying elements is relatively soft and ductile and becomes hard, strong, and tough when subjected to an aging treatment at around 480°C. 

Bayonet Heaters A bayonet-tube element consists of an outer and an inner tube. These elements are inserted into tanks and process vessels for heating and cooling purposes. Often the outer tube is of expensive alloy or nonmetalhc (e.g., glass, impeivdous graphite), while the inner tube is of carbon steel. In glass construction, elements with 50.8- or 76.2-mm (2- or 3-in) glass pipe [with lengths to 2.7 m (9 ft)] are in contact with the external fluid, with an inner tube of metal. 

The addition of small amounts of alloying materials greatly improves corrosion resistance to atmospheric environments but does not have much effect against liquid corrosives. The alloying elements produce a tight, dense adherent rust film, but in acid or alkaline solutions corrosion is about equivalent to that of carbon steel. However, the greater strength permits thinner walls in process equipment made from low-alloy steel. 

As you can see from the tables in Chapter 1, few metals are used in their pure state -they nearly always have other elements added to them which turn them into alloys and give them better mechanical properties. The alloying elements will always dissolve in the basic metal to form solid solutions, although the solubility can vary between <0.01% and 100% depending on the combinations of elements we choose. As examples, the iron in a carbon steel can only dissolve 0.007% carbon at room temperature the copper in brass can dissolve more than 30% zinc and the copper-nickel system - the basis of the monels and the cupronickels - has complete solid solubility. 

Carbon is the cheapest and most effective alloying element for hardening iron. We have already seen in Chapter 1 (Table 1.1) that carbon is added to iron in quantities ranging from 0.04 to 4 wt% to make low, medium and high carbon steels, and cast iron. The mechanical properties are strongly dependent on both the carbon content and on the type of heat treatment. Steels and cast iron can therefore be used in a very wide range of applications (see Table 1.1). 

Low-carbon, low-alloy steels are in widespread use for fabrication-welded and forged-pressure vessels. The carbon content of these steels is usually below 0.2%, and the alloying elements that do not exceed 12% are nickel, chromium, molybdenum, vanadium, boron and copper. The principal applications of these steels are given in Table 3.8. 

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