Physical Metallurgy of Steel - University of Plymouth

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Class Notes and lecture material For

MSE 651.01--

Physical Metallurgy of Steel

Notes compiled by: Notes revised by:

Web installation by:

Glyn Meyrick, Professor Emeritus Robert H. Wagoner, Distinguished Professor of Engineering Wei Gan, Graduate Research Associate

Last revision date: 1/8/01

STEEL

Foreword This document is intended to augment formal lectures on the general topic of the physical metallurgy of steels, presented within the MSE Department during the Fall Quarter, 1998. It is based on a variety of texts and published articles and also on personal experience. Specific references to sources are made within the document. However, the material is often in the form of knowledge that has been accumulated by the work of many people and is "well-known" by experts in the field. A detailed acknowledgment of the work of each contributor to the field is not attempted because that would be an awesome task. This document is not intended for publication and is restricted for use in MSE 651.01.

Texts: Steels; Microstructures and Properties by R.W.K. Honeycombe (Edward Arnold) Principles of the Heat Treatment of Steel by G. Krauss (ASM) The Physical Metallurgy of Steel by W.C. Leslie (McGraw Hill) The ASM Metal Handbooks. Handbook of Stainless Steels, Peckner and Bernstein (eds.) McGraw Hill 1977 Tool Steels Roberts and Cary, Edition 4, ASM, 1980 Ferrous Physical Metallurgy A. K. Sinha, Butterworths 1989.

Introduction

Steel is a family of materials that is derived from ores that are rich in iron, abundant in the Earth's crust and which are easily reduced by hot carbon to yield iron. Steels are very versatile; they can be formed into desired shapes by plastic deformation produced by processes such as rolling and forging; they can be treated to give them a wide range of mechanical properties which enable them to be used for an enormous number of applications. Indeed, steel is ubiquitous in applications that directly affect the quality of our lives. Steel and cement constitute about 90% of the structural materials that are manufactured ( Westwood, Met and Mat Trans, Vol. 27 A, June 1996, 1413).

What, then, is steel?

A precise and concise definition of steel is not an easy thing to present because of the very large variety of alloys that bear the name. All of them, however, contain iron. We might reasonably begin by describing a steel as an alloy which contains iron as the major component. This is only a beginning because there are alloys in which iron is the major constituent, that are not called steels; for example, cast irons and some superalloys. The major difference between a cast iron and a steel is that their carbon contents lie in two different ranges. These ranges are determined by the maximum amount of carbon that can be dissolved into solid iron. This is approximately 2% by weight (in FCC iron at 1146 ?C). Steels are alloys that contain less than 2% carbon. Cast irons contain more than 2 % carbon. Many steels contain specified minimum amounts of carbon. This does not mean that all steels must contain substantial quantities of carbon; in some steels the carbon content is deliberately made very small and, also, the amount actually in solution is reduced further by the addition of alloying elements that have a strong tendency to combine with the carbon to form carbides.

Steels can be divided into two main groups; plain carbon steels and alloy steels. The latter can then be subdivided into many groups according to chemistry ( e.g. standard low alloy steels), applications (e.g. tool steels ) or particular properties (e.g. stainless steels) etc. Let us begin with

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plain carbon steels; this group is the simplest to understand and it comprises steels that are used in the greatest tonnage.

Plain carbon steels

A plain carbon steel is essentially an alloy of iron and carbon which also contains manganese and a variety of residual elements. These residual elements were either present within the raw materials used in the production process e.g. iron ore and scrap steel additions, or they were added in the production process for a specific purpose, e.g. deoxidization by means of silicon or aluminum. Hence they are called residual elements to distinguish them from alloying elements that are deliberately added according to specified minimum amounts. The American Iron and Steel Institute (AISI) has defined a plain carbon steel to be an alloy of iron and carbon which contains specified amounts of Mn below a maximum amount of 1.65 wt. %, less than 0.6 wt. % Si, less than 0.6 wt. % Cu and which does not have any specified minimum content of any other deliberately added alloying element. It is usual for maximum amounts (e.g. 0.05 wt. %) of S and P to be specified. ( see The Making, Shaping and Treating of Steel, USS 1970)

The manufacture of steel is a subject that is well beyond the ambit of these notes. But we should be aware that various manufacturing practices can affect the oxygen, nitrogen and sulfur content and hence the cleanliness of the product. The term "cleanliness" usually refers to the amounts of various phases such as oxides, sulfides and silicates that can be present in steel. The smaller the amount of these phases, the cleaner is the steel. These phases are in the form of particles called inclusions and they can have significant effects on the properties of steel and are normally undesirable. Thus, variations in the manufacturing process can have significant effects on the properties of the steel, so some brief comments are in order. For many years steels have been produced by casting the molten steel into molds and allowing it to solidify into ingots which were then processed by rolling etc. Steel produced by ingot metallurgy is subdivided into four categories according to the deoxidization process used. These categories are rimmed, capped, semi-killed and killed steel. When an undeoxidized steel is cast into an ingot, carbon monoxide is evolved during solidification because the solubility of oxygen decreases as the temperature decreases. By adding enough ferrosilicon and aluminum, much of the oxygen can be removed from solution and also iron oxide is reduced by the formation of silicon and aluminum oxides so that no gas bubbles form and the steel is quiescent as it solidifies. The result is a fully killed steel. Plain carbon steels containing more than 0.3% C , produced by ingot metallurgy, are usually fully killed. Semi-killed steels are those which are partially deoxidized. Ingots for which the evolution of bubbles of carbon oxide gas is permitted during solidification produce rimmed steel, while capping means the interruption of the rimming process by freezing the top of the ingot by putting a cast iron cap on it. The different practices produce ingots that differ with respect to segregation, inclusion content and as-cast microstructure.

There have been major advances in the production of steel during the last 20 years and continuous casting, in which great attention is being paid to the cleanliness of the steel, has become the dominant production method. According to R. D. Pehlke in Metallurgical Treatises, published by the Metallurgical Society of the AIME and reprinted in 1983, most steel produced by continuous casting is fully killed. Vacuum deoxidization is also being used to eliminate the oxygen, and the steel is protected by argon atmospheres in covered tundishes thereby yielding a cleaner steel with respect to its inclusion content. This is beneficial with respect to the mechanical properties and uniformity of the final product. Continuous casting also produces a product that is much closer to the final shape that is required for the application of the steel.

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More and more steel sheet is now being produced in what are called mini-mills. These mills charge electric arc furnaces with scrap steel and iron, melt the charge, control the composition, and, via a tundish continuously cast it into slabs several inches thick and a few feet wide. This slab is immediately fed through a long furnace in which it is heat treated and from which it emerges at the temperature desired to begin hot-rolling and passes directly into a series of hot rolls. Problems can arise in controlling the residual element contribution to the composition of the product because of variations in the quality of the scrap steel charged into the electric furnaces. Copper is particularly undesirable because it is not easily removed from liquid steel, and as its concentration increases, it can produce cracks due to hot-shortness. This phenomenon arises because when the hot steel is exposed to air the iron is oxidized to form scale, much of which spalls off but the copper is not oxidized and accumulates beneath the scale at the surface of the steel. The copper penetrates the steel along grain boundaries and causes grain boundary cracking to occur during rolling.

Alloy steels

It follows from the definition of a plain carbon steel, that if one or more additional elements are deliberately added to produce specified minimum contents then the product is an alloy steel. In general, this is so, but we should also note that small additions of rare earth elements such as cerium can be made to a plain carbon steel for inclusion control. ( One of the important roles of manganese is to combine with sulfur to prevent the formation of iron sulfides which can embrittle the steel. It has been discovered that additions of rare earth elements can produce high-melting point, complex sulfides and oxysulfides that are smaller and less harmful to the mechanical properties of steels than manganese sulfides can sometimes be.) The steel would not then be referred to as an alloy steel. The definition given is a very broad one and it indicates that a clear, concise, nice little subdivision scheme to describe all steels is not easily produced. As we shall see, there is a group of low-alloy steels for which the compositions are specified in this country according to schemes originally recommended by the AISI and the Society of Automotive Engineers(SAE), but alloy steels are also classified on the basis of some important property ( e.g. stainless steels) or on the basis of use (e.g. tool steels) or even as a consequence of a particular heat-treatment (e.g. maraging steel). To add to the confusion, there are steels that have particular names coined by the manufacturer and also steels that are characterized by an American Society for Testing Materials (ASTM) specification, e.g. A306 which specifies the mode of manufacture, permissible amounts of S and P but leaves the manufacturer to choose the carbon levels necessary to achieve specified tensile properties. The tensile properties determine the grade of steel, e.g. A306 grade 50 specifies a tensile strength of 50 to 60 KSI. The picture appears to be quite confusing. Our objective is to eliminate the confusion by understanding how the properties of steel are related to their microstructures, how particular microstructures are produced and what effects are produced by the alloying elements. There is, of course, a wide range of properties. Mostly we will concentrate on mechanical properties. The ASM. handbooks and the book "Making, Shaping and Refining of steel" put out by the old U. S.. Steel Co. and in a revised tenth edition in 1985 by W. T.Lankford, Jr. et al. published by the Association of Iron and Steel Engineers, present many details of properties and production processing. Our purpose is to develop a fundamental understanding. In order to do this, I propose to begin with pure iron, proceed to Fe-C, consider plain carbon steels, put in alloying elements and finally to select some particular classes of steel for examination.

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Iron and its Solid Solutions

At normal atmospheric pressure, iron, in equilibrium, is BCC below 910?C, FCC between 910?C and 1390?C and then BCC up to its melting point of 1536?C. Hence, at normal ambient temperatures and below, iron is BCC. Its mechanical properties, as determined by testing in tension at room temperature and at typical strain rates of 10-3 to 10-4 sec-1, are functions of purity, grain size and dislocation content. The yield strength is also temperature dependent, increasing with a reduction in temperature as shown in figure 1 (W.C. Leslie, Met. Trans. 3, 9, 1972.) These results were obtained with iron to which Ti had been added to getter the interstitial elements. ( For example, the carbon that combines with titanium is removed from solid solution. It has, therefore been "got" by the titanium which explains why the verb "to getter" was coined.) Above 300 K, dislocation glide can occur relatively easily so that the magnitude of the macroscopic yield stress is controlled by the effects of long range internal stress fields on dislocation motion. Therefore, the temperature dependence of the yield stress in this temperature realm is due to the temperature dependence of the shear modulus of iron which decreases gradually as the temperature is increased and vice versa. From 250 K downwards, the variation of the macroscopic yield stress with temperature increases linearly and much more rapidly than the variation above room temperature. The rapid increase in the flow stress at low temperatures appears to occur because the atomic displacements involved in the movement of individual dislocations by glide becomes difficult very rapidly. As the yield stress increases, the ductility decreases, and, if the strain rate is also increased, dislocation glide is replaced by twin formation and brittle fracture by cleavage can occur. The temperature where brittle behavior replaces ductile behavior depends upon the strain rate. Dislocation glide can still occur at temperatures as low as 4.2 K when the strain rate is small.

Interstitial solutes in iron

See also Chapter 2 in Leslie's book and chapter 1 in "Principles of Heat Treatment of Steel by G. Krauss, ASM, 1980.

FCC crystals have octahedral interstitial sites at coordinates 0.5, 0.5, 0.5 and the equivalent 0.5,0,0 positions in the unit cell, see figure 2. These are symmetric sites; i.e. in the hard sphere-central force model for metallic crystals, the centers of these sites are equidistant from the centers of the nearest neighboring atoms. Using 2.52 angstroms for the diameter of FCC iron atoms ( 1 angstrom = 1010 meters) we find that 1.04 angstroms is the diameter of a sphere that will just fit in the octahedral site. We do this by noting that if the lattice parameter is "a" then the length of the diagonal of a

face of the unit cell is a

2

.

This

is

also

equal

to

2DF e ,

where

D Fe

is

the

diameter

of

an

iron

atom

that is assumed to behave as a hard sphere. Thus we can evaluate "a" and then the diameter of the

octahedral hole which is equal to ( a - DFe). The tetrahedral sites are at 0.25, 0.25, 0.25, for which we find the diameter of the hole to be 0.27 Angstroms. Because of this difference in size, the

interstitial solutes C and N ( of which C has a radius of about 0.8 angstroms

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