ME2151-1

LAB MANUAL

ME 2151-1 COOLING RATE EFFECT

SESSION: 2012/2013

Department of Mechanical Engineering

National University of Singapore CONTENTS Page No. TABLES OF CONTENTS

(i)

TERMINOLOGY

(ii)

INTRODUCTION

1

THEORY

1

DESCRIPTION OF APPARATUS

7

EXPERIMENTAL PROCEDURE

9

REFERENCES

9

APPENDIX I

10

APPENDIX II

11

APPENDIX III

12

TERMINOLOGY The term phase may be defined as a chemically and structurally homogeneous region of material. A phase diagram is a graphical representation of the phases present and the ranges in composition, temperature, and pressure over which the phases are stable.

Phases in the Fe-Fe3C System _____________________________________________________________________ Atomic Phase packing Description and comments _____________________________________________________________________ Liquid

d.r.p

Liquid solution of C (carbon) in Fe (iron)

(densely random packing)

δ(also called delta iron)

b.c.c

(body- centered cubic)

γ(also called austenite)

f.c.c

(face- centered cubic)

α(also called ferrite)

Fe3C(also called iron carbide or cementite) (α + Fe3C) Pearlite

b.c.c

orthorhombic

-

Random interstitial solid solution of C in b.c.c. Fe. Random interstitial solid solution of C in f.c.c. Fe. Maximum solubility is 2 wt% at 11470C. It is normally not stable at room temperature. Random interstitial solid solution of C in b.c.c. Fe. Maximum solubility is 0.02 wt % at 7230C. It is the softest structure appears on the diagram. A hard and brittle interstitial compound of Fe and C containing 25 atomic % (6.7 wt%) C.

It is very fine platelike or lamellar eutectoid mixture of ferrite and cementite containing 0.8 wt% C and formed at 7230C on very slow cooling _____________________________________________________________________

Martensite- It is a supersaturated solid solution of carbon in b.c.t (body-centered tetragonal) iron. This meta-stable phase is formed under very rapid cooling.

INTRODUCTION The purpose of this manual is to provide instructions to enable student to investigate the effects of cooling rate and carbon content on the microstructure and hardness of commercial steel. This manual contains some fundamental theory for understanding the experiment and description of the apparatus for hardness measurement. THEORY Pure iron is undergone polymorphic changes depending upon temperature, as indicated in Fig.1. When liquid iron first solidifies at 15390C, it is in δ form. Upon further cooling at 14000C, a phase change occurs and the atoms rearrange themselves into the γ form, which is nonmagnetic. When the temperature reaches 9100C, another phase change occurs from γ to α iron (magnetic). Alloys of iron and carbon (with other elements intentionally added for special purposes) comprise the commercially important ferrous-base alloys known as steels and cast irons. These alloys, particularly the steels, are susceptible to heat-treatment, and a wide range of properties can be obtained by proper variation and timing of heating and cooling cycles.

Fig.1 Temperature ranges in which allotropic forms of iron exist under equilibrium conditions. Carbon atoms are small compared to iron atoms, and have a radius ratio (carbon to iron) of 0.63. Consequently, any solute carbon forms an interstitial solution. Since the biggest interstices in gamma (f.c.c) iron are larger (0.52 A radius) than the largest in alpha (b.c.c) iron (0.36 A radius). We expect greater solubility of carbon in γ than in α. This does occur as indicated in Fig. 2, the so-called Iron-Carbon Equilibrium Phase Diagram

1

which indicates that γ can dissolve a maximum of 2.0 wt% carbon at 1147 0C, while α can dissolve a maximum of only 0.02 wt% carbon at 7230C.

Fig.2 The Fe-Fe3C meta-stable portion of iron-carbon equilibrium phase diagram. Plain carbon steels are generally considered to contain up to 2 wt% carbon whereas cast irons range from 2 to 5 wt% carbon. Consider in detail three alloy steels containing of 0.4% carbon, 0.8% carbon, and 1.2% carbon cooled from slightly above the upper critical temperature. At the temperature of starting cooling all three alloys are in the form of austenite. The lowest temperature for an alloy of iron and carbon to exist in the form of 100% austenite or gamma iron is 7230C and the alloy composition is austenite containing 0.8% carbon. At 7230C the austenite changes to a structure known as pearlite. The structure of pearlite consists of alternate layers of ferrite and cementite. When a steel is cooled below 7230C there is a driving force for the eutectoid reaction of: γ (f.c.c iron + 0.8 wt% dissolved carbon) → α (b.c.c iron + 0.02 wt% dissolved carbon) + Fe3C (6.67 wt% carbon). Transformation of austenite to pearlite starts by formation of cementite nuclei at austenite grain boundaries (see Fig.3). Carbon diffuses from the surrounding austenite to the cementite, and the growth of carbide begins. As carbon diffuses, the adjacent austenite is depleted in carbon and transforms to ferrite. With formation of ferrite, there is rejection of carbon from the ferrite region, i.e., effective enrichment of the adjacent austenite. This results in the formation of additional nuclei of cementite. Because of the alternate formation of cementite and ferrite, cementite can only grow away from the boundary of the original austenite grain as a platelet.

2

Fig. 3 Schematic representation of pearlite formation by nucleation and growth; (a) through (d) indicate successive steps in time sequence. Nucleation and growth of alternate plates of cementite and ferrite occur at several points along the austenite grain boundaries. This forms pearlite colonies, which are approximately hemispherical regions of alternate parallel plates of cementite and ferrite. These pearlite colonies grow until the entire austenite grain has been consumed and has become a pearlite structure. The process of pearlite formation is sometimes referred to as sidewise nucleation and edgewise growth. All alloys cooling from the austenite range down to room temperature are compelled to produce some pearlite as part of their final structure. Alloy 1 is a hypoeutectoid steel containing 0.4% carbon i.e. the initial carbon content of the alloy is below the 0.8% carbon value. The γ starts to transform as soon as the alloy enters the austenite,γ + ferrite,α field (Fig.4). "Primary" α nucleates at γ grains boundaries and grows as the steel is cooled from A3 to A1. At A1 the remaining γ (which is now of eutectoid composition) transforms to pearlite as usual. The room temperature microstructure is then made up of primary α + pearlite.

3

Fig. 4 The sequences of change of phases of the three alloys cooling from the austenite region to room temperature. Alloy 2 is a eutectoid steel containing of 0.8% C. Hence no transformation takes place until the steel is cooled to eutectoid temperature (7230C), where though eutectoid reaction austenite to pearlitic transformation takes place. Hence at room temperature we get 100% pearlite in the microstructure (see Fig.4). Alloy 3 is a hypereutectoid steel containing 1.2% carbon i.e. in excess of 0.8% carbon and therefore during cooling from the austenite range this extra carbon must be removed from the austenite. The excess carbon is precipitated out of solid solution but during the process it brings with it ferrite atoms and the final precipitate is not pure

4

carbon but is cementite. As cooling and cementite precipitation continue the remaining austenite becomes gradually lower in carbon content until only 0.83% carbon content. Then it changes to pearlite the reaction temperature being 7230C. Finally we get a room-temperature microstructure of primary Fe3C plus pearlite (Fig.4). Under slow or moderate cooling rates, the carbon atoms are able to diffuse out of the austenite structure resulting proeutectoid ferrite, proeutectoid cementite, and/or pearlite, This transformation takes place by a process of nucleation and growth and is time dependent. With a very rapid cooling rate, insufficient time is allowed for the carbon atoms to diffuse out of solution, and although some movement of the iron atoms takes place, the structure cannot become b.c.c while the carbon is trapped in solution thus transform austenite into a metastable phase known as martensite which is a singlephase, supersaturated solution of carbon in ferrite with carbon atoms located interstitially in a body-centered tetragonal lattice; i.e., the excessive supersaturation distorts the normal b.c.c structure to body-centered tetragonal. The lattice distortion is reflected in mechanical properties of high strength and hardness and low ductility. Figure 5 shows that the hardness of martensite increases rapidly with carbon content.

Fig. 5 The hardness of martensite increases with carbon content because of the increasing distortion of the lattice. The cooling rate for martensite formation must exceed a critical value, which depends on composition and metallurgical history. Martensite will not form unless austenite is cooled below a certain critical temperature (Ms), which depends on composition. Formation continues only if temperature continues to decrease. There is a lower temperature (Mf) at which transformation to martensite is complete. Martensite formation is essentially independent of time. On the TTT (time temperature transformation), also called CCT (continuous cooling transformation) diagram (see Fig.6) curves X, Y and Z, representing different continuous cooling rates that are superimposed.

5

Fig. 6 The relationship between true TTT curves and those representing continuous cooling conditions. Curve X represents a rate of cooling such as might prevail during a normalising (air cooling) process. Transformation of the unstable austenite begins at K and is complete at N and product is fine pearlite. A very rapid cooling is represented by curve Y which just touches the modified transformation-begins curve so unstable austenite persists until at O (on the Ms line) transformation direct to martensite begins. It should be noted that the above discussion is mainly for 0.8% C steel. If the carbon content is either above or below this amount the curves are displaced to the left so that the CCR (critical cooling rate) required to produce a fully martensite structure will be even greater. Ms denotes the start and Mf the finish. It should be noted that if steels contain more than 0.7% C the Mf temperature lies below 00C so that there is some retained austenite unless sub-zero treatment is performed.

Description of Equipment

6

Rockwell hardness test: Hardness may be defined as the resistance of the material to penetration/indentation. The Rockwell hardness test is based on the measurement of the depth of penetration of the indenter into the specimen. The salient features of a typical Rockwell Hardness Testing Machine are shown in Fig. 7.

7

Major load is supplied by a dead weight loading lever (L) having a ratio of 25 to 1. Rockwell number is read from the dial gauge (D), connected to the plunger system in the head of the tester by means of the Index Lever (M) having 5: 1 ratio. The dial gauge is a 1 mm gauge, i.e. one revolution of the pointer equals 1 mm travel of the dial rack. There are 100 divisions to a revolution and as the lever ratio is 5:1, each division on the dial represents a depth of 0.002 mm. When minor load is applied there is a fixed zero or set point. Two scales are provided on the dial gauge. The outer circle is in black, and all readings with diamond indenter are taken on this. The inner circle is in red, and all readings with ball indenters are taken from this. The set point is always same regardless of the scale. Fig. 8 shows steel ball and diamond cone indenter.

The general principals of the Rockwell hardness test are illustrated in Fig. 9 (ball indenter) and Fig. 10 (diamond indenter) and the accompanying Tables 1 and 2 (in appendix I).

Experimental Procedure

8

You will be provided four medium carbon steel specimens having composition of 0.45 % carbon with different cooling rates (furnace cooled, air cooled, fan cooled and water quenched). 1. Observe the microstructures under the optical microscope, as high magnification as possible. 2. Sketch neatly the different phases and structures of each specimen. 3. Determine the hardness of each sample using Rockwell Hardness Tester (procedure and precautions are given in appendix II). 4. Convert all the harness values to Vickers' hardness (HV) through the conversion table given in appendix III. 5. Analyse the results and establish the relationships amongst cooling rate, microstructure and hardness of specimens. References 1. R. A. Higgins, "The Properties of Engineering Materials". 2. M. F. Ashby & D. R. H. Jones, "An Introduction to Microstructures, Processing and Design". 3. S. H. Avner, "Introduction to Physical Metallurgy". 4. C .O. Smith, "The Science of Engineering Material”

Appendix I

9

Table 1 Symbols and Designation Associated with Fig. 9. __________________________________________________________________________________ _ Number Symbol Designation __________________________________________________________________________________ _ 1 3 4 5 6

--P0 P1 P ---

Diameter of ball = 1/16 in. (1.588 mm) Preliminary test force = 10 kgf Additional force = 90 kgf Total test force = P0 + P1 = 10 + 90 = 100 kgf Depth of penetration under test force before application of additional load 7 --Increase in depth of penetration under additional load 8 e Permanent increase in depth of penetration under preliminary test force after removal of additional force, the increase being expressed in units of 0.002 mm. 9 xx HRB Rockwell B hardness = 100-e __________________________________________________________________________________ _

Table 2 Symbols and Designation Associated with Fig. 10. __________________________________________________________________________________ _ Number Symbol Designation __________________________________________________________________________________ _ 1 2

-----

Angle at the top of the diamond indenter (120) Radius of curvature at the tip of the cone (0.200 mm) 3 P0 Preliminary test force = 10 kgf 4 P1 Additional force = 90 or 140 kgf 5 P Total test force = P0 + P1 = 10 + 140 = 150 kgf 6 --Depth of penetration under test force before application of additional load 7 --Increase in depth of penetration under additional load 8 e Permanent increase in depth of penetration under preliminary test force after removal of additional force, the increase being expressed in units of 0.002 mm. 9 xx HRC Rockwell C hardness = 100-e __________________________________________________________________________________ _

10

Appendix II Measurement of Harness 1. 2. 3. 4. 5. 6. 7. 8. 9.

Place the specimen on the plane anvil. Raise the anvil by rotating the capstan slowly until the tip of the indenter touches the specimen. Further elevate the anvil gently until the small pointer in the dial gauge points to the red dot i.e. set position. Turn the outer ring of the indicator gauge until the large pointer reads 0 on the appropriate scale. Push the crank handle to apply the major load. The load is to be maintained for about 10 seconds before pushing crank handle in the reverse direction to remove the major load. Read the hardness number while the minor load is still being applied. Turn the capstan in the reverse direction to lower the anvil and remove the specimen. The next test is carried out now as before.

Precautions 1. 2.

Ensure that both surfaces of the specimens are flat and positioned securely on the anvil. Rotate the capstan gently during elevation of the anvil. Otherwise due to abrupt strike of the indenter tip with the sample, the indenter may be destroyed.

11

Appendix III

12