Mech 473 Lectures

Professor Rodney Herring

Microstructure of a Slowly Cooled Hypoeutectoid Steel

Hypoeutectoid steels have a carbon content of < 0.77

Eg., 0.52 %C in the figure below, At temperatures “a”, the steel is 100% austenite At temperature “b”, it enters the two-phase ferrite + austenite field and ferrite nucleates heterogeneously at grain boundaries – see A) At temperature “c”, the ferrite grains have grown in size – see B) At temperature “d”, the austenite has transformed to pearlite – see C), the ratio of ferrite:pearlite is the same as ferrite:austenite at temperature “c” On cooling from “d” to room temperature, no further changes occur in the microstructure.

A) B)

C

)

Application of the Lever Rule

• •

The ratio of the components of the 0.52 C hypoeutectoid steel can be expressed in terms of: Either the observed microstructure constituents – ferrite and pearlite Or the phases in equilibrium – ferrite and cementite Microstuctural constituents At temperatures “

d

”, the intersection of the tie lines show that: (proeutectoid) ferrite = 0.022 %C and pearlite = 0.77 %C Hence according to the lever rule:

proeutecto id ferrite  0.77

0.77

0.52

0.022

 33 % pearlite  0.52

0 .

77  0.022

0 .

022  67 %

This is also the ratio of proeutectoid ferrite:pearlite at room temperature, “

e

” .

Application of the Lever Rule

Phase equilibria At temperature “

d

”, the compositions are: ferrite = 0.02 and cementite = 6.67

.

Again using the lever rule:

total ferrite  6.67

6.67

0.52

0.022

 92 % total cementite  0.52

6 .

67  0.022

0 .

022  8 %

The amount of ferrite in the pearlite , ie., “ eutectoid ferrite ”, is given by the percentage ferrite in pearlite times the percentage pearlite in the steel (67%).

In a previous calculation, the constitution of pearlite was shown to be eutectoid ferrite = 88% and cementite = 12% Hence: The amount of eutectoid ferrite = 88% x 67% = 59% Note: the total ferrite = 33% proeutectoid + 59% eutectoid = 92% , which is the same amount as given above.

The amount of eutectoid cementite = 12% x 67% = 8% , which is the total cementite in the steel, as calculate above.

Isothermal Transformation Diagram for Hypereutectoid Steel The diagram below refers to a steel with 1.13 %C and 0.3% Mn The line m-n , top left corner, is the temperature dependence of the start of the precipitation of the proeutectoid cementite from austenite .

After longer holding times, the line m-n asymptotically approaches the equilibrium temperature for the precipitation of cementite.

Precipitation of proeutectoid cementite ceases at the eutectoid temperature , which is marked A s TTT diagram.

on the The pearlite and bainite reactions are of the same form as those described previously for the eutectoid steel (0.79 %C, 0.76 % Mn)

Continuous Cooling Transformations

The TTT diagrams are useful to identify the phases which can form in steels, but in practice it is very expensive to heat treat steels isothermally.

The economic technique is to cool steels continuously to room temperature, which shifts the transformation lines from the dashed to solid lines.

If the steel is cooled at a rate of 1 on the diagram, after 6 s, it will be at point “b” because it has cooled more slowly than a direct quench to this temperature.

The structure of the steel will thus be that of an isothermal sample at “a”.

The finish of the pearlite reaction will be depressed from “c” to “d” .

Hence two further curves can be drawn representing the start and finish of the pearlite reaction during continuous cooling .

Continuous Cooling Transformations

A sample cooled at rate 1 will cross both the pearlite start and pearlite finish curves and thus be 100% pearlite at room temperature .

A sample cooled at the faster rate of 2 will cross the pearlite start curve and thus transform partly to pearlite but will not cross the pearlite finish curve.

Since the bainite curves will also be moved downward and to the right, no bainite will form and the RT structure will be pearlite + martensite .

Effect of Different Continuous Cooling Rates

A sample cooled at a rate to just miss the knee of the continuous cooling pearlite start curve will be 100% martensite at RT, which is the critical cooling rate .

Cooling rates for different heat treatments are shown, which give a mixture of pearlite and martensite, but only a negligible amount of bainite.

Effect of Sample Size on Cooling Rates

On continuous cooling, a plain carbon steel is composed essentially of a mixture of pearlite and martensite depending on the cooling rate.

However, the centre of a relatively large size sample will generally cool at a slower rate than the outside.

For a slow cooling rate , such as full annealing, the surface and centre will be at much the same temperature throughout the cooling process.

For a rapid quench the temperature difference will be much greater.

In the extreme, the outside may be martensite and the centre pearlite , as shown in the graph.

The critical cooling rate and the cooling rate to give 50/50 pearlite/martensite (50/50 P/M) are also shown.

50/50

Hardness of Steels with Various Microstructures

Notes: The shaded area refers to the loss of hardness due to retained austenite.

Pearlite structures are significantly less hard than martensite .

Tempered steels with spheroidized carbide structures have the lowest hardness.

Hardness of Quenched Samples

The change in microstructure from pearlite to martensite on a transverse path from the centre to the surface of the bar results in a progressive increase in hardness from Rockwell C-40 to C-65.

The value of Rockwell C-54 = the hardness of 50/50 P/M in a eutectoid steel , which intersects the hardness transverse in the region of its greatest slope.

The depth below the surface equivalent to this hardness value can thus be determined with some precision .

The depth of 50/50 P/M can be determined metallographyically by examining the structure under a microscope or by macroscopic etch contrast since pearlite is darker than martensite.

Variables Affecting 50/50 P/M Depth

1) Size of the sample 2) Severity of the quench 3) Composition of the steel 4) Grain size of the austenite 5) Sample Size – for a given range of rapidly quenched bar shaped samples, there will be a unique diameter at which the centre of the bar will be exactly 50/50 P/M .

This is known as the “ critical diameter ”, D , and is used as a measure of the ability of the steel to harden during heat treatment = “ hardenability ” .

The critical diameter depends on 1) the severity of the quench, 2) the composition of the steel and 3) the austenite grain size.

diameter of bar (inches), D = 1”

Significance of Hardenability

High hardenability is desired for a component, which must be martensitic through the entire diameter and then tempered to enhance ductility.

Large structural components cannot be heat treated in this manner, so medium strength with good ductility is obtained by having a mixture of martensite and pearlite, i.e., with a moderate hardenability . Welded steels require a low hardenability to avoid the formation of martensite, which causes a brittle region adjacent to the weld.

There is not a unique answer to the question, “which is the best hardenability?” so first define the component in terms of its operational use and then specify the steel and its heat treatment.

Severity of the Quench

During a quench, the liquid (oil, water) adjacent to the hot surface is usually instantly evaporated and can form an insulating layer, which delays further cooling.

This can be prevented by agitation, which continuously removes the bubbles.

Lowering the temperature, lowers the vaporization, eg., iced brine vs water.

The severity of the quench can thus be expressed in terms of a parameter, H .

H Value 0.20

0.35

0.50

0.70

1.00

1.50

2.00

5.00 Infinity Quenching Condition Poor oil quench – no agitation Good oil quench – moderate agitation Very good oil quench – good agitation Strong oil quench – violent agitation Poor water quench – no agitation Very good water quench – strong agitation Brine quench – no agitation Strong brine quench – violent agitation Ideal quench (t=0)

Ideal Critical Diameter

The effect of the severity of the quench can be eliminated by referring all cooling rates to a standard condition – know as the “ ideal quench ”.

This is a hypothetical cooling treatment , which would bring the sample instantly to the desired temperature, i.e., an infinite cooling rate.

The diameter of a sample, which would have 50/50 P/M at its center when cooled by an ideal quench is known as the “ideal quench diameter” = D 1 The effect of the ideal quench on the microstructure of the steel is computed in comparison to known quenching conditions.

This comparison is expressed in terms of the critical diameters – D and D 1 , for different quenching severities, as expressed by the parameter, H .

Eg., a critical diameter D = 1.0 derived from a quench of H = 2.0 (iced brine) is equivalent to an ideal critical diameter of D 1 of 1.4

Factors Influencing Hardenability

A steel with a high hardenability will transform from austenite to martensite rather than pearlite even at relatively slow cooling rates.

A steel with a low hardenability will only form martensite when it is rapidly quenched from the austenite phase.

A high hardenability is thus related to the suppression of the pearlite reaction or a shift of the TTT curves in continuous cooling diagrams to the right, i.e., toward higher time values .

• • •

The factors, which can cause such a shift are: 1) Grain size of the austenite 2) The carbon content of the steel 3) Other alloying elements We shall look at these factors in turn .

Grain Size of Austenite

When a steel is austenitized the ferrite and carbide react to form a single phase ( increase in temperature from two phases to one ).

The austenite grains are nucleated heterogeneously at ferrite carbide interfaces.

With large interfacial area between the ferrite and carbide, a large number of nuclei are formed, which leads to a small austenite grain size unless the steel is held for a long time, which will allow grain growth to occur.

Grain size is measured by the ASTM grain size number – N – defined by

n  2 N 1

Where n is the number of grains per square inch seen at 100 x mag.

Grain Size of Austenite

The larger the grain size number , the larger the number of grains per inch and hence the smaller the actual grain size .

ASTM Grain Size Number (N) 1 2 3 4 5 6 7 8 Avg. No. of Grains/in 2 at 100x magnification 1 2 4 8 16 32 64 128

Effect of Austenite Grain Size on Hardenability

Pearlite nucleates heterogeneously at austenite grain boundaries.

The number of pearlite nuclei that form per unit time, ie., nucleation rate, is directly proportional to the grain boundary surface area , which is inversely related to the austenite grain size .

Hence, a fine grain steel, with ASTM No 7 will have four times the grain boundary area of a course grained steel with ASTM No 3.

The rate of nucleation of pearlite will be much more rapid in the fine grained steel , which will consequently have a lower hardenability.

The use of a large grain size to increase hardenability is counterproductive however, because this increases the risk of quench defects and usually causes a loss of ductility if not actually creating brittleness.

Effect of Carbon Content on Hardenability

For a given austenite grain size, the hardenability of a steel is strongly increased with increasing carbon content .

However, it should be noted that the hardenability of all plain carbon steels is in fact very low , because of their propensity to form pearlite.

Eg., a eutectoid steel with 0.8 %C and ASTM No 8 has D 1 = 7.1 mm (0.28 in) To obtain 100% martensite even by a very rapid brine quench would thus require a bar less than 6.35 mm (0.25 in) in diameter .

This is a consequence of the fact that the knee of the pearlite continuous cooling curve lies very close to the left hand vertical axis .

Effect of Alloying Elements on the TTT Curves

• •

Of the interstitial solutes Carbon has a strong effect Boron has a much greater effect than carbon

• •

Of the substitutional solutes Mn > Mo > Cr > Si > Ni > Cu in terms of the shift of the TTT curves to the right.

V and Co have a negative effect, ie., they shift the TTT curves to the left.

(See also the future notes on slide 39 and 40 in Lecture 9)

Effect of Carbon on TTT Curves

Hypoeutectoid steel Hypereutectoid steel Note also the change in Ms Note presence of bainite and mixed grain size

Magnified View

Effect of Manganese (.46 - .91) on TTT Curves

Effect of Molybdenum (.53 – 1.96) on TTT Curves

Note the retardation of ferrite and retension of bainite.

Effect of Chromium (0.93 – 12.18) on TTT Curves

Note the removal of bainite and increased hardenability to form 100% martensite.

Effect of Silicon (2.32 – 3.8) on TTT Curves

Note the slight enhancement of bainite and reduction in pearlite formation.

Effect of Nickel (0.99 – 3.9) on TTT Curves

Note the decreased hardenability by the reduction in pearlite formation.

Effect of Copper (0.46 – 1.49) on TTT Curves

Note the slight enhanced hardenability by the reduction in pearlite formation.

Effect of Ni-Cr (3.33, 1.52) to Ni-Mo (1.84, 0.23)on TTT Curves 100% Martensite Note the significant decrease in hardenability by the enhancement in pearlite formation.

Effect of Cr-V (0.92, 0.16) and Cr-Mo (0.98, 0.21) on TTT Curves Note slight increase in hardenability and loss of pearlite.

Effect of Multiple Alloying Additions on TTT Curves

Note significant reduction in hardenability and enhancement of proeutectoid ferrite.

Effect of Multiple Alloying Additions on TTT Curves

Note reduction in hardenability and enhancement of proeutectoid ferrite and pearlite.

Calculations of D

1

from Chemical Composition

The critical diameter, D 1 of a steel containing < 1 % C and < % of common alloying elements can be determined using a series of multiplication factors derived from a large number of TTT curves .

The critical diameter, D 1 , of an equivalent plain carbon steel is first obtained from a knowledge of its carbon content and grain size , and this value is then successively multiplied by constants for the compositions of the different alloying elements.

Multiplying Factor

Example of D

1

Calculation from Composition

Using the compositions given below and assuming a grain size of ASTM No. 7, calculate D 1 .

C = 0.38 – 0.43% Mn = Si = Ni = Cr = 0.75 – 1.00% 0.20 – 0.35% 0.40 – 0.70% 0.40 – 0.60% Mo = 0.15 – 0.25% Using the maximum compositions for C (previous provided figure - see next slide) and Mn (from the values listed in the table) gives D 1 for an equivalent plain carbon steel . D 1 = 0.221 x 4.333 inch = 0.96 inch Taking into account the full alloy content gives, D 1 = 0.221 x 4.333 x 1.245 x 1.255 x 2.296 x 1.75 = 6.00 inch.

- a significant difference in D 1 .

Results of D

1

Calculation from Composition

So, even trace amounts of alloy solutes have a large effect on hardenablity .

A similar calculation using the minimum alloy compositions gives D 1 = 2.72 inch Which is less than half the value for the maximum permitted compositions.

Steel manufactured in a given plant are usually made to a much tighter specification within these permitted limits.

If a particular D 1 is required, rather than order a steel of the general specification from a warehouse, which would likely cost a lot, it is better to examine the output from various mills and order the one which manufactures to the required tolerance with the specification.