HARDENABILITY

Dr. H. K. KhairaProfessor in MSME

MANIT, Bhopal

Introduction

• Hardenability is one of the most important properties of a steel because it describes the ease with which a given steel can be quenched to form martensite or the depth to which martensite is formed on a given quench.

• It is an important property for welding, since it is inversely proportional to weldability, that is, the ease of welding a material.

Introduction

• The ability of steel to form martensite on quenching is referred to as the hardenability.

• Hardenability is a measure of the capacity of a steel to be hardened in depth when quenched from its austenitizing temperature.

• Steels with high hardenability form martensite even on slow cooling.

• High hardenability in a steel means that the steel forms martensite not only at surface but to a large degree throughout the interior.

Introduction

• For the optimum development of strength, steel must be fully converted to martensite.

• To achieve this, the steel must be quenched at a rate sufficiently rapid to avoid the decomposition of austenite during cooling to such products as ferrite, pearlite and bainite.

Introduction

• Hardenability of a steel should not be confused with the hardness of a steel.

hardness hardenabilty

Introduction

Hardenability• The Hardness of a steel is a

measure of a sample's resistance to indentation or scratching,

Hardness• Hardenability refers to its

ability to be hardened to a particular depth under a particular set of conditions.

Hardenability• It is a qualitative measure of the rate at which hardness

drops off with distance into the interior of a specimen as a result of diminished martensite content.

• Hardenability is more related to depth of hardning of a steel upon heat treat.

• The depth of hardening in a plain carbon steel may be 2-3 mm vs 50 mm in an alloy steel.

• A large diameter rod quenched in a particular medium will obviously cool more slowly than a small diameter rod given a similar treatment. Therefore, the small rod is more likely to become fully martensitic.

Hardenability

• The hardenability of a steel is the maximum diameter of the rod which will have 50% martensite even in the core when quenched in an ideal quenchant. This diameter is known as Di or ideal diameter.

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Relation between cooling curves for the surface and core of an oil-quenched 95 mm diameter bar

Determination of Hardenability

• There are TWO methods to determine hardenability of steels

– Grossman’s Method– Jominy end quench method

Grossman’s method • In Grossman’s method, we use round bars of different

diameters.• These bars are quenched in a suitable quenchant.• Further, we determine the critical diameter (dc) which is the

maximum diameter of the rod which produced 50% martensite on quenching.

• The ideal diameter (DI) is then determined from the curve. • This type of experiment requires multiple austenitization

and quenching treatments on specimens of varying diameter just to quantify the hardenability of a single material.

Radial hardness profile of cylindrical steel samples of different diameter and composition.

0.4C+1.0Cr+0.2Mo→

Quench in water

0.4C only →

Effect of Composition Diameter

Hardenability Curves

Jominy End Quench Method• Grossmans method requires multiple austenitization and quenching treatments

on specimens of varying diameter just to quantify the hardenability of a single material.

• An alternative approach is to develop a more convenient standard test method that can be used for relative comparison of hardenability. The Jominy end-quench test is one such approach.

• The jominy end-quench test is specified in ASTM standard A255 and is a widely used method for quantifying hardenability. Its wide use adds to its value, since the utility of empirical relations and data comparison becomes more reliable as more data are accumulated.

• Moreover, Jominy data have been collected on a large enough scale to offer a high degree of statistical certainty for many steels.

• These data have been correlated with measurements and/or calculations of dc. • By using these correlations, a single Jominy test can be used to estimate dc and DI

for a given steel (and austenite grain size).

The Jominy End Quench Test

• The most commonly used method for determining hardenability is the end quench test developed by Jomini and Boegehold.

• The details of the test are covered in IS : 3848 – 1981 and ASTM A 255.

The Jominy End Quench Test

• The Jominy End Quench Test measures Hardenability of steels.

• Information gained from this test is necessary in selecting the proper combination of alloy steel and heat treatment to minimize thermal stresses and distortion when manufacturing components of various sizes.

Principle• The hardenability of a steel is measured by a Jominy test: • A round metal bar of standard size is transformed to 100%

austenite through heat treatment, and is then quenched on one end with room-temperature water.

• The cooling rate will be highest at the end being quenched, and will decrease as distance from the end increases.

• The hardenability is then found by measuring the hardness along the bar: the farther away from the quenched end that the hardness extends, the higher the hardenability.

Jominy Test

The Jominy bar measures the hardenbility of a steel

Softest

Hardest

Cooling Rates at Different Jominy Distances

Cooling rate and Jominy distance (distance from the quenched end) do not change with alloying elements as the rate of heat transfer is nearly independent of composition

Steps in Jominy End Quench Test

• First, a sample specimen rod either 100mm in length and 25mm in diameter, or alternatively, 102mm by 25.4mm is obtained.

• Second, the steel sample is normalized to eliminate differences in microstructure due to previous forging, and

• Then it is austenitised. This is usually at a temperature of 800 to 900°C. • Next, the specimen is rapidly transferred to the test machine, where it is

held vertically and • Sprayed with a controlled flow of water onto one end of the sample.

This cools the specimen from one end, simulating the effect of quenching a larger steel component in water. Because the cooling rate decreases as one moves further from the quenched end, you can measure the effects of a wide range of cooling rates from vary rapid at the quenched end to air cooled at the far end.

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Hardenability

• How is the hardenability of steels assessed?

– Jominy End-Quench Test

– Test bar is heated to form 100% austenite. It is then quenched directly at one end with a stream of water

Jominy End Quench Test

Details of Jominy Test for Hardenability

All dimensions are in inches

Jominy End Quench Test• After end quenching, longitudinal Flat Surfaces are ground on opposite

sides of the test piece as per dimensions. The specimen is ground flat along its length to a depth of .38mm (15

thousandths of an inch) to remove decarburized material.This grinding is very important for correct positioning of the sample in the

fixture and also for accurate repeatable and reliable test results.

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• Jominy end quench test to measure hardenability.

24°C water

specimen (heated to phase field)

flat ground4”

1”

Fig. 14.5

Hardenability of Steels

Jominy End Quench Test

•The hardness is measured at intervals along its length beginning at the quenched end. Hardness at equal intervals (1 mm or 1/16”) to be checked and noted.

Plotting of Result

Plot the resulting data on graph paper with hardness value as ordinate (Y axis) and distance from the quenched end as abscissa (X axis).

Hardenability CurveHa

rdne

ss, H

RC

Distance from quenched end

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Heat Treatment of Steels: HardenabilityThe cooling rate varies throughout the length of the bar, the rate being highest at the lower end which is in direct contact with water.The hardness along the length of the bar is then measured at various distances from the quenched end and plotted in a graph.The greater the depth to which the hardness penetrates, the greater the hardenability of the alloy.

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Figure 12.23 The hardenability curves for several steels.

Hardenability Curve

• Because the cooling rate decreases as one moves further from the quenched end, we can measure the effects of a wide range of cooling rates from vary rapid at the quenched end to air cooled at the far end.

• By comparing the curves resulting from end quench tests of different grades of steels, their relative hardenability can be established. Thus the flatter the curve, the greater the hardenability.

Hardenability Curve

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Cooling Curves and Phases at different Jominy Distances

• A correlation may be drawn between position along the Jominy specimen and continuous cooling transformations.

• For example, figure shows a continuous cooling transformation diagram for a eutectoid iron-carbon alloy onto which is superimposed the cooling curves at four different Jominy positions, and corresponding microstructure that result from each.

Cooling curves from Jominy Distances

Cooling Curves and Phases at different Jominy Distances

Determination of Hardenability from Jominy Test Graph

• After plotting the Jominy distance Vs Hardness curve, the Jominy distance having hardness equal to 50 % martensite is determined.

• Then the diameter of a rod having cooling rate similar to the cooling rate at the Jominy distance having 50 % martensite is determined from the graph corelating the Jominy distance with the diameter of the rod having similar cooling rate for water quenching .

• This diameter gives the hardenability of the steel in water quenching (having H value equal to 1).

• Hardenability in any other quenchant can be determined from the same graph.

• Di (hardenability in ideal quenching medium) can also be determined in a similar manner.

• We can determine hardenability for any other amount of martensite in the core in any quenchant in a similar way.

Grossman chart used to determine the hardenability of a steel bar

For Jominy distance 4, the hardenability in water quenching is 1.1 Inch.

Hardenability Curves

Dr. Ken Lewis ISAT 430 40Spring 2001

Quenching Media

• The fluid used for quenching the heated alloy effects the hardenability.– Each fluid has its own thermal properties

• Thermal conductivity• Specific heat• Heat of vaporization

– These cause rate of cooling differences

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Coefficient of severity of quench: H

• Cooling capacities (Severity of quench) of quenching medium is known as H value.• H values of some of the quenchants are given below.• Cooling rates are at the center of a 2.5 cm bar.

H Value Cooling Rate (0C/s)– Ideal Quench ∞ ∞– Agitated brine 5 230– Brine (No agitation) 2 90– Agitated Water 4 190– Still water 1 45– Agitated Oil 1 45– Still oil 0.25 18– Cold gas 0.1 -– Still air 0.02 -

Effect of Agitation on Coefficient of severity of quench: H

Cooling MediumAgitation Oil Water Brine

Violent 0.8-1.1 4.0 5.0Strong 0.5-0.8 1.6-2.0Good 0.4-0.5 1.4-1.5Moderate 0.35-0.40 1.2-1.3Mild 0.30-0.35 1.0-1.1 2.0-2.2None 0.25 - 0.30 0.9-1.0 2.0

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Ideal Quenchant

• Ideal quenchant is one which brings down the surface temperature to room temperature instantaneously and keeps it at that temperature thereafter.

Grossman chart can be used to determine the hardenability of a steel bar for different quenchants.

• ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.

If a steel is having 1.1” hardenability, it will have 1.6” hardenability in a quenchant with H value equal to 5.

Similarly, it will have 0.4” and 0.9” hardenability in quenchants with H value 0.2 and 0.5 respectively.

Effect of DI and “H” on D

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Factors affecting Hardenability

• Slowing the phase transformation of austenite to ferrite and pearlite increases the hardenability of steels.

• The most important variables which influence hardenability are– 1. Austenite grain size – 2. Carbon content– 3. Alloying elements

Austenitic Grain Size

• The hardenability increases with increasing austenite grain size, because the grain boundary area which act as nucleating site is decreasing.

• This means that the sites for the nucleation of ferrite and pearlite are being reduced in number, with the result that these transformations are slowed down, and the hardenability is therefore increased.

Effect of austenite grain size onHardenability

• The more γ-grain boundary surface the easier it is for pearlite to form rather thanmartensite

• Smaller γ-grain size → lower hardenability• Larger γ-grain size → higher hardenability

Effect of Austenitic Grain size

Carbon Content

• Carbon is primarily a hardening agent in steel.• It also increases hardenability by slowing the

formation of pearlite and ferrite. • But its use at higher levels is limited, because of

the lack of toughness which results in greater difficulties in fabrication and, most important, increased probability of distortion and cracking during heat treatment and welding.

Carbon and Hardenability

Hardenability of a steel increases with increase in C content TTT diagram moves to the right.

Effect of Austenitic Grain size and Carbon Content on Di

Effect of Alloying Elements• most metallic alloying elements slow down the ferrite and pearlite

reactions, and so also increase hardenability. However, quantitative assessment of these effects is needed.

• Chromium, Molybdenum, Manganese, Silicon, Nickel and Vanadium all effect the hardenability of steels in this manner. Chromium, Molybdenum and Manganese being used most often.

• Boron can be an effective alloy for improving hardenability at levels as low as .0005%. – Boron is most effective in steels of 0.25% Carbon or less. – Boron combines readily with both Nitrogen and Oxygen and in so doing its

effect on hardenability is sacrificed. – Therefore Boron must remain in solution in order to be affective. – Aluminum and Titanium are commonly added as "gettering" agents to react

with the Oxygen and Nitrogen in preference to the Boron.

Effect of Alloying Elements• The most economical way of increasing the hardenability

of plain carbon steel is to increase the manganese content, from 0.60 wt% to 1.40 wt%, giving a substantial improvement in hardenability.

• Chromium and molybdenum are also very effective, and amongst the cheaper alloying additions per unit of increased hardenabilily.

• Boron has a particularly large effect when it’s added to fully deoxidized low carbon steel, even in concentrations of the order of 0.001%, and would be more widely used if its distribution in steel could be more easily controlled.

Effect of Alloying Elements

Cr, Mo, W, Ni

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• Hardenability of a steel increases with addition of alloying elements such as Cr, V, Mo, Ni, W TTT diagram moves to the right.

Jominy hardenability curves: Hardenability improves with increasing Mo content

Hardenability curves of 6 steels

Effect of Alloying Elements• all steels have 0.4wt% C, but with

different alloying elements.– At the quenched end all alloys have

the same hardness, which is a function of carbon content only.

– The hardenability of the 1040 is low because the hardness of the alloy drops rapidly with Jominy distance. The drop of hardness with Jominy distance for the other alloys is more gradual.

– The alloying elements delay the austenite-pearlite and/or bainite reactions, which permits more martensite to form for a particular cooling rate, yielding a greater hardness.

Effect of Alloying Elements

Hardness of 42 at center is obtained in bars of different diameters in different steels indicating different hardenabilities.

Effect of Alloying Elements

Hardness at center of a 3 inch bar is different for different steels indicating different amounts of martensite at the center

Hardenability Multiplying Factor• The Hardenability Multiplying Factor shows the rate at

which the hardening depth is increased with the percentage of the alloying element

• The ideal diameter (DI ) is calculated from:

DI = DIC * ƒMn *ƒSi *ƒNi*ƒCr *ƒMo

Where DIC is the basic DI factor for carbon and ƒx is the multiplying factor for the alloying element x.

Multiplying Factors For The Calculation Of Ideal Diameter

Base ideal diameter, DIjominy Carbon grain size Alloying factor, fX

% No. 6 No. 7 No.8 Mn Si Ni Cr Mo0.05 0.0814 0.0750 0.0697 1.167 1.035 1.018 1.1080 1.150.10 0.1153 0.1065 0.0995 1.333 1.070 1.036 1.2160 1.300.15 0.1413 0.1315 0.1212 1.500 1.105 1.055 1.3240 1.450.20 0.1623 0.1509 0.1400 1.667 1.140 1.073 1.4320 1.600.25 0.1820 0.1678 0.1560 1.833 1.175 1.091 1.5400 1.750.30 0.1991 0.1849 0.1700 2.000 1.210 1.109 1.6480 1.900.35 0.2154 0.2000 0.1842 2.167 1.245 1.128 1.7560 2.050.40 0.2300 0.2130 0.1976 2.333 1.280 1.246 1.8640 2.200.45 0.2440 0.2259 0.2090 2.500 1.315 1.164 1.9720 2.350.50 0.2580 0.2380 0.2200 2.667 1.350 1.182 2.0800 2.500.55 0.2730 0.2510 0.2310 2.833 1.385 1.201 2.1880 2.650.60 0.284 0.262 0.2410 3.000 1.420 1.219 2.2960 2.800.65 0.295 0.273 0.2551 3.167 1.455 1.237 2.4040 2.950.70 0.306 0.283 0.260 3.333 1.490 1.255 2.5120 3.100.75 0.316 0.293 0.270 3.500 1.525 1.273 2.6200 3.250.80 0.326 0.303 0.278 3.667 1.560 1.291 2.7280 3.400.85 0.336 0.312 0.287 3.833 1.595 1.309 2.8360 3.550.90 0.346 0.321 0.296 4.000 1.630 1.321 2.9440 3.700.95 - - - 4.167 1.665 1.345 3.0520 -1.00 - - - 4.333 1.700 1.364 3.1600 -

Hardenability Multiplying Factor

Exceptions

• S - reduces hardenability because of formation of MnSand takes Mn out of solution as MnS

• Ti - reduces hardenability because it reacts with C to form TiC and takes C out of solution; TiC is very stable and does not easily dissolve

• Co - reduces hardenability because it increases the rate of nucleation and growth of pearlite

Hardenability Band

• The industrial products of steels may change composition and average grain size from batch to batch, therefore, the measured hardenability of a given type of steel should be presented as a band rather than a single line, as demonstrated by the Figure at right.

Hardenability Band

• Hardenabilily data now exists for a wide range of steels in the form of maximum and minimum end-quench hardenability curves, usually referred to as hardenability bands. This data is, available for very many of the steels listed in specifications such as those of the American Society of Automotive Engineers (SAE), the American Iron and Steel Institute (AISI) and the British Standards.

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Hardenability Band

During the industrial production of steel, there is always a slight, unavoidable variation in composition and average grain size from one batch to another. This variation results in some scatter in measured hardenability data, which frequently are plotted as a band representing the max and min values.

Effects of composition variation and grain size change on the hardenability of alloy steels

Hardenability (as Range of Di Values) of Various Steels

Example

• Calculate the approximate hardenability of an 8630 (0.3%C, 0.3%Si, 0.7%Mn, 0.5%Cr, 0.6%Ni,

0.2%Mo) alloy steel with an ASTM grain size of 7

Solution

• Find out base DI for 0.3% carbon• Calculate multiplying factors for each element• Ideal critical diameter found by multiplying

base diameter by the multiplying factors

Summery• The hardenability of ferrous alloys, i.e. steels, is a function of the carbon

content and other alloying elements and the grain size of the austenite. • The relative importance of the various alloying elements is calculated by

finding the equivalent carbon content of the material. • The fluid used for quenching the material influences the cooling rate

due to varying thermal conductivities and specific heats. • Substances like brine and water cool much more quickly than oil or air. • Additionally, if the fluid is agitated cooling occurs even more quickly. • The geometry of the part also affects the cooling rate: of two samples of

equal volume, the one with higher surface area will cool faster.

Numerical problem -1

• Predict the center hardness in a water quenched 3” bar of 8640

Solution to Numerical Problem - 1The cooling rate at the center of a 3” dia bar in water quenching will be same as that at Jominy distance 17 mm.

Water Quenched Oil Quenched

Jominy Distance =17mm

Effect of Alloying Elements

Hardness produced at Jominy distance 17 mm in 8640 steel will be 43 HRC.

Therefore, the hardness at center of a 3 inch bar will be 43 HRC

Cooling rate and Jominy distance do not change with alloying elements as the rate of heat transfer is nearly independent of composition

Equivalent bar diameter when quenched

• When the end-quench hardness curve of a steel has been found, this table enables the user to estimate the hardnesses that would be obtained at the centers of quenched round bars of different diameters, when that same steel is quenched with various severities of quench. For each successive 1/16 in. position, the hardness obtained in the end-quench test would be found at the center of the bar size.

Equivalent bar diameter when quenched

Cooling Rate at Each Jominy Positionfor Room Temperature Water

Distance from water quenched end Cooling Rate 1/16 in. 0C/s1 2702 1703 1104 705 436 317 238 189 1410 11.912 9.114 6.916 5.618 4.620 3.9

Jominy test and CCT diagrams

Influence of quench medium and sample size on the cooling rates at different locations.

• Severity of quench: Water > Oil > Air, e.g. for a 50 mm diameter bar, the cooling rate at center is about 27°C/s in water, but, 13.5 °C/s in oil.

• For a particular medium, the cooling rate at center is lower when the diameter is larger. For example, 75mm vs. 50mm.

Dr. Ken Lewis ISAT 430 82Spring 2001

Other quenching concerns

• Fluid agitation– Renews the fluid presented to the part

• Surface area to volume ratio• Vapor blankets

– insulation• Environmental concerns

– Fumes– Part corrosion

Correlation of carbon and martensite content withRockwell hardness

Alloy Factors For The Calculation Of Ideal Diameter

Interconversion of ideal bar diameter as a function of shape

Thanks