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44 Hardenability “Hardenability” is the ability of Fe-C alloy to be hardened by the forming martensite as a result of a given heat treatment. For every different steel alloy there is a specific relationship between the mechanical properties and the cooling rate. Hardenability is not “hardness. It is a qualitative measure of the rate at which hardness decreases with distance from the surface because of decreased martensite content. High hardenability means the ability of the alloy to produce a high martensite content throughout the volume of specimen. The Jominy End-Quench Test One standard procedure that is widely utilized to determine hardenability is the Jominy end-quench test. With this procedure, except for alloy composition, all factors that may influence the depth to which a piece hardens (i.e., specimen size and shape, and quenching treatment) are maintained constant. A cylindrical specimen 25.4 mm (1.0 in.) in diameter and 100 mm (4 in.) long is austenitized at a prescribed temperature for a prescribed time. After removal from the furnace, it is quickly mounted in a fixture as diagrammed in Figure 2.31a. The lower end is quenched by a jet of water of specified flow rate and temperature. Thus, the cooling rate is a maximum at the quenched end and diminishes with position from this point along the length of the specimen. After the piece has cooled to room temperature, shallow flats 0.4 mm deep are ground along the specimen length and Rockwell hardness measurements are made for the first 50 mm (2 in.) along each flat (Figure 2.31b); for the first 12.8 mm (1/2 in.), hardness readings are taken at 1.6 mm (1/16 in.) intervals, and for the remaining 38.4 mm ( 1 1/2 in.), every 3.2 mm (1/8in.). A hardenability curve is produced when hardness is plotted as a function of position from the quenched end.

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Hardenability

“Hardenability” is the ability of Fe-C alloy to be hardened by theforming martensite as a result of a given heat treatment. For everydifferent steel alloy there is a specific relationship between themechanical properties and the cooling rate.Hardenability is not “hardness. It is a qualitative measure of the rate atwhich hardness decreases with distance from the surface because ofdecreased martensite content.High hardenability means the ability of the alloy to produce a highmartensite content throughout the volume of specimen.

The Jominy End-Quench Test

One standard procedure that is widely utilized to determine hardenabilityis the Jominy end-quench test. With this procedure, except for alloycomposition, all factors that may influence the depth to which a piecehardens (i.e., specimen size and shape, and quenching treatment) aremaintained constant. A cylindrical specimen 25.4 mm (1.0 in.) indiameter and 100 mm (4 in.) long is austenitized at a prescribedtemperature for a prescribed time. After removal from the furnace, it isquickly mounted in a fixture as diagrammed in Figure 2.31a. The lowerend is quenched by a jet of water of specified flow rate and temperature.Thus, the cooling rate is a maximum at the quenched end and diminisheswith position from this point along the length of the specimen. After thepiece has cooled to room temperature, shallow flats 0.4 mm deep areground along the specimen length and Rockwell hardness measurementsare made for the first 50 mm (2 in.) along each flat (Figure 2.31b); for thefirst 12.8 mm (1/2 in.), hardness readings are taken at 1.6 mm (1/16 in.)intervals, and for the remaining 38.4 mm ( 1 1/2 in.), every 3.2 mm(1/8in.). A hardenability curve is produced when hardness is plotted as afunction of position from the quenched end.

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Figure 2.31 Schematic diagram of Jominy endquench specimen(a) mounted during quenching and (b) after hardness testing from the

quenched end along a ground flat.

Hardenability CurvesA typical hardenability curve is represented in Figure 2.32. The quenchedend is cooled most rapidly and exhibits the maximum hardness; 100%martensite is the product at this position for most steels. Cooling ratedecreases with distance from the quenched end, and the hardness alsodecreases, as indicated in the figure. With diminishing cooling rate moretime is allowed for carbon diffusion and the formation of a greaterproportion of the softer pearlite, which may be mixed with martensite andbainite. Thus, a steel that is highly hardenable will retain large hardnessvalues for relatively long distances; a low hardenable one will not. Also,each steel alloy has its own unique hardenability curve.

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Figure 2.32 Typical hardenability plot of Rockwell C hardness as afunction of distance from the quenched end.

Sometimes, it is convenient to relate hardness to a cooling rate rather thanto the location from the quenched end of a standard Jominy specimen.Cooling rate [taken at 700°C] is ordinarily shown on the upper horizontalaxis of a hardenability diagram; this scale is included with thehardenability plots presented here.This correlation between position and cooling rate is the same for plaincarbon and many alloy steels because the rate of heat transfer is nearlyindependent of composition. On occasion, cooling rate or position fromthe quenched end is specified in terms of Jominy distance, one Jominydistance unit being 1.6 mm ( in.). A correlation may be drawn betweenposition along the Jominy specimen and continuous coolingtransformations. For example, Figure 2.33 is a continuous coolingtransformation diagram for a eutectoid iron–carbon alloy onto which aresuperimposed the cooling curves at four different Jominy positions, andcorresponding microstructures that result for each. The hardenabilitycurve for this alloy is also included.

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Figure 2.33 Correlation of hardenability and continuous coolinginformation for an iron–carbon alloy of eutectoid composition.

The hardenability curves for five different steel alloys all having 0.40wt% C, yet differing amounts of other alloying elements, are shown inFigure 2.34. One specimen is a plain carbon steel (1040); the other four(4140, 4340, 5140, and 8640) are alloy steels. The compositions of thefour alloy steels are included with the figure.Several details are worth noting from this figure. First, all five alloys haveidentical hardness at the quenched end (57 HRC); this hardness is afunction of carbon content only, which is the same for all these alloys.

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Probably the most significant feature of these curves is shape, whichrelates to hardenability. The hardenability of the plain carbon 1040 steelis low because the hardness drops off precipitously (to about 30 HRC)after a relatively short Jominy distance 6.4 mm, (1/4 in.). By way ofcontrast, the decreases in hardness for the other four alloy steels aredistinctly more gradual.For example, at a Jominy distance of 50 mm (2 in.), the hardness of the4340 and 8640 alloys are approximately 50 and 32 HRC, respectively;thus, of these two alloys, the 4340 is more hardenable.A water quenched specimen of the 1040 plain carbon steel would hardenonly to a shallow depth below the surface, whereas for the other fouralloy steels the high quenched hardness would persist to a much greaterdepth.The hardness profiles in Figure 2.34 are indicative of the influence ofcooling rate on the microstructure. At the quenched end, where thequenching rate is approximately 600°C/s, 100% martensite is present forall five alloys. For cooling rates less than about 70°C/s or Jominydistances greater than about 6.4 mm (1/4 in.), the microstructure of the1040 steel is predominantly pearlitic, with some proeutectoid ferrite.However, the microstructures of the four alloy steels consist primarily ofa mixture of martensite and bainite; bainite content increases withdecreasing cooling rate.

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Figure 2.34 Hardenability curves for five different steel alloys, eachcontaining 0.4 wt% C. Approximate alloy compositions (wt%) are asfollows: 4340–1.85 Ni, 0.80 Cr, and 0.25 Mo; 4140–1.0 Cr and 0.20 Mo;8640–0.55 Ni, 0.50 Cr, and 0.20 Mo; 5140–0.85 Cr; and 1040 is anunalloyed steel.

This disparity in hardenability behavior for the five alloys in Figure 2.34is explained by the presence of nickel, chromium, and molybdenum in thealloy steels. These alloying elements delay the austenite-to-pearlite and/orbainite reactions, as explained above; this permits more martensite toform for a particular cooling rate, yielding a greater hardness. The right-hand axis of Figure 2.34 shows the approximate percentage of martensitethat is present at various hardness for these alloys.The hardenability curves also depend on carbon content. This effect isdemonstrated in Figure 2.35 for a series of alloy steels in which only theconcentration of carbon is varied. The hardness at any Jominy positionincreases with the concentration of carbon. Also, during the industrialproduction of steel, there is always a slight, unavoidable variation incomposition and average grain size from one batch to another.

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This variation results in some scatter in measured hardenability data,which frequently are plotted as a band representing the maximum andminimum values that would be expected for the particular alloy. Such ahardenability band is plotted in Figure 2.36 for an 8640 steel.An H following the designation specification for an alloy (e.g., 8640H)indicates that the composition and characteristics of the alloy are suchthat its hardenability curve will lie within a specified band.

Figure 2.35 Hardenability curves for four 8600 series alloys of indicatedcarbon content.

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Figure 2.36 The hardenability band for an 8640 steel indicatingmaximum and minimum limits.

Influence of Quenching Medium, Specimen Size, and Geometry onHardenability:

Quenching medium: Cooling is faster in water then oil, slow in air. Fastcooling brings the danger of warping and formation of cracks, since it isusually accompanied by large thermal gradients.

The shape and size of the piece: Cooling rate depends upon extractionof heat to specimen surface. Thus the greater the ration of surface area tovolume, the deeper the hardening effect. Spheres cool slowest, irregularlyshaped objects fastest.

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Figure 2.37 Radial hardness profiles of cylindrical steel bars

Example 2.5: Design of a Wear-Resistant Gear

A gear made from 9310 steel, which has an as-quenched hardness at acritical location of HRC 40, wears at an excessive rate. Tests have shownthat an as-quenched hardness of at least HRC 50 is required at that criticallocation. Design a steel that would be appropriate.

Figure 2.38 Thehardenability curves for

several steels.

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Figure 2.37 Radial hardness profiles of cylindrical steel bars

Example 2.5: Design of a Wear-Resistant Gear

A gear made from 9310 steel, which has an as-quenched hardness at acritical location of HRC 40, wears at an excessive rate. Tests have shownthat an as-quenched hardness of at least HRC 50 is required at that criticallocation. Design a steel that would be appropriate.

Figure 2.38 Thehardenability curves for

several steels.

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Figure 2.37 Radial hardness profiles of cylindrical steel bars

Example 2.5: Design of a Wear-Resistant Gear

A gear made from 9310 steel, which has an as-quenched hardness at acritical location of HRC 40, wears at an excessive rate. Tests have shownthat an as-quenched hardness of at least HRC 50 is required at that criticallocation. Design a steel that would be appropriate.

Figure 2.38 Thehardenability curves for

several steels.

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Solution:

From Figure 2.38, a hardness of HRC 40 in a 9310 steel corresponds to aJominy distance of 10/16 in. (10oC/s). If we assume the same Jominydistance, the other steels shown in Figure 2.38 have the followinghardnesses at the critical location:

1050 HRC 28 1080 HRC 36 4320 HRC 31

8640 HRC 52 4340 HRC 60

In Table 2-1, we find that the 86xx steels contain less alloying elementsthan the 43xx steels; thus the 8640 steel is probably less expensive thanthe 4340 steel and might be our best choice. We must also consider otherfactors such as durability.

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Example 2.6: Design of a Quenching Process

Design a quenching process to produce a minimum hardness of HRC 40at the center of a 1.5-in. diameter 4320 steel bar.

Figure 2.39 The Grossman chart used to determine the hardenabilityat the center of a steel bar for different quenchants.

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Example 2.6: Design of a Quenching Process

Design a quenching process to produce a minimum hardness of HRC 40at the center of a 1.5-in. diameter 4320 steel bar.

Figure 2.39 The Grossman chart used to determine the hardenabilityat the center of a steel bar for different quenchants.

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Example 2.6: Design of a Quenching Process

Design a quenching process to produce a minimum hardness of HRC 40at the center of a 1.5-in. diameter 4320 steel bar.

Figure 2.39 The Grossman chart used to determine the hardenabilityat the center of a steel bar for different quenchants.

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Solution:

Several quenching media are listed in Table 2-2. We can find anapproximate H coefficient for each of the quenching media, then useFigure 2.39 to estimate the Jominy distance in a 1.5-in. diameter bar foreach media. Finally, we can use the hardenability curve (Figure 2.38) tofind the hardness in the 4320 steel. The results are listed below.

The last three methods, based on brine or agitated water, are satisfactory.Using an unagitated brine quenchant might be least expensive, since noextra equipment is needed to agitate the quenching bath. However, H2O isless corrosive than the brine quenchant.