Harden Ability

METALLURGY TRAINING MODULE 3 REV. A01

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11/12/12 Hardenability

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Introduction Hardenability can be defined as the ease with which a given type of steel can form martensite and bainite when it is quenched from the austenitic region. The properties that can be achieved at any given location within a steel part are dependent on a number of factors including the the alloy content of the steel and the rate of cooling at that specific location. The cooling rate at a given location is dependent on the size of part, the type of quenchant, the amount of agitation, and a number of other factors. Clearly hardenability is a complex topic, but a very important one for engineers to grasp. Knowing a steel’s hardenability is important because it allows you to determine if a steel is capable of achieving the properties you need where you need them in a given size part after heat treatment. In order to form martensite and bainite, the chief hardening constituents in our steels, we must be able to cool the austenitized steel faster than a certain critical cooling rate. The critical cooling rate is dependent mostly on composition, but is also affected by a number of other factors including austenitic grain size. Alloying elements such as chromium, nickel, manganese, silicon, molybdenum, and especially carbon greatly increase the hardenability of steel. When a large steel bar is quenched after austenitizing, the surface of the bar will cool at the fastest rate while the center cools at the slowest. The surface may cool at a rate faster than the critical rate for martensite/bainite formation and thus harden. Depending on the size of the bar, a point may be reached below the surface of the bar where the cooling rate drops below the critical value and 100% hardening does not occur. Soft ferrite and pearlite begin to appear as the cooling rate decreases below the critical value. This is the reason why strength and hardness of large steel parts are typically highest at the surface and then decrease towards the center. Consider two large, identical sized bars: one made of steel A and the other of steel B. Steel A has twice Cr and Mo contents of steel B, but is otherwise similar in composition. If we austenitize and quench both A and B together, we would expect steel A to have a greater depth of hardening than steel B even though both were cooled at the same rate. The increased Cr and Mo contents give steel A better hardenability. If we quench a series of different size bars, we’ll find that we can through harden a larger diameter of steel A than we can steel B. The increased hardenability of steel A allows for a slower critical cooling rate so a larger diameter bar can be quenched. ARGUS Subsea’s material specifications typically call out a specified minimum yield strength (SMYS). It is very important that you understand that this strength requirement must be met in the Qualification Test Coupon (QTC) at the location specified in the material specification, but it does not necessarily reflect the strength of the production parts at all locations. Under API 6A rules, a 4” X 4” QTC can represent any size parts. The QTC may very well through harden while the production parts may have a sudden drop off in strength and hardness somewhere below the surface where the cooling rate drops below the critical value. The purpose of a QTC is not to represent the mechanical properties of the production parts, but


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rather to represent the thermal response of the specific heat of material used in making the production parts. It is a capability test that shows that if a specific heat of material in a standard 4” X 4” test bar responds to a prescribed heat treatment in a predictable and acceptable manner, then it can be assumed that the same heat in other sizes and configurations will also respond to heat treatment in a known and predictable manner. Don’t be misled by a material test report that states that a large, quenched and tempered, 4130 gate valve forging has a reported yield strength of 75ksi when qualified by a QTC! In reality you may only have 75ksi yield material only within ½” of the outer surface. When you design a part and specify a steel, you must make certain that the hardenability of the steel is such that it will give you the strength where it is needed within the part. With the possible exceptions of stems, hangers, fasteners, and running tools, most of the parts we deal with have their highest stresses on the surface due to bending. Seldom is it necessary (except for those parts just noted) to have the same SMYS throughout the entire cross section of a part. In this module we’ll look at some of the ways hardenability can be quantified and used to predict properties and aid in material selection. We’ll discuss the pro’s and con’s of pre-heat treat machining and give some guidelines on how dimensions should be specified on pre-heat treat machining drawings. And finally data will be presented showing typical mechanical properties at different locations for different size bars for the types of steel commonly used by ARGUS Subsea.

The Jominy Test The Jominy end quench test is probably the most widely used means of characterizing the hardenability of a particular heat of steel. Here in the U.S. ASTM A255 is the most common standard for performing a Jominy test. ASTM A255 uses a cylindrical specimen 1” in diameter and 3-4” long that is machined from the heat in question (see Figure 1). One end may have an oversized button head, threaded hole, or other arrangement to facilitate suspending the specimen in the vertical position during the test. The test specimen is heated up to a temperature in the austenitic region (actual temperature varies by the type of steel) in a lab furnace, removed from the furnace, and then suspended over a jet of water so that it is quenched from one end only. The apparatus for doing this is shown in Figure 2. The water nozzle has a ½” ID. Water pressure is adjusted so that the free height of the unimpeded water column above the nozzle is 2-1/2”. The hot specimen removed from the furnace is positioned in the water column such that its bottom end is ½” directly above the nozzle. The specimen is then allowed to cool for 10 minutes or longer.


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Figure 1: ASTM A255 Jominy Test Specimen (From Wikipedia:

http://en.wikipedia.org/w/index.php?title=File:Jominy-test.svg&page=1)

Figure 2; Jominy Test Appuratus (From Wikipedia:

http://en.wikipedia.org/wiki/File:Jominy_en.png) After the specimen has cooled down, two longitudinal flats 180

o apart are

milled/ground on the specimen about 1/8” below the original OD. Rockwell C hardness tests are then made starting 1/16” from the quenched end at 1/16

th

intervals along each flat for the first 1”, and then as agreed upon for the rest of the length (see Figure 3).

http://en.wikipedia.org/w/index.php?title=File:Jominy-test.svg&page=1


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Figure 3: Jominy Bar Rockwell C Hardness Tested (From Wikipedia :

http://en.wikipedia.org/wiki/File:Probka_Jaminy_hor.jpg) The Rockwell hardness data is typically plotted against the distance from the quenched end as shown in Figure 4. This type of Jominy curve has many practical uses. The highest hardness, as expected, will always be on the end being quenched and then decrease along the length of the bar as the cooling rate decreases. Hardness

Rockwell C

Figure 4: Developing A Jominy Curve


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The highest as-quenched hardness is primarily a function of the steel’s carbon content and is relatively independent of its other alloying content. Don’t confuse high as-quenched surface hardness with high hardenability! A highly hardenable steel will have better depth of hardening than one with poor hardenability, but not necessarily a higher as-quenched hardness. For example 4130 low alloy steel develops a higher as-quenched surface hardness than does F22 because of its higher carbon content (about twice that of F22), yet F22 has much better hardenability because of its increased chromium and molybdenum content. The cooling rate decreases with distance from the quenched end (see Table 1), although the cooling rate is always constant at a given location. If we can correlate the quench rate at given location in a steel bar with the cooling rate on the same steel’s Jominy curve, we can presume that the resulting hardnesses at both locations will be the same.

Table 1

Cooling Rate At Jominy Points

Distance from

Quenched End,

1/16th

Inch

Cooling

Rate 1

oF/sec

1 490

2 305

3 195

4 125

5 77

6 56

7 42

8 33

9 26

10 21.4

12 16.3

14 12.4

16 10.0

18 8.3

20 7.0

Note 1: Assumes room temperature water How can we estimate what the cooling rate is at a particular location within a given steel bar as it is being quenched? Obviously the size of the bar matters. Also important is the severity of the quench. One way of quantifying the severity of the quench is by the use of the Grossman H-Factor. The Grossman H-Factor is defined as:

H = h/(2k)


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Where H = Grossman H-Factor h = Effective heat transfer coefficient at the part surface k = Thermal conductivity of metal. Table 2 shows the H-Factor values of various quenchants with differing amounts of agitation. The higher the H-Factor value, the more severe the quench.

Table 2

H Values

Agitation Quenchant H Value

Oil Water Brine Air

None 0.20-0.30 0.9-1.00 2.00 0.02

Mild 0.30-0.35 1.00-1.10 2.00-2.20

Moderate 0.35-0.40 1.20-1.30

Good 0.40-0.50 1.40-1.50

Strong 0.50-0.80 1.60-2.00

Violent 0.80-1.00 4.00 5.00

An “ideal” or “infinite” quench is one where the surface of the part immediately cools down to the quenchant temperature and remains there for the duration of the quench. An ideal quench has an H-Factor value of infinity. It is a concept that we will use later on to calculate ideal diameters. A Croft-Lamont transformation curve shows the correlation between the cooling rate at a given Jominy location and at a given position in various size bars for different H-Factors. The Croft-Lamont transformations curves shown in Figure 5 are for the center location of round bars. Other curves are available in heat treating handbooks for other locations. To illustrate the use of these curves, let’s assume we have run a Jominy test on a heat of steel and plotted out the results as shown in Figure 6. ___________________________________________________________________

Example #1 What will be the hardness in the center of a 5” round bar of our example steel if it is quenched in strongly agitated water? From Table 2, strongly agitated water has a H-Factor of about 2. In Figure 5, draw a horizontal line through 5” diameter on the Y-axis and find where it intersects the H-Factor = 2 curve. Now drop a vertical line down from the intersection point to find the corresponding Jominy location. In this example it will be about 1-3/8” from the quenched end or 22/16”. We now go to Figure 6 and see what the hardness of our example steel is at the 22/16” Jominy location. It’s approximately 33HRC. The center of our 5” diameter steel bar will thus have a Rockwell C hardness of about 33HRC after quenching in strongly agitated water.


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Figure 5: Croft-Lamont Transformation Curves for Centers of Round Bars

Figure 6: Example Jominy Curve __________________________________________________________________


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Example #2

What is the largest diameter bar we can quench in still brine and get a minimum hardness of 40HRC in the center? From Table 2, the H-Factor for still brine is 2. From Figure 6, 40HRC corresponds to a Jominy position of 11/16” from the quenched end. Find 11/16” on the X-axis in Figure 5, draw a vertical line through it, and find the point of intersection with the H-Factor = 2 curve. This point corresponds roughly to 3.25” on the Y-axis. The largest bar we can quench in still brine and still get at least 40HRC in the center is approximately 3.25”. ___________________________________________________________________ There are other useful curves available in heat treating handbooks that equate the cooling rates at the surface, mid-radius, center, and other positions of various sized bars quenched in different media to the equivalent Jominy position. Once this is known, then the hardness can be predicted at these locations if a Jominy curve for the steel is available. Steel bars are sometimes ordered with specific hardness requirements at specified Jominy locations. Most steels have a relatively broad range for each alloying element. As a consequence, a “lean” heat with chromium, molybdenum, etc. on the low side with have much lower hardenability than a heat “rich” in these elements. By ordering to prescribed Jominy location hardnesses, the purchaser can be assured that the composition of the steel he is receiving has adequate hardenability for its intended purpose.

Hardenability Band Curves The American Iron and Steel Institute (AISI) publishes hardenability band curves for “H” grades of steels that show the Jominy limits for lean and rich heats of a particular steel. The “H” grades are steels that are specially processed to prescribed minimum and maximum hardenability limits. The composition ranges of the H-grades may vary from standard grades. The hardenability band curves for the different grades can be found in AISI’s Steel Products Manual. The curves were developed by Jominy testing many heats of material. Figure 7 shows the hardenability bands for 4140H steel. Note at the top of the curves there are several horizontal axes containing round bar sizes that have equivalent cooling rates with each other and equal to that associated with the distance from the quenched end as a function of location and quenchant. For example in Figure 7, a point at 3/4R from the center of a 2.9” bar quenched in mildly agitated water has the same cooling rate as a point at the center of a bar 1.6” in diameter quenched in mildly agitated water. This cooling rate will be the same for a point on the surface of a 2.5” round bar, for a point at 3/4R from the center of a 1.6” bar, and for a point at the center of a 1” bar all quenched in mildly agitated oil.


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The cooling rate at these locations correspond to the cooling rate at a Jominy distance of 6/16” from the quenched end. The as-quenched hardnesses at these locations would be expected to fall within the limits of the hardenability bands at 6/16” or from 50-58.5HRC in Figure 7. Hardenability band curves can be used in many ways. They are often utilized in material selection to see if the alloy is capable of producing the desired hardness at the desired locations.

Figure 7: Hardenability Bands for 4140H (©American Iron And Steel

Institute, 1986, Steel Products Manual)


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Ideal Diameter

The ideal diameter, or DI, of a steel is the theoretical, maximum size round that will through harden given an ideal or infinite quench. “Through hardened” here means that the center of the bar contains at least 50% martensite. It is a very useful concept for comparing hardenability in different heats of the same steel grade or between different grades of steels. The hardenability of a steel is primarily a function of its alloying content and grain size. The ideal diameter of a steel is one way to quantify the overall effect of its alloying content and grain size on its hardenability. It is calculated using a series of multiplying factors that are based on the effects of individual alloying elements. The formula to calculate the ideal diameter, DI, of a steel is:

DI = DIC X fMN X fSi X fNi X fCr X fMo

Where DIC is the basic DI factor for carbon

f is the multiplying factor for each element Table 3 gives the alloy factors for calculating DI’s. Let’s calculate the ideal diameters for some common carbon and low alloy steels used in in the Oil Patch as see how they compare. We’ll assume nominal (mid-range) content of each alloying element and use the in Table 1 interpolating when necessary. We’ll use a 12% carbon (mid-range for the ARGUS Subsea spec) for the F22. The carbon factor will be based on a grain size of #7.

AISI 1020 Carbon Steel Nominal composition C=0.20%, Mn=0.45%

DI = (0.1509) X (2.50) = 0.38”

AISI 1040 Carbon Steel Nominal composition C=0.40%, Mn=0.75%

DI = (0.2130) X (3.50) = 0.75”

AISI 4130 Low Alloy Steel Nominal composition C=0.30%, Mn=0.50%, Si=0.22%, Cr=0.95%, Mo=0.20%

DI = (0.1849) X (2.667)X(1.60)X(3.0520)X(1.60) = 3.85”

AISI 4140 Low Alloy Steel Nominal composition C=0.40%, Mn=0.88%, Si=0.22%, Cr=0.95%, Mo=0.20%

DI = (0.2130) X (3.900)X(1.60)X(3.0520)X(1.60) = 6.49

ASTM A182, Grade F22 (2-1/4Cr-1Mo) Low Alloy Steel Nominal composition C=0.12%, Mn=0.45%, Si=0.22%, Cr=2.25%, Mo=1.00%

DI = (0.1200) X (2.500) X (1.60) X (5.00) X (4.00) = 9.6”

Table 3


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Alloying Element Multiplying Factors

%

Base Ideal Diameter

for Carbon & Grain

Size

Alloying Factor f

#6 #7 #8 Mn Si Ni Cr Mo

0.05 0.0814

0.0750

0.0697

1.167 1.035 1.018 1.1080

1.15

0.10 0.1153

0.1065

0.0995

1.333 1.070 1.036 1.2160

1.30

0.15 0.1413

0.1315

0.1212

1.500 1.105 1.055 1.3240

1.45

0.20 0.1623

0.1509

0.1400

1.667 1.140 1.073 1.4320

1.60

0.25 0.1820

0.1678

0.1560

1.833 1.175 1.091 1.5400

1.75

0.30 0.1991

0.1849

0.1700

2.000 1.210 1.109 1.6480

1.90

0.35 0.2154

0.2000

0.1842

2.167 1.245 1.128 1.7560

2.05

0.40 0.2300

0.2130

0.1976

2.333 1.280 1.146 1.8640

2.20

0.45 0.2440

0.2259

0.2090

2.500 1.315 1.164 1.9720

2.35

0.50 0.2580

0.2380

0.2200

2.667 1.350 1.182 2.0800

2.50

0.55 0.2730

0.2510

0.2310

2.833 1.385 1.201 2.1880

2.65

0.60 0.2840

0.2620

0.2410

3.000 1.420 1.219 2.2960

2.80

0.65 0.2950

0.2730

0.2551

3.167 1.455 1.237 2.4040

2.95

0.70 0.3060

0.2830

0.2600

3.333 1.490 1.255 2.5120

3.10

0.75 0.3160

0.2930

0.2700

3.500 1.525 1.273 2.6200

3.25

0.80 0.3260

0.3030

0.2780

3.667 1.560 1.291 2.7280

3.40

0.85 0.3360

0.3120

0.2870

3.833 1.595 1.309 2.8360

3.55


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0.90 0.3460

0.3210

0.2960

4.000 1.630 1.321 2.9440

3.70

0.95 4.167 1.665 1.345 3.0520

3.85

1.00 4.333 1.700 1.364 3.1600

4.00

2.25 5.0000

Clearly the 2-1/4Cr-1Mo (F22) low alloy steel is the winner here! The carbon steels have very poor hardenability which is why carbon steels are seldom used for the thick wall components used in wellhead equipment. The 4130 has the poorest hardenability of the three low alloy steels, yet tonnage wise, it is the most commonly used low alloy steel in the Oil Patch. How come? Again the answer is because it can provide the strength where it’s needed. Through hardening is usually not required for our designs. 4130 is cheap, readily available, easily forged, easily machined, and easily welded. 4140 is very difficult to weld (very prone to cracking) and has rather poor toughness. The 2-1/4Cr-1Mo low alloy steel has the best hardenability; is easily forged, machined, and welded; and has great toughness, but it can cost up to 40% more than 4130. 4140 and 2-1/4Cr-1Mo low alloy steels should only be used when your design requires their better hardenability. Continuous Cooling Curves

Continuous cooling curves (also called continuous cooling transformation diagrams or CCT diagrams) show the decomposition products of austenite during quenching. There are several ways of presenting this information. The first is a plot of transformation products as a function of transformation temperature and cooling time (time after quenching). This is illustrated for an AISI 4140 steel in Figure 8. Transformation temperature is always plotted on the vertical axis and the cooling time along the horizontal axis. Time is plotted on a logarithmic scale in order to make the diagram more compact.


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Figure 8: AISI 4140 CCT Diagram Let’s determine what type of structure (and hence properties) we would have in the center of 2-1/4" diameter bar of 4140 after austenitizing and then oil quenching. First we superimpose a plot of temperature versus time corresponding to the cooling rate at the center of a 2-1/4" diameter bar during an oil quench onto Figure 8 (curve A). The austenite begins to transform to proeutectoid ferrite at point 1 (after 20 seconds of cooling). At point 2 (after approximately 45 seconds), 5% of the austenite has transformed into ferrite. The remaining austenite begins to transform into bainite at point 2. The bainite transformation is complete at point 3. There is now roughly 5% ferrite, 55% bainite, with the remainder being untransformed austenite. Point 3 marks the start of martensite formation. The remaining austenite will transform to martensite by the time we reach room temperature. The final structure consists of roughly 5% proeutectoid ferrite, 55% bainite, and 40% martensite. Another way of presenting CCT diagrams is illustrated in Figure 9, 10, and 11. Here the horizontal axis represents the size of bar being quenched. There are three scales: one for each type of quenching medium. Again the horizontal scales are logarithmic to keep the diagrams compact. The vertical axis is the transformation temperature. The horizontal curves in the diagrams represent the start of transformation; the points where the transformation is 10%, 50%, and 90% complete, and then the finish of transformation. This type of CCT diagram is a convenient means of determining the microstructure in the center of a round bar that has been austenitized and then air, oil, or water quenched.


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Figures 9, 10, and 11 are very powerful tools for predicting the microstructure and hardness in the center of a quenched bar as a function of the quench media. For example look at the 2-1/4Cr-1Mo diagram in Figure 11. What is the microstructure in the center of a 50mm (2”) round bar quenched in water? To ascertain this we’ll draw a vertical line through the 50mm point on the Y axis for water. Starting in the austenitic range and then following our vertical line down through the diagram, we see that the austenite begins to transform at approximately 510C. The diagram shows that the initial transformation product is bainite. It’s not until we reach about 390C that some of the remaining austenite begins to transform into martensite. At this point roughly 92% of the austenite has already transformed into bainite so only 8% is available to form martensite. At roughly 320C the austenite has completely transformed. The center of our bar will thus have a microstructure consisting of approximately 92% bainite and 8% martensite. Note in Figure 11 that the hardness of 100% martensite is the highest. The hardness of 100% bainite is slightly lower. There is a significant drop in hardness if the cooling rate is so slow that the austenite begins to transform into ferrite and/or pearlite. For the types of steels that ARGUS Subsea utilizes, a bar having a center microstructure consisting of mostly martensite and bainite after quenching may be considered through hardened. From Figure 9, a predominately martensitic/bainitic microstructure in a 4130 bar is possible with a water quench up to about 75mm (3”). For 4140, a bar up to 280mm (11”) will be predominately martensitic/bainitic after an oil quench. For a 2-1/4Cr – 1Mo low alloy steel, the largest possible bar diameter for a predominately martensitic/bainitic microstructure in the center after a water quench is around 500mm (20”). Clearly the 2-1/4Cr – 1Mo low alloy steel has the best hardenability of the three alloys. Note that the 4140 has the highest as-quenched hardness at the surface because it has the highest carbon content. The 2-1/4Cr – 1Mo low alloy steel has the lowest surface hardness because it has the least carbon. Figures 9, 10, and 11 illustrate the fact that alloying elements such as chromium and molybdenum enhance the hardenability of steels by shifting the austenitic transformation curves to the right on continuous cooling transformation diagrams. This means that there is more time allowed during cooling before the onset of ferrite and pearlite formation: a slower cooling rate is possible while still missing the ferrite/pearlite “knee” of the curves. Highly hardenable steels have a lower critical cooling rate and can thus be hardened in larger sizes.


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Figure 9: AISI 4130 CCT Diagram (From Atlas of Continuous Cooling

Transformation diagrams for Engineering Steels, © ASM International, 1980.

Reprinted with Permission of ASM International. All Rights Reserved.

www.asminternational.org)

http://www.asm/


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Figure 10: AISI 4140 CCT Diagram (From Atlas of Continuous Cooling




http://www.asm/


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Figure 11: 2-1/4Cr-1Mo CCT Diagram (From Atlas of Continuous Cooling




http://www.asminternational.org/


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Putting It All Together

It is vital for ARGUS Subsea engineers to understand the concept of hardenability. Most design calculations are based upon the yield strength of the material. From our discussions above it is apparent that the yield strength in a large steel part may vary considerably from the surface to the center of the part depending on the specific alloy, the size of the part, the specific heat treatment, and the type and severity of the quench. Why not specify an alloy that has the necessary hardenability to produce the desired mechanical properties throughout the entire cross section of the part? Why not standardize on 2-1/4Cr – 1Mo low alloy steel for all components instead of using 4130? These are both good questions and deserve careful responses. The alloying elements used to increase a steel’s hardenability come at a price. They are expensive so steels with higher amounts of them will cost more. Often increasing these elements may lead to difficulties in welding, machining, galling, etc. Hardenability is not that only material parameter to consider when selecting a steel! In general you want to select a steel that is just sufficiently alloyed to give you the properties you need where you need them using a standard heat treatment. Most (but not all!) wellhead components typically have their highest service stresses at their outer surfaces due to a combination of bending, structural, and pressure containing loads. This means that it is seldom necessary to meet the specified minimum yield strength (SMYS) throughout the entire wall section of a part. As long as we have enough alloying content to meet the SMYS at the high stress locations at the part’s surface, we can get away with lower strengths at deeper locations and can tolerate lower hardenability. We calculated the ideal diameter for 4130 (3.85”). If we need a specified minimum yield strength of 75ksi throughout the entire cross section of the part, then we will be limited to a bar of this diameter. What if we need just 60ksi SMYS? Then we can go to a bar up to 12” or 14” in diameter. Remember that ideal diameter reflects the largest size bar that can be given an ideal quench resulting in at least 50% martensite in the center. Depending on the alloy, the standard API 6A minimum design strengths of 60ksi and 75 ksi may be achieved in the center of bars having greater than the ideal diameter even though the microstructure in the center has less

than 50% martensite. The Useful Data section at the end of this module will show the limits for obtaining different SMYS at different locations for commonly used steels. API 6A requires a 60ksi SMYS for equipment rated for 10ksi. We can machine a bonnet directly from a 12” round of quenched and tempered 4130. 4130 has sufficient alloying to gives us close to 60ksi yield strength throughout the cross section. There’s no reason to spec out the more expensive 2-1/4Cr – 1Mo low alloy steel with its better hardenability – it’s not needed here. But what if the equipment is going to be rated for 15ksi? Now API 6A requires that we have 75ksi SMYS material. A 12” round, quenched and tempered 4130 bar with a surface hardness of 22HRC


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(thus NACE compliant), will have a 75ksi yield strength only in the outer ½” or so of material. Go deeper than this and this yield will drop precipitously to around 55-60ksi. API 6A requires a surface hardness of 197HBW minimum for 75ksi SMYS material. If we machine our bonnet from 4130, the bonnet neck will be well below the OD of the bar. All the 75ksi material will end up as chips on the floor and the surface hardness will be well below the required 197HBW. Does this mean we can’t use 4130? Are we now forced to go to an alloy with more horsepower such as 2-1/4Cr – 1Mo low alloy steel? Now the engineer is at a fork in the road. He or she can certainly choose to go with the more hardenable (and more expensive) 2-1/4Cr – 1Mo low alloy steel that is capable of giving 75ksi throughout the entire cross section. There is an alternative, however, that would permit the use of the cheaper 4130. We can rough machine the bonnet from the 12” – 4130 bar prior to heat treating. This reduces the wall section so the cooling rate during quenching is faster. If we rough machine close to the final dimensions (but retaining some cover to allow for the distortion that occurs in heat treating), then we minimize the amount of material that must be removed during finish machining after heat treatment and we’ll retain the high strength material near the heat treated surface. As long as the design requires 75ksi yield strength just near the surface, but can tolerate lower strengths at deeper locations, then preheat treat machining would be a viable option that would permit the use of 4130.

Preheat Treat Machining Preheat treat machining is done to reduce the wall section of a part in order to increase the cooling rate during quenching and thereby get better properties. It is typically done in order to allow the use of a low cost steel with low hardenability and still meet the mechanical requirements. It may be done to optimize mechanical properties in high stress areas like gate valve seat pockets or flange necks. Preheat treat machining may not be the answer. It is sometimes better to go with the more expensive alloy with better hardenability for both technical and economic reasons than to use a low cost alloy that requires preheat treat machining. Here are some things to consider when deciding if preheat treat machining is the appropriate approach:

Is there really a need to preheat treat machine given the hardenability of the alloy and the engineering requirements of the part? Even though a given alloy may not through harden, can it achieve the required mechanical properties where they are needed without preheat treat machining?

Consider the cost per pound of an alloy requiring preheat treat machining versus a more expensive alloy with better hardenability that does not.


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Consider other costs associated with preheat treat machining including additional handling, additional machine set-ups, etc.

Non-destructive testing may be more difficult, take longer, and cost more after heat treatment on a part that was preheat treat machined than on a simple shape such as a bar.

Will the part have to undergo one or more stress reliefs after fab welding or cladding? Sometimes the more expensive, higher alloyed steel will be more temper resistant and thus not fall below the minimum hardness after multiple stress reliefs.

Higher alloyed steels with sufficient hardenability to negate the need for preheat treat machining may be more difficult to weld, more likely to gall, etc.

Can a steel with low hardenability be effectively preheat machined to obtain the desired properties without increasing the risk of quench cracking or distortion to an unacceptable level?

For a narrow, allowable hardness range, it may be more economical to go with a more expensive steel with high hardenability that is easier to heat treat than with a low hardenability steel that is preheat treat machined. There will be more scatter in the hardness values of the steel with low hardenability and reheat treatment may be necessary.

Once the decision is made to preheat treat machine, a preheat treat machining drawing must be prepared. The drawing should be identified with the words “Preheat Treat Machining Drawing” and include a note that the specified dimensions must be met prior to heat treatment. Do not use “Rough Dimensions” or similar words to identify the drawing. A vendor could interpret this as allowing rough machining after heat treatment. The exact dimensions to include on the drawing require a great deal of thought. We want to minimize the preheat treat machining. We want to keep the configuration as simple as possible. We need to remove material just in those areas that are critical. We need to provide sufficient heat treat cover to allow for the growth and distortion of the material during heat treatment. We need to minimize stress risers that may lead to quench cracking. Remember that steels, depending on their carbon content, may undergo a volumetric expansion of up to 4% as the austenite transforms during quenching. Here are some guidelines for developing a preheat treat drawing.

Minimize the number of features that must be preheat treat machined. Preheat treat machine only where it’s needed. Often a single bore is all that is required.

Bores should have a uniform diameter throughout the length of the part.


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Only put in a single bore. For dual bore equipment, put in the one bore that will be most effective in increasing the cooling rate where enhanced properties are needed.

Do not put intersecting bores in parts with carbon contents over 0.20% because of the risk of quench cracking where they intersect. For example, in a 4130 gate valve body, preheat treat machine the through bore or the gate cavity, but not both. Select the one that will do the most good.

Avoid making blind holes. These may trap steam and greatly reduce the effectiveness of the quench. Where a blind hole is unavoidable, there should be a note on the drawing showing that the part must be oriented with the blind hole opening on top during quenching.

Generally a heat treat cover of 0.25” on both the interior and exterior surfaces (including end faces) is sufficient to account for the growth and movement that occurs in the part during heat treatment. It may be possible to reduce this cover slightly for simple cylindrical shapes.

Use a generous radius on both interior and exterior corners. A ½” minimum is a good rule of thumb. It should never be less than ¼”.

Generally no special provision is necessary for the increase in length that occurs in the part during heat treatment except for long components with features on either end such a flanged spool piece. Here the back face-to- back face dimension between the flanges may be critical in the finished part. It may be necessary to increase the heat treat cover on the back face of each flange to ½” or more to insure clean-up to the correct finished dimension after heat treatment and final machining. It’s always a good idea to review this with the vendor and his heat treater and let them comment on the adequacy of the proposed heat treat cover for clean-up.

For parts with fixed dimensions that will not be altered by pre-heat treat machining (the length of a closed die gate valve forging, for instance), only the pre-heat treat machining dimensions and any necessary reference dimensions should be included on the drawing.

Try to keep uniform wall thicknesses along the length of the part. Removing too much material from just one side may result in a long part becoming bowed (banana shaped).

Remember that parts must be nondestructively tested after heat treatment. Avoid unnecessary holes, curved surfaces with varying radii, angular surfaces, and keep wall sections as uniform as possible (keep opposite surfaces as parallel as possible).


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Keep the configuration as simple as possible. Avoid abrupt changes in wall sections or any feature that may be a stress riser during quenching.

For long parts with a small, preheat treat machined bore (under about 3”), consider adding a note to the drawing requiring the use of a water lance during quenching.

Consider adding preferred a preferred orientation during quenching so the most critical area gets the most effective quench.

Consider adding hardness test locations to the drawings to insure that the critical areas attained the desired properties.

Fixturing may be required for long parts preheat treat machined to thin or non-uniform walls to prevent sagging during austenitizing, or distortion during quench.

Consider having different hardness ranges specified on the drawing depending on the mechanical property requirements in each area. For example, a flanged block valve with a 15ksi rating for sour service must meet 197-237HBW (for 75ksi SMYS) on the flange and flange neck. The heavy main section of the body may only require 60 or 65ksi SMYS (it’s governed by the manufacturer’s design and not API). Specify a lower minimum hardness here. This gives the heat treater a break and lets him tailor the heat treatment to the more critical area.

Ruling Sections

The following data in Table 4 is provided to help engineers select an appropriate steel and to decide when preheat treat machining is necessary. A ruling section is defined as the largest size bar that can be expected to have the required specified yield strength at the desired test location within the stated parameters. Please note that the mechanical properties are typical values and not guaranteed minimums. The actual values for a given part will vary with actual composition, the specific heat treatment, and the severity of the quench.


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Table 4

Ruling Sections

Alloy SMYS

ksi

Surface

Hardness

Maximum Diameter Where SMYS Can

Be Typically Achieved At The Stated

Test Location With Midrange Surface

Hardness 1

MR Center

4130 60 22HRC max. 14” 8”

4130 75 22HRC max. 5” 3”

4130 85 22HRC max. 3” 2”

4130 105 35HRC max. 3” 2”

4140 60 22HRC max. 20” 14”

4140 75 22HRC max. 14” 8”

4140 85 22HRC max. 10” 6”

4140 105 35HRC max. 10” 6”

4340 105 35HRC max 12” 8”

8630MOD 60 22HRC max. 14” 10”

8630MOD 75 22HRC max. 12” 7”

8630MOD 85 22HRC max. 8” 4”

8630MOD 105 35HRC max. 8” 4”

F22 60 22HRC max. 24” 20”

F22 75 22HRC max. 20” 12”

F22 85 22HRC max. 14” 8”

F22 105 35HRC max. 14” 8”

410 60 22HRC max. 24” 24”

410 75 22HRC max. 24” 24”

410 85 22HRC max. 24” 18”

410 105 35HRC max. 20” 16”

F6NM 60 23HRC max. 24” 24”

F6NM 75 23HRC max. 24” 24”

F6NM 85 23HRC max. 24” 20”

F6NM 105 35HRC max. 24” 20”

Note 1: The sizes given show what can be expected for a mid-range surface hardness and mid-range chemical composition. Actual sizes are of course dependent on chemical composition, quench severity, tempering temperature, etc. and may vary from the stated values.

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