Hardenability Hardenability

HARDENABILITYDr. H. K. Khaira Professor 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. hardness  hardenabilty .Introduction • Hardenability of a steel should not be confused with the hardness of a steel. • Hardenability refers to its ability to be hardened to a particular depth under a particular set of conditions. .Introduction Hardenability Hardness • The Hardness of a steel is a measure of a sample's resistance to indentation or scratching. • The depth of hardening in a plain carbon steel may be 2-3 mm vs 50 mm in an alloy steel. • Hardenability is more related to depth of hardning of a steel upon heat treat. • A large diameter rod quenched in a particular medium will obviously cool more slowly than a small diameter rod given a similar treatment.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. . the small rod is more likely to become fully martensitic. Therefore. This diameter is known as Di or ideal diameter.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. 8 . 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 . • Further. • 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. • These bars are quenched in a suitable quenchant. we use round bars of different diameters. we determine the critical diameter (dc) which is the maximum diameter of the rod which produced 50% martensite on quenching.Grossman’s method • In Grossman’s method. 4C only → .2 Mo→ 0. Effect of Quench in water Diameter Composition 0.0Cr+0.Radial hardness profile of cylindrical steel samples of different diameter and composition.4C+1. . Hardenability Curves . • Moreover. • 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 is specified in ASTM standard A255 and is a widely used method for quantifying hardenability. • These data have been correlated with measurements and/or calculations of dc. Jominy data have been collected on a large enough scale to offer a high degree of statistical certainty for many steels. .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. The Jominy end-quench test is one such approach. • An alternative approach is to develop a more convenient standard test method that can be used for relative comparison of hardenability. since the utility of empirical relations and data comparison becomes more reliable as more data are accumulated. Its wide use adds to its value. • The details of the test are covered in IS : 3848 – 1981 and ASTM A 255.The Jominy End Quench Test • The most commonly used method for determining hardenability is the end quench test developed by Jomini and Boegehold. . 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. the higher the hardenability. • The cooling rate will be highest at the end being quenched. and will decrease as distance from the end increases. and is then quenched on one end with room-temperature water. • The hardenability is then found by measuring the hardness along the bar: the farther away from the quenched end that the hardness extends. .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. 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 . a sample specimen rod either 100mm in length and 25mm in diameter. 102mm by 25. and • Then it is austenitised. the steel sample is normalized to eliminate differences in microstructure due to previous forging. simulating the effect of quenching a larger steel component in water. • Next. . 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. 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. or alternatively.4mm is obtained. Because the cooling rate decreases as one moves further from the quenched end. the specimen is rapidly transferred to the test machine. • Second.Steps in Jominy End Quench Test • First. This is usually at a temperature of 800 to 900°C. It is then quenched directly at one end with a stream of water 22 .Hardenability • How is the hardenability of steels assessed? – Jominy End-Quench Test – Test bar is heated to form 100% austenite. 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. Hardenability of Steels • Jominy end quench test to measure hardenability. 1” specimen (heated to  phase field) 24°C water flat ground 4” Fig. 14.5 26 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). HRC Hardenability Curve Distance from quenched end .Hardness. Heat Treatment of Steels: Hardenability The cooling rate varies throughout the length of the bar. 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. the rate being highest at the lower end which is in direct contact with water. 30 . a division of Thomson Learning. Inc. .23 The hardenability curves for several steels.©2003 Brooks/Cole. Thomson Learning™ is a trademark used herein under license. Figure 12. • By comparing the curves resulting from end quench tests of different grades of steels. the greater the hardenability. 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. .Hardenability Curve • Because the cooling rate decreases as one moves further from the quenched end. Thus the flatter the curve. their relative hardenability can be established. Hardenability Curve . 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. • For example.Cooling Curves and Phases at different Jominy Distances • A correlation may be drawn between position along the Jominy specimen and continuous cooling transformations. and 34 corresponding . Cooling curves from Jominy Distances . Cooling Curves and Phases at different Jominy Distances . • We can determine hardenability for any other amount of martensite in the core in any quenchant in a similar way. . • Hardenability in any other quenchant can be determined from the same graph. • 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 . the Jominy distance having hardness equal to 50 % martensite is determined. • Di (hardenability in ideal quenching medium) can also be determined in a similar manner. • This diameter gives the hardenability of the steel in water quenching (having H value equal to 1).Determination of Hardenability from Jominy Test Graph • After plotting the Jominy distance Vs Hardness curve. 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 . Ken Lewis ISAT 430 40 . – Each fluid has its own thermal properties • Thermal conductivity • Specific heat • Heat of vaporization – These cause rate of cooling differences Spring 2001 Dr.Quenching Media • The fluid used for quenching the heated alloy effects the hardenability. 02 41 .Coefficient of severity of quench: H • Cooling capacities (Severity of quench) of quenching medium is known as H value. 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. • Cooling rates are at the center of a 2.5 cm bar.1 – Still air 0. • H values of some of the quenchants are given below.25 18 – Cold gas 0. 1 4.5 Moderate 0.2 None 0.0 .Effect of Agitation on Coefficient of severity of quench: H Cooling Medium Agitation Oil Water Brine Violent 0.4-1.1 2.30 0.4-0.30-0.25 .0 Good 0.35-0.0 Strong 0.35 1.9-1.0-2.5-0.5 1.40 1.2-1.0 5.0 2.8-1.6-2.0.3 Mild 0.0-1.8 1. 43 .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. .6” hardenability in a quenchant with H value equal to 5. If a steel is having 1. it will have 0.9” hardenability in quenchants with H value 0. Inc. • ©2003 Brooks/Cole. Similarly.5 respectively. Thomson Learning™ is a trademark used herein under license. it will have 1.2 and 0.1” hardenability. a division of Thomson Learning.4” and 0. Effect of DI and “H” on D . Alloying elements 46 . Carbon content – 3.Factors affecting Hardenability • Slowing the phase transformation of austenite to ferrite and pearlite increases the hardenability of steels. Austenite grain size – 2. • The most important variables which influence hardenability are – 1. • This means that the sites for the nucleation of ferrite and pearlite are being reduced in number. because the grain boundary area which act as nucleating site is decreasing. with the result that these transformations are slowed down.Austenitic Grain Size • The hardenability increases with increasing austenite grain size. and the hardenability is therefore increased. . Effect of austenite grain size on Hardenability • The more γ-grain boundary surface the easier it is for pearlite to form rather than martensite • Smaller γ-grain size → lower hardenability • Larger γ-grain size → higher hardenability . Effect of Austenitic Grain size . most important. . because of the lack of toughness which results in greater difficulties in fabrication and. increased probability of distortion and cracking during heat treatment and welding. • But its use at higher levels is limited.Carbon Content • Carbon is primarily a hardening agent in steel. • It also increases hardenability by slowing the formation of pearlite and ferrite. 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 . • Chromium. – Aluminum and Titanium are commonly added as "gettering" agents to react with the Oxygen and Nitrogen in preference to the Boron. – Boron combines readily with both Nitrogen and Oxygen and in so doing its effect on hardenability is sacrificed. However. Manganese. and so also increase hardenability. Molybdenum and Manganese being used most often. – Therefore Boron must remain in solution in order to be affective. Nickel and Vanadium all effect the hardenability of steels in this manner. .0005%.25% Carbon or less. – Boron is most effective in steels of 0. quantitative assessment of these effects is needed. Chromium. Silicon.Effect of Alloying Elements • most metallic alloying elements slow down the ferrite and pearlite reactions. • Boron can be an effective alloy for improving hardenability at levels as low as . Molybdenum. and amongst the cheaper alloying additions per unit of increased hardenabilily. giving a substantial improvement in hardenability. • Boron has a particularly large effect when it’s added to fully deoxidized low carbon steel. from 0. and would be more widely used if its distribution in steel could be more easily controlled.60 wt% to 1.001%.Effect of Alloying Elements • The most economical way of increasing the hardenability of plain carbon steel is to increase the manganese content. • Chromium and molybdenum are also very effective. even in concentrations of the order of 0.40 wt%. . Ni. V. W  TTT diagram moves to the right. Ni time . temperature Cr. W. Mo.Effect of Alloying Elements • Hardenability of a steel increases with addition of alloying elements such as Cr. Mo. Jominy hardenability curves: Hardenability improves with increasing Mo content . Hardenability curves of 6 steels . yielding a greater hardness. The drop of hardness with Jominy distance for the other alloys is more gradual.Effect of Alloying Elements • all steels have 0.4wt% C. . which permits more martensite to form for a particular cooling rate. – At the quenched end all alloys have the same hardness. which is a function of carbon content only. but with different alloying elements. – The hardenability of the 1040 is low because the hardness of the alloy drops rapidly with Jominy distance. – The alloying elements delay the austenite-pearlite and/or bainite reactions. 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. . 260 3.5120 3.321 - Mo 0.1560 1.50 0.073 1.0995 1.2960 2.0750 0.333 1.293 0.0697 1.350 1.306 0.1623 0.25 0.315 1.280 1.3240 1.291 2.667 1.036 1.35 0.284 0.15 0.4040 2.1315 0.2510 0.1065 0. 6 No.40 0.018 1.1700 2.4.700 1.1080 1.270 3.8360 3.30 0.1991 0.10 0.6480 1.2160 1.1600 -  .500 1.00 0.140 1.6200 3.105 1.9440 3.312 0.20 0.45 0.1880 2.210 1.333 1.336 0.2259 0.262 0.167 1.665 1.273 0.40 0.346 - 0.2551 3.490 1.1976 2.167 1.667 1.182 2.2300 0. DIjominy Carbon grain size Alloying factor.65 0.2154 0.2090 2.80 0.2310 2.4320 1.55 0.2440 0.0814 0.9720 2.109 1.455 1.595 1.1509 0.05 0.50 0.273 2.278 3.000 1.1400 1.1820 0.8 Mn Si Ni Cr 0.035 1.245 1. fX % No.10 0.70 .283 0.667 1.333 1.500 1.2000 0.070 1.1413 0.8640 2.90 0.95 0.0800 2.167 1.500 1.45 0.60 0.95 1.2380 0.295 0.4.345 3.85 0.1842 2.000 1.90 0.525 1.2410 3.7560 2.333 1.560 1.1678 0.1849 0.055 1.237 2.15 0.2580 0.246 1.5400 1.175 1.75 0.60 0.255 2.128 1.364 3.0520 .1212 1.420 1.833 1.80 0.316 0.2130 0.2730 0.091 1. 7 No.303 0.1153 0.000 1.167 1.630 1.75 0.287 3.70 0.385 1.Multiplying Factors For The Calculation Of Ideal Diameter Base ideal diameter.296 4.219 2.20 0.7280 3.321 2.201 2.309 2.55 0.35 0.25 0.833 1.326 0.30 0.65 0.05 0.2200 2.833 1.164 1. Hardenability Multiplying Factor . TiC is very stable and does not easily dissolve • Co .Exceptions • S .reduces hardenability because it increases the rate of nucleation and growth of pearlite .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. the measured hardenability of a given type of steel should be presented as a band rather than a single line.Hardenability Band • The industrial products of steels may change composition and average grain size from batch to batch. as demonstrated by the Figure at right. therefore. . This data is. the American Iron and Steel Institute . available for very many of the steels listed in specifications such as those of the American Society of Automotive Engineers (SAE). usually referred to as hardenability bands.Hardenability Band • Hardenabilily data now exists for a wide range of steels in the form of maximum and minimum end-quench hardenability curves. which frequently are plotted as a band 67 representing the max . there is always a slight.Hardenability Band During the industrial production of steel. This variation results in some scatter in measured hardenability data. unavoidable variation in composition and average grain size from one batch to another. Effects of composition variation and grain size change on the hardenability of alloy steels . Hardenability (as Range of D i Values) of Various Steels . 3%C.3%Si.Example • Calculate the approximate hardenability of an 8630 (0.7%Mn. 0. 0. 0.5%Cr.2%Mo) alloy steel with an ASTM grain size of 7 .6%Ni. 0. 0. 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 . the one with higher surface area will cool faster. if the fluid is agitated cooling occurs even more quickly. steels. • Substances like brine and water cool much more quickly than oil or air. i. • Additionally. • The geometry of the part also affects the cooling rate: of two samples of equal volume. . is a function of the carbon content and other alloying elements and the grain size of the austenite. • The fluid used for quenching the material influences the cooling rate due to varying thermal conductivities and specific heats.e.Summery • The hardenability of ferrous alloys. • The relative importance of the various alloying elements is calculated by finding the equivalent carbon content of the material. Numerical problem -1 • Predict the center hardness in a water quenched 3” bar of 8640 . Jominy Distance =17mm Water Quenched Oil Quenched .Solution to Numerical Problem 1 The cooling rate at the center of a 3” dia bar in water quenching will be same as that at Jominy distance 17 mm. the hardness at center of a 3 inch bar will be 43 HRC .Effect of Alloying Elements Hardness produced at Jominy distance 17 mm in 8640 steel will be 43 HRC. Therefore. Cooling rate and Jominy distance do not change with alloying elements as the rate of heat transfer is nearly independent of composition . this table enables the user to estimate the hardnesses that would be obtained at the centers of quenched round bars of different diameters. . the hardness obtained in the end-quench test would be found at the center of the bar size. For each successive 1/16 in.Equivalent bar diameter when quenched • When the end-quench hardness curve of a steel has been found. when that same steel is quenched with various severities of quench. position. Equivalent bar diameter when quenched . 9 .1 14 6.Cooling Rate at Each Jominy Position for Room Temperature Water Distance from water quenched end Cooling Rate 1/16 in.9 12 9.9 16 5.6 20 3.6 18 4. 0C/s 1 270 2 170 3 110 4 70 5 43 6 31 7 23 8 18 9 14 10 11. Jominy test and CCT diagrams . 5 °C/s in oil. the cooling rate at center is about 27°C/s in water. For example. 75mm vs. • Severity of quench: Water > Oil > Air. . for a 50 mm diameter bar. 50mm.g. the cooling rate at center is lower when the diameter is larger. but. • For a particular medium.Influence of quench medium and sample size on the cooling rates at different locations. 13. e. 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 Spring 2001 Dr. Ken Lewis ISAT 430 82 . Correlation of carbon and martensite content with Rockwell hardness . Alloy Factors For The Calculation Of Ideal Diameter . Interconversion of ideal bar diameter as a function of shape . Thanks .