Volume 01 - Properties and Selection Irons, Steels, and High-Performance Alloys Episode 7 pps

Fig. 1 Effect of carbon on the hardness of martensite structures The hardenability of steel is governed almost entirely by the chemical composition (carbon and alloy content) at the austenitizing temperature and the austenite grain size at the moment of quenching. In some cases, the chemical composition of the austenite may not be the same as that determined by chemical analysis, because some carbide may be undissolved at the austenitizing temperature. Such carbides would be reflected in the chemical analysis, but because the carbides are undissolved in the austenite, neither their carbon nor alloy content can contribute to hardenability. In addition, by nucleating transformation products, undissolved carbides can actively decrease hardenability. This is especially important in high-carbon (0.50 to 1.10%) and alloy carburizing steels, which may contain excess carbides at the austenitizing temperature. Consequently, such factors as austenitizing temperature, time at temperature, and prior microstructure are sometimes very important variables when determining the basic hardenability of a specific steel composition. Certain ingot casting and hot reduction practices may also develop localized or periodic inhomogeneities within a given heat, further complicating hardenability measurements. The effects of all these variables are discussed in this article. Hardenability of Carbon and Low-Alloy Steels Revised by Harold Burrier, Jr., The Timken Company Hardenability Testing The hardenability of a steel is best assessed by studying the hardening response of the steel to cooling in a standardized configuration in which a variety of cooling rates can be easily and consistently reproduced from one test to another. The Jominy end-quench test fulfills the cooling rate requirements of hardenability testing of a broad range of alloy steels. The test specimen, a 25.4 mm (1.000 in.) diam bar 102 mm (4 in.) in length, is water quenched on one end face. The bar from which the specimen is made must be normalized before the test specimen in machined. The test involves heating the test specimen to the proper austenitizing temperature and then transferring it to a quenching fixture so designed that the specimen is held vertically 12.7 mm (0.5 in.) above an opening through which a column of water can be directed against the bottom face of the specimen (Fig. 2a). While the bottom end is being quenched by the column of water, the opposite end is cooling slowly in air, and intermediate positions along the specimen are cooling at intermediate rates. After the specimen has been quenched, parallel flats 180° apart are ground 0.38 mm (0.015 in.) deep on the cylindrical surface. Rockwell C hardness is measured at intervals of 1 16 in. (1.6 mm) for alloy steels and 1 32 in. (0.8 mm) for carbon steels, starting from the water-quenched end. A typical plot of these hardness values and their positions on the test bar, as shown in Fig. 2(b), indicates the relation between hardness and cooling rate, which in effect is the hardenability of the steel. Figure 2(b) also shows the cooling rate for the designated test positions. Details of the standard test method are available in ASTM A 255 and SAE J406. Fig. 2 Jominy end-quench apparatus (a) and method for presenting end-quench hardenability data (b) The Carburized Hardenability Test. It is often necessary to determine the hardenability of the high-carbon case regions of carburized steels. Such information is important in controlling carburizing and quenching practice and in determining the ability of a specific steel to meet the microstructural and case depth requirements of the carburized component manufactured from the steel. As a general rule, adequate core hardenability does not ensure adequate case hardenability, especially when it is required to reheat for hardening after carburizing rather than to quench directly from the carburizing furnace. Two factors are responsible for this fact. The first is that equal alloying additions do not have the same effect on the hardenability of all carbon levels of alloyed steels. The second factor (as noted earlier) is that the high- carbon case regions do not always achieve full solution of alloy and carbides, as is normally achieved in the austenite of the low-carbon core region, prior to quenching. Accordingly, direct measurements of case hardenability are very important whenever a carburizing steel must be selected for a specific application. Measurements of case hardenability are performed as follows. A standard end-quench bar is pack carburized for 9 h at 925 °C (1700 °F) and end quenched in the usual manner. A comparison bar is simultaneously carburized in the same pack to determine carbon penetration. Successive layers are removed from it and analyzed chemically to determine the carbon content at various depths. When a carbon-penetration curve is established, depths to various carbon levels can be determined in the Jominy bar, assuming that the distribution of carbon in the end-quench specimen is the same as in the carbon gradient bar. Longitudinal flats are then carefully ground to various depths on the end-quench bar (usually to carbon concentrations of 1.1, 1.0, 0.9, or 0.8%, and in some cases to as low as 0.6%), and hardenability is determined at these carbon levels by hardness traverses. In grinding, care must be exercised to avoid overheating and tempering, and in conducting hardness surveys, similar concern must be shown to ensure that the hardness level corresponds to a single carbon level by remaining in the exact center of the flat. Rockwell A hardness readings are preferable to Rockwell C readings because they minimize the depth of indentor penetration into softer subsurface layers. Rockwell A values are converted into Rockwell C values for plotting, as illustrated in Fig. 3, which shows the curves of carburized hardenability of an EX19 steel. In the higher-carbon layers of carburized specimens, the hardness will be influenced by the presence of retained austenite. Therefore, it is often useful to evaluate the microstructure/depth relationship by metallographically polishing and etching the ground flats. The Jominy distance to some chosen level of nonmartensitic transformation product can then be used as a measure of hardenability. Fig. 3 Carburized hardenability, EX19 steel. Composition: 0.18 to 0.23% C, 0.90 to 1.20% Mn, 0.40 to 0.60% Cr, 0.08 to 0.15% Mo, 0.0005% B (min) The case hardenability of steels that are carburized and then reheated for hardening at temperatures below 925 °C (1700 °F), such as 8620, 4817, and 9310, can also be determined by using a modification of this technique. The carburized end- quench specimens and companion gradient bars are oil quenched together from carburizing, but are then reheated in an atmosphere furnace to the desired austenitizing temperature for a total of 55 to 60 min, which should ensure at least 30 to 35 min at temperature. The hardenability specimen is then end quenched, and the carbon gradient bar is oil quenched and tempered to facilitate machining for carbon gradient determination, as described above. It is recommended that case hardenability tests be performed on no fewer than two test specimens. A more detailed description of the case hardenability measurement technique appears in SAE J406. Air Hardenability Test. Occasionally, the hardening performance either of a steel cooled at a rate slower than that applied to the end-quench bar or of steels of very high hardenability must be determined. An air hardenability test method described in Ref 1 can be employed for this purpose. In this test, a machined and partially threaded round test specimen, 25.4 mm (1.000 in.) in diameter and 254 mm (10 in.) long, is inserted to a depth of 152 mm (6 in.) in a hole drilled in a bar 152 mm (6 in.) in diameter and 381 mm (15 in.) long, thus leaving 102 mm (4 in.) of the test bar length exposed (Fig. 4). A second test specimen can be inserted at the opposite end of the bar holder to serve as a duplicate. With both test bars securely in place, the assembly is heated to the proper austenitizing temperature, after which it is transferred to a convenient location for cooling in still air. This cooling procedure results in very slow and ever decreasing cooling rates along the length of the test bars. Hardness is then measured at discrete intervals along each test bar and plotted against distance from the exposed end on charts specifically designed for this purpose. Fig. 4 Dimensions (given in inches) of components in air hardenability test setup Continuous-Cooling-Transformation Diagrams. The use of continuous-cooling-transformation diagrams determined dilatometrically, for example, can also be helpful in evaluating the cooling behavior of high-hardenability steels. Reference cited in this section 1. C.F. Jatczak, Effect of Microstructure and Cooling Rate on Secondary Hardening of Cr-Mo-V Steels, Trans. ASM, Vol 58, 1965, p 195 Hardenability of Carbon and Low-Alloy Steels Revised by Harold Burrier, Jr., The Timken Company Low-Hardenability Steels In plain carbon and very low-alloy steels, the cooling rate at even the 1.6 mm ( 1 16 in.) position on a standard Jominy bar may not be fast enough to produce full hardening. Therefore, this test lacks discrimination between these steels. Tests that are more suited to very low hardenability steels include the hot-brine test and the surface-area-center (SAC) test. In the hot-brine test proposed by Grange, coupons (Fig. 5) are quenched in brine maintained at a series of different temperatures. As shown in Fig. 6, the resulting hardnesses provide a very sensitive test of hardenability. Fig. 5 Hot-brine hardenability test specimen. (a) Specimen dimensions. (b) M ethod of locating hardness impressions after heat treatment. Dimensions given in millimeters. Source: Ref 2 Fig. 6 Typical results of the hot- brine hardenability test. Steel composition: 0.18% C, 0.81% Mn, 0.17% Si, and 1.08% Ni. Austenitized at 845 °C (1550 °F). Grain size: 5 to 7. RT, room temperature. Source: Ref 2 In the SAC test, a 25.4 mm (1.000 in.) round bar is normalized by cooling in air and then reaustenitized for water quenching. Hardnesses are measured on a specimen cut from the center of the 100 mm (4 in.) length. Hardness is determined on the surface, the center, and at 1.6 mm ( 1 16 in.) intervals from surface to center. An area hardness is then computed as the sum of the average hardness in each interval × 1 16 (Fig. 7). The resulting set of three-digit numbers, for example, SAC No. 63-52-42, indicates a surface hardness of 63 HRC, a Rockwell-inch area of 52, and a center hardness of 42 HRC. Testing details are given in SAE J406. Fig. 7 Surface-area-center estimation of area Reference cited in this section 2. R.A. Grange, Estimating the Hardenability of Carbon Steels, Metall. Trans., Vol 4, Oct 1973, p 2231 Hardenability of Carbon and Low-Alloy Steels Revised by Harold Burrier, Jr., The Timken Company Calculation of Hardenability The hardenability of a steel is primarily a function of the composition (carbon, alloying elements, and residuals) and the grain size of the austenite at the instant of quenching. If this relationship can be determined quantitatively, it should be possible to predict the hardenability of a steel through a relatively simple calculation. Such a technique was published by Grossmann in 1942, based on his observation that hardenability could be expressed as the product of a series of composition-related multiplying factors (Ref 3). The result of the calculation is an estimate of D I , the ideal critical diameter of the steel. The multiplying-factor principle is still used today in several hardenability calculation techniques (see the article "High-Strength Structural and High-Strength Low-Alloy Steels" in this Volume for examples of multiplying factors for quench and tempered low-alloy steels). Other researchers have developed methods based on regression equations and on calculation from thermodynamic and kinetic first principles. To date, none of the hardenability prediction methods has proved to be universally applicable to all steel types; that is, different predictors are more suited to steels of given alloying systems, carbon contents, and hardenability levels. In addition, it is often necessary to fine-tune the predictions based on the characteristics (residuals, melt practice, and so on) of a particular steel producer. Some excellent discussions of current thinking on this subject are available in Ref 4 and 5. Properly used, hardenability calculations can provide a valuable tool for designing cost-effective alternative steels, for deciding the disposition of heats in the mill prior to rolling, and possibly for replacing the costly and time-consuming measurement of hardenability. References cited in this section 3. M.A. Grossmann, Hardenability Calculated from Chemical Composition, Trans. AIME, Vol 150, 1942, p 227 4. D.V. Doane and J.S. Kirkaldy, Ed., Hardenability Concepts With Applications to Steel, The Metallurgical Society, 1978 5. C.S. Siebert, D.V. Doane, and D.H. Breen, The Hardenability of Steels, American Society for Metals, 1977 Hardenability of Carbon and Low-Alloy Steels Revised by Harold Burrier, Jr., The Timken Company Effect of Carbon Content Carbon has a dual effect in hardenable alloy steels: It controls maximum attainable hardness and contributes substantially to hardenability. The latter effect is enhanced by the quality and type of alloying elements present. It might be concluded, therefore, that increasing the carbon content is the least expensive approach to improving hardenability. This is true to a degree, but several factors weigh against the use of large amounts of carbon: • High carbon content generally decreases toughness at room and subzero temperatures • It produces harder and more abrasive microstructures in the annealed conditions, which makes cold shearing, sawing, machining, and other forms of cold processing more difficult • It makes the steel more susceptible to hot shortness in hot working • It makes the steel more prone to cracking and distortion in heat treatment. Because of these disadvantages, more than 0.60% C is seldom used in steels for machine parts, except for springs and bearings, and steels with 0.50 to 0.60% C are used less frequently than those containing less than 0.50% C Figure 8 shows the differences between minimum hardenability curves for six series of steels. In each series, alloy content is essentially constant, and the effect of carbon content on hardenability can be observed over a range from 0.15 to 0.60%. The hardness effect is shown by the vertical distance between the curves at any position on the end-quench specimen, that is, for any cooling rate. This effect varies significantly, depending on the type and amounts of alloying elements. For example, referring to Fig. 8 (d) to (f), an increase in carbon content from 0.35 to 0.50% in each of the three series of steels causes hardness increases (in Rockwell C points) at four different end-quench positions, as shown below: Distance from quenched surface, in. Series 1 16 4 16 8 16 12 16 41xxH 8 10 17 20 51xxH 8 13 9 8 86xxH 8 12 18 12 Fig. 8 Effect of carbon content on the minimum end-quench hardenability of six series of alloy H- steels. The number adjacent to each curve indicates the carbon content of the steel, to be inserted in place of xx in alloy designation. The hardenability effect of carbon content is read on the horizontal axis in Fig. 8. If the inflection points of the curves are used to approximate the position of 50% martensite transformation, the effect of carbon content on hardenability in 8650 versus 8630 steel can be expressed as + 4 16 ; that is, the inflection point is moved from the 5 16 position to the 9 16 position. Similarly, with nominal carbon contents of 0.35 and 0.50%, the hardenability effect of carbon is seen to be less ( 2 16 ) in 51xx series steels and more ( 6 16 ) in 41xx steels. Considering the combined hardening and hardenability effects in terms of quenching speed, the cooling rate (or quenching speed) required to produce 45 HRC is affected more by 0.15% C with certain combinations of alloying elements than it is by other combinations. For example, in a steel containing 0.75 Cr and 0.15 Mo (a 41xxH series steel, for example), increasing the carbon content by 0.15% lowers the required or critical cooling rate to obtain 45 HRC from 25 to 4.6 °C (45 to 8.3 °F) per second, while in a steel containing 0.75% Cr and no molybdenum (51xxH series), the same increase in carbon content lowers the cooling rate from 47 to 21 °C (85 to 37 °F) per second. The practical significance of the effect of carbon and alloy contents on cooling rate is considerable. In a 51 mm (2 in.) diam bar of 4150 steel, a hardness of 45 HRC can be obtained at half-radius using an oil quench without agitation. In a 4135 steel bar of the same diameter, to obtain the same hardness at half-radius would require a strongly agitated water quench. Comparing 32 mm (1 1 4 in.) diam bars of 5135 and 5150 steel, an agitated water quench will produce a hardness of 45 HRC at half-radius in the 5135 bar; the identical condition can be obtained in the 5150 bar using an oil quench with moderate agitation. Thus, an increase or decrease in carbon content or an alloying addition, such as 0.15% Mo, affects the results obtained both in terms of the quenching severity required and the section size in which the desired results can be obtained. Figure 9 shows how steels are rated on the basis of ideal critical diameter by expressing the effect of carbon and alloy content on the section size that will harden to 50% martensite at the center, assuming an ideal quench. An ideal quench is defined as one that removes heat from the surface of the steel as fast as it is delivered to the surface. In general, the relation between hardness and carbon content that is important in practice is obscured in this rating method because the steel is rated to a constant microstructure. Hardness decreases continuously with lower carbon contents. Fig. 9 Effect of carbon content on ideal critical diameter, calculated for the minimum chemical composition of each grade Hardenability of Carbon and Low-Alloy Steels Revised by Harold Burrier, Jr., The Timken Company [...]... 1 2 4815, 872 0, 4621, 8622, 1050(b) 0.9 2.35 0 .7 1.5 0.6 1.3 46B12, 48 17, 4320, 8625, 5046 1.05 2.6 0.8 1.6 0 .7 1.45 40 37, 1541, 471 8, 8822 1.2 2.9 0.9 1.8 0.85 1.6 94B15, 86 27, 4042, 1541, 15B35 1.4 3.2 1.1 1.9 1.0 1 .7 94B 17 4820, 1330, 4130, 8630, 1141 1 .7 3.8 1.4 2.2 1.25 2.0 9130, 5130, 5132, 40 47 1.85 1.5 2.4 1.35 2.1 1335, 50B46, 15B 37 2.0 1 .7 2.5 1.5 2.2 5135 2.1 1.8 2 .7 1.6 2.35... 0.9 0.65 4028, 472 0, 8620, 40 27, 1042, 1045, 1146, 1050, 1524, 1526 0.4 1.5 1.1 0.8 9310, 46B12, 4320, 6120, 872 0, 4621, 8622, 8625, 4032, 4815 0.6 1.8 1.2 0.3 0.95 4815, 48 17, 94B 17, 5046, 1050(b), 478 1, 8822 0 .7 2.05 0.5 1.4 0.45 1.1 86 27, 40 37 0.9 2.35 0 .7 1.5 0.6 1.3 94B15, 4042, 1541 1.05 2.6 0.8 1.6 0 .7 1.45 4820, 1330, 4130, 5130, 8630, 5132, 1141, 50B46, 40 47, 15B35, 94B 17 1.2 2.9 0.9 1.8... 2.6 874 2, 8645, 5160, 9262 2.5 2.2 3.3 1.9 2 .7 6150, 50B60 2 .7 2.35 3.5 2.1 2.9 4140 2.8 2.4 3 .7 2.15 3.0 81B45, 8650, 5152 2.9 2.5 3.8 2.25 3.1 12 86B30 13 51B60 3.25 2.8 2.45 3.4 14 8655 3.45 2.95 2.6 3.55 15 4142 3.65 3.1 2 .7 3 .7 5 1 2 6 6 1 2 7 7 1 2 8 8 1 2 9 9 1 2 10 1 2 11 11 1 2 875 0 3 .75 3.2 2 .75 3.8 18 4145, 8653, 8660 3.45 19 9840, 86B45 3.45 20 41 47 ... 1.4 3.2 1.1 1.9 1.0 1 .7 3 3 1 2 4 4 1 2 5 5 1 2 6 6 4140, 8645 7 1 2 8 1.2 2.1 1.1 1.85 9261, 50B44, 5155 1 .7 3.8 1.4 2.2 1.25 2.0 1.85 1.5 2.4 1.35 2.1 5160, 9262, 50B50 2.0 1 .7 2.5 1.5 2.2 4142, 81B45, 8650 7 3.5 51 47, 6150 1 2 1.55 2.1 1.8 2 .7 1.6 2.35 1 2 5152, 50B60 2.2 1.9 2.9 1 .7 2.45 8 1 2 43 37, 875 0, 8655 2.5 2.2 3.3 1.9 2 .7 9 4145, 51B60 2.6 2.3 3.4 2.0 2.8 9840 2 .7 2.35 3.5 2.1 2.9... 1.5 0.6 1.3 5 86 37, 1340, 5140, 50B46, 4053, 9260, 15B 37 1.2 2.9 0.9 1.8 0.85 1.6 5145, 4063 1.4 3.2 1.1 1.9 1.0 1 .7 4135, 4640, 4068, 1345 1.55 3.5 1.2 2.1 1.1 1.85 8640, 874 0, 5150, 94B30 1 .7 3.8 1.4 2.2 1.25 2.0 41 37, 8642, 6145, 9261, 50B40 1.85 1.5 2.4 1.35 2.1 874 2, 50B44, 5155 2.0 1 .7 2.5 1.5 2.2 8645, 51 47 2.1 1.8 2 .7 1.6 2.35 4140, 6150, 5160, 9262, 50B50 2.2 1.9 2.9 1 .7 2.45 50B60 2.35... 8635, 40 37, 1042, 1146, 1045 0.9 0.8 0.45 4135, 1541, 15B35, 15B 37 1.2 0.9 0.65 50 HRC 1 1 1 2 2 2 1 4 1050(b) 0.3 1.3 1.0 0 .7 1 2 4042 0.4 1.5 1.1 0.8 2 86 37, 5140, 5046, 40 47 0.6 1.8 1.2 0.3 0.95 41 37, 1141, 1340 0 .7 2.05 0.5 1.4 0.45 1.1 4640, 5145, 50B46 0.9 2.35 0 .7 1.5 0.6 1.3 8640, 874 0, 4053, 9260 1.05 2.6 0.8 1.6 0 .7 1.45 8642, 4063, 1345, 50B40 1.2 2.9 0.9 1.8 0.85 1.6 874 2, 6145,... 1.05 2.6 0.8 1.6 0 .7 1.45 1335, 50B46, 15B35 1.2 2.9 0.9 1.8 0.85 1.6 3 3 1 2 4 4 5 1 2 8635, 5140, 4053, 15B 37 1.4 3.2 1.1 1.9 1.0 1 .7 1340, 9260, 4063 1.55 3.5 1.2 2.1 1.1 1.85 86 37, 5145, 1345 1 .7 3.8 1.4 2.2 1.25 2.0 4640, 4068 1.85 1.5 2.4 1.35 2.1 8640, 5150 2.0 1 .7 2.5 1.5 2.2 4135, 874 0, 50B40 2.1 1.8 2 .7 1.6 2.35 6145, 9261, 50B44, 5155 2.2 1.9 2.9 1 .7 2.45 41 37, 8642, 51 47, 50B50, 94B30... frequent intervals because dragout and thermal breakdown may affect their quenching efficiency Table 1 Quenching severities, H, for various media and quenching conditions Typical flow rates Typical H values m/min sfm Air Mineral oil Water Brine None 0 0 0.02 0.2 0-0 .30 0. 9-1 .0 2.0 Mild 15 50 0.2 0-0 .35 1. 0-1 .1 2.1 Moderate 30 100 0.3 5-0 .40 1. 2-1 .3 Good 61 200 0.05 0.4 0-0 .60 1. 4-2 .0 Quenchant agitation Fig... 1.4 3.2 1.1 1.9 1.0 1 .7 5135 1.55 3.5 1.2 2.1 1.1 1.85 15B 37 7 8635, 1340, 5140, 4053 1.85 1.5 2.4 1.35 2.1 8 4063, 1345, 5145 2.1 1.8 2 .7 1.6 2.35 86 37 2.2 1.9 2.9 1 .7 2.45 35 HRC 1 1 2 2 2 1 2 3 3 1 2 4 4 1 2 5 5 1 2 6 6 8 1 2 1 2 9 4640, 4068, 50B40 2.35 2.0 3.1 1.8 2.6 8640, 50B44, 5150 2.5 2.2 3.3 1.9 2 .7 874 0, 9260 4135,50B50 2 .7 2.35 3.5 2.1 2.9 13 41 37 3.25 2.8 2.45 3.4... 4620, 4320, 472 0, 8620, 872 0, 1038, 1522, 1526, 4621 0.9 0.8 0.45 8622, 8625, 40 27, 1045, 1524, 4028, 471 8 1.2 0.9 0.65 9 1 2 10 10 1 2 40 HRC 1 1 1 2 2 2 1 4 1146 0.3 1.3 1.0 0 .7 1 2 4820, 86 27, 4032, 1042, 1050 0.4 1.5 1.1 0.8 2 40 37, 8822 0.6 1.8 1.2 0.3 0.95 4130, 5130, 8630, 5046, 1050(b), 1541 0 .7 2.05 0.5 1.4 0.45 1.1 1330, 5132, 4042 0.9 2.35 0 .7 1.5 0.6 1.3 5135, 1141, 40 47 1.05 2.6 . = B-[(N- 0.002)-Ti/5-Zr/15] ≥0. Source: Ref 5 Reference cited in this section 5. C.S. Siebert, D.V. Doane, and D.H. Breen, The Hardenability of Steels, American Society for Metals, 1 977 . None 0 0 0.02 0.2 0-0 .30 0. 9-1 .0 2.0 Mild 15 50 . . . 0.2 0-0 .35 1. 0-1 .1 2.1 Moderate 30 100 . . . 0.3 5-0 .40 1. 2-1 .3 . . . Good 61 200 0.05 0.4 0-0 .60 1. 4-2 .0 . . . Fig 205, and 150 mm (12, 10, 8, and 6 in.) in diameter. Each bar size was evaluated by tests on end-quench specimens cut from five locations (center, quarter-radius, half-radius, three-quarter-radius,