2 Cooling Rate and Hardenability of Steels Chapter 13Cooling Rate and Hardenability of SteelsCooling Rate • Hardenability
3 The face-centered cubic crystal structure of austenite transforms by shear into the body-centered tetragonal cubic crystal structure of martensite.Martensite is formed by a diffusionless, shear-type mechanism. The face-centered cubic crystal structure of austenite is transformed by shear into the body-centered tetragonal structure of martensite. See Figure This transformation mechanism allows cooperative movement of atoms from one crystal structure to the other. The new structure is equivalent to a supersaturated solution of carbon in alpha iron (ferrite).
4 The hardness of martensite is a function of the carbon content of the steel. The hardness and strength of martensite increases with increasing carbon content of the steel. See Figure Low-carbon martensite in low-carbon steel is soft and does not necessarily require tempering to improve its toughness. For example, carbon content is the principal factor in determining whether fusion-welded steel components require preheating and postweld heat treatment to maintain toughness of the weld heat-affected zone.
5 Lath martensite and plate martensite require the use of an electron microscope for complete resolution.The morphology of martensite may be either lath martensite or plate martensite, depending on the carbon content of the steel. Lath martensite forms in steels with <0.5% C (the majority of engineering steels) and has the appearance of bundles of laths (narrow strips). Plate martensite forms in steels with >1% C and has the appearance of lens-shaped needles packed in different orientations. Steels between 0.5% C and 1% C exhibit mixed structures of lath and plate martensite. Both types of martensite exhibit an indistinct grain structure because the individual grains of laths or plates tend to merge into one another at the grain boundaries. The detail within the laths and plates is hard to resolve in the optical microscope, even at extremely high magnification. An electron microscope must be used to determine the morphology type. See Figure 13-3.
6 The morphology of upper bainite consists of a feathery structure, and that of lower bainite consists of a needle-shaped structure.The morphology of bainite consists of upper bainite and lower bainite. Upper bainite forms closer to the temperature range for pearlitic products and has a feathery structure. Lower bainite forms closer to the martensite and has a needle-shaped structure. See Figure 13-4.
7 Hypoeutectoid steels, eutectoid steel, and hypereutectoid steels are the three main groupings of steels identified on the iron-carbon diagram.The iron-carbon diagram indicates the phase changes that occur when carbon steel is slowly cooled from the austenitizing temperature. The phase transformation occurs by the diffusion of carbon out of austenite to form ferrite, pearlite, or cementite. Hypoeutectoid steels, eutectoid steel, and hypereutectoid steels are the three main groupings of steels identified on the iron-carbon diagram. See Figure 13-5.
8 As the cooling rate increases, diffusion of carbon has less time to occur, which results in a slight change in the shape of the iron-carbon diagram. The eutectoid composition is shifted to the left for hypoeutectoid steels and to the right for hypereutectoid steels.As the cooling rate increases, diffusion of carbon has less time to occur, which results in a slight change in shape of the iron-carbon diagram. See Figure The principal changes are the reduction of the upper and lower critical temperatures for cooling and the movement in the eutectoid composition. The eutectoid composition shifts to the left for hypoeutectoid steels and to the right for hypereutectoid steels. Faster cooling rates lead to finer pearlite grain size, which is harder and tougher than the coarse pearlite formed at slower cooling rates and higher transformation temperatures.
9 Isothermal transformation (I-T) diagrams typically exhibit three distinctive regions and the temperature and time boundaries for the transformation of austenite.I-T diagrams are developed for each specific steel composition. The shapes of I-T diagrams vary with the steel composition, but they typically exhibit a nose, a bay, and/or a flat bottom shape. See Figure The experimental method involves studying the transformation products obtained in small steel samples. The samples are austenitized and then held for set time periods at various temperatures below the lower critical temperature.
10 The microstructure of steel depends on the rate that it cools to the isothermal transformation temperature.A variety of microstructures are obtained from the isothermal transformation of a eutectoid steel. The microstructure formed depends on the rate that the steel cools to the isothermal transformation temperature. See Figure If the steel is quenched rapidly enough to avoid the nose of the start boundary line to a temperature below Mf, then 100% martensite is formed. If the steel is cooled to a temperature above the nose and is allowed to transform isothermally through the start and finish lines, then 100% pearlite is formed. If it is quenched rapidly enough to miss the nose of the first boundary and is allowed to transform isothermally at a temperature above Ms, then 100% bainite is formed.
11 I-T diagrams for hypoeutectoid steels include a region for proeutectoid ferrite. I-T diagrams for hypereutectoid steels include a region for proeutectoid cementite.I-T diagrams for hypoeutectoid and hypereutectoid steels contain an additional region not present on the eutectoid diagram. This region is located above the nose and between the start and finish boundaries. See Figure For a hypoeutectoid steel, the region consists of austenite plus proeutectoid ferrite. For a hypereutectoid steel, the region consists of austenite plus proeutectoid cementite.
12 The products of transformation are indicated along the bottom of C-T diagrams and on the right-hand side of I-T diagrams.Despite the general similarity of shape between C-T diagrams and I-T diagrams for identical steels, the data is presented differently. On C-T diagrams, phase changes are recorded within the start and finish boundaries, whereas on I-T diagrams, this region indicates the transforming phases. On C-T diagrams the products of transformation appear at the bottom of the diagram, whereas on I-T diagrams, they are indicated on the right-hand side of the finish boundary. See Figure
13 A 1080 steel develops higher surface hardness when quenched, but a 4140 steel has higher hardenability because it retains hardness across the section thickness.The difference between hardenability and hardness can be illustrated by heat treating bars of 1080 carbon steel (0.8% C) and 4140 low-alloy steel (0.4% C plus small amounts of chromium and molybdenum). The bars are approximately 50 mm in diameter by 100 mm in length (2″ in diameter by 4″ in length).The bars are austenitized, quenched, and then sectioned diametrically (across the diameter of the bar) at their midpoints. A slice from each is carefully wet ground to avoid overheating. Diametrical hardness traverses are then made across the ground face of each slice using a Rockwell hardness tester. See Figure
14 The critical cooling rate is the slowest cooling rate that misses the nose of the I-T or C-T diagram.In any section thickness of steel, the cooling rate decreases from the surface toward the center. To achieve 100% martensite throughout the section, the critical cooling rate must be exceeded throughout the section. On the I-T diagrams and C-T diagrams, the cooling rate throughout the section must miss the nose of the curve. See Figure
15 The Jominy end-quench specimen is austenitized and quenched under standardized conditions. The Jominy end-quench test is a laboratory procedure for determining the hardenability of steels. The standards for the test procedure are found in ASTM A255 and SAE J406, End-Quench Test for Hardenability of Steel. See Figure
16 On an end-quench hardenability curve, hardness is plotted against distance from the quenched end of the Jominy bar.In an end-quench test, two longitudinal flat surfaces are ground on opposite sides of the quenched specimen, and hardness readings (HRC) are taken at 1.5 mm (0.0625") intervals for the first inch from the quenched end, and at greater intervals beyond that point until a hardness level of 20 HRC or a distance of 50 mm (2") from the quenched end is reached. An end-quench hardenability curve is created to determine the hardenability of the specimen. An end-quench hardenability curve is a plot of hardness readings on the y-axis versus the distance from the quenched end on the x-axis. See Figure
17 The ASTM graph paper used for plotting end-quench hardenability curves indicates the variation of cooling rate with distance from the quenched end of the Jominy bar.Each test specimen is subjected to a series of cooling rates that vary continuously from very rapid at the quenched end to very slow at the air-cooled end. The ASTM graph paper used for plotting the end-quench hardenability curve indicates the cooling rate as a function of distance from the quenched end. See Figure
18 High-hardenability steels exhibit hardness that is maintained for greater distances from the quenched end of the Jominy bar than low-hardenability steels.Hardenability differences between specific grades of steel are readily compared if end-quench hardenability curves are available. High-hardenability steels maintain their as-quenched hardness values to greater distances along the Jominy specimen. See Figure
19 A hardenability band indicates the maximum and minimum hardenability boundaries for a given grade of steel.A hardenability band is a band that defines the boundaries for the minimum and maximum end-quench hardenability curves for standard steels. They indicate the range of hardenability expected from such alloys. Hardenability bands have been established by the AISI and SAE by analysis of end-quench hardenability curves collected from hundreds of heats (ingot) of standard steels. See Figure
20 Correlation of the end-quench hardenability curve with the matching C-T diagram enables the phases formed at different locations along the Jominy end-quench specimen to be predicted.C-T diagrams indicate the phases that form in a steel at different cooling rates. Correlating the end-quench hardenability curve with the C-T diagram for a particular steel makes it possible to identify the various phases expected at different locations along the Jominy end-quench specimen. See Figure
21 The most common criterion for hardenability on the end-quench hardenability curve is the point of inflection (50% martensite). Cooling rates at given distances from the quenched end of the Jominy bar can be correlated to the cooling rates at four different locations in the quenched specimen.The most common criterion for hardenability is the distance along the end-quench specimen where the microstructure consists of 50% martensite. This value is selected because 50% martensite is easy to distinguish in the microstructure and is clearly indicated as the point of inflection on the end-quench hardenability curve. For example, with an 8650 alloy steel end-quench hardenability specimen, the point of inflection occurs at 16 mm (0.625) from the quenched end. See Figure
22 Severity of quench increases from air, to oil, to water, to brine Severity of quench increases from air, to oil, to water, to brine. The amount of agitation of the quenching medium also increases the severity of quench.Severity of quench (H) is a quantitative measure of the cooling power of a quenching medium. See Figure The higher the value of H, the more severe the quench. The order of increasing severity in common quenching media is as follows: air, oil, water, and brine. Increasing the circulation or agitation of any quenching medium increases its severity of quench.
23 Curves of Du/D versus HD are used for estimating the severity of quench (H) of the quenching medium. A graph of Du/D versus D is plotted for the oil-quenched and water-quenched samples. To obtain severity of quench, the oil-quench or water-quench curves are matched to one of the calculated curves of Du/D versus HD. These are available for a wide range of quench severities. See Figure
24 The hardenability of various steels is rated using the ideal critical diameter (DI) values. The higher the DI, the greater the hardenability.Ideal critical diameter (DI) is the largest diameter of any specific steel bar that is hardened to 50% martensite by a perfect quench. Perfect quench is a theoretical quench in which the surface of the bar cools instantaneously from the austenitizing temperature to the temperature of the quenching medium. A perfect quench has a severity of quench value of infinity (H = ∞). Ideal critical diameter values allow quantitative ranking of the hardenability of steel. The higher the ideal critical diameter, the greater the hardenability of the steel. See Figure Ideal critical diameters are expressed as a range of values. This allows for acceptable variations of chemical composition and grain size within the steel specification.
25 Ideal critical diameter (DI) is related to the actual critical diameter (D) by the severity of quench (H). For a perfect quench (H = ∞), DI and D are equal.Ideal critical diameter is related to the actual critical diameter (D) for any specific quench by the severity of quench. For example, for a perfect quench, the actual critical diameter is equal to the ideal critical diameter. See Figure
26 Increasing carbon content significantly lowers the Ms and Mf temperatures. Carbon is the principal alloying element that influences the hardenability of steel. Additions of carbon increase hardenability by increasing the time available for martensite to form. The effect of increasing carbon content is the movement of the nose of the C-T diagram along the time axis and additionally to alter the Ms and Mf temperatures. The C-T diagram is shifted to the right with the increase in carbon content. Carbon also causes significant lowering of Ms and Mf temperatures. For steels with greater than approximately 0.6% C, the Mf falls below room temperature. See Figure
27 Like carbon, most alloying elements have a depressing effect on the Ms temperature. In most cases, increasing the alloy content in a steel delays the start of transformation and increases the time available for its completion. Like carbon, most alloying elements have a depressing effect on the Ms and Mf temperatures. The effect of the alloying elements is not as great as the effect produced by carbon. See Figure A primary reason for alloying steel is to increase its hardenability at acceptably low carbon contents. This is equivalent to making martensite form more easily by delaying the start of transformation so that slower cooling rates can be used in heat treatment operations. This leads to fewer problems.
28 Retained austenite is usually difficult to resolve in the optical microscope, but it is sometimes observed as white patches in a martensite structure.Retained austenite is usually difficult to detect in the microstructure but can sometimes be observed as white patches in a martensite structure. See Figure The patches are difficult to resolve in the optical microscope. X-ray diffraction is used to measure the amount of retained austenite in steels.