Alteration of Microstructure / 73steel was to combine with sulfur in the form of manganese sulfide, Internal hot shortness can be created by the presence of iron MnS, to prevent hot shortness. Without sufficient manganese, the sulfides. Iron sulfides have a lighter gray appearance than man- sulfur would combine with the iron and create iron sulfides. Iron ganese sulfides, as shown in Fig. 3.41. In this micrograph, taken sulfide has a much lower melting point than manganese sulfide from a normal AISI/SAE 1015 steel casting, the iron sulfide and thus remains liquid at rolling temperatures. Also, iron sulfides constituent surrounds a manganese sulfide particle. The dove gray are more brittle than manganese sulfides and degrade the me- color of a normal manganese sulfide inclusion can be seen in Fig. chanical properties of the steel. Manganese sulfides, on the other 3.42 for comparison. In Fig. 3.41, small cracks can be seen in the hand, are easily deformed at rolling temperatures and are not as brittle iron sulfide rim, and the rim has separated from the steel deleterious to mechanical properties. matrix. The iron sulfide has formed around (nucleated on) the manganese sulfide, because it was liquid when the manganese sulfide was solid during the solidification process. Upon heating to Fig. 3.38 Microstructure of segregation along a prior austenite grain Fig. 3.39 Macrograph of an AISI/SAE 1035 steel showing surface cracking boundary in the 0.7% C-3% Cr steel shown in Fig. 3.36. 1000⫻ due to a hot-shortness condition caused by copper. 2⫻ Fig. 3.40 Microstructure of the steel shown in Fig. 3.39 with elemental copper along grain boundaries (see arrow). This is a condition known as hot shortness. Note: the micrograph is out of focus, because the ferrite matrix was chemically attacked by the etch, thus leaving the focused Fig. 3.41 Microstructure of an AISI/SAE 1015 casting showing iron sulfide copper in relief. 2% nital etch. 500⫻ surrounding a manganese sulfide inclusion. Unetched. 2000⫻ 74 / Metallographers’ Guide hot-rolling temperatures, the iron sulfide would remelt and create has serious consequences for a stainless steel is the result of an internal flaw. This condition can lead to internal cracking. exposure to temperatures in the range of 425 to 870 °C (800 to Because they are undesirable in steel, a metallographer must be 1600 °F). During exposure, chromium carbides form at the grain able to identify iron sulfides. However, with modern steelmaking boundaries and deplete the regions near the boundaries of chro- technology, it is quite rare to see iron sulfides in steel. mium. The longer the exposure, the greater the depletion of Sensitization. Stainless steels rely on their chromium content chromium, until eventually the level drops below 12% locally, and to prevent corrosion. Generally, about 12% Cr is needed for this corrosion along grain boundaries can result. The process of task. One example of unintentional microstructural alteration that chromium loss is called sensitization. An example of sensitization in an AISI 316 austenitic stainless steel sheet is shown in Fig. 3.43. In this micrograph, the austenite grain boundaries are “decorated” with chromium-rich carbides (Cr,Fe)23C6. This stain- less steel sheet was exposed to 675 °C (1250 °F) for 12 days. A more severe example of sensitization is shown in Fig. 3.44, where the same AISI 316 stainless steel sheet was exposed to 730 °C (1350 °F) for two months. Here, the chromium carbides have precipitated on annealing twins in addition to the austenite grain boundaries. Because of the long-term exposure, large chromium carbides have also formed. The metallographer must be aware of the possibility of a sensitized condition in a stainless steel that has been exposed to an elevated temperature. In the early stages, it is very difficult to detect the carbides at the grain boundaries, and careful observation at high magnification is required. It is important to know that the condition of sensitization can be eliminated by another heat treatment at temperatures above 870 °C (1600 °F), where all the carbides dissolve in the austenite. A typical treatment is heating a sensitized stainless steel to 980 °C (1800 °F) for four hours, followed by rapid cooling. Temper Embrittlement. If an alloy steel is slow cooled from above 595 °C (1100 °F), it may become embrittled. The specific Fig. 3.42 Microstructure of an AISI/SAE 1020 cast bloom with a normal embrittlement temperature range is about 375 to 575 °C (710 to manganese sulfide inclusion. Note the color difference of iron 1070 °F). Indications of embrittlement are evident in the Charpy sulfide in Fig. 3.41. Unetched. 1000⫻ Fig. 3.43 Microstructure of an AISI/SAE 316 stainless steel showing sensitization. Note the chromium carbides at the austenite grain Fig. 3.44 Microstructure of an AISI 316 stainless steel showing severe boundaries. The steel was exposed to 675 °C (1250 °F) for 12 days. sensitization. Exposed to 730 °C (1350 °F) for two months. HCl/HNO3/H2O etch. 1000⫻ HCl/HNO3/H2O etch. 1000⫻ Alteration of Microstructure / 75 V-notch impact test, where very low values of toughness are 3.45. This example is a 3% Cr steel. Many of the austenitic grain recorded. The embrittlement takes place at the austenite grain boundaries can be seen as the flat, faceted surfaces in the SEM boundaries. Phosphorus segregation to the austenite grain bound- micrograph. This type of fracture is called intergranular fracture. aries is usually attributed as the cause of the problem. An easy In this case, phosphorus has accumulated on the austenite grain way to detect temper embrittlement is to observe the fracture boundaries and created a weakness in the bond between the surface of a broken Charpy bar or other fractured surface in the austenite grains. In most chromium-bearing steels, an addition of SEM. An example of such a fracture surface can be seen in Fig. 0.5 to 1.0% Mo is sufficient to eliminate the problem. The Fig. 3.45 A SEM micrograph of the fracture surface of a 3% Cr steel showing intergranular fracture, indicating a condition of temper Fig. 3.46 Microstructure of a 1.4% C steel showing numerous micro- cracks (dark lines) in the martensite plates. The white-appearing embrittlement. 500⫻ constituent is retained austenite. 12% sodium metabisulfite tint etch. 500⫻ Fig. 3.48 Microstructure of a cold-drawn and spheroidized AISI/SAE 1095 Fig. 3.47 Microstructure of an AISI/SAE 1080 steel showing a microcrack steel bar showing regions of graphite (elongated, dark bands). at the center of the micrograph (see arrow) in a martensite plate. 4% picral etch. 1000⫻. Courtesy of S. Lawrence, Bethlehem Steel’s Homer 12% sodium metabisulfite tint etch. 1000⫻ Research Center 76 / Metallographers’ Guide molybdenum changes the activity of phosphorus in the steel each other during transformation, the plates can crack at the point (austenite) and minimizes its ability to segregate to the grain of impingement. The result is a microcrack. An example of boundaries. numerous microcracks that have formed in a 1.4% C steel is Microcrack Formation. When higher-carbon steels (above shown in Fig. 3.46. In this micrograph, one can see the small about 0.6% C) are rapidly cooled from the austenitic state, plate cracks in the martensite plates. These cracks can grow to larger martensite can form. Plate martensite differs from the lath cracks when a component with microcracks is used in service, martensite that forms in lower-carbon steels. The differences have especially under a cyclic loading environment. Cyclic loading will been described in Chapter 2. One problem with plate martensite is eventually lead to a fatigue failure. A microcrack can also be seen that it is very brittle. When two or more martensite plates strike in Fig. 3.47, representing both lath and plate martensite in a Fig. 3.49 Microstructure of a 1.2% C steel that has formed graphite (dark etching constituent), or “graphitized,” after exposure to 700 °C (1290 °F) for (a) 190, (b) 375, and (c) 565 h. 4% picral etch. 500⫻. Courtesy of B. Lindsay and A.R. Marder, Lehigh University Alteration of Microstructure / 77 quenched AISI/SAE 1080 steel rod containing centerline segre- particles. At 375 hours (Fig. 3.49b), more graphite forms, replac- gation (a region of higher hardenability enhancing the formation ing some of the cementite particles, and at 565 hours (Fig. 3.49c), of martensite). In this figure, the few martensite plates can be seen all the cementite particles have decomposed into graphite. This (arrows). Whenever a metallographer observes plate martensite in latter treatment is unusually long, but illustrates the graphitization a microstructure, he or she should carefully look for microcracks, process. because their presence will be an important indication of a The only way to remove graphitization is to retreat the steel in potential problem. If microcracks form, the part may have to be the austenite regime to dissolve the graphite and to redistribute the scrapped. It is possible that a component with microcracks can be carbon uniformly in the steel. From the iron-carbon equilibrium salvaged by annealing for an extended period of time to allow diagram shown in Fig. 3.2, one can see that carbon is very soluble healing of the cracks. This treatment, however, does not always in austenite. Once the graphite dissolves in the austenite, the steel work. can be cooled to room temperature. Figure 3.50 shows the result Graphitization of Steel. It is obvious that cast irons contain of reversing the graphitization process by reheating the 1.2% C free graphite. However, it is also possible for a steel to contain steel at 980 °C (1800 °F) for two hours. The dark, rounded region graphite. An example of graphite formation in steel is shown in in the center is a void. One of the results of dissolving the graphite Fig. 3.48, where the graphite is shown as dark, elongated particles. into austenite is the development of voids, because of differences This steel is AISI/SAE 1095 that has been annealed, hot rolled, in specific volume between graphite and austenite. and spheroidized. However, the spheroidization treatment was extended too long, and the cementite (Fe3C) phase decomposed to Microstructures Altered during Cutting/Machining pure carbon (graphite) and iron. This graphitization process occurs when a steel is held for long times at temperatures just Many times, the microstructure of a steel or cast iron is below the lower critical temperature (A1). The optimal tempera- unintentionally altered during a cutting operation using an oxy- ture range for graphitization is between 600 and 700 °C (1110 and gen-acetylene torch. The microstructural alteration could be 1290 °F). Graphitization can also occur in steam power plant detrimental. This section shows examples of how a microstructure components that are exposed to elevated temperatures for ex- can be altered simply by cutting the steel. tended periods of time. Graphitization is not desirable, because Improper Cutting. Many steel products are cut to a specific the regions of graphite are very soft and provide locations of size or length by an oxygen-acetylene or plasma-arc cutting torch. weakness within the steel. However, this practice may not always be the best procedure. The development of graphite in a 1.2% C steel is shown in Fig. Let’s look at an example of a plasma-arc-cut steel plate. Figure 3.49. This steel was given a spheroidizing heat treatment at 700 3.51 shows the microstructure at the cut surface of an AISI/SAE °C (1290 °F) for 190, 375, and 565 hours. At 190 hours (Fig. 1020 steel plate. The dark surface region is 100% lath martensite, 3.49a), very small regions of graphite begin to form on cementite as shown in Fig. 3.52(a). Adjacent to the lath martensite surface Fig. 3.50 Microstructure of a 1.2% C steel that was graphitized after Fig. 3.51 Microstructure of a plasma-arc-cut surface of an as-rolled exposure to 700 °C (1290 °F) for 190 h, then heat treated at 980 AISI/SAE 1020 steel plate showing surface damage (top). Re- °C (1800 °F) to dissolve the graphite. Note void (dark region) in center of gions “A”, “B”, “C” are shown at higher magnification in Fig. 3.52. 2% nital micrograph. 4% picral etch. 500⫻ and 4% picral etch. 100⫻ 78 / Metallographers’ Guide region is a region of lath martensite and ferrite, as shown in Fig. microcracks during transformation (as shown in a previous 3.52(b). The base microstructure is shown in Fig. 3.52(c). The lath section on microcracks). Microcracks form when martensite martensite layer has a hardness of 349 HK (35 HRC), whereas the plates impinge on one another. This condition would be unaccept- base steel has a hardness of 165 HK (80 HRB). This hard layer at able, because the microcracks would create larger cracks during the surface may cause problems in machining the material. service. Residual stresses will also develop, because of the formation of a Improper Machining. Machining can induce cold work into martensitic layer in a ferritic-pearlitic component. These stresses the surface layers of a part. An example is shown in Fig. 3.53(a), and the higher hardness level can be reduced by a stress-relief heat which represents the surface layer of a 13 mm (0.5 in.) diameter treatment. In a higher-carbon steel, the martensite would be in the bar of AISI 316 stainless steel. The surface layer consists of a high form of plate martensite, which would have a tendency to form density of mechanical twins that were induced during machining. Fig. 3.52 Microstructure of the affected layers in the plasma-torch-cut AISI/SAE 1020 steel plate in Fig. 3.51 showing (a) lath martensite at the surface, (b) lath martensite and ferrite just below the surface, and (c) ferrite and pearlite of the base steel. 4% picral etch. 500⫻ Alteration of Microstructure / 79 The microstructure at the center of the bar is shown in Fig. microscope was necessary to reveal this sensitized condition. This 3.53(b). Here, the microstructure contains only a few twins. In an is a caution to the metallographer who could miss the subtle effort to remove the mechanical twins at the surface layers, the bar features if only a cursory examination is used. was heat treated at 800 °C (1470 °F) for 0.5 hours. Unfortunately, This is a unique sample in that not only is the surface cold the mechanical twins were not removed, and the bar was worked, but the remaining bar is sensitized. The only way to inadvertently sensitized, as shown in the high-magnification salvage this material would be to anneal the steel to remove both micrograph in Fig. 3.54. Sensitization was discussed in the section the cold work and the sensitized condition. “Microstructures Altered by Improper Heat Treatment.” In this micrograph, one can see carbides that formed at the grain Microstructures Altered under Service Conditions boundaries and twins. A very high magnification in the light In many cases, the original microstructure is altered in service. When steels and cast irons are exposed to high temperatures, oxidation and corrosion processes produce microstructural changes that can adversely affect the properties of the material. Also, deformation and friction can create high temperatures in steel. These temperatures can alter microstructure. The metallog- rapher must be aware of the effects of these service conditions on microstructure. A few examples are given in this section to illustrate some of these changes. Friction Effects. A metallographer examining a specimen taken from a railway track should expect to find a fully pearlitic microstructure. However, sometimes the pearlitic microstructure is unintentionally altered in an unusual manner in service. For example, the running surface of a rail (the surface in contact with the railway wheel) can be heated into the austenitic range by the friction of a seized or slipping locomotive wheel rubbing along the rail surface. This can take place during slippage, where the locomotive wheels are turning, but the train does not move. Also, severe friction could occur if the locomotive wheel is stationary during emergency breaking, with the wheels sliding along the track. The surface damage to the rail is termed a “wheel burn.” Fig. 3.53 Microstructure of a machined AISI 316 stainless steel bar Fig. 3.54 A sensitized condition found in the central region of the AISI showing (a) deformation bands at the surface and (b) annealing 316 stainless steel bar in Fig. 3.53. Note the carbides on the twins at the center. Electrolytic etch of 10% oxalic acid in water, stainless steel grain boundaries and annealing twins. Electrolytic etch of 10% oxalic acid in cathode, 6V, 10 s. 200⫻ water, stainless steel cathode, 6V, 10 s. 1500⫻ 80 / Metallographers’ Guide Upon cooling from the austenitic field, the microstructure that metallographer the complexity of microstructures that can de- forms is not pearlite, because the cooling rate is very high due to velop within one sample. This is why it is important to understand the self-quenching effect of the underlying steel. Martensite and the origin of microstructures in steels and cast irons. This basic bainite layers form from the austenite. There is also retained understanding is vital to metallographic interpretation. The pre- austenite contained within the martensite layer. An example of a vious example is a case where the light microscope has reached wheel burn is shown in Fig. 3.55. At this low magnification, the the limit of resolving power, and an electron microscope is microstructural constituents are not resolved, but one can see a necessary to add to the interpretation through higher magnifica- shallow, nonetching white layer at the surface, several surface tion and resolving power. cracks due to the thermal stresses, and a dark layer followed by a Void Formation during Creep. When steel and cast iron light etching layer and a gray layer. The thin white layer and a components are exposed to a stress or load for a long period of portion of the dark etching layer are shown in Fig. 3.56(a). The time at an elevated temperature, a process known as “creep” can white layer has been the focus of many metallurgical investiga- take place. Generally, the stress is well below the normal yield tions, and its identity is controversial. It is usually considered as strength of the material. Metal flow takes place by mechanisms martensite. In some cases, it may be highly deformed ferrite. In related to grain-boundary sliding and void formation at grain the example shown in Fig. 3.56, the white layer is martensite. The boundaries. An example of the beginning of the creep process is dark etching layer in Fig. 3.56(a) and the region just below is shown in Fig. 3.57. The alloy is a 0.28% C, 2.5% Ni, 1 % Cr, 1% tempered martensite, as shown in Fig. 3.56(b). The light etching Mo steel that was held at 565 °C (1050 °F) under a stress of 210 regions within the martensite are retained austenite. The light MPa (30 ksi). Voids can be seen nucleating and growing at grain etching region below the tempered martensite is as-quenched boundaries in Fig. 3.57(a). In Fig. 3.57(b), the voids begin to link martensite and mixtures of as-quenched martensite and pearlite, as up, and in Fig. 3.57(c), they form separations along grain seen in Fig. 3.56(c). The dark etching constituent is the base boundaries. Once this grain-boundary separation occurs, the pearlitic microstructure (see Fig. 3.56d). This example shows the material will begin to fail at an accelerated rate. Under these conditions, it took about 5000 hours to develop the stage of extensive grain-boundary separation. Creep occurs in parts such as rotors in steam-powered generators and in the blades of aircraft gas turbine engines. Once in the final stages of creep, the parts are prone to fail in a catastrophic manner. Special techniques are used to monitor the life of parts that are exposed to environments that promote creep. One of these techniques is field metallography, where a small area of the part is polished and etched while the part is still in service. Field metallography is used when the part or a sample cannot be brought to the metallographic laboratory. Hydrogen Damage. Damage due to hydrogen gas can occur under many circumstances. Hydrogen can be picked up in steels through the steelmaking process where the liquid steel is exposed to moisture from the furnace/ladle/tundish refractories, from exposure to humidity in the air, and from moisture in alloying additions. Also, in cast steels and cast irons, sand binders and cores containing hydrocarbons can break down to form hydrogen gas upon exposure to molten steel and cast iron. Hydrogen can also be absorbed into steel by exposure to acids and electrolytic plating processes. Hydrogen can also enter a weld from moisture- laden electrodes. Thus, hydrogen can come from many sources, and if hydrogen damage is found in a steel, the metallographer may have to find clues to determine the source of hydrogen. Hydrogen, being the smallest element, is easily absorbed in the iron lattice as elemental hydrogen (H⫹). To eliminate this form of hydrogen, it can be easily diffused from the steel under controlled conditions, for example, vacuum degassing of liquid steel and slow cooling from an elevated temperature, for example, 540 °C (1000 °F), immediately after rolling or forging (not allowing the component to cool to room temperature first). However, when the hydrogen atoms combine into the molecular form (H2), diffusion Fig. 3.55 Microstructure of a “wheel burn” condition on a railroad rail is almost impossible, and hydrogen is thus trapped inside the steel. (eutectoid steel) showing the surface “white layer” and other layers. These layers are shown at higher magnification in Fig. 3.56. 2% nital Once inside the steel, hydrogen prefers to reside at a surface such etch. 32⫻ as an inclusion interface, for example, a manganese sulfide. At