Asm Iron and Steel Par 51-100. 44-50 PDF

Alteration of Microstructure / 81Fig. 3.56 Microstructure of the various layers of the rail steel in Fig. 3.55. (a) The white layer at the surface (unattacked by the etchant), (b) tempered plate martensite, (c) as-quenched plate martensite and pearlite (dark), and (d) pearlite base microstructure. 4% picral etch. 1000⫻ 82 / Metallographers’ Guide these sites, sufficient hydrogen gas pressure can create a separa- structure can obscure the cracks, because there are so many other tion at the steel/inclusion interface. As the component is stressed, features present. Also, differential interference contrast (Nomar- the separation can extend into a crack. Hydrogen “flakes” or ski) can be very helpful in locating cracks. cracks form in this manner. An example of hydrogen flakes in an Note the vast improvement in the same AISI/SAE 1080 steel AISI/SAE 1080 steel bar is shown in Fig. 3.58. From these shown in both bright-field (Fig. 3.59a) and differential interfer- micrographs, it is evident that hydrogen cracks can more easily be ence contrast (Fig. 3.59b). One way to detect hydrogen damage is detected in a sample in the unetched condition (Fig. 3.58a) than in in the fracture appearance of through-thickness tensile specimens. the etched condition (Fig. 3.58b). Many times, the etched micro- If hydrogen flakes are present, “fisheyes” appear on the fracture Fig. 3.57 Microstructure of a Ni-Cr-Mo steel held at 565 °C (1050 °F) under a load of 210 MPa (30 ksi), showing (a) initial void formation at the austenite grain boundaries, (b) void linkup, and (c) separation of an austenite grain boundary. 4% picral and HCl etch. 500⫻ Alteration of Microstructure / 83 surface. Fisheyes are shiny, rounded regions that have developed (decarburization) along the crack surface, seen as the white by internal cracks caused by hydrogen damage. These regions nonetching constituent in Fig. 3.60. were there before the tensile fracture took place. An example of severe hydrogen damage is shown in Fig. An unusual example of a decarburized region surrounding a 3.61(a), representing an ASTM A516 steel plate. In this micro- hydrogen flake in an AISI/SAE 1080 steel can be seen in Fig. graph, one can see cracks that developed by grain-boundary 3.60. In this case, the component was heat treated after the separation along the banded regions. Usually these cracks are not hydrogen crack formed. Here, exposure to 870 °C (1600 °F) for noticed until the part is in service, where it is exposed to some five hours allowed the hydrogen gas to react with the carbon in the kind of stress, that is, thermal or mechanical stress. Under steel to form methane gas (CH4). The result was carbon depletion continued stress, the part usually fails as the crack or cracks grow Fig. 3.58 Hydrogen flakes (cracks—see arrows) found in an AISI/SAE Fig. 3.59 Another example of a hydrogen flake (crack) in an AISI/SAE 1080 steel bar in the (a) unetched and (b) etched condition. 4% 1080 bar showing (a) the crack in bright-field illumination and picral etch. 1000⫻ (b) in differential interference contrast (Nomarski). Unetched. 1000⫻ 84 / Metallographers’ Guide in length. Also, as atomic hydrogen diffuses into the steel, it forms the dissolution of the pearlite colonies adjacent to the cracks molecular hydrogen that cannot leave, because in its molecular caused by the reaction of the hydrogen with the iron carbide. Once state, the diffusivity is extremely low. As hydrogen diffusion grain-boundary separation occurs, the strength of the tube will continues, the molecular hydrogen begins to form gas pockets that decrease, and the tube will eventually fail. are under very high pressure. In the previous example, one can see Corrosion Effects. Corrosion is generally thought to affect only in the the enlarged view of Fig. 3.61(b) that the gas pressure the outer surface layer of a part. This is not always the case. For actually expanded the cracks and plastically deformed the regions example, as in the case of a gray cast iron, corrosion can penetrate between cracks (note the bending of the ferrite/pearlite bands into the interior of a component, for example, an underground between the two expanded cracks). Once a hydrogen-related crack water pipe, due to the connectivity of the graphite flakes. An forms, the process is irreversible, and the part must be taken out of service before catastrophic failure occurs. Another form of hydrogen damage can occur in service from electrochemical corrosion reactions. Such a reaction can occur in steel boiler tubes exposed to high temperature and high-pressure steam. Water can react with iron at high temperature to create atomic hydrogen (as opposed to molecular hydrogen, H2) and iron oxide. The atomic hydrogen can easily diffuse into the steel boiler tube and cause damage. One form of damage occurs when hydrogen can combine with the carbides (cementite) in the steel to form methane gas, CH4, which can cause internal grain-boundary separation similar to the hydrogen flakes described previously. An example of this type of damage is illustrated in a SA 210 boiler tube exposed for hundreds of hours to superheated steam at 360 °C (675 °F) under a pressure of 18 MPa (2.6 ksi). Figure 3.62(a) shows the microstructure of the boiler tube in its original condition, and Fig. 3.62(b) shows the result of the methane damage. The damage is in the form of grain-boundary separation created by the increased pressure developed by the reaction of hydrogen with cementite. The methane gas is in molecular form, and the molecules are too big to easily diffuse in the steel. Note Fig. 3.60 An internal hydrogen flake (crack) in an AISI/SAE 1080 steel bar that was exposed to a temperature of 870 °C (1600 °F) for 5 h. The crack surface was decarburized (white area surrounding crack) by the Fig. 3.61 Microstructure of an as-rolled ASTM A516 steel plate showing reaction of the hydrogen with the carbon in the steel. Pearlite matrix. 4% hydrogen flakes along the pearlite bands. 2% nital and 4% picral etch. 1500⫻ picral etch. (a) 50⫻ and (b) 400⫻ Alteration of Microstructure / 85 example is shown in Fig. 3.63(a). Here, corrosion products can be Corrosion engineers often call this type of corrosion “graphiti- found surrounding the flake graphite and graphitic cells (a cell is zation,” which, to a metallurgist, is obviously incorrect, because a connected network of graphite flakes). Figure 3.63(b) shows the the graphite was present before the corrosion took place. What has corrosion product and graphite flakes at higher magnification. The happened in this case is selective leaching of the iron matrix penetration of corrosion, in a way, proves that the graphite flakes around the graphite flakes. In very severe cases, the entire outer are interconnected. skin of the corroded pipe or component is graphite, with all the Fig. 3.62 Microstructure of an ASME SA 210 steel tube consisting of (a) ferrite (light etching constituent) and pearlite (dark etching constituent) and (b) a hydrogen-damaged region showing cracks (arrows) at the pearlite/ferrite interfaces. 4% picral etch. 1000⫻ Fig. 3.63 Microstructure of a gray cast iron water pipe with corrosion penetrating below the surface along graphite flake networks (cells) (see arrows). (a) unetched, 50⫻ and (b) 4% picral etch, 500⫻ 86 / Metallographers’ Guide iron leached from the surface. From a corrosion viewpoint, the • The Heat Treaters Guide—Standard Practices and Procedures graphite and iron act as a galvanic cell, where the graphite is the for Steel, American Society for Metals, 1982 cathode and the iron is the anode. Because of the galvanic action, • Heat Treating, Vol 4, ASM Handbook, ASM International, corrosion is enhanced next to the graphite flakes. In a previous 1991 section in this chapter, the term “graphitization” has been shown • G. Krauss, Principles of Heat Treatment, 2nd ed., ASM to be the formation of graphite during a long-term exposure of a International, 1993 carbon steel to temperatures between 315 and 370 °C (600 and • Properties and Selection: Irons, Steels, and High-Perfor- 700 °F). mance Alloys, Vol 1, ASM Handbook, ASM International, 1990 • L.E. Samuels, Optical Microscopy of Carbon Steels, American SELECTED REFERENCES Society for Metals, 1980 • Atlas of Microstructures of Industrial Alloys, Vol 7, Metals • H. Thielsch, Defects and Failures in Pressure Vessels and Handbook, 8th ed., American Society for Metals, 1972 Piping, Reinhold Publishing, 1965 Metallographer's Guide: Practices and Procedures for Irons and Steels Copyright © 2002 ASM International® Bruce L. Bramfitt, Arlan O. Benscoter, p87-107 All rights reserved. DOI:10.1361/mgpp2002p087 www.asminternational.org CHAPTER 4 The Metallographer and the Metallographic Laboratory MATERIALS PLAY A MAJOR ROLE in the world economy ing wrought iron, wrought iron armor plate, and blister steel. He and in the development of nations. This is particularly true of found that the samples had definite microstructural features. A metals, with steels and cast irons being the most widely used. copy of Sorby’s 1864 macrograph of blister steel can be seen in Metals not only enhance our life-style, but have become a Fig. 4.2. This macrograph, taken at 9⫻, shows distinct grain necessity of modern life. Science and engineering, particularly in boundaries. This discovery was significant, because it eventually the areas involving metal, ceramic, polymeric, electronic, and led to the realization that microstructural features imparted certain superconducting materials, have advanced rapidly and will con- properties to steel. The development of ferrous physical metal- tinue to outpace technology as a whole. In this swirl of activity, a lurgy, and physical metallurgy in general, depended on this metallographer is vital to our basic understanding of the link important link. between microstructure and the properties of these materials. By understanding microstructure and its origin, one can begin to develop a basis on how to achieve specific properties tailored for a particular engineering application. For example, an application for an earth-moving shovel in mining requires a steel with very high tensile strength and hardness, combined with a high degree of wear resistance and some toughness. What optimal microstruc- ture would provide these properties? This book provides a beginning to the basic understanding of the development of microstructure and gives instructions on how to employ proper techniques to reveal microstructure, that is, the techniques of metallography. In this book we, of course, restrict our attention to the metallography of iron and steel. The Metallographer What is a metallographer? In general terms, a metallographer is a person who has the skill to properly prepare a specimen of a metal or alloy in order to allow examination and interpretation of its microstructure. In this technological age, the term “metallog- rapher” is becoming somewhat of a misnomer, because today it covers not only metals but ceramics and the other materials mentioned previously. The term at some time in the future may be changed to “materiallographer,” but for the moment, the term “metallographer” is still in place. The field of metallography, which involves the study of the microstructure of metals, is almost a century and a half old. It all began with Henry Clifton Sorby on July 28, 1863. Sorby, whose photograph can be seen in Fig. 4.1, was an English geologist, petrographer, and mineralogist who was the first person to examine polished and chemically etched metal samples under the microscope. His samples included Swedish wrought iron, Bowl- Fig. 4.1 Henry Clifton Sorby, the father of metallography