ASM Metals Handbook - Desk Edition (ASM_ 1998) WW part 14 doc

Constant displacement (K-decreasing) tests do not have the problems of the K-increasing tests. The plastic zone ahead of the crack tip does not increase with increasing crack size, so that the stress condition always remains in the plane-strain mode. Also, the constant displacement tests can be self-loaded, and thus external testing equipment is not needed. Because in these tests the stress-intensity factor, K ISCC , can be easily determined by exposing a number of specimens loaded to different initial K I values. This can even be accomplished by crack arrest in one specimen. A major problem with this test method occurs when corrosion products form in the crack, blocking the crack mouth and interfering with the environment at this crack tip. Moreover, the oxide can wedge open the crack and change the originally applied displacement and load. Measurement of Crack Growth. In order to quantify the crack growth behavior in precracked stress-corosion specimens, the crack length needs to be monitored, so that the crack velocity (da/dt) can be calculated, and the relationship between the increasing K and the crack velocity can be determined. There are basically three methods to monitor the growth of stress-corrosion cracks: visual/optical measurements, measurement of the crack-opening displacement using clip gages, and the potential drop measurement, which monitors the increase in resistance across two on either side of the propagating crack. Tests for Hydrogen Embrittlement HYDROGEN EMBRITTLEMENT is a time-dependent fracture process caused by the absorption and diffusion of atomic hydrogen into a metal, which results in a loss in ductility and tensile strength. Hydrogen embrittlement is distinguished from stress-corrosion cracking generally by the interactions of the specimens with applied currents. Cases where the applied current makes the specimen more anodic and accelerates cracking are considered to be stress-corrosion cracking, with the anodic-dissolution process contributing to the progress of cracking. On the other hand, cases where cracking is accentuated by current in the opposite direction, which accelerates the hydrogen-evolution reaction, are considered to be hydrogen embrittlement. Tests for hydrogen embrittlement are performed to determine the effect of hydrogen damage in combination with residual or applied stresses. In the past decade, conventional testing methods have been modified to incorporate fracture mechanics, and the various types of hydrogen damage have been classified further in terms of crack nucleation, crack growth rates, and threshold stress-intensity measurements. Testing Methods As described in the section "Tests for Stress-Corrosion Cracking" in this article, the cantilever beam test and the wedge- opening load test result in a parameter called K ISCC , which is the threshold stress intensity for SCC. Many different designations, such as K th , K IHE , and K SH , denote this parameter for steels that undergo a similar phenomenon in which the mechanism is internal hydrogen embrittlement. The threshold stress intensity for hydrogen stress cracking is designated by K IHE , and K ISCC is used for SCC. The mechanisms are different in that SCC occurs under anodic polarization conditions, whereas hydrogen embrittlement and hydrogen stress cracking occur under cathodic polarization conditions, which normally are generated to protect steels from corrosion. Such is the case when a sacrificial anode is galvanically coupled to the steel hull of a ship to prevent the hull from corroding. In such a couple, the steel is the cathode and hydrogen is produced at the cathode in an electrochemical reaction. This results in a steel structure, apparently free of corrosion (with a clean, metallic luster), that fails by intergranular cracking due to internal diffusion of hydrogen generated at the surface. This type of hydrogen embrittlement is found in types 410 and 17-4PH stainless steel and AISI type 4340 steel. The cantilever beam test is a constant-load test in which a V-notched specimen is inserted along a portion of the beam and enclosed by an environmental chamber (Fig. 9). A crack at the root of the V-notch is initiated and extended by fatigue before testing. Notch-root thickness is prescribed by ASTM, although the requirement often is excessive for high- toughness steels. The specimen is subjected to a constant load over a preset time period. As the crack grows, the stress intensity increases. Time to time failure is plotted versus applied stress intensity. The lower limit of the resultant curve is a threshold for hydrogen embrittlement (Fig. 10). Fig. 9 Fatigue-cracked cantilever beam test specimen and fixtures Fig. 10 Procedure to obtain K IHE with precracked cantilever beam test specimen The K IHE results of a cantilever beam test depend on how much time elapses before the test is terminated. Recommended testing periods to establish the true stress-intensity threshold vary, ranging from 200 h, which is typical for hydrogen embrittlement testing, to as long as 5000 h. Another limitation of this testing method is that it can be expensive in terms of materials and machining. As many as 12 specimens, placed under different loads in separate test machines, are needed per test to obtain valid K IHE values. The wedge-opening load test applies a constant wedge or crack opening displacement; as the crack extends, stress intensity decreases until crack arrest occurs (Fig. 11). The initial load is assumed to be slightly above K IHE . The specimen is maintained under these conditions for about 5000 h to establish the threshold. The crack grows to a point after which further growth is not measured (K IHE ). However, it is difficult to determine precisely when the "no growth" criterion is met. Crack tip opening displacement should also be monitored. Corrosion reactions accompanied by expansion in volume may occur at the crack tip. This changes the opening displacement and increases the load, thus altering desired testing conditions. Fig. 11 Schematic showing basic principle of modified wedge-opening load test specimen As subcritical crack extension occurs, stress intensity increases in the cantilever beam test and decreases in the wedge- opening load test (Fig. 12). Generally, the threshold stress intensity measured with the wedge-opening load test is lower than with the cantilever beam test. The advantage of the wedge-opening load test is that only a single specimen is required to measure K IHE . Fig. 12 Influence of time, crack extension, and load on stress-intensity behavior of modified wedge- opening load, cantilever beam, and contoured double-cantilever beam test specimens The contoured double-cantilever beam test is used to measure crack growth rate a constant stress-intensity factor. This test simplifies the calculation of stress intensity by using a contoured specimen so that stress intensity is proportional to the applied load and is independent of the crack length. Under a constant load, stress intensity also remains constant with crack extension. For the test geometry shown in Fig. 13, the stress-intensity factor equals 20 times the load (K = 20P). Fig. 13 Dimensions and configuration for double-cantilever beam test specimen. Specimen contoured to 3a 2 /h 3 + 1/h = C, where C is a constant. All values given in inches (1.0 in. = 25.4 mm). Data on hydrogen embrittlement can be obtained with subthickness specimens, even in excess of the ASTM requirement of < 0.4 B/(YS) 2 (where B is thickness and YS is yield strength of the specimen), by using side grooves, which provide additional constraint on the material being tested. Side grooves enable the maintenance of a plane-strain condition in a thin specimen by enhancing stress triaxiality. This method has been used extensively to study the effect of heat treatment (hardness) and environment on hydrogen stress cracking of AISI type 4340 steels (Fig. 14). Fig. 14 Hydro gen embrittlement crack growth rate as a function of applied stress intensity for two different hardnesses and environments for an AISI 4340 steel contoured double-cantilever beam test specimen The contoured double-cantilever beam test has also been used to study the stress-history effect that produces an incubation time before hydrogen stress cracking. Figure 15 shows that incubation time is dependent on the type of steel. A decrease in the stress-intensity factor from 44 to 22 MPa (40 to 20 ksi ) may change the incubation time from less than 1 h for AISI type 4340 steel to about 1 year for type D-6AC steel. Fig. 15 Incubation time prior to hydrogen stress cracking for AISI type 4340 and type D- 6AC steel contoured double-cantilever beam test specimens as a function of decrease in stress intensity Three-Point and Four-Point Bend Tests. The contoured double-cantilever beam test uses a constant load to maintain a constant stress-intensity factor with crack extension. The same effect can be produced by using a three- or four-point bend test under displacement control. These tests use heavily side-grooved Charpy V-notch specimens (Fig. 16). Because crack opening displacement is constant as the crack extends, the load decreases, so that there is a slight initial increase in stress intensity to a maximum value that drops slightly as the ratio of crack depth to specimen width exceeds 0.5. Typically, stress intensity is constant, within a small range. Figure 17 compares the change in stress-intensity factor with crack extension as a function of load control to that of displacement control for a three-point bend specimen. Fig. 16 Standard side-grooved Charpy V-notch test specimen used for three- and four-point bend tests Fig. 17 Use of three-point bend displacement control as constant-K specimen The rising step-load test provides a stress intensity that is different at each load but remains constant with crack extension as each load level is sustained. Crack initiation is signaled by a drop in load (Fig. 18). The rising step-load test was developed as an accelerated low-cost test to measure resistance of steels (particularly weldments) to hydrogen embrittlement. The threshold obtained by this method will be somewhat high, as test duration at each load is short. Fig. 18 Typical load-time record for four-point rising step-load test To index susceptibility to hydrogen-assisted cracking, the test should last no longer than 24 h, and the hydrogen source should reflect the most aggressive environment. In one experiment, a 3.5% sodium chloride solution was selected to simulate seawater, and a cathodic potential of -1.2 V (saturated calomel electrode) was used to generate hydrogen to reproduce the extreme conditions of sacrificial anodic protection generally found on a ship hull. A Charpy specimen was chosen, because such specimens are small and easy to machine and handle. In this test, however, the specimen was modified. Instead of using a fatigue precrack, the notch-root radius was machined to less than 7.6 m (3 mil). This was done to lower the cost and give less ambiguous environmental conditions at the crack tip. Also, hydrogen cracks nucleate below the surface. The specimen was deeply side grooved, a common practice used in hydrogen stress cracking tests to prevent the crack from branching. Side grooves are also used in crack opening displacement or J-integral testing to cause load-displacement curves to increase monotonically to fracture by inducing a highly triaxial stress field at the crack tip. Because a Charpy specimen is small, deep side grooves produce a triaxial stress field at the notch to promote hydrogen stress cracking. The extent of the side grooving is such that the remaining ligament is only 40% of the original thickness. The modified Charpy specimen dimensions are shown in Fig. 16. The specimen was loaded by means of beams and an instrumented bolt (Fig. 19). Four-point bending under constant displacement control and stress intensity produced crack growth. Once cracking initiated at the notch (a/W = 0.2, where a is crack length and W is width of the specimen), arrest did not occur until the crack was nearly through the specimen. The load was increased manually at 1 h intervals. An environmental chamber encompassed the specimen and included a potentiostat to produce hydrogen while under stress. Fig. 19 Loading frame used for rising step-load test The rising step-load test was used to evaluate high-strength HY ship steels and weldments in an environment simulating seawater under conditions of cathodic protection commonly used to protect ship hulls. Samples from the heat-affected zone and other locations in the weld metal were tested. Interlayer gas tungsten arc heating was evaluated as a means of providing a refined, homogeneous, tempered microstructure with improved resistance to hydrogen stress cracking. As a baseline, comparison was made between HY-130 and HY-180 steels. Figure 20 plots rising step-load test results for HY-130 and HY-180 base metals, in addition to combinations of modified HY steel compositions and programmed-cooling-rate thermal cycles for the base metal and weld wire. The vertical axis is a plot of a parameter derived from the specimen strength ratio in ASTM E 399, "Test Method for Plane-Strain Fracture Toughness of Metallic Materials" i.e., 6 P max /B(W - a) 2 YS, where P max is the maximum load that the specimen is able to sustain, B is the specimen thickness, W is the specimen width, a is the crack length, and YS is the yield strength in tension. For the data shown in Fig. 13, P max was replaced by the crack initiation load. The horizontal axis is a ratio of K IHE /YS, measured in a separate test program with cantilever beam and wedge-opening load specimens. Fig. 20 Analytical correlation of strength ratio with threshold stress-intensity data The resistance to hydrogen embrittlement of the two base metals and six locations in HY-130 weldments was ranked using this testing method. Test results showed that HY-180 is more susceptible to hydrogen stress cracking than HY-130 and that the resistance to hydrogen embrittlement of specimens taken from the heat-affected zone and fusion line is consistently higher than that of weld-metal specimens. The resistance of the weld metal is affected by the grain structure; interlayer gas tungsten arc reheating homogenized the weld structure, but did not temper the weld metal. Specimens from the gas tungsten arc reheated weldment consistently exhibited higher hardness and lower resistance to hydrogen embrittlement than similar specimens from the standard HY-130 weld metal. The disk-pressure testing method measures susceptibility to hydrogen embrittlement of metallic materials under a high-pressure gaseous environment. The test is used for the selection and quality control of materials, protective coatings, surface finishes, and other processing variables. A thin disk of the metallic materials to be tested is placed as a membrane in a test cell and subjected to helium pressure until the bursts. Because helium is inert, the fracture is caused by mechanical overload; no secondary physical or chemical action is involved. An identical disk is placed in the same test cell and subjected to hydrogen pressure until it bursts. Metallic materials that are susceptible to environmental hydrogen embrittlement fracture under a pressure lower than the helium-burst pressure; materials that are not susceptible fracture under the same pressure for both hydrogen and helium. The ratio (S ) between the helium-burst pressure (P He ) and the hydrogen-burst pressure (P ) indicates the susceptibility of the material to environmental hydrogen embrittlement: If S is equal to or less than 1, the material is not susceptible to environmental hydrogen embrittlement. When S is greater than 2, the material is considered to be highly susceptible. At values between 1 and 2, the material is moderately susceptible, with failure expected after long exposure to hydrogen; therefore, the material must be protected against exposure. Slow strain-rate tensile test can be used to evaluate many product forms, including plate, rod, wire, sheet, and tubing, as well as welded parts. Smooth, notched, or precracked specimens can be used. The principal advantage of this standardized test is that the susceptibility to hydrogen stress cracking for a particular metal-environment combination can be assessed rapidly. A variety of specimen shapes and sizes can be used; the most common is a smooth bar tensile coupon, as described in ASTM E 8, "Methods of Tension Testing of Metallic Materials." The specimen is exposed to the environment and is stressed under displacement control. For stainless steel in chloride solution, the strain rate is 10 -6 s. One or more of the following parameters are applied to the tensile test at the same initial strain rate; time-to-failure; ductility, as assessed by reduction in area or elongation to fracture, for example; maximum load achieved; and area bounded by a nominal stress- elongation curve or a true stress-true strain curve. Potentiostatic Slow Strain-rate Tensile Testing. The use of dissociated water under potentiostatic conditions that produce hydrogen on the surface of the tensile test specimen while under slow strain-rate displacement control has been studied. Results suggest that hydrogen is the most significant parameter in stress cracking under conditions of hydrogen sulfide stress-corrosion cracking found in oil fields. Selected References • R.H. Jones, Ed., Stress-Corrosion Cracking: Materials Performance and Evaluation, ASM International, 1992 • G.H. Koch, Stress-Corrosion Cracking and Hydrogen Embrittlement, Fatigue and Fracture, Vol 19, ASM Handbook, ASM International, 1996, p 483-506 • G.M. Ugianski and J.H. Payer, Ed., Stress-Corrosion Cracking The Slow Strain-Rate Technique, STP 665, ASTM, 1979 Metallographic Practices Generally Applicable to All Metals Edited by George F. Vander Voort, Buehler Ltd. Metallographic Methods THE METHODS AND EQUIPMENT described in this article cover the preparation of specimens for examination by light optical microscopy (LOM), scanning electron microscopy (SEM), electron microprobe analysis (EMPA) for microindentation hardness testing, and for quantification of microstructural parameters, either manually or by the use of image analyzers. In this article, it is assumed that the specimen or specimens being prepared are representative of the material to be examined. Random sampling, as advocated by statisticians, can rarely be performed by metallographers. Instead, systematically chosen test locations are employed based on convenience in sampling. In failure studies, specimens are usually removed to study the origin of the failure, to examine highly stressed areas, and to examine secondary cracks. All sectioning processes produce damage; some methods (such as flame cutting) produce extreme amounts of damage. Traditional laboratory sectioning procedures using abrasive cut-off saws introduce minor damage that varies with the material being cut and its thermal and mechanical history. This damage must be removed if the true structure is to be examined. However, because abrasive grinding and polishing steps also produce damage, where the depth of damage decreases with decreasing abrasive size, the preparation sequence must be carefully planned and performed. Otherwise, preparation-induced artifacts will be interpreted as structural elements. A properly prepared specimen has the following characteristics: • Deformation induced by sectioning, grinding, and polishing is removed or shallow enough to be removed by the etchant. • Coars e grinding scratches are removed; fine polishing scratches are tolerated in routine metallographic studies. • Pullout, pitting, cracking of hard particles, and smear are avoided. • Relief (i.e., excessive surface height variations between structural features of different hardness) is minimized. • The surface is flat particularly at edges (if they are to be examined) and at coated surfaces to permit examination at high magnifications. • Specimens are cleaned adequately between preparation steps, after preparation, and after etching. Preparation of metallographic specimens generally requires five major operations: sectioning, mounting (optional), grinding, polishing, and etching (optional). Sectioning Many metallographic studies require more than one specimen. For example, a study of deformation in wrought metals usually requires two sections one perpendicular and the other parallel to the direction of deformation. A failed part may best be studied by selecting a specimen that intersects the origin of the failure, if the origin can be identified. Depending on the type of failure, it may be necessary to take several specimens from the area of failure and from adjacent areas. Sampling. Bulk samples for sectioning may be removed from larger pieces or parts using methods such as core drilling, band or hack sawing, flame cutting, etc. However, when these techniques are used, precautions must be taken to avoid alteration of the microstructure in the area of interest. Laboratory abrasive-wheel cutting is recommended to establish the [...]... tungsten-filament lamps, tungsten-halogen lamps, quartz-halogen lamps, and xenon arc bulbs Tungsten-filament lamps generally operate at low voltage and high current They are widely used for visual examination because of their low cost and ease of operation Tungsten-halogen lamps are the most popular light source today due to their high light intensity They produce good color micrographs when tungsten-corrected... eliminated by operating in an inert-gas atmosphere; however, to prevent surface oxidation or contamination, the inert gas generally must be of very high purity A serious problem in high-temperature microscopy is potential damage to the microscope parts, particularly the objective lens, by the high temperatures This danger is partly corrected by water cooling the parts of the hot stage near the objective;... of one-tenth of a displacement, which means that differences in height of about 27 nm can be measured Fig 18 Principle of two-beam interferometry Figure 19 shows the principles of multiple-beam interferometry Instead of creating interference between two light beams, the multiple-beam method produces interference among many beams An optically flat reference plate that is partly transmitting and partly... films can be used as well as electronic media Historically, wet-processed films have yielded the finest results, and that is still true today If enlargements are required, particularly for sizes greater than 8 by 11 in (21.5 by 28 cm), a medium-format film or a large-format film is better Generally, panchromatic films are best for black-and-white work For color work, the film type must be compatible... neutral density filters Microscopic Techniques Most microscopic studies of metals are made using brightfield illumination In addition to this type of illumination, several special techniques (oblique illumination, darkfield illumination, opaque-stop microscopy, phase-contrast microscopy, and polarized-light microscopy) have particular applications for metallographic studies Köhler Illumination Most... windows To overcome this, long-working-distance objectives have been used The most widely used type employs a reflecting concave mirror in conjunction with a standard objective Low-Temperature Microscopy Certain reactions that occur in metals at low temperatures can be observed by microscopy Stages have been constructed for this purpose; most are either adaptations of high-temperature stages or similar... found in Metallography, Structures, and Phase Diagrams, Volume 8, Metals Handbook, 8th ed., p 3 0-3 1; Metallography and Microstructures, Volume 9, ASM Handbook, p 5 2-5 3; and ASTM E 1558 Other compilations of electrolyte compositions may be found in Metallography: Principles and Practice, by G.F Vander Voort, McGraw-Hill, 1984 Preferred (or sometimes required) characteristics of an electrolyte are: • • •... observed with brightfield illumination Polarized-Light Microscopy is particularly useful in metallography, because many metals and metallic and nonmetallic phases are optically anisotropic Polarized light is obtained by placing a polarizer in front of the condenser lens of the microscope and placing an analyzer before the eyepiece (Fig 17) The polarizer produces plan-polarized light that strikes the surface... (Fig 8) are often used for final polishing, particularly with more difficult to prepare materials, for image analysis studies, or for publication-quality work Automatic Polishing Mechanical polishing can be automated to a high degree using a wide variety of devices ranging from relatively simple systems to rather sophisticated minicomputer- or microprocessor-controlled devices (Fig 10) Units also vary... cloths of different fabrics (woven or nonwoven) with a wide variety of naps (or napless) are available for metallographic polishing Napless or low-nap cloths are recommended for rough polishing using diamond abrasive compounds Low-, medium-, and occasionally high-nap cloths are used for final polishing, but this step should be as brief as possible to minimize relief Polishing Abrasives Polishing usually . between HY-130 and HY-180 steels. Figure 20 plots rising step-load test results for HY-130 and HY-180 base metals, in addition to combinations of modified HY steel compositions and programmed-cooling-rate. stress-intensity factor with crack extension. The same effect can be produced by using a three- or four-point bend test under displacement control. These tests use heavily side-grooved Charpy V-notch. V-notch test specimen used for three- and four-point bend tests Fig. 17 Use of three-point bend displacement control as constant-K specimen The rising step-load test provides a stress intensity