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Metallographic Preparation of Titanium and Its Alloys

Experiments were conducted using three-step preparation procedures for titanium and its alloys.  For CP titanium and alpha-titanium alloys, use of an attack-polishing agent in the third step was required to obtain good results.  The experiments defined optimum surfaces for each step and operating conditions.  Two-phase, α-ß alloy specimens are significantly easier to prepare than a single-phase α specimen. The method does yield perfect polarized light response with α-phase alloys, such as commercial-purity titanium.

Titanium and its alloys have become quite important commercially over the past fifty years due to their low density, good strength-to-weight ratio, excellent corrosion resistance and good mechanical properties.  On the negative side, the alloys are expensive to produce. Titanium, like iron, is allotropic and this produces many heat treatment similarities with steels.  Moreover, the influences of alloying elements are assessed in like manner regarding their ability to stabilize the low temperature phase, alpha, or the high temperature phase, beta.  Like steels, Ti and its alloys are generally characterized by their stable room temperature phases – alpha alloys, alpha-beta alloys and beta alloys, but with two additional categories: near alpha and near beta.

Martensite and the Control of Retained Austenite

Formation of martensite in fine-grained steels is probably the most common goal in heat treatment of components. The carbon content of the parent austenite phase determines whether lath (low-carbon) or plate (high-carbon) martensite, or mixtures of the two will be produced, assuming the quench rate and steel hardenability are adequate for full hardening. Lath martensite produces higher toughness and ductility, but lower strengths, while plate martensite produces much higher strength, but may be rather brittle and non-ductile.

For a given alloy content, as the carbon content of the austenite increases, the martensite start, Ms, temperature and the martensite finish, Mf, temperature will be depressed which results in incomplete conversion of austenite to martensite. When this happens retained austenite, which may be either extremely detrimental or desirable under certain conditions, is observed.  The amount of retained austenite present depends upon the amount of carbon that can be dissolved in the parent austenite phase and the magnitude of the suppression of the Ms and Mf temperatures. This paper examines the conditions under which austenite is retained and the problems associated with it presence, with detecting it and with measuring it.

Determining the Nodularity of Graphite in Ductile Iron Using ASTM E2567

ASTM Committee E-4 on Metallography began a program to develop a test method for rating the nodularity of graphite in ductile iron in 1986. The initial effort centered upon using the sphericity equation to assess the shape. However, an interlaboratory study showed that the perimeter measurement varied with the magnification used. A perimeter-free shape factor based on the maximum Feret’s diameter was determined to be magnification independent and reliable.

Additionally, two types of “convex perimeters” were proposed over the ensuing years, but they were demonstrated to be highly biased towards yielding high nodularity ratings regardless of the irregularity of the particle’s shape. The study reported here compared the use of the maximum Feret’s diameter in the shape equation to the mean Feret’s diameter, 100X vs. 200X, a minimum shape factor limit for a particle to be a nodule of 0.5 vs. 0.6, and calculation of an area-based vs. a number-based % nodularity.

Fracture of a 17th Century Japanese Helmet

There was a crack in the helmet which is not visible in this image (some associated damage can just be seen in the lower left side of the helmet visor). The crack was opened and the fracture began at a streak with mostly intergranular fracture and then propagated by cleavage as shown below.

Note the intergranular fracture in the center foreground. The walls show transgranular cleavage the propagated from the intergranular origin. Next to the fracture, we see a region of columnar grains at the surface with a small region of finer, more equiaxed grains below and the very coarse columnar grains blow that, as shown below.

Influence of the Equations Defining HV and HK on Precision

The basic equations defining (see equations 1 and 2) the Knoop (HK) and Vickers (HV) hardness, where the applied force is multiplied by a geometric constant and then divided by the long diagonal squared or the mean diagonal squared, respectively, cause an inherent problem in measuring small indents, that is diagonals ≤20 µm in length.  Figure 1 shows the calculated relationship between the diagonal and load and the resulting hardness for Knoop indents while Figure 2 shows this relationship for Vickers indents. As the test load decreases, and the hardness rises, the slope of the curves for diagonal versus hardness becomes nearly vertical. Hence, in this region, small variations in diagonal measurements will result in large hardness variations.

If we assume that the repeatability of the diagonal measurement by the average user is about ±0.5 µm, which is quite reasonable, and we add and subtract this value from the long diagonal length or the mean diagonal length, we can then calculate two hardness values. The difference between these values is ΔHK and ΔHV, shown in Figures 3 and 4. From these two figures, we can see how the steepness of the slopes shown in Figures 1 and 2 will affect the possible range of obtainable hardness values as a function of the diagonal length and test force for a relatively small measurement imprecision, ±0.5 µm. These figures show that the problem is greater for the Vickers indenter than for the Knoop indenter for the same diagonal length and test force. For the same specimen and the same test force, the long diagonal of the Knoop indent is 2.7 times greater than the mean of the Vickers’ diagonals, as shown in Figure 5.

Propeller Shaft of the USS Monitor

March 9, 1862 marks the date when the USS Monitor and the CSS Virginia (formerly the USS Merrimack) fought an indecisive naval battle at Hampton Roads that changed naval warfare from wood and sails to iron and steam. The USS Monitor sunk off the Outer Banks of North Carolina during a storm on December 31, 1862 but its remains were discovered in 1973. The wreck site, the Monitor National Marine Sanctuary, is managed by the National Oceanic and Atmospheric Administration (NOAA).

The Confederates began construction of an “ironclad” ship at the Gosport Yard of Hampton Roads in 1861. This was well known to the Union Navy Department. The US Army had actually launched ironclad gunboats in the summer of 1861 to patrol the Mississippi River; but none were available in the east to counter the Virginia. On August 3, 1861, Gideon Wells (Secretary of the Navy) requested design proposals for ironclad warships. Swedish inventor John Ericsson had designed an ironclad in 1854 for Napoleon III that incorporated a revolving cupola turret. Cornelius Bushnell promoted this design to Abraham Lincoln.

Metallographic Examination of Bronze Bracelets from Hasanlu

Hasanlu is an early Iron Age settlement located in northwestern Iran. It dates back to the second millennium B.C., ~1450 B.C., until it was destroyed around 800 B.C. Although the site attracted the attention of the British archeologist Sir Aurel Stein due to artifacts recovered from burial mounds in the 1930s, it was not given substantial attention until the discovery of the “Hasanlu Golden Bowl” in 1958. The Hasanlu archeology project began in 1957 and was greatly stimulated by this discovery. It ended in 1977. The excavations were sponsored by The University of Pennsylvania Museum and the Metropolitan Museum of Art of New York in cooperation with the Archeological Service of Iran.

The author obtained six specimens from the University of Pennsylvania Museum. The six specimens consisted of three cast (No. 1) and three wrought bronze bracelet sections (Nos. 2-4), as defined in Table 1. The three cast specimens were from the same bracelet. For simplicity, they will be referred to as specimens 1, 2, 3 and 4. As there are three specimens of the cast bracelet in mount 1 (HAS 60-617), they will be referred to by their size. The chemical analysis shows that they are similar in composition although the tin content of the fourth specimen is somewhat higher than the other three.

Obtaining Consistent Vickers Hardness at Loads ≤ 100 Grams Force

One of the most serious limitations to Vickers hardness testing in the micro-load range (10-1000 gf or 0.098-9.81 N) has been the variability in measured hardness with loads ≤ 100 gf (≤ 0.98 N). In the literature four HV-load trends have been reported for this range.  In the order of most common to least common, the trends are: the hardness decreases with decreasing load; the hardness increases slightly and then decreases; the hardness increases with decreasing load; and, the hardness is constant. Many publications have concentrated on the most common trend and attributed it to material factors. Samuels [1] stated, however, that these problems were due to microscope limitations, such as limited contrast and resolution, and visual perception limitations. At the same symposium, Westrich [2] showed that the SEM could be used to measure small Vickers indents and yield virtually constant hardness as a function of load.

Decarburization

Decarburization occurs when carbon atoms at the steel surface interact with the furnace atmosphere and are removed from the steel as a gaseous phase (1-8). Carbon from the interior will then diffuse towards the surface, that is, carbon diffuses from a region of high concentration to a region of low concentration to continue the decarburization process and establish the maximum depth of decarburization (MAD).

Because the rate of carbon diffusion increases with temperature when the structure is fully austenitic, the MAD will increase as the temperature increases above the Ac3. For temperatures in the two phase region, between the Ac1 and Ac3, the process is more complex. The diffusion rates of carbon in ferrite and in austenite are different and are influenced by temperature and composition.

Methanol Pipeline Failure in the Canyon Express Pipeline System

The Canyon Express Pipeline System (CEPS) was started up in November 2002 in the Gulf of Mexico, south of Louisiana. It is owned by six oil companies and collects hydrocarbons from ten wells at depths of ~6100, 7100 and 7200 feet.

The flow line system consists of two 12” diameter gas pipelines (“east” and “west”) connected to a header system, which carries the hydrocarbons 57 miles north to a fixed platform, the Canyon Station, in about 500 feet of water. Tankers come into the Canyon Station to fill up and carry the hydrocarbons to refineries. Just west of the “east” flow line is a 2.875” diameter, X-70 line pipe that carries methanol from the station to the header where it is injected to prevent freezing of the hydrocarbons. To the left of the methanol line is a 6” diameter umbilical line containing electrical power and hydraulic lines.