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Using the Control Chart Approach to Evaluate Hardness Tester Performance

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wrought-7-Mo wsThe control chart data analysis approach is an ideal method to evaluate the quality of test data using a specific tester, such as a microindentation hardness tester, over a period of usage time. The method described in ASTM E2554 was used for this work.  This analysis is done by plotting a means and a standard deviation control chart of the weekly/periodic verification data obtained with certified hardness test blocks at a specific test force and a specific hardness level. The method is illustrated using a Knoop hardness test block certified at a test force of 500 gf and with a long diagonal length of 116.18 μm (527.1 HK). From this data, one can easily calculate the uncertainty of the measurements.

The Control Chart method described in ASTM E2554, and discussed by Neil Ullman, is an ideal procedure for evaluating the performance of hardness testers as it will detect any abnormalities that may occur with usage time. The control chart concept was first developed by Walter Shewhart in 1931 to define the state of statistical control and to detect random or special problems. In 1933, ASTM Committee E1 produced STP 15, “ASTM Manual on Presentation of Data.” This was supplemented in 1935 with “Presentation ± Limits of Uncertainty of an Observed Average,” the first use of the term “uncertainty” in statistical analysis of test data. Today, additional information on control charts is provided by E2587.

Image caption: Microstructure of wrought 7-Mo Plus duplex stainless steel (Fe – <0.03% C – 27.5% Cr – 4.2% Ni – 1.75% Mo – 0.25% N) electrolytically etched with aqueous 20% NaOH (3 V dc, 5 s) to color the ferrite blue. There is some light yellow-tan coloring of the austenite. The average Knoop hardness of the austenite was 361.8 HK and that of the ferrite was 263.5 HK.  Magnification bar is 20 µm in length. 7-Mo Plus is a registered trademark of Carpenter Technology Corp., Reading, Pennsylvania.

Vacuum Deposition Processes

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Vacuum deposition is a generic term used to describe a type of surface engineering treatment used to deposit layers of material onto a substrate. The types of coatings include metals (e.g., cadmium, chromium, copper, nickel, titanium) and nonmetals (e.g., ceramic matrix composites of carbon/carbon, carbon/silicon carbide, etc.), deposited in thin layers (i.e. atom by atom or molecule by molecule) on the surface.

Vapor deposition technologies include processes that put materials into a vapor state via condensation, chemical reaction, or conversion. When the vapor phase is produced by condensation from a liquid or solid source, the process is called physical vapor deposition (PVD). When produced from a chemical reaction, the process is known as chemical vapor deposition (CVD). These processes are typically conducted in a vacuum environment with or without the use of plasma (i.e., ionized gas from which particles can be extracted), which adds kinetic energy to the surface (rather than thermal energy) and allows for reduced processing temperature.

The Use of Tungsten in Vacuum Furnace Applications

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Tungsten is used in vacuum furnaces when there is a need for structural integrity at elevated temperature and/or in situations where other materials may degrade, such as when lower melting point eutectics are a concern. One example of its use in is roller rail assemblies in which graphite wheels are positioned between molybdenum rails using tungsten axles.

Tungsten (chemical symbol W) is a member of the family of refractory metal (Mo, Nb, Re, Ta, W) and has the highest melting point and vapor pressure of this group. Due to this unique property, it is commonly used as a material of construction in specific areas of vacuum furnace hot zones operating above 1315ºC (2400ºF). Tungsten can also be used for heating elements given that it has the highest duty temperature, typically 2800°C (5075°F). In practice, this rating is often downgraded as it is for all heating element material choices. Tungsten will become brittle, however, if exposed to oxygen or water vapor and is sensitive to changes in emissivity. In general, tungsten is resistant to corrosion below 60% relative humidity.

Measurement of the Volume Fraction of Beta Phase in Naval Brass

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Measurement of the amount of phases or constituents in metals and alloys is probably the most commonly performed quantitative microstructural test. The amount present is usually referred to as the volume fraction, although it is rarely expressed as a fraction but usually as a percentage. The volume fraction, or VV, in stereological terms, is the volume per unit volume of the phase or constituent. However, there is no simple direct way to measure the volume fraction. Instead we measure the area fraction, AA, a lineal fraction, LL, or a point fraction, PP, which can be measured and correlate with the volume fraction: VV = AA = LL = PP (1).

Areal analysis was first described by Delesse, a French geologist, in 1848. As the minerals were rather coarse in size, he could measure the area fraction of the grains of interest compared to the total two-dimensional area. As microstructures are rather fine in size, this is not a simple method to perform manually. Delesse suggested that a linear ratio of dimensions could also be used, but he thought that the accuracy would not be as good and did not try to develop a lineal analysis method. Rosiwal, a German geologist, was the first to publish a lineal fraction method in 1898 to assess the volume fraction. The point counting method to assess the volume fraction came much later and was proposed by Thompson in 1933, by Glagolev in 1933 and by Chalkey in 1943 – each working in different countries and different fields of science.

Metallographic Procedures for Cast Irons

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Metallographic techniques for cast irons are similar to those for steels; with the exception that graphite retention is a more challenging task. Recommended procedures to prepare cast irons are given. Colloidal silica is an excellent final polishing abrasive for many metals and alloys. However, for pearlitic cast iron grades, colloidal silica often produces small etch spots on the specimen surface. In this case, OP-AN alumina suspension yields excellent results, much better than standard alumina abrasive powders made by the calcination process. Examples of cast iron structures revealed using a variety of etchants is presented.

New concepts and new preparation materials have been introduced that enable metallographers to shorten the process while producing better, more consistent results. But first, the specimens must be sectioned. Many metallographers do not use a blade designed for metallography work, and the depth of damage will be much greater when production-type abrasive saws are used. So, as a first rule, produce a cut with the least possible amount of damage. If an automated device is used that holds a number of specimens rigidly (central force), then the first step must remove the sectioning damage on each specimen and bring all of the specimens in the holder to a common plane. This first step is often called “planar grinding.” SiC paper can be used for this step, although more than one sheet may be needed. Alternatively, the metallographer could use MD-Piano 120 or 220 (for specimens with hardness >150 HV) for the initial grind, followed (if desired) by MD-Piano 600 for a second grinding step. If the cast iron has a low hardness (<250 HV), one can planar grind with MD Primo 220. Alternatively, MD-Allegro could also be used to planar grind for specimens >150 HV hardness. If the hardness is <150 HV, MD-Largo can be used.

The Use of Molybdenum in Vacuum Furnace Applications

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Vacuum furnace hot zones are manufactured using materials that can withstand temperatures in the range of 1315ºC (2400ºF) and higher. Of the various types of refractory metals in use, none is more common than molybdenum.

The popularity and widespread use of molybdenum in vacuum furnaces is due to the wide range of properties that it exhibits, namely: high melting point, 2620ºC (4748ºF), low vapor pressure, high strength at elevated temperature, low thermal expansion, high thermal conductivity, high elastic modulus, high corrosion resistance, and elevated recrystallization temperature, between 800º – 1200ºC (1470º – 2190ºF). Mechanical properties of molybdenum are influenced by purity, type and composition of any alloying elements and by microstructure. Properties such as strength, ductility, creep resistance and machinability are enhanced by additions of alloys such as titanium, zirconium, hafnium, carbon and potassium along with rare earth element (La, Y, Ce) oxides.

Deformation and Annealing of Cartridge Brass

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Copper and its alloys are among the most malleable metals and alloys in existence. Cartridge brass, Cu – 30% Zn, has been used for many years to produce cartridge cases for ammunition due to its superior cold forming characteristics. This article shows the microstructure and hardness of cartridge brass from the fully annealed to the heavily cold worked condition. Then, it illustrates the influence of annealing temperature and time on removing the effect of the cold work and returning the alloy to a very low hardness annealed structure.

Cartridge brass, Cu – 30% Zn, is a single-phase Cu-based alloy where the addition of zinc increases the strength of copper by solid solution strengthening. The maximum solubility of zinc in copper at ambient temperature is slightly above 30% Zn. Higher levels of Zn, for example, 40% Zn, produce two phased α-β brass which is less malleable than the single phase, α-Cu cartridge brass. Cartridge brass, as the name states, has been used for many years to make cartridges for bullets due to its excellent formability and good cold formed mechanical properties. As an example, Figure 1 shows the microstructure of the starting cup with an annealed α-Cu grain structure, exhibiting annealing twins, used to cold form cartridge cases. Figure 2 shows the firing pin end of a formed 338 caliber cartridge case revealing a heavily cold worked microstructure. Color etching is far more effective than black & white etching to reveal the complete grain structure and deformation. Comparisons of color vs. B&W etching will be presented later.