Introduction

— A foundry needs accurate chemical data to assure product quality. In other words, accurate knowledge of molten metal com­position is basic to the achievement of casting quality.

— Since, determination of molten metal composition of both ferrous and non-ferrous metals, prior to pouring, is possible, this exercise is essential in order to manufacture quality castings and to com­pete in the national and international markets.

— SELECTING the most appropriate method of chemical analysis can be difficult, largely because many foundrymen and others in the metal-working industry lack basic information on available methods and their advantages and disadvantages.

— The classical “wet” methods, using acids and reagents, gradually are being replaced by emission spectroscopy, X-ray fluorescence, and atomic absorption spectroscopy. A rudimentary knowledge of these techniques may help to avoid costly wrong decisions, particularly if services of a commercial laboratory are utilized.

— Foundrymen also have come to appreciate the advantages of liquid metal analysis. Instrumentation is available commercially to analyze and display digitally the numerical values for carbon equi­valent, total carbon content, and silicon level in members of the cast iron family. This procedure is made possible by correlations based on the liquidus and solidus arrest temperatures of cast high-carbon ferrous alloys. Results are obtained within seconds.

— Liquid metal monitoring of many nonferrous alloys, by relating the cooling curve data to chemical composition, also is possible based on recent developments. A typical example is the quanti­tative determination of copper in 70-30 brass.

— Dissolved oxygen in liquid steel likewise can be analyzed accu­rately and displayed digitally. Determination of molten metal composition of both ferrous and nonferrous metals, prior to pouring, is possible, moreover, with expendable cartridges.

WET ANALYSIS

—                This method is accurate, but time consuming. For common alloy­ing elements in most popular commercial alloys, the main advant­age of wet analysis is its accuracy. The percentage of almost any element can be determined, regardless of the amount of element present, and the accuracy of the measurement is not affected by the metallurgical history of the sample — that is, no “matrix effect” influences wet analysis results.

Another advantage is that any form of sample can be used, including solid, powder, drillings, and chips.

The main disadvantage is the time required to perform a complete analysis of a sample. Analysis time for a single element can range from minutes to more than a day with “manual” methods. Analysis time has been shortened considerably, however, by instrumented and sometimes automated techniques.

Wet techniques for iron-base and copper alloys are straightforward. Consequently, few common steels, irons, brasses, or bronzes cannot be analyzed completely in a day. Problem areas include measuring the low contents of boron in boron-treated steels and magnesium in ductile iron. Determining carbon contents of less than 0.04% may require use of a special analyzer.

Procedures for nickel-base and aluminum alloys are more complex, and complete analyses typically require more than a day. Some nickel-base alloys are particularly difficult to dissolve — for example, much time is spent just getting the analysis started.

In summary, wet analysis can be useful for irons, steels, and copper alloys, unless on-the-spot, rapid determination is required. With a trained technician, a company could set up and successfully operate a wet lab for these materials at a relatively modest capital investment.

It should be pointed out that “referee analyses” — tests conducted to settle disputes over a material’s composition—must be conducted by the wet chemical method. It is the only referee technique acceptable to most government agencies and major manufacturers.

SPECTROSCOPY

— Spectroscopy means, literally, looking at spectra. Spectroscopy is the science of interactions between radiation and matter. Spectro­chemistry is that branch of spectroscopy where the measurement of radiation is used to obtain information on the composition of a material. In atomic emission (or absorption) spectroscopy, the atomic spectra emitted by a sample are used to determine its quantitative elemental analysis. A quantometer carries out auto­matic quantitative analysis.

Atomic spectra are emitted by atoms, and they are a form of radia­tion, physically, energy per unit time. An atom consists of a positively charged nucleus surrounded by an equal amount of negative charges.

—  Some definitions related to spectroscopy are given below:

Spectrograph — The complete spectrum is recorded on a black-and-white 35 mm film. With proper calibration, many measurements can be made on the film. Densitometer then measures “blackness” of lines. From this, concentration can be calculated.

Spectrometer—Only selected wavelengths are recorded through entrance slits, are dispersed by a grating, then go through exit slits for a particular set of elements.

Airpath Spectrometer—Does not allow determination of carbon and sulfur.

Vacuum Spectrometer—Allows analysis of carbon and sulfur.

Spectroscope — An optical device for producing and observing a spectrum of light or radiation from any source. (Several types of instru­ments for producing and viewing spectra today employ diffraction grating instead of prism).

X-ray Fluorescence — Surfaces of samples are bombarded with a source of hard X-rays that penetrate surface and emit soft X-rays at various angles and wavelengths.

—                In both optical emission and X-ray fluorescence spectroscopy, wavelengths of radiation from the sample identify the elements present. Intensity of the radiation is proportional to concen­tration of the elements. The instrumentation must provide a mechanism for exciting the sample to emit radiation, and it has to have a means of identifying the wavelengths (or energy) of the radiation and measuring the intensity.

In optical emission, the spark source is used to “sample” the speci­men — blast finely divided material into the analytical gap, in which it is vaporized into atoms that are excited to emit light. Light is emitted in the range of 170 to 780 nanometers, from the far ultraviolet to the deep red.

That radiation falls on the slit of the spectrometer and then on a prism or grating that disperses the light into a spectrum. The wavelengths that will identify the elements likely to be present in the sample are isolated by exit slits. Intensity of the radiation is measured by photo-multiplier tubes that produce currents proportional to concentrations of elements in the sample.

In X-ray fluorescence, the sample is illuminated by a primary intense beam of X-rays from an X-ray tube. The sample produces fluorescent X-rays. They are identified by wavelengths or by photon energy, and again the intensities of each radiation measure concentrations of the elements in the samples.

Atomic Emission

— Atomic emission spectroscopy long has been the standard for routine metal analysis in many analytical applications. A basic principle underlies the method. A minute part of the sample is vaporized and excited to the point of light emission by either an electric arc or spark, a direct current argon plasma, an induction-coupled argon plasma, or a laser. The emission spectrum is focused onto the spectrometer’s entrance slit. Light derived from the vaporized, excited material is dispersed into its component parts in the spectrometer.

At the exit aperture, the light either is photographed on a plate or film or is recorded by a photodetector. Each element produces a series of spectral lines of specific wavelengths. Identification of an element is pos­sible by studying the lines according to their respective locations. Intensity of the lines is a function of quantity of the specific element observed.

According to manufacturers of equipment, no single excitation source is best for all applications. For solid samples, arc excitation is more sensitive, and spark excitation is more stable.

Surface Spectroscopy

—    Surface spectroscopy involves probing a sample target with a flux of energetic particles and detecting characteristic particles

emitted from the surface after interaction. The probe beam may be photons, electrons, or ions.

Surface analysis has many applications, such as absorption of conta­minants, identification of corrosion products, and detection of impurities that may affect embrittlement of metals, alloys, and grain boundaries.

Digital computers have become integral components of modern methods of analysis. Applications of these devices to analytical instru­mentation have increased with advances in computer technology. Most recently, microcomputers are influencing instrument design as well as analytical methods. Understanding of the computer’s role in a specific instrumental method requires that interactions between instrument, com­puter, and analyst be considered.

Instrumental Analysis

— Metal that is to be analyzed by either X-ray fluorescence (XRF) or optical emission spectroscopy (OES) requires a specially poured, chill-cast sample to retain elements in solution and to be homo­geneous.

There are, however, important differences between the two tech­niques. XRF is most accurate at concentrations of about 0.1 to 50 or 60%. OES is considered more appropriate for concentrations of 0.0001 to 2.0%. During 1980′s, however, XRF has been extended down, and OES has been extended to high concentrations.

Instruments incorporate an X-ray tube, the beam of which scans the surface of a specially prepared specimen. X-rays impinging on the sample cause it to emit X-rays characteristic of the matrix and the elements present. Data readouts can be manual, on a strip chart recorder, or elec­tronic, via a computer printout.

Analysis time is short. A manual readout takes 1 to 3 min per element, but a computerized system often can give a complete analysis in a minute. XRF spectroscopy works well for quantitative analyses of steels and nickel-base alloys.

Users must be aware of the “matrix effect”. The “matrix”, which includes the sample’s metallurgical composition, form, and history, affects the way light is emitted in the arc. Analyzing a cast alloy against a wrought alloy standard therefore can give misleading results, eventhough both materials have identical chemical compositions.

It also is good practice to compare a solid sample against a solid standard and to insure that both have equivalent heat-treatments. Solid samples are preferred, although other forms such as chips and drillings can be analyzed after special, sometimes time-consuming preparation.

Choosing a Method

Carbon and sulfur in iron and steel are special cases. Atomic absorption cannot measure either. X-ray spectrometers cannot measure carbon, although vacuum units can measure sulfur. Direct-reading, vacuum-emission spectrometers can measure both, but a nonhomogeneous carbon distribution, as may occur in gray iron, will hinder accuracy. Carbon present as graphite requires special treatment. Best results for carbon and sulfur are likely to be obtained from commercially available, automated wet chemical analysis instruments designed specifically to measure concentrations of these two important elements. No generalization can be made, however, because level of con­centration is important.

Quality, speed, and cost, in that order, should be considered in selection of an analysis method. If the highest quality is desired, and an analysis time of up to about two days can be tolerated for a single determination, wet chemical analysis still is the best. For a secondary statistically valid analysis — that is, checking a material against a com­position certified by its producer — a spectroscopic analysis method will be acceptable.

SPOT TEST TECHNIQUES

Spot test analysis is a generic term referring to sensitive and selective tests based on chemical reactions, wherein use of a drop of the reagent solution is an essential step. The tests are microanalytical in nature and are applicable to the investigation of metallic materials. An important part of spot test analysis is played by the actual manipulation of unknown substances and reagents, and the method is not dependent on the use of auxiliary optical magnification.

In general, spot tests are the ultimate in simplicity. Elegance of the method derives from the nature of the reagents used, together with the advantageous use of reaction conditions. The utmost sensitivity and selectivity can be obtained with a minimum of physical and chemical operations.

Essential requirements for the successful application of spot test procedures include a knowledge of the chemical basis of all details of the tests used, strict observance of trustworthy experimental conditions, scrupulous cleanliness of the laboratory and equipment, and use of only the purest reagents available. Tests should be repeated to assure repro­ducibility. Both blanks and controls always should be run.

A NEW INSTRUMENT

Highly sophisticated instrumentation is available commercially that combines the advantages of a direct-reading spectrometer with the guid­ing “hand” of a computer. The computer is hidden inside the instrument,

and the operator works only with the externally mounted keyboard and the
video screen.                                                                                     ,

The spectrometer is run by menu-driven software that, according to equipment manufacturers, is easy to learn and use. These tools for testing metal composition can be used to analyze a wide variety of alloys as well as refractories, oils, and slags. They appear to be versatile and require a minimum of maintenance.

No matter what the model designation or which type of computer is used, these spectrometers basically work on the physical principle of the prism: A high-voltage spark source excites a metal sample so that it emits light. This light then is separated into its component wavelengths by a diffraction grating, and each wavelength is measured electronically and translated into concentration readings by a computer, eliminating the human error altogether.

The operator then knows from the readout whether the heat he is about to pour has the correct chemical composition and is within prescribed specification limits.

Use of Standards

Both primary standards and setting-up samples are used in spectrographs analysis. They are manufactured under strictly controlled conditions to produce material that is homogeneous in composition over the analytical surface and to a guaranteed depth.

Standards have three main uses: (1) Constructing primary calibra­tion curves (the basis of all subsequent analysis). (2) Determining the analytical performance of an instrument. (3) Periodically checking day-to­day setting-up calibrations.

Primary calibration curves should be checked for compatibility with both spectrographic and wet chemical analysis of many production sam­ples. If a reproducible bias away from the primary calibration is detected, corresponding corrections can be made either to all subsequent results or to the calibration itself to produce a working curve.

AN EVALUATION

In the view of many chemists traditional wet chemical analysis methods remain important for the foundry industry although they are supplanted frequently by rapid instrumental techniques. The predominant uses of wet methods include independent checks of spectrometric data, establishing the composition of standard materials for instrument calibration, and action as referee in disputes between a foundry and a customer.

At their best, gravimetric, volumetric, and photometric methods provide unsurpassed accuracy, and the American Society for Testing and Materials annually publishes detailed procedures for determining many elements in cast ferrous and nonferrous engineering metals and alloys. Many chemists, however, find strict adherence to ASTM standards un­necessarily cumbersome and laborious. Most commercial laboratories use simplified versions of referee methods for routine work.

For unquestionably reliable analyses, a good standard method should be adopted and used consistently. In addition, the type of sample chosen obviously is of primary importance. Without a proper, repre­sentative sample, no analytical technique can be effective. Foundrymen should be aware of the differing sample requirements for each technique. The AFS Chemical Analysis Committee prepared a reference table (Modern Casting, October 1970 p. 70) that lists characteristics of common chemical analysis samples.

Repeatability, reproducibility, and accuracy are statistical consider­ations, and numerical estimates are calculated, based on a given set of analytical data.