Chemically analyzing white iron.

With the help of 12 participants, a spectrographic round-robin study was held to examine white iron analysis.

The quality of all cast metals is largely dependent upon chemical content. In measuring that content, most foundries are aided by the use of acknowledged standards for the results of compositional analysis.

The lab technician uses standards initially to calibrate the testing instrument to establish accuracy, and then to refer back to them to determine the acceptability of the results. Individual needs may vary, but with certified compositional guidelines, gauging is relatively simple.

Testing Concerns

Compositional analysis is an important manufacturing step--especially in the unique process of pouring white iron. Due to their generally high alloy content and carbon in particular, white irons offer the best abrasion resistance of any commercially available cast product. And their unique properties almost demand that a foundry specialize in white iron to ensure a consistent product.

One drawback to such specialization is that compositional standards for analysis are currently limited to one or two comprehensive sets that are classified only as reference materials and not certified.

Another problem arises in choosing the best method of analysis. Most producers use some form of spectroanalysis--most commonly the optical emission spectrometer. This type of instrument measures the elements in a metal sample in low to moderate concentrations, but its accuracy and precision are questionable at high concentrations. In such cases, other instruments--such as x-ray fluorescence spectrometers--are favored tools.

Therefore, producers of high-alloy white irons have justifiable concern about the accuracy of their analyses. Their products can have alloy contents as high as 30%, placing them in the questionable range for optical emission spectrometers. However, the standards available for calibration do not command the respect of a certified reference material (CRM), recognized by a certifying organization--most often, the federal government.

TABLE 1. ASTM A 532 Specifications for Test Samples


Class/Type           Description      Specimen Code

IA                   (Ni Hard 1)           N
ID                   (Ni Hard 4)           B
IIB                  (15% Cr-Mo)           TB
IID                  (20% Cr-Mo)           CA
(2) IIIA             (25% Cr)             CB, TA
TABLE 2. Entries of Spectrographic Results for a Sample, with Wet Analysis
Results at Bottom

Sample CA
Foundry            C       Mn    Si     P     Cr      Ni     Mo    Cu     S

1.               2.70     .46   .87   .022   19.23    .95   2.92   NA    .050
2.               2.67     .45   .79   .013   18.51    .88   3.27   .24   .034
3.               2.73     .27   .75   .008   18.53    .91   2.13   .19   .033
4.               2.74     .37   .93   .025   18.54   1.00   2.51   .24   .046
5.               2.59     .40   .76   .011   19.74    .86   3.10   .23   .038
6.               NA       .45   .74    NA    18.5     .87   3.08   .23   NA
7.               2.57     .46   .78   .012   18.74    .98   2.70   .24   .036
8.               NA       .47   .53   .010   20.4     .93   3.30   .21   NA
9.               2.71     .45   .74   .011   19.34    .74   3.56   .23   .029
10.              2.66     .42   .73   .015   18.3     .91   3.43   NA    .037
11.              2.66     .55   .78   .006   18.77    .91   2.85   .25   .035
12.              2.65     .46   .77   .014   18.96    .85   3.36   .22   .037

Average          2.67     .43   .76   .013   18.96    .90   3.02   .23   .038
Std. Dev.         .05     .06   .09   .005     .59    .06    .40   .02   .006
Wet Analysis
Armco            2.75     .43    .74  .010   19.03    .95   3.26   .22   .042
USBM              .73   18.8    3.30

Establishing a Study

It was that concern that led the AFS Special Irons Committee (5-D) to conduct a spectroanalytical round robin to evaluate the accuracy and compatibility of analysis among a large number of producers.

The study would have two main goals. The first would be to establish the comparability of analyses done with different spectrometers. The second: to determine the accuracy of test results from a number of separate foundries. Ten foundries that regularly run tests on white iron were selected to participate:

* Babcock & Wilcox, Barberton, Ohio;

* Bradley & Foster, West Midlands, England;

* Capitol Castings, Inc., Phoenix, Arizona;

* Columbia Steel Casting, Portland, Oregon;

* Eagle Foundry, Eagle Creek, Oregon;

* Esco Corp., Portland, Oregon;

* Norcast, New Liskeard, Ontario;

* Denver Thomas, Birmingham, Alabama;

* United Foundries, Inc., Canton, Ohio;

* Weatherly Cast & Machine Co., Weatherly, Pennsylvania.

In addition, two highly respected laboratories were included:

* Armco R&T, Middletown, Ohio;

* U.S. Bureau of Mines, Albany, Oregon.

Setting the Rules

At the outset of the project, the committee decided on certain rules and procedures. Because the intent was to examine the ability of producers to measure the contents of their own products, the samples would reflect just that--actual foundry furnace or ladle samples taken as they would be in production. Four member foundries cast the specimens using their own individual techniques.

Next, it was decided that a set of six individual specimens would be provided to every participant. In this way, all participants would work on the project simultaneously and keep their set once finished. (Earlier committee experiences with similar projects concluded that providing only a single set to be worked on sequentially--with time for transshipment in between tests--failed to produce results. Interest waned and activity almost ceased.)

Once the specimens were collected from the four supplying foundries, each one was individually sparked with the same instrument prior to being distributed. Any that differed significantly from others in their group were discarded, ensuring that the test specimens were homogenous.

The contents of the specimens loosely conformed to specific ASTM A 532 class and types, and were coded accordingly. Each specimen was engraved with its respective code and a discrete sequence number explicitly identifying it. Once assembled, the sets had one specimen for each code, but all in the set bore the same sequence number. As part of the instructions accompanying the package, participants were asked to identify their results according to these marks. In that way, all responses would be attributed correctly.

The different participants were to analyze the specimens using their own instruments, calibrated with standards they normally use and taking an average of three exposures per specimen. No extraordinary procedure was to be used for these analyses. To complete the data collection, participants were asked to submit the make and model of their spectrometers and the standards with which the instrument was set up.

To evaluate the accuracy of the instrumental results, Armco and the U.S. Bureau of Mines conducted conventional quantitative analyses of chips milled from the spectro specimens. These values were presumed to be correct measures of the various contents and offered an independent reference point from which to gauge any error in instrumental results.

Satisfactory Results

The report format was a single page per specimen with element symbols as column headings and 12 lines identified by sequence numbers 1-12. Coded like this, the data would be recognizable only by sequence number and participants knew only their own. An average for each specimen column was taken, along with a standard deviation. Of about 600 element entries, only 12 seemed unreasonable and were thrown out so as not to unduly affect the average.

Results culled from the testing included:

* Ten of the 12 participants used optical emission spectrometers with nearly all manufacturers of those instruments represented. The remaining two were x-ray fluorescence spectrometers, which are incapable of direct carbon or sulfur analysis.

* The most commonly used standards were sets of high-chrome iron (FCR) and ni-hard (NH), produced by the Foundry Industry Technical center, France (CTIF). Other standards included those from the National Institute of Standards and Technology (NIST), Bureau of Analyzed Samples, Ltd. and MBH Analytical, Ltd. Most participants also used secondary in-house standards for routine standardization.

* Precision, as measured by % RSD (standard deviation x 100 / average), was good. Elements C, Cr and Ni were all single-digit RSD. Elements Mn, Si and Mo were in the low to midteens.

* Accuracy, as judged by comparison of element averages between spectro and wet chemical analyses, was very good in virtually all cases.

* The committee concluded that within the limits of experimental error, optical emission spectrometers using presently available standards yield satisfactory results in high-alloy white iron analysis.

Spectrometers: Simplifying Analysis

Spectrometry, used mainly in iron, steel and aluminum production, offers important advantages to the foundryman. It is the fastest, most convenient comprehensive chemical analysis available. Prior to its development, chemical analysis of foundry melts could be done only after the casting was poured. Thus, the only data with which the foundryman could make on-the-floor decisions was historical.

The instant results spectrometers provide allow for last-minute chemical adjustments while the metal is still in the furnace. Evolving technology has produced compact, portable units rugged enough for use right on the melt floor. Allowing time for taking and cooling the sample, the entire process is less than five minutes.

Spectrometers work simply. Shortly after pouring a molten metal disk sample, the cooled sample is placed in a chamber of the spectrometer next to an electrode. The electrode then strikes an arc with the sample, creating enough energy to cause a portion of the sample to vaporize and burn.

While this spark appears collectively as white light, each element of the metal actually causes light of specific wave lengths. These wavelengths bounce off a refraction grating system that separates them, much like the action of a prism.

The wave lengths, each representing an element, are then read by the spectrometer and gauged for intensity. Essentially, the brighter the light, the higher the concentration of the element. The spectrometer is calibrated to whatever standard metallurgical measurements might be used for the elements. On newer models, the data is sent to a PC for graphic display or printout.