Ferrite Determination in Stainless Steel Welds Advances ...files.aws.org/wj/supplement/WJ_1997_01_s24.pdf · ISO 8249 Duplex Stainless Austenitic Stainless Ferrite Standards 1996

Ferrite Determination in Stainless Steel Welds Advances since 1 974

Developments over the last 20 years have extended Ferrite Number determinations to duplex ferritic-austenitic stainless steel weld metals

BY D. J. KOTECKI

ABSTRACT. Since the Comfort A. Adams Lecture of W. T. DeLong in 1974, significant advances in the determination of ferrite content in stainless steel welds have taken place. These are reviewed in the present Comfort A. Adams Lecture.

Examination of MagneGage Number 3 Magnet strengths led to a concept for extending, by extrapolation, the calibration range of AWS A4.2-74 to ferrite levels above 28 FN. Ferrite Numbers could then be assigned to thinner coating thickness standards for primary calibration of MagneGages over the extended range. Round robin studies were conducted to establish the reproducibility of measurement of Ferrite Numbers in duplex stainless steel welds. The method was incorporated into ANSI/AWS A4.2-91.

Calibration using primary standards (coating thickness standards) is limited to a very few instruments, due to the difference in distribution of ferromagnetic material in coating thickness standards vs. that in stainless steel weld metal. Sec- ondary standards, covering the range from near zero to about 100 FN, became available for calibrating additional instruments at the beginning of 1995. A round robin of tests established that the interlaboratory reproducibility of measurement after calibration by the secondary standards is similar to that observed with MagneGages calibrated by use of primary standards.

Excessive ferrite in duplex stainless steel weld metals has adverse effects on

D. J. KOTECKI is Technical Director for Stain- less and High Alloy Product Development, The Lincoln Electric Company, Cleveland, Ohio. Paper presented as the Comfort A. Adams Lec- ture at the AWS 77th Annual Meeting, April 23-25, 1996, Chicago, III.

weld properties. The utility of the Ferrite Number measurement system for duplex stainless steels is thus established. De- velopment of a solid link between Ferrite Number and ferrite percent, determination of ferrite in the heat-affected zone (HAZ) of duplex stainless steel weldments, and further development of pre- dicting diagrams remain for the future.

Background

In the 1984 Comfort A. Adams Lec- ture, Olson (Ref. 1) reviewed the numerous diagrams that had been used to predict the amounts of various phases in stainless steels. Most were developed by relating metallographic "percent ferrite "1 to chemical composition. At the time of World War If, there was much emphasis on avoiding sensitivity to hot cracking when using nominally austenitic stainless steel weld metals for joining armor steels. The presence of at least "5% ferrite" in weld metal was considered to be sufficient evidence of freedom from hot cracking sensitivity, and the well-known

KEY WORDS

Stainless Steels Weld Metal Ferrite Measurement ANSI/AWS A4.2 ISO 8249 Duplex Stainless Austenitic Stainless Ferrite Standards 1996 Adams Lecture

Schaeffler Diagram (Ref. 2) became the tool of choice for both estimating and specifying weld metal ferrite.

The situation regarding ferrite became more complicated when ferrite in nominally austenitic stainless steel weld metals was found to have other influences in addition to eliminating sensitivity to hot cracking. There are applications in which very little ferrite can be tolerated. Examples include those applications requiring high toughness at very low tem- peratures, such as might be encountered in superconducting magnet structures for fusion energy devices, those requiring low magnetic permeability, such as naval mine sweepers, and those requiring service environments that are selectively ag- gressive toward ferrite, such as are encountered in urea manufacture (Refs. 3, 4). During postweld heat treatment (PWHT), or during elevated temperature service, ferrite was found to transform to embrittling phases such as sigma, so that an upper l imit on ferrite (sometimes "15%", or even "10%") might be speci- fied if such conditions were anticipated for a weldment (Ref. 3). These restrictions on weld metal ferrite content led to a dif- ficult situation for those involved in filler metal specification, supply and quality assurance. At least three parties (the fab- ricator or filler metal specifier, the filler metal supplier and one or more other organizations, such as the final weldment owner, an insurance company or even possibly the Nuclear Regulatory Com-

1. Note that, throughout this document, when a weld metal ferrite content is expressed as a percent, the expression is enclosed in quotation marks to indicate doubt concerning the value, because there is no reproducible method of determining weld metal percent ferrite.

24-s I JANUARY 1997

mission) had to agree that the specification was met.

Ferrite determination to demonstrate compliance with a specification involves some mutually agreed upon form of measurement. Initially, ferrite determination required metallographic polishing of a weld metal sample, careful etching and some form of estimating the area fraction of ferrite in a microscopic view of the weld metal. This process is destructive: the sampled piece cannot be used as the production weldment. In addition, round robin studies of measurement results in- dicated poor reproducibility among laboratories (Ref. 5). The causes of poor interlaboratory reproducibility of metallographic ferrite determinations are considered to include the following: that ferrite distribution is inhomogeneous in a weld, that ferrite in a weld is very fine and very irregularly shaped, and that etching to distinguish between ferrite and other phases is an imperfect art in which un- deretching tends to fail to reveal all ferrite while overetching tends to falsely identify some other phase(s) as ferrite. Figure 1 clearly shows the irregular shape of ferrite in several weld metals of various ferrite contents 2. Computer-auto- mated point counting of the fraction of a given number of superimposed grid in- tersections falling on ferrite has been used in recent years for "ferrite percent" determinations on metallographically prepared weld specimens, but the procedure is sensitive to etch quality, elec- tronic interpretation of the ferrite signal and interpretation of grid points falling on a boundary between two phases.

Furthermore, any constitution diagram, such as the Schaeffler Diagram, provides, at best, only a reasonable estimate of the weld metal ferrite content. The accuracy of the estimate depends on the accuracy of the chemical analysis of the sample under test (again, a destructive test), the accuracy of the diagram it- self (including the underlying assump- tions made in preparing the diagram and the accuracy of the chemical analyses that went into developing the diagram in the first place) and the degree to which chemical composition varies within the weld. Composition variation within the weld, or from manufacturer's certifica- tion for a filler metal to the actual production weld metal, is an often-over- looked variable. Nitrogen pickup during welding is outside of the control of the filler metal manufacturer, and the litera- ture offers numerous examples of significant reduction of weld metal ferrite levels below expected values due to

2. Note that Ferrite Numbers are not given for the microstructures shown in Figs. 1D and 1E because, in the technology of 1974, Ferrite Numbers were not defined for such welds.

accidental, or purposeful, nitrogen pickup (Refs. 6, 7, for example). In submerged arc welding, chromium loss or gain can occur, depending on the flux used and on the ratio of flux melted to wire melted (Ref. 8), and can cause a sig- nificantly different ferrite content in the weld than might be predicted from the wire composition.

As a result of these limitations, a nondestructive method of ferrite determination was developed by the WRC Sub- committee on Welding Stainless Steels as AWS A4.2-74 (Ref. 5). This standardized the technique by assigning a Ferrite Number (FN) on the basis of magnetic attraction. A range of magnetic attraction was obtained by using various thicknesses of a nonmagnetic coating over a ferromagnetic substrate. The coating thickness standards were available from the U.S. National Bureau of Standards (today known as the National Institute for Standards and Technology, or NIST). Round robin investigations of measurements in the 5 to 15 Ferrite Number range using MagneGages calibrated according to this Ferrite Number system (Ref. 5) showed interlaboratory reproducibility to less than +1 Ferrite Number, much better than could be achieved by metallographic determination of "ferrite percent." The Ferrite Number system became the subject of William T. DeLong's Adams Lecture in 1974 (Ref. 9) and became internationally recognized as ISO 8249 (Ref. 10). With the 1986 revision of A4.2, it became an American National Standard, ANSI/AWS A4.2-86.

The DeLong Diagram (Ref. 6) was the first to predict weld metal ferrite content from deposit composition in terms of Ferrite Numbers instead of "ferrite percent." Siewert, McCowan and Olson (Ref. 11 ), sponsored by the Welding Re- search Council and using data provided by the WRC Subcommittee on Welding Stainless Steels, developed a diagram that became known as the WRC-1988 Diagram, also in terms of Ferrite Num- bers. This diagram was subsequently shown (Ref. 4) to predict, more accu- rately than the DeLong Diagram, the measured Ferrite Numbers of some two hundred samples not included in the original database used in developing the WRC-1988 Diagram. Kotecki and Siew- ert (Ref. 12) made a minor modification to the WRC-1988 Diagram to include copper in the nickel equivalent, without moving the isoferrite lines, and this last version, known as the WRC-1992 Dia- gram, was off icial ly adopted by the ASME Boi le r and Pressure Vessel Code in its Winter 1994 Addendum.

It should be noted that a Ferrite Num- ber is an arbitrary value defined by the calibration system, not an absolute "ferrite percent." However, the assignment of a

Fig. 1 - - Microstructure of five stainless steel weld metals, X500. Murakami's etch darkens ferrite. (A) 0.05% C 18.3% Cr, 10.3% Ni, (B) 0.04% C, 19.4% Cr, 10.8% Ni, (C) 0.07% C 20.8% Cr, 9.4% Ni, (D) 0.08% C, 30% Cr, 9.5% Ni, and (E) 0.03% C, 22.5% Cr, 9.0% Ni, 2.9% Mo, 0.14% N.

WELDING RESEARCH SUPPLEMENT I 25-s

Fig. 2 - - MagneGage with a set of four coating thickness standards.

Fig. 3 - - Laboratory beam balance, with a MagneGage Number 3 Magnet, ready for ferrite measurements on a weld sample.

given Ferrite Number to a given level of magnetic attraction (nonmagnetic coating thickness) was strongly influenced by the average value of "ferrite percent" obtained in metallographic round robin measurements compared to magnetic measurements on the same weld metal. Therefore, AWS A4.2-74 could state that, at low Fer- rite Numbers, the Ferrite Number of a

given weld metal seemed to be a good approximation of the "ferrite percent."

in the Ferrite Number system, calibration by use of primary standards (nonmagnetic coating thickness standards) can only be applied to a few instruments. The Ferrite Number assigned to a given coating thickness standard depends upon the specific instrument being calibrated. For example, 0.279 mm (0.011 in.) of nonmagnetic coating thickness corresponds (Ref. 5) to 20.3 FN with a MagneGage, to 29.9 FN with an Inspec- tor Gage and to 17.2 FN with a particular model of Fischer Feritscope (a.k.a. Ferritescope). This difference occurs because the probe of each instrument senses a hemispherical volume of the primary standard, or of the weld metal under test, but the radius of the hemispherical volume depends upon the strength of the magnet or other instrument probe. Because the distribution of ferrite in a given weld is far different from the distribution of ferromagnetic material in a coating thickness standard, it should come as no surprise that different instrument probes produce different correlations between nonmagnetic coating thickness and weld metal Ferrite Num- ber. A problem that has concerned the WRC Subcommittee on Welding Stain- less Steels is that instrument manufacturers, from time to time, change probe design, or even the entire instrument design, rendering a painstakingly ac- quired calibration table obsolete.

The Ferrite Number system, defined by AWS A4.2-74 or ANSI/AWS A4.2-86, provides for calibration of instruments that cannot be satisfactorily calibrated using primary standards as well as calibration when primary standards are not available, by introducing secondary standards, distinct from the nonmagnetic

coating thickness primary standards. Secondary standards have been prepared from weld metals, to each of which a Fer- rite Number is first assigned by making measurements on the secondary standard with an instrument calibrated by use of primary standards. Over the years, there have been several commercial sources of secondary standards. In round robin tests, instruments properly calibrated by use of secondary standards provide virtually the same degree of interlaboratory reproducibility of measurement on weld metal "unknowns" as do instruments calibrated with primary standards (Ref. 5).

Other Methods of Ferrite Determinat ion

Stalmasek (Ref. 13) has extensively reviewed other methods proposed for ferrite determination. These include X-ray diffraction, M~ssbauer Effect (described in detail in Ref. 14), magnetic permeability, and magnetic saturation. Of these, X-ray diffraction and M~ssbauer Effect measurements have apparently only been applied in laboratory experi- ments. Magnetic permeability, rather than magnetic attraction force, is the measurement principle used in several commercial devices that can be calibrated in Ferrite Numbers according to ANSI/AWS A4.2, including the Fischer Feritscope and the Dr. Foerster meter. Magnetic saturation measurements offer the possibility to determine the absolute volume ferrite percent in weld metal, but the magnetic saturation response of a given volume of ferrite-containing metal depends upon not only the volume ferrite percent but also the composition of that ferrite (Refs. 15, 16). in general, ferrite of lower alloy content provides a higher level of magnetic saturation than does

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Fig. 4 - - FN measured on a particular sample vs. strength of the magnet used for the measurement.

Fig. 5 - - FN assigned by ANSI/AWS A4.2-86, for calibration of a Mag- neGage, vs. nonmagnetic coating thickness.

26-s I JANUARY 1997

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Fig. 6 - - FN assigned to coating thickness standards by ANSI/AW5 A4.2- 86 vs. tear-off force for a particular MagneGage Number 3 Magnet, measured using the beam balance arrangement shown in Fig. 3.

Fig. 7 - - MagneGage with the dust cover removed and a counterweight added at the hole in the balance beam being calibrated for ferrite measurements above 30 FN, on nonmagnetic coating thickness standards.

ferrite of higher alloy content. Then magnetic saturation determination of ferrite content as volume ferrite percent requires knowledge of the ferrite composition and application of a corresponding "correction factor" to the instrument reading. Further, magnetic saturation measurements require machining of a precise volume (usually a cylinder) from the metal under test, which makes this also a destructive test. Nevertheless, a ferrite determination system based on magnetic saturation measurements is used as the standard method in the countries of the former Soviet Union and COMECON (Ref. 17).

The Need To Expand the FN System

AWS A4.2-74 (and ANSI/AWS A4.2- 86) considered ferrite determination in nominally austenitic stainless steels, and therefore only needed to cover the range up to 28 FN. This applied to the vast ma- jority of stainless steel weld metals of that time, quite satisfactorily. An excep- tion is Type 312 weld metal, which rou- tinely exceeds 28 FN. Part D of Fig. 1 shows the microstructure of a 312 weld metal. Clearly, there is a great deal more ferrite than in part A (1.9 FN), part B (4.1 FN) or part C (13.4 FN), but it is not possible to say how much more using ANSI/AWS A4.2-86. Type 312 weld metal is most often used for maintenance and repair applications for joining diffi- cult-to-weld steels. In most cases, if the weld does not crack, then it is satisfactory. If a covered electrode were to be classified as E312-16 according to ANSI/AWS A5.4-92, the all-weld-metal tensile test would have to meet 22% elongation, minimum, which indirectly imposed an upper limit on ferrite.

Since 1974, a new class of stainless

steels has come into prominence - - duplex ferritic-austenitic stainless steels, typified by Alloy 2205 (UNS $31803). Weld metals for these steels exceed 28 FN, also. Figure 1E shows such a weld metal, which is qualitatively similar in microstructure, including ferrite content, to the 312 weld metal in Fig. 1D. The rapidly advancing technology of these steels has been chronicled by four major international conferences (St. Louis, 1982; The Hague, 1986; Beaune, 1991 ; and Glasgow, 1994). A fifth international conference on duplex stainless steels is scheduled for Maastricht in 1997. These steels, and their weld metals, contain nearly equal amounts of ferrite and austenite. In particular, their ferrite contents exceed the 28 FN maximum of ANSI/AWS A4.2-86.

In duplex stainless steel weldments, excessive ferrite content is associated with inferior mechanical properties and inferior corrosion resistance (Ref. 3). Likewise, insufficient ferrite can also produce inferior mechanical properties and inferior corrosion resistance. So again, a need arose for a means of ferrite determination. Metallographic determination of "ferrite percent" has been used, with the same problem of lack of interlaboratory reproducibility as was observed in nominally austenitic stainless steel welds (Ref. 18). In view of this reproducibility problem, as well as in view of the destructive nature of metallographic ferrite determination, extension of the Ferrite Number system of nondestructive ferrite determination was undertaken by the author, with considerable encouragement and assistance from the WRC Subcom- mittee on Welding Stainless Steels and from Subcommission II-C of the Interna- tional Institute of Welding (IIW), beginning in the late 1970s. What follows is a review of that work.

Origin of the Ferrite Number System Extension

In contrast to ANSI/AWS A4.2, the international standard for Ferrite Numbers, ISO 8249 (Ref. 10), avoids reference to specific commercial instruments such as the MagneGage. ISO 8249 defines the Number 3 Magnet of the MagneGage in a generic form, and provides for the measurement of the force ("tear-off force") necessary to pull this magnet free from a standard or weld metal sample by any suitable means. One such means involves modification of a simple laboratory beam balance to suspend the magnet over the specimen and increase the force applied until detachment is observed. Then this tear-off force is related to Ferrite Number in the same fashion as is used to relate the White Dial reading of a MagneGage to Ferrite Number following ANSI/AWS A4.2. The White Dial reading of a MagneGage is proportional to the tear-off force, but it is usually con- verted directly to FN. In fact, the Magne- Gage is essentially a modified beam balance that uses a torsion spring to vary the force applied to the magnet. Figure 2 shows a MagneGage, and Fig. 3 shows a laboratory beam balance modified to accept a MagneGage Number 3 Magnet and measure the force necessary to tear the magnet free from a specimen.

As part of the development of ISO 8249, IIW Subcommission II-C felt it necessary to specify magnet strength in defining its generic version of the Mag- neGage. Because the MagneGages in ex- istence produced excellent reproducibility of FN measurement, the approach adopted was to determine the strength range of MagneGage Number 3 Magnets in circulation and then build the ISO 8249 magnet strength definition around


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22 1 Calibration by Extrapolation 22 1 Calibration by Extrapolation 20 ~ Calibration by Coating Thickness 20 I-7 Calibration by Coating Thickness

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26.7 32.5 48.1 57.9 65.0 30.0 33.9 39.0 44.7 49.4 58.3 61.1 65.4 68.5

WRC Round Robin Interlaboratory Average FN IIW Round Robin Interlaboratory Average FN

Fig. 9 - - Percent variability in WRC round robins: calibration by use of Fig. 10 - - Percent variability in IIW round robins: calibration by use of extrapolation compared to calibration by use of coating thickness stan- extrapolation compared to calibration by use of coating thickness standards, dards.

this range. Kotecki (Ref. 19) borrowed a total of 21 MagneGage Number 3 Mag- nets from various owners of these instruments around the United States. With a set of eight coating thickness standards spanning the 0 to 28 FN range and a laboratory beam balance, a calibration line, relating tear-off force to assigned FN, was developed for each of the 21 magnets. In all 21 cases, a regression line of the form of Equation 1 below could be fit with a correlation coefficient R of at least 0.995:

FN = m'TF (1) where FN is the Ferrite Number assigned to the coating thickness standard according to ANSI/AWS A4.2-86, TF is the

3. Note that only 17of the magnets were used to measure FN on the single "unknown," as can be seen in Fig. 4. Four magnets had been returned to their owners before this part of the work was conceived.

measured tear-off force (grams), and m (FN/g) is the slope of the best-fit line, as determined by linear regression analysis of the data.

The value of m, then, is a convenient measure of magnet strength. Magnet strength is inversely proportional to the value of m. A larger value of m means a weaker magnet, while a smaller value of m means a stronger magnet. All but one of the 21 magnets examined had values of m within the range of 5.0 +_ 0.5 FN/g, and the average for all 21 was 4.95 FN/g. Each individual magnet 3 was then used with the beam balance and that magnet's own special calibration line to measure the Ferrite Numbers of three weld metal "unknowns," to examine reproducibility of measurement as compared to the requirements of ANSI/AWS A4.2-86. The scatter of results was found to comply with those requirements. A slight tendency was noted in this study (Ref. 19) for

a weaker magnet to produce a higher FN measurement on the weld metal "unknowns" than would a stronger magnet, as can be seen in Fig. 4. This tendency provided justification for rather narrowly defining the required magnet strength. Accordingly, a magnet strength require- ment of 5.0 +0.5 FN/g was incorporated into ISO 8249.

If one plots Ferrite Number, assigned by either ANSI/AWS A4.2-86 or ISO 8249, vs. coating thickness of the primary standards (the relationship is identical in the two standards), one obtains what ap- pears to be something of an inverse relationship - - Fig. 5. The curvature of this relationship makes extension of the calibration beyond 28 FN uncertain. But, as observed in the study of 21 magnets, if one plots Ferrite Number vs. tear-off force, one obtains a straight line relationship, passing through the origin, with very little scatter about the regression line - -

28-s I JANUARY 1997

Fig. 6. This ability to define a straight calibration line for a MagneGage Number 3 Magnet solely by its slope, m, over the range of 0 to 28 FN, as defined by the coating thickness standards of ANSI/AWS A4.2-86 and ISO 8249, leads directly into the notion of extending the calibration line beyond 28 FN by extrapolation. If this could be done, then the extension to higher Ferrite Numbers would be continuous with the calibration up to 28 FN, which was considered very desirable.

Extension of the Ferrite Number Sys- tem by Extrapolation

Extrapolation of the FN system beyond 28 FN was investigated, both with a laboratory beam balance using Number 3 Magnets taken from several MagneGages and with the MagneGages themselves (Ref. 20). Originally, the measurements so made were reported as extended Ferrite Numbers (EFNs), but it was not long before the WRC Subcommittee on Welding Stainless Steels and IIW Subcommission II-C agreed to apply the term Ferrite Num- ber to these measurements, and for sim- plicity, that term is used in this review.

Extrapolation of the calibration line, when a laboratory beam balance is used for tear-off force measurements, is con- ceptually straightforward. But, in practice, it is not so easily done because the beam balance, as a means of determining tear-off force, turns out to be very sensitive to premature magnet detachment from the sample, due to very minor vibrations. Even over the range of 0 to 28 FN, premature magnet detachment was a problem that required many determinations at a given location on the sample and accepting only the highest observed tear-off force as the true value for that location on the sample. First attempts to measure Ferrite Numbers above 28 FN with a beam balance turned out to be dif- ficult, requiring many repeat measurements before concluding that the highest value at a given magnet placement had indeed been found. Nevertheless, the method did permit measurements even on fully ferritic samples (100% ferrite) (Ref. 20).

With a MagneGage, over the range of 0-28 FN, little tendency was observed for premature magnet detachment due to small vibrations (Ref. 20). In its normal condition, it can apply a force from 0 to about 6 g. The upper limit is imposed by a stop in the mechanism that prevents loading the torsion spring beyond its elastic limit. However, there is a small hole in the balance beam, on the side of the fulcrum opposite the point of magnet attachment, that permits a counterweight to be added easily. Figure 7 shows a Mag- neGage with such a counterweight added. This counterweight can be used

Table 1 - - Summary of WRC Round Robin of Ferrite Measurements, after MagneGage Calibration by Extrapolation, on Duplex Stainless Steel Weld Metals (Ref. 21)

Sample number 9294-988-5 9294-988 9276-985-1 9276-904-5 8483

Interlaboratory 27.0 33.0 48.5 57.9 65.5 average FN

Highest 29.0 35.5 52.7 63.0 72.1 laboratory FN

Lowest 24.7 28.0 45.7 52.2 57.6 laboratory FN

Interlaboratory 4.3 7.5 7.0 10.8 14.5 range, FN

2 Standard 2.6 4.4 4.4 6.2 9.7 deviations (SD), FN

Percent 9.2% 15.7% 9.1% 10.7% 14.8% variability (a)

(a) Percent variability = (100 x 2 SD's)/(Interlaboratory average FN).

to change the range of tear-off forces applicable. Kotecki (Ref. 20) developed a procedure to measure the tear-off force without a counterweight, as well as with addition of various counterweights. This procedure involved hanging various ferromagnetic weights from the magnet and determining the corresponding instrument reading at balance. By judicious choice of counterweight (about 8 g), the range of tear-off forces could be changed to 6-12 g, corresponding to about 30-60 FN. Another counterweight (about 16 g) could make the range of force be 12-18 g, corresponding to about 60-90 FN. (Note that, because the distance from the fulcrum to the counterweight attachment point is about three-quarters of the distance from the fulcrum to the magnet attachment point, the force applied to the magnet by the counterweight is only about three-quarters of the weight of the counterweight.) Use of a counterweight increased the tendency for premature magnet detachment, so that the practice of repeating measurements at a given location on the sample and accepting only the highest value as the true value for that location had to be adopted, though it was not nearly as time con- suming as with the laboratory beam balance.

With appropriate counterweights and five different MagneGages having magnets con- forming to the strength requirements of ISO 8249, measurements were made on a series of nine fully ferritic steels. Extrapolation to 100% ferrite would constitute the extreme of extrapolation in this extension. The range of FN determinations (average for each magnet) on each of the nine samples varied by no more than +7.5% of the average value for

all five MagneGages on that sample. A sixth MagneGage, with a considerably weaker magnet than ISO 8249 would allow (m -- 7.97 FN/g), consistently produced much higher FN values than did the other five MagneGages (Ref. 20), con- firming the trend to higher FN with weaker magnets noted previously. So long as the magnet met the strength requirements of ISO 8249, the procedure then appeared to offer reasonable reproducibility even when extrapolation was carried out to the extreme of 100% ferrite.

All of the above work was, however, done by a single person. To evaluate interlaboratory reproducibility of measurements using the extrapolation procedure with counterweighted MagneGages, the Welding Research Council Subcommit- tee on Welding Stainless Steels (Ref. 21) and the IIW Subcommission II-C (Ref. 22) organized separate round robin studies of measurements on samples of duplex stainless steel weld metals. In the WRC round robin, five laboratories, using a total of nine MagneGages, provided results, which are summarized in Table 1. It can be seen that the percent variability (twice the standard deviation, divided by

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Fig. I I - - Ferrite measurement points on sample blocks machined from centrifugally chi l l cast stainless steel rings.


Table 2 - - Summary of I IW Subcommission II-C Round Robin of Ferrite Measurements, after MagneGage Calibration by Extrapolation, on Duplex Stainless Steel Weld Metals (Ref. 22)

Sample A B C D E F G H I

Interlaboratory average FN 29.6 33.8 38.8 44.0 48.9 57.9 59.7 64.9 67.6 Interlaboratory standard 2.1 2.1 2.7 3.6 5.2 7.3 7.4 8.9 8.3

deviation (SD), FN Percent variability (a) 7.1% 6.2% 7.0% 8.2% 10.6% 12.6% 12.4% 13.7% 12.3%

(a) Percent variability = (100 x 2 SD's)/(Interlaboratory average FN).

the inter laboratory average, and expressed as a percent) is as high as 15%, appreciably greater than that observed above with a single operator and a number of MagneGages.

The IIW round robin results are summarized in Table 2. In this round robin, the maximum percent variability is only sl ightly smaller than that in the WRC round robin, 13.7%. Note that the samples evaluated in the IIW round robin pro- ject are not the same samples as were evaluated in the WRC round robin. The results of these two round robins do establish a basis for understanding the interlaboratory reproducibil ity of measurements made with MagneGages calibrated by use of the extrapolation procedure. However, it would clearly be desirable to improve the interlaboratory reproducibility of ferrite measurements made on duplex stainless steel weld metals.

Extension of the Ferr i te N u m b e r System by Coat ing Thickness Standards

Lake (Ref. 23) originally proposed that th inner coating thickness standards might be found than those used for calibrating MagneGages over the range of

0-28 FN, and these thinner standards might be used to calibrate MagneGages for ferrite measurements above 28 FN. Seven such standards were obtained (Ref. 24), with coating thicknesses rang- ing from 0.143 mm (0.0056 in.) to 0.0074 mm (0.00029 in.). With four different MagneGages (whose Number 3 Magnets complied with the ISO 8249 strength requirements), calibrated by use of extrapolation, FN measurements were made on each standard, and the average value of the four MagneGages was taken as the "true" FN for that coating thickness standard. The highest average FN so determined was 152, and the greatest difference from that value, among the four MagneGages, was about 8 FN (5.3% of the average value), so the reproducibil i ty of these "true" FN values was considered to be quite high.

Then these "true" FN values for thin coatings were used, along wi th 13 thicker coating thicknesses and their FN values assigned by ANSI/AWS A4.2-86, to try to devise, by regression analysis, a mathematical relationship between coating thickness and FN that would encompass the 0-28 FN range and Ferrite Num- bers above 28 FN wi th a single continuous mathematical relation. Such

Table 3 - - Calculated FN, Using Equation 2 or Equation 3, vs. Measured FN or FN from ANSI/AWS A4.2 (Ref. 24)

Coating Thickness Standard Calculated FN Using Measured or mm mils (0.001 in.) Equation 2 or 3 Assigned FN FN Difference

0.0074 0.29 153.3 152.2 -1.1 0.0127 0.50 132.8 134.9 2.1 0.0257 1.01 97.9 97.9 0.0 0.0358 1.41 81.9 82.1 0.2 0.048 1.90 68.9 67.5 -1.4 0.084 3.31 48.5 48.4 -0.I 0.143 5.62 33.7 34.3 0.6 0.204 8.05 25.8 26.1 0.3 0.178 7.0 28.7 28.3 -0.4 0.203 8.0 26.0 25.9 -0.1 0.229 9.0 23.7 23.8 0.1 0.254 10.0 21.8 22.0 0.2 0.305 12.0 18.8 18.9 0.1 0.381 15.0 15.6 15.6 0.0 0.508 20.0 12.1 12.0 -0.1 0.762 30.0 8.2 8.1 -0.1 1.016 40.0 6.0 6.0 0.0 1.270 50.0 4.6 4.6 0.0 1.524 60.0 3.7 3.7 0.0 1.778 70.0 3.0 3.0 0.0

a relation would al low the appropriate FN to be assigned to any coating thickness standard, and would permit the cal- ibrat ion table for MagneGages in ANSI/AWS A4.2-86 to be expanded to cover the needs of duplex ferri t ic- austenitic stainless steels. Essentially by trial and error, a satisfactory relation was found, equating the natural logarithm of the FN to a fourth order polynomial in the natural logarithm of the coating thickness (Ref. 24).

In S.I. units, this relation is

In(FN) = 1.8059 - 1 .11886[ In(T)] - 0 .17740[ In(T) ] 2 - 0.03502[In(T)]3 - 0.00367[In(T)]4 (2)

where T is the coating thickness in mm. In US customary units, this relation is

In(FN) = 4.5891 - 0.50495[In(T)] - 0.08918[In(T)]2 + 0.01 917[ln(T)]3 - 0.00371 [In(T)] 4 (3)

where T is the coating thickness in mils (i.e., increments of 0.001 in.).

Either Equation 2 or Equation 3 can be used to calculate the FN for all of the coating thicknesses used in developing these equations, and the results can be compared wi th the "true" FN values measured for the thinner coating thickness standards, and with the assigned FN values for the thicker coating thickness standards of ANSI/AWS A4.2-86. Table 3 (Ref. 24) does this. The agreement between calculated FN values and either the measured or A4.2 FN values is excellent [R 2 = 0.9999 for Equation 2 or 3]. Figure 8A plots Equation 2 over the full range over which the coating thickness standards could be applied. Figure 8B re- plots Equation 2 in terms of FN vs. coating thickness. Both curves of Fig. 8 are smooth, without inflection points, which seems sensible from the point of v iew of the physical situation.

Two more round robins were organized to examine more fu l ly the approach of calibration of MagneGages by coating thickness standards for ferrite determinat ions in duplex stainless steel weld metals. In the WRC Subcommittee on Welding Stainless Steels the same samples used in the WRC extrapolation

30-s I JANUARY 1997

Table 4 - - Coating Thickness Standards Used for Calibration of MagneGages in WRC Round Robin of Ferrite Measurements on Duplex Stainless Steel Weld Metals (Ref. 21)

Nonmagnetic Coating

Thickness Standard mi ls Assigned Identification mm (0.001 in.) FN

2619-CD 0.204 8.05 25.8 1311-A 0.151 5.9 32.5 2619-CC 0.143 5.62 33.7 1311-B 0.137 5.4 34.7 1311-C 0.132 5.2 35.6 1311-D 0.089 3.5 46.8 2619-CB 0.084 3.31 48.5 1310-E 0.080 3.1 50.7 1310-F 0.063 2.5 58.2 1310-G 0.056 2.2 63.0 1310-H 0.050 2.0 66.8 4560-AD 0.048 1.90 68.9 1310-1 0.043 1.7 73.6 1306-J 0.041 1.6 76.2 2619-CA 0.036 1.41 79.1

round robin (Ref. 21) were reevaluated by the same participants, using the same MagneGages. For this calibration, an as- sortment of coating thickness standards, outside the common ranges, were obtained from the U.S. National Bureau of Standards. Their identifications, nonmagnetic coating thicknesses, and Ferrite Numbers assigned by Equation 2 or Equation 3 are listed in Table 4. These coating thickness standards were provided to each participant in the round robin, to be used in a second calibration of the various MagneGages. Then the Ferrite Numbers of the five weld metals of Table 1 were determined from this new calibration. The results are summarized in Table 5. Comparison of Table 5 with Table 1 shows that calibration by use of coating thickness standards did not change the interlaboratory average FN by a significant amount for any of the five

Table 5 - - Summary of WRC Round Robin of Ferrite Measurements, after MagneGage Calibration Using Coating Thickness Standards, on Duplex Stainless Steel Weld Metals (Ref. 21)

Sample number 9294-988-5 9294-988 9276-985-1 9276-904-5 8483

Interlaboratory average FN 26.7 32.5 48.1 57.9 65.0 Highest laboratory FN 27.5 35.0 50.4 62.3 73.0 Lowest laboratory FN 26.0 29.4 46.6 52.9 61.4 Interlaboratory range, FN 1.5 5.6 3.8 9.4 11.6 2 Standard deviations (SD), FN 1.0 3.3 2.0 4.3 6.9 Percent variability (a) 3.7% 10.2% 4.2% 7.4% 10.6%

(a) Percent variabil i ty = (100 x 2 SD'sV[Interlaboratory average FN),

duplex stainless steel weld metal samples (the maximum change was 0.5 FN). However, calibration by use of coating thickness standards markedly reduced the percent variability as compared to calibration by extrapolation, for all five samples, as shown in Fig. 9. It could be concluded that a significant improve- ment in interlaboratory reproducibility was therefore obtained by calibration with coating thickness standards as compared to calibration by extrapolation.

The WRC round robin of measurements after calibration by use of coating thickness standards only involved a single set of coating thickness standards, as detailed in Table 4. So a possible contri- bution to interlaboratory variability was lacking: variability due to different sets of coating thickness standards. This was addressed by a new IIW round robin (Ref. 22). As a result of the successful im- provement in interlaboratory reproducibil i ty observed in the WRC round robins when calibration was done with coating thickness standards as compared to extrapolation, the author had per- suaded the U.S. National Institute of Standards and Technology (NIST, formerly the National Bureau of Standards) to offer sets of coating thickness standards to cover duplex stainless steel weld metal needs (above 28 FN). These sets are detailed in Table 6 (Ref. 25).

Each round robin participant was in- structed to obtain sets of these coating thickness standards from NIST, so that each used a different group of standards for MagneGage calibration. Nine laboratories from six countries participated in this round robin, and the same nine duplex stainless steel weld metal "unknowns" of the IIW round robin of measurements after calibration by use of ex- tr~polation (Table 2) were evaluated. Thv new round robin results are detailed in Table 7. Comparison of these results to those of the WRC round robin (Table 5), shows that the largest percent variability is the same in both calibration by coating thickness standards round robins, 10.6% of the interlaboratory average at about 65 FN interlaboratory average. So it could be concluded that calibration with different sets of standards in different laboratories did not detract from interlaboratory reproducibility, as compared to the WRC round robin that used only one set of coating thickness standards for all participants.

The samples used in the IIW round robin summarized in Table 7 are the same samples used in the earlier IIW round robin summarized in Table 2. Therefore, a direct comparison can be made between Table 2 measurements (calibration by extrapolation) and Table 7 measurements (calibration by coating

Fig. 12 - - A set o f the new ferrite secondary standards made f rom chi l l cast rings.

190

~ 180

170 .=

" 160 ~e o

15o

,z 140

130 60

o

e'5 7'5 80 8's o'o ,'5 loo % Iron in Alloy

Fig. 13 - - Ferrite Number o f 100% ferrite steels (Ref. 20).


Table 6 - - NIST Coating Thickness Standards Sets Applicable to Duplex Stainless Steel Weld Metals (Ref. 2 5 ) (a)

Set SRM 1323 SRM 1322 SRM 1321

Nonmagnetic coating thickness 0.001 mm 174 135 113 95.8 79.8 70.3 60.2 54.4 45.6 38.9 37.9 35.1 0.001 in. 6.86 5.31 4.44 3.77 3.14 2.77 2.39 2.14 1.79 1.53 1.49 1.38

Assigned FN 29.1 35.1 40.2 44.5 50.2 54.5 59.9 64.1 71.4 78.2 79.4 82.9

(a) Since 1988, when the above sets were available, NIST has changed both the number of sets available and the target thickness of individual coatings. For the current situation regarding coating thickness standards, it is appropriate to contact the National Institute of Standards and Technology, Standard Reference Materials Program, Gaithersburg, MD 20899-0001. (TeLephone: 1-301-975-6776). Telefax: 1-301-948-3730. E-mail: SRMINFO[@enh.nist.gov.)

Table 7 - - Results of I IW Round Robin of Duplex Stainless Steel Weld Metal Ferrite Measurements after MagneGage Calibration with Individually Obtained Sets of Coating Thickness Standards (Ref. 22)

Sample number A B C D E F G H 1

Interlaboratory average FN 30.0 33.9 39.0 44.7 49.4 58.3 61.1 65.4 68.5 Highest laboratory FN 32.8 37.4 40.6 47.9 54.4 62.8 65.0 71.8 75.1 Lowest laboratory FN 28.7 32.4 37.6 43.5 46.9 54.5 58.8 58.7 63.6 Interlaboratory range, FN 4.1 5.0 3.0 4.4 7.5 8.3 6.2 13.1 11.5 2 Standard deviations (SD), FN 2.2 2.6 2.0 2.9 4.0 4.3 3.5 6.9 6.4 Percent Variability (a) 7.3% 7.7% 5.1% 6.5% 8.1% 7.4% 5.7% 10.6% 9.3%

(a) Percent variability = (100 x 2 SD's)/(Intedaboratory average FN).

thickness standards). These two round robins were not conducted at the same time, but more than a year apart. This comparison, as in the two WRC round robins, shows no significant change in the average FN value reported for any of the "unknowns" (maximum change 1.4 FN for sample G). However, the percent variability is reduced for seven of the nine samples, as can be seen readily in Fig. 10.

As a result of the WRC round robin of ferrite measurements on duplex stainless steel weld metals with MagneGages calibrated by use of coating thickness standards, ANSI/AWS A4.2-91 was prepared by the WRC Subcommittee on Welding Stainless Steels, to incorporate Equations 2 and 3 into the standard and to expand the calibration range up to about 90 FN (Ref. 26). Table 9 in that standard indi- cates a percent variability of +10% can be expected from 25 up to 90 FN, on the basis of the WRC round robin of Table 5. The IIW round robin of Table 7 offers further support for this level of variability.

Secondary Standards for Ca l ib ra t ion of O t h e r Ins t ruments

As noted in the Background section, instrument calibration by use of coating thickness standards (primary standards) is very specific to a particular instrument with a particular magnet or other probe. This is due to the fact that the distribution of ferromagnetic material in a given coating thickness standard is very different from the distribution of ferrite in a nominally austenitic or duplex ferritic- austenitic stainless steel weld metal. An alternate approach, first envisioned by

the AWS A4.2-74 standard, is to use an instrument (or, better, several instruments), calibrated by use of primary standards, to assign Ferrite Numbers to a series of metal samples of various ferrite contents, each of which has a morphology of ferrite that is similar to the weld metals to be measured after calibration. These samples would then in turn be used as "secondary standards" for calibrating a host of other instruments. Be- cause the ferrite morphology is similar to that of the weld metals to be subsequently measured, the Ferrite Numbers assigned to these secondary standards would be independent of the instrument to be subsequently calibrated. In particular, it would not be necessary that the instruments to be calibrated with these secondary standards sense the same volume of the secondary standard, provided that the ferrite distribution in the secondary standard was reasonably homogeneous within the volume to be sensed.

Secondary standards, machined from multiple-layer shielded metal arc welding (SMAW) deposits and covering from nearly 0 FN to about 28 FN, were offered for commercial sale by The McKay Com- pany (which became Teledyne McKay) until about 1980. Some of the McKay secondary standards consisted of only one weld pass per layer, while others consisted of three passes per layer. It was recognized that there was some variation in FN from point to point on the machined surface of these standards (Ref. 27), so it was necessary to precisely mark the points for instrument probe placement on this surface to obtain a valid calibration. In particular, the interface between two weld passes in the same layer

was to be avoided because the FN at that interface was lower than that at other points on the machined surface. Ferrite Numbers were assigned to these standards by taking the average FN measured with two different calibrated Magne- Gages at the various marked points on the machined surface. These secondary standards provided sufficiently accurate calibration of other instruments that measurements made after calibration were interchangeable with those made directly by MagneGages calibrated with primary standards.

In the 1980s, the International Insti- tute of Welding (IIW) offered secondary standards covering near 0 FN to about 28 FN, prepared from submerged arc welding (SAW) strip clad weld overlays by B6hler Schweisstechnik (Austria), and assigned Ferrite Numbers and distributed by The Welding Institute (United Kingdom). These secondary standards consisted of only one weld pass per layer, and their machined surfaces were found to be more homogeneous than the McKay secondary standards (Ref. 27). TWl assigned Ferrite Numbers to these secondary standards as the average of readings obtained by two different operators, each using two MagneGages calibrated by using primary standards (Ref. 10). One hundred such sets were distributed worldwide by TWl. The supply was exhausted in 1989. In view of the now broadened range of desired ferrite calibration (up to nearly 100 FN) brought about by the duplex stainless steel interests, it seemed apparent that more levels of ferrite would be needed as secondary standards.

Commission II of the IIW set out to de-

32-s I JANUARY 1997

Table 8 - - Results of I IW Round Robin of Duplex Stainless Steel Weld Metal Ferrite Measurements after Feritscope Model MP-3 Calibration with Individually Obtained Sets of Secondary Standards (Ref. 32)

Sample

K L E C G F B M H A J N D

Laboratory Ferrite Number A 4.1 8.0 14.1 20.4 24.9 30.3 46.4 50.4 52.4 62.3 66.3 77.7 103.0 B 4.4 7.9 14.6 19.4 26.7 35.8 50.6 51.2 59.2 65.7 75.7 76.7 99.0 C 4.4 7.8 14.4 19.7 25.8 32.2 46.8 50.8 53.8 65.4 69.7 79.4 95.0 D 3.8 7.7 15.4 20.2 26.3 32.6 46.2 50.0 52.3 68.9 73.9 78.2 92.8 E 4.0 7.6 16.4 21.5 28.4 33.8 48.6 57.3 59.9 67.7 69.8 80.4 96.4

Interlaboratory Summary Average FN 4.1 7.8 15.0 20.2 26.4 32.9 47.7 51.9 55.5 66.0 71.1 78.5 97.2 Standard deviation, FN 0.23 0.14 0.83 0.73 1.16 1.82 1.67 2.71 3.34 2.26 3.34 1.30 3.51 Percent variability 11.3 3.6 11.1 7.2 8.8 11.1 7.0 10.4 12.0 6.8 9.4 3.3 7.2

velop a new source of secondary standards to address these needs. The Russ- ian Delegation came forward with a pro- posal to provide large numbers of samples, wh ich could become secondary standards, in the form of rectan- gular blocks wi th weld-metal- l ike microstructure, machined from centr i fugal ly chi l l cast stainless steel rings (Ref. 28). Each ring was about 500 mm (20 in.) in outer diameter and length, wi th a 20-mm (3/4-in.) wal l thickness. The inside and outside surfaces of the ring were machined away so that the samples were taken from the central part of the ring wall thickness. With this approach, a large number of samples, of nearly the same ferrite content, could be machined from a single chil l cast ring. Samples from a few rings of various ferrite contents were offered to IIW Sub- commission II-C for evaluation. A round robin was conducted on these samples within II-C to evaluate reproducibil i ty of ferrite measurement by calibrated Mag- neGages and to examine homogeneity of the samples that were provided as 10-12

mm (0.4-0.5 in.) thick (ring wall thickness direction) by 12 mm (0.5 in.) wide (ring wall circumference direction) by 20 mm (0.8 in.) long blocks. In the round robin, each participant made measurements at the face center and at the quarterdiagonal points of all six faces of the samples - - Fig. 11. No significant difference was found between the face center measurements and the quarterdiagonal measurements, or from one face to another, so that the sample homogeneity was concluded to be excellent (Ref. 22). In addition, the microstructure of these blocks was examined and was confirmed to be very similar to that of weld metal (Ref. 29). A few casting defects (pores and cold laps) were discovered in some samples, and improvements were subsequently made in the casting procedure to eliminate these. New rings were cast and confirmed by radiography to be sound. This provided sufficient ferrite levels for two sets of secondary standards. One type was to consist of eight standards distributed over the range of about 1-30 FN, for calibration of instruments to measure

ferrite in nominal ly austenitic stainless steel weld metals. The second type of set was to consist of eight standards distributed over the range of about 30-100 FN, for calibration of instruments to measure ferrite in duplex ferritic-austenitic stainless steel weld metals.

At the beginning of 1994, l lW Com- mission II undertook to raise some $60,000 in the form of loans, mainly from the various weld ing f i l ler metal manufacturers around the world, to help fund the Russian work. The loans were to be repaid, in part by transfer of com- pleted sets of standards to those lenders who so chose, and in part by proceeds from sales of sets of standards. Ten fil ler metal manufacturers and the Nickel De- velopment Institute responded to the call with loans, and the funding goal was met in 1994 (Ref. 30). Enough samples to provide 55 sets (17 in the below 30 FN range, and 38 in the above 30 FN range) were supplied to The Welding Institute for assignment of Ferrite Numbers. The same procedure of using two operators, each using two MagneGages to deter-

i O

45 40 z 35

"E 30 ~. 2s

20 m

i s lO

o 5 z = 0

• - 50

0 5 10 15 20 25 30 35 40 45 50 Ferrite Percent by ASTM E562 Point Counting

Fig. 14 - - "Normal ized" Ferrite Number vs. percent ferrite by ASTM E562 for CF8 and CF8M castings (Ref. 20).

120 IF

100Ii : , g

80

,0

20

Tensile 1 • • Yield

0 20 40 60 80 100 120

A l l o y 2 2 0 5 W e l d Me ta l FN

Fig. 15 - - Tensile and yield strength vs. FN for A l loy 2205 al l -weld metal (Ref. 40).

W E L D I N G RESEARCH SUPPLEMENT I 33-s

Table 9 - - Ferrite Numbers of 100% Ferrite Steels (Ref. 20)

Sample N o m i n a l Calculated Number Composition % Fe FN

95928 29Cr-4Mo-2Ni 64.1 138 8-4638 29Cr-4Mo 66.4 146 1783C 25Cr-5Ni-3~Mo 66.4 149 2125A 25Cr-3~Mo 70.6 154 1274E 21Cr-3Mo 74.2 158 31606 18Cr-2Mo 77.7 166 U2 18Cr-2Mo 78.3 166 9317-448 13Cr-~Ti 86.2 174 Ingot iron Fe 100 183

mine the assigned FN, was used as with the earlier strip clad secondary standards. These sets of standards began to be distributed worldwide at the beginning of 1995. Figure 12 shows one of the new sets of secondary standards. Additional materials to prepare more sets of secondary standards, as needed, are held in Russia.

IIW Commission 11 also undertook a new round robin of ferrite measurements, using the new secondary standards to calibrate instruments other than MagneGages, notably the Fischer Fer- itscope, Model MP-3, which has been applied rather widely to "ferrite percent" measurements in duplex stainless steel welds, although the "ferrite percent" calibration is uncertain. That round robin consists of measurement of Ferrite Num- ber in a series of thirteen weld metal "unknowns" distributed over the whole range of near 0 to about 100 FN. The ob- jective here is to establish the interlaboratory reproducibil i ty of FN measurements after calibration with the secondary standards (Ref. 31). That round robin is still underway, but prelim- inary results (Ref. 32), shown in Table 8, indicate interlaboratory reproducibility of FN measurement that is comparable to that of the MagneGages calibrated by using primary standards (Table 7).

F e r r i t e N u m b e r vs. Fe r r i t e P e r c e n t

As noted in the Background section, the Ferrite Number scale is arbitrary. It has been defined in terms of magnetic attraction to nonmagnetic coating thickness standards, although low FN values were once considered to approximate ferrite percent (Ref. 5). While it has not yet been possible to obtain agreement among numerous laboratories as to the true ferrite percent in weld metals, the relationship between Ferrite Number and ferrite percent has been explored (Ref. 20). As a point of departure, a series of fully ferritic steels (essentially 100% ferrite) were obtained, mostly as wrought "superferritic" stainless steels. The Ferrite

Table 10 - - Ferrite Measurements on Cast CF8 and CF8M Alloys (Ref. 20)

Ferrite FN of 100% Percent

Alloy Ferrite of Normalized by ASTM Sample Type % Fe FN Same % Fe FN E562

45T CF8M 64.41 0.2 142.0 0.14 0.23 6882 CF8 68.34 5.1 149.0 3.42 4.40 6851 CF8M 66.27 8.5 145.3 5.85 7.04 3492 CF8 67.12 12.5 146.8 8.51 8.98 3496 CF8M 68.84 22.9 149.9 15.28 17.16 3491 CF8M 65.69 30.1 144.3 20.86 20.85 3485 CF8 65.67 32.0 144.2 22.19 22.46 3494 CF8 69.38 33.4 150.8 22.15 23.94 3499 CF8 68.91 39.0 150.0 26.00 27.98 3631 CF8M 70.04 59.4 152.0 39.08 35.56 3488 CF8M 65.70 54.2 144.3 37.56 40.78 3611 CF8 64.16 57.8 141.6 40.82 41.30 3501 CF8 69.12 58.8 150.4 39.10 42.43 3489 CF8M 63.83 65.4 141.0 46.38 45.45 3495 CF8M 64.99 64.2 143.0 44.90 48.64

Number of each was measured with the beam balance procedure described above, and it was noted that the Ferrite Number of 100% ferrite depends strongly upon the iron content of the given sample. The results obtained are shown in Table 9 and Fig. 13. From these results, it can be concluded that there is not a simple relationship between Ferrite Number and ferrite percent - - the relationship depends upon the composition of the ferrite.

Lyman (Ref. 33), as well as others, has shown that the composition of the ferrite in a stainless steel weld metal is considerably different from the nominal composition of the alloy. The ferrite is en- riched in chromium and other ferrite-stabilizing elements, whi le it is depleted in nickel and other austenite- stabilizing elements, as compared to the nominal composition of the weld metal. However, the iron content of the ferrite turns out to be nearly the same as the nominal iron content of the weld metal. So, at least to a first approximation, it can be assumed that the Ferrite Number of 100% ferrite, of the same composition as the ferrite in a given weld metal, would be the same as the Ferrite Number of an alloy of 100% ferrite of the same iron content as the nominal iron content of the weld metal. This assumption provides a basis for "normalizing" the Ferrite Number of a sample against the Ferrite Number of 100% ferrite of the same iron content to obtain an estimated ferrite percent (Ref. 20).

As noted in the Background section, it has been impossible to obtain agreement among various laboratories as to the ferrite percent in a weld metal by metallographic measurement due to the fine- ness, irregular shape and nonuniform etching of the ferrite. Brantsma and Ni- jhof (Ref. 18), in a relatively recent round robin of ferrite measurements, reinforced

this finding and concluded that Ferrite Numbers were clearly preferable to "ferrite percents" for determination of ferrite in duplex stainless steel weld metals. Such is not the case, however, with cast alloys, in which the ferrite is much coarser and more regularly shaped than in weld metal. The Steel Founders Soci- ety produced a series of 15 Type CF8 and CF8M castings in which the ferrite percent had been determined by the metal- Iographic point counting method of ASTM E562 (Ref. 34). The Ferrite Num- ber of each sample was determined by use of the balance beam method with a Number 3 Magnet from a MagneGage, and this Ferrite Number was then "normalized," using the composition of the casting and the data of Fig. 13 (Ref. 20). These results, along with the ferrite percent 4 determined by the Steel Founders Society, are reproduced in Table 10. It can be seen that "normalization" amounts to dividing the Ferrite Number by a factor of about 1.5 for the CF8 casting, and 1.4 for the CF8M castings considered herein. The "normalized" Ferrite Numbers are plotted against ferrite percent in Fig. 14. The numerical agreement between "normalized" Ferrite Number and ferrite percent for these castings is re- markable. The regression line included in Fig. 14 reflects an almost perfect 1:1 relation between metallographic ferrite percent (FP) and "normalized" Ferrite Number ("normalized" FN = 0.97.FP, with correlation coefficient R 2 = 0.987).

While it is tempting to accept "normalization" as a means of converting Fer- rite Numbers to ferrite percents, there is no assurance that this relationship wil l

4. Note that, for castings, where the ferrite content can be determined with known preci- sion following ASTM E 562, ferrite percent is used without quotation marks.

34-s [ JANUARY 1997

40

3O

o = 20

==

10 Q .

=:

0 20 40 60 80 100 120 Alloy 2205 Weld Metal FN

Fig. 16 - - Tensile ductility vs. FN for Alloy 2205 all-weld metal (Ref. 40).

o 8O ¢O

'T 7O

~ 6O

~ 5o ~ 4o

e -

N 3O 0

~ 2o

m , ,c u 0

• o

t •

0 20 40 60 80 100 120 Alloy 2205 Weld Metal FN

Fig. 17 - - Charpy V-notch energy at ~16 °C vs. FN for Alloy 2205 all-weld metal (Ref. 40).

hold up for weld metal as it did for castings. Shinozaki, et al. (Ref. 35), corre- lated metallographic "ferrite percent" (FP) with Ferrite Numbers for duplex stainless steel weld metals, above about 25 FN up to nearly 150 FN and obtained a relation that converts to

FN = 1.67.FP- 10 (4)

Liljas and Qvarfort (Ref. 36), considering Alloy 2205 weld metals of 41 to 110 FN, obtained a slightly different relation, which converts to

FN = 1.69.FP- 7.6 (5)

Both Relation 4 and Relation 5, though quite consistent with one another, are problematic from the point of view of the physical world, because zero Ferrite Number would not correspond to zero "ferrite percent." Recall again the uncer- tainty of determining "ferrite percent" in weld metal, as noted above.

Another approach to relating Ferrite Number to "ferrite percent" is via magnetic saturation measurements, as has been attempted by Merinov, et al. (Refs. 37, 38). This method also requires taking composition of the ferrite into account. They conclude that, for 19Cr-9Ni weld metal,

FN = 1.7.FP (6)

Because their weld metal would be similar in composition to the CF8 castings, their factor of 1.7 should be compared to the factor of about 1.5 obtained (Ref. 20) for CF8 castings. The result of Merinov, et al., is reasonable from a physical point of view, because 0 FN corresponds to 0 FP. This approach requires an adjustment in the relation between FN and FP depending upon composition of the metal.

Still another approach to relating Fer-

rite Number to ferrite percent is by selectively dissolving the austenite electro- chemically, then weighing the residual ferrite and converting the result to volume ferrite percent. Gill, et al. (Ref. 39), did this with four Type 316 weld metals. Averaging their results leads to

FN = 1.45.FP (7)

While the various correlations agree that the FN overstates the ferrite percent, they do not agree on the amount of the overstatement. Subcommission II-C of the IIW plans further examination of the relation between FN and FP.

Correlation of Ferrite Number with Duplex Stainless Steel Weld Properties

If there were no correlation between weld properties and ferrite content, then ferrite determination might be only an academic exercise. Lefebvre (Ref. 3) notes that both strength and resistance to stress corrosion cracking may be reduced when there is less than 30 FN in duplex stainless steel welds.

Connections between high Ferrite Numbers and various properties of duplex stainless steel weld metals have been considered by several investigators. Loss of all-weld-metal ductility to below minimum requirements of ANSI/AWS A5.4 was noted for E312-16 weld deposits above about 80 FN (Ref. 20). Loss of both weld metal ductility and toughness were noted for Alloy 2205 (UNS $31803) and Alloy 255 (UNS $32550) duplex stainless steel weld metals above about 70 FN (Ref. 40). Figures 15, 16, and 17 show Alloy 2205 all-weld-metal strength, tensile ductility and Charpy V- notch toughness at -46°C (-50°F), re-

spectively. The strength is not affected by FN (Fig. 15), but the ductility (Fig. 16) and toughness (Fig. 17) are strongly affected. Similar results were obtained for Alloy 255 weld metals in this same study. Fur- thermore, Shinozaki, et al. (Ref. 35), found diffusible hydrogen damage in Alloy 2205 welds above about 70 FN but not in welds of lower FN.

Hoffmeister and Lothongkum (Ref. 41 ) examined the pitting potential of sim- ulated weld heat-affected zones in 22% Cr and 25% Cr duplex stainless steels. They showed chromium nitride precipitation occurring in ferrite when the cooling rate was rapid enough to suppress most austenite formation. With more than 50-60 "ferrite percent" (which possibly corresponds to about 70-85 FN), pitting resistance was severely degraded.

Ferrite measurements can also be use- ful for evaluating the success of postweld annealing of a duplex stainless steel weldment. Kotecki (Ref. 42) demon- strated that annealing weldments of Alloy 2205 or Alloy 255 at 1040°C (1900°F), while satisfactory for the 5% Ni base metal, can result in extensive transformation of the 9% Ni weld metal ferrite to sigma phase. Because the sigma phase is nonmagnetic, a ferrite measurement after a sigmatizing "anneal" will reveal little or no ferrite in the weld. Such welds were shown to be extremely brittle. Non- embrittling annealing procedures were developed, which resulted in nearly the same weld metal FN after annealing as before annealing for 9% nickel weld metals. Lower nickel weld metals showed some ferrite reduction as a result of annealing, which successfully avoided sigma formation, but nothing like the ferrite loss due to improper annealing, which produced extensive sigma phase formation.

There is clearly a quality assurance


role to be played by Ferrite Number determination in duplex ferritic-austenitic stainless steel weld metals.

Unfinished Business

Despite more than 50 years of atten- tion to the determination of ferrite in stainless steel weld metals, it is clear that there are still several questions that need answers. The relationship between Fer- rite Number and ferrite percent calls for further study, though it does not prevent the Ferrite Number system from being used effectively. There is ample experi- ence and experimental data to support setting appropriate FN limits for a variety of weldment applications (Ref. 3), which can satisfy the engineer. But the scientist wants to know the ferrite percent and how it relates to the Ferrite Number. The centrifugal chill cast secondary standards, produced by the Russian Delega- tion to IIW Commission II, may provide a means of finding the relation. In contrast to other secondary standards that have been produced over the years, these are very homogeneous. So a surface FN measurement, and a volumetric ferrite percent measurement by magnetic saturation, may be made in which the material sampled in each case is the same. Previous multipass weld secondary standards always contained reheated zones known to be lower in ferrite than the last weld pass, which may corrupt relation- ships developed by Merinov, et al. (Refs. 37, 38). The composition of each chill cast secondary standard can also be determined and taken into account. As noted above, this remains in the work program of IIW Subcommission II-C.

Secondly, the Ferrite Number system is quite adequate for weld metal, but it is not usable in its present form for measurements in the heat-affected zone (HAZ) of duplex ferritic-austenitic stainless steel welds, because the HAZ is too thin. Often, the critical region of the HAZ is only a single ferrite grain thick. This is a matter of no small concern because the ferrite content in the HAZ ad- jacent to the weld interface can vary widely depending upon cooling rate. A rapid cooling rate results in very high ferrite content, which leaves nitrogen trapped in the ferrite. The trapped nitrogen subsequently produces chromium nitride precipitation in the ferrite, with severe loss of corrosion resistance (Ref. 41). At present, specifications call for metallographic estimation of HAZ ferrite content (Ref. 3). But this is a destructive test, and reproducibil ity is doubtful. While it can be used to indicate that a given welding procedure will produce satisfactory results, it cannot be used to demonstrate that the procedure was ac- tually followed in fabrication. The tiny

samples and the method used by Elmer and Eagar (Ref. 16) might be considered for sophisticated study of HAZ ferrite content in duplex stainless steel weldments, but this is also a destructive test. A nondestructive method of ferrite determination in the HAZ of duplex stainless steel welds is needed.

Thirdly, constitution diagrams for pre- dicting weld metal ferrite content need further development in two areas. One aspect concerns the adequacy of existing diagrams. There are a number of alloy elements that have not been considered in the most accurate diagram to date, the WRC-1992 Diagram. Silicon may not be adequately handled, and elements such as tungsten (which has been used in significant amounts in some newer duplex stainless steels) and titanium (which is used in Type 321 stainless steel) are not considered at all. The alloy element base of constitution diagrams needs to be broadened. Hull (Ref. 43) made a promising beginning, but the work was done in terms of "ferrite percent," so the relation to Ferrite Numbers is doubtful.

The second aspect of constitution diagrams needing consideration is that, to date, they have not well addressed cooling rate effects. David, et al. (Ref. 44), has considered cooling rate effects in nominally austenitic stainless steel weld metals using the Schaeffler Diagram (which is outdated) and have shown that, only at very high cooling rates obtained with laser welds, there is a significant difference between real behavior and the predictions of the original constitution diagram. Alloys such as Type 316, which normally solidify as primary ferrite and leave ferrite in the weld metal at room temperature, were found, under rapid cooling rates from laser welding, to solidify in a fully austenitic mode and be free of ferrite at room temperature, though cracking was not observed in these alloys. Lippold (Ref. 45) has subsequently shown cracking in pulsed laser welds when the solidification mode was changed from primary ferrite to primary austenite by more rapid cooling. But neither David, et al., nor Lip- pold, examined duplex stainless steel weld metals, in which changes in ferrite content are known to occur at much lower cooling rates. One might expect that there is a gradual transition, from compositions (e.g., Type 304 or 316 stainless) that are only affected by cooling rate at the very high cooling rates of laser welds to compositions (e.g., high- alloy duplex stainless steels) that show significant effects of cooling rate on ferrite content even under normal arc welding conditions. Cooling rate effects need to be considered more thoroughly in constitution diagrams for stainless steels.

Conclusions

Starting from the original Ferrite Num- ber system in place in 1974, significant progress has taken place. The method has been extended to encompass duplex ferritic-austenitic stainless steel weld metals and is published as ANSI/AWS A4.2-91. The interlaboratory reproducibility of measurement has been established at +10% of the interlaboratory mean FN, or better, by round robin tests. Primary standards and secondary standards for calibration of instruments are now available. Correlation of duplex stainless steel weld metal properties with Ferrite Numbers is established, providing a basis for specification of ferrite requirements.

Acknowledgments

A number of mentors and organizations have profoundly influenced my professional growth. At the top of the list are Professor Richard A. Moll and the late Professor Norman R. Braton, who were advisors to me at the University of Wis- consin in Madison. At Battelle Memorial Institute, Dr. David G. Howden and the late Robert E. Monroe were instrumental in further growth. Then, at Teledyne McKay, Paul Kammer, Ed Szuma- chowski, and, of course, Bill DeLong were very important to me. While all professional associations are important, the counsel, sharing, and support of the members of two associations have been particularly instrumental in the studies that underlie this progress report: the Welding Research Council Subcommit- tee on Welding Stainless Steels and the International Institute of Welding, Sub- commission II-C. Joel Feldstein, Frank Lake, Dr. Tom Siewert and Doug Vander- griff from the WRC Subcommittee and Dr. Pavel Merinov, Leo van Nassau, Dr. Gunther Rabensteiner and Dr. Trevor Gooch of the IIW Subcommission have been especially helpful. Dr. Siewert reviewed the draft of this manuscript during its development and made many helpful suggestions. NIST (formerly NBS) cooperated by making coating thickness standards available. Harold Bezanson, of MagneGage Sales and Service, and Peter Raffelsberger, of Fischer Technology, offered information about their companies' ferrite-measuring instruments and loaned instruments. I am indebted to these indi- viduals, to the organizations they repre- sented, and to The Lincoln Electric Com- pany, for giving me the professional guidance and the opportunity to explore the world of welding stainless steels to the fullest extent possible.

Peg Lawrence prepared some of the graphics presented herein. John McLane prepared the metallographic specimens

36-s I JANUARY 1997

and pho tomic rog raphs . Jenny Ogbo rn made the necessary photographs of instruments and standards.

Final ly, and most impor tan t l y , it wou ld not have been possible to devote the t ime and energy to these pursui ts w i t h o u t a suppor t ive fami ly . Kate, my love of more than 33 years, and my chi l - dren, Nathan and Amanda, prov ided the home life that made it possible.

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W E L D I N G RESEARCH SUPPLEMENT I 37-s

Documents

Ferrite Determination in Stainless Steel Welds Advances ...files.aws.org/wj/supplement/WJ_1997_01_s24.pdf · ISO 8249 Duplex Stainless Austenitic Stainless Ferrite Standards 1996