Ferite Check

7/28/2019 Ferite Check

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II – 1531 -04 (IIC-275-04)(IXH-590-04)

The Measurement of Ferrite Number (FN) in Real Weldments – Final Report

J C M Farrar

1. Introduction.The amount of ferrite in stainless steel weld metals can have an influence on a widerange of properties including corrosion resistance, toughness, long term high

temperature stability, resistance to hot cracking etc. It is therefore essential for someapplications that the ferrite level is controlled within specified limits and that methods

are available for checking that specified levels have been achieved (1).There are a number of ways of doing this, of which the most commonly used are:

a) Direct microscopic measurement on suitably prepared specimens. This is adestructive method, which cannot readily be used on a completed welded

fabrication, but it can be used on representative welding procedure samples. In principal, both manual and automatic image analysis techniques are available but

at higher ferrite levels, above about 20FN, it becomes increasingly difficult to

quantify the complex structures, which are present in the reheated regions of some

multipass stainless steel welds.

b) A magnetic method using equipment such as an instrument making use of a permanent magnet and measuring tearing-off force (e.g., a MagneGage) or an

instrument utilizing eddy current to measure magnetic properties (e.g., Fisher

Feritscope) and a probe, which is in direct contact with the specimen or weld

metal surface. This is a non-destructive method, which requires the use of

appropriate secondary standards in order to calibrate the equipment and enableaccurate measurements of FN to be made.

c) Prediction of FN can be carried out based on the chemical composition of the

weld metal. The most widely used predictive diagram, and the one recognised by

the ASME code is the WRC-1992 diagram (2).A number of other systems areavailable, including some based on Neural Networks. All these methods depend

on an accurate chemical analysis of the actual weld deposit. Certifiedcompositions of the welding consumable can be used, but it must be recognised

that these may well be original heat or cast analyses (in the case of solid wires androds) and will not necessarily be the same as the deposit composition, particularly

if there is significant dilution by parent materials and different welding

parameters have been used.

For the reasons given above it is widely accepted that direct measurements using suitablycalibrated magnetic instruments provide the most reliable assessment of the FN level in



stainless steel weld metals (3). The instruments cover the practically important range of 0to about 50FN, and even higher, and IIW certified secondary calibration standards to

cover this range are readily available from the National Institute of Standards and

Technology, Gaithersburg, MD, USA.In 1992 and 1993 round robins, to measure FN using magnetic instruments, were

completed by the members of IIW sub commissions II-C and IX-H. Ten laboratoriestested small, fairly homogenous samples of weld metal with a smooth surface. The FN

values of the different samples ranged from about 1 to about 105FN. Further details of the results of these round robins carried out are given in reference 4. The overall

conclusion was that the accuracy of measurement of FN was about ±10% of the measuredvalue. That means a nominal measurement of 5FN would be reported as about 4.5 to 5.5and a nominal value of 40FN would be reported as about 36 to 44FN by the various

laboratories. An ISO technical report (5) is available for those who require further background.

It must be emphasised that the above exercise was carried with small, idealisedspecimens with a reasonably homogenous microstructure and a smooth test surface. Real

welds, particularly multi-pass welds, are seldom homogenous and often have a rippled

rather than smooth surface. In1998, IIW sub commissions II-C and IX-H decided to carryout a further FN Round Robin using real weldments rather than small laboratory

specimens. It was also decided to include chemical analysis of the weld deposit andreview prediction of FN via the chemical analysis. This document is an attempt to

summarise the results of this large project and to present the findings in a way, which will

be useful to both the specifiers and manufacturers of stainless steel fabrications and those

who have an interest in the control and measurement of FN in weld metals.

2. BackgroundThis Round Robin was organised to measure ferrite (FN) levels in the weld metal of five

weldments, designed for different nominal FN levels in the range from zero to about

40FN which it was believed would cover most commonly used specifications. The five

weldments were identified as follows:

• A – 308H- A controlled ferrite 308H type weld metal, where FN is typically

specified in the range 3 – 8FN to give long term thermal stability combinedwith hot cracking resistance. Base material was type 304 stainless steel.

• B – 2205- A 22 % Cr austenitic-ferritic duplex stainless steel weld metal,

where FN is typically specified in the range 30 – 50FN to give a combination

of corrosion resistance and mechanical properties. Base material was a 22%Cr duplex stainless steel.

• C - 309Mo- A 'mixed welding', 309Mo weld metal commonly used for

welding carbon steels to austenitic stainless steels. FN levels are not routinelyspecified but levels have to be sufficiently high, typically 15 – 20 FN toaccommodate dilution from the carbon steel. Base materials were a type 316L

stainless steel and plain 0.2% carbon steel.



• D - 16-8-2- A controlled ferrite 'lean 316H' weld metal with a similar design

philosophy and FN range to 308H, but with improved pitting corrosion

resistance for some applications. Base material was type 316L stainless steel.

• E - 316NF- A 316 weld metal with negligible ferrite designed for non-

magnetic and cryogenic applications. Increasing the manganese contentreduces the risk of hot cracking. Base material was type 316L stainless steel.

The specimens were designed as 25mm single-sided butt weldments and were weldedwith realistic dilutions from the base plates. They were not intended to simulate research-

grade FN pads with negligible dilution used in previous exercises.. Welding was carried

out using GTAW with 1.6mm diameter solid wire for the root run and one fill pass, with

argon gas purging of the back of the weld root. Shielded metal arc welding, using 4mm

diameter electrodes, was used for the remainder of the weld. Each weldment was sliced

into several pieces, and various areas on the surfaces of these specimens were markedand identified for measurements by the Round Robin participants, of which there were

18. The general arrangement of the specimens is shown in Figure 1.

In addition, some locations on the weld caps and weld roots were finished by common

surface preparation techniques to allow an assessment of the effect of these surfacefinishes on the measurements. These were:

a) As-welded with light manual wire brushing to remove any residual slag.

b) Lightly polished with an abrasive ‘flap wheel’ to remove weld ripples but not to

change the overall weld profile and reinforcement.

c) Ground flush to remove all weld reinforcement and leave the weld flush with the

plate surfaces.

d) Filed flush to produce a surface finish typical of that specified by AWS for all-

weld metal FN measurement samples.

Figure 1 (below) shows the general arrangement of the test weldments including the following:

a) The different surface finishes applied.

b) The FN measurement locations numbered 1 to 22 and

c) The location of the OES chemical analysis.



19

20

21

Chemical analysis

A

AA

A

1

2

3

7

89

10

11

12

13

14

15

Ground

surface

Light

polished

As-

welded

4

5

6

Filed

surface

A

1

2

1617

18

19

22

Ground

surface

Light

polished

As-welded

Large sample

Small samp le



The overall project was divided into three parts.

Part I -Results of FN Measurements . Each laboratory was asked to carry out FN

determinations for each of 22 designated areas on each sample, using their own in-houseequipment calibrated against IIW secondary standards, and using the procedure outlined

in reference 6. The participants were asked to make 5 or 10 measurements in eachspecified area, depending on the size of the area, and supply the mean value as well as

maximum and minimum values for each area.

Part II - Results of Chemical Analyses. Each laboratory was asked to carry out achemical analysis using optical emission spectroscopy (OES) on a slice of each of the

weldments. For those elements for which OES is not considered sufficiently accurate(e.g. nitrogen), metal chip sample were supplied so that a dedicated combustion methodcould be used.

Part III - Prediction of FN values. Finally, each participant was asked to use their own

analysis to predict an FN for each specimen using their own preferred method, e.g. WRC-

92 diagram, commercial software, etc.

3.Background Reports

The following documents give the data generated by the round robin and the statistical

analyses carried out.

J C M Farrar, and Z Zhang, “Preliminary Summary Report on Round Robin Experiment

– Ferrite Measurement and Prediction in Real Weldments,” IIW Documents II-1440-01and IX-H-512-01.

T A Siewert, E A Siewert, J C M Farrar and Z Zhang, ‘Statistical Evaluation of a Round-

Robin Experiment: uncertainties in Ferrite Measurement in Weldments.’ IIW DocumentsII-C-224-02 and IX-H-531-02.

J C M Farrar and Z Zhang, ‘Round Robin on ferrite measurement and chemical analysis- Statistical Review of chemical analysis.’ IIW Documents II-C-238-02 and IX-H-541-02

J C M Farrar and Z Zhang, ‘Round Robin on ferrite measurement and chemical analysis- Further review of chemical analysis.’ IIW Documents II-1510-03 and IX-H-562-03.

4.FN Measurements

4.1 Overall Results

It is not intended to include detailed information in what is intended to be practical guide.For this reason the results are summarised for each of the five samples and some general

conclusions are given.



308H The results fall into three groups. The bulk of the data are in the range 3 to 6 FN,

which are within the specified design limits for the consumable, but there are two

groups of higher values at ~8 and ~10 FN, which are from the high dilution rootarea. With the exception of one or two outlying points, the data are all within about

±1.5 FN of the mean for each area. Some laboratories appear to consistently

overestimate FN level whilst others underestimate relative to the mean. The dilution

effect is picked up on the cross sections with increasing FN towards the root.

Overall scatter is about ±20% of the measured value.

2205 The mean values for the areas range from 30 FN to about 42 FN with some slight

increase in scatter between the various laboratories as the FN increases. Scatter is

±7 FN at the 35 FN level, increasing to ±9 FN at the 42 FN level, again about ±20%

of the measured value. Values of 30 to 40FN are typical for this weld metal andwithin the design range.

This weldment also exhibited increased FN levels arising from dilution in the root

area, whether measured directly on the root or on the cross sections.

Again there is evidence that some laboratories consistently over-estimate the FN,

whilst others underestimate.

309Mo This sample was a mild-steel to stainless steel dissimilar joint made with type

309Mo consumables. The effects of dilution are most marked in the root (with FNvalues down to 1), and a similar trend evident on the cross sections.

For areas that produced means between 1 and 5 FN, the scatter of results about themean is fairly small and is about ±1.5 FN - with the exception of one laboratory.

The other data are in the range 13 to 18 FN with a scatter of ±3 FN. Values are aswould be expected for this weldment.

This weldment presents the clearest evidence that some laboratories consistentlyover-estimate the results, whilst others underestimate, and this is particularly

apparent in the root area.

16.8.2 The base material for this sample was type 316L plate, and the root area as

measured directly and in the cross sections show an increase in FN in the range 4 to

5.5 FN, again with a scatter of about ±1 FN (ignoring two outlying results). Theresults fall into two populations with the bulk weld metal values in the range 1.5 to

3 FN which is within the specified design limits. Scatter at this level is similar to

other samples at about ±1 FN.

316NF This sample was made with a nil ferrite type 316 consumable. All laboratories

reported either zero ferrite or very small fractional values in all positions.



In general terms it can be seen that between laboratory scatter for real weldments is about±20% of the measured value, compared with about ±10% for the homogenous laboratory

samples tested in the previous Round Robin.

4.2 Effects of weld surface finishA statistical analysis of the data showed that surface finish was a significant variable.However there was no consistent trend across the four samples analysed. The 316NF

sample was not considered because all FN results were zero or very close to zero.

The two lower FN samples, 308H and 16.8.2 both showed a small increase in FN as thesurface finish changed from polished, to as-welded, to ground and finally filed. In the308H sample the increase was just over 2FN with a mid-range value of about 4.5FN. In

the 16.8.2 sample the increase was only about 0.5FN with a mid-range value of about2FN. These results would appear to show an effect of strain induced martensite arising

from cold work during the surface grinding operations. Martensite gives a magneticresponse which is additional to that from the ferrite and this would explain the small rise

in the ground and filed samples.

With the two higher FN samples, 2205 and 309Mo both showed a larger increase in FN

as the surface finish changed from ground, to filed, to polished and finally as-welded. Inthe 2205 sample the increase was about 8FN with a mid-range value of about 34FN. In

the 309Mo sample the increase was about 3.5FN with a mid-range value of about 17FN.

These results might suggest some short range variation within the top layer if a weldment

with FN decreasing as more weld metal is removed.

In general terms, the results indicate that different surface finishes can give rise tovariations of about ±10-12% of the mean measured value. A slightly higher percentage

variation was found with the 308H sample but absolute differences were still quite small.

Unfortunately there is no consistent pattern arising from the effect of surface finish but in

practical terms it would be desirable to test the weld surface as close to its originalcondition as possible consistent with good probe contact and consistent readings.

5. Chemical AnalysisChemical analyses were carried out using OES on the same sample from each of the 5

weldments by 17 of the 18 participating laboratories. An all-weld metal area was marked

on a cross sectional slice taken through each butt weld to indicate the area to be analysed.Each laboratory analysed the same nominal area but it is recognised that the need to re-

prepare the specimen between individual analyses would introduce a small degree of

variability. The variability of the results is summarised in Figure 1.

This next part of the report assesses the actual chemical analysis results in terms of

recognised specifications and compares the precision of the results with those given inASTM E1086 (6). This specification is based upon the results of optical emission

spectroscopy, carried out on certified reference materials.



Nb S Cu P C Si N Mo Mn Ni Cr

Sample D

Sample A

Sample CSample E

Sample B

0

0

0

0

0

1

1

1

IQR

Element

Sample

Variability of Element Analysis by Sample (Logged Data)

Inter Quartile Range (IQR) Plotted

Sample D

Sample A

Sample C

Sample E

Sample B

Figure 2.The above plot shows variability of element analysed by sample with the most variable

elements shown to the left. It is particularly interesting to note that the major stainless steel alloying elements (Cr, Ni, Mn, Mo, Si, N & C) show lower variability than residuals

and non-deliberate alloying additions. It should be noted that a number of labs did not report niobium and therefore this data set is limited.

The sample plots are arranged with those with most overall variability at the back of thediagram and those with least at the front. From this it can be seen that Sample B wasmost variable (2205) whilst sample D (16-8-2) was least.

5.1 ScopeThe elements considered most important were Carbon, Manganese, Silicon, Chromium,

Nickel, Molybdenum and Nitrogen since these are all relevant to the formulae used for prediction of FN. Residual and trace elements are not considered here but results can be

found in the original reports.

Nitrogen was not included in the precision data since it was not considered in E1086,

.

The precision values considered in this document are equal to two standard deviations for

the appropriate element taken from E1086 Table 2 Precision Data. This would beexpected to encompass about 95% of the measured results. The values used were those,

which were related most closely to the compositions in this report. The data in E1086 was

obtained from a Round Robin involving 15 laboratories and certified reference material(parent stainless steels, not weld metals). If the 17 labs participating in the FN round

robin were similar in precision to those reporting in E1086, one or possibly two resultsmight be expected to lie outside the limits.



5.2 Summary of results

The results are best summarised in a series of tables, which for each element analysed,

and for each sample, show how many labs gave results outside the specification limits,and how many were outside the precision limits based on E1086.

Table 1. Summary of results for main elements analysed and showing number of laboratories

reporting results outside specification limits and outside precision limits expected based onE1086.

Carbon

Sample Type Nominal % Labs out of

spec

Labs out of

precisionA 308H 0.05 1 3

B 2205 0.04 4 5C 309Mo 0.03 2 2

D 16-8-2 0.05 1 2

E 316NF 0.025 0 0

Manganese


spec

Labs out of


B 2205 1.2 0 8

C 309Mo 0.9 0 4

D 16-8-2 1.1 0 1

E 316NF 3..2 1 5

Silicon


spec

Labs out of


B 2205 0.4 0 14

C 309Mo 0.6 0 10

D 16-8-2 0.5 1 7

E 316NF 0.2 0 7

Chromium


spec

Labs out of

precision

A 308H 19 0 1

B 2205 23 0 1

C 309Mo 23 1 3

D 16-8-2 17 2 4

E 316NF 19 0 2

Nickel

Sample Type Nominal % Labs out of Labs out of



spec precisionA 308H 9 3 0

B 2205 9 3 3

C 309Mo 12 2 3D 16-8-2 8 0 0

E 316NF 15 1 1

Molybdenum

Sample Type Nominal % Labs out of spec

Labs out of precision

A 308H 0.1 0 0

B 2205 3.5 1 1

C 309Mo 2.5 0 2

D 16-8-2 1.2 2 10

E 316NF 3 0 4

Nitrogen

Sample Type Nominal % Labs out of spec

Labs out of precision*

A 308H - - -

B 2205 0.17 1 -

C 309Mo - - -

D 16-8-2 - - -

E 316NF 0.1 1 -

* not in E1086

Analysis of these tables shows that:

a) Out of a total of about 450 individual analyses, about 6% gave values outside the

specified ranges. The worst elements were C and Ni with about 2% each, but it

should be noted that the aim nickel levels in the test samples tended to be towardsthe lower end of the specification range, which would increase the risk of some

results falling outside the range. Carbon measurements made with OES, used by

some participants, tend to be less accurate than those made using some other

techniques.

b) Of the 450 results, about 25% were outside the precision limits given in E1086.

Since these E1086 limits are based on ± two standard deviations, only about 5%would be expected if the round robin laboratories were as precise as those in

E1086. It should be noted that E1086 data was based on homogeneous certified

reference materials, whereas the round robin analyses were made on weld metal,

with each lab analysing a slightly different sample after re-preparation.

c) The worst elements from a precision point of view were Silicon, Manganese and

Molybdenum. Fortunately the first two are at quite low levels and have a minor

effect on FN predictions. Of more concern is the Mo precision, but can be seen



from the above table that one sample gave rise to most of the problems. Sample Dhad a Mo of about 1.2%, compared with about 3% in samples B,C and E. It may

be that a number of laboratories have limited experience in measuring Mo at this

level in stainless steels.

d) It is pleasing to note that precision levels for two of the most critical elementsused in FN prediction, namely Cr and Ni were relatively good with only about

12% of round robin labs reporting results outside the E1086 limits.

6. Prediction of FN using chemical analysis

Each of the participants was asked to calculate the FN number for each sample based on

their own individual analysis. The thirteen laboratories, which complied with this request,

chose to quote FN based on the WRC-92 diagram.

Using the WRC-92 formulations, namely Creq = %Cr + %Mo + 0.7×%Nb, Nieq = %Ni +

35×%C + 20×%N + 0.25×%Cu, the Chromium and Nickel equivalents* from the analysis

data provided by the participants FN values were calculated.

The overall mean predicted value was compared with the mean measured values taken

from the undiluted weld metal regions of the cross sections which were chosen to be

representative of the area used for the chemical analysis. The results are shown below:

Table 2. Comparison of mean predicted FN and mean measured FN.

Sample Weld Metal Mean Predicted FN Mean Measured FN

A 308H 3.9 4.5

B 2205 45 36.5

C 309Mo 15.9 15.5

D 16.8.2 2 2.5

E 316NF 0 0

6.1 Comparison between measured and predicted FN results

Sample A

(308H)

The predictions using the WRC diagram ranged from 1.4FN up to 11.2FN, although the

overall mean of 3.9 agreed quite closely with the measured mean of 4.5.

*: In Creq calculation, 0.02% was used as the value for Nb content.



Nine laboratories predicted FN values within ±2FN of the measured mean.

Sample B

(2205)

The predictions using the WRC diagram ranged from 29.2FN up to 78.6FN, although the

overall mean of 45FN was closer to the measured mean of 36.5FN.

Only seven laboratories predicted FN values within ±10FN of the measured mean.

Sample C

(309Mo)

The predictions using the WRC diagram ranged from 10.9 up to 27.6. However, the

overall mean of 15.9 agreed very well with the measured mean of 15.5.

All but two laboratories gave WRC predictions within ±5FN of the measured mean.

Sample D(16.8.2)

The predictions using the WRC diagram ranged from 1.4FN up to 7.1FN. The overallmean prediction was 2FN which is in good agreement with the mean measured value of 2.5FN.

At this low FN value all but three of the laboratories predicted FN values within ±1FN of the measured mean.

Sample E

(316NF)

The predictions using the WRC diagram ranged from -5.4FN up to 1.2FN. The overall

mean prediction was effectively zero (or a small negative FN) and the measured meanwas also zero.

Since this weld metal was designed to be fully austenitic, the results are in line withexpectations.

7. What does all this data tell us?

7.1 Direct measurement of FN

Scatter

Previous work on smooth surface, homogenous laboratory specimens showed that scatter

between laboratories was about ±10% of the measured mean value.

This work on real weldments shows that scatter between laboratories is about ±20% of the measured mean value. In many cases the same laboratories took part in both exercises

using the same equipment, procedures and calibration samples.



Dilution

The weldments also showed clear effects of dilution from the parent material. Results,

from root regions and areas close to the sidewalls of non-matching parent materials, werevery different from those of the bulk weld metal.

Surface finish

Surface finish of the area being measured has a significant but inconsistent effect. Tests

carried out on a number of typical finishes ranging from as welded (deslagged) through to

ground flush and smooth, showed a scatter of about ±10% of the measured mean value.

Practical implications and recommendations

Magnetic instruments are suitable for measuring the FN value of weld metal in real welds

provided the following points are taken into account:

a) The equipment used should be properly calibrated against certified secondarystandards with designated FN values relevant to the material being tested.

b) To avoid effects of dilution, measurements should be made in the center of the

weld run away from root areas and weld toes unless these regions are beingspecifically investigated.

c) There is a significant effect of surface finish on results, and if the effects of

possible strain induced martensite or FN gradients in the top weld layer are to beavoided, then the welds should be tested in their original condition. Preparation

prior to testing should be kept to minimum consistent with good probe contactand consistent readings.

d) The important factor is to remove any surface debris, such as slag, and to check that there is good and consistent contact between the probe and the weld surface.A number of consistent readings along an undiluted weld bead are a good

indicator.

e) Even when all the above are taken into account, scatter between laboratories of about ±20% of the mean measured value should be expected.

In practical terms this means that measurements up to about 10FN, which should

cover all controlled ferrite specifications for long term high temperature or cryogenic

toughness applications, may exhibit between laboratory scatter of about ±2FN. This is

approximately in line with typical specifications, which often indicate a range of say

3 to 8FN. At the zero FN level laboratory agreement is good.The other area of practical interest is that of duplex weld metals where FN values in

the range 30 to 60FN are typically specified. Measurements in this range may exhibit

between laboratory scatter of about ±10FN. These relatively high scatter values

suggest that the specification ranges for duplex weld metals should be about 20FN if differences between laboratories are to be taken into account. Fortunately such

relatively high variations in FN value do not have a major effect on critical properties

such as strength, toughness and corrosion resistance.



7.2 Chemical Analysis

Differences between laboratories

Differences between laboratories analysing the same sample must be accepted. Thesamples analysed covered a reasonable range of stainless steel compositions although

they did differ somewhat from the most common grades such as 308L and 316L.Statistically, about 6% of the entire individual element analyses would have fallen

outside the specification limits. However this figure is strongly influenced by the

results for carbon and nickel. The carbon levels were 0.05% or less and at this level,

OES is not the most accurate method. In cases of dispute over compliance with

specifications it is recommended that a combustion method be used.

Nickel levels in the 5 samples tended to be towards the lower end of the specification

range so that a normal distribution of scatter would lead to more readings being

outside the lower limits of the specification range. Nickel levels towards the middle

of the specification range should not present a problem.

If the above effects are discounted, the overall result is that about 3% or one result in33 for the elements and compositions tested might be expected to fall outside

specified limits. In the light of the ‘special’ nature of some of the compositions tested,

it might be argued that even this prediction is a little pessimistic and that a morereasonable expectation might be about 2%, or one individual result in 50.

Users of analytical results should not be surprised if, from time to time, results for individual elements fall outside specification limits

Comparisons with other Laboratories

Some indication of the accuracy to be expected for a given element in a givenstainless steel compositions can be found in ASTM E1086. Based on the precision

limits given in E1086, statistically about 5% of all results would be expected to fall

outside these limits. In fact about 25% were outside the limits. Since there is noreason to believe that the two sets of laboratories were fundamentally different in

their performance, the most likely explanation is that weld metal is less homogenous

than wrought certified reference samples and in the case of the welds, the test area

would be very slightly different for each test.

The practical implication of this finding is that more variability in weld metals should be expected, when compared with wrought materials and that differences in sampling

position will lead to yet further variability.

It should also be noted that some laboratories gave consistently good results across allelements and samples whereas others were much more variable. It is impossible to

predict whether a laboratory will be more consistent or accurate than any other.

However, important considerations are:

i) Independent accreditation of equipment and procedures.

ii) Use of relevant alloy calibration standards and procedures.



iii) Prior experience with the alloy composition in question, particularly if issomewhat unusual.

7.3 Prediction of FN using chemical composition.The prediction of FN by plotting on a diagram such as the WRC-92 diagram or using acomputed version of this or any other system is subject to two sources of error:

i) The errors in input chemical composition.

ii) Inherent errors in the predictive system.

The agreement between mean measured and mean predicted results for each sample,

when tested in the same area and away from dilution effects, was reasonably good asshown in Table 2. However there is much greater spread of predicted results when

compared with measured values. At FN values of about 5 or less the scatter of predicted

values are at least ±60% of the mean value. Even at higher FN values of 15 to 35 thescatter is still about ±30% of the mean. These are the extremes of scatter and many

laboratories appear to be able to predict values significantly closer to the mean.

In general terms it is reasonable to expect that predicted FN based on individual

laboratory chemical analyses are less accurate than magnetically measured values by a

factor of about two on average.

8. General Conclusions.

Magnetic measurements using small laboratory samples can be expected to give scatter

between laboratories of 10% of the mean value.

When magnetic measurements are carried out on real welds this scatter can be

expected to increase to about ±20%.

When predictions of FN based on the WRC-92 diagram, are made using individuallaboratory chemical analyses, then the scatter can be expected to increase to about ±40%of the mean value.

For the most accurate assessment of weld metal FN, magnetic methods using appropriate

equipment should be used. The equipment should be calibrated using certified secondary

calibration samples appropriate to the values being measured. Users should be aware of the effects of parent material dilution and surface finish, when real welds are being tested.

Even when using magnetic methods, some scatter of results must be expected.

Fortunately the levels of scatter are comparable with the weld metal specification rangeslikely to be selected for critical applications.



9. Acknowledgements

Firstly I must thank all those members of sub-commissions II-C and IX-H and their

colleagues who participated in the Round Robin and provided the data, without whichthis report would not have been possible. Their support and encouragement over the years

the project has been in progress is much appreciated.Secondly I would like to thank Drs T & E Siewert of NIST and my good friend Dr John

Brew of Warwick University for their invaluable help with the statistical analysis of avery large database – a task, for which, I am totally unqualified!

Finally I should thank my colleagues in the welding laboratory at Metrode Products Ltdwho produced the excellent test pieces and Dr Zhang who most efficiently managed thevery large database.

10. References

1. Lefebvre, J ‘Guidance on specifications of ferrite in stainless steel weld metal.’

Welding in the World 1993, 31 (6): 390 to 406.

2. Kotecki, D. J., and Siewert, T. A. 1992. ‘WTRC-1992 Constitution Diagram for

Stainless Steel Weld Metals: A Modification of the WRC-1988 Diagram,’ Welding Journal, 71(5): 171-s – 178-s.)

3. ISO 8249:2000 ‘Welding – Determination of Ferrite Number (FN) in austenitic and

duplex ferritic-austenitic Cr-Ni stainless steel weld metals,’ ISO, Geneva, Switzerland.

4. Rabensteiner G. ‘Summary of 5th round robin of FN measurements.’ IIW Document II-C-902-92. International Institute of Welding.

5. ISO TR22824, ‘Ferrite Specification’ (published Dec 2003), ISO, Geneva,

Switzerland.

6. Kotecki, D. J. FN ‘Measurement Round Robin using Shop and Field Instruments after

calibration by Secondary Standards – Final Summary Report.’ IIW document II–1345-98.

International Institute of Welding.

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