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ABSTRACT. Duplex stainless steel cladmetals contain delta ferrite, which is ex-pressed in terms of Ferrite Number (FN).The amount of ferrite present in the de-posit is a function of chemical compositionof the filler and base metals, weldingprocess, type of shielding gas, welding pro-cedure, and heat input during cladding.Excessive ferrite in duplex stainless steelcladdings can result in poor ductility,toughness, and corrosion resistance. Like-wise, insufficient ferrite can also produceinferior mechanical and corrosion resis-tance properties. Hence, control of ferritein duplex stainless steel cladding is essen-tial to obtain the required mechanical andcorrosion-resistant properties.

This paper highlights the application ofresponse surface methodology to developmathematical models and to analyze vari-ous effects of flux cored arc welding(FCAW) process parameters on the FN ofduplex stainless steel clad metals. The ex-periments were conducted based on four-factor, five-level, central composite rotat-able design with full replicationstechnique and mathematical models de-veloped using multiple regression tech-nique. The developed mathematical mod-els are very useful for predicting andcontrolling the FN in duplex stainless steelcladding. The main and interaction effectsof input process parameters on calculatedFN (by WRC-1992 diagram) and mea-sured FN have been presented in graphicform, which helps in selecting FCAW

process parameters to achieve the re-quired FN.

Introduction

Weld cladding is a process in which athick layer of a weld metal is depositedonto a carbon- or low-alloy-steel basemetal to provide a corrosion-resistant sur-face. It finds extensive use in numerous in-dustries such as paper, chemical, fertilizer,nuclear, food processing, petrochemical,and other allied industries. The desirablecharacteristics of cladding material arereasonable strength, weldability to thesteel, resistance to general and localizedcorrosion attack, and good corrosion fa-tigue properties (Ref. 1). In recent years,duplex stainless steel is extensively usedfor weld cladding because it has excellentchloride stress corrosion cracking resis-tance, pitting and crevice corrosion resis-tance, yield strength, ductility, impacttoughness, and weldability (Ref. 2).

The composition and properties ofclad metals are strongly influenced by di-lution, as illustrated in Fig.1. It is theamount of base metal melted (B) dividedby the sum of the filler metal added andbase metal melted (A+B). Dilution re-

duces the alloying elements and increasesthe carbon content in the clad layer, whichreduces corrosion resistance propertiesand causes other metallurgical problems.It also affects the ferrite content incladdings. The amount of ferrite presentin duplex stainless steel clad metals influ-ences mechanical and corrosion proper-ties. Both strength and stress corrosioncracking resistance may be reduced whenthe FN is less than 30, and there is a lossof both ductility and toughness of the cladmetal when the FN is above 70 in duplexstainless steel weld claddings (Ref. 3).Hence, control of the FN is essential toachieve optimum corrosion resistance andmechanical properties (Ref. 4). This canbe effectively done by properly selectingthe process parameters after thoroughlyunderstanding the direct and interactioneffects of process parameters on dilution.

The economics of stainless steel weldcladding are dependent on achieving thespecific chemistry at the highest practicaldeposition rate in a minimum number oflayers (Ref. 5). Heat input also affects theFN. Ferrite Number increases with a de-crease in heat input. With increasing cool-ing rate (low heat input), the solid-statetransformation is suppressed, and theresidual ferrite content increases.

In stainless steel weld cladding, it is es-sential to understand how the dilution af-fects the composition and FN so as to con-trol the corrosion resistance andmechanical properties.

During the last two decades, four con-stitution diagrams have found the widestapplication in predicting ferrite contentfrom weld deposit composition. These in-clude the Schaeffler Diagram, DeLongDiagram, WRC-1988 Diagram, andWRC-1992 Diagram. The Schaeffler Dia-gram, published in 1947, has been exten-

SUPPLEMENT TO THE WELDING JOURNAL, MAY 2006Sponsored by the American Welding Society and the Welding Research Council

Prediction of Ferrite Number of DuplexStainless Steel Clad Metals Using RSM

Response surface methodology (RSM) was used to establish a relationship between process parameters and Ferrite Number for

duplex stainless steel clad metals

BY T. KANNAN AND N. MURUGAN

KEYWORDS

Duplex Stainless SteelFCAWFerrite NumberHeat InputMathematical ModelsResponse Surface Methodology

T. KANNAN (kannan_kct@yahoo.com) is anassistant professor of mechanical engineering, Ku-maraguru College of Technology, Coimbatore,Tamil Nadu, India. N. MURUGAN (drmuru-gan@yahoo.com) is professor of mechanical en-gineering, Coimbatore Institute of Technology,Coimbatore, Tamil Nadu, India.

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sively used for estimating the ferrite con-tent of stainless steel weld metals and weldmicrostructure from its composition (Ref.6). This diagram does not consider thepowerful effect of nitrogen in promotingaustenite, which tends to seriously overes-

timate the weldmetal ferrite (Ref.7). Also, the Scha-effler Diagram wasused to predict theferrite in terms of“percent ferrite,”which is imprecise.The second widelyused prediction di-agram, the De-Long Diagram,was published in1974 and incorpo-rated some im-provements (Ref.8). It has a FNscale and includesa coefficient for ni-trogen in thenickel equivalent.This diagram isspecifically de-signed for 300-series stainlesssteel welds con-taining smallamounts of ferrite.

Prediction of the FN in duplex stainlesssteel welds is not possible using the De-Long Diagram because nickel andchromium equivalents fall outside therange of this diagram (Ref. 4).

The WRC-1988 Diagram overcomesmany of the problems associated with theSchaeffler and DeLong Diagrams (Ref.4). It was developed with data measuredby the most recent definition of the FNscale. In recent years, duplex stainlesssteels have been used more frequently.Some of these steels and their weld metalscontain significant amounts of copper,which is not included in nickel equivalentof WRC-1988 Diagram, and hence, its FNprediction accuracy is less.

The WRC-1992 Diagram (Ref. 9) is themost recent of the diagrams mentionedabove, and it is officially adopted by theASME Boiler and Pressure Vessel Code forpredicting the FN when the FN cannot bemeasured.

Among the four constitution diagramsused for estimating the FN of duplex stain-less steel weld/clad metals from its com-position, the WRC-1992 Diagram is moresuitable due to the reasons below.

∑ It has a Ferrite Number (FN) scale. ∑ It includes a coefficient for nitrogen

in the nickel equivalent.∑ Nickel and chromium equivalents fall

inside the range of this diagram.∑ Copper term is included in nickel

equivalent, hence, FN prediction accuracyis improved.

∑ Axes of the diagram can be extendedto predict dilution effects in dissimilar

Fig. 1 — Weld bead geometry. Fig. 2 — Typical cladded plate (Trial Nos. 13 and 2).

Fig. 3 — Locations of the FN and chemical analysis measurements.

Table 1 — Chemical Composition of Filler and Base Metals

Material Elements, % FNC Si Mn P S Al Cr Mo Ni N2 Cu

IS: 2062 0.150 0.160 0.870 0.015 0.016 0.031 — — — — — —E2209T1-4/1 0.023 0.760 1.030 0.024 0.002 — 23.14 3.05 9.22 0.13 0.09 47

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welding/cladding applications.The above diagrams do not accurately

reflect the effects of welding process, tech-nique, and cooling rate on the FN. There-fore, a study on the effects of welding pa-rameters on the calculated FN (byWRC-1992 Diagram) and the measuredFN of duplex stainless steel clad metalsmay be useful. However, there is littlepublished information available with re-gard to the effect of welding parameterson the FN of duplex stainless steel cladmetals.

This paper highlights an experimentalstudy carried out to analyze the effects ofvarious FCAW process parameters on thecalculated FN (by WRC-1992 Diagram)and the measured FN of duplex stainlesssteel cladding.

Experimental Procedure

The experiments were conducted using

a constant voltage programmable weldingmachine (UNIMACRO 501C). In thismachine, welding current can be set di-rectly instead of changing wire feed ratefor changing current level. Test plates ofsize 200 x 150 x 20 mm were cut from low-carbon structural steel (IS: 2062) plate,and its surfaces were ground to removeoxide scale and dirt before cladding. Fluxcored duplex stainless steel welding wireof 1.2 mm diameter was used for deposit-ing the weld beads. The chemical compo-sition of filler and base metals is given inTable 1. Carbon dioxide (CO2) gas at aconstant flow rate of 18 L/min was used forshielding. The experimental setup usedconsisted of a traveling carriage with atable for supporting the specimens. Thewelding gun was held stationary in a framemounted above the worktable, and it wasprovided with an attachment for both upand down movement and angular move-ment for setting the required contact tip-

to-workpiece distance and welding gunangle, respectively. The experiments wereconducted by laying three beads using astringer bead technique with a constantoverlap of 40%. An interpass temperatureof 150°C was maintained during thecladding experiments.

Among the many independently con-trollable primary and secondary processparameters affecting the FN, the primaryprocess parameters viz welding current (I)and welding speed (S), and the secondaryprocess parameters viz contact tip-to-workpiece distance (N) and welding gunangle (T), were selected as process para-meters for this study. The welding current,welding speed, and voltage are the pri-mary parameters contributing to the heatinput, influencing FN variations in thecladdings. The machine used for this studywas constant voltage type, hence it was de-cided to select the welding current andwelding speed as primary process parame-ters. So far, few studies have been carriedout on the effects of contact tip-to-work-piece distance and welding gun angle onthe FN, therefore it was decided to selectthe contact tip-to-workpiece distance andwelding gun angle (push angle) as sec-ondary process parameters. The workingranges of all selected parameters werefixed by conducting trial runs. This wascarried out by varying one of the factorswhile keeping the rest of them at constantvalues (Ref. 10). The working range ofeach process parameter was decided uponby inspecting the bead for a smooth ap-pearance without any visible defects suchas surface porosity, undercut, etc. The

Fig. 4 — A — Scatter diagram of calculated FN model; B — scatter diagram of measured FN model.

Table 2 — Welding Parameters and Their Levels

Parameter Unit Notation Factor levels

–2 –1 0 +1 +2

Welding current A I 200 225 250 275 300Welding speed cm/min S 20 30 40 50 60

Contact tip-to- mm N 22 24 26 28 30workpiece distance

Welding gun (push) angle degree T 70 75 80 85 90

A B

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upper limit of a factor was coded as +2,and the lower limit was coded as –2. Thecoded values for intermediate values werecalculated using the equation

Xi = 2[2X – (Xmax + Xmin)] / (Xmax – Xmin)(1)

Where Xi is the required coded value of avariable X; X is any value of the variablefrom Xmin to Xmax; Xmin is the lower limit ofthe variable and Xmax is the upper limit ofthe variable. The chosen levels of the se-lected process parameters with their unitsand notations are given in Table 2.

The design matrix chosen to conductthe experiments was a central compositerotatable design (Ref. 11), which is shownin Table 3. In this work, 31 deposits weremade using a cladding condition corre-sponding to each treatment combinationof parameters as shown in Table 3 at ran-dom. At the end of each run, settings forall four parameters were disturbed andreset for the next deposit. This was essen-tial to introduce variability caused by er-rors in experimental settings (Ref. 12).

A typical cladded plate is shown in Fig.2. To measure the FN of the clad metals, thecladded plates were cross sectioned at theirmidpoints to obtain test specimens of 20mm wide. The top surface of each specimenwas ground flat. The FN measurement wascarried out using a Ferritescope. Six read-ings were taken on the top surface along thelongitudinal axis of the three beads with theFerritescope calibrated in accordance withprocedures specified in ANSI/AWS A4.2.The average values found are given in Table3. Using the same specimens, three testburns were taken on the top surface of thecladdings to find out chemical compositionusing an optical emission spectrometer(ARL 3460). The average of three readingswas calculated and tabulated in Table 4. Fig-ure 3 shows the locations of the FN andchemical analysis measurements. Thechromium and nickel equivalents (Creq andNieq) of WRC-1992 Diagram were calcu-lated using Equations 2 and 3. These valuesare also given in Table 4.

Fig. 5 — Effect of heat input on FN. Fig. 6 — Effect of dilution on FN.

Table 3 — Design Matrix with FN and Dilution

Trial Design matrix Calculated FN Measured DilutionNo. (by WRC-1992) FN (%)

I S N T

01 –1 –1 –1 –1 33 37 07.8602 +1 –1 –1 –1 27 31 12.1003 –1 +1 –1 –1 31 36 11.3504 +1 +1 –1 –1 22 24 11.9805 –1 –1 +1 –1 33 38 06.5406 +1 –1 +1 –1 28 34 08.8207 –1 +1 +1 –1 29 37 09.6908 +1 +1 +1 –1 24 25 11.1609 –1 –1 –1 +1 30 38 08.9710 +1 –1 –1 +1 29 31 13.7511 –1 +1 –1 +1 21 21 18.5212 +1 +1 –1 +1 19 20 20.5813 –1 –1 +1 +1 32 37 07.4614 +1 –1 +1 +1 29 34 09.1415 –1 +1 +1 +1 28 25 18.0016 +1 +1 +1 +1 20 24 14.8017 –2 0 0 0 34 47 05.8618 +2 0 0 0 24 25 16.4819 0 –2 0 0 33 36 05.3120 0 +2 0 0 19 18 17.3521 0 0 –2 0 24 33 11.7122 0 0 +2 0 33 37 09.0123 0 0 0 –2 25 34 10.5424 0 0 0 +2 24 29 13.9825 0 0 0 0 29 32 10.3326 0 0 0 0 25 30 13.6027 0 0 0 0 30 33 10.7328 0 0 0 0 28 33 11.7129 0 0 0 0 25 30 13.7630 0 0 0 0 24 33 10.9931 0 0 0 0 24 33 10.67

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Creq = Cr+Mo+0.7Nb (2)

Nieq = Ni+35C+20N+0.25Cu (3)

From the calculated chromium andnickel equivalents, FN values were pre-dicted using WRC-1992 Constitution Dia-gram in Table 3. To study the effect ofprocess parameters on dilution, dilutionvalues for all specimens were measuredusing the following procedure. Each weldwas cross sectioned at mid length, pol-ished and etched with 2% Nital. The beadprofiles were traced using a reflective typeoptical profile projector at a magnificationof 10X, and then the deposit area andplate fusion area were measured using adigital planimeter. The measured dilutionvalues are given in Table 3.

Development of MathematicalModels

The response function representingFN can be expressed using the equation

Y = f (X1, X2, X3, X4) (4)

where Y = response [FN], X1 = weldingcurrent (I) in A, X2 = welding speed (S) incm/min, X3 = contact tip-to-workpiecedistance (N) in mm, and X4 = welding gunangle (T) in degrees.

The second order response surfacemodel (Ref. 13) for the four selected pa-rameters is given by Equation 5.

(5)

The above second order response surfacemodel could be expressed as follows:

(5A)Where bo is free term of the regressionequation, the coefficients b1, b2, b3, and b4

are linear terms, the coefficients b11, b22,

Y I S N T I

S N To= + + + + +

+ + +

b b b b b bb b b

1 2 3 4 112

222

332

4422

12 13

14 23 24 34

+ ++ + + +

b bb b b b

IS INIT SN ST NT

Y X

X X X

o ii

i

iii

i iji

i j

= +

+ +

=

= =

Â

 Â

b b

b b

1

4

1

42

1

4

Fig. 7 — Effect of welding current on calculated and measured FN. Fig. 8 — Effect of welding speed on calculated and measured FN.

Fig. 9 — Effect of contact tip-to-workpiece distance on calculated andmeasured FN.

Fig. 10 — Effect of welding gun angle on calculated and measured FN.

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b33, and b44 are quadratic terms, and thecoefficients b12, b13, b14, b23, b24, and b34 areinteraction terms. The coefficients werecalculated using QA six sigma software

and the same was verified by using SYS-TAT 10.2 software. After determining thecoefficients, the mathematical models(Equations 6 and 7) were developed as

follows:Calculated FN (By WRC-1992 Diagram)= 26.429 – 2.458I – 3.125S + 1.208N – 0.875T+ 0.674I2 – 0.076S2 + 0.549N2

Fig. 11 — Interaction effects of welding current and welding gun angle on cal-culated FN.

Fig. 12 — Interaction effects of welding current and welding gun angle onmeasured FN.

Table 4 — Cladding Compositions

Trial Elements, % EquivalentsNo. Cr Mo Nb Ni C N Cu Mn Si Creq Nieq

1 21.64 3.05 0.018 8.86 0.037 0.132 0.058 0.902 0.598 24.71 12.812 21.10 3.08 0.018 8.56 0.041 0.140 0.056 0.903 0.575 24.20 12.803 21.24 3.06 0.018 8.76 0.035 0.137 0.055 0.925 0.633 24.31 12.744 19.96 2.87 0.017 8.08 0.053 0.119 0.052 0.930 0.598 22.85 12.325 21.88 3.20 0.018 9.32 0.036 0.134 0.056 0.892 0.581 25.09 13.086 21.22 3.00 0.017 8.38 0.037 0.134 0.053 0.9916 0.570 24.23 12.857 21.28 3.13 0.018 8.73 0.038 0.155 0.056 0.959 0.651 24.43 13.178 20.50 2.80 0.017 8.00 0.041 0.138 0.054 0.937 0.602 23.32 12.219 21.59 3.02 0.018 8.99 0.029 0.142 0.057 0.928 0.590 24.63 12.8610 20.78 3.03 0.017 8.41 0.041 0.113 0.052 0.839 0.537 23.82 12.1211 19.16 2.86 0.017 7.74 0.053 0.107 0.049 0.899 0.557 22.03 11.7312 19.02 2.73 0.017 7.70 0.052 0.110 0.049 0.900 0.555 21.76 11.7413 21.85 3.10 0.019 9.08 0.029 0.141 0.059 0.919 0.626 24.96 12.7314 21.03 3.16 0.018 8.67 0.039 0.134 0.056 0.936 0.583 24.20 12.7315 20.21 2.94 0.017 7.85 0.047 0.104 0.051 0.934 0.571 23.17 11.5916 19.21 2.71 0.017 8.24 0.047 0.117 0.050 0.858 0.527 21.92 12.2317 21.94 3.28 0.019 8.34 0.031 0.131 0.060 0.978 0.676 25.23 12.7518 19.98 2.98 0.017 7.61 0.053 0.132 0.052 0.912 0.568 22.97 12.6619 21.48 3.27 0.018 8.85 0.030 0.144 0.057 0.866 0.545 24.76 12.7820 19.27 2.80 0.016 7.46 0.043 0.156 0.048 0.934 0.579 22.07 12.4821 20.54 2.91 0.017 8.40 0.041 0.116 0.052 0.939 0.636 21.02 12.1722 21.37 3.15 0.018 8.71 0.036 0.117 0.053 0.901 0.542 24.53 12.3223 20.31 2.99 0.017 8.31 0.045 0.119 0.053 0.917 0.559 23.31 12.2924 19.81 2.90 0.017 8.09 0.050 0.112 0.053 0.909 0.560 22.72 12.0925 20.76 2.98 0.018 8.63 0.033 0.124 0.055 0.914 0.603 23.75 12.2826 21.04 2.95 0.018 8.08 0.055 0.139 0.050 0.890 0.534 23.70 12.8027 21.05 3.11 0.018 8.63 0.033 0.124 0.055 0.982 0.642 24.17 12.2828 20.61 3.12 0.018 8.44 0.041 0.114 0.053 0.916 0.608 23.74 12.1729 20.43 2.95 0.018 8.63 0.033 0.124 0.055 0.928 0.619 23.40 12.2530 20.13 2.95 0.018 8.27 0.034 0.123 0.052 0.935 0.610 23.10 11.9431 20.18 2.95 0.018 8.63 0.033 0.124 0.055 0.942 0.598 23.14 12.28

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– 0.451T2 – 0.562IS – 0.188IN + 0.688IT+ 0.312 SN -1.062ST +0.562 NT (6)

Measured FN = 32.000 – 3.750I – 4.333S+ 1.000N – 1.750T + 0.729I2 – 1.521S2

+ 0.479N2 – 0.396T2 – 0.375IS – 0.375IN + 1.375IT + 0.250SN – 2.000ST + 0.250NT

(7)To develop final mathematical models,

the insignificant coefficients were elimi-nated without affecting the accuracy of thedeveloped models by using t-test. This isdone by back elimination technique, usingQA six sigma software and the same wasverified by using SYSTAT 10.2 software.

The final mathematical models wereconstructed by using these coefficients.The developed final mathematical models(Equations 8 and 9) with process parame-

ters in coded form are given below.

Calculated FN (By WRC-1992 Diagram)= 25.942 – 2.458I – 3.125S + 1.208N– 0.875T + 0.725I2 + 0.600N2 + 0.688IT– 1.062ST (8)

Measured FN = 31.596 – 3.750I – 4.333S+ 1.000N – 1.750T + 0.771I2 – 1.479S2

+ 0.521N2 + 1.375IT – 2.000ST (9)

It was found that the reduced modelsare better than the full models because thereduced models have higher values of R2

(adjusted) and lesser values of standarderror of estimates than that of full models.The values of R2 (adjusted) and standarderror of estimates for full and reducedmodels are given in Table 6.

The adequacies of the developed mod-els were tested using the analysis of vari-ance (ANOVA) technique (Ref. 14). Asper this technique, if the calculated F-ratiovalues for the developed models do not ex-ceed the standard tabulated values of F-ratio for a desired level of confidence(95%), and the calculated R-ratio valuesof the developed models exceed the stan-dard tabulated values of R-ratio for a de-sired level of confidence (95%), then themodels are said to be adequate within theconfidence limit. The calculated values ofF-ratio and R-ratio are given in Table 7.This shows that the developed models areadequate. The validity of these modelswere again tested by drawing scatter dia-grams as shown in Fig. 4A and B, whichshow the observed and predicted values of

Fig. 13 — Contour plot and response surface plot for interaction effects of weld-ing current and welding gun angle on calculated FN.

Fig. 14 — Contour plot and response surface plot for interaction effects ofwelding current and welding gun angle on measured FN.

Fig. 15 — Interaction effects of welding speed and welding gun angle on calcu-lated FN.

Fig. 16 — Interaction effects of welding speed and welding gun angle onmeasured FN.

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calculated and measured FN.Conformity tests were conducted using

the same experimental setup to conformthe results of the experiments. The resultsof the conformity tests shown in Table 8depict the accuracy of the developed mod-els, which is above 94%.

Results and Discussions

The above developed models (Equa-tions 8 and 9) can be used to predict thecalculated FN (by WRC-1992 Diagram)and the measured FN by substituting thecoded values (–2, –1, 0, +1, +2) of the re-spective process parameters. The effectsof heat input and dilution on FN are rep-resented in graphical form in Figs. 5 and 6.The responses calculated using the devel-oped mathematical models for each set ofcoded welding parameters are also repre-sented in graphical form in Figs. 7–18.

Effects of Heat Input on FN

Heat input is a relative measure of theenergy transferred per unit length of weld.It is an important characteristic because itinfluences the cooling rate, which may af-fect the mechanical properties and metal-lurgical structure of the weld and heat-affected zones. Heat input is typically cal-

culated as the ratio of the power (i.e., volt-age x current) to the velocity of the heatsource (i.e., the arc). In this study, the heatinput values were calculated for specimenswelded at cladding conditions correspond-ing to trial numbers 2, 7, 17, and 28, whichare given in Table 5. These are representedin graphical form in Fig. 5. It is evident fromFig. 5 that the FN increases with a decreasein heat input. At higher cooling rates (lowheat input), the transformation of ferrite toaustenite will be suppressed, resulting inhigher residual ferrite content in thecladdings (Refs. 15 and 16).

Effects of Dilution on FN

The composition and properties of cladmetals are strongly influenced by the dilu-tion obtained. Control of dilution is very im-portant in cladding, where low dilution istypically desirable. When the dilution is low,the final deposit composition is close to thatof the filler metal, and the corrosion resis-tance of the cladding is maintained.

The calculated values of dilution fortrial numbers 2, 7, 17, and 28 (Table 5) arerepresented in graphical form in Fig. 6. Itis evident from Fig. 6 that FN decreaseswith increases in dilution. An increase indilution enhances the C content and re-duces the Cr and Ni content of the

cladding. The reduction of Cr and Ni andthe enhancement of C in the cladding withthe increase of dilution occurred primar-ily because the base metal had no Cr andNi and higher C with respect to the chem-ical composition of the duplex stainlesssteel electrode.

The change in dilution governingchemical composition of the cladding af-fects chromium and nickel equivalents(Ref. 17) estimated by the WRC-1992 Di-agram. The increase in dilution reduceschromium equivalent and moderately de-creases the nickel equivalent of thecladding, which results in reduced FN.The lower dilution in claddings resulted inhigher FN in the clad metals (Ref. 18).

Direct Effects of ProcessParameters on Calculated andMeasured FN

Direct Effects of Welding Current on Calculated and Measured FN

From Fig. 7 it is evident that calculatedand measured FN decrease with an in-crease in welding current. This may be dueto an increase in heat input and dilutionwith an increase in welding current. An in-crease in welding current resulting in en-hanced heat input and higher current den-sity causing a larger volume of the baseplate to melt and hence increased dilution.This decreases the FN of the claddings.

Direct Effects of Welding Speed onCalculated and Measured FN

From Fig. 8 it is evident that the calcu-lated and measured FN decrease with anincrease in welding speed. This may bedue to increased dilution of the base metalin the pool with an increase in weldingspeed, since the weight of deposited metalper unit of length decreases while the

Fig. 17 — Contour plot and response surface plot for interaction effectsof welding speed and welding gun angle on calculated FN.

Fig. 18 — Contour plot and response surface plot for interaction effects ofwelding speed and welding gun angle on measured FN.

Table 5 — Heat Input and Corresponding Values of Dilution and FN

S. Trial I S N T V Heat Input Dilution Calculated MeasuredNo. No. (Volts) (kJ/mm) (%) FN FN

1 17 200 40 26 80 32 0.96 05.86 34 472 07 225 50 28 75 38 1.00 09.69 29 373 28 250 40 26 80 40 1.50 11.71 28 334 02 275 30 30 75 43 2.37 12.10 27 31

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cross section of the bead decreases verylittle (Refs. 19 and 20). The speed, there-fore, exerts an influence on the composi-tion of the weld bead analogous to that ofcurrent. The effect of dilution is moredominant than the effects of heat input onthe FN with increased welding speed,hence the FN decreases with the increasein welding speed.

Direct Effects of Contact Tip-to-Workpiece Distance onCalculated and Measured FN

It is evident from Fig. 9 that calculatedand measured FN increase slightly with anincrease in contact tip-to-workpiece dis-tance. An increase in contact tip-to-work-piece distance increases the circuit resis-tance, which reduces the welding current.This decrease of welding current reducesthe penetration of the arc, and hence, re-duces the dilution (Ref. 21). The decreasein dilution increases the FN of the clad met-als. The changes in contact tip-to-workpiecedistance do not affect the heat input much.

Direct Effects of Welding GunAngle on Calculated andMeasured FN

From Fig. 10 it is evident that calculatedand measured FN increase with the in-crease in welding gun angle. The reason iswhen the angle is increased in forehandwelding, the arc force pushes the weld metalforward, i.e., toward the cold metal, whichreduces penetration and dilution (Ref. 22).This results in increased FN of the clad met-als. The changes in welding gun angle do notaffect the heat input much.

Interaction Effects of ProcessParameters on Calculated andMeasured FN

Interaction Effects of Welding Current andWelding Gun Angle on Calculated andMeasured FN

From Figs. 11 and 12 it is evident that theFN decreases with an increase in weldinggun angle when welding current is from 200to 275 A, and the rate of increase in FN alsodecreases with an increase in welding cur-rent up to 275 A. But when welding currentis 300 A, the FN decreases with a decreasein welding gun angle. Figures 13 and 14show the response surface and the contourplot of FN for the interaction of weldinggun angle and welding current. From thecontour surface, it is found that the FN hasits lowest value when welding current is atits maximum value and the welding gunangle is at its maximum value. The highestvalue of FN is obtained when welding cur-rent is at its minimum value and the weld-ing gun angle is at its minimum value.

Interaction Effects of WeldingSpeed and Welding Gun Angleon Calculated and Measured FN

From Figs. 15 and 16 it is evident that

FN increases with a decrease in weldinggun angle when welding speed is from 40to 60 cm/min. The rate of increase in FNalso decreases with the increase in weldingspeed up to 40 cm/min, but when weldingspeed is from 20 to 40cm/min, FN de-creases with a decrease in welding gunangle. Figures 17 and 18 show the re-sponse surface and the contour plot of FNfor interaction of welding gun angle andwelding speed. From the contour surface,it is found that the FN has the lowestvalue, when welding speed is at its maxi-mum value and the welding gun angle is atits maximum value. The FN has highestvalue when welding speed is at its mini-mum and the welding gun angle is at itsmaximum value.

Conclusions

The effects of welding current, weldingspeed, contact tip-to-workpiece distance,and welding gun angle on the FN in duplexstainless steel deposits were investigated.The following are the conclusions derivedfrom this investigation:

1) A five-level, four-factor full-factor-ial design matrix based on the central com-posite rotatable design technique can beused for the development of mathematicalmodels to predict calculated (by WRC-1992

Table 6 — Comparison of R2 Values and Standard Error of Estimates for Full and ReducedModels

Parameters R2 values Standard error of estimatesFull models Reduced models Full models Reduced models

Calculated FN 0.757 0.780 2.160 2.053Measured FN 0.833 0.863 2.529 2.290

Table 8 — Results of Conformity Tests

Process parameter in Predicted Values of Actual Values of FN Error, %Coded form FN (using models)

I S N T FN(C) FN (M) FN(C) FN (M) FN (C) FN (M)–0.11 –0.22 0.09 –0.3 27 33 28 32 3.70 –3.03–0.79 –0.35 0.94 1.02 30 36 30 34 0.00 –5.56–0.66 0.03 0.90 1.03 28 39 28 40 0.00 2.56

% Error = Actual value – Predicted value x 100. FN(C) – Calculated FN, FN (M) – Measured FNPredicted value

Table 7 — Analysis of Variance for Testing Adequacy of the Models

Parameter 1st order 2nd order Lack of fit Error terms F-ratio R-ratio Whether modelTerms Terms is adequate

SS DF SS DF SS DF SS DFFN(C) 433 4 67 10 36.92 10 37.8 6 0.547 9.590 adequateFN (M) 852 4 194 10 91.00 10 12.0 6 3.406 59.82 adequate

FN(C) – Calculated FN, FN (M) – Measured FN, SS – Sum of squares, DF – Degrees of freedomF-ratio (10, 6, 0.05) = 4.09, R-ratio (14, 6, 0.05) = 3.96

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WELDING RESEARCH

MAY 2006-s100

Diagram) and measured FN of duplexstainless steel cladding deposited by FCAW.

2) The Ferrite Number calculated fromthe cladding compositions using WRC-1992Diagram agrees reasonably well with themeasured FN.

3) The Ferrite Number decreases withthe rise in heat input and dilution.

4) The Ferrite Number decreases with arise in welding current and welding speedand increases with a decrease in weldinggun angle and rise in contact tip-to-work-piece distance.

5) A decrease in welding gun angle in-creases the FN when welding current waslow, but the FN decreases slightly with a de-crease in welding gun angle when weldingcurrent was high.

6) A decrease in welding gun angle in-creased the FN when welding speed washigh, but the FN decreased slightly with adecrease in welding gun angle when weld-ing speed was low.

Acknowledgments

The authors wish to thank M/S Bohlerwelding, Austria, for providing flux coredduplex stainless steel welding wire for thiswork. The financial support for this workfrom All India Council of Technical Educa-tion and University Grants Commission aregratefully acknowledged. The authors alsowish to thank the management of Coimbat-ore Institute of Technology and Ku-maraguru College of Technology for pro-viding all the necessary facilities to carry outthis research work.

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