• Corresponding author: Rohollah Solhdost, E-mail address: seyed_ebrahim_vahdat@yahoo.com Doi: http://dx.doi.org/10.11127/ijammc2017.04.04Copyright@GRIET Publications. All rights reserved

Advanced Materials Manufacturing & Characterization Vol. 7 Issue 1 (2017)

Advanced Materials Manufacturing & Characterization

journal home page: www.ijammc-griet.com

Determining Impact of Groove Depth on the Repairing of Groove on the

Surface of General Structural Steel by Using Taguchi Method

Rohollah Solhdost1, Seyed Ebrahim Vahdat2,*

1 MSc Student, Department of Engineering, Bandar Abbas Branch, Islamic Azad University, Bandar Abbas, Iran. 2 Assistant Professor, Department of Engineering, Ayatollah Amoli Branch, Islamic Azad University, Amol, Iran,

A B S T R A C T

If pipelines are damaged or fail to function normally, downstream units will stop operation, daily activities will be disrupted, or the process will shut down. Awareness of and knowing about the condition of pipelines in service as well as recognizing the cause of failure and then taking timely measures to make appropriate changes can increase productivity. One of the appropriate measures is to repair the surface crack of steel pipelines using different welding methods. Two variables of crack width and crack depth are common in all the repair methods. In this research, the first variable is the crack depth and other variables are considered the second variables. Thus, the effect of crack depth on the repairing of steel surface crack is determined using Oxyacetylene Flame Welding with Spray of Pure Iron Powder. The test results are analyzed and then compared using the Taguchi test design method with one factor at one level (one-factor ANOVA). Investigating and analysis of the results showed that the contribution of crack depth variable in Oxyacetylene Flame Welding with Spray of Pure Iron Powder is 34.2%, 14.6% and 91.5% to achieving the yield strength, tensile strength and elongation of base metal, respectively. Thus, to achieve strength (tensile or yield) in the base metal, studying the effect of crack depth is not enough by itself; however, to achieve relative elongation of the base metal, the most effective variable is crack depth (91.5% contribution).

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Keywords: Flame Welding, One level, One variable, Oxyacetylene

1.Introduction

Pipelines are the safest and most economical way for

transporting gas, crude oil, and petroleum products all over the

world, with less negative environmental effects. Therefore,

energy transportation through pipelines is extremely important.

Islamic Republic of Iran, with 14000 km of oil transportation and

more than 22000 km of gas transportation pipelines, has the

longest network of oil and gas pipelines in the Middle East.

Moreover, the unique conditions of Iran in terms of hydrocarbon

resources as well as position of the country among its

neighboring countries and the Middle East have made Iran a safe

energy transportation pathway and the intersection of energy

resources in the Middle East to European and southeastern

Asian markets.

The test design concepts that are currently used are the results

of Fisher's research [1] in optimizing agricultural operations

nearly half a century ago. Optimization using Fisher's method

requires too much time and is not economical. Hence, methods

such as fractional factorial tests, which merely examine a

fraction of total possible combinations, are usually used for this

purpose. Using this method, the goal can be achieved much faster

with lower costs. However, the use of such methods

requiresnumerous mathematical calculations and operations in

test design steps and analysis of results. In test deigns using

fractional factorial methods, each tester can provide a different

design for performing a test with a specified purpose. The

Taguchi method [2] is also named factorial design in some

publications. It is designed to improve the quality of products

and processes. Test design methods are helpful in achieving

optimum quality in manufacturing products. In this method, no

matter how the quality of product is measured, it will include

one of the following three features:

-The bigger is the better.

-The smaller is the better.

-The closer to the nominal value is the better.

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In this research, the closer the results of repairing the surface

crack of metal components to the nominal value (base metal or

control sample) is the better.

In case of any accident in the pipelines, there will be irreparable

human and environmental disasters. The main cause of

accidents is the formation of cracks mainly at transportation

lines. The early detection of surface cracks in the transportation

lines and their repair can prevent the occurrence of accidents,

human and financial losses, and waste of energy resources

[3,4,5]. Thus, identifying the repair variables of surface cracks of

metal components and their contribution is highly important.

Different methods are used for repairing the surface crack in

industrial components [6, 7], such as body of heavy diesel

engines [8]. Each method has its own variables in addition to

crack opening size and depth [9, 10, 11].

So far, the effect of crack opening size on the tensile strength and

elongation of the structural steel repaired using spray welding

with oxyacetylene flame and pure iron powder has been

evaluated. In this evaluation, the contribution of crack opening

size is 10.15% for achieving the tensile strength of the base

metal, and 66.20% for achieving the elongation of the base metal

[12]. In this research, the effect of crack depth (the second

variable) will be examined.

In addition, the effect of crack opening size on the tensile

strength and elongation of the structural steel repaired byspray

welding with magnetic induction and pure iron powder is

evaluated, in whichthe contribution of crack opening size is

18.37% for achieving the tensile strength of the base metal,

and51.95% for achieving the elongation of the base metal [13].

In this research, the effect of crack depth (the second variable)

will be examined using spray welding with oxyacetylene flame

and pure iron powder.

In this research, determining the contribution of crack opening

size using spray welding by oxyacetylene flame and pure iron

powder to achieve the yield strength, tensile strength, and

elongation of the base metal (control sample) is considered

using Taguchi method with one factor on one level (single-factor

ANOVA). Thus, the crack depth is ranked based on effectiveness

and used in future studies.

Materials and Methods

In order to ensure accuracy in this study, all the tests were

carried out in Razi Metallurgical Research Center (RMRC).

Moreover, to ensure precision, all the tests were repeated for at

least three times.

A structural steel (St37) plate with the thickness of 6mm was

purchased from Mobarakeh Steel Company and chemical

composition determined with ARL for controlling the material of

the plate, as listed in Table (1). Tensile test using GOTECH 7100L

was employed to assess the flexural strength. Tensile test

samples were prepared with the thickness of 6 mm in

accordance with the ASTM A37 [12] standard.

Table (1): Chemical composition of St37

Element Fe C Si Mn

Wt% Balance 0.15 0.18 0.65

Element P S Co Al

Wt% 0.020 0.008 0.004 0.002

Element Cu Cr Ni V

Wt% 0.008 0.31 0.008 0.003

Results and Discussion

In this research, the quality of the product (repaired samples)

was defined in accordance with the base metal (control sample).

Thus, the purpose was to achieve the yield strength, tensile

strength, and elongation of the base metal (control sample).

In order to study the efficiency of repairing structural steel

surface groove using spray flamewelding with pure iron powder,

five sets of samples including a control sample without the

groove, and four sets of grooved samples withthe groove depths

of 0.5, 0.8, 1, and 1.3 mm, but equal groove lengths (10 mm) and

widths (0.5 mm), were prepared. Each set of the samples

included four samples, three of which were used for tensile test

and one for metallographic test. Mean yield strength, mean

tensile strength, and mean elongation of the control and grooved

samples with the depths of 05, 0.8, 1, and 1.3 mm were

determined and listed in Table (2). For repair, iron powder with

the purity of >99.5% and average grain size of 40μm was

purchased from Pourian Chemical Corporation.

Table (2): Comparison of tensile properties of the repaired specimens with control specimen

Average of elongation

%

Average of tensile strength

MPa

Average of yield strength MPa

Groove depth

37.8 442 293

0 mm (Control, without groove)

24.3 458 317 0.5 mm

16.3 422 297 1 mm

There were two variables in this research. A common variable,

i.e. crack depth, was considered in all the repair methods. Thus,

the second variable included all the other variables with errors.

According to Tables (3) and (4), the L12 array was used for two

variables (crack depth and other variables with errors) with four

levels (0.5, 0.8, 1, and 1.3 mm). In the L12 array, there were

twelve independent test positions, specified by row numbers. In

the spray flame welding method using pure iron powder, other

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variables were welding temperature, welding time, crack

opening size, powder grain size, powder material, preheat time,

preheat temperature, and surface preparation. In this research,

these variables were equal.

Table (3): Variables and levels in the L12 array

Level 4 Level 3 Level 2 Level 1 Variables

1.3 mm 1 mm 0.8 mm 0.5 mm Crack depth

1 1 1 1 The others and error

In this research, the quality of the product (repaired samples)

was defined in accordance with the base metal (control sample).

Thus, the purpose was to achieve the mean yield strength of 293

MPa, tensile strength of 442 MPa, and elongation of the base

metal (control sample) of 37.8%. Therefore, the results were

recorded in the right column of the orthogonal array. Since there

was only one implementation in each test situation, the results

were recorded in one column. Based on Table (4), the L12

orthogonal arrays had 12 rows, representing 12 combinations of

variables and their levels.

Table (4): L12 orthogonal array with yield strength, tensile strength, and elongation per array

Elongation %

Tensile strength

MPa

Yield strength

MPa

The others and error

Crack depth

No.

23.0 458 325 1 0.5 1

27.0 461 310 1 0.5 2

23.0 457 318 1 0.5 3

27.0 459 310 1 0.8 4

31.3 456 298 1 0.8 5

27.2 458 304 1 0.8 6

14.0 431 315 1 1 7

20.0 409 285 1 1 8

16.5 425 291 1 1 9

18.0 414 313 1 1.3 10

17.5 401 272 1 1.3 11

18.1 412 286 1 1.3 12

37.8 442 293 Goal values is to obtain

Data obtained from single-factor tests were achieved by single-

factor analysis of variance. In order to organize the analysis of

variance ratio, it is necessary to calculate values such as degree

of freedom, sum of squares, and mean squares.

Therefore, the contribution of crack depth was only 14.6% in

achieving the tensile strength of the base metal. As a result, in

order achieve the tensile strength of the base metal, the crack

depth was not sufficient and other variables (85.4%

contribution) such as opening size had to be studied. Similarly,

calculations were carried out for yield strength and elongation,

as shown in Table (5). The contribution of crack depth was

34.2% for achieving the yield strength of the base metal. Thus, in

order to achieve the yield strength of the base metal, crack depth

was not sufficient, and other variables (65.8% contribution)

such as crack opening size had to be studied. Nevertheless, the

contribution of crack depth was considerable (91.5%) for

achieving the elongation of the base metal. Thus, in order to

achieve the elongation of the base metal, it was sufficient to

study crack depth.

Table (5): Comparison of crack depth and variance ratio to achieve the desired tensile properties

34.2%Contribution of variable of crack depth Achiving of Yield

stregth 65.8% Contribution of the others and error

3.68 Variance ratio 14.6%Contribution of variable of crack depth Achiving of

Tensile strength

85.4% Contribution of the others and error 0.54 Variance ratio

91.5% Contribution of variable of crack depth Achiving of Elongation

8.5% Contribution of the others and error 96.48 Variance ratio

For a simpler comparison of crack depthand variance ratio to

achieve the desired tensile properties after repairing the general

structural steel using flame spray welding with pure iron

powder, the results of Table (5) are depicted in Figure (1). It can

be seen that by increasing the variance ratio, the contribution of

crack depth was also increased.

Figure (1): Comparison of crack depth and variance ratio to achieve the desired tensile properties

The effect of differential of tensile properties calculated and

presented in Table (6). In addition, the percentage of them

calculated and presented in Table (7). The results confirm that

elongation will have the highest contribution rather than the

others. Because of when variable of crack depth changes,

elongation will change 21.3% while yield strength and tensile

strength will change 3% and 2.7%, respectively.

Table (6): The effect of differential of tensile properties

0.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

80.00

90.00

100.00

yield strength tensile strength elongation

contribution of variable of

crack depth

variance ratio of tensile

properties

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Differential of Yield strength

MPa

Tensile strength

MPa

Elongation

%

0.5 to 0.8 -13.67 -1.00 4.17

0.8 to 1 -7.00 -36.00 -11.67

1 to 1.3 -6.67 -12.67 1.03

Average -9.11 -16.56 -2.16

Table (7): Percentage of the effect of differential of tensile properties

Differential of Yield strength Tensile strength Elongation

0.5 to 0.8 4.3% 0.2% 17.0%

0.8 to 1 2.3% 7.8% 41.0%

1 to 1.3 2.2% 3.0% 6.0%

Average ≈3.0% ≈2.7 ≈21.3%

Conclusion

In this research, using the Taguchi method with one factor on

one level (single-factor ANOVA), the effect of one of the two

common variables in all the surface crack repair methods in

metal components, i.e. crack depth, was examined. The purpose

of surface crack repair of the metal component was to achieve

the yield strength, tensile strength, and elongation of the base

metal. Results of the test in spray flame welding using pure iron

powder demonstrated that the contribution of crack depth in

achieving the yield strength, tensile strength, and elongation of

the base metal was 34.2%, 14.6%, and 91.5%, respectively. Thus,

in order to achieve the yield and tensile strengths of the base

metal, it was not sufficient to study the effect of crack depth

alone. Nevertheless, in order to achieve the elongation of the

base metal, the most effective variable (91.5%) was crack depth.

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