INTRODUCTION

A welding procedure should include descriptions on numerous welding process

variables. It is important for the variables associated with welding to be described in the

welding procedure with sufficient detail in order to allow for the reproduction of the weld and

also give clear explanation on the parameters of the weld. These variables often come in two

documents which are:

i) Welding Procedure Specification (WPS)

ii) Procedure Qualification Record (PQR)

This project will be referring to the standard code of ASME Section IX for the

application of pressure vessel because the material chosen for the weld process was

determined by following this standard.

Welding Procedure Specification

A WPS is a formal written document that includes a description on the welding

procedures which provides direction to the welder or welding operators that would enable

them to produce sound and quality weld as required by the code. The objective of this

document is to guide welders to do the weld according to a proven and accepted procedure so

that repeatable and trusted welding techniques are used. A WPS is developed for each

material alloy and for each welding type used, normally guided by specific codes or

engineering societies. A WPS is supported by a Procedure Qualification Record (PQR).

Before a WPS is produced, a preliminary WPS are usually proposed according to the

standard, a test weld specimen are prepared and weld tests such as non-destructive tests and

destructive tests are done on the specimen.

Procedure Qualification Record

The American Welding Society (AWS) define PQR as a record of welding variables

used to produce an acceptable test weldment and the results of tests conducted on the

weldment to qualify a Welding Procedure Specification. The American Society of

Mechanical Engineers (ASME) similarly defines welding PQR as a record of variables

recorded during the welding of the test coupon. The record also contains the test results of the

tested specimens. The test includes mechanical tests such as destructive test and non-

destructive test. All of the testing requires the endorsement from third party inspection bodies

such as Lloyd consultation, meaning the tests are conducted and approved under supervision.

The PQR must be in compliance with the standard of the task carried out. This is what allows

the PQR to be able to support and validate a WPS.

Welder Qualification Record (WQR)

Welder certification or WQR is a process that examines and documents a welder’s

capability to produce welds of acceptable quality while following a well defined welding

procedure. Specially designed tests are used to determine a welder’s skill and ability to

deposit sound weld metal. The welder’s test consist of many variables which include specific

welding process, type of metal, thickness, joint design and position among other things. Often

times the test is conducted according to a particular code. It can be administered under the

auspices of national or international organization, such as AWS or ASME, but manufacturers

may specify their own standards and requirements.

Application of WPS

For this project, the application that was chosen was the weldment of outer shell for

pressure vessel, therefore the standard that must be referred to is the standard code ASME

section IX. Below is a diagram that shows a pressure vessel.

Figure 1: Pressure Vessel undergoing fixing

PRELIMINARY WPS PROPOSAL

This section lists out and discusses all of the parameters that were chosen for the

proposal of the pWPS including justification for choices that were made.

The Standard Code

In order to produce the pWPS and conduct the welding process, the ASME Boiler and

Pressure Vessel Code (BPVC) Section 9 was followed. This code only acts as guideline for

the preparation of the parameters of the welding process and also the tests to be done on the

weldment. To prove the competence of the welding process or the WPS however must be

done by an Independent Inspection Body, or in our case, a lecturer or technician that has the

required qualification.

Selection of Welding Process

Gas metal arc welding (GMAW) was proposed for the pWPS. GMAW was selected

because it is suitable for welding most ferrous and non ferrous metals, not to mention that

GMAW is a process that is used extensively and most common in the metal fabrication

industry. This process was also chosen after considering the availability of the equipment and

the skill needed by the welder to perform it. GMAW can be used to weld in any position, uses

direct current electrode positive (DCEP) polarity and due to the equipment having automatic

arc control, the welder only need to pay attention to the gun positioning and travel speed.

Shielding Gas

The proposed shielding gas to be used for the GMAW process is Carbon Dioxide

(CO2) with 100% purity with a flow rate of 15 – 20 l/min. Carbon dioxide is the most

common among the reactive gases often used and it is the only type of gas that can be used in

its pure form without the addition of inert gas. CO2 is also the least expensive when

compared to other common shielding gases while still being able to provide very deep weld

penetration which is useful for welding thick material.

However the CO2 shielding gas is not without weakness, such as producing a surface

weld that is usually heavily oxidized. Therefore certain countermeasures must be planned in

order to produce better quality weld such as the properly choosing the type of electrode that

has higher amounts of deoxidizing elements to compensate for the loss of alloying elements

across the arc.

Base Metal

The material used in the fabrication of the pressure vessel is carbon steel. For the

exact material steel code and the P-numbers assigned, we refer to the ASME Section IX

(QW/QB-422 table at appendix). From this we were able to determine the base metal

characteristics such as the composition, weldability and mechanical properties. The thickness

chosen for the base metal is 6 mm.

Joint Design

The proposed joint design was a single V butt joint. Single V was chosen to allow for

bigger opening compared to single bevel, and this will allow for better penetration to the root

of the weld, considering the diameter of the electrode is quite big at 1.2 mm. The root

opening has a gap of 2 mm, which is small but enough because of the single V configuration

that allows the weld metal to penetrate deep. The weld is also designed to have a double pass,

as 1 pass would not be enough to cover the whole groove due to the thickness of the metal

which is bigger than 4 mm.

Electrical Characteristic

As mentioned before, the proposed current polarity is direct current electrode positive

also known as reverse polarity for the GMAW process. This type of connection allows for a

stable arc, smooth metal transfer, relatively low spatter loss and good weld bead

characteristics. After careful consideration and calculation, the current proposed for the weld

is 110 – 130 A for the first pass and 120 – 150 A for the second pass. This produces a heat

input from the range 0.8 to 1.2 which is considered to be on the low side but is preferred in

order to reduce the tendency of distortion from happening.

Welding Position

The welding position of choice is 1G which is a flat position. This position was

proposed because it is the easiest one to perform. The holding of the welding gun and the

angle however is dependent on the skill of the welder.

Welding Technique

In terms of welding technique, the proposed technique for pass 1 is the weaving weld.

This is to ensure that the weld can properly fill the gap in the groove and penetrate to the root.

The second pass is then proposed to be using the linear technique. The most important thing

in ensuring the techniques can produce a sound weld bead is the speed of the weld which

must be controlled by the welder.

Figure: Cutting Process Figure: Milling Process

Figure: Welding Process

PREPARATION AND THE WELDING PROCESS

The preparation began with the choosing of the material, and in this case a carbon

steel plate with a thickness of 6mm was selected. The material came in size that is much

bigger than our intended use; therefore we proceed to cutting the material into the desired

dimension. The specimens were then machined using milling machine to make the groove

angle of 30˚ for each side, having a single V groove of 60˚. Before proceeding to welding

process, the specimens were grinded to remove the unwanted chips and to produce smooth

edges.

Once the specimens are prepared, the welding process can begin. Tack weld was

performed on the specimens to ensure that they stay in position during the welding process,

making sure the distance between the two plates remain the same and so on. With the

welding parameters required as in the pWPS, the welding test was performed. Lastly the

actual parameters used in full welding was recorded and stated in Procedure Qualification

Record (PQR). The welding process was performed by an experienced welder of UiTM

FKM.

Figure: Welding Result

Once all the required specimens have been welded, they were given a visual

inspection, which is to inspect any defects that exist on the physical surface of the welded

part. After deciding that the specimens have all passed the visual inspection, we then proceed

with the non-destructive testing (NDT) by performing liquid penetrant test (LPT). LPT was

performed to inspect the tiny defects on the surface that cannot be seen with our naked eyes.

Afterwards, guided bend test and tensile test were performed in the lab for destructive testing

methods (DT). The purpose of DT is to determine the weld quality and to measure the

strength of the material while making sure they pass the required value as stated in the

standard. All of the tests performed were recorded in the PQR forms.

PQR DESCRIPTION

This section provides information recorded during the all tests that were done. Before the

WPS can be endorsed, the welding coupon provided need to be filled, which is done by

completing the tests. The completion and the success of the PQR would allow the pWPS to

be considered acceptable and legal for use.

Visual Inspection

Visual inspection is the easiest and cheapest method of inspection. This method is

mostly effective in saving time to determine whether the specimen should proceed to the

other tests or not.

When designing an inspection plan, we need to establish the most appropriate areas to

apply our inspection. Visual inspection can often be utilized to prevent welding problems

from happening in the first place. The welding inspection function is often divided into three

areas. First, and often the least utilized, is inspection before welding. This type of inspection

can often provide us the opportunity to detect and correct unacceptable conditions before they

develop into actual welding problems such as material specification, material condition,

cleaning, inter-pass temperature, root fusion and penetration. Second, inspection during the

welding operation can often prevent problems in the completed weld through verification of

the welding conditions and procedural requirements. Third, inspection after welding which is

a relatively easy method of conducting completed weld quality evaluation. The inspection

may done with evaluate the cleaning, bead appearance, weld penetration, reinforcement, weld

width and other parameter that related.

Non Destructive Test

The definition of NDT is to evaluate the properties of a material, component or

system without causing damage, allowing the material to still be used for any application.

There are a variety of NDT such as liquid penetrant test, magnetic particles test, radiographic

test, ultrasonic test and eddy current. In this project, we have decided to choose liquid

penetrant test because it is a widely applied method and is considered a low-cost inspection

method used to locate surface-breaking defects such as fine crack, surface porosity and

fatigue cracks.

The penetrant may be applied to all non-ferrous materials and ferrous materials,

although for ferrous components magnetic-particle inspection is often used instead for its

subsurface detection capability.

Dye penetrant process

Destructive Testing

The definition of DT is to evaluate the properties of a material, component or system

while the material undergoes destruction in order to understand the specimen's structural

performance or material behavior under different loads. Once a specimen has undergone a

destructive test, it can no longer be used. These tests are usually much easier to carry out, and

are easier to interpret than NDT. There are five types of DT which are transverse tensile test,

bend test, hardness test, impact toughness test, macroscopic test, and nick break test. In this

project, we only focus on two of the five; tensile test and bend test.

Tensile Test

Tensile test can be defined as the material testing to evaluate the strength and

behavior of material when the sample is subjected to a controller tension until failure. Figures

on the next page show the specimen and process. The data from this test was recorded and

tabulated in table 1.

Figure: tensile test specimen

The specimen breaks at maximum load

Table 1: experimental data

Details A36

Width (mm) 20 mm 21 mm

Thickness (mm) 6 mm 6 mm

Gauge length (mm) 45 mm 45 mm

Cross-sectional area (mm2) 120 mm2 126 mm2

Ultimate load (N) 53.16 kN 54.94 kN

Ultimate Tensile strength (MPa) 443 MPa 436 MPa

Tensile extension at maximum load (mm) 10.647 mm 10.479 mm

Tensile extension at break (mm) 11.869 mm 11.682 mm

Tensile strain at maximum load (%) 23.660 % 23.287 %

Type of failure location Base metal Base metal

Sample calculation

To calculate Normal stress

σ = P / A

Where; σ is the engineering stress

P is the external axial tensile load

A is the original cross-sectional area of the specimen

To calculate ultimate tensile strength

σts = Pmax / A0

Where; σts is the engineering stress at maximum load

Pmax is the external axial tensile load maximum

A0 is the cross-sectional area of the specimen

σy = 53.16 k / 120

σy = 443 MPa

To calculate strain at maximum load

s = (final length - initial length) / initial length

s = 10.647 / 45

s = 0.2366

Therefore, the strain percentage at maximum load acting on specimen was 23.66%.

Bend test

Root and face bend tests are also considered as a simple and low cost method of

testing. The results are observed physically and will directly show any signs of poor fusion or

weaknesses such as porosity within the weld. To perform face and root bend test, the part of

testing should be at tension load. Once bended, the specimen is inspected visually, and if the

weld doesn’t break or show signs of cracking, it shows that the weldment was good. The strip

should show a nice even radius. Figure below shows the results of the two bend tests.

From the figure, we can see that there are no cracks found on the face and root of the

weld after the bend tests. The surface of the weld metal seems to look very smooth without

any defect.

DISCUSSION & RECOMMENDATION

Figure: face bend test (FBT)

Figure: root bend test (RBT)

Some of the parameters that have been suggested in the pWPS are far from perfect

and has many more rooms for improvement. For example the welding techniques chosen for

the first and second pass which are weave and linear movement respectively would be better

if both passes were to be done only using weave technique. This is because during the linear

weld of the second pass, the travel speed of the weld had to go lower than recommended in

order to properly seal the groove with a beautiful bead. This allowed for higher heat input on

the base metal which caused a little bit of distortion. This distortion however was fixed

during the flashing of the weld bead for the preparation of the dog bone for tensile test.

The dye penetrant test gave a better look on the condition of the weld surface;

however the addition of arc spark spectrometer test would give a better understanding of the

homogenousity of the weld and help in determining the most suitable filler metal to be used.

The tensile test was a success; however the thickness of the material was slightly

lower than 6 mm after it was cut into the dog bone shape due to the act of grinding the weld

bead and the surface of the base metal. There is a worry that the lack of thickness could

somehow affect the end result of the tensile test from acquiring the most accurate result.

CONCLUSION

In conclusion, the project was a success according to the all the visual inspection,

non-destructive and destructive tests, as we were able to produce results that are required by

the standard. The WPS produced by this project if used as parameters for welding materials

that are similar or in this case the fabrication of outer shell pressure vessel, should be able to

produce sound weld that is safe.

REFERENCE

[1] ASME Boiler and Pressure Vessel Code (BPVC) Section 9 handbook (2010)

[2] The Practical Welding Engineer Handbook by J. Crawford Lochhead and Rodgers

[3] Weld Inspection & Repair. The Goodheart-Willcox Co., Inc (2006)

[4] Welding Inspection Handbook by A W S Third Edition (2000)

APPENDICES