PROJECT REPORT

(Project Semester January – May 2014)

Analysis of Defects in Tube Mill Process

Submitted by Vaibhav Gupta

STUDENT ID – 11108037

Under the Guidance of

Mr. Sanjeet Mr. R N Saini

Dept. of Materials and Metallurgical Engineering Senior Engineer

PEC University of Technology Bhushan Steels Ltd.

Chandigarh Sahibabad

Department of Materials and

Metallurgical Engineering

PEC University of Technology, Chandigarh

January to May, 2014

DECLARATION

I hereby declare that the project work entitled “To Analyse the defects in various grades of steels in

the tube mill process” is an authentic record of my own work carried out at Bhushan Steels

Ltd., Gurgaon as requirements of sixth semester project for the award of degree of B.E. (Materials

and Metallurgical Engineering), PEC University of Technology (Deemed University),

Chandigarh, under the guidance of Mr. Vikrant singh (Industry Coordinator) and Mr. Shalom

Akhai (Faculty Coordinator) during January to May,2014.

Vaibhav Gupta

Date: ______________ 11108037

Certified that the above statement made by the student is correct to the best of our knowledge and

belief.

Mr. Shalom Akhai Mr. Vikrant Singh

Dept. of Materials and Metallurgical Engineering Senior Engineer

PEC University of Technology Bhushan Steels Ltd.

Chandigarh Sahibabad

ACKNOWLEDGEMENT

It gives me a great pleasure to take this opportunity to thank Bhushan Steels Ltd. for giving me an

opportunity to work in their esteemed organization. I deem it my privilege to have carried out this

dissertation work under this well-known quality conscious organization.

I would like to thank Dr. Manoj Arora (Director, PEC University of Technology) and Dr. Uma

Batra (Head of Department of Materials and Metallurgical Engineering) for providing me the

cognitive base required to undergo my semester training. I want to express my sincere gratitude to

my guide Mr. Vikrant Singh, Project Leader for all his guidance. He was with me throughout the

term of the project and rendered me all the help required.

My gratitude also extends to all those people, apart from the above, who met me during this

endeavor and enriched me with their support and knowledge, in one way or the other and gave a

helping hand for the project.

I would also like to take this opportunity to express my sincere gratitude to my faculty co-

coordinator, Mr. Shalon Akhai, for his constant guidance, valuable suggestions and moral support.

NO OBJECTION CERTIFICATE

(To Whomsoever It May Concern)

This is to certify that Mr. Vaibhav Gupta, STUDENT ID: 11108037, student of PEC University

of Technology, Chandigarh, has successfully completed his PROJECT Training at Bhushan

Steels Ltd., Sahibabad from 10th January 2014 to 24th May 2014.

His report does not contain anything that can endanger the secrecy and the working of the

company. We have “NO OBJECTION” for the matter and drawings contained in the report.

__________________________

Vikrant Singh

Senior Engineer

Bhushan Steels Ltd.

Sahibabad

ABOUT BHUSHAN STEEL LTD.

Bhushan Steel Ltd is engaged in the steel business. The company has portfolio of flat products,

which are manufactured at steel processing facilities at Sahibabad, Uttar Pradesh. The company is

producing cold rolled close annealed coils (CRCA), galvanized sheets, precision tubes, high tensile

steel, hardened and tempered steel strip (H&T strips), wire-rods, color-coated sheets and galume.

They also produce, sponge iron, pig iron, billets, slabs, HRC and power. Bhushan Steel Ltd was

incorporated on January 7, 1983 with the name Jawahar Metal Industries Pvt Ltd. In January 14,

1987, Brij Bhushan Singal and his sons Sanjay Singal, Neeraj Singal and associate companies took

over the management of the company by acquiring the entire stake. In the year 1989, the company

became a deemed public limited company. In the year 1992, the company was renamed as Bhushan

Steel and Strips Ltd after diversifying into wide-width cold-rolled (CR) steel strips. Also, they

completed the cold rolling plant during the year. In the year 1993, the company came out with their

first public issue to finance their forward integration project for the manufacture of 1,00,000 tpa of

continuous annealed/ galvanized steel strips. In January 1994, the company commissioned the

galvanising plant with a capacity to manufacture 120000 tonnes per annum of wide width cold rolled

steel strips and 100000 tonnes per annum of galvanised sheets.

HISTORY OF THE COMPANY

Year events 1983 - The company was incorporated on 7thJanuary, under the name of Jawahar Metal

Industries Private Limited for the manufacture of cold rolled steel strips and steel ingots at

Sahibabad Industrial Area, District Ghaziabad.

1987 - On 14th January, Brij Bhushan Singal and his sons Sanjay Singal and Neeraj Singal and

associate companies took over the management of the company by acquiring the entire share capital

of the company.

1989 - The company undertook the setting up of a new plant for the manufacture of wide width Cold

Rolled Steel Strips with integrated plant facilities.

1992 - The name of the company was changed to the present name of Bhushan Steel & Strips

Limited and fresh Certificate of Incorporation was issued on 9th June.

1993 - The company made its maiden Public Issue of 22 lakh equity shares of Rs.10 each at a

premium of Rs.55 share aggregating Rs. 1430 lakh in September/October.

1994 - The galvanizing plant was commissioned in January. Presently the company has facilities for

the manufacture of 1,20,000 tones per annum of wide width cold rolled steel strips and 1,00,000

tones per annum of galvanized sheets.

1995 - The Cold Rolling Expansion the Company is installing state of the art 1600mm width 6HI

combination Universal Crown Mill (UCM) of Hitachi, Japan with sophisticated features for shape

control and surface finish to cater to the requirements of the automobile and white goods sector.

1996 - The Part B of 68,94,800 14% unsecured fully convertible Debentures aggregating Rs 8375

Lakhs have been converted into Equity Shares w.e.f. 1st April.

1998 - With the commissioning of the new plant recently set up at company's existing site at

Sahibabad (UP), the company is now exploring further growth possibilities of setting up a modern

Cold Rolling cum Galvanizing Unit at West Coast of the Country.

1999 - During the year, the company has set up a dedicated service centre for large OEM customers

at Sahibabad so as to ensure supplies to them on 'just in time' concept.

2000 - The Delhi-based Bhushan Steel and Strips' to set up a Rs 750 crores cold rolled steel plant is

likely to hit a road block.

2002-Strikes an important position in the market for cold rolled steel for automobiles, feeding over

70% of demand for car bodies.

2003-Enters into a strategic alliance with Sumitomo Metal Industries of Japan under which, the latter

has further extended process know-how for the manufacture of automotive steel sheets for a period

of six years

2004-Bhushan Steel awards Rs 36 crore order for BHEL

2006-Bhushan Steel & Strips Ltd has informed that Sh. Sanjay Singal, has ceased to be a Director of

the Company w.e.f. October 18, 2006.

2007-Company name has been changed from Bhushan Steel & Strips Ltd to Bhushan Steel Ltd

2008-Bhushan Steel Ltd has informed that w.e.f. September 23, 2008, Sh. B B Tandon has been

appointed as an Additional Director on the Board of the Company as a Independent Non-Executive

Director.

2009-Bhushan Steel buys Aussie exploration firm

2010- Bhushan Steel Ltd has informed that Life Insurance Corporation of India has appointed Smt.

Sunita Sharma, their representative as a Nominee Director on the Board of the Company.

COMPANY PROFILE

Type: Private

Founded in: 1987

Headquarters: India

Key persons: Brij Bhushan Singhal (Chairperson) Neeraj Singhal (Managing Director)

Industry: Steel

Website: www.bhushangroup.com

VISION OF THE COMPANY

The key to Vision is to use rigorous conceptual framework and to understand how that framework

connects to the underlying DNA of enduring great companies.

A well-conceived vision consists of two major components—“CORE IDEOLOGY” and an

“ENVISIONED FUTURE”. A good vision builds on the interplay between these two

complementary Yin-and-Yang forces; it defines “What we stand for and Why we exist” that does not

change the Core Ideology and sets forth “What we aspire to become, to achieve.

It is true to say that most of our vision statements express an element of ambition. BSL’s vision of

total integration is a lot closer to realization today. Through seamless backward integration, BSL is

consolidating its position on the entire steel value chain from iron ore to specialized is surging

ahead.

POLICIES OF THE COMPANY

Integrated Quality, Environment, Occupational Health & Safety Management System Policy

Bhushan Steel Ltd. commits to produce cold rolled and galvanized steel sheets of world class quality

in a safe, healthy and clean environment by involving employees with continual improvements in

system implementation, technological advancement, operational integration, prevention of pollution

& hazards maintaining legal compliance and satisfying needs & expectations of Customers.

• For environment management system we have ISO 14001:2004 certification

• For quality system we have ISO/TS 16949:2002 certification

• For safety management system we have OHSAS

MAJOR CUSTOMERS

Manufacturing Sites

PRODUCTS

The automotive industry in India is booming, and there is a growing requirement for PT Tubes

across the industry. Bhsuhan Steel has been producing automotive grade steel for decades and has

established itself as a preferred manufacturer, supplier and technology leader.

Bhushan Steel is amongst the most prominent manufacturers of automotive tubes to the 2/3 wheeler

industry in India. Our constant focus on innovation, customer satisfaction and widening our product

offering has enabled us to win the trust of all major manufactures in India, and expand our customer

base further.

India has the largest production and consumer base for motorcycles. This segment is an important

part of our growth strategy. In the two-wheeler category we focus on all three segments – Motorcy-

cles, Scooter, and Mopeds. We constantly work to improve our product offering and jointly work

with our customers to develop new products for their upcoming model ranges, as per their require-

ments. We are also prominent suppliers of automotive tubes to the three-wheeler industry.

We supply all kind of frame tubes, head pipes, telescopic front forks and other ERW or CDW tubes

for two and three Wheelers

.

Applications

Frame

Handle Bar

Saree / Leg guard

Stand pipe

Seat Frame

Hood Pipe

Attributes

Superior flexibility & bend-ability

Higher strength

Excellent surface finish

Closer Dimensional Tolerances

Uses of tubes in cars

Precision Tubes division has specialized product offerings for passenger and commercial vehicles.

We are constantly working to expand our product base to fulfil the emerging needs of customers. We

have dedicated teams to work with our customers right from the inception of a particular model, and

best serve their tubes component requirement.

We supply propeller shaft tubes, steering linkage tubes, shock absorbers, chassis tubes etc for

passenger and commercial vehicles.

Applications

Steering Linkages

Steering Column

Shock Absorber

Propeller Shaft

Tie Rod

Axel Tube

Bush Pipe

The Process of Manufacturing pipes

There are two main types of pipes for industrial use. One is the seamless pipe and the other is

produced in the form of a pipe by welding. The most widely used categories of welding methods for

pipe-making are gas metal submerged arc welding and electric resistance/induction welding. The

work presented in this report is particularly focused on the steel pipes produced by high frequency

induction welding. It needs to be mentioned here that this technology is also suitable for tubing made

of other metals for different purposes.

The same process is described in phases here under :

a) The strips will be available is 60/80 feet folded lengths. The folded raw materials will be available

in bundles. The bundles will be open and straightened to facilitate welding for joining the strips to

have a continuous feeding to the machine.

b) Then the joined strip will be feeded to machine in the first stage the machine will remove the

bends and straighten the strip for the correct formation of pipe. The pipe making mill will be

connected to a slippering motor to have movement to the various rolls and to the raw material feeded

to the machine. The speed of the movement will depend upon the feeded Raw Materials width the

thickness. Because of this movement there will be friction between the rolls and strip. Because of the

friction the Rolls and some parts of the machine gets heated. Hence the mill will be connected by an

efficient and continuous water circulation system to cool the rolls and machine parts.

c) The next phase in the passage of raw materials through slitting zone to remove the excess and

uneven edges.

d) The next phase is the passages of raw materials through various rolls to convert into open seem

pipe.

e) The next phase is the passage of open seem pipe through welding rolls where the mill will be

connected to an automatic electrical welding unit which releases required heat to melt the edges of

the open seem pipe and through the pressure from rolls the edges and gap will be closed and

becomes closed pipe. Then by using a special cutting tool the weld flash will be removed.

f) The welded pipe/tube will be made to pass through cooling zone where there will be a continuous

cool water supply to control the heat caused by automatic electrical welding. The manufacturing

process requires continuous cool water supply to control the heat arises due to manufacturing

process.

g) The next phase will be the sizing and straightening of the Pipes/Tubes. Here the pipes and tubes

will be made to pass through rolls to control the bends of pipes/tubes.

h) The next phase will be the passage of pipes/tubes through cutting machine where the pipes/tubes

will be cut into required sizes and removing the pipes /tubes from machine bed with the finished

goods.

PICTORIAL REPRESENTATION OF THE PROCESS:

PICTORIAL REPRESENTATION OF PARTS OF TUBE MILL:

Cold Saw Machine:

A cold saw is a sawing machine that uses a circular saw blade to cut metal. The name "cold saw"

comes from the cutting process they employ. These sawing machines transfer the heat generated by

cutting to the chips created by the saw blade. Therefore, the blade and material being cut

remain cold, unlike an abrasive saw, which abrades the metal and creates a great deal of heat in the

metal and cutting blade. Cold saws use either a solid high speed steel (HSS) or tungsten carbide-

tipped, resharpenable circular saw blade. They are equipped with an electric motor and a gear

reduction unit to reduce the saw blade's rotational speed while maintaining constant torque. This

allows the HSS saw blade to feed at a constant rate with a very high chip load per tooth. A cold saw

cut produces minimal burr, no sparks, no discoloration and no dust. The material being cut must be

mechanically clamped to prevent movement during the cutting process. Extra care should be taken to

choose the appropriate number of teeth, saw blade type, cutting speed and feed rate. All of these

variables are based on the type and size of material being cut. Cold saws are capable of machining

most ferrous and non-ferrous alloys. Cold saws are intended to be used with a flood coolant system

to keep the saw blade teeth cooled and lubricated.

Horizontal uncoiler

The horizontal uncoiler was developed to suit special ways of coil storage. The coil handling of the

horizontal coil can be fully automated. Using the horizontal uncoiling version reduces the risk of

accidents and avoids the damage of strip edges.

High frequency induction welding

High frequency induction welding is a modern manufacturing method, by which hot-rolled plates of

high strength steels can be made into pipes for long distance gas and oil transportation. Both

longitudinal and spiral seam scan be welded by this technology.

The hot rolled steel plates are curled into a tube shape by pressure rolls, the two abutting edges of the

plate are heated up during their passage through the induction coils, and pressed together by the

welding station pressure rolls. The convergence point and the separated abutting edges in front of it

forms a V shape.

This point is called the ‘Vee’ apex, which defines the onset of the joint formation that leads to a

weld. The high frequency current flows along the outside surface of the tube and along the edges of

the Vee, so that the electrical circuit is completed. This circuit is formed in the context of three

principal features of high frequency induction heating. This circuit is formed in the context of three

principal features of high frequency induction heating.

1. An induction effect allows the contactless transmission of power to the work piece with the

aid of an alternating magnetic field. The induction coil generates this alternating field

according to:

Pi=k √ f · A2

Where Pi is the induced power (kW cm−2), k is a constant, A2 represents the Ampere-turns

per cm of the inductor and f is the frequency (Hz).

2. A skin effect occurs because at high frequencies, electrical currents and magnetic fields can

exist only in a thin layer at the conductor’s surface. The thin layer is defined as a skin depth

in cm:

ε=5030√ ϑμf

Where € is the specific resistance (# cm), and μ is the relative permeability of the material of

which the conductor is made.

3. The proximity effect means that the high-frequency currents always flow along the path of

least resistance. Two currents flowing in opposite directions on the same material are

mutually attracted. The position of welding should be within 6 to 14 mm upstream from the

centreline of the induction coils, so that Vee angle is kept within an acceptable range.

Only the heating along the edges of the Vee is useful for welding. The temperature rise is localized

there because of the combination of the skin and proximity effects. The current flowing at the tube

periphery gives rise to a heat loss which must be kept small. This is achieved by reducing the

resistance at the tube circumference through substantially broadening the current path. A suitably

designed induction coil is essential. The distribution and penetration of heat from the high frequency

current are controlled by the skin and proximity effects together with the frequency of the power

supply and the mill speed, so the welding frequency and mill speed must be chosen carefully.

Although the proximity effect makes the two abutting edges mutually

attract, it can be impeded by

1. Roughness on the contacting surfaces.

2. An oxide layer or foreign matter.

3. A thin layer of absorbed gas on the oxide surface.

4. Relative positions of the abutting edges during the introduction of the high frequency

current to achieve heating.

By applying pressure from the rolls, the melted material with relatively high content of impurities is

expelled from the joint. The metal at the junction flows towards the inner and outer surfaces of the

pipes. The quantity expelled is defined as the difference between the circumferences measured at the

Vee angle in front of the rolls and at pipes at the rear of the rolls, which is usually 1 to 5 mm. The

extent of squeezing can affect the weld strength and toughness. After welding, the rejected materials

are mechanically trimmed off from both the inner and outer surfaces of the pipe.

Certain mill adjustments and roll designs also determine the quality of the welding. Figure below

shows five different configurations of the pressure rolls used in the welding station. Some criteria

were followed in the design, such as, the fact that the top flange must be large enough in diameter to

fully contain the top edges, and the head rolls located on top should be as thin as possible since they

should not do any edge forming in bending. The top head rolls can be used to correct only minor

edge mismatches.

The ideal disposition of the abutting edges, their melting and two kinds of mismatch are shown in

figure below. both vertical and angular mismatch result in uneven heating and heat distribution.

In figure above

a) Ideal relationship is to have the two abutting edges parallel

and matched in the vertical direction

b) The electro-magnetic force rejects hot metal during the high frequency pipe welding.

c) Two edges are mismatched in the vertical direction.

d) Two edges are mismatched angularly

The narrow weld joint and heat affected zone resulting from high frequency induction welding is

stronger than the wider weld from many other welding processes because of the absence of cast

structure and the minimization of the distortion of the joining parts. In order to guarantee the quality

of the welded joint, a lot of details need to be controlled.

Reasons for HF Welding

High-frequency welded pipes are widely applied due to advantages such as

1. no weld filler needed

2. small heat-affected zone (HAZ),

3. high-precision pipe forming

4. high-efficiency continuous production.

Different Parameters that effect the Weld

1. Effect of Magnet Radius on Heating Time Required

the effective magnetic flux concentrated inside the pipe increases with an increase of the magnetic

bar radius, the time required for the minimum temperature in the directionof wall thickness at the V-

shaped weld cross section to reach the welding temperature is gradually reduced, and the heating

efficiency gradually increases.

2. Effect of Magnet Position on Heating Time Required

as the coil becomes smaller, the distance between the coil and pipe is reduced, and the missing

magnetic flux between the coil and tube is reduced such that the effective conversion rate between

the magnet and power will increase, thus increasing the heating efficiency. But taking the coil

cooling conditions, production adjustment, and other factors into account, the coil cannot be made

with too small a size. The specific size should be based on actual production while minimizing the

opening between the induction coil and steel pipe.

3. Effect of Coil Position on Heating Time Required

the closer the coil is to the V point, the stronger the proximity effect becomes, and the higher

the heating efficiency becomes. However, the coil work environment is deteriorated.

It cannot be placed too close to the V point considering the constraints of splashed sparks at the V-

shaped area and squeezing roll size.

4. Effect of Frequency on Heating Time Required

with the increase in welding frequency, the skin effect becomes more intense, the energy is

more concentrated in the range of the HAZ, the required heating time is less, and the heating

efficiency is higher.

In summary, if the site conditions allow, in order to improve the heating efficiency of high-

frequency welded pipe :

1. magnetic bars with a larger radius should be used;

2. the horizontal distance between the magnetic bar and pipe V point should be reduced;

3. the magnetic bar should be shifted toward the pipe entrance side;

4. the coil diameter and distance between the coil and V-shaped point should be reduced;

5. and there should be an attempt to increase the welding frequency and loaded current so as to

shorten the heating time and obtain the highest possible heating efficiency.

Effect of the Main Parameters on Temperature Difference

There are many factors that affect the weld quality, and high-frequency welding is the most

important process to determine the pipe quality in pipe manufacturing. In the high-frequency

welding process, the most critical issues constraining the quality of high-frequency welding and pipe

yield rate are the overheated edges and poor cold weld in the center of the joint wall thickness due to

the unique skin effects and ring effect in electromagnetic fields.

The key to solving this problem is to eliminate the excessive temperature difference in the

direction of weld wall thickness, that is,

1. to try to minimize the temperature difference at the weld cross section when passing the V- shaped

welding point through the pipe, and

2. ensure that the minimum temperature in the direction of the cross section of weld thickness

reaches welding temperature so as to avoid the undesirable phenomena of overheated sheet metal

edges under the premise of ensuring that the center weld is penetrated .

1. Effect of Magnet Radius on Temperature Difference

Magnetic field lines gathered at the downstream edge of the weld joint increase with the increase in

magnet radius, the increase rate of the surface temperature under the weld is higher with the same

heating time, the temperature difference between inner and outer surfaces is reduced in a certain

range, and the temperature difference between the sections becomes slightly lower.

With the continuous increase of magnetic bar radius, the surface temperature rate becomes

excessively high and the heat of the inner surface can’t be transferred to the weld joint center in

time, resulting in a progressive increase in the wall temperature difference, thereby reducing the

weld quality.

2. Effect of Magnet Position on Temperature Difference

the temperature difference is a minimum when the magnetic bar end is placed about 4 mm from the

V-shaped point.

Heating efficiency of magnetic field lines is a maximum here. The heating effect of the downstream

surface is the most obvious if the magnetic bar end is placed at the location, reducing the

temperature difference between inner and outer surfaces in a certain range, and also reducing some

of the temperature difference in the wall thickness direction.

3. Effect of Frequency on Temperature Difference

The higher the Welding frequency, the stronger the skin effect, and the larger the temperature

difference in the cross section in the direction of the wall thickness.

4. Effect of Current on Temperature Difference

With the gradual increase in density of the current loaded to the coil, the ring effect between the coil

and tube becomes stronger, causing the eddy current excited by the induction coil to become highly

concentrated in the direction of the tube edge wall thickness, resulting in an excessive temperature

increase rate at the tube surface such that the heat cannot be transferred to the wall thickness center

in time, leading to an excessively large wall temperature difference.

Quality Control

There are several parameters which must be controlled at the welding station in order to make the

products of steel tubes/pipes satisfy the requirements of service under the gas or fluid pressure. Some

of these parameters are trimmed width of the material entering the mill, circumferential reduction in

the forming stands and welding station, and size and shape of the un-welded steel entering the

welding station.

The circumference of the welded and outer diameter trimmed pipe must be less than the width of the

un-welded tube, so as to make sure that a certain amount of material is squeezed out of the weld

junction. All these efforts aim to avoid defects due to improper power adjustment, which can cause a

cold, pasty weld, or no weld at all, or a hot weld with blow hole type voids.

The optimum welding condition for the input power could be determined experimentally using an

electric resistance welding simulator, non destructive defect inspection and impact energy

measurements. The optimum heat input range should be re-established whenever the material

conditions are changed.

The requirement for pipes of higher strength and ductility has led to an increase in manganese

contents in the hot rolled sheet steels. However, higher manganese content greater than 1.2 % and a

greater wall-thickness are likely to induce “penetrator” defects in the welded zone. These defects are

generally classified as:

1. Residual FeO-MnO-SiO2-(Al2O3) oxides left without being squeezed out from the joint;

2. Exposed cracks on the external surface of the pipe due to cavity formation and hot

cracking;

3. blow-holes including oxides.

Penetrator defects become more frequent as the heat input is increased and the mill speed reduced,

and the Mn/Si ratio. The manganese and silicon content at the welded joint noticeably decreases with

the heat coefficient Q, which is defined as:

A weld joint in an induction welded pipe may contain defects caused by environmental factors or

inappropriate power duringseam welding. A lot of previous work was done to detect and analyse

these defects from the aspect of fatigue properties, since they can initiate fatigue failure. The fatigue

crack propagation rate of a welded joint is generally lower than that of base metal. This may be

caused by the hardened microstructure and distorted fatigue crack propagation path within

irregularly arrayed coarse grains in the weld joint.

Fine oxides formed at the weld joint at optimum input power did not lead to a deterioration of the

fatigue propagation rate.

Steel cleanliness requires low sulphur contents to optimise the pipe- body fracture toughness and

avoidance of clustered alumina inclusions to minimise the occurrence of ultrasonic testing

indications in the vicinity of the longitudinal weld seam. Thus the maximum sulphur content is

restricted to 0.005 wt% and the actual level of calcium is typically lowered from 0.0035 wt% post

injection to 0.0008 wt% post vacuum degassing during steel-making.

Common HF Weld Defects

There are many types and variations of HF welding defects and each one is known by many different

names around the industry. Regrettably, there is no common term with which everyone will agree so

the following defect names are followed by another common name for the same defect.

1. Entrapments (black penetrators)

2. Pre-arcs (white penetrators)

3. Lack of Fusion (open seam)

4. Lack of Fusion on edges (puckers)

5. Lack of Fusion in centre (cold centre)

6. Paste Weld (cold weld)

7. Cast Weld (brittle weld)

8. Porosity (pin holes)

9. Stitching

Entrapments (Black Penetrators)

This type of defect is usually a metal oxide that has been trapped on the bond plane instead of being

squeezed out with the molten metal. These oxides are formed on the surface of the molten metal

edges in the vee. In the vee, if the approach velocity of the strip edges is less that the melt rate, i.e.,

the edges are melting faster than they are being squeezed, a pocket forms behind the vee apex which

will contain both molten metal and metal oxides. The normal squeeze out does not completely

remove the larger than normal liquid volume and an entrapment results.

The entrapment is readily observable when the weld is broken open. The surface of the entrapment is

generally a dark color and fairly flat in comparison to the rather woody surface of the weld line.

They can occur individually or in strings. It has been observed that the incidence of entrapments is

increased when the vee is narrow, e.g., less than 4 degrees or when the ratio of manganese to silicon

in the strip is less than 8:1. The effect of Mn/Si ratio is difficult to consistently reproduce suggesting

that other factors may be involved.

Prevention of Entrapments

1. Maintain a 4-6 degree vee angle.

2. Maintain a stable vee length with proper tooling and mill set-up.

3. Maintain the lowest welding temperature possible that achieves a sound weld.

4. Avoid steel chemistries that have a Mn/Si ratio of less than 8:1

Pre-Arcs (White Penetrators) The use of the term “Penetrator” is inappropriate for this type of

defect because nothing is actually trapped on the bond plane. It is a very short lack of fusion caused

by a pre-arc. A pre-arc occurs when the HF current jumps across the vee ahead of the vee apex,

usually as a result of a sliver or bit of scale falling across the vee. The short-circuit diverts the

current momentarily, robbing the vee of heat.

The very short duration of the diverted current leaves only a short defect, often no longer than the

wall thickness. It is easily observed when the weld is broken open and has a flat, shiny surface

surrounded by the woody fracture of the rest of the weld area.

It is possible with very high operating voltages common to the vacuum tube welders to experience

pre-arcs in a narrow vee without the presence of scale or slivers to facilitate the short circuit. The

very high potential between the edges can result in the same type of arc-over with the same defect.

Prevention of Pre-Arcs

1. Maintain a vee angle of 4-6 degrees.

2. Use good slitting practice to minimize slitter burrs.

3.Use good handling practice to minimize damage to the edges.

4. Keep coolant clean and directed away from the vee area.

Lack of Fusion (Open Seam)

As the name implies, this is the failure of the two strip edges to fuse to form a sound weld. The

edges of an open seam usually show a blue heat tint suggesting that some heat was input.

However, the edge face remains flat and smooth, showing no signs of having been molten. The

obvious cause of the defect is insufficient weld heat and several factors need be considered. Power

setting, vee angle and length, impeder placement and condition, and coil size influence Weld power.

All of these factors can work independently or as a group to create a problem. Occasionally, the

appropriate heat is input and the seam remains open. This is likely to be related to insufficient

squeeze out. Here the edges show evidence of being molten but have not fused because of the

oxidized metal remaining on the surface of the molten edges prevents bonding. As the weld passes

beyond the squeeze rolls, the strips natural spring back opens the seam.

Prevention of Lack of Fusion

1. The actual power setting must be consistent with the speed and material gage;

2. The impeder must be placed 1/8th inch past the weld roll center line and kept cool;

3. The vee length should not exceed approximately 1 tube diameter;

4. The vee angle should not exceed 7 degrees;

5. The coil inside diameter should not exceed the tube diameter by more than 1⁄4 inch.

6. The strip width must be appropriate and consistent for the diameter tube being produced.

Lack of Fusion at Edges (Puckers)

Lack of fusion at the edges of the weld is usually caused by non-metallics on the bond plane. This

may be similar to a penetrator that is confined to the outer or inner edges. The defect gets its name

from its appearance when the tube is crushed with the weld at the 3:00 o’clock position. When

broken open, the pucker area is dark and flat. It can also be the manifestation of peaked edge

forming where the OD edges are not heated as hot as the ID edges, in which case the pucker fracture

can be silvery in color. Puckers are variations of the entrapment and lack of fusion defects.

Prevention of Puckers

1. Maintain flat, parallel edges.

2. Use slightly more squeeze out.

3. If pucker is silver, also use more weld heat.

Lack of Fusion at Mid-Wall

When a lack of fusion weld is broken open, the fracture at mid-wall appears to be a flat, dull, silvery

band. The edges appear woody and fibrous. This condition is usually caused by running at speeds

just beyond the rated power of the welder. There simply wasn’t enough time to heat the entire cross-

section of the edge to the full temperature and depth required for a sound forge weld.

Lack of mid-wall fusion can also be the result of insufficient squeeze-out although the bond plane

for this situation would exhibit some un-extruded molten metal.

Prevention of Lack of Mid-Wall Fusion

1. Increase weld power.

2. Increase weld upset.

3. Increase vee length or reduce line speed.

Paste Weld (Cold Weld)

Paste welds are perhaps the most dangerous of all HF weld defects because they are virtually

invisible to current Non-Destructive Test (NDT) methods. The paste weld is sufficiently bonded to

transmit an ultrasound signal but not strong enough to pass normal crush or flare tests.

Electro- Magnetic Inspection (EMI) cannot see it because there is no opening in the bond plane.

When broken open, the paste weld is very flat and brittle, showing very little of the woody, fibrous

structure common to a full fusion weld. Some evidence of the slit edge may still be visible. If looked

at in a transverse metallographic section, it would exhibit a very narrow HAZ, no white bond plane

and very little upset of the flow lines.

Prevention of Paste Welds

1. Use sufficient weld power for the gage and speed of the mill.

2. Use sufficient squeeze and/or increase strip width.

Cast Weld

A cast weld is the result of failure to eject all of the molten metal from the bond plane. The

remaining cast metal on the bond plane likely contains metal oxides similar to the penetrator.

The appearance of the fracture surface will vary with the amount of cast metal remaining but will

almost always be flat and brittle looking. If examined by metallographic section, the cast metal

would be visible on the bond plane.

The cast weld usually fails the crush or flare tests. Since there is obviously ample power to melt the

edges, this defect has a fairly simple solution.

Prevention of a Cast Weld

1. Increase squeeze out.

2. Increase strip width.

Porosity (Pinholes)

Porosity on the bond plane is the result of high welding temperatures and insufficient squeeze out.

The fracture surface would appear to be woody and fibrous with shiny, spherical voids randomly

distributed across the edge. Where the voids intersect the OD, the surface of the void may be black

due to oxidation. Small pinholes may be visible on the OD bead before scarfing.

After scarfing, the pinholes may be visible on the bond line.

Prevention of Porosity

1. Reduce weld heat.

2. Increase squeeze out.

Stitching

Stitching defects can be manifested in a variety of ways but common to all is the fact that the defects

are regularly spaced and almost continuous (FIG 14). Usually, the defect takes the shape of puckers

on the OD and are spaced some multiple of the power line frequency (60 cycles).

If, for example, a line is running at 120 feet per minute and the defects are 4” apart we get:

120 fpm x 12”/foot = 1440 inches / minute,

1440 ipm / 4” = 360 which is a multiple of 60.

It is sometimes possible to get what appears to be stitching with no relationship to a line frequency

multiple. In this case, the spacing may be equal to the diameter of a bad roll or a bent shaft, whose

periodic movement causes a small defect.

Prevention of Stitching

1. Add additional filtering to weld circuit.

2. Check voltages across incoming phases.

3. Check rolls and shafts.

SOURCES OF DEFECTS

It is likely that several variables may be conspiring to create the defect you are experiencing. A

slightly narrow vee would not cause penetrators unless the squeeze out was also just slightly less

than necessary. The smaller squeeze out may be the result of slightly narrow slit width or worn

tooling or even a bad setup.

Also, the cause of the problem may have its origin outside of the immediate weld area. For example,

a cold weld may be the result of an impeder pump cavitating. As the pump fails to deliver adequate

cooling water, the impeder momentarily gets hot. When the impeder gets hot, it

becomes less effective in focusing the current in the vee. As the current is allowed to spread around

the backside of the tube, heat in the vee drops and a cold weld occurs. Turning up the weld heat may

prevent the cold weld until the pump fails altogether and the impeder looses all power to focus the

current.

DESTRUCTIVE TESTING OF WELDED STEEL TUBES

Steady evolution of Non-Destructive Testing equipment has dramatically reduced the number of

weld defects being sent to customers. However, even the best NDT equipment does nothing to pre-

vent those defects from occurring. Total reliance on NDT technology can lead to serious losses in

productivity when inspection is used to replace prevention. Early detection of defects is essential to

rectifying the problem in order to minimize the defective footage produced.

Because the interpretation of NDT results can be made in error, destructive methods are often used

as verification. The destructive methods can also supply a “quick and dirty” evaluation for immedi-

ate use by production personnel. While destructive meth-ods cannot evaluate an entire run of pipe as

can NDT, they can give a fair evaluation of the mill setup, steel quality, and welding and normaliz-

ing practice.

TRANSVERSE WELD AREA EVALUATION

One of the best tests for the weld setup is the TWA evaluation. This test is simple, quick and should

be performed at every gage or setup change to ensure that the strip edges are coming together flat

and parallel into weld rolls. A cutting torch is used to cut the Vee area out of the pipe (Fig 1). The

Vee is split open and the section is viewed looking at the edge which is half welded and half un-wel -

ded. The welded portion will be bright and shiny and the unwelded edge will be dark from heat tint-

ing. If the line be-tween the light and dark areas (welded and unwelded) is vertical, the edges are

meeting square and parallel. If the line is sloped, the edges are coming together peaked. Peaked

edges are prone to create bond line defects such as entrapments as well as cold welds on the O.D.

Additionally, welding with peaked edges takes more power than welding with parallel edges and the

overheated inside corners may melt off and destroy impeder casings. The ID bead on peaked welds

is usually larger than beads from welding with parallel edges so ID scarfing is more difficult and

more metal is wasted in squeeze out. Note that peaked edges can be the result of improper fin de-

sign or because of spring back.

WELD AREA MICROSTRUCTURES

A weld area microstructure can provide a wealth of information on weld quality and edge presenta-

tion. The sample should be taken with care using a cutting torch. The sample should be large enough

that the heat from the cut does not influence the micro-structure. The degree of upset, the uniformity

of the squeeze, flow angles and micro-structural constituents can be determined using standard me-

tallurgical preparation techniques and a metallurgical microscope. Figure 2 illustrates some of the

basic con-ditions observable.

The most obvious structure to be observed is the flow angles. The lines or bands visible in the steel

are the result of the rolling operation at the steel mill. These lines are usually straight, running paral-

lel to the rolling direction, i.e., longitudinally. When the weld is made, the hot steel “bulges” (upsets)

in the weld area. The angle and symmetry of the flow lines is an indication of the degree to which

the edges are presented flat and parallel.

The specimens should be ground, polished and lightly etched to show the grain structure and flow

angles. Viewing at low power, 50-100X is best.

Figure shows a normal HF weld area. The hourglass heat affected zone and flow lines are symmet-

rical around the bond line. Figure shows a weld area resulting from non-parallel edges resulting in a

skewed bond line and hourglass. Not shown is the undercutting of the ID and OD which results

when off-set edges are scarfed. This undercutting may seriously reduce the wall thickness in the

weld area. Figure shows the weld area of a peaked weld. Because the inside edges are closer together

than the outside edges as they pass through the vee, the ID gets hotter than the OD. The double vee

created may also encourage entrapments on the bond plane. Figure 2-4 shows a typical hook crack

and bond line defects such as penetrators or entrapments.

If the flow angles are very steep, i.e., approaching vertical, preferential corrosion may attack the up-

turned fibers, penetrating the wall along side the weld. If the flow lines turn up and then turn down,

you may be seeing the result of edge deformation in slit-ting being compounded by the normal weld-

ing upsetting

If the hourglass is very narrow in the center, too much squeeze is being applied by the weld box. If

the bond line shows a cast metal structure, too little squeeze is being ap-plied. Both situations can

result in a weak bond or brittle welds which can fail flattening and flare tests.

If your process includes seam normalizing, regular samples should be taken to evaluate the center-

ing, penetration and heat effects. Samples for evaluation should be prepared with the same care as

described above for weld area micros with special care taken not to overheat the specimen when cut -

ting or grinding it. Figure 3 illustrates some common effects of seam normalizing.

Figure illustrates a properly centered seam normalizing heat affected zone. It penetrates the full wall

thickness and is centered over the weld hourglass. If the proper temperature has been achieved, the

grain structure after normalizing will be very similar to the parent metal. The ferrite bond line should

be obliterated or significantly reduced. Figure shows insufficient depth of penetration of the seam

normalizing heat. The top half of the weld hourglass has been normalized but the bottom half re-

mains as welded. Figure shows an off center HAZ caused by improper position of the weld line rel-

ative to the inductor bar. While the heat has penetrated full depth, it has not completely affected the

entire weld area. Figure 3-4 shows the results of using a very high normalizing temperature. Above

1750 Deg F grain growth may occur which may weaken the weld area or lead to weld area corro-

sion.

FLATTENING TESTS

The flattening test is often performed on several or all pipe in each coil. While being a poor substi-

tute for a full NDT inspection of the weld, it does give a good evaluation of the weld area ductility

and can occasionally alert you to the presence of bond line de-fects. The flattening test can test both

the ID and the OD by changing the orientation of the weld from the 12 o’clock position to the 3

o’clock position. Crushing the ring in the 12 o’clock position puts the ID in tension and the OD in

compression; the 3 o’clock orientation does the opposite. In any case the sample should be at least as

long as the pipe diameter up to a maximum of about 4” long. Rough edges and burrs can be re-

moved prior to crushing. Figure illustrates the two positions.

Any flattening test apparatus should be designed with operator safety in mind and interlocks

incorporated into its operation that preclude actuation while the operator’s hands are in the

press.

Typically, when specified by an organization such as ASTM or API, the pipe sample must be

crushed to a specified height (expressed as a dimension or % of diameter) without fracture in

the weld. Brittle welds due to improper normalizing or welding will usually fail before the

minimum height is reached.

FLARE TESTS

One of the least valuable and most popular tests is the flare test. This test involves flaring one

end of a short sample of tube by forcing it over a mandrel or expanding it. See Figure 5. In

theory, the test looks like it closely resembles the manufacturing pro-cesses the tube will un-

dergo and should be a valuable evaluation of the tube. However, several problems limit the

usefulness of the test.

First, a small defect at the very end of the tube may fail while a larger one further in from the

edge does not. Which defect is most critical? Deep scarfing may reduce the wall thickness

causing a weld area failure. Conversely, high weld bead may reinforce a weak weld and al-

low it to pass. Also, if the bead is left in place during the test, the mandrel will force the bead

into the body of the tube and may cause premature failure or possibly reinforce a bad weld

and allow it to pass the test.

Even if done perfectly, the test is sensitive to the yield strength of the tube and may fail in a

soft weld which is free of defects and pass on a hard weld with defects. Flange tests and ex-

panding plug test suffer from similar limitations.

BEND TESTS

The most common bend test is the reverse bend. A short piece of pipe or tube is cut and then

slit at 90 degrees to the weld.The section with the weld is flattened out and mounted in a vice

and bent backward to put the ID in tension. It is used to evaluate ID defects and normalizing

depth of penetration but the flattening test is easier, faster and just as useful.

Destructive testing is a valuable tool when used in support of a defect prevention pro-

gram and NDT systems. Operating personnel must have all necessary equipment at their

work stations to conduct the tests quickly and efficiently. Proper training in the methodology

is very important. The results should be documented so that statistical evaluations can be per-

formed. All efforts to improve quality should start with the goal of preventing the defect from

occurring. Relying on finding the defects will invariably add cost and reduce productivity.

EDDY Current Testing

An eddy current flaw detection system is suitable for detecting discontinuities in tube and

pipe during the production process. Understanding about eddy current system principles and

this technology's capabilities and limitations can help tube and pipe producers learn how to

Eddy-current flow follows a closed-loop pattern unless interrupted by a crack, pin-hole, or

similar discontinuity.

Eddy currents are alternating electrical currents that can be induced to flow in any electrically

conducting material, which covers all metals. Eddy current flow follows a closed-loop pattern

unless it is interrupted or diverted by a nonconductivity area such as a crack, pin-hole, or

similar discontinuity .

Eddy current testing is the science of detecting flaws while ignoring other influences on the

flow pattern created by dimensional variations, stress, chemistry changes, magnetic

properties, electrical interference, mechanical movement, vibration, etc.

Collectively, the signals to be ignored are termed as "noise," while the ones which are of

interest are called "signals."

The state of the art is to achieve significant improvement in signal-to-noise ratio to obtain a

desirable minimum of 3 to 1 for satisfactory in-line operation. Often, signals are totally

drowned out by noise at their source, and this must be corrected by various physical,

mechanical, and electrical means to optimize the end result.

The depth of penetration of eddy currents on tubes is influenced by test frequency,

conductivity, and other variables.

Much can be done with electronic filtering and transducer design, but tackling the

interference at the source may still be necessary in many instances, and expert evaluation of

the proposed inspection site may be required.

Depth of penetration of eddy currents on tubular products is a complex matter influenced by

test frequency, coupling factors, inside diameter (ID) to outside diameter (OD) relationships,

and the electromagnetic characteristics of the material .The lower the test frequency, the

greater the penetration but the poorer the sensitivity to defects. One or two kilohertz is the

normal practical base frequency.

When testing ferromagnetic material with optimized magnetic saturation of the material, the

maximum wall that eddy currents penetrate is about 0.322 inch, but this may be augmented

by magnetic effects from flaws on the ID. Tubing and pipe up to 0.500-in. wall has been

successfully tested using a combination of eddy current and magnetic flux leakage effects.

On higher conductivity materials such as aluminum, copper, and brass, penetration is much

less, ranging only up to 0.080 in. as the practical limit at which sensitivity to flaws is still

reasonable.

Testing Tube and Pipe

Eddy current testing is widely used for non destructive testing in the tube and pipe industry. It

is relatively simple to

install and operate and can

detect a range of defects

and discontinuities at

varying mill speeds.

Once calibrated, modern

drift correction techniques

help ensure that systems operate for periods of years with little maintenance or attention,

except for size changing.

for periods of years with little maintenance or attention, except for size changing.

Depending on their shape and construction, eddy-current transducers check just a few degrees

of the tubular shape or the entire circumference or some amount in between.

There is no physical contact between the transducer and the material under test, so wear is not

a factor, although damage sometimes results from misalignment or from crashes on the mill.

With seam or spiral welded products, the most vulnerable area is the weld itself. Flare and

flattening tests are essential tests on any mill, but 100 percent inspection of the heat-affected

zone (HAZ) indicates anomalies or deviation in the process as early as possible. This allows

the operator to correct the process and contribute to better overall quality and reduced

potential scrap.

Eddy current inspection can be made fully automatic with accurate tracking, marking, and

rejection of defective sections. However, the main focus must be on motivating the mill

Tube and pipe mills have four likely locations for inspection head installation.

operator to respond and correct early trend signals, since this can significantly improve

efficiency.

The Charpy Test

While most commonly used on metals, it is also used on polymers, ceramics and composites.

The Charpy test is most commonly used to evaluate the relative toughness or impact

toughness of materials and as such is often used in quality control applications where it is a

fast and economical test. It is used more as a comparative test rather than a definitive test.

What Does the Charpy Test Involve?

The Charpy test involves striking a suitable test piece with a striker, mounted at the end of a

pendulum. The test piece is fixed in place at both ends and the striker impacts the test piece

immediately behind a a machined notch.

Figure 1. Schematic of the Charpy impact test.

Determination of Charpy Impact Energy

At the point of impact, the striker has a known amount of kinetic energy. The impact energy

is calculated based on the height to which the striker would have risen, if no test specimen

was in place, and this compared to the height to which the striker actually rises.

Tough materials absorb a lot of energy, whilst brittle materials tend to absorb very little

energy prior to fracture.

Factors Affecting Charpy Impact Energy

Factors that affect the Charpy impact energy of a specimen will include:

•         Yield strength and ductility

•         Notches

•         Temperature and strain rate

•         Fracture mechanism

Yield Strength and Ductility

For a given material the impact energy will be seen to decrease if the yield strength is

increased, i.e. if the material undergoes some process that makes it more brittle and less able

to undergo plastic deformation. Such processes may include cold working or precipitation

hardening.

Notches

The notch serves as a stress concentration zone and some materials are more sensitive

towards notches than others. The notch depth and tip radius are therefore very important.

Temperature and Strain Rate

Most of the impact energy is absorbed by means of plastic deformation during the yielding of

the specimen. Therefore, factors that affect the yield behaviour and hence ductility of the

material such as temperature and strain rate will affect the impact energy.

This type of behaviour is more prominent in materials with a body centered cubic structure,

where lowering the temperature reduces ductility more markedly than face centered cubic

materials.

Ductile to Brittle Transition

Some materials such as carbon steels undergo what is known as a ‘ductile to brittle

transition’. This behaviour is obvious when impact energy is plotted as a function of

temperature. The resultant curve will show a rapid dropping off of impact energy as the

temperature decreases. If the impact energy drops off very sharply, a transition temperature

can be determined. This is often a good indicator of the minimum recommended service

temperature for a material.

Olympus Spectrometer

Handheld XRF spectrometer or spectrogun is a very useful device and is nowadays used a lot in the industries to determine the constituents of metals. It is based on the principle of excitation of atoms using x- rays.

Rockwell hardness tester

The Rockwell scale is a hardness scale based on indentation hardness of a material. The Rockwell test determines the hardness by measuring the depth of penetration of an indenter under a large load compared to the penetration made by a preload. There are different scales, denoted by a single letter, that use different loads or indenters.

Operation

Force diagram of Rockwell test

The determination of the Rockwell hardness of a material involves the application of a minor

load followed by a major load. The minor load establishes the zero position. The major load

is applied, then removed while still maintaining the minor load. The depth of penetration

from the zero datum is measured from a dial, on which a harder material gives a higher

number. That is, the penetration depth and hardness are inversely proportional. The chief

advantage of Rockwell hardness is its ability to display hardness values directly, thus

obviating tedious calculations involved in other hardness measurement techniques.

It is typically used in engineering and metallurgy. Its commercial popularity arises from its

speed, reliability, robustness, resolution and small area of indentation.

In order to get a reliable reading the thickness of the test-piece should be at least 10 times the

depth of the indentation. Also, readings should be taken from a flat perpendicular surface,

because convex surfaces give lower readings. A correction factor can be used if the hardness

of a convex surface is to be measured.

Visual Tests

In practice, the welding process is often adjusted depending on the welder with years of

experience who will observe the

1. splash of weld sparks,

2. weld glitch shape,

3. and color of a thermally affected zone.

However, this mode of production not only requires a welder with a high technical level, but

it is also unable to guarantee continuous and stable weld quality. It is often required to take

samples of the formed tubes so as to perform tests of metallurgical and mechanical

parameters to investigate whether the quality of the welding area can meet the manufacturers

requirements, not only increasing the product cost, but also extending the production cycle

and reducing the production efficiency.

About IS 3074:2005

This Indian Standard (Second Revision) was adopted by the Bureau of Indian Standards, after

the draft finalized by the Steel Tubes, Pipes and Fittings Sectional Committee had been

approved by the Metallurgical Engineering Division Council.

This standard was first published in 1965 and subsequently revised in 1979. As a result of the

experience gained since its publication, it has been decided to revise this standard

incorporating the following changes:

a) Provision relating to surface protection of pipes during transit, permissible fm height,

flattening test and bundling and marking have been included.

b) A reference has been made to IS 5429.

c) Due consideration has been given in this revision with regard to steel making practices and

the end use characteristics as required by the purchaser.

d) The composition given for different types of steel tubes in this standard have been aligned

with IS 10748.

Section 3 describes standards for high induction electric welding

This section of the standard covers three grades of electric resistance welded/high frequency

induction welded steel tubes, designated as ERW-1,ERW-2 and ERW-3.

Condition:

Tubes shall be supplied as welded with the external fin removed. In fin cut condition the

maximum height of the fin shall not be more than 0.25 mm or otherwise as agreed to between

the manufacturer and the purchaser. All tubes shall be of works man like finish free from

injurious defects and shall be reasonably straight. The tubes may be supplied in heat treated

condition provided that all the requirements of this standard are complied with.

Chemical composition

The Chemical composition must be according to table below:

Mechanical Properties

The mechanical properties obtained from test pieces selected and prepared as specified in 8

shall be within the limits specified in Table.

Hydraulic Test:

When required by the purchaser each tube shall be subjected to an internal hydraulic pressure

test in accordance with the following requirements:

a) Each tube shall be hydraulically tested to a pressure of 7 Mpa (0.7 kgf/cm-) or I Y2 times

the working pressure whichever is greater provided that in no case shall the pressure induce a

stress in the wall of the tube greater than 0.9 times the minimum yield stress specified for that

tube based on the following formula:

b) P =2tf/D

where, P = pressure, in MPa (kgf/cm-);

t = thickness of wall of tube, in mm;

f = 0.9 times yield stress, in MPa (kgf/cm/); and

D = outside diameter of tube, in mm.

c) The pressure shall be maintained in each tube for at least 5 seconds and there shall be no

sign of leakage during the test.