General Information on Welding For the benefit of business and people Shanghai August 2007

General Information on Welding

For the benefit of business and people

Shanghai

August 2007

2FOR BUREAU VERITAS INTERNAL USE ONLY – MARINE TRAINING ON MATERIALS


1

Welding is similar with metallurgy;

• In arc welding, energy is transferred from the welding electrode to the base metal by the electric arc.

• When the welder starts the arc, both the base metal and the filler metal are melted to create the weld.

• This melting is possible because a sufficient amount of power (energy transferred per unit time) and energy density is supplied to the electrode.

WeldingWelding



Electric Arc and Type of Current2

• High current

• Low voltage

-+

Anode Cathode

Arc Plasma

Electrons e

------------

Positive gas ions

+++++++

Welding machine

Source AC and/or DC

and controls

The electric arc is an electric current flowing between two electrodes through an

ionized column of gas called a plasma. The hottest part of the plasma is the central

column (above 3000°C).

The current used to make the electric arc can be :

• Alternating Current (AC)

• Direct Current with the Reverse Polarity (DCRP)

• Direct Current with the Straight Polarity (DCSP)

The choice of current and polarity depends on the welding process, the type of

electrode, the arc atmosphere, and the metal being welded.


Weld Assembly3 General Information on Welding

Sufficient heat is to be produced to melt the metal.

Welding of two pieces of metal using or not a filler metal produces three zones known

as unaffected base metal – Heat Affected Zone – Weld Metal.


4 Temperature CycleGeneral Information on Welding

TEMPERATURE CYCLE covers the whole range from liquid to solid at ambient temperature;

Zone d ; Weld metal is a solidification structure distinct from base metal.

Zone c ; Transformed to austenite on heating as temperature has exceed 910°C.

Zone b ; Partially transformed to austenite on heating as temperature has exceed 723°C.

Zone a ; Not significantly affected as temperature was below 723°C.

Resulting microstructure in each region depends on cooling rate.



Temperature covers the whole range from liquid to solid at ambient temperature;



Temperature and Hardness of Welding Seams of Mild Steel

a). Center of Weld b), Semi-fusing zone c) Heat Affected Zone



Temperature and Hardness of Welding Seams of Mild Steel

d). Normalize Zone e), Cristalized zone c) Base Metal


8 Weldability of Carbon-Manganese Steels


Weldability represents the possibility to make satisfactory welded joints.

Weldability varies with the grade, chemistry and mechanical properties of the steel.

For carbon-managenese (like hull grades) and low alloy steels, weldability is related

inversely to hardenability and maximum attainable hardness in the heat affected

zone. Several models have been proposed for predicting the hardness of the heat affected

zone. These models use term known as Carbon Equivalent. A widely used formula is the International Institute of Welding (IIW) carbon

equipvalent equation :

This formula is valid only if the alloy contents are less than the following :

0.50% C 3.50% Ni 1.00% Cr

1.60% Mn 0.60% Mo 1.00% Cu

The value of the carbon equivalent is one of the a variable used for the determination

of the preheat temperature. (To have a good weability CEQ should be less than 0.43).


9 PreheatingGeneral Information on Welding

Preheating involves heating the base metal to a specific desired temperature prior to

welding.

When preheat is required, temperature of base metal during welding operation is not

to fall below the preheat temperature.

Main primary reasons to utilize preheat are :

Preheating can be applied using banks of heating torches, electrical strip heaters, or

induction or radiant heaters.

• To lower the cooling rate in the weld metal and the base metal in order

to form metallurgical structure with greater resistance to cracking

• Lower cooling rate allows time for hydrogen to diffuse out

• To reduce the shrinkage stresses in highly restrained joints

Preheating is to be applied throughout the thickness of the parts to be welded and

over a reasonable distance on both sides of the weld (typically four times the

thickness with maximum 50mm for thickness below 50mm). Temperature is to be

controlled using heat sensitive crayon or other suitable equipment.


10 Interpass TemperatureGeneral Information on Welding

Interpass temperature (minimum and maximum) is the temperature of the material in

the weld area immediately before the second and each subsequent pass of a

multiple pass weld.

When preheating is specified, the minimum interpass temperature is to be equal to

the preheat temperature.

Maximum interpass temperature for welding carbon-manganese steels is typically in

the range 180°C to 250°C ; Excessive interpass temperature can affect the weld metal

properties.

Interpass temperature is to be controlled using sensitive crayon or other suitable

equipment. Location of the measure can be weld metal or adjacent base metal

(typically at 25mm from the weld toe). Temperature is to be controlled using heat

sensitive crayon or other suitable equipment.


11 Diffusible HydrogenGeneral Information on Welding

If present in the atmosphere surrounding the arc, hydrogen can dissolove in the

molten pool. When the weld metal solidifies, the hydrogen diffuses out into the Heat

Affected Zone and atmosphere. Depending on the quantity of hydrogen and cooling

conditions, some hydrogen will be trapped into the weld metal and the HAZ.

Hydrogen trapped in steel has the particularity to cause a severe embrittlement and

can cause cracking of sensible midcrostructures.

The principal source of hydrogen during welding operations in optimal conditions is

the welding proces and the consumables used.

Consumables are tested and graded according to the diffusible hydrogen content in

the deposited weld metal (see Part D Chapter 5 Section 2 Item 2.5).

The hydrogen grades are :

• H15 (H) when diffusible hydrogen content is found less than 15ml/100g acc. ISO 3690

• H10 (HH) when diffusible hydrogen content is found less than 10ml/100g acc. ISO 3690

• H5 (HHH) when diffusible hydrogen content is found less than 5ml/100g acc. ISO 3690


12Preheating, Diffusible Hydrogen

and Hydrogen CrackingGeneral Information on Welding

Preheating is a variable used to control hydrogen cracking tendencies typically in

heat affected zone of base metals ;

Hydrogen cracking may also be called cold cracking or delayed cracking ; For some

cases, it can be necessary to delay the non-desctructive examination of about

48hours after the weld has cooled to ambient temperature ;

Factors which have an effect on the risk of cracking ;

• Weld metal diffusible hydrogen

• Base metal composition

• Base metal thickness

• Level of restraint

• Heat input


13 General Information on Welding

Expansion and contraction of heated material create non-uniform stresses in the

component. If the stresses exceed the yield strength, localised plastic deformation

occurs. The volume of the weld metal plus the heat affected zone will be reduced as

temperature falls from the melting point to room temperature (about 7% of reduction

for C-Mn steels). Distortion is affected by parent material properties, amount of restraint, joint design,

fit-up and welding procedure.

• avoiding overwelding by adequate preparation

• use of intermittent welds

• place welds near neutral axis

• plan the welding sequence ; place welds at different points to counteracts the shrinkage

forces of weld already made like welding alternately on both sides of the neutral axis in

making a butt weld.

Weld Distortion

Distortion can be minimized by several practical ways like the following :


14Post Weld Heat Treatment

& Stress ReliefGeneral Information

on Welding

Very large welded structures like ship hull are not stress relieved.

Local stress relief can be required in special cases.

Heating a welded fabrication, like pressure vessels or components with thick welded

sections, to a suitable temperature to reduce residual stresses is commonly

required.

The stress relief temeperature is chosen below the transformation temperature so

there will be no major changes in the microstructure (typical range is 580°C~620 °C

for most carbon steels).

Heating rate and cooling rate are to be slow (typically not exceed 220°C/h) and

soaking time is typically one hour per 25mm of thickness.


15 Type of JointGeneral Information on Welding


16 Type of WeldsGeneral Information on Welding

A weld of approximately triangular cross section joining two surfaces approximately at right angles to each other in a lap joint, T-joint or corner joint.

A weld made in a groove between the workpieces

Butt Weld

Fillet Weld


17Type of Preparation for

Groove-Butt WeldsGeneral Information

on Welding


18Preparation of

Welds - TermsGeneral Information

on Welding


19 Butt Weld DescriptionGeneral Information on Welding


20 Fillet Weld DescriptionGeneral Information on Welding


21Welding Positions

Butt Welds on Plates – European Standard Designation




Welding Positions Butt Welds on Tubes –

European Standard Designation

PA

Rotating tube with horizontal axis

Welding in flat position

PF

Fixed tube with horizontal axis

Welding in vertical upwards position

PG


Welding in vertical downwards position

PC

Fixed tube with vertical axis

Welding in horizontal position

H-L045


Welding in inclined upwards position


23Welding Positions

Fillet Welds on Plates – European Standard Designation




Welding Positions Fillet Welds on Tubes –

European Standard Designation

PB

Rotating tube with horizontal axis

Welding in flat position

PG


Welding in vertical downwards position

PF


Welding in vertical upwards position

PB


Welding in horizontal vertical position

PD


Welding in inclined overhead position



Welding Positions for Plates

US Standard Designation



Welding Positions for Pipes

US Standard Designation


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4.2.1. Weather protection Adequate protection from the weather is to be provided to parts being welded ;

in any event, such parts are to be dry.

4.2.3. Surface condition

The surfaces to be welded are to be free from rust, moisture and other

substances, such as mill scale, slag caused by oxygen cutting, grease or paint,

which may produce defects in the welds.

Effective means of cleaning are to be adopted particularly in connections with

special welding procedures ; flame or mechanical cleaning may be required.

The presence of a shop primer may be accepted, provided it has been approved

by the Society.


General Conditions for Welding (BV Class Rules Part B Chapter 12 Section 1)

In welding procedures using bare, cored or coated wires with gas shielding, the

welding is to be carried out in weather protected conditions, so as to ensure that

the gas outflow from the nozzle is not disturbed by winds and draughts.


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4.2.11 Interpass cleaning After each run, the slag is to be removed by means of a chipping hammer and a

metal brush ; the same precaution is to be taken when an interrupted weld is

resumed or two welds are to be connected.


General Conditions for Welding (BV Class Rules Part B Chapter 12 Section 1)

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• Heat Input

13


TITLE CHAPTER 113 Heat InputHeat Input

What is Heat Input ?

• In arc welding, energy is transferred from the welding electrode to the base metal by an electric arc. When the welder starts the arc, both the base metal and the filler metal are melted to create the weld.

• Hat input is a relative measure of the energy transferred per unit length of weld.

• It is an important characteristic because, like preheat and Inter-pass temperature, it influences the cooling rate, which may effect the mechanical properties and metallurgical structure on the weld and the HAZ.



Heat Input calculation ;

• Heat input is calculated as the ratio of the power (i.e. voltage x current) to the velocity of the heat source (i.e. the arc) as follows; however, the true heat input is obtained by multiplication by efficiency of process.

• This equation is useful for comparing different welding procedures for a given welding process. However, heat input is not necessary applicable for comparing different processes, unless additional data are available such as the heat transfer efficiency. (For example, efficiency of SMAW is in the range 77 to 87 %).


TITLE CHAPTER 113

How is Heat Input Measured ?

• Heat Input can not be measured directly. It can be calculated from the measured values of the arc voltage, current and travel speed.

• In determining the arc voltage, the voltage should be measured as close to the arc as possible, as opposed to the value displayed on the welding machine voltmeter. Measuring the voltage across the arc provides the actual voltage drop across the welding arc. The welding machine voltmeter reading is always higher than the arc voltage due to the resistance of the welding cables. The machine voltage meter can be used only for approximate calculation.

• The welding current is measured with either an inductance meter or a shunt with appropriate metering equipment. The current used I the calculation should be the average value of minimum value and maximum value.

• The travel speed is the forward velocity of the arc measured in mm per minute. A continuous welding time of 30 seconds or 1 minute is suggested.

Heat InputHeat Input



Welding size is related to Heat Input :

• The cross-sectional area of a weld is generally proportional to the amount of heat input. (More energy is supplied to the arc, more filler metal and base metal will be melted per unit length, result in larger weld bead.

• Two welds which are kept with same welding current and voltage, weld bead of which with a slow travel speed will be larger than the one with a faster speed, due to higher heat input.

• The precise relationship between heat input and fillet weld size also depends on other variables, such as process and polarity,

• When reviewing welding procedure, if a minimum fillet weld size is specified, then the corresponding minimum heat input can be determined and controlled.

• An approximation formula between the filler weld leg size and heat input is shown in below;



Cooling Rate and Heat Input

• The cooling rate is a primary factor that determines the final metallurgical structure of weld and heat affected zone (HAZ), an it is especially important with heat-treated steels.

• When welding quenched and tempered steels, slow cooling rates (result form high heat input) can soften the material adjacent to the weld (HAZ), reducing the load-carrying capacity of the connection.

• As either the heat input or the preheat temperature increases, the rate of cooling decreases for a given base metal thickness.


TITLE CHAPTER 113

The rate of cooling in a given material will depend on three major factors ;

• The heat input to the weld

• The thickness of the parts welded.

• The initial temperature of the parts (preheat or interpass temperature)


Effects of preheating temperature and heat inputon cooling rate of welds on plate 19mm thick



Calculation of the Cooling Rate with the Heat Input ;

• The following proportionally function shows the relationship between preheating temperature, heat input and cooling rate;



How does Heat Input affect Mechanical Properties?

• Varying the heat input typically will affect the material properties in the weld. The following table shows how the listed properties change approximately with increasing heat input from the minimum to maximum value (6 kJ/cm to 43 kJ/cm).


TITLE CHAPTER 113

Other issues related to the Heat Input and Mechanical Properties ;

• The change in notch toughness is not just tied to the heat input, but is also significantly influenced by the weld bead size. As the bead size increases, which corresponds to a higher heat input, the notch toughness tends to decrease.

• In multiple-pass welds, a portion of the previous weld pass is defined, and the toughness improved, as the heat from each pass tempers the weld metal below it. If the beads are smaller, more grain refinement occurs, resulting in better notch toughness.

• No significant correlation between heat input and mechanical property was established for the Submerged Arc Welding (SAW) with heat input levels of 50 to 90 kJ/in (20 to 36 kJ/cm).

• If high heat input welding is used, the HAZ can be significantly weakened due to high temperatures and slower cooling rates..




Other issues related to the Heat Input and Mechanical Properties ;

• The heat input can vary widely depending on the process and welding condition.

• With Manual welding process, the technique influences the energy input.

• Depositing stringer beads using a small diameter electrode involves a lower heat input than a weaving technique or a technique using a larger diameter electrode.


TITLE CHAPTER 113

Heat Input and qualified Welding Procedure Specifications (WPS) ;

• Most Industrial Codes and Classification Rules have requirement for re-qualification of the WPS if the heat input over range, due to concerns regarding to the potential alternation of the weld metal and HAZ mechanical properties;

» AWS S1.1 Structural Welding Code – Steel ;

– If the procedure used in production has a corresponding heat input that is 10% or greater than that recorded in the Welding Procedure Qualification Records (WPQR), then the qualified WPS must be re-qualified.

» IACS UR W28 & Bureau Veritas Rules Part D, Chap. 5 Section 4,

– The upper limit of heat input approved is 25% greater than that used in welding the test piece or 55kJ/cm whether is smaller, except that the upper limit is 10% greater than that for high heat input processes over 50kJ/cm.

– The lower limit of heat input approved is 25% lower than that used in welding the test piece.


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Documents

General Information on Welding For the benefit of business and people Shanghai August 2007