welding procedure

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Basics in Welding and Weldability

HGRS/MTC

2"Basics in Welding and Weldability"Maintenance training

Objective

Outline the most important concepts and rules of welding in the cement industry

Create awareness around the main key success factors of high quality welding

Provide examples from the cement industry

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Agenda

Welding process

Weldability of steels

Welding techniques

Welding defects

Practical examplesKiln shell crack repairHydrogen induced cracks

Conclusions


Welding process

The process of welding employs the heat of an external source (usually electrical arc) to bring metals to be welded to a molten state. The metals fusing takes place almost instantaneously

From this point of view, welding has many similarities with steel heat treatments, especially Quenching and this has to be taken in consideration

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Welding process

. Quenching is a heat treatment which

objective is to form a very hard and resistant (but brittle) martensitestructure: The material

is heated until it reaches its austenitic

structure and then directly cooled down to

the ambient temperature

Carbon – Iron Diagram


Welding process

1400°C1000°C900°C600°C

200°C

3mm

In a welding process, there are 2 main operations :

1. Melting and fusing in the zone under the arc

2. Heat treatment in the area around

The higher the cooling speed is, the higher is the amount of martensite structure formed

Martensite is a very hard, resistant but brittle steel structure

Preheating is used to reduce the cooling rate (speed) after welding

Heat Affected Zone : HAZ

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Welding process


Welding process

In general molten steels have a strong affinity for oxygen, nitrogen and hydrogen : if the welding atmosphere is not controlled, the molten metal will pick-up some oxygen or/and hydrogen, forming oxides/nitrides or/and other structures as the weld solidifies. These are impurities which will embrittlethe weld and weaken it

For this last reason shielded welding techniques (or welding under controlled atmospheres) were developed : SAW (Submerged Arc Welding), MIG (Metal Inert Gas), …

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Weldability of steels

The term weldability is relative : practically all metals are weldable. Some however require special welding procedures in order to preserve the properties and characteristics designed for

The quality of welding may be affected by any one of the following factors :

OxidationNon metallic inclusionsChange of structureGas solubility of metalCoefficient of thermal expansionOperator, welding current/voltage, …


Classification of steels

The classification used in this document is based on the chemical composition (DIN & AFNOR), Examples from the Cement industry :

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Ordinary Steels

Iron/Carbon alloys (C<1.2% in general), with small quantities of Mn (<0.8%) and Si (<0.5%) and other ‘impurities’ (S and P)

< 0.25% Carbon, there are no precautions to be taken for welding

%CTensile Strength

kg/mm2Yield Strength

kg/mm2Elongation

% Brinell Hardness

Low Carbon < 0.15 35 - 42 20 28 - 30 < 120

Mild 0.15 - 0.3 40 - 45 20 - 25 22 - 25 120 - 140

Medium Carbon 0.3 - 0.6 45- 65 22 - 35 22 - 15 140 - 180

High Carbon 0.6 - > 0.8 65 - > 85 40 - > 50 14 - 6 180 - > 220

Classification according to DIN and AFNOR (based on Chemical composition)


Ordinary Steels0.25 – 0.4% Carbon, Preheating is needed because the critical cooling speed which forms the martensite might be exceeded

Above 0.4% Carbon, preheating is a must. The temperature of preheating is around 75 – 350°C depending on the thickness (see low alloy steels)

For welding casehardened steels, the casehardened layer must be removed even with preheating

For quenched steels, preheating, maintaining at stable temperature during welding and slow cooling process are needed


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Low Alloy Steels

At least one alloy element is added in a quantity < 5% to improve the steel properties (Mechanical Resistance, Corrosion and Abrasion)

The Carbon content is not sufficient to predict the weldability, some alloy elements participate also in the process of martensite formation : the Carbon Equivalent % has to be considered

Minimal Content in %

Mn 1.2Ni 0.5Cr 0.25V 0.05Si 1Mo 0.1

Co, Ti, Al, Cu, W 0.3



Low Alloy Steels

Actually, it is the cooling velocity around the welding seam which must be controled :

Preheating which objective is to reduce the cooling rate after weldingCovering the area welded for slow coolingIn some special cases (steels with high quenching aptitude), the cooling is controled in a furnace (50°C per hour)

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Low Alloy Steels

The preheating level depends on :The material composition : Carbon equivalent (%CE)The thicknessSéférian Diagram (Thickness, %CE, preheating T°)

carbon equivalent

0

50

100

150

200

250

300

350

400

0 50 100 150thickness in mm

tem

pera

ture

in C

0.2

0.25

0.3

0.4

0.5

0.6

0.7

0.8

%CE = C+Mn/6+(Ni+Cu)/15

+(Cr+Mo+V)/5


High Alloy SteelsThe content of at least one alloy element exceeds 5%

The weldability of such steels has to be analyzed case by case

Maximal content in %

Ni 30Cr 30Mn 14Si 4W 20Mo 8V 5

Cu 2Al 12Co 18Ti 1.6


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High Alloy Steels

13% Mn Steels : Abrasion/Impact resistant steels very sensitive to precipitation of carbides around 250°C, the welding sequences must be controlled in order to limit the temperature increase

Application : Hardfacing of crusher hammers and impact bars

Example


High Alloy SteelsChromium, Molybdenum materials : Usually >1%C, 2-30% Cr, and 0-3% Mo.The carbides formed (Mainly C + Cr, 10 – 40%) are very hard and resistant but very fragile (notch effect due to carbides)

The structure of the matrix (martensite, austenite, …), and the form and dimensions of carbides depend on the heat treatment (especially the solidification speed) and has a large influence on the behavior under wear and impact

In most of the cases, welding is not recommendedor should be performed by specialized third parties

Application : liners and inlet wall for ball mills

Example

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Welding techniques

The common techniques used in cement are :Manual Arc weldingSubmerged arc welding : SAWMIG/TIG


Manual Arc Welding

Most common technique

Deposition rate around 1.5 kg/h

Better control of post-welding deformation

Less sensitive to lack of fusion defectsThe welding electrode coating role is :

. Stabilizing the arc

. shielding the arc: prevent from

atmospheric contamination (O2, N2)

. Scavenging and deoxidizing :

produce a slag blanket to protect the

molten crater (and reduce its cooling)

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For special works (I.e. kiln shell replacement), the welder must be qualified and certified according to DIN/EN 287-1, ASME9 or equivalent

Manual Arc Welding

5 -10°


Manual Arc Welding

The deposition rate can be increased by using more welders (place restrictions may apply)

The oscillating welding technique is unfavorable for residual stresses, the straight line technique gives better results

To avoid thermal distortions, the welding sequence must be determined carefully

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For special works (i.e. kiln shell replacement), the electrodes have to be heated (dried) 2 hours at 200-350°C and must be kept at this temperature while welding

It is very important

To define and stick to

The welding

Procedure rules

Manual Arc Welding


Semi automatic / Automatic Arc Welding

In the semi-automatic arc welding, the electrode is mechanically fed through a welding gun into the arc from a continuously wound coil. The operation still needs an operator

In the fully automatic arc welding, the electrode is fed through welding jaws into the arc from a continuously wound coil. The welding doesn’t need an operator

These 2 techniques increase welding speed, reduce welding time and thus allow lower welding cost

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Submerged Arc Welding

Very high deposition rates : 10kg/h, the welding velocity is around 80cm/mn

Semi-automated process

Very high quality of weld with

specialized operator

Wire and flux must

correspond to the material


Submerged Arc Welding

Commonly used for welding kiln shells external and internal

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Metal Inert Gas (MIG) Welding

Semi automated process with Gas protecting the weld pool

This Welding process is sensitive to lack of side-wall fusion and cold lapping defects

Defect occurs in case of lack of gas

Certified people

Deposition rate: 2.5 kg/h


Welding defects

The most common welding defects are :Cold cracking, or hydrogen induced crackingSolid inclusionsPorositiesLack of fusionShape defects (design related defects)Lamellar tearingHot cracking

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Hydrogen induced cracking : Cold cracking

Hydrogen induced cold cracking : occurs at low temperatures (< 150°C), usually originates from the weld toe, and it is caused by the diffusion of hydrogen from the weld deposit to embrittle the existing martensite structure

The hydrogen comes mostly from moisture associated with fluxes :

Usage of low hydrogen consumables. TIG and MIG prcessess, being fluxless, give lower levels of hydrogenUsage of austenitic electrodes (I.e. 2222) : the hydrogen diffusion rate is lower in austenite structure


Hydrogen induced cracking : Cold cracking

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Solid inclusions

The most important is slag Inclusion which arises because it is difficult to ensure that all pockets of slag are removed from the weld surface (particularly when access is difficult). Slag deposits are produced by the welding flux

Oxide inclusions usually result from inadequate precleaning of the joint surface


Porosities

Porosity occurs when a weld is saturated with a particular gas (hydrogen, nitrogen, CO) which forms bubbles on discontinuities in the metal

Gases originate from air entrainment in the arc atmosphere (hydrogen, nitrogen), grease and moisture on joint faces or welding consumables (hydrogen) or chemical reaction in the weld (CO)

Factors of influence : instability of the arc, inefficient cleaning of the surface, and bad control of the arc at stops and starts

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Lack of fusion and lack of penetration

The welding arc is not sufficiently ‘penetrating’ to ‘wet’ the edge, or does not completely fill the joint

Generally depend on the electrode manipulation, joint design, arc current and surface preparation

Example : shell joining section


Shape defects

Example : undercut, poor profile, misalignment

Consequence of poor electrode manipulation, bad fit-up, or/and incorrect procedure (current, voltage, speed, …)

Undercut is caused by too high current or too low speedToo low current can cause excessive spatter

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Lamellar tearing

Development of cracks in the base metal in parallel with the seam surface : The bonding between the inclusions and the matrix in the base metal is weak and some inclusions will brittle, therefore strains in the thickness direction will cause de-cohesion and fracture of inclusions


Lamellar tearingPreventing measures :

Modification of welding procedure and joint design to reduce strains in the thickness direction

Usage of low inclusion levels (I.e. low sulphur steels)

Special heat treatments that modify the shape of porosities

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Hot cracking

Occurs during or just after solidification below 1200°C

Due to the presence of low melting point components such as iron sulphides or phosphidewhich mechanical resistance is lowered under temperature

It is admitted to be due to shrinkage strains before cohesion

Inter-granular


Weld seam design

The weld design

has a direct impact

on its resistance :

Fatigue Strength

Load

cycles

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Weld seam design


Welding Material

The most special electrodes commonly used in the cement industry are :

2222 of Castolin : kiln tires, rollersE7018 : kiln shell

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Welding with 2222

Electrode that provides an optimum combination of strength and ductility

Factors to consider :Linear Coefficient of thermal expansionEffects of dilutionSigma phase precipitation

170 - 210Hardness HB40 - 45Elongation %

390Yield strength N/mm2650 - 690Tensile strength N/mm2

Eutectic 2222


Welding with 2222Linear Coefficient of Thermal Expansion (TEC)

The Thermal Expansion Coefficient increases with temperatureAt 300°C, the TEC of the following materials are :- Austenitic steel (stainless) : 17.2 10-6/°C- Eutectic 2222 : 14.4 10-6/°C- 0.2% Carbon steel : 13.4 10-6/°C- Chromium Iron : 10 10-6/°CThe stainless steel welding will expand and contract significantly, this will increase the residual stresseswhilst distorting the jointEutectic 2222 has a TEC very close to the Carbon steel, in operation, this results in maximum safety margin against cracks

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Welding with 2222

Effect of dilutionDilution is expressed as the amount of parent metal ingressed in the weld depositWhen conventional stainless steel electrodes are used to weld carbon steels, the weld metal will inevitably be diluted with the parent metal by at least 20 – 30%, this creates a weld deposit which may be embrittled due to martensite formationThe 2222 weld is capable of ‘dissolving’ a high proportion of steel (~50%) whilst retaining its natural austenitic structureReference : Schaeffler diagram Cr equivalent vs. Ni equivalent


Welding with 2222

Sigma phase precipitationBrittle Iron-Chromium intermetallic which is very dependent on temperature and time kinetics. It can form at 900°C within minutes and after several days at 500°CIt often occurs when welding thick carbon steel section with with 309 or 310 based electrodesEutectic 2222, even when diluted can not reach the sigma zones and is therefore resistant to embrittlementReference : ternary diagram Nickel/Chromium/Iron

Example of application : kiln tires and rollers

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Welding with E7018E7018 : American Welding Society numbering system

E : Metal arc welding electrode70 : Minimum allowable tensile strength of deposits expressed in 000 pounds / inch2

1 : All welding positions8 : arc characteristics and polarity (ac and dc)

Iron powder – low hydrogen electrode : rimmed steel core wire upon which a lime covering is applied

Yield strength: 470 MPa, Tensile strength: 540 MPa, Elongation: 30%


Welding with E7018

Because this covering is slightly thicker than normal, the arc is shorter and moderately penetrating, the slag is heavy and friable and the deposited metal lies in a flat bead providing a very reduced tendency for underbead cracking (cold cracking)

E7018 belongs to the Low Moisture Absorption electrodes family

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Welding with E7018

Preheat is generally not required to prevent hydrogen induced cracking

Preheat should be used with hardenable steels to prevent the formation of hard heat-affected zones and eliminate tendencies toward quench cracking on cooling

Preheat may be required in welding heavy sections

Basics in Welding and weldability

Practical example 1

kiln shell crack repair

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Fatigue and Crack formation

Kilns are exposed to dynamic loads : fluctuating stresses that can be bending, torsion, tensile, compression or combinations of these

Stresses like these can lead to crack formation and fracture without any kind of deformation and real overload


Fatigue and Crack formation

The load and stress changes are higher because of heat deformations and possible incorrect adjustment (typical case : crank in the tube)

Examples of areas where cracks can start : shell welding joints, toe of welds between shell and supports, manholes, satellite cooler supports, …

Circumferential weldsLoose tires pads

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Crack repair


Crack repair

When a crack is detected, the first actions are :1. Finding the length of the crack2. Finding the depth of the crackThis is done with NDT

methods as UT, MP or DP

If the crack is throughgoing,

it is necessary to work from both

inside and outside

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Crack repair procedure

The normal procedure is the following :1. Gouging from one side to about 2/3 of the thickness

2. The gouging must start a little away from the crack ends in sound material, gouging towards the crack, to avoid crack propagation

Crack end

Smooth transition radius

Gouging direction


Crack repair procedure3. Careful cleaning by grinding

4. Preheating : If the result of the Carbon equivalent is higher than 0.41 it is necessary to preheat. It must be pointed out that even if the result is lower than this, it does not harm to preheat, especially with heavy sections and if the weather is cold

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Crack repair procedure5. Welding with a normal basic electrode as AWS E

7016 or E 7018 (The electrodes have to be dry)

6. Each run should be overpeenedto reduce stresses, and carefully cleaned from slags


Crack repair procedure7. Grinding down the surface of the weld : Root weld

8. The process has to be repeated from the other side controlling that the root is completely clean

9. Control with UT, MP and/or DP

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Crack repair procedure

After the repair, in the case of long cracks, the shell can be reinforced by adding some joining plates :

Basics in Welding and Weldability

Practical example 2

Hydrogen-induced cracking - kiln shell welding

December 2003

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Situation

Welding joint N°A between a 75mm thick new shell and a 60mm thick old section

Root weld and outside welding completed, internal gouging and DP done

While welding internally, a loud bang was heard : A crack was found inside the partially welded area


Situation

A 3.5m long crack was found in the HAZ in the old shell during the UT of the outside completed welding

The crack was not fully removed after gouging and grinding 20mm

A 1.6m long crack was also found in the partially welded groove on the inside of the kiln

MP was carried out and cracks were found also in the root weld

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Investigation : External consultant

Preliminary finding was that the dye penetrantresidue was probably not fully removed

A hydrogen-induced crack (HIC) could have been developed due to welding over the DP residueA Hydrogen removal heat treatment (HRHT) should be carried out before continuing

Excess hydrogen in parent base metalMany repairs were done successfully in the pastThe joint at the other end of the 60mm thick old shell was welded to a new section without problemHypothesis not probable



HIC (or Cold cracking)

Occurs after welding has been completed

4 main Conditions :

1. Hydrogen is introduced by diffusion during the welding to the weld or HAZ :

. Moisture in the coating of the electrodes

. Oil, grease, dirt, paint, water … in the surface

. Degreasing fluids used to clean surfaces, DP or wet MP powder residues. Hydrogen in the parent metal : original casting process, heat treatment, or corrosion (example H2S)

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2. Low preheating temperatures (<150°C)

3. Hard HAZ microstructure : Soft microstructures tolerate more hydrogen before cracking occurs. The E9018 weld metal is susceptible to HIC

4. Existence of tensile stresses on the weld seam originating from thermal contraction of the cooling or from external efforts


Investigation : External consultantSome indications :

1. Position and morphology : very fine and some times interrupted cracks, usually develop internally, at the toe of the adjacent weld passes and on the root weld metal

2. HIC propagate

3. More HIC develop and propagate during attempts to repair without HRHT

4. No cracking occur after the HRHT and other precautions

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Investigation : Contractor

Crack only in the 60mm thick old shell

Steel : 0.24%C, 1.01%Mn, …

The crack was propagating

Cracks had not been found in any of the other kiln joints (other sections thinner !)

All cracks are hydrogen-induced and a HRHT must be carried out before any repair

It is not a standard practice to remove the DP lacquer before gouging and welding (Not acceptable statement)


Information from the DP supplier

The product is a paint that contains an organic resin that incorporates hydrogen in its formulation

Welding over this product must be avoided to prevent hydrogen-induced cracking

The product datasheet does not permit gouging and welding over the lacquer

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RepairBefore the HRHT, for safety reasons, the contractor recommended :

Complete welding of the 3.5m long and 20mm deep groove on the externalInstallation of strong backs to prevent further cracking

HRHT : 450°C – 20 hours (december 23)


RepairAfter the HRHT :

In some areas the lacquer was not completely removed The 1.6m and 3.5m cracks propagated and others were created

It was reported afterwards that the contractor didn’t perform UT on the entire section : difficulty to know the history of the defects !

UT on the entire section :2 lack of fusions : 380mm and 190mm3 cracks : 35mm, 1200mm and 1500mmLaminations : 1200mm long

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Repair

No additional cracks were developed after :HRHTStrict storage and handling of the E9018 welding electrodesProper cleaning of the joints before weldingRepair weldingDP was replaced by MP


Welding procedureWelding Procedure Specification WPS :

The ASME WPS applied by the contractor was qualified for a thickness range of 4.8mm to 56mm :- E9018-D1 electrodes (min tensile strength of 90kpsi),

preheating at 150°C, 3.2mm for the root and 5mm for the filler

The FOX EV 51 (E9018-D1) standard low hydrogen electrodes (lime covered) were used for the root and filler passes, This electrode when well prepared (backing at 300°C for 2 hours) has a diffusible hydrogen of <10ml/100g

The electrodes to be used in such applications are the special extra-low hydrogen content ESAB OK 74.78 or E7018 with a diffusible hydrogen of < 5ml/100g

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Welding procedure

Welding Procedure Specification WPS :According to the hydrogen Control Method, the minimum preheating temperature should have been 180°C for diffusible hydrogen less than 9.5ml/100g to prevent HIC

The risk of HIC increased substantially when FLS replaced the low strength E7018 electrodes with the higher strength E9018-D1


Welding procedure

Drying and holding ovens : A temperature of 130°C was measured in the large oven that contained many electrodesThe drying and main holding oven was stored outside and directly exposed to rainIt is possible that some electrodes which were used were not baked (dried) : information from welding registers

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Learning from the practical case 2

The WPS has to be prepared including all details prior to the welding

Selection of an extra low diffusible hydrogen electrode <5ml/100g : Equivalent to E7018

Preheating the shell sections at the right temperature depending on the C equivalent and thickness in all the areas between –100 and +100mm of the joint

Proper drying and storage of the electrodes has to be checked : 300 to 350°C during 2 hours


ConclusionsWelding is an important Maintenance activity performed in cement plants

Welding is not as simple as we imagine, special care must be given when welding alloyed materials

The welding efficiency (quality and time) impacts directly the equipment availability

The main welding parameters have to be known and prepared in advance :

Material to weld Welding technique and electrode to be usedWelding procedure : preheating temperature, cooling speed, preparation of the electrodes

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Conclusions

Once developed, the Welding procedure has to be strictly followed

The contractors involved in special welding jobs must have the right experience

The NDT method to be applied has to be checked in advance

Other HGRS complementary documents :Kiln shell replacementGirth gear repair procedureMill end repair procedure

welding procedure

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rules of welding

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special welding procedures

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