Weldability Of Steels Section 21

Welding Inspector

Weldability Of Steels

Section 21

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Weldability of SteelsDefinition

It relates to the ability of the metal (or alloy) to be welded with mechanical soundness by most of the common welding processes, and the resulting welded joint retain the properties for which it has been designed.

is a function of many inter-related factors but these may be summarised as:

•Composition of parent material

•Joint design and size

•Process and technique

•Access

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Weldability of Steels

The weldability of steel is mainly dependant on carbon & other alloying elements content.

If a material has limited weldability, we need to take special measures to ensure the maintenance of the properties required

Poor weldability normally results in the occurrence of cracking

A steel is considered to have poor weldability when:

• an acceptable joint can only be made by using very narrow range of welding conditions

• great precautions to avoid cracking are essential (e.g., high pre-heat etc)

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The Effect of Alloying on SteelsElements may be added to steels to produce the properties required to make it useful for an application.

Most elements can have many effects on the properties of steels.

Other factors which affect material properties are:

•The temperature reached before and during welding

•Heat input

•The cooling rate after welding and or PWHT

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Steel Alloying Elements

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Iron (Fe): Main steel constituent. On its own, is relatively soft, ductile, with low

strength.

Carbon (C): Major alloying element in steels, a strengthening element with

major influence on HAZ hardness. Decreases weldability.

•typically < ~ 0.25%

Manganese (Mn): Secondary only to carbon for strength, toughness and

ductility, secondary de-oxidiser and also reacts with sulphur to form

manganese sulphide.

< ~0.8% is residual from steel de-oxidation

•up to ~1.6% (in C-Mn steels) improves strength & toughness

Silicon (Si): Residual element from steel de-oxidation.

•typically to ~0.35%


Phosphorus (P): Residual element from steel-making minerals. difficult to reduce below < ~ 0.015% brittleness

Sulphur (S): Residual element from steel-making minerals

< ~ 0.015% in modern steels

< ~ 0.003% in very clean steels

Aluminium (Al): De-oxidant and grain size control

•typically ~ 0.02 to ~ 0.05%

Chromium (Cr): For creep resistance & oxidation (scaling) resistance for elevated temperature service. Widely used in stainless steels for corrosion resistance, increases hardness and strength but reduces ductility.

•typically ~ 1 to 9% in low alloy steels

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Nickel (Ni): Used in stainless steels, high resistance to corrosion from acids, increases strength and toughness

Molybdenum (Mo): Affects hardenability. Steels containing molybdenum are less susceptible to temper brittleness than other alloy steels. Increases the high temperature tensile and creep strengths of steel. typically ~ 0.5 to 1.0%

Niobium (Nb): a grain refiner, typically~ 0.05%

Vanadium (V): a grain refiner, typically ~ 0.05%

Titanium (Ti): a grain refiner, typically ~ 0.05%

Copper (Cu): present as a residual, (typically < ~ 0.30%) added to ‘weathering steels’ (~ 0.6%) to give better resistance to atmospheric corrosion

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Classification of Steels

Mild steel (CE < 0.4)• Readily weldable, preheat generally not required if low hydrogen

processes or electrodes are used

• Preheat may be required when welding thick section material, highrestraint and with higher levels of hydrogen being generated

C-Mn, medium carbon, low alloy steels (CE 0.4 to 0.5)

• Thin sections can be welded without preheat but thicker sections willrequire low preheat levels and low hydrogen processes or electrodesshould be used

Higher carbon and alloyed steels (CE > 0.5)

• Preheat, low hydrogen processes or electrodes, post weld heating andslow cooling may be required

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Process Cracks

• Hydrogen Induced HAZ Cracking (C/Mn steels)

• Hydrogen Induced Weld Metal Cracking (HSLA steels).

• Solidification or Hot Cracking (All steels)

• Lamellar Tearing (All steels)

• Re-heat Cracking (All steels, very susceptible Cr/Mo/V steels)

• Inter-Crystalline Corrosion or Weld Decay (stainless steels)

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CrackingWhen considering any type of cracking mechanism, three elements must always be present:

• Stress

Residual stress is always present in a weldment, throughunbalanced local expansion and contraction

• Restraint

Restraint may be a local restriction, or through platesbeing welded to each other

• Susceptible microstructure

The microstructure may be made susceptible tocracking by the process of welding

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Cracks

Hydrogen Induced Cold Cracking

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May occur:

• up to 48 hrs after completion

• In weld metal, HAZ, parentmetal.

• At weld toes

• Under weld beads

• At stress raisers.

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Also know as:

Cold Cracking, happens when

the welds cool down.

HAZ cracking, normally occurs

in the HAZ.

Delayed cracking, as it takes

time for the hydrogen to

migrate. 48 Hours normally but

up to 72,

Under-bead cracking, normally

happens in the HAZ under a

weld bead


There is a risk of hydrogen cracking when all of the 4 factors occur together:

•Hydrogen More than 15ml/100g of weld metal

•Stress More than ½ the yield stress

•Temperature Below 300oC

•Hardness Greater than 400HV Vickers

•Susceptible Microstructure (Martensite)

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Precautions for controlling hydrogen cracking

• Pre heat, removes moisture from the joint preparations, and slows down the cooling rate

• Ensure joint preparations are clean and free from contamination

• The use of a low hydrogen welding process and correct arc length

• Ensure all welding is carried out is carried out under controlled environmental conditions

• Ensure good fit-up as to reduced stress

• The use of a PWHT

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• Hydrogen is the smallest atom known

• Hydrogen enters the weld via the arc

• Source of hydrogen mainly from moisture pick-up on

the electrodes coating, welding fluxes or from the

consumable gas

H2

H2

H2

H2H2

Moisture on the electrode or grease on the wire

Water vapour in the air or in the shielding gas

Oxide or grease on the plate

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Hydrogen absorbed

in a long, or

unstable arc

Hydrogen introduced in

weld from consumable,

oils, or paint on plate

Cellulosic electrodes

produce hydrogen as a

shielding gas

Hydrogen

crack

Martensite forms from γ H2 diffuses to γ in HAZ

H2H2



Susceptible Microstructure:

Hard brittle structure – MARTENSITE Promoted by:

A) High Carbon Content, Carbon Equivalent (CE)

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Heat input (Kj/mm) = Amps x Volts x arc time

Run out length x 103 (1000)

CEV = %C + Mn + Cr+Mo+V + Ni+Cu

6 5 15B) high alloy content

C) fast cooling rate: Inadequate Pre-Heating

Cold Material

Thick Material

Low Heat Input.


Typical locations for Cold Cracking

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•HSLA or Micro-Alloyed Steels are high strength steels

(800MPa/N/mm2) that derive their high strength from small

percentage alloying (over-alloyed Weld metal to match the

strength of parent metal)

•Typically the level of alloying is in the elements such as

vanadium molybdenum and titanium, nickel and chromium

Strength. are used. It would be impossible to match this micro

alloying in the electrode due to the effect of losses across an

electric arc (Ti burn in the arc)

•It is however important to match the strength of the weld to

the strength of the plate, Mn 1.6 Cr Ni Mo

HICC in HSLA steels

Hydrogen Scales

List of hydrogen scales from BS EN 1011:part 2.

Hydrogen content related to 100 grams of weld metal deposited.

• Scale A High: >15 ml

• Scale B Medium: 10 ml - 15 ml

• Scale C Low: 5 ml - 10 ml

• Scale D Very low: 3 ml - 5 ml

• Scale E Ultra-low: < 3 ml

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Potential Hydrogen Level Processes

list of welding processes in order of potential lowest hydrogen content with regards to 100g of deposited weld metal.

•TIG < 3 ml

•MIG < 5 ml

•ESW < 5 ml

•MMA (Basic Electrodes) < 5 ml

•SAW < 10ml

•FCAW < 15 ml

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Weldability

Solidification Cracking

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Usually Occurs in Weld Centerline

Solidification CrackingAlso referred as

Hot Cracking: Occurring at high temperatures while the weld is hot

Centerline cracking: cracks appear down the centre line of the bead.

Crater cracking: Small cracks in weld centers are solidification cracks

Crack type: Solidification cracking

Location: Weld centreline (longitudinal)

Steel types: High sulphur & phosphor concentration in steels.

Susceptible Microstructure: Columnar grains In direction of solidification

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Factors for solidification cracking

• Columnar grain growth with impurities in weld metal (sulphur,

phosphor and carbon)

• The amount of stress/restraint

• Joint design high depth to width ratios

Liquid iron sulphides are formed around solidifying grains.

High contractional strains are present

High dilution processes are being used.

There is a high carbon content in the weld metal

• Most commonly occurring in sub-arc welded joints


• Sulphur in the parent material may dilute in the weldmetal to form iron sulphides (low strength, low meltingpoint compounds)

• During weld metal solidification, columnar crystals pushstill liquid iron sulphides in front to the last place ofsolidification, weld centerline.

• The bonding between the grains which are themselvesunder great stress and may now be very poor to maintaincohesion and a crack will result, weld centerline.

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Solidification CrackingAvoidance

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Deep, narrower weld bead

On solidification the

bonding between the grains

may now be very poor to

maintain cohesion and a

crack may result

Shallow, wider weld bead

On solidification the

bonding between the

grains may be adequate to

maintain cohesion and a

crack is unlikely to occur

HAZ HAZ

Intergranular liquid filmColumnar grains Columnar

grains


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Precautions for controlling solidification cracking

•The first steps in eliminating this problem would be to choose a low

dilution process, and change the joint design

Grind and seal in any lamination and avoid further dilution????

Add Manganese to the electrode to form spherical Mn/S which form

between the grain and maintain grain cohesion

As carbon increases the Mn/S ratio required increases

exponentially and is a major factor. Carbon content % should be a

minimised by careful control in electrode and dilution

Limit the heat input, hence low contraction, & minimise restraint


Precautions for controlling solidification cracking

• The use of high manganese and low carbon content fillers

• Minimise the amount of stress / restraint acting on the joint during welding

• The use of high quality parent materials, low levels of impurities (Phosphor & sulphur)

• Clean joint preparations contaminants (oil, grease, paints and any other sulphur containing product)

• Joint design selection depth to width ratios

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Solidification cracking in Austenitic Stainless Steel

• particularly prone to solidification cracking

• large grain size gives rise to a reduction in grain boundary area withhigh concentration of impurities

• Austenitic structure very intolerant to contaminants (sulphur,phosphorous and other impurities).

• High coefficient of thermal expansion /Low coefficient of thermalconductivity, with high resultant residual stress

• same precautions against cracking as for plain carbon steels with extraemphasis on thorough cleaning and high dilution controls.

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Cracks

Lamellar Tearing

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Lamellar Tearing

Factors for lamellar tearing to occur

Cracks only occur in the rolled plate !

Close to or just outside the HAZ !

Cracks lay parallel to the plate surface and the fusion boundary of the weld and has a stepped aspect.

• Low quality parent materials, high levels of impurities

• Joint design, direction of stress

• The amount of stress acting across the joint during welding

• Note: very susceptible joints may form lamellar tearing under very low levels of stress

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Lamellar Tearing

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Tee fillet weld Tee butt weld

(double-bevel)

Corner butt weld

(single-bevel)

Susceptible joint types combined with susceptible rolled plate

used to make a joint.

High stresses act in the through thickness direction of the plate

(know as the short transverse direction).

T, K & Y joints normally end up with a tensile residual stress

component in the through thickness direction.

Lamellar Tearing

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Critical area

Critical

area

Critical area

Lamellar Tearing

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Modifying a corner joint to avoid lamellar tearing

Susceptible Non-Susceptible

Prior welding both plates may be grooved to avoid lamellar tearing

An open corner joint may be selected to avoid lamellar tearing

Lamellar Tearing

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Precautions for controlling lamellar tearing

• The use of high quality parent materials, low levels of

impurities

• The use of buttering runs

• A gap can be left between the horizontal and vertical

members enabling the contraction movement to take

place

• Joint design selection

• Minimise the amount of stress / restraint acting on the

joint during welding

• Hydrogen precautions

Lamellar TearingCrack type: Lamellar tearing

Location: Below weld HAZ

Steel types: High sulphur & phosphorous steels

Microstructure: Lamination & Segregation

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Occurs when:

High contractional strains are through the short

transverse direction. There is a high sulfur content in

the base metal.

There is low through thickness ductility in the base

metal.

There is high restraint on the work

Short Tensile (Through Thickness) Test

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The short tensile test or through thickness test is a

test to determine a materials susceptibility to

lamellar tearing

Friction Welded Caps

Short Tensile Specimen

Through

Thickness

Ductility

Sample of Parent Material

The results are given as a STRA value

Short Transverse Reduction in Area

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High contractional

strains

Lamellar tear

Restraint

Lamellar Tearing

Documents

Weldability Of Steels Section 21