WELD DEFECTS

Weldability and defects in weldments

Subjects of Interest Reviews of weld design and weldability Residual stresses and weld distortion Weld metal in homogeneities(Porosities, poak marks etc.,) Cracks, Hydrogen Cracking Incomplete fusion/penetration Burn through Common weld defects and their remedies

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This session aims to :

Understand the causes of common weld defects

Preventing these weld defects

Repair of weld defects.

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Weld design joint type

A Butt -A joint between two members aligned approximately in the same plane.

B Butt

-A joint between two members located approximately at right angles to each other in the form of an L.

C Butt

-A joint between two members located approximately at right angles to each other in a form of a T.

D Butt

- A joint between two overlapping members located in parallel

E Butt

-A joint between the edges of two or more parallel or nearly parallel members.

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Weld design 8 Weld types

Fillet weld Groove weld Back weld Slot weld Spot weld Seam weld Stud weld Surface weld -On the joint - In the joint -Made on the backside of the joint - Used with prepared holes - Weld at the interface of the members - Without prepared holes - Welding a metal stud - Weld beads deposited on the base metal or broken surface

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Weldability
The capability of a material to be welded under the imposed fabrication conditions into a specific, suitably designed structure and to perform satisfactorily in the intended service. Weldability depends on Nature of metals Joint design Welding techniques Skills All metals are weldable only some are more difficult than others Steel is readily weldable than other metals like Al, Cu Cu is not easily weldable due to its high thermal conductivity requires pre heating to ~ 300 Deg C Al is not easily weldable having oxide layer

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Weldability

Steel

Contd.

Weldability of steels is inversely proportional to its hardenability, due to martensite formation during heat treatment

% Carbon

Hardenability

Weldability

There is a trade-off bet materials strength and Weldablity

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Defects in Weldments
It is unusual for the weldments to be completely sound They normally contain small defects such as porosity, slag, oxide inclusions, lack of fusion, undercut, crack, distortion etc.,

Understand the cause

Solve/Prevent the pbm

Furthermore, diff metals have diff weldability hence understand the nature of the metal to be welded

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Defects in Weldments

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Defects in Weldments
Residual stresses(internal) are stresses that would exist in a body after removing all external loads (normally due to non uniform temp change during welding in this case Weld metal and adjacent base metal are restrained by the areas further away from the weld metal due to expansion and contraction. Weld metal and adj. base metal Residual tensile stress Res. Compressive stress

Areas away from weld metal

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Effect of temperature and time on stress relief of steel weld

Stress relief temperature

% Relief of initial stress

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Weld Distortion is due to solidification shrinkage and thermal contraction of the weld metal during welding

Distortion in welded structure

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Angular distortion
Thin plates

Thick plates

Upward angular distortion usually occurs when the weld is made from the top of the work piece alone. The weld tends to be wider at the top than the bottom, causing more solidification shrinkage and thermal contraction

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Remedies for angular distortion

There are several techniques used to reduce angular distortion. Reducing volume of weld metal Using double-V joint and alternate welding Placing welds around neutral axis Controlling weld distortion

Placing weld around neutral axis

Reducing volume of weld metal and by using singlepass deep penetration welding

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Remedies for angular distortion

Balancing the angular weld distortion on either side of double V joint

Double V Balance the shrinkage to almost same amount on contraction on sides(a) Asymetrical double V the first weld always produces more angular distortion. The 2nd side is larger to pull back(b) Single U uniform weld with through the section c
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Remedies for angular distortion

Methods for controlling weld distortion Presetting : by compensating the amount of distortion to occur in welding Elastic pre springing can reduce angular changes after restraint is removed Preheating and post weld heat treatment Mechanical methods : Straightening press, jacks, clamps Welding short lengths on a planned or random distribution

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WELD DEFECTS - Porosity

Single pore Separated by at least their own diameter along the axis of the weld

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Uniformly Scattered Porosity

Typically judged by diameter and proximity to a start or stop Often caused by low amperage or short arc gap or an unshielded weld start.

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Weld defects - Porosity

Cluster porosity -Typically viewed as a single large discontinuity Linear Porosity - Being linear greatly affects the severity of this discontinuity

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Porosity - Remedies

Preheat will help eliminate May need an electrode with more deoxidizers

Cleaning the joint before weld(free from oil/greasy)

Restart on top of previous weld and grind off lump

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Weld Defects - Incomplete Fusion

Definition : Where weld metal does not form a cohesive bond with the base metal Cause : Low amperage, improper electrode angles, fast travel speed, lack of preheat, unclean base metal, short arc gap

Prevention: Eliminate the above said potential causes Repair. Remove and re weld

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Weld Defects - Incomplete Fusion

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Weld Defects Incomplete inter-run fusion/penetration

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Weld defects
Definition : When the weld metal does not extend to the required depth into the joint root
: Low Amp, Low preheat, tight root opening, fast travel speed, short arc : Correct the contributing factors : back gouge and back weld

Cause

Prevention Repair

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Weld Defects Cracks

Longitudinal Transverse Crater Throat Toe Root Under bead and HAZ HOT Cold or delayed
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Weld Defects Longitudinal Cracks

Definition : A crack running in the direction of the weld axis either in weld or metal Cause : Preheat or fast cooling. Stresses(shrinkage) in high constraint areas

Prevention : Preheat to even out cooling rates. Weld toward areas of less constraint Repair : Remove and re weld.

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Weld Defects

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Weld defects - Transverse

Transverse Crack A crack running into or inside a weld, transverse to the weld axis direction This happens basically due to weld metal hardness.

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Weld Defects Crater

Definition: A crack, generally in the shape of an X which is found in the crater. These are normally hot cracks The centre of the weld pool becomes solid before the outside of the weld pool, pulling the centre apart during cooling Use crater fill, fill the crater at the weld termination. Also even out the cooling of the puddle

Cause:

Prevention:

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Weld defects

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Weld defects Toe crack

Definition: A crack in the base metal beginning at the toe of the weld Cause : Transverse shrinkage stresses. Indicates a HAZ brittleness problem

Prevention: Increase preheat if possible, or use a more ductile filler material.

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Weld defects Throat crack

Definition :A longitudinal crack located in the weld throat area Cause : Transverse stresses, probably from shrinkage, indicates inadequate filler metal selection or welding procedure. May be due to crater crack propagation.

Prevention: Correct initial cause. Increasing preheat may prevent it but crater should not be left. Use more ductile filler material Repair : Remove and re weld.

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Hydrogen cracking (cold cracking)

Hydrogen in the weld metal sources: moisture from metal surface, tools, atmosphere, flux, High stresses Susceptible microstructure : martensite (HAZ of carbon steels due to lower diffusion coefficient of hydrogen in austenite than in ferrite), combination of hydrogen + martensite promotes hydrogen cracking. Relatively low temperature. (-100-200oC) cold cracking or delayed cracking.

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Remedies for hydrogen cracking

Controlling welding parameters: proper preheat and interpass Temperature Use consumables with low hydrogen(H2R)

Post weld treatment: stress relief.

Use proper welding processes and Materials (consumables),

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Weld Defects Burn Through

Definition : An undesirable open hole has been completely melted through the base metal. The hole may or may not be open Cause : Excessive heat input

Prevention: Reduce heat input by increasing speed, use of a heat sink or welding parameters. Repair : Will be defined by standards. Some standard may require special filler metal and/or PWHT

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Ready Reckoner on weld defects

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Ready Reckoner on weld defects

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Thank You.

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