advanced metaullrgy final welding

1

Advanced Metallurgy(Welding Metallurgy)

Presented by Marwan Faisal

Mohamed Abdel Moniem

2 Outlines Basics of welding metallurgy

Microstructure of weld o Iron-carbon phase diagram

Welding Metallurgyo Fusion zoneo Heat affected zoneo Different zones in steel weld

3 Outlines Problems Associated with Re-melting and

Solidification in Welding o Formation of grains in fusion zone o Macro segregation o Hot Cracking o Cold Cracking o Lameller Tearing o Reheat Cracking

Welding inspectiono Non Destructive Testing

Visual Inspection Weld Penetrant examination Radiographic examination References

References

4

Welding metallurgist will examine the changes in physical characteristics that happen in short periods.

The solubility of gases in metals and between metals and the effect of impurities are all of major importance to the welding metallurgist.

Basics Of Welding Metallurgy

5

The overall arrangement of grains, grain boundaries, phases present in an alloy is called its microstructure. It is largely responsible for the properties of the metal.

The microstructure is affected by the composition or alloy content and by other factors such as hot or cold working, straining, heat treating etc.

The microstructure of weld metal and adjacent metal is greatly influenced by the welding process, which influence the properties of the weld.

Microstructure

6

Equilibrium diagrams are used to determine the phases that are present and the percentage of each, based on the alloy composition at a temp. And changes by increasing and decreasing temp. Most of them are designed for alloy system containing two elements.

In welding because of rapid changes in temperatures, equilibrium conditions are rarely occur. In an equilibrium condition, the metal is stable at the particular point on the diagram based on relatively slow heating and cooling

Iron-carbon equilibrium diagram provides an insight of the behavior of steels in connection with welding thermal cycles and heat treatment. This diagram represents the alloy of iron with carbon, ranging from 0% to 5% carbon.

Microstructure

7

Iron-carbon phase diagram

Microstructure

8

Pure iron is relatively weak but ductile metal. When carbon is added in small amounts, the iron acquires a wide range of properties and uses and becomes the most popular metal, ‘steel’. 0% carbon, pure iron,

above 1540ºC, in liquid state, no crystalline structure < 1540 ºC, solidification starts, BCC structure, Delta iron < 1400 ºC, transformation occurs, FCC structure, Gamma

iron < 910 ºC, iron back to BCC, alpha iron until room temp Iron and carbon form a compound known as iron carbide

(Fe3C) or cementite. When iron carbide or cementite is heated above 1115 ºC, it

decomposes into liquid iron saturated with graphite, which is a crystalline form of carbon.

Microstructure

9 Ferrite This phase has a Body Centre Cubic structure (B.C.C) which

can hold very little carbon; typically 0.0001% at room temperature. It can exist as either: alpha or delta ferrite.

Austenite This phase is only possible in carbon steel at high temperature. It has a Face Centre Cubic (F.C.C) atomic structure which can contain up to 2% carbon in solution.

Cementite Unlike ferrite and austenite, cementite is a very hard intermetallic compound consisting of 6.7% carbon and the remainder iron, its chemical symbol is Fe3C. Cementite is very hard, but when mixed with soft ferrite layers its average hardness is reduced considerably.

Pearlite A mixture of alternate strips of ferrite and cementite in a single grain. The name for this structure is derived from its mother of pearl appearance under a microscope. A fully pearlitic structure occurs at 0.8% Carbon. It is a lamellar structure, which is relatively strong and ductile.

Microstructure

10

Figure 1 Figure 2

Microstructure

11

When a weld is made, following factors occur: A-the changes of temperature B-The growth of dimensions C-The phase transformation etc.

The rate of cooling or quench is of primary importance and this is controlled by the process, procedure, metal and mass.

Example: The electroslag has the lowest cooling rate among welding methods, while the gas metal arc has a much faster cooling rate.

Welding Metallurgy

12

The rate of change decreases as the distance from the center of the weld increases.

It is obvious that many different cooling rates occur and that different microstructures will result. Also different phases occur in the base metal adjacent to the weld. o (a)Mixture of ferrite and pearlite grains o (b)Pearlite transformed to Austenite o (c)Full Austenite transformation o (d)Completely liquid state

Welding Metallurgy

13

In addition to the complications created by the rapid cooling, there is also the complication of composition variations.

As weld metal is deposited on a base metal, some of the base metal melts and mixes with the weld metal, producing a dilution of metal.

If the compositions of the weld metal and the base metal are not identical, variation of composition at the interface can be observed and also it causes variation of cooling rates. This results variation of microstructures.

Welding Metallurgy

14 Fusion ZoneWelding Metallurgy

15 Fusion Zone The fusion zone (referred to as FZ) can be

characterized as a mixture of completely molten base metal (and filler metal if consumable electrodes are in use) with high degree of homogeneity where the mixing is primarily motivated by convection in the molten weld pool.

Similar to a casting process, the microstructure in the weld fusion zone is expected to change significantly due to remelting and solidification of metal at the temperature beyond the effective liquidus temperature.

The weld interface, which is also referred to as mushy zone, is a narrow zone consisting of partially melted base material which has not got an opportunity for mixing. This zone separates the fusion zone and heat affected zone.

Welding Metallurgy

16

The area between the interface of the deposited weld metal, and extending into the base metal far enough that any phase change occurs, is know as the heat-affected-zone (HAZ).

HAZ is a portion of the weld since it influences the service life of the weld.

HAZ is the most critical in many welds. For instance, when welding a hardenable steel, HAZ can increase in hardness to an undesirable level. When welding a hardened steel, HAZ can become a softened zone since the heat of the weld has annealed the hardended metal

Heat affected zoneWelding Metallurgy

17 Different zones in steel weld The fusion zone and heat affected zone of

welded joints can exhibit very different mechanical properties from that of the unaffected base metal as well as between themselves .

the fusion zone exhibits a typical cast structure while the heat affected zone will exhibit a heat-treated structure involving phase transformation, recrystallization and grain growth .

The unaffected base metal, on the other hand, will show the original rolled structure with a slight grain growth.

Welding Metallurgy

18Welding Metallurgy

Different zones in steel weld

19 Formation of grains in fusion zone

In fusion welding, the existing base metal grains at the fusion line serves as the substrate for nucleation of crystals during solidification of the fusion zone.

Thus, new crystals or grains are formed by arranging the atoms from the base metal grains without altering their crystallographic orientations. This feature is referred to as epitaxial solidification, which is usual, in particular, in the autogeneous welding (i.e. without filler material).

Problems Associated With Re-melting And Solidification In Welding

20

Non-epitaxial solidification occurs when fusion welding is done with a filler material or with two different metals and the new grains start forming on heterogeneous sites at the fusion boundary. The boundary of the fusion zone will exhibit random misorientations between the base metal grains and the weld metal grains such that the later may not follow any special orientation relationships with the base metal grains they are in contact with.


Formation of grains in fusion zone

21 Macro segregation Macro segregation is a solidification related problem that

occurs during solidification primarily due to lack of weld pool mixing by convection during fusion welding of dissimilar metals or alloys.

Macro segregation can be avoided by (a) stirring the liquid weld pool either applying a magnetic field or in any other way to give a better mixing in the weld pool, (b) using direct current electrode negative (DCEN) mode in gas tungsten arc welding process for a deeper weld penetration and mixing, (c) using proper filler metals, (d) facilitating enough time for the weld pool to be melt, (e) reducing weld speed.


22 Hot Cracking Hot cracking occur during the terminal stage of solidification

due to contraction of solidifying metal and thermal contraction. Weld joint geometry and the impurity elements in weld pool primarily contribute to solidification cracking.

For example, when the penetration to width ratio is very high, hot cracking occurs due to excessive transverse contraction stress.

Impurities such as sulfur and phosphorus drastically lower the solidification temperature. As a result, solidification occurs at a much lower temperature along the weld centerline, where sulfur and phosphorus tend to segregate, leading to the formation of cracks.

The tendency to solidification cracking can be reduced by using filler metals or flux-wire with high manganese content as manganese combines with sulfur to form manganese sulfide particles reducing the amount of free sulfur available to segregate to the centerline.


23 Cold Cracking

Cold cracking occurs by the combination of the martensite that might have formed of the heat affected zone of a steel weld with entrapped hydrogen leading to the formation of cracks specially in steel welds.

Hydrogen can come from moisture, or flux in the flux-cored, shielded-metal, and submerged arc welding processes, welding consumables, paint, mill oil, degreasing fluids etc.

This defect can be avoided by reducing the cooling rate hence reducing the formation of martensite or by the post-weld heat treatment which convert hard martensitic structure to tempered martensitic structure.


24 Lameller Tearing In the processing of steel, sulfur combines with

manganese to form MnS inclusions in the ingot. When the ingot is rolled, these inclusions elongate

into stringers reducing the strength of the steel in the direction transverse to these stringers.

Subsequently, the transverse stress produced during solidification of welds can cause cracks if the weld is made in the rolling direction, parallel to the stringers. This is referred to as lameller tearing.


25 Reheat Cracking

Welds in thick sections of high strength low alloy in combination with the presence of residual stress and low creep-ductility are susceptible to reheat cracking in the heat affected zone during post weld heat treatment.

The cracks form along the grain boundaries in the heat affected zone, particularly in the coarse-grained region near the fusion line.


These techniques use the application of physical principles from the detection of flaws or discontinuities in materials without impairing their usefulness.

In the field of welding, 4 nondestructive tests are widely used:

1-Dye-penetrant testing and Fluorescent-penetrant testing

2-Magnetic particle testing3-Ultrasonic testing4-Radiographic testing

26 Non Destructive testingWelding Inspection

Visual inspection : It is the most widely used nondestructive testing technique.

It is extremely effective and is the least expensive inspection method.

It is an effective quality control method that will ensure procedure conformity and will catch errors at early stages.

Visual examinations of the finished weldment: weld size (using weld gauges), defects (surface cracks, surface porosity), base metal defects etc.

27Visual weld inspectionWelding Inspection

Weld Penetrant examination Liquid-penetrant examination

is a highly sensitive, nondestructive method for detecting minute discontinuities(flaws) such as cracks, pores, and porosity, which are open to the surface of the material being inspected.

The applied surface must be cleaned from dirt and film. So, discontinuities must be free from dirt, rust, grease, or paint to enable the penetrant to enter the surface opening.

28Welding Inspection

Weld Penetrant examination A liquid penetrant is applied to the surface of the

part to be inspected. The penterant remains on the surface and seeps into any surface opening.

The penetrant is drawn into the surface opening by capilary action. The parts may be in any position when tested.

After sufficient penetration time elapsed, the surface is cleaned and excess penetrant is removed.

The pentrant is usually a red color; therefore, the indication shows up brilliantly against the white background. Even small defects maybe located


Radiographic examinations Radiography is a nondestructive examination

method that uses invisible X-ray, or Gamma radiation to examine the interior of materials.

It gives a permanent film record of defects that is relatively easy to interpret.

Although this is a slow and expensive method of nondestructive examination, it is a positive method for detecting porosity, inclusions, cracks, and voids in the interior of castings, welds, other structures.

X-ray generated by electron bombardment of tungsten, and gamma rays emitted by radioactive elements are penetrating radiation whose intensity is modified by passage through a material.


Radiographic examinations The amount of energy absorbed by a material

depends on its thickness and density. Energy not absorbed by the material will cause exposure of the radiographic film.

Those area will be dark when the film is developed. Areas of material where the thickness has been changed by discontinuities, such as porosity or cracks, will appear as dark outlines on the film.

All discontinuities are detected by viewing shape and variations in the density of the processed film.


Radiographic examinations32Welding Inspection

Welding Handbook, Vol. 3, 7th ed., American Welding Society, Miami, FL, 1980, pp. 170–238.

Mendez, P. F., and Eagar, T. W., Advanced Materials and Processes, 159: 39, 2001. Welding Handbook, Vol. 1, 7th ed., American Welding Society, Miami, FL, 1976, pp. 2–

32. Welding Workbook, Data Sheet 212a, and Weld. J., 77: 65, 1998. Welding Handbook, Vol. 2, 7th ed., American Welding Society, Miami, FL, 1978, pp. 78–

112, 296–330. Lyttle, K. A., in ASM Handbook, Vol. 6, ASM International, Materials Park, OH, 1993, p.

64. Schwartz, M. M., Metals Joining Manual, McGraw-Hill, New York, 1979, pp. 2–1 to 3–40. Lesnewich, A., in Weldability of Steels, 3rd ed., Eds. R. D. Stout and W. D. Doty, Welding

Research Council, New York, 1978, p. 5. Gibbs, F. E., Weld. J., 59: 23, 1980. Fact Sheet—Choosing Shielding for GMA Welding, Weld. J., 79: 18, 2000. Jones, L. A., Eagar, T.W., and Lang, J. H., Weld. J., 77: 135s, 1998. Blackman, S. A., and Dorling, D.V., Weld. J., 79: 39, 2000.

33References

Thank you

34

advanced metaullrgy final welding

Engineering