Modelling the Evolution of Microstructure

Modelling the Evolution of Microstructure
in Steel Weld Metal

H. K. D. H. Bhadeshia and ^*L.-E. Svensson

University of Cambridge
Materials Science and Metallurgy
Pembroke Street, Cambridge CB2 3QZ, U. K.
www.msm.cam.ac.uk/phase-trans
.
^*ESAB AB, Gothenburg, Sweden

Abstract:

Physical models for the development of microstructure have the potential of revealing new phenomena and properties. They can also help identify the controlling variables. The ability to model weld metal microstructure relies on a deep understanding of the phase transformation theory governing the changes which occur as the weld solidifies and cools to ambient temperature. Considerable progress has been made with the help of thermodynamic and kinetic theory which accounts for the variety of alloying additions, non-equilibrium cooling conditions and other many other variables necessary to fully specify the welded component. These aspects are reviewed with the aim of presenting a reasonably detailed account of the methods involved, and of some important, outstanding difficulties.

It is now well established that extremely small concentrations of certain elements can significantly influence the transformation behaviour of weld metals. Some of these elements are identical to those used in the manufacture of wrought microalloyed steels, whereas others enter the fusion zone as an unavoidable consequence of the welding process. The theory available to cope with such effects is as yet inadequate. Methods for incorporating the influence of trace elements such as oxygen, aluminium, boron, nitrogen, titanium and the rare earth elements into schemes for the prediction of microstructure are discussed. The very high sensitivity of modern microalloyed steels to carbon concentration is also assessed. Some basic ideas on how the approximate relationships between weld microstructure and mechanical properties can be included in computer models are discussed.

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Possible E ects of Stress on Steel Weld Microstructures

Mathematical Modelling of Weld Phenomena, eds H. Cerjak, H. Bhadeshia, Institute of Materials, London,
1995, pp. 71-118

Possible Eects of Stress on Steel Weld Microstructures

H. K. D. H. Bhadeshia
University of Cambridge
Materials Science and Metallurgy
Pembroke Street, Cambridge CB2 3QZ, U. K.
www.msm.cam.ac.uk/phase-trans

Abstract.
Little is known about the eect of stress on the development of microstructure in steel welds. This paper contains an assessment of published data together with a description of the theory that is available for dealing with stress{aected transformations in steels. Attention is focused on those transformations which have the greatest potential for interaction with an externally applied stress. These include the solid{state transformation products of austenite, such as Widmanstatten ferrite, acicular ferrite, bainite and martensite.

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TECHNICAL GUIDANCE NOTES

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Fillet Welds

The fillet weld (pronounced "FILL-it," not "fil-LAY") is used to make lap joints, corner joints, and T joints. As its symbol suggests, the fillet weld is roughly triangular in cross-section, although its shape is not always a right triangle or an isosceles triangle. Weld metal is deposited in a corner formed by the fit-up of the two members and penetrates and fuses with the base metal to form the joint. (Note: for the sake of graphical clarity, the drawings below do not show the penetration of the weld metal. Recognize, however, that the degree of penetration is important in determining the quality of the weld.)

The perpendicular leg of the triangle is always drawn on the left side of the symbol, regardless of the orientation of the weld itself. The leg size is written to the left of the weld symbol. If the two legs of the weld are to be the same size, only one dimension is given; if the weld is to have unequal legs (much less common than the equal-legged weld), both dimensions are given and there is an indication on the drawing as to which leg is longer.

The length of the weld is given to the right of the symbol.

If no length is given, then the weld is to be placed between specified dimension lines (if given) or between those points where an abrupt change in the weld direction would occur (like at the end of the plates in the example above).

For intermittent welds, the length of each portion of the weld and the spacing of the welds are separated by a dash (length first, spacing second) and placed to the right of the fillet weld symbol.

Notice that the spacing, or pitch, is not the clear space between the welds, but the center-to-center (or end-to-end) distance.

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For more information, see ANSI/AWS A2.4, Symbols for Welding and Nondestructive Testing.

Deciphering Weld Symbols

When welds are specified on engineering and fabrication drawings, a cryptic set of symbols is used as a sort of shorthand for describing the type of weld, its size, and other processing and finishing information. The purpose of this page is to introduce you to the common symbols and their meaning. The complete set of symbols is given in a standard published by the American National Standards Institute and the American Welding Society:

ANSI/AWS A2.4, Symbols for Welding and Nondestructive Testing.

Our thanks to Dr. Kent L. Johnson, past Chairman of the AWS Chicago Section, for his many helpful comments on the content of our welding pages.

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The structure of the welding symbol

The horizontal line--called the reference line--is the anchor to which all the other welding symbols are tied. The instructions for making the weld are strung along the reference line. An arrow connects the reference line to the joint that is to be welded. In the example above, the arrow is shown growing out of the right end of the reference line and heading down and to the right, but many other combinations are allowed.

Quite often, there are two sides to the joint to which the arrow points, and therefore two potential places for a weld. For example, when two steel plates are joined together into a T shape, welding may be done on either side of the stem of the T.

The weld symbol distinguishes between the two sides of a joint by using the arrow and the spaces above and below the reference line. The side of the joint to which the arrow points is known (rather prosaically) as the arrow side, and its weld is made according to the instructions given below the reference line. The other side of the joint is known (even more prosaically) as the other side, and its weld is made according to the instructions given above the reference line. The below=arrow and above=other rules apply regardless of the arrow's direction.

The flag growing out of the junction of the reference line and the arrow is present if the weld is to be made in the field during erection of the structure. A weld symbol without a flag indicates that the weld is to be made in the shop. In older drawings, a field weld may be denoted by a filled black circle at the junction between the arrow and the reference line.

The open circle at the arrow/reference line junction is present if the weld is to go all around the joint, as in the example below.

The tail of the weld symbol is the place for supplementary information on the weld. It may contain a reference to the welding process, the electrode, a detail drawing, any information that aids in the making of the weld that does not have its own special place on the symbol.

Types of welds and their symbols

Each type of weld has its own basic symbol, which is typically placed near the center of the reference line (and above or below it, depending on which side of the joint it's on). The symbol is a small drawing that can usually be interpreted as a simplified cross-section of the weld. In the descriptions below, the symbol is shown in both its arrow-side and other-side positions.

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Fillet welds		Groove welds		Plug welds and slot welds

Free Energy of a Corrosion Reaction

In electrical and electrochemical processes, electrical work is defined as the product of charges moved (Q) times the potential (E) through which it is moved. If this work is done in an electrochemical cell in which the potential difference between its two half-cells is E, and the charge is that of one mole of reaction in which n moles of electrons are transferred, then the electrical work (-w) done by the cell must be nE. In this relationship, the Faraday constant F is required to obtain coulombs from moles of electrons. In an electrochemical cell at equilibrium, no current flows and the energy change occurring in a reaction is expressed in equation:

Under standard conditions, the standard free energy of the cell reaction DG⁰ is directly related to the standard potential difference across the cell, E⁰:

For solid or liquid compounds or elements, standard conditions are the pure compound or element; for gases they are 100 kPa pressure; and for solutes they are the ideal 1 molar (mol/L) concentration.

Electrode potentials can be combined algebraically to give cell potential. For a galvanic cell, such as the Daniell cell, a positive cell voltage will be obtained if the difference is taken in the usual way, as equation.

The free energy change in a galvanic cell, or in a spontaneous cell reaction, is negative and the cell voltage positive. The opposite is true in an electrolytic cell that requires the application of an external potential to drive the electrolysis reaction, in which case E_cell would be negative.

Other thermodynamic quantities can be derived from electrochemical measurements. For example, the entropy change (DS) in a cell reaction is given by the temperature dependence of DG:

hance

where DH is the enthalpy change and T the absolute temperature (K).

The equilibrium constant (K_eq) for the same reaction can be obtained withthe following equation: