Metallurgy and Mechanics of Welding

Processes and Industrial Applications

Regis Blondeau

• Publisher: Wiley-ISTE
• Year: 2013
• Print ISBN:
  9781848210387
• Edition: 1
• eBook ISBN:
  9781118623749
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

This book offers a comprehensive overview on the subject of welding. Written by a group of expert contributors, the book covers all welding methods, from traditional to high-energy plasmas and lasers. The reference presents joint welding, stainless steel welding, aluminum welding, welding in the nuclear industry, and all aspects of welding quality control.

Chapter 1

Traditional Welding Processes ¹

1.1. Introduction

To avoid any misunderstandings, the definitions of the terms which appear in this text are those proposed in the document entitled “Terms and definitions used in welding and related techniques” published by the “Publications of Autogeneous Welding and the International Council of the French Language” [COL 96].

It has been specified in the preface to this book that welding makes it possible to reconstitute metallic continuity between the components to be assembled. This reconstitution involves the re-establishment of the interatomic metal bonding forces which requires at the same time a connection of the nodes of the crystal lattices and the absence of any foreign body likely to constitute a screen.

This chapter will successively cover the physical conditions necessary to create the metallic bond and the industrial processes which make it possible to establish this bond.

1.2. Conditions to create metallic bonding

Creating the metal bond consists, theoretically, of bringing the surfaces to be linked closer so that the surface atoms are at a distance of the order of the internodal distances of their own crystalline system.

This operation, which would assume at the beginning that surfaces are chemically clean and in a specular state of polish, is not practically feasible.

To mitigate this industrial impossibility, the surfaces to be joined will have to be activated with a view to eliminating the foreign bodies and elements likely to obstruct the creation of the bond.

1.2.1. Activation of surfaces

The most effective surface activation is fusion which can simultaneously ensure their cleaning. The metallic bond is created by solidification. Different procedures can be employed:

a) the two parts to be assembled undergo a surface fusion and thus contribute to the formation of a molten metal pool (possibly with the addition of a filler) which solidifies without mechanical action;

b) the two parts to be assembled undergo a surface fusion but an external mechanical action expels the molten metal and creates the assembly by placing the surfaces in contact at the solidus temperature;

c) the two parts to be assembled undergo a localized fusion and take part in the formation of a captive molten metal core which during its solidification is compacted by the action of an external effort of compression.

The activation of surfaces can also be obtained by heating without fusion. In general it is then supplemented by a mechanical action which enables, moreover, cleaning and improvement in contact of the surfaces to be assembled. It is possible to distinguish between:

a) the case where the heating and the cleaning of surfaces to be assembled are simultaneously carried out by mechanical friction (which implies the assembly of axisymmetric parts) and is followed, after stopping the latter, by a crushing (“forging”) by axial compression; and

b) the case where the heating is carried out by external heating and the close contact is ensured by an effort perpendicular to the joint plane.

Finally, activation can result from a mechanical action without total heating of the parts to be assembled. This mechanical action causes a plasticization of the outer layer of each surface and generates a very localized heating which finally allows the establishment of the metallic bond. This process simultaneously requires a relative displacement of the surfaces to be assembled, parallel to the mating plane, coupled with a compressive force perpendicular to this same plane. It is necessary to carry out a careful surface preparation and/or to make sure that relative displacements of the latter cause the rejection of the products which pollute them.

1.2.2. Elimination of obstacles to bond creation

Obstacles to the creation of the metallic bond can be of various kinds:

– geometrical surface irregularities,

– pollution of the surface (oxides, grease, moisture, etc.),

– chemical elements brought in by the surrounding air.

Surface irregularities are likely to disrupt the creation of metallic bonds in all the cases where there is not surface fusion of the parts to be assembled. It will then be necessary to carry out a surface preparation by mechanical means (grinding, machining, etc.).

All pollution of surfaces to be assembled will have to be eliminated by mechanical action (sanding, grinding) or by chemical means (solvents, scouring, drying, etc.).

It is necessary to neutralize the possible effects of chemical elements brought in by the surrounding air. Welding operations generally being carried out in atmospheric conditions, it is especially oxygen, nitrogen and hydrogen (carried in the air’s humidity) which can be harmful.

Oxygen can react with the elements volatilized by the arc and in this way contribute to the creation of welding fumes. Furthermore, it can especially dissolve in the molten metal and, during solidification, contribute to the formation of:

– metallic oxides which constitute inclusions in solidified metal;

– porosities in the molten metal due to the drop in solubility which accompanies cooling and solidification. This formation of porosities can be aggravated by a reaction developing with an element contained in the metal and leading to the formation of a gas compound (for example, formation and release of CO during steel welding without protection against the atmosphere).

Protection against oxygen in the air can be ensured by the interposition of a neutral gas, a molten slag or by fixing in the form of oxides by the addition of oxygen hungry elements (silicon especially). In the vicinity of the molten metal, the surface of the parent metal raised to a high temperature can also react with oxygen and be covered with oxides, which is a further justification for using protective means, including at the back of the weld.

Nitrogen can dissolve in the molten metal and contribute to:

– either the formation of porosities in the molten metal due to the drop in solubility which accompanies cooling and solidification;

– or the formation of metal nitrides which, according to the conditions in which they appear, constitute inclusions or precipitates more or less hardening and weakening in nature;

– or for the part which remains in solid solution, a process of weakening by ageing.

Protection against nitrogen can be ensured by interposing a neutral gas or a molten slag.

Hydrogen dissolves in the molten metal and its concentration can reach high levels, even reaching saturation if precautions are not taken to limit its presence. Hydrogen, the solubility of which decreases when the temperature drops, can then contribute to the formation:

– of porosities during solidification;

– of cracks, in a solid state, when, oversaturated, it gathers in the form of gas molecules on the structural defects of a not very ductile metal.

Protection against hydrogen primarily consists of limiting its introduction into the molten metal by lowering the atomic or ionic hydrogen content of the plasma arc. To do this it is necessary to minimize the water content of the surrounding air (no welding in a damp atmosphere), to interpose a gas low in hydrogen between the surrounding air and the arc, to eliminate compounds supplying water (hydroxides, condensation, greases, basic non-dried coatings, fluxes, etc.) and other sources of hydrogen (cellulose or rutile coatings and grease).

1.2.3. How can we classify the various welding processes?

At this stage we are led to adopt a system of grading the welding processes according to the modes of action and means of protection against the atmosphere (see Table 1.1).

Table 1.1. Classing the welding processes according to modes of action and means of protection against the atmosphere

Activation	Complementary action	Protection
fusion	none	imperative
	compression	not essential
heating	compression	possible
friction	compression	none

Actually, the various welding processes are above all classified according to more practical criteria, which are:

– the energy source applied: flame, electric arc, plasma, Joule effect, spark, induction, friction, explosion, etc.;

– the means of protecting the hot metal: gas or slag.

1.3. Industrial welding processes

Industrial welding processes are set out here according to the criteria defined above, namely:

– processes utilizing the fusion without mechanical action;

– processes utilizing the fusion combined with mechanical action;

– processes utilizing heating without fusion but with a mechanical action;

– processes utilizing a mechanical action without heating.

A classification akin to industrial practices will be presented at the end of this chapter.

1.3.1. Processes using local fusion of the parts without mechanical action

For welding processes operating without voluntary mechanical action, the local fusion of the parts to be assembled can be described by distinguishing the mode of heating used and the means of protecting the molten metal against the chemical action of the surrounding air. Thus, it is possible to list:

– flame or gas welding;

– plasma welding;

– arc welding;

– vertical electroslag welding;

– aluminothermic welding.

It should be noted that, in all these processes, the molten metal weld pool is contained in a crucible formed by the shape of the parts to be assembled adjacent in the mating plane (sometimes the complete closure of the “crucible” is ensured by specific tools, e.g. slat, slides, mold). In this way a non-molten section of the parts, in the vicinity of the molten metal, is brought up to temperatures, according to its distance from the latter, between the temperature of the solidus of the metal and the initial temperature of the parts. The fraction of this volume (nearest to the molten metal), of which the structure and therefore the properties change because of this heating, is called the heat affected zone (HAZ).

1.3.1.1. Flame welding

Figure 1.1. Fusion of parent metals and filler metals obtained with a ...

Table of contents

• Cover
• Title Page
• Copyright
• Preface: Welding: The Permanent Bond Between Two Solid Bodies
• Chapter 1: Traditional Welding Processes
• Chapter 2: High Density Energy Beam Welding Processes: Electron Beam and Laser Beam
• Chapter 3: Thermal, Metallurgical and Mechanical Phenomena in the Heat Affected Zone
• Chapter 4: Molten Metal
• Chapter 5: Welding Products
• Chapter 6: Fatigue Strength of Welded Joints
• Chapter 7: Fracture Toughness of Welded Joints
• Chapter 8: Welding of Steel Sheets, With and Without Surface Treatments
• Chapter 9: Welding of Steel Mechanical Components
• Chapter 10: Welding Steel Structures
• Chapter 11: Welding Heavy Components in the Nuclear Industry
• Chapter 12: Welding Stainless Steels
• Chapter 13: Welding Aluminum Alloys
• Chapter 14: Standardization: Organization and Quality Control in Welding
• List of Authors
• Index