Chapter 25 Welding Metallurgy

Objectives List the crystalline structures of metals and explain how grains form Work with phase diagrams List the five mechanisms used to strengthen metals Explain why steels are such versatile materials Describe the types of weld heat-affected zones Discuss the problems hydrogen causes during steel welding

Objectives (cont’d.) Discuss the heat treatments used in welding Explain the cause of corrosion in stainless steel welds

Introduction Skilled welders Metals mechanical and chemical properties Need to understand the materials being welded Need to learn metallurgy Metals mechanical and chemical properties Result from alloying and heat-treating Welding operations heat the metals Change structure and properties

Heat, Temperature, and Energy Heat and temperature Describe quantity and level of thermal energy Heat: quantity of thermal energy Temperature: level of thermal activity Independent values Material can have a large quantity of heat energy but a low temperature Material can be at a high temperature but have very little heat

Heat Amount of thermal energy in matter Two forms Measured in the British thermal unit (BTU) Two forms Sensible (measurable) As it changes a change in temperature can be sensed or measured Latent Absorbed by a material as it changes from one state to another Also occurs with a change in structure

FIGURE 25-1 There is no change in temperature when there is a change in state. © Cengage Learning 2012

Temperature Measurement of frequency of atoms in matter Matter becomes warmer: atoms vibrate at a higher frequency Temperature: determined by frequency of light produced by vibrating atoms FIGURE 25-4 Visible and invisible light. © Cengage Learning 2012

Mechanical Properties of Metal All of a metal's properties interact with one another Significant mechanical properties Hardness: resistance to penetration Brittleness: ease metal cracks or breaks without noticeable deformation Ductility: ability of a metal to be permanently twisted, drawn out, bent, or changed in shape Toughness: allows a metal to withstand forces Strength: property of a metal to resist deforming Tensile, compressive, shear, or torsional

Other Mechanical Concepts Include: Strain: deformation caused by stress Elasticity: ability of a material to return to its original form Elastic limit: maximum load with a deformation directly proportional to the load Impact strength: ability of a metal to resist fracture under a sudden load

Structure of Matter Solid matter: two basic forms Crystalline Orderly arrangement of atoms Amorphic No orderly arrangement of atoms into crystals Both look and feel like solids Sophisticated testing equipment is required to tell the difference

Crystalline Structures of Metal Atoms arranged in very precise three-dimensional patterns are called crystal lattices Smallest identifiable group of atoms is the unit cell Some metals change their lattice structure when heated above a specific temperature Crystal structures are studied by polishing and etching small pieces of metal

FIGURE 25-9 Body-centered cubic unit cell. © Cengage Learning 2012

FIGURE 25-10 Face-centered cubic unit cell. © Cengage Learning 2012

FIGURE 25-11 Hexagonal close-packed cubic unit cell. © Cengage Learning 2012

Phase Diagrams Most engineering metals are alloys Phases and temperatures at which alloys exist Summarized in phase diagrams Also called equilibrium or constitution diagrams Describe constituents present at temperature equilibrium

Lead-Tin Phase Diagram Many similarities with iron-carbon phase diagram Used for steel Chart areas Liquid phase Solid phase Liquid-solid phase Solid-solution phase Eutectic composition Lowest possible melting temperature of an alloy

Iron-Carbon Phase Diagram More complex than lead-tin phase diagram Very small changes in the percentage of carbon produce major changes in the alloy's properties Iron is called an allotropic metal Pure iron forms body-centered cubic crystal below a temperature of 1675 degrees Fahrenheit Iron changes to face-centered cubic crystal above 1675 degrees Fahrenheit

FIGURE 25-15 Iron-carbon phase diagram. © Cengage Learning 2012

Strengthening Mechanisms Metal strength Most important physical characteristic Pure metals are relatively weak Structures built with pure metals would be massive and heavy Welders must understand numerous methods used to strengthen metals

Solid-Solution Hardening It is possible to replace atoms in crystal lattice with atoms of another metal Not all metals have lattice dimensions that allow substitution of other atoms Does not change lattice structure as a result of thermal treatments Alloys are generally weldable

Precipitation Hardening Solubility increases with temperature Until alloy system reaches its limit Heat treatment involving three steps: Heating alloy to dissolve the second phase Quenching alloy rapidly: producing a supersaturated solution Reheating alloy Process is used to strengthen many alloys

Mechanical Mixtures of Phases Two phases may exist in equilibrium Depends on alloy’s temperature and composition Room temperature Iron-carbon alloy has two forms Alpha iron ferrite: ductile but weak Cementite: strong but brittle In combination: cementite strengthens ferrite

FIGURE 25-22 Change in mechanical properties caused by beta (silicon phase) in mechanical mixture with alpha (aluminum phase). © Cengage Learning 2012

Quench, Temper, and Anneal Quenching rapidly cools a metal Methods Molten salt quenching Air quenching Oil quenching Water quenching Brine quenching Tempering reheats a part that has been hardened and quenched Reduces some brittle hardness

Martensitic Reactions Martensite characteristics Hardest of transformation products of austenite Has an acicular structure Formation can be minimized by preheating steel to slow cooling rates Can be tempered to a more useful structure Tempering time/temperature is increased: structure changes to spheroidized microstructure

Cold Work Metals are deformed at room temperature Grains are flattened and elongated Increases strength and decreases ductility Cold-worked structure Can be annealed by heating above the recrystallization temperature Final annealed structure Weaker than cold-worked structure

Grain Size Control Grain growth Common to all metals and alloys Growth rate increases with temperature and time Coarse grains are weaker and more ductile Allotropic transformation requires the creation of fresh grains Grain refinement: quickly heated above critical temperature and then quickly cooled Not all metals exhibit allotropic transformation

Heat Treatments Associated with Welding Welding specifications Frequently call for heat treating joints before welding or after fabrication Welders should understand the reasons for these heat treatments

Preheat Reduces the rate at which welds cool Amount of preheat Lowers residual stress Reduces cracking Amount of preheat Increased when welding stronger platesor in response to higher levels of hydrogen contamination Most commonly used preheat temperature range is between 250 and 400 degrees Fahrenheit

Stress Relief, Process Annealing Residual stresses are unsuitable in welded structures Significant effects Yield strength of steels Decreases at higher temperatures Temperature range for stress relief steel 1100 to1150 degrees Fahrenheit Time at temperature Important factor

Annealing Referred to as full annealing Involves heating the structure of a metal to turn it completely austenitic After soaking to equalize temperature: cooled in furnace at slowest possible rate Austenite transforms to ferrite and pearlite Metal is now its softest with small grain size

Normalizing Consists of heating steels to slightly above Ac3 Holding for austenite to form Followed by cooling in still air On cooling: austenite transforms Somewhat higher strength and hardness Slightly less ductility than in annealing

Thermal Effects Caused by Arc Welding Liquid metal is deposited on base metal Some base metal melts from contact with liquid weld metal and arc, flame, etc. Metallurgic changes in heated region are inevitable Lowest temperature at which such changes occur defines the heat-affected zone (HAZ)

Thermal Effects Caused by Arc Welding (cont'd.) Exact size and shape of HAZ are affected by: Type of metal or alloy Method of applying welding heat Mass of the part Pre- and postheating HAZ produces fine grains as a result of the allotropic transformation Welder must control the HAZ

Gases in Welding Many welding problems and defects result from undesirable gases that can dissolve in weld metal Gases that dissolve in the molten weld pool have a high solubility in liquid metal During freezing process: dissolved gases try to escape High solidification rates: become trapped in the metal Intermediate rates: trapped as bubbles

Hydrogen Many sources Troublesome in aluminum and steel Moisture in electrode coatings Fluxes Very humid air Damp weld joints Organic lubricants Rust on wire or joint surfaces Troublesome in aluminum and steel Problems are avoidable

Nitrogen Comes from air drawn into the arc stream Primary problems GMAW: results from poor shielding or strong drafts SMAW: results from an excessively long arc Primary problems Porosity Embrittlement Improves strength of stainless steel Sometimes intentionally added

Oxygen Common source of oxygen contamination is air Metallurgic changes cause most effects of oxygen Oxygen causes the loss of oxidizable alloys Causes oxide formation on aluminum welds About two percent of oxygen is added intentionally to stabilize the GMAW process Amount of oxygen used is carefully controlled

Carbon Dioxide Oxygen substitute for stabilizing GMAW process using argon shields Carbon in carbon dioxide is a potential contaminant Causes problems with corrosion resistance Carbon dioxide levels below five percent do not seem to increase carbon content of stainless steel

Metallurgic Defects Cold cracking Hot cracking Carbide precipitation Result of hydrogen dissolving in weld metal Hot cracking Caused by tearing metal along partially fused grain boundaries of welds Carbide precipitation Occurs when chromium carbides deplete steel of free chromium Carbon dioxide shield gases can cause a similar problem, especially with ELC grades

Summary Understanding metallurgy Enables a welding engineer to design better weldments Welding engineers know chemical elements that make up a metal alloy As metals are thermally cycled their physical and mechanical properties change You must know the importance of controlling temperature cycles during welding Understanding metallurgy will aid you in avoiding welding problems