Metallography of Deformed Welded and Surface Hardened Structures

Metallography of Deformed, Welded and Surface Hardened Structures MSE 206 -Materials Characterization I Lecture-8

Metal Fabrication Methods FORMING • Forging • Rolling (I-beams, rails) (wrenches, crankshafts) often at elev. T • Drawing (rods, wire, tubing) • Extrusion (rods, tubing)

Metal Fabrication Methods: Forming Methods 1 -Forging A bulk deformation process in which the workpiece (preform) is squeezed between two opposing dies, thus the shape of the dies is imparted on the work part. It can be done cold or hot.

Metal Fabrication Methods: Forming Methods 2 -Rolling A bulk deformation process in which the thickness of the work part is reduced by compressive forces exerted by two opposing rolls.

Metal Fabrication Methods: Forming Methods 3 -Extrusion is a bulk deformation process in which material is forced to flow through a shape-forming die. – In extrusion, work part is pushed to flow through die opening rather than pulling. – Extrusion can be cold or hot. – Cross-section of the part must be uniform along its extruded length. Typical extrusion parts Direct extrusion

Metal Fabrication Methods: Casting CASTING • Sand Casting (large parts, e. g. , auto engine blocks) • Investment Casting (low volume, complex shapes e. g. , jewelry, turbine blades) plaster die formed around wax prototype • Die Casting (high volume, low T alloys) • Continuous Casting (simple slab shapes)

Metal Fabrication Methods: Casting Process in which molten metal flows by gravity or other force into a mold where it solidifies in the shape of the mold cavity. Term “casting” also denotes the part made in the process. 1. Melt the metal 2. Pour the metal in cavity 3. Let the metal freeze

Metal Fabrication Methods: Joining JOINING • Welding (when one large part is impractical) • Heat affected zone: (region in which the microstructure has been changed).

Metal Fabrication Methods: Forming Temperature of Forming q When the deformation is achieved at a temperature above the recrystallization temperature ( 0. 5 Tm) the process is called hot working otherwise, it is cold working q For most forming techniques, both hot and cold working procedures are possible.

Metal Fabrication Methods: Cold Working • Strength is increased considerably due to strain hardening. • Ductility is decreased. • Quality of the surface is higher when compared to hot worked metals. • Better control of the dimensions of the finished piece is possible.

Metal Fabrication Methods: Cold Working Effect of Cold Rolling On Grain Shape q As a result of cold working, the equiaxed grains of the metal are deformed into elongated grains q The elongation is along the direction of applied stress

Metal Fabrication Methods: Cold Working Effect of %Reduction by Cold Rolling on hypoeutectoid Steel

Annealing after cold working q Annealing is a heat treatment to negate the effects of cold work and increase the ductility of a strain hardened metal. q Strain hardening is a result of increased dislocation interactions due to increased dislocation density. Heating of a strain hardened metal for annealing results in: 1 -Recovery (dislocation and vacancy annihilation) 2 -Recrystallization (nucleation and growth of strain and dislocation free grains)

Annealing after cold working Cold rolled annealed AISI 1010 steel (a) (b) (c) a) Cold rolled, %90 deformation. Grains elongated along the rolling axis can not be differentiated easily in the optical microscope b) Annealed at 650ºC for 5 min, recrystallized 10 -40%. New dislocation free ferrite grains are nucleating and growing. c) Further annealed, recrystallized 80%. Recrystallized ferrite grains can be seen clearly.

Annealed Structures of Non-ferrous Alloys a) Fully annealed homogeneous hexagonal equiaxed grains. b) Cold worked metal with flattened grains. c) Annealed after cold working showing twinned grains. d) Cold worked again, after annealing, showing distorted twin lines and strain lines in the grains.

Annealed Structures of Non-ferrous Alloys Effect of Annealing Temperature α-brass (70 Cu-30 Zn) -- Cold worked annealed at (a) 450 ºC (b) (c) 550 ºC 750 ºC

Annealed Structures of Non-ferrous Alloys Annealing twins in α-brass

Metal Fabrication Methods: Hot Working q. In Hot Working, which is carried out at temperatures over the recrystallization temperature, Large deformations and successive operations are possible because the dislocations (hardening) generated by deformation are simultaneously annihilated by dynamical recovery and recrystallization. q Deformation energy requirements are less compared to those for cold working. q Surface oxidation may cause loss of material and poor surface finish.

Metal Fabrication Methods: Hot Working Banding in Hot Worked Alloys q During solidification of steels elemental segregation of C, S, P, Mn may occur. q When such steels are hot-rolled, elongated bands of different carbon and mangenese content occurs. q Bands of ferrite and pearlite forms along the direction of rolling.

Metal Fabrication Methods: Hot Working Banded Structures in Steel

Surface Hardening: Used to obtain hard wear resistant surface without effecting the relatively soft, tough interior. Case: Hard wear resistant surface Core: Relatively soft, tough inside q Surface hardening is usefull in parts such as cam or ring gear that must have a very hard surface to resist wear, along with a tough interior to resist the impact.

Surface Hardening of steels Layer addition Substrate treatment Hardfacing - Fusion hardfacing - Thermal spray Diffusion Methods - Carburizing - Nitriding - Carbonitriding and cyaniding - Nitrocarburizing - Boriding - Titanium-carbon diffusion - Toyota diffusion process Coatings - Electrochemical plating - Chemical vapor deposition (electroless plating) - Thin films (physical vapor deposition, sputtering, ion plating) - Ion mixing Selective hardening methods - Flame hardening - Induction hardening - Laser hardening - Electron beam hardening - Ion implantation - Selective carburizing and nitriding - Use of arc lamps

Surface Hardening Surface is hardened to; Ø increase wear resistance Ø increase surface strength for load carrying capacity (crush resistance) Ø impart favorable residual compressive stresses Ø improve fatigue resistance Ø produce tough core for resistance to impact Some of the surface hardening methods; Carburizing Nitriding Cyaniding and carbonitriding Flame hardening Induction hardening Diffusion methods; Change the chemical composition Selective hardening methods; don’t change the chemical composition. The steel must be capable of being hardened.

Surface Hardening Carburizing q In the austenic temperatures, carbon is introduced to the surface of a low carbon steel component by diffusion to increase the surface carbon content and produce a hard martensitic surface layer after quenching. q Traditionally, the carbon is supplied from (a) hydrocarbon gas atmosphere (b) coke packed around (c) carburizing salt bath. Case depth ≈ K √D*t

Surface Hardening Gas Carburizing q The steel is heated in contact with CO and/or hydrocarbon which is readily decomposed at the carburizing temperature. Hydrocarbon gas • Methane • Propane • Natural Gas • Vaporized fluid hydrocarbon The commercial practice is to use a carrier gas, such as obtained from an endothermic generator and enrich it with one of the hydrocarbon gas. q High surcafe carbon concentration may be decreased by turning of the gas and keeping the samples in furnace for a period of time.

Surface Hardening Nitriding q Nitrogen is introduced to the surface of a special alloy steel (with Al and Cr) component by diffusion to form alloy nitride precipitates to produce a hard surface layer. It is done at around 525°C and since hardening is by precipitates rather than martensite quenching is not necessary. q Traditionally, nitrogen is supplied from amonia gas atmosphere or cyanide bath.

Surface Hardening Flame Hardening q In Flame hardening a thin surface shell of medium carbon steel or cast iron is heated rapidly to austenite by an intense high temperature flame of propane and oxygen gas mixture, then quickly quenched to obtain martensite of high hardness. q In flame hardening chemical composition of the steel doesn’t change

Surface Hardening Induction Hardening q Surface of the part is heated very fast by induction heating and then quenched. Part is put in a work coil composed of several turns of water cooled copper. By passing current through this coil, an eddy current is induced on the surface of the part, which heats the surface. Then the part with the austenitized surface is quenched by using water jets. q As in flame hardening, chemical composition of the steel doesn’t change in induction hardening

• Instead of carburizing, induction hardening etc. sometimes metals are coated with hard ceramics such as Ti. N, Ti. Al. N, Cr. N to increase the wear resistance of the part

Joining: Welding q A technique for joining metals in which actual melting of the pieces to be joined occurs in the vicinity of the bond. A filler material may be used to facilitate the process. q Welding is very similar to casting since it also consists of melting and solidification of the metal

Joining: Welding • Weld Root: Dendritic microstructures may be observed. • HAZ (Heat Affected Zone): Change in grain size may be observed • Parent Material: Welding has no effect on the microstructure of this region

Joining: Welding HAZ Before welding After welding Hypoeutectoid steel (a) Before welding, parent metal consists of a pearlitic and ferritic matrix. (b) In the regions exposed to high temperatures (HAZ), pearlite loses its lamellar morphology, the layers break up leading to a structure known as divorced pearlite.

Joining: Welding HAZ q Pointed edged widmanstatten ferrites form on grain boundaries and grow into the grains. Black regions are unresolved pearlitic areas.

Joining: Welding Fusion Zone q The upper left part is the parent metal, lower right part belongs to the filler material.

Joining: Welding Weld zone q Filler materials generally contain very low amounts of carbon, therefore only ferrite forms upon solidification. The structure is directly solidified from the liquid phase, resulting in a dendritic morphology.

Joining: Welding Discontinuities • • Cold Cracking Hot Cracking Porosity in Weld regions Slag and Inclusions