Annealing Processes

– All the structural changes obtained by hardening and tempering may be eliminated by annealing.

• to relieve stresses

• to increase softness, ductility, and toughness

• to produce a specific microstructure

– Process consists of

• heating to the desired temperature

• holding

• cooling to room temperature

– annealing time must be long enough to allow for any necessary transformation reactions

– Normalizing - used to refine the grains

• cooling in air, less expensive, some sections of a part may cool too fast

– Full anneal: Utilized in low and medium carbon steels that will be machined or plastically deformed

• cooling in furnace to room temperature

• final product is coarse perlite (soft and ductile)

– Spheroidizing

• for medium and high carbon steels

• Fe3C will turn into the spheroids

– Some parts should be hard on the surface but soft and ductile inside

• shafts, gears, guideways of machine tools

• carious surface hardening processes

• heating on the surface only and

• quenching it

• flame hardening (by torch)

• induction hardening

• carburizing

Precipitation Hardening

• Hardening of non-allotropic alloys– The properties of nonferrous substances that do not readily form

allotropes cannot be changed by controlled cooling.

– Such substances are known as non-allotropic alloys, and include aluminum, copper, and magnesium alloys as well as stainless steels containing Ni.

– The method of hardening is called precipitation hardening and age hardening.

– Precipitation involves the formation of a new crystalline structure through the application of controlled quenching and tempering. Precipitation disperses hard particles throughout the existing more ductile material.

– These particles disrupt long dislocation planes of the material, restricting the movement of dislocations and increasing the strength and stiffness of the alloy.

– The final step is to hold the material at a specific temperature for a given amount of time.

– When aluminum is quenched in water to room temperature, the solubility of copper is drastically decreases and a compound of copper aluminide forms that slowly disperses along grain boundaries and slip planes to harden the alloy.

• Summary– primarily interest to increase YS

– it means increase the load-carrying capacity of elements

– the methods to increase YS are based on the various mechanisms of interfering with dislocation movements.

– Methods:

• Obtaining fine grain material

– combination of hot working and recrystallization

– annealing after cold work

• Cold work (strain hardening)

– the total amount of strain is limited by a total loss of ductility

• Solid solution treatment

– for alloys in which a solute-rich solid-solution phase exists at room temperature

• Precipitation hardening

– for alloys in which substantial solid solubility exists at an elevated temperature

– followed by quenching it yields a supersaturated solution that releases fine, coherent precipitate

• Allotropic hardening

– only for steels

– heating up to austenite and quenching leads to a martensitic structure (very hard and brittle)

– tempering is followed in order to get some ductility

Engineering Metals

– This section describes the most common metallic alloys, their properties, and their usage. The alloys considered are steels, cast irons, alloys of aluminum, copper, titanium, and Ni- and Co-based superalloys.

– The production volumes in tons per year in the United States of the individual classes of metals are given below:

• Steels and cast irons: 100 million• Aluminum alloys: 36 million• Copper alloys: 1 million• Ni-based alloys: <100,000

– The significance of the individual classes of metals is not entirely expresses by these tonnages. For example, the Ni-based alloys are extremely important for gas turbines and jet engines; without them modern aircraft would not exist, and there would not be such a great need for aluminum alloys.

• Steels:– Carbon Steels

• no alloying elements other than C

• impurities: Si, S, and P

– adverse effect on ductility and toughness

• Mn improves hardenability

• low carbon steels (nonhardenable) with less than 0.2% C

– thin sheet steel for car bodies, appliances, and for sidings of houses, heavy steel plates for ships and tanks, for the structures and frames of heavy machinery

• hardenable steels

– medium carbon steels, with more than 0.3% C

– high carbon steels

• hot rolled carbon steels

• cold rolled carbon steels

• Low alloy steels

– almost always used in a heat treated (quenched and tempered) state

• High alloy steels

– contain over 5% of alloying elements

– austenite could be present at room temperature, Ni widens the field, Cr narrows

– Two classes

» stainless steels

» tool steel

• Tools steels:– Each grade of tool steel is designed for a specific purpose, and as

such, there are few generalizations that can be made about tool steel. Each tool steel exhibits its own blend of the three main performance criteria: toughness, wear resistance, and hot hardness.

– Some of the few generalizations possible are listed here:

• An increase in carbon content increases wear resistance and reduces toughness.

• An increase in wear resistance reduces toughness.

• Hot hardness is independent of toughness.

• Hot hardness is independent of carbon content.

• Aluminum alloys

– pure Al is light, about three times lighter than iron

– excellent electric conductor

– excellent ductility

– good resistance to corrosion

– Tm = 660 degree C

– could be strengthened by cold work

– used from electric wire to extruded structural shapes for housing construction

– it is usually used as an alloy

– alloying elements: Si, Cu, Mn, and Mg

– above 3% of Si improves fluidity

– above 12% Si improves hardness and wear resistance

– Cu improves the age hardenability, it is a primary element in achieving high mechanical strength in aluminum alloys at elevated temperatures.

• Magnesium

– its density is 2/3 that of Al

– Tm = 650 degree C

– is alloyed with Al, Zn, Mn, Ce, and Ag

– main use in the aerospace industry

– UTS below 350 MPa

– it is difficult to cold form

– extrusion, forging, and deep drawing possible on higher temperature