Material Science: Processing of Metals

Material science: Non-Ferrous metals and alloys
August 7, 2017
Information – Processing, Storage, Security
August 14, 2017

1. Introduction

2. Mechanical fabrication

2.1 Metal casting

2.2 Forming

2.3 Machining

2.4 Joining

2.5 Powder metallurgy

3. Thermal processing of metals

3.1 Annealing process

3.2 Quenching

3.3 Tempering

4. Case hardening

4.1 Induction hardening

4.2 Flame hardening

4.3 Laser hardening

4.4 Carburizing

4.5 Cyaniding

4.6 Nitriding

5. FAQs

 

1. Introduction

Metals are processed by different means to achieve metals and alloys of desired shapes and characteristics. These processing techniques are further divided based on the different procedure employed.

2. Mechanical Fabrication

Metal fabrication techniques are mainly four kinds:

Process Purpose
Casting To give a shape by pouring in liquid metal into a mold that holds the required shape, and letting harden the metal without external pressure
Forming To give shape in solid state by applying pressure;
Machining In which material is removed in order to give it the required shape
Joining Where different parts are joined by various means.

One of the most important miscellaneous techniques is powder metallurgy.

2.1 Metal casting

This technique is employed when

  • Product is large and/or complicated shape
  • Particular material is low in ductility.

This is also employed as it is usually economical compared with other techniques.

Different casting techniques include: sand, die, investment, continuous casting

Sand casting: The common casting method where sand is used as casting material. A two piece mold (cope and drag) is formed by compact packing of sand around a pattern of required shape. An addition gating is provided for proper distribution of liquid metal.

Die casting: Here metal is forced into mold by external pressure at high velocities. Usually a permanent two-piece mold made of steel is used. In this technique rapid cooling rates are achieved, thus inexpensive.

Investment casting: In this pattern is made of wax. Then fluid slurry of casting material is poured over which eventually hardens and holds the required shape. Subsequently, pattern material is heated to leave behind the cavity.

   This technique is employed when high dimensional accuracy, reproduction of fine details, and an excellent finish are required.

Examples: Jewelry, dental crowns, and gas turbine blades jet engine impellers.

Continuous casting: After refining metals are usually in molten state, which are later solidified into ingots for further processing like forming. In continuous casting, solidification and primary forming process are combined, where refined metal is cast directly into a continuous strand which is cooled by water jets.

 This technique is highly automated and more efficient. Uniform composition throughout the casting is achievable when compared with ingot-cast products.

2.2 Metal forming

In these techniques, a metallic piece is subjected to external pressures (in excess of yield strength of the material) to induce deformation, thus material acquires a desired shape.

These are basically two types

  • Cold working – one that performed at relatively low temperatures
    • In this, fine details are achieved along with material getting strengthened.
  • Hot working – performed at high temperatures
    • this  is responsible mainly for substantial change in cross section without material getting strengthened,

Most common forming techniques are: forging, rolling, extrusion, and drawing.

Different forming processes.

Forging: This involves deforming a single piece of metal, usually, by successive blows or continuous squeezing.

  • In open die forging, two dies having same shape is employed, usually, over large work-pieces
  • while in closed die forging, there may be more than two pieces of die put together having finished shape.

Properties:

  • Forged products have outstanding grain structures and very good mechanical properties.

Typical products:

  • Crane hooks, wrenches, crankshafts, connecting rods.

Rolling: Most widely used forming technique. It involves passing a piece of metal between two rotating rolls. Deformation is terms of reduction in thickness resulting from applied compressive forces.

Typical products:

  • Typically employed to produce sheets, strips, foil, I- beams, rails, etc.

Extrusion: In this technique a piece of material is forced through a die orifice by a compressive force. Final product emerging from die will have the desired shape and reduced cross sectional area, and will constant cross-section over very long lengths.

    Two varieties of extrusion are direct extrusion and indirect extrusion, where distinction limits to movement of tool and final product and consequent changes in required force.

Typical products:

  • Rods, (seamless) tubes, complicated shapes for domestic purpose.

Drawing: It is pulling of material though die orifice using tensile forces. Again a reduction in cross-section results with corresponding change in length.

Drawing die entrance is at angle against to extrusion die which is usually rectangular.

Typical products:

  • Rods, wire, and tubes are commonly produced using drawing technique.

2.3 Machining

This technique employs removable of metal from selected areas of the workpiece to give final shape to the product. Machining usually is employed to produce shapes with high dimensional tolerance, good surface finish, and often with complex geometry.

   And another important note is that when number of product pieces required is small, machining is preferred over forming as special tool cost will be less

2.4 Joining

There been many joining techniques, especially for metallic materials. These include:

  • Welding, brazing, soldering, and riveting.

Joining mechanism:

  • Adhesive/cohesive bonding
    • Welding, brazing, and soldering involve melting of either parent metal or external metallic liquid (filler material) which upon cooling provides cohesive bonds
  • Mechanical locking
    • In riveting, pieces are put together by mechanical locking.

In these techniques, two pieces are joined together either by adhesive/cohesive bonding and/or mechanical locking. Welding, brazing, and soldering involve melting of either parent metal or external metallic liquid (filler material) which upon cooling provides cohesive bonds. In riveting, pieces are put together by mechanical locking.

2.5 Powder metallurgy

Where to use powder metallurgy?

  • When the metals with low ductility/high melting points.
  • Close dimensional tolerance of complicated shapes.

Steps involved in this process are

Compaction:

  • Metal powders or mixture of metal powders at desired relative amounts are compacted into the desired shape,

Sintering:

  • In a controlled atmosphere to produce a denser product.
  • It is makes it possible to produce a virtually non-porous product where diffusional processes controls the efficiency of the process.

3. Thermal processing of metals and alloys

Metals are very often subjected to thermal processing apart from mechanical processing.

Reasons for thermal processing

  • To refine grain structure/size
  • To minimize residual stresses
  • To impart phase changes
  • To develop special phases over external surfaces, etc.

Thermal processing is also known as heat treatment.

Factors affecting heat treatment:

  • Temperature up to which material is heated
  • Length of time that the material is held at the elevated temperature
  • Rate of cooling
  • The surrounding atmosphere under the thermal treatment.

All these factors depend on material, pre-processing of the material’s chemical composition, size and shape of the object, final properties desired, material’s melting point/liquidus, etc.

Thermal processes are classified based on cooling rates from elevated temperatures

Annealing cooling the material from elevated temperatures slowly
Quenching & tempering very fast cooling of the material using cooling medium like water/oil bath to retain the phase change

3.1 Annealing processes

  • High temperatures allow diffusion processes to occur fast.
  • The time at the high temperature (soaking time) must be long enough to allow the desired transformation to occur.
  • Cooling is done slowly to avoid the distortion (warping) of the metal piece, or even cracking, caused by stresses induced by differential contraction due to thermal inhomogeneities.

Benefits of annealing:

  • relieve stresses
  • increase softness, ductility and toughness
  • produce a specific microstructure

Depending on the specific purpose, annealing is classified into various types:

  • process annealing.
  • stress relief.
  • full annealing.
  • normalizing.

Annealing classification

Process annealing:

  • It is primarily applied to cold worked metals to negate the effects of cold work.
  • During this heat treatment, material becomes soft and thus its ductility will be increased considerably.
  • During this, recovery and recrystallization are allowed whereas grain growth was restricted.

Stress relief:

  • This Operation removes the stresses that might have been generated during
    • plastic deformation
    • non-uniform cooling or
    • phase transformation.
  • Unless removed, these stresses may cause distortion of components.
  • Temperature used is normally low such that effects resulting from cold working are not affected.

Full annealing:

  • It is normally used for products that are to be machined subsequently, such as transmission gear blanks.
  • After heating and keeping at an elevated temperature, components are cooled in furnace to effect very slow cooling rates.

Typically, the product receives additional heat treatments after machining to restore hardness and strength.

Normalizing:

  • It is used to refine the grains and produce a more uniform and desirable size distribution.
  • It involves heating the component to attain single phase (e.g.: austenite in steels), then cooling in open air atmosphere.

3.2 Quenching

Quenching is heat treatment process where material is cooled at a rapid rate from elevated temperature to produce Martensite phase. This process is also known as hardening.

How to achieve rapid cooling?

  • Rapid cooling rates are accomplished by immersing the components in a quench bath.
  • General quench baths are either water or oil, accompanied by stirring mechanism.

Quenching process is almost always followed by tempering heat treatment.

3.3 Tempering

  • It is the process of heating martensitic steel at a temperature below the eutectoid[FAQ] transformation temperature to make it softer and more ductile.
  • During the tempering process, Martensite(a very hard form of steel crystalline structure) transforms to a structure containing iron carbide particles in a matrix of ferrite.

Below are modified quenching processes:

  • Martempering
    • used to minimize distortion and cracking that may develop during uneven cooling of the heat-treated material.
    • It involves cooling the austenized steel to temperature just above Ms temperature, holding it there until temperature is uniform, followed by cooling at a moderate rate to room temperature before austenite-to-bainite transformation begins.
    • The final structure of martempered steel is tempered Martensite.
  • Austempering
    • it involves austenite-to- bainite transformation.
    • the structure of austempered steel is bainite.
    • Advantages
      • improved ductility
      • decreased distortion
    • Disadvantages
      • need for special molten bath
      • process can be applied to limited number of steels.

4.Case Hardening

In case hardening, the surface of the steel is made hard and wear resistant, but the core remains soft and tough. Such a combination of properties is desired in applications such as gears.

case hardening summary

4.1 Induction hardening

  • An alternating current of high frequency passes through an induction coil enclosing the steel part to be heat treated. The induced emf heats the steel.
  • The depth up to which the heat penetrates and raises the temperature above the elevated temperature is inversely proportional to the square root of the ac frequency.
  • In induction hardening, the heating time is usually a few seconds.
  • Immediately after heating, water jets are activated to quench the surface.

Uses:

  • Martensite is produced at the surface, making it hard and wear resistant.
  • The microstructure of the core remains unaltered.
  • Induction hardening is suitable for mass production of articles of uniform cross-section.

4.2 Flame hardening

For large work pieces and complicated cross-sections induction heating is not easy to apply. In such cases, flame hardening is done by means of an oxyacetylene torch. Heating should be done rapidly by the torch and the surface quenched, before appreciable heat transfer to the core occurs

4.3 Laser hardening

  • As laser beams are of high intensity, a lens is used to reduce the intensity by producing a defocused spot of size ranging from 0.5 to 25 mm.
  • Proper control of energy input is necessary to avoid melting.

Advantages of laser hardening:

  • It has the advantage of precise control over the area to be hardened
  • an ability to harden reentrant surfaces.
  • very high speed of hardening
  • no separate quenching step. The disadvantage is that the hardening is shallower than in induction and flame hardening

4.4 Carburizing

Carburizing is the most widely used method of surface hardening.

  • The surface layers of low carbon steel are enriched with carbon up to 0.8-1.0%.
  • The source of carbon may be a solid medium, a liquid or a gas.
  • In all cases, the carbon enters the steel at the surface and diffuses into the steel as a function of time at an elevated temperature.
  • Carburizing is done at 920-950o C.

Why carburizing always done in austenitic state?

  • If carburizing is done in the ferritic region, the carbon, with very limited solubility in ferrite, tends to form massive cementite particles near the surface, making the subsequent heat treatment difficult.
  • For this reason, carburizing is always done in the austenitic state, even though longer times are required due to the diffusion rate of carbon in austenite being less that in ferrite at such temperatures.

4.5 Cyaniding

  • Cyaniding is done in a liquid bath of NaCN, with the concentration varying between 30 and 97%.
  • The temperature used for cyaniding is lower than that for carburizing and is in the range of 800-870o C.
  • The time of cyaniding is 1-3 hr to produce a case depth of 0.25 mm or less

4.6 Nitriding

  • Nitriding is carried out in the ferritic region.
  • No phase change occurs after nitriding.

Process:

  • During nitriding, pure ammonia decomposes to yield nitrogen which enters the steel.
  • The solubility of nitrogen in ferrite is small.
  • Most of the nitrogen, that enters the steel, forms hard nitrides (e.g., Fe3N).
  • The temperature of nitriding is 500-590o C.

Uses of Nitriding:

  • In addition to providing outstanding wear resistance, the nitride layer increases the resistance of carbon steel to corrosion in moist atmospheres.

5. FAQs

 

 

References

  1. William D. Callister, Jr.David G. Rethwisch: Materials Science and Engineering: An Introduction, Wiley publication, 2014
  2. NPTEL material science material by Satish Vasu Kailas (IISc)

 

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