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Chapter: Mechanical - Engineering materials and metallurgy - Heat Treatment Process

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Heat Treatment Process

1 Basic principles of heat treatment 2 Hardening 3 Annealing 4 Normalizing 5 Hardening process 6 Thermo chemical process 7.Tempering 8 Martempering and Austempering

 

HEAT TREATMENT PROCESS


1 Basic principles of heat treatment

2 Hardening

3 Annealing

4 Normalizing

5 Hardening process

6 Thermo chemical process

7.Tempering

8 Martempering and Austempering



1.BASIC PRINCIPLES OF HEAT TREATMENT

 

Heat treatment of a metal or alloy is a technological procedure, including controlled heating and cooling operations, conducted for the purpose of changing the alloy microstructure and resulting in achieving required properties.

 

There are two general objectives of heat treatment: hardening and annealing.

 

2.HARDENING

 

Hardening is a process of increasing the metal hardness, strength, toughness, fatigue resistance.

 

·        Strain hardening (work hardening) – strengthening by cold work (cold deformation)

 

Cold plastic deformation causes increase of concentration of dislocations, which mutually entangle one another, making further dislocation motion difficult and therefore resisting the deformation or increasing the metal strength.

 

·        Grain size strengthening (hardening) - strengthening by grain refining.

 

Grain boundaries serve as barriers to dislocations, raising the stress required to cause plastic deformation.

• Solid solution hardening- strengthening by dissolving an alloying element.

Atoms of solute element distort the crystal lattice, resisting the dislocations motion. Interstitial elements are more effective in solid solution hardening, than substitution elements.

 

• Dispersion strengthening – strengthening by adding second phase into metal matrix.

 

The second phase boundaries resist the dislocations motions, increasing the material strength. The strengthening effect may be significant if fine hard particles are added to a soft ductile matrix (composite materials).

 

Hardening by result of Spinodal decomposition. Spinodal structure is characterized by strains on the coherent boundaries between the Spinodal phases causing hardening of the alloy.


• Precipitation hardening (age hardening) - strengthening by precipitation of fine particles of a second phase from a supersaturated solid solution.

 

The second phase boundaries resist the dislocations motions, increasing the material strength. The age hardening mechanism in Al-Cu alloys may be illustrated by the phase diagram of Al-Cu system. When an alloy Al-3%Cu is heated up to the temperature TM, all CuAl2 particles are dissolved and the alloy exists in form of single phase solid solution (α-phase). This operation is called solution treatment.

 

Slow cooling of the alloy will cause formation of relatively coarse particles of CuAl2 intermetallic phase, starting from the temperature TN.However if the the cooling rate is high (quenching), solid solution will retain even at room temperature TF. Solid solution in this non-equilibrium state is called supersaturated solid solution.

 

Obtaining of supersaturated solid solution is possible when cooling is considerably faster, than diffusion processes. As the diffusion coefficient is strongly dependent on the the precipitation of CuAl2 from supersaturated temperature, solution is much faster at elevated temperatures (lower than TN).This process is called artificial aging. It takes usually a time from several hours to one day. When the aging is conducted at the room temperature, it is called natural aging. Natural aging takes several days or more.

Precipitation from supersaturate d solid solution occurred in several steps:

 

·        Segregation of Cu atoms into plane clusters. These clusters are called called Guinier-Preston1 zones (G-P1 zones).

 

·   Diffusion of Cu atoms to the G -P1 zones and formation larger clusters, called GP2 zones or θ” phase. This phase is coherent with the matrix .

 

·        Formation of ‘θ’ phase which is partially coherent with the matrix. This phase provides maximum hardening.

 

3.ANNEALING

Annealing is a heat treatment procedure involving heating the alloy and holding it at a certain temperature (annealin g temperature), followed by controlled cooling.

Annealing results in relief of internal stresses, softening, chemical homogenizing and transformation of the grain structure into more stable state.

Annealing stages:

 Stress Relif - a relatively low temperatu re process of reducing internal mechanical stresses, caused by cold-work, casting or welding.

 During this process atoms move to more stable positions in the crystal lattice. Vacancies and interstitial defects are eliminated and some dislocations are annihilate d.

Recovery heat treatment is used mainly for preventing stress-corrosion cracking and decreasing distortions, caused by internal stresses.

Recrystallation -alteration of the grain structure of the metal.

If the alloy reaches a par ticular temperature (recrystallization or annealing temperature) new grains start to grow from the nuclei formed in the cold worked metal. The new grains absorb imperfections an d distortions caused by cold deformation. The grains are equi-axed and independent to the ol d grain structure.

 As a result of recrystallization mechanical properties (stre ngth, ductility) of the alloy return to the pre-cold-work level. The annealing te mperature and the new grains size are dependent on the degree of cold-wor k which has been conducted. The more the cold-work degree, the lower the annealing temperature and the fine recrystallization grain structure. Low d egrees of cold-work (less than 5%) may cause formation of large grains.Usually the annealing temperature of metals is between one-third to one-half of t he freezing point measured in Kelvin (absolute) tem perature scale.

 

Grains Growth: (ov er-annealing, secondary recrystallization) - growth of the new grains at the expense of their neighbors, occurring at temperature, above the recrystallization temperat ure.

This process results in coarsening grain structure and is undesirable.

 

THE SOFTENING PROCESSES

 Heat Treatment is the controlled heating and cooling of metals to alter their physical and mechanical properties without changing the product shape. Heat treatment is sometimes done inadvertently due to manufacturing processes that either heat or cool the metal such as welding or forming. Heat Treatment is often associated with increasing the strength of material, but it can also be used to alter certain manufacturability objectives such as improve machining, improve formability, restore ductility after a cold working operation. Thus it is a very enabling manufacturing process that can not only help other manufacturing process, but can also improve product performance by increasing strength or other desirable characteristics. Steels are particularly suitable for heat treatment, since they respond well to heat treatment and the commercial use of steels exceeds that of any other material. Steels are heat treated for one of the following reasons:

 

Softening

Softening is done to reduce strength or hardness, remove residual stresses, improve toughnesss, restore ductility, refine grain size or change the electromagnetic properties of the steel.

 

Restoring ductility or removing residual stresses is a necessary operation when a large amount of cold working is to be performed, such as in a cold- rolling operation or wiredrawing.

Annealing — full Process, spheroidizing, normalizing and tempering— austempering, martempering are the principal ways by which steel is softened.

 

Hardening:

Hardening of steels is done to increase the strength and wear properties. One of the pre-requisites for hardening is sufficient carbon and alloy content. If there is sufficient Carbon content then the steel can be directly hardened. Otherwise the surface of the part has to be Carbon enriched using some diffusion treatment hardening techniques. Material Modification: Heat treatment is used to modify properties of materials in addition to hardening and softening. These processes modify the behavior of the steels in a beneficial manner to maximize service life, e.g., stress relieving, or strength properties, e.g., cryogenic treatment, or some other desirable properties, e.g., spring aging.

 

Annealing

Used variously to soften, relieve internal stresses, improve machinability and to develop particular mechanical and physical properties.In special silicon steels used for transformer laminations annealing develops the particular microstructure that confers the unique electrical properties.Annealing requires heating to above the As temperature, holding for sufficient time for temperature equalisation followed by slow cooling. See Curve 2 in Figure.1


Stress from the forming operations can affect both rimfire and centerfire cartridge cases. For many cases, especially those with bottlenecks, the stresses are so great that high-temperature annealing must be used.After forming, a bottleneck case may appear perfectly serviceable. However, massive stresses are likely to remain in these areas. If the ammunition is loaded and stored without addressing these stresses, cracks can appear in the bottleneck area.


 

Case bottlenecks are normally flame-annealed by the following process:

Case bottlenecks are normally flame-annealed by the following process:

 

Placed on a moving rail or rotary disk system, the case passes before a set of gas burners that rapidly heat the neck and shoulder area to glowing.

 

As the becomes incandescent the brass grains grow larger. he heated area of the case is immediately tipped into a water bath to quench the case, establishing the large grain size.

 

The treatment causes a dark, but harmless, discoloration to the neck area. In commercial ammunition, this dark area may be polished out for cosmetic reasons; in U.S. military ammunition, the discoloration remains vi sible.

 

The application of heat treatment technology to vary the grain size gradually, from small grains in the head area to large o nes at the case mouth, determines c ase hardness. All high pressure cases must have variable metallurgical properties depending on the part of the case, as follows:

 

 Head - must be t ough and relatively unyielding, small brass grains contribute to the t oughness.

 

4.NORMALISING

Also used to soften and relieve internal stresses after cold work and to refine the grain size and m etallurgical structure. It may be used t o break up the dendritic (as cast) str ucture of castings to improve their m achinability and future heat treatment response or to mitigate banding in rolled steel. This requires heatin g to above the As temperature, holding for sufficient time to allow tem perature equalization followed by air co oling. It is therefore similar to annealing but with a faster cooling rate. Curve 3 in Figure I would give a normalized structure.

 

5.THE HARDENING PRO CESSES

Hardening

In this process steels which contain sufficient carbon, and perhaps other alloying elements, are cooled (quenc hed) sufficiently rapidly from above the transformation temperature to produce Martensite, the hard phase already described, s ee Curve 1 in Figure 1.There is a range of quenching media of varying severity, water o r brine being the most severe, through oil and synthetic products to air which is the least severe.

 

Tempering

After quenching the steel is hard, brittle and internally stre ssed. Before use, it is usually necessar y to reduce these stresses and increa se toughness by 'tempering'. There will als o be a reduction in hardness and the selection of tempering temperature dictates The final properties. Tempering curves, which are plots of hardness against tempering temperature. exist for all commercial steels and are used to select the correct tempering temperature. As a rule of thumb, within the tempering range for a particular steel, the higher the tempering temperature the lower the final hardness but the greater the toughness. It should be noted that not all steels will respond to all heat treatment processes, Table 1 summaries the response, or otherwise, to the different processes.


 

Boronised substrates will often require heat treatment to restore mechanical properties. As borides degrade in atmospheres which contain oxygen, even when combined as CO or C02, they must be heat treated in vacuum, nitrogen or nitrogen/hydrogen atmospheres.

 

PROCESSING METHODS

In the past the thermochemical processes were carried out by pack cementation or salt bath processes. These are now largely replaced, on product quality and environmental grounds, by gas and plasma techniques. The exception is boronising, for which a safe production scale gaseous route has yet to be developed and pack cementation is likely to remain the only viable route for the for some time to come.

The gas processes are usually carried out in the now almost universal seal quench furnace, and any subsequent heat treatment is readily carried out immediately without taking the work out of the furnace. This reduced handling is a cost and quality benefit.

TECHNIQUES AND PRACTICE

 

As we have already seen this requires heating to above the As temperature, holding to equalise the temperature and then slow cooling. If this is done in air there is a real risk of damage to the part by decarburisation and of course oxidation. It is increasingly common to avoid this by ‗bright„ or ‗close„ annealing using protective atmospheres. The particular atmosphere chosen will depend upon the type of steel.

 

NORMALISING

In common with annealing there is a risk of surface degradation but as air cooling is common practice this process is most often used as an intermediate stage to be followed by machining, acid pickling or cold working to restore surface integrity.

 

HARDENING

With many components, hardening is virtually the final process and great care must taken to protect the surface from degradation and decarburisation. The ‗seal quench„ furnace is now an industry standard tool for carbon, low and medium alloy steels. The work is protected at each stage by a specially generated atmosphere.

Some tool steels benefit from vacuum hardening and tempering; salt baths were widely used but are now losing favour on environmental grounds.

 

7.TEMPERING

Tempering is essential after most hardening operations to restore some toughness to the structure. It is frequently performed as an integral part of the cycle in a seal quench furnace, with the parts fully protected against oxidation and decarburisation throughout the process. Generally tempering is conducted in the temperature range 150 to 700°C, depending on the type of steel and is time dependent as the microstructural changes occur relatively slowly.

Caution: Tempering can,in some circumstances, m a k e the steel brittle which is the opposite of what it is intended to achieve.

There are two forms of this brittleness Temper Brittleness which affects both carbon and low alloy steels when either, they are cooled too slowly from above 575°C, or are held for excessive times in the range 375 to 575°C. The embrittlement can be reversed by heating to above 575°C and rapidly cooling.

Blue Brittleness affects carbon and some alloy steels after tempering in the range 230 to 370°C The effect is not reversible and susceptible steels should not be employed in applications in which they sustain shock loads. If there is any doubt consult with the heat treater or in house metallurgical department about the suitability of the steel type and the necessary heat treatment for any application.

 

8.MARTEMPERING AND AUSTEMPERING

 

It will be readily appreciated that the quenching operation used in hardening introduces internal stresses into the steel. These can be sufficiently large to distort or even crack the steel.

Martempering is applied to steels of sufficient hardenability and involves an isothermal hold in the quenching operation. This allows temperature equalisation across the section of the part and more uniform cooling and structure, hence lower stresses. The steel can then be tempered in the usual way.

 

Austempering also invo lves an isothermal hold in the quenching operation, but the structure formed, whilst hard and tough, does not require further tempering. The process is mostly applied to high carbon steels in relatively thin sections for springs o r similarparts . These processes are shown schematically in the TTT Curves, (figures 2a and 2b). there is sufficient heat sink in the part and an external quench is not needed. There is a much lower risk of distortion associated with this practice, and it can be highly automated and it is very reproducible


Body - the case walls must combine flexibility and strength to contribute to the obturati on system.

Mouth - must be softer (larger brass grains) to prevent cracks from the strain of holding a bullet.

 

 

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