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Mechanical and Technological Properties of Metals

Updated on September 18, 2016

Example of Metal

A picture of metal which illustrates its mechanical strength
A picture of metal which illustrates its mechanical strength | Source

Introduction

What are the mechanical and technological properties of metals? Metals are of great importance to human race even before the birth of Christ (BC). Their great importance is the reason why there is multiplication of metallic industries in every year. Steel, which is the most used metal alloy, in many industries have been playing lots of good roles in many departments of technological developments.


Their applications (metals) can be found in medical, aerospace, agricultural, electronics, automobile, building, and even in educational departments of the world. In medical sector, most medical equipments are products of metal. One which is not farfetched is the needle which is attached with the syringe for injection purposes. Most thermometers, microscopes, inlays, and other instruments used by doctors and their co-health workers are made of metal. Almost all the parts of aircrafts are made of metals. Agricultural and electronics machines cannot work efficiently without metals having their functions to perform in one or two areas. The same thing appears in automobile, building and also in education sectors.


Before any metal is used for any important application, there are certain properties they must possess. These properties that they are expected to posses in other to function properly in any areas of their applications is termed mechanical properties of metals. In this article, both the mechanical and technological properties of metals are to be explained. The features (properties) make metals to have high strength, corrosion resistance, ability to withstand stress and others.



Mechanical Properties of Metals

It is very good to know the technological or physical properties of metals for research and other purposes.What are the mechanical properties of metals?

The mechanical or physical properties of metals are:

  • Hardness;
  • Hardenability;
  • Toughness;
  • Tenability;
  • Stiffness;
  • Creep; and
  • Endurance.


Hardness

It simply implies the ability of any material to resist indentation. Any metal with low hardness receives high indentation depth while that with higher hardness is likely to possess lesser indentation distance into the metal. Hardness can also be defined as the ability of a metal to resist notch by any object. It is the property of any metal that enables it to resist indentation.


Where can hardness be tested on any metal? Hardness can either be tested on either the surface or edge of any metal. It is a physical or mechanical property of metals.


The hardness or degree of hardness of any metal can be determined by the nature of heat treatment given to the metal. The hardness of a metal quenched in furnace cannot be the same when that metal is quenched in brine or air. Quenching is rapid cooling of hot metal in any medium. The medium in this context can be oil, air, furnace, or brine.


What is or are the application of hardness of any metal? The hardness of metals determines suitable places the metals can be used. There is specific hardness required of metals that can be used for construction of bridges when compared with for metal used in construction of gathers for just six bedrooms flat located in any timid area where there is no attack by criminals.


Hardness of metals with low strength can be improved by alloying, strain-hardening and heat treatment processes. Almost all metals which are used for construction functions are not used in their pure state but as alloys. The reason for alloying, which is the mixture between a base element and other minors is to improve the mechanical strength of the metal. Strain-hardening process is also done to increase strengths of metals. It hinders dislocation movement in materials. In heat treatment, metals are subjected to high temperature and slowly or rapidly cooled to improve strength. Heat treatments include quenching, annealing, tempering, normalizing and others.


Hardenability

It is another mechanical property of metals, which is the depth to which metals can be hardened. According to William D. Callister, Jr, and David G. Rethwisch in their book entitled Materials Science and Engineering, they defined hardenability as the depth of hardness penetration.


There are factors that determine hardenability and also tests which is applied in determining the hardenability of materials. The tests used in determining hardenability of metals are called hardenability tests. These tests are Jominy end-quench test, hot brine test, and Grossman’s method. Among all these three, the easiest and most frequently used is the Jominy end-quench method. Again, factors that determine the hardenability of metals are amount of alloying elements, austenitising temperature and time, nature of coolants, criterion of hardenability, grains size, and amount of carbon content. Note that metals with very high hardenability can fracture by means of cracking is the metal is very brittle.


Toughness

It is the property of metal that enable it to absorb maximum energy before it fractures. The mechanical term, toughness can be used in many contexts. Before any structural material is produced, its toughness must be taken into consideration.


Toughness can also be defined as the ability of any material to resist fracture when cracks are noticed on the material. For notch toughness, it is tested by what is called impact test. What is impact test? Impact test or testing is a test done on materials to determine their fracture characteristics under high loading rates. Impact test is done because of inability of laboratory tensile test to correctly give accurate result. Tensile testing is conducted under low loading rates.


Note that toughness of any material can be determined using either laboratory tensile test or impact test. The kind of test that is employed is dependent on the strain rate of the material. It is important to have in mind that for practical purposes, toughness decreases with rise in temperature of the materials.


Tenability

It is a physical property of metals, which enables them to resist attack on metals by any substance. This property is very important to prevent corrosion in metallic materials. Care is taking when manufacturing materials to be used in corrosive environment. Corrosion resistant elements, like chromium, are used during manufacturing of steel product to reduce the materials susceptibility to corrosion.


Stiffness: What is stiffness? It is the property of metal by which it resists deformation. The word, stiffness, is used in most cases with strength of materials. As the stiffness of any material is increased, it strength is also increased. It is the mechanical properties of metals related to applied load or force.


If a material has a load of 2000kg loaded on it and it takes many years for the material to deform, the material is said to have high stiffness. Applying Young’s modulus:

E = σ/ϵ

The symbol, E, stands for the stiffness of any metal, and is known as the Young’s modulus. Young Modulus states that whenever stress is applied on elastic material, the material is forced to undergo plastic deformation and extends. As the stress is increased, the extension of the material increases. The symbol, ϵ, stands for the extension which is caused by the stress, σ.



Creep

It is the property of metal by virtue at which it deforms continuously under steady load. It is a long time deformation process. Creep occurs in metallic materials and it is dependent on temperature. Before creep is being discussed in detail, the metal must first be subjected to high temperature and under static mechanical stress.


Creep can also be defined as undesirable phenomenon in materials, which is time-dependent and permanent deformation of materials when subjected to a constant load or stress.


Creep becomes important for metals whose temperature is greater than 0.4 Tm. Tm stands for absolute melting temperature of the metal. Any material whose temperature is 0.4Tm and below does not really need its creep considered or its consideration is of no importance. Plastic and rubbers, which are amorphous polymers, are sensitive to creep.

Diagram explaining creep behaviour
Diagram explaining creep behaviour | Source

Creep Characteristics or Behaviour

Typical creep curve of strain versus time at constant temperature is shown above. The minimum creep rate, Δϵ/ Δt, is the slope of the linear segment in the secondary region. Rapture lifetime, tr, is the total time to rupture. It is the total time taken for rapture to occur on metallic material subjected to static tensile stress at constant temperature. It is also called rupture lifetime. Creep test conducted to determine rupture time is called creep rupture tests. When a metallic material is subjected to constant temperature, the strains are obtained and then plotted against the individual time intervals. The test done to achieve the result is called constant-stress test.


After plotting of the strains against the time, the slope can be obtained, and there are three major divisions of the curve. The slope obtained from this experiment is called the minimum creep rate. The three regions of the creep curve are the primary, secondary, and tertiary strain-time regions.


The primary creep is also called the transient creep. It is the first region as shown in the curve. What happens at primary creep? In primary creep, there is continuous decreasing creep rate. It means that the slope of the curve at this region diminishes with time. The material undergoing creep experiences increase in creep resistance and strain hardening.


Secondary creep can also be called steady-state creep. This region experiences the longest creep in the curve and the creep rate at this point is constant. The plot of strain against the time at this region gives linear result. In secondary region, the material becomes more elastic.


The last of the three main regions is the tertiary creep. There is accelerated rate and ultimate failure of the metallic sample at this stage. The failure is termed rupture and it happens because the load acting on the material is much to the extent that the specimen could not withstand it again. The rupture can result from internal crack, grain boundary separation and even voids in the metallic structure.


The slope of the secondary creep is the most important parameter in creep test. This is identified in the strain-time creep curve. It is an engineering design parameter applicable in nuclear power plant.


What is the application of creep characteristics? Creep characteristics enables design engineers to find out the stability of any material use. In other words, the characteristic helps design engineers to know the best place where the material can be used.


Factors that affect Creep Behaviour

In any material that undergoes or is undergoing creep, there are two major factors that affect the character of the creep. The two major factors are:

  • Stress; and
  • Temperature

When there is increase in stress acting on a material at constant temperature, the time for the rupture become decreased. At temperature below 0.4 Tm, after the initial deformation of any material, the strain of the material is almost independent of time. With increase in temperature or the stress of any material, the instantaneous strain at the time of stress application increase.


When stress and temperature on a metallic material increased, the secondary creep rate is also increased. This means the rate of at which strain changes with time gets ‘accelerated’. With increasing stress and temperature on a material, the rupture lifetime or time to rupture begins to diminish.

A steel metal
A steel metal | Source

Endurance

It is the property of metal by virtue at which it can withstand varying stresses. Endurance limit is the maximum value of stress or the largest value of fluctuating stresses that will not cause failure for essentially an indefinite number of cycles or times. No fatigue failure occurs below endurance limit. The fatigue limits of many steels are between 35% to 60% of the steels tensile strengths (Materials Science and Engineering by William D. Callister, Jr. and David G. Rethwisch). Fatigue limit can be defined as the stress level at which failure cannot occur in metal. Endurance limit is an important parameter in reciprocating engines. Metals like copper, aluminium, magnesium, andmost non-ferrous alloys have no fatigue limit.


Technological Properties of Metallic Materials

Technological properties are those properties that apply during manufacturing and forming processes using metal. These properties can also be said to be mechanical properties of metals based on angle of consideration. Most authors refer these properties of metals as the mechanical properties, and therefore it is not bad to consider these properties as the mechanical properties of metals. Hence, the technological properties of metals are:

  • Malleability;
  • Machinability;
  • Weldability;
  • Castability; and
  • Ductility


Malleability

It is the property of metal that enables it to be deformed into thin sheet by rolling or hammering without the material being ruptured. Malleability is dependent on the crystal structure of the material. The higher the grain sizes of the metal the higher the malleability. Again, the smaller the grain sizes of any metallic material the lesser the malleability. How are grain sizes of metals obtained? With electron microscopic view of metals that have undergone metallography, the grain sizes can be obtained. This property is very important in engineering applications of metals.



Machinability

It is the ability of any metal to be machined or the ability of metals to be cut by machine tools. Examples of machine operations are turning, milling, and boring.


Machinability of metals is dependent on the chemical and physical compositions of the metals. There are three elements which are more machinable than others. Zinc (Zn), Aluminium (Al), and Magnesium have excellent machinability.


Copper-Aluminium alloys have good machinability property while low alloy steels have low machinability. There are metals which have poor machinability, and among them are high speed steels and wrought iron. Monel alloy (alloy of nickel that contains about 60% nickel, 35% copper and other elements which among them include iron and molybdenum) and wrought iron. Monel alloy have very poor machinability while white cast irons are not machinable.


White cast irons are very hard. What makes them to be very hard is because of the large content of cementite during its formation. Cementite is interstitial intermediate solid compound with fixed carbon content of 6.67%. As a result of the composition of large amount of cementite, the material becomes brittle and then non-machinable. Due to the nature of monel, it is a no- ductile material.


Factors Affecting the Machinability of Metals

There are certain factors that affect machinability of metals or materials. Whether a metal is machineable or not, machinability of all metals is dependent on these factors. Hence, factors that determine machinability are:

  • Composition
  • Grain size
  • Hardness
  • Size and shape
  • Machine tool used
  • Coolant/lubricants
  • Coefficient of friction

Composition of the metal: The composition of metal in this context includes both physical and chemical compositions. Whether a metal will be machinable is dependent on its composition.


Taking malleability as physical/mechanical property of a metal, any material that is malleable is likely to be machinable. Again, any metal that can be drawn into wire, ductility, has good machinable property. In the contrast, brittleness, as a property of a metal affects machinability negatively. A very brittle metal can hardly be machined.


Grain size and microstructure: Grain size and microstructure of any metal affects the machinability of that metal. Fine-grained metals are tougher and harder. But, coarse-grained metals are less tough and less hard. So, the grain size of any metal that is coarse is more machinanable than those whose grain size are small or fine.

Hardened metal surface
Hardened metal surface | Source

Hardness: Hardness is the property of metals which enable them to resist indentation or notch. A less hardened metal can easily be machined. On the contrast, as the hardness of metal is increased, the machinability of the metal gets decreased. When a metal is hardened too much, it makes the metal brittle which in return can make the metal unmachinable. Increasing the carbon content in steel can lead to increase in its hardness, and hence make it brittle and less machinable. The nature of heat treatment given to metals after casting by either sand casting process, investment casting or by other processes determine the hardness of metals.


In ceramics, brittleness is another important property. Some ceramics are brittle and this makes them unmachinable. A little indentation on these materials can lead to crack which results to failure.


Size and shape of the metal: This is another vital factor that determine how easily or difficult metals can be machine. Between metals of 50mm and 400mm thick, which do you think can easily be machine? That which is 50mm in size can be machined easily than the other because its thickness is lesser than that of the other.


Metals shapes on the other hand affect their ease or difficulty to be cut by machine tool. Metal which is of hollow shape can easily be machined than square shaped thick block of metal.


The type of machine tool used: Machinability is dependent on the kind of machine tool used during the cutting process. The time taken to cut a metal that operates at the speed of 20 rev/sec cannot be the same when the same metal is cut with machine that has speed of 50 rev/sec. That with 50 rev/sec will have higher machinability than that with revolution of 20 per second. The reason is because the former's cutting disc covers more distance into the metal than the later.


Furthermore, the hardness of machine tool also determines how machinable a material can be. It is clearly known that machining with machine tools which are made of diamond cannot be the same when same material is machined by machine tools which are of low strengths. Diamond being the hardest known material can easily machine many materials because of its strength.


Coolants /lubricants: Coolants and lubricants play important role during machining operation on material. As lubricants are applied in machining operation, the machinability of metals is increased. This is because lubricants lower the frictional forces acting between the machine and the material being machine.


Also, coolants play their own roles in reducing the temperature of the machining tool and the material being machined. It reduces hot-tearing operation which may result due to excessive heat generated. Examples of coolants that can be used in machining are liquid hydrogen, liquid nitrogen, and liquid helium.


Coefficient of friction: With the idea of Physics, it is noted that the coefficient of friction, µ, is directly proportional to the frictional force, F, from the formula, µ = F/R. In the formula, R is the normal reaction between the work piece surface and the surface of the cutting machine tool.


Based on the formula, as the coefficient of static friction is increased, the frictional force between the two surfaces in contact also gets increased. So, higher coefficient of friction, which results from high frictional force, increases machinability of metals. In this context, frictional force is the force needed to overcome friction between two surfaces in contact during machining operation. Note that µ cannot be one i.e 100 per cent.


Weldability

It is the property of metal, which indicates the ease with which two similar or dissimilar metals are joint by fusion (with or without application of pressure) and with or without the use of filler metals. Capillary action must take place between the metals to be welded together.


Before welding, the portions to be welded together are made clean from impurities. The impurities can be in form of grease, oil, or even oxides. Grease on the area to be welded can be eliminated using degreasing agents. Oxides impurities can be removed by fluxing process. If the impurities are not removed, the weldability of the metal will be affected before and after the welding.


When dealing with welding that includes the use of filler metals, to attain optimum weldability, the filler metal to be used must match with the base metals to be welded together. There are suitable filler metals for certain base metal joining. For instance, copper can be used as filler metal when making steel joints.


Factor Affecting Weldability

There are unique factors that affect the weldability of joints. These factors include:

  • Composition of the metal;
  • Thermal properties;
  • Welding technical;
  • Fluxing; and
  • Proper treatment before and after welding

Composition of the metal: The elements that are composed in a metal affect its weldability. Materials with higher sulphur and phosphorus contents have lower weldability. Taking grey iron for instance, its weldability is low because of high content of sulphur and phosphorus in it. Again, when the interest is on spheroid graphite cast iron, its weldability is more than that of grey cast iron because it contains less proportion of sulphur and phosphorus.


Thermal properties: The word, thermal can be used in place of heat energy. The rate at which any metal conducts heat affects the weldability of that metal. Any metal that has good thermal conductivity has good weldability as well.


Welding technical: A special knowledge of welding also determines the weldability of any joint. There is clear different between the welding done by a welder who has had good experience in welding for many years to a ‘young boy’ who is just learning how to weld. When an expert welds a joint, its weldability looks good and stronger than when it is conducted by a mere learner. So, the weldability of any joint is dependent on the welding skill applied as well as the technique used in the welding process.


Fluxing and filler metal: Fluxing is done on metals during welding to remove impurities; especially oxide impurities. The material to be fluxed determines the kind of flux to be used on it. When fluxing is done on parts to fuse together, capillary action on the metal can easily take place.


Proper treatment before and after welding: This is an important factor that determines the weldability of any metal. The treatment given to surfaces before welding proceeds can determine how weldable the joint can be. Parts of the treatment can be proper assembling of the two metals to be welded and also smoothening the two points to ensure good bonding between the two. Again, certain treatments on the portion welded can make the portion last longer.


Castability

What is castability? Castability is the property of metal, which indicates the ease at which castings can be made with less cost, less defects, and at less time. In making any casting, there must be tendency for the casting to be made without any defect. This is one of the reasons why castings are subjected to inspection after they are produced. A defect-free casting cannot easily fail as a result of corrosion or stress.


  • Ductility: This is the ability of a metal to be drawn into thin sheet or into wire. It can be achieved by mounting pressure on the metal and then drawn under heavy force or application of force by pulling operation. Cast or fabricated metals generally display directional properties, especially as regard to ductility, and usually, a cast metal is less ductile than one in the forged or annealed condition (Mechanical Properties of Metals by Donald Mclean).


Conclusion

Engineers critically study mechanical properties of metals for many purposes including their area of applications. Detailed out in this article are the mechanical and technological properties of metals. These properties are important and play good part during metals applications into many areas for good use and in production processes. Some of the factors that affect the mechanical and technological properties of metals were also detailed out.

References on the Topic

•New School Physics by M. W Anyakoha
•Materials Science and Engineering: An Introduction by William D. Callister, Jr. and David G. Rethwisch
•Metallurgy of Welding by J. F Lancaster
•Mechanical Properties of Metals by Donald Mclean

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