Outline: Mechanical Properties of Steel

 

I.                    Mechanical properties and their definitions

II.                 The basics of steel

III.               Impurities of steel

IV.              Alloying elements of steel

V.                 Changing the mechanical properties by various processes

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Mechanical Properties and Their Definitions

Materials are all around us all the time.  We use them daily no matter what our occupation or profession.  And there is hardly a technical or industrial field where subject matter in one way or another does not relate to materials.  We live in a world of man-made materials.  Steel, plastics, ceramics, glass, and all others are the concrete substance of our ideas, our designs, our product plans, and blueprints.  Because of their pervasive importance, knowledge of the mechanical properties and processing of all types of steel is essential today.  The mechanical properties of a material describe how it will react to physical forces.  Mechanical properties occur as a result of the physical properties inherent to each material, and are determined through a series of standardized test. 

            If we consider the material as a property system, the input is the applied mechanical energy or force, which is usually expressed as load or stress.  The output is the net change that the material undergoes as a result of the input.  This change can deformation and/or fracture or rupture (see figure 1).  Mechanical properties are expressed in three ways: (1) in terms of the input-output load, stress, or energy- for a specified amount of output (i.e. tensile strength); (2) in terms of output for a specified amount of input and/or set conditions (i.e. ductility); and (3) in terms of ratio between input and output (i.e. modulus of elasticity).

            The most common properties include strength, yield strength, elastic limit, proportional limit, ultimate tensile strength, ductility, toughness, fatigue ratio, and loss coefficient. 

Strength

      Strength has several definitions depending on the material type and application.  For metals the most common measure of strength is the yield strength.  The yield strength is the minimum stress, which produces permanent plastic deformation.  The yield strength is usually defined at a specific amount of plastic strain, or offset, which may vary by material and/or specification.  The offset is the amount that the stress-strain curve deviates from the linear elastic line.  The most common offset for structural metals is 0.2%.

 

Figure 1.  A model of Mechanical Properties

 

 

 

 

 

 

 

 

 

 

 

 

 

Elastic Limit

      The elastic limit is the highest stress at which all deformation strains are fully recoverable.  For most materials and applications this can be considered the practical limit to the maximum stress a component can withstand and still function as designed.  Beyond the elastic limit permanent strains are likely to deform the material to the point where function is impaired.  The proportional limit is the highest stress at which stress is linearly proportional to strain.  This is the same as the elastic limit for most materials.

Ultimate Tensile Strength

      The ultimate tensile strength is an engineering value calculated by dividing the maximum load on a material experienced during a tensile test by the initial cross section of the other tensile test data the ultimate tensile strength helps to provide a good indication of a material’s toughness but is not by itself a useful design limit,  Conversely, this can be construed as the minimum stress that is necessary to ensure the failure of a material. 

 

 

Toughness

      Toughness describes a material’s resistance to failure.  It is often expressed in terms of the amount of energy a material can absorb before fracture.  Tough materials can absorb a considerable amount of energy before fracture while brittle materials absorb very little.  Toughness is not a single property rather a combination of strength and ductility.  The toughness of a material can be related to the total area under its stress-strain curve.  A comparison of the relative magnitudes of the yield strength, ultimate tensile strength, and percent elongation of different materials will give a good indication of their relative toughness.  Materials with high yield strength and high ductility have high toughness.  The most common test for toughness is the Charpy Impact test.

Ductility

      Ductility is a measure of how much a deformation or strain a material can withstand before breaking.  The most common measure of ductility is the percentage of change in length of a tensile sample after breaking.

Fatigue Ratio and The Loss Coefficient

      Fatigue ratio is a dimensionless measurement, which is the ratio of the stress required to cause failure after a specific number of cycles to the yield stress of a material.  Fatigue tests generally run through 10^7 or 10^8 cycles.  A high fatigue ratio indicates materials, which are more susceptible to crack growth during cycle loading.  The loss coefficient is another important parameter in cyclic loading.  It is the fraction of mechanical energy lost in a stress-strain cycle.  The loss coefficient fir each material is a function of the frequency of the cycle.  The loss coefficient is also an important factor in resisting fatigue failure.  If the loss coefficient is too high, cyclic loading will dissipate energy into the material leading to fatigue failure.  Figure 2 explains the regions on a stress-strain diagram of the aforementioned properties.

 

 

 

 

 

Figure 2.  Stress-strain Diagram for a typical steel in tension (not to

              scale).

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The Basics of Steel

            In order to understand the mechanical properties of steel, we must first explore its elements.  Steel is an alloy of iron and carbon in which the carbon content amounts to about 2% or less (see figure3).  The addition of this tiny amount of carbon results in a substance that exhibits great strength, hardness, and other valuable mechanical properties.  Metallurgical terms used in defining the various phases of the iron-carbon alloys are as follows:

1.      Cementite is a compound of iron and carbon found in high-carbon steels, often called iron carbide.  Cementite contains 6.67% carbon and 93.3% iron and is very hard and brittle.

2.      Ferrite is the excess iron in low-carbon steels, and it gives the steel softness, which enables it to be cold-worked.

3.      Pearlite is a mixture of ferrite and cementite in the proportion of 12% cementite and 88% ferrite.

 

Figure 3.  Iron-carbon Equilibrium Diagram.

 

 

 

4.      Graphite occurs in small flakes or nodules of carbon, which become mixed with steel.

5.      Eutectic of the carbon-steel alloy is the combination, which freezes at the lowest temperature.  The euctectic at 4.3% carbon freezes at 1130°C and contains solid-solution austentite and solid cementite.  Below 723°C the solid-solution austentite changes to pearlite and cementite.

6.      Eutectoid is the solid solution of iron and carbon, which changes to a solid at the lowest temperature.  Pearlite is the euctectoid and changes to a solid at 723°C.

Pure iron is about as soft as copper and is of little structural value, but it is greatly hardened by the addition of small amounts of carbon.  Because steel only contains up to 2% carbon, a higher content of carbon creates a different metal and very different properties.  Based on the quantity of alloying elements, steels can be grouped into three major classes: low-alloy steels, high-alloy steels, and plain-carbon steels. 

Low-alloy steels, which contain up to 8% alloying elements, are exceptionally strong and are used for machine parts (see table 1).  High-alloy steels, which contain more than 8% alloying elements, are prized for unusual properties such as resistance to corrosion and oxidation.  An example of a high-alloy steel is stainless steel (see table 2).

Plain carbon steels are the most widely used and account for about 90% of the world’s steel production.  By definition, plain-carbon steels are those that contain up to 1% carbon, not more than 1.65% manganese, 0.60% silicon, and 0.60% copper, and residual amounts of other elements, such as sulfur (0.05% maximum).  For all plain-carbon steels, carbon is the principal determinant of many performance properties.  As Figure 4 shows, carbon has a strengthening and hardening effect.  At the same time, it lowers ductility, which is evident from a decrease in elongation and reduction of area.  In addition, a rise in carbon content lowers machinability and decreases weldability. Plain carbon steels are commonly divided into carbon content:

Low Carbon- up to 0.30%

Medium Carbon- .31 to .55%

High Carbon- .56 to 1%

 

Table 2. Mechanical Properties of Typical Stainless Steels