Mild SteelIntroduction:Steel is not a single alloy rather it a generic name for iron based alloys or an alloy which has Iron (Fe) as the major component. Based on the alloying elements in iron we have different classes of steel.
For example Fe with 12 or more wt% of chromium (Cr) is stainless steel, which again is a generic name for a large group of alloys depending upon wt% of alloying elements like Cr, Ni (nickel), Mn (manganese), C (carbon), N (nitrogen), Cu (copper) etc. Historically, the first steels were Fe-C alloys with C up to 2.0 wt% and these are called simply steel or carbon steel. Further, carbon steels with carbon upto 0.
35 wt% is termed as mild steel, which is the topic of this paper. This is the largest tonnage metallic material produced and consumed today. This is because; it is one of the cheapest metallic materials and has wonderful mechanical as well as chemical properties to suit a variety of industrial applications. It has excellent formability and weldability coupled with moderate strength and excellent ductility.
Historically, carbon steels were produced using Bessemer converter and then using open hearth furnaces. With introduction of LD converters, the old routes were phased out and today, practically all mild steel production is through LD converter route. In this converter, pig iron from blast furnace is converted into mild steel by oxidizing the carbon in pig iron using oxygen.I must clarify that the value 0.
35% is not a sharply defined limit; rather it is just a guiding value. Thus mild steel is Fe-C alloy having up to 0.35 wt% carbon. Besides, there are other alloying elements like Mn, Si etc.
which are introduced in the different stages of manufacturing, mainly during deoxidation by ferrosilicon, ferromanganese etc. after LD converter. Besides, there are impurities like sulfur, phosphorous etc. Because, mild steel is primarily a Fe-C alloy therefore, it is essential to look at Fe-C phase diagram or precisely speaking Fe-Fe3C phase diagram.
Iron-Carbon Phase Diagram:Figure 1, below from wikipedia (http://en.wikipedia.org/wiki/Ferrite_(iron)) presents Fe-C phase diagram and Figure 2 again from wikipedia (http://en.wikipedia.
org/wiki/Carbon_steel), presents a part of the Fe-C phase diagram relevant to this essay. Figure 1: Fe-C phase diagramFigure 2: A part of Fe-C phase diagram coveringsolid state reactions upto 1.6 wt% C When mild steel is cooled from molten stage, then depending upon its composition either d phase or g phase (austenite) precipitates out of liquid. Subsequently, this d phase is converted completely into g phase and we have all g phase (Figure 1).
Now referring to figure 2, when the mild steel is further cooled, a phase (ferrite) precipitates out of g phase. Because a phase can dissolve very less of carbon, practically zero, therefore, C concentration in remaining g phase goes on increasing and reaches upto 0.8%, when it converts into a mixture of ferrite (a phase) and Fe3C (cementite). This composite microstructure of ferrite and cementite is consisting of alternate lamellae of these two phases and is known as pearlite.
Thus final microstructure of mild steel consists of pearlite lamellae dispersed in ferrite matrix.Now, we will briefly discuss the structure of different constituting phases of the microstructure of mild steel.d phase: This is a high temperature phase. It has Body Centered Cubic (BCC) structure of Iron atoms with carbon atoms occupying interstitial locations.
It can dissolve upto approximately 0.2 wt% carbon.g phase or Austenite: this is a soft and ductile phase. It has Face Centered Cubic (BCC) structure of Iron atoms with carbon atoms occupying interstitial locations.
It can dissolve upto approximately 2.0 wt% carbon.a phase or ferrite: It is a soft and ductile phase. It has Body Centered Cubic (BCC) structure of Iron atoms with carbon atoms occupying interstitial locations.
It can dissolve practically no carbon.Cementite or Fe3C: It is an intermetallic compound of Fe and C. It is hard and brittle. It imparts strength to pearlite and therefore, to steel.
Now we will briefly discuss some of the heat treatments relevant to mild steel.Annealing: This is done for a variety of purposes and accordingly the procedure varies. Full annealing is done to get a soft and refined microstructure. For this, the steel is heated to fully austenitic phase field i.
e. above AC3 (figure 2) temperature and subsequently cooled slowly to the lower temperatures in the furnace itself. This gives near equilibrium microstructure which is free of stresses and hence renders the final product stress free. Besides, there can be stress relieving annealing, in which the steel is heated to higher temperature, between 0.
5 Tm (Tm is melting temperature) to below A1 (figure 2) temperature, held there for sometime and cooled slowly again in the furnace to lower temperatures in the furnace itself. This relieves the stresses. Sometimes, this is used in between the intermediate processing steps, like between cold rolling passes) and then it is termed as process annealing.Normalizing: In this treatment, the steel is heated to fully austenitic phase field, for ease of mechanical processing as it is easier to mechanically process a single phase material, that too at high temperature.
The steel is then processed in the desired shape by processes like hot rolling, hot forging etc. and then allowed to cool to lower temperatures in the air itself. The final microstructure is harder than a normalized product and is having some internal stress. It is the most prevalent heat treatment on mild steel as it is a part and parcel of the mechanical processing itself.
Hardening: In this treatment, the steel is heated to fully austenitic phase field and then quenched in water. This rapid cooling, rather quenching leads to formation of a metastable phase known as Martensite. It has Body Centered Tetragonal (BCT) structure; this structure can be seen as extended c-axis of a BCC structure. This structure forms because, BCC iron or ferrite cannot dissolve carbon in its lattice at room temperature and quenching does not give sufficient time to carbon atoms to escape out of it, thus the carbon atoms distort the BCC structure into BCT structure.
This phase is a hard phase and imparts hardness to steel. However, with it comes loss in ductility. Hardness or strength of martensite depends on its carbon content. Besides, there is considerable residual stress associated with hardening.
Tempering: This is done to relieve residual stress associated with hardening. For this the hardened steel is heated to ~200 oC held there for sometime and then cooled in air. The time and temperature of tempering determines final microstructure, strength an d ductility of mild steel. Properties:Mild steel has excellent combination of strength and ductility.
Its strength and ductility properties are weighted average of the properties of ferrite phase and pearlite phase. The properties of ferrite and pearlite are presented in table 1 and 2 respectively.Table 1: Properties of ferrite (http://info.lu.
farmingdale.edu/depts/met/met205/fe3cdiagram.html)Tensile Strength40,000 psiElongation40 % in 2 in gage lengthHardnessLess than Rockwell C 0 or less than Rockwell B 90.ToughnessLowTable 2: Properties of pearlite (http://info.
lu.farmingdale.edu/depts/met/met205/fe3cdiagram.html)Tensile Strength120,000 psiElongation20 % in 2 in gage lengthHardnessRockwell C 20 or BHN 250-300Strength and ductility of mild steel is dependent on carbon content.
With increasing carbon content more of pearlite and correspondingly less of ferrite is present in the steel microstructure. As pearlite is stronger than ferrite; therefore, increasing carbon content and hence pearlite content, strength and hardness of steel increases. The opposite is true about ductility. Because, pearlite is less ductile than ferrite therefore, increasing carbon content and hence pearlite content, ductility of steel decreases.
These trends are clear in the figure 3, below, taken from http://www.roymech.co.uk/Useful_Tables/Matter/Phase_diagram.htmlFigure 3: Variation in strength, hardness and impact toughness of carbon steel References:http://en.wikipedia.org/wiki/Carbon_steel; retrieved on April 21, 2007http://en.wikipedia.org/wiki/Ferrite_(iron); retrieved on April 21, 2007http://info.lu.farmingdale.edu/depts/met/met205/fe3cdiagram.html; retrieved on April 21, 2007http://www.roymech.co.uk/Useful_Tables/Matter/Phase_diagram.html; retrieved on April 21, 2007