Mechanical Working and Rolling Process

Introduction

  • 1.1 The aim of mechanical working operation is not only to get the required shape, but to enhance the mechanical properties of material. Hundreds of mechanical forming methods have been evolved for particular metal working applications.
  • 1.2 Mechanical working involves plastic deformation of either cold or heated metal by the external action of specific tools, such as mill rolls, forging dies, flat hammer dies or extruding dies.
  • 1.3 The mechanical deformation in cold state leads to strain or work hardening. Strain hardening is, not only increase the tensile strength and hardness of the metal, but is also responsible for a measurable reduction in some of its characteristics mechanical properties like, elongation and impact strength. If the forming of metal is still required to be continued, then increase in strength of metal necessities to apply higher strain over the metal for further plastic deformation. After heavy strain hardening of metal, further deformation in cold state may be become impossible due to development of discontinuities of metal i.e., crack, tears and other defects. Deformation then must be immediately stopped. Metal will then ready for further plastic deformation, when it further softened by heating to a definite temperature.

Hot and Cold Working

Forming process is commonly classified into hot working and cold working.

  • 2.1 Cold working is the plastic de formation of metal at a tempera true and rate such that Strain hardening is produced. When the temperature at which deformation takes place is high enough to produce softening of metal, simultaneously with the strain hardening, which effect on the working process gets nullified. Plastic deformation under this condition is called hot working. So, hot working can be defined as “Plastic deformation at a temperature and rate such that no strain hardening is produced.”
  • 2.2 Softening principally is due to the phenomenon known as “Recrystallization”, in which old strained ciystals get altered into new strain-free ciystals.
  • 2.3 The minimum crystallization temperature for the given working conditions creates the dividing line between hot working and cold working. The recrystallization temperature is not constant for a particular metal, but it depends upon the time, temperature, the amount of previous deformation and other variables also.
  • 2.4 There is no specific or defined temperature of deformation, wherein a distinction can be made between hot or cold working. Hot working operation is performed at a high temperature to get a speedy rate of recrystallization in most of the cases. However, for lead and tin, working at room temperature called as hot working, similarly working Tungsten at 750°C, which is the hot working range for steel, but it is a cold working range for tungsten.

Theory of Microscopic Plasticity

A few important observations, based upon stress and strain is placed below:

  • 3.1 During plastic deformation, volume of metal continues to remain the same, hi other words, the sum of principal strain rate, stretches and compression is zero. This is due to plastic flow effects because of mechanism of slip, which does not require any volume change.
  • 3.2 When maximum shear stress in some direction or plane attains the critical value, then yielding takes place at that particular point.
  • 3.3 The direction of the greatest shear strain rate coincides with direction of greatest shear stress.
  • 3.4 The amount of plastic working is generally enhanced by using compressive rather than tensile methods.

Methods of Metal Working

Steel is an alloy generally made of non i.e., Fe (approx, by 99% wt) and carbon

i.e., C (<1.0% wt), sometimes it also contain small amounts of various other

alloying elements.

• Iron atoms are much bigger than carbon atoms, which fill the gaps (interstitial).

Carbon atoms in gaps

Fig. 2.1 Carbon atoms in gaps.

  • • Iron atoms tiy to pack as closely to one another as possible.
  • • The most efficient way to achieve this is through face-centered cubic packing.
Face centered cubic packing

Fig. 2.2 Face centered cubic packing.

  • • Pure Fe is in its most stable form at high temperatures in a FCC arrangement.
  • • FCC has 26% free volume.
Phase transformation from Austenite (FCC) to Ferrite (BCC)

Fig. 2.3 Phase transformation from Austenite (FCC) to Ferrite (BCC).

Allotropy of Iron

The effects of carbon addition are closely associated with the allotropic changes of iron. At temperature above 1540°C, iron is in liquid form. Liquid iron starts solidifying at 1540 °C. The temperature remains constant till solidification is completed. When solidification is completed, the iron is in delta (8) form, with a BCC structure. When the temperature drops down to 1395°C, an allotropic modification of iron takes place from delta (8) iron to gamma (y) iron with FCC structure. No change takes place with the further drop in temperature from 1395°C to 910°C. However, at 910°C the gamma (y) iron changes into alpha (a) iron with BCC structure. The alpha iron at 910°C is non-magnetic and remains so till temperature of 768°C is reached. Beta (p) iron is the nonmagnetic version of alpha (a) iron. At 768°C, it is termed as curie temperature which transform the non-magnetic beta iron into magnetic alpha iron.

Allotropy of Iron

Fig. 2.4 Allotropy of Iron

 
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