Cover

Chapter 1

RAW MATERIALS

2.1 AN INTRODUCTION:

The raw material is the first requirement to start the production as per planning. For an economical production the raw material should be of good quality and should be of the required specifications.

The raw material used in STELCO is hot rolled coils, which are imported from Jamshedpur. The hot rolled coils are inspected in the chemical analysis lab for the content of carbon and manganese. The hot rolled coils are bigger in size, which are then slitted as per required width by H.R. slitting machine.

GRADE

C%

Mn%

Si%

P%

C-40

0.35-0.45

0.50-0.70

0.15-0.35

0.04-0.04

C-55

0.50-0.60

0.50-0.80

0.15-0.35

0.04-0.04

C-60

0.55-0.65

0.50-0.80

0.15-0.35

0.04-0.04

C-80

0.75-0.85

0.60-0.90

0.15-0.35

0.04-0.04

C-120

1.1-1.3

0.60-0.90

0.15-0.35

0.04-0.04

Width-As per the requirement of the customer.

2.2 DUCTILITY OF PLAIN CARBON STEELS

Carbon Content

Elongation in tensile test

Nil

0.2

0.4

0.6

0.8

1.2

42%

37%

31%

22%

17%

03%


Chapter 2

HEAT TREATMENT

AN INTRODUCTION:

It may be defined as an operation of heating and cooling of metals or alloys in the solid state to induce certain desired properties into them. Heat treatment can alter the mechanical properties of steel by changing the shape and size of grains of which it is composed, or by changing its micro-constituents. It is generally employed for following purposes:-

  1. To improve machinability
  2. To change or refine the grain size
  3. To relieve the stresses of the metal induced during cold or hot working.
  4. To improve the mechanical properties e.g. tensile strength, hardness, ductility, shock resistance etc.
  5. To improve magnetic and electrical properties.
  6. To increase resistance to wear, heat and corrosion.
  7. To produce a hard surface on a ductile interior.

Heat treatment refers to a combination of heating and cooling of a metal or alloy in the solid state for the purpose of obtaining desired properties. Changes in properties result from micro structural changes in the material produced by heat treatment operations.

The various improvements from heat treatment are a result of the development of certain micro constituents which have desired qualities. These micro constituents are obtained by different rates of heating and cooling.

The most commonly used operations of heat treatment are as follows:-

  1. Annealing
  2. Hardening
  3. Tempering
  4. Normalizing
  5. Carburizing
  6. Cyaniding
  7. Nitriding
  8. Induction hardening
  9. Flame hardening

PROCESSES

4.1 ANNEALING

4.1.1 AN INTRODUCTION:

Annealing is one of the most important widely used operations in the heat treatment of steel. The purpose of annealing is to obtain one or more of the following effects:

  1. Soften the steel
  2. Improve machinability
  3. Increase of restore ductility and toughness
  4. Relieve internal stresses
  5. Reduce or eliminate structural inhomogeneity.
  6. Refine grain size.
  7. Prepare steel for subsequent heat treatment.

The various types of annealing that serve one or more of these problems are explained in the following discussions.

4.1.2 FULL ANNEALING

This process is known as full annealing because it wipes out all traces of previous structure by complete phase recrystallisation. Full annealing consists of:

  1. Heating the steel slightly above the critical point
  2. Holding it at this temperature for a considerable period
  3. Slowly cooling

4.1.3 PROCESS ANNEALING

When steel is cold worked the hardness and elastic properties particularly considerably increase, while the ductility remarkably suffers and the steel becomes unsuitable for further plastic deformation. The ductility of steel may then be restored by so-called recrystallisation or process annealing. Furthermore, the effect of process annealing is to relieve internal successes resulting from any previous heat treatment.

Process annealing consists in heating the steel to a temperature usually in the range of 5000C to 7000C holding at this temperature for a prolonged period and slow cooling. This causes the grains of steel, broken up or distorted in the process of deformation, to come back into their normal state. Process of recrystallisation annealing is frequently applied in the production of cold rolled steel which, in deep drawing, and in wire drawing operations is an intermediate process with the aim of increasing the plasticity of the steel. It should be pointed out that this process is applied to low carbon steels only. Medium and high carbon steels, however, require full annealing for recrystallisation.

In process annealing, new equiaxial grains gradually grows from the fragments of the original elongated grains, forming centers, or nuclei. These new grains become equiaxial. It must, therefore, be remembered that this process does not produce ay new structure by phase transformations but produces only new crystals of the same structure.

Fig.4.1 Annealing Tank

4.2. HARDENING:

4.2.1 AN INTRODUCTION:

The operation of hardening is applied to all tools and some important machine parts intended for especially heavy duty service as well as to all machine parts made of alloy steel.

The purposes of hardening with subsequent tempering are:

  1. To develop high hardness to resist wear and to enable it to cut other metals.
  2. Improve strength, elasticity, ductility, and toughness.

The process consists of:

  1. Heating the steel to a temperature above critical point.
  2. Holding at this temperature for a considerable period.
  3. Quenching (rapid changing) in water, oil or molten salt bath.

Hypoeutectoid steels are heated from 300C to 500C above points Ac3 (higher critical) while hypereutectoid steels are heated about the same amount above Ac1 (lower critical).

Rapid cooling should enable the austenite to e supercooled to the martensitic point.

Alloy steels and high speed steels are heated for hardening to about 11000C to 13000C, and cooled in a current of air. The alloying elements increase the stability of austenite and retard the decomposition of austenite into martensite. This means that with less drastic speed of cooling the same harder (martensite) constituent can be obtained as would be developed in carbon steel by a faster rate of cooling.

According to American Society of Material Testing, it is defined as the heat-treating process in which steel is heated at 200 C above the transformation range, soaking at this temperature for a considerable period to ensure thorough penetration of the temperature inside the component, followed by continuous cooling to room temperature by quenching in water, oil or brine solution.

Fig. 4.2 Hardening

Heating is carried out (200 C) above the upper critical temperature of steel in case of hypo-eutectoid steel and some degree above the lower critical point of steel in case of

hyper-eutectoid steel and some degree above the lower critical point of steel in case of hyper-eutectoid steel. The former consists of ferrite and pearlite while the latter consists of pearlite and cementite. On heating above the critical temperature, these get changed into a single structure known as austenite structure containing a considerable part of cementite.

Upon cooling which in this case is a critical cooling (2000 C/minute), the austenite is changed into a fine needle like microstructure known as martensite. Martensite is a supersaturated solution of carbon in alpha-iron. Hardness in steel is due to this very microstructure.

The hardness produced by hardening treatment depends upon the carbon

Impressum

Verlag: BookRix GmbH & Co. KG

Tag der Veröffentlichung: 11.01.2018
ISBN: 978-3-7438-4981-5

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