semanas c

43 4.1 INTRODUCTION

STEEL CLEANLINESS OF RAILS:

In order to obtain the satisfactory cleanliness of steel it is necessary to control and improve a wide range of operating practices throughout the steelmaking processes like deoxidant- and alloy additions, secondary metallurgy treatments, shrouding systems and casting practice.

Table 4-1: The importance of clean steel with respect to mechanical properties of the product [12]

Element Form Mechanical Properties Affected

S, O Sulfide and oxide inclusions  Ductility, Charpy impact value, anisotropy

 Formability (elongation, reduction of area and bendability)

 Cold forgeability, Drawability

 Low temperature toughness

 Fatigue strength

C, N Solid solution  Solid solubility (enhanced), hardenability Settled dislocation  Strain aging (enhanced), ductility and toughness

(lowered)

Pearlite and cementite  Dispersion (enhanced), ductility and toughness (lowered)

Carbide and nitride precipitates  Precipitation, grain refining (enhanced), toughness (enhanced)

 Embrittlement by intergranular precipitation P Solid solution  Solid solubility (enhanced), hardenability

(enhanced)

 Temper brittleness

 Separation, secondary work embrittlement

Rail steel needs to conform to stringent quality standards described in the standards owing to its critical nature of its application. Chemical composition range of Grade 880, which is a common rail grade as per IRS-T12, is shown in Table 4-2.

44 Table 4-2: Chemical composition of Grade 880 rails as per IRS T-12 2009 specifications

Grade %C %Mn %Si %S %P %Al %Nb H in

Hydrogen in rail is restricted to a maximum of 1.6 ppm which makes degassing necessary.

As far as inclusions are concerned, it is well known that they are detrimental to rails. IRS T-12 2009 specifies that the inclusion rating level of rails, when examined as per IS: 4163, shall not be worse than 2.5 A, B, C, D thin or 2.0 A, B, C, D thick.

EFFECT OF INCLUSIONS TO THE PHYSICAL CONTINUITY OF RAILS:

Inclusions act as the barrier to the physical continuity of metal. The area in the vicinity of inclusion develops a local residual stress field; so that the initiation & propagation of crack gets driven. Fatigue is the result of progressive initiation & subsequent propagation of crack.

Initiation is typically accepted to involve crack development- microcracks (size ranging from micrometer to millimetre) transforming into macro cracks (greater than millimetre, & up to as long as sizeable fraction of a metre). The really important crack dimension, which determines fatigue life, is penetration into the load bearing area. Initiation is dependent on slip processes, governed by cyclic shear stresses. Propagation is generally governed by cyclic tensile stresses

& is caused by repeated plastic stretches & blunting at the crack tip. The classic explanation is that, when a flat crack is open by tensile stresses, stretching occurs normal to the crack tip, thereby advancing its position. In a generally compressive field, such as that under a wheel contact, early growth by shear is the only possible mechanism available to advance the crack.

Later, under the influence of bulk bending stresses in the body of rail, the crack grows by tensile opening & closing. The extremely high contact stresses & the enormous power density (i.e the power passing through per unit) concentrated at the contact under the vertical loads, are enhanced by lateral (curving) longitudinal (traction & braking) loads. In these

circumstances, the initiation of crack is almost inevitable [21].

45 Fig. 4.1 Force applied by a Wheel on Rail

A wide variety of inclusion always exists in the rail steels of the composition shown in Table 2. The most common of which includes those of MnS, Al2O3 and SiO2. Large inelastic inclusions, such as those comprising of Ca, Al, Si and O tends to act as a nucleation site for crack growth below the surface of the rail head. These inclusions which are themselves brittle in nature; under the influence of stresses can shear in a brittle manner; thus leading to loss of serviceability. Rail industry has been constantly working in this regard to lower down the size

& amount of inclusion prevailing. MnS inclusions can become crack initiators as they deform in a non-uniform manner to produce long thin inclusions. Studies reveal that MnS inclusions, present in the material are considerably elongated by the loading of the rail in service and contribute to spontaneous cracking, subsequently resulting in failure. ^[14]

This study assesses the level and type of inclusions in rail steels produced at JSPL and tries to minimise the inclusion level by carrying out appropriate modifications in steel making &

simultaneously carrying out the comparative study between VD & RH processed heat.

46 4.2 EXPERIMENTAL PROCEDURE

SAMPLE PREPARATION

4.2.1 A 20mmX20mmX10mm sample is cut from the standard location of the 60-100mm long rail sample, as per IS: 4163 by using Abrasive Cutter Machine. The polished area of the specimen shall be approximately 200mm2. It shall be parallel to the longitudinal axis of the product. It shall be located halfway between the outer surface and the center.

4.2.2 Rough filing is done on the surface to be polished by using stone grinder to remove the cut marks.

4.2.3 The specimen is polished by using coarse emery papers of size 240, 320, 400 to get the surface free from scratches.

4.2.4 Again it is polished by using fine emery papers of size 1/0, 2/0, 3/0 and 4/0 to get further smooth and scratch free surface.

4.2.5 Fine polishing of the rail sample is done by using Cloth Polishing Machine where the polishing media is Alumina powder to get mirror surface. Then it is washed with water and dried by using blower.

Fig 4-2 Sample images taken @ TSD, JSPL for inclusion rating

47 DETERMINATION OF CONTENT OF INCLUSION

4.2.6 Inclusion content determination is done by using Optical Microscope at 100 magnification.

4.2.7 The following types of inclusion are determined in this method.

 Group A (Sulphide Type) – highly malleable, individual grey particles and generally rounded ends.

 Group B Alumina - Numerous and non-deformable, angular, black or bluish particles (at least 3) aligned in the deformation direction.

 Group C Silicate - highly malleable, individual black or dark grey particles and generally sharp ends.

 Group D Globular Oxide – non deformable, angular or circular, black or bluish randomly distributed particle.

4.2.8 The image is projected on the ground glass and a clear plastic overlay is placed over the ground glass projection screen.

4.2.9 The image within the test square is compared with the standard chart diagrams of IS:

4163 Specification.

4.2.10 The entire polished surface is examined. Randomly any ten numbers of worst fields are chosen and each field is compared with the standard chart for each type of inclusion.

4.2.11 In each worst field, for each type of inclusion, total length of the inclusion is measured and corresponding severity number is noted down from the comparison chart of IS: 4163 specification

48 4.2 RESULT

Table 4-3 Inclusion Rating Results

Heat ID A type B type C type D type

Thin Thick Thin Thick Thin Thick Thin Thick

1 1.5 0.5 1.0

To confirm that the inclusions are of sulphide type, SEM-EDS analysis was also carried out.

Fig. 4.3: (a) SEM image of inclusion in Heat ID 1 at 799X magnification (b)EDS spectrum of point 3 shown in image

(a)

(b)

49

Fig. 4.3: Spectral imaging of inclusion in Heat ID 2 at 3210X magnification

SEM-EDS analysis confirms the results of inclusion rating and reveals that the inclusions are Manganese Sulphide (MnS) stringers.

The control of sulphur and its associated level of sulphide inclusions in rail steel is a challenge in spite of RH-degassing. This can be attributed to the silicon killing practice adopted in rail steels and RH-degasser’s limitations for desulphurization understanding the effect of secondary refining parameters on desulphurization and inclusion removal.

50

CONCLUSION AND FUTURE