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ROLE OF PARTICLES IN FRACTURE MECHANICS:

EVOLUTION OF ONE APPROACH

Nenad Radović

Department of Metallurgical Engineering, Faculty of Technology and Metallurgy,

University of Belgrade, Begrade, Serbia

1. INTRODUCTION

Since the early days of fracture mechanics, the importance of the particles present in

microstructure was greatly emphasized. The particles in steels are either impurities

(sulphides, silicates. aluminates, oxides), or second phase particles (predominantly

carbides and nitrides). To describe the role of the particles, it was necessary to take into

account the chemical composition, shape and size distribution. Primarily, all particles

were treated as stress concentrators, due to its sharp edges which were formed during

solidification. Therefore, the main challenge for metallurgical engineers was how to avoid

the presence of the sharp elongated particles in steels. The problem was mainly solved by

increasing the steels purity; decrease in sulphur and phosphorous content is a permanent

activity in steel production. Later, concurrently with the development of physical

metallurgy, this approach was modified. The new approach was provided by

achievements in industrial practice related to the possibilities of particles control. For

that, the control of the particles had enabled two new different strategies:

(i) control of the particle shape, and

(ii) control of the particle chemical composition.

The shape control was applied primarily on sulphur based particles, mainly MnS. The

addition of Ca led to the change in shape and increase of the Young’s modulus.

Therefore, MnS particles became spheroid and non deformable, i.e. MnS did not act as

stress concentrator, in spite of the presence of sulphur. This approach was applied for the

large particles (non-metallic inclusions). The other route was based on the knowledge of

nucleation of the phases with high toughness. The nucleation of these phases preferen-

tially starts on matrix/particles surface. Also, the relationship between particles and

phases is well established. These particles are carbides, nitrides, oxides or carbonitrides of

alloying elements, sub micrometer in size. Therefore, the control of particle precipitation

will lead to the final microstructure with high toughness and/or very small grain size. This

approach has successfully enabled the development of microalloyed steels and significant

improvement of the toughness of the weld metal and welded joints.

Inclusions have undergone a long way from undesirable features in early days (stress

concentrators) to beneficial factor for excellent toughness in steel. This evolution is the

subject of this presentation.

For the last several decades, fracture mechanics had been one of the main tools for

prediction the materials resistance to fracture. Equation 1 quantifies and correlates

materials property, service conditions and the size of features that act as stress

concentrators /1-3/.

= ⋅

⋅

(1)