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This solution, originally developed for castings, had significant positive impact on

improvement of welded structures. First, the toughness of weld metal was increased and

secondly, the temperature interval for solidification was narrowed, leading to smaller

chances for solidification hot cracking /19/.

4.1 Influence of particles on grain size

One of the most important problem in welding of low carbon steels was abnormal

grain growth in the heat-affected-zone (HAZ). The weld pool, as the heat source, would

rise the temperature of steel very close to melting point. In order to find practically

applicable solution, the main idea was to modify the alloying and introduce elements that

would produce precipitates on the grain boundaries. Once the particles that are present on

the grain boundary would be able to suppress the grain boundary mobility and delay the

grain growth to times much longer than time for welding. The solution was found in Ti

addition /10-14,19/.

As indicated before, titanium is an element, which exhibits a strong tendency to form

oxides and sulphides as well as nitrides and carbides. Titanium nitride, considering most

typical nitrogen levels in steel, will be formed before or during solidification. Such

particles, already formed in the liquid steel can be separated into the slag and have no

effect on the properties of the steel. If not, they are relatively large as a result of their high

formation temperature and must be considered as inclusions, having a ductility-impairing

effect on the steel properties. With a larger particle size, the ability to refine the micro-

structure is diminished. However, a positive effect on the steel properties remains from

the formation of TiO and TiN by the reduction of any free oxygen and nitrogen, which

are harmful elements with regard of the toughness of steel /20/.

On very high temperatures in steel, atoms of titanium would react with atoms of

nitrogen in solid solution. The newly formed precipitates will occur on the grain

boundaries, since they are preferential places for precipitation, due to thermodynamic

reasons (Eq. 3).

The solubility (maximal amount of nitrogen and titanium in solid solution at

respective temperature) can be calculated using Eq. (4) /20-22/

[ ][ ]

Ti N

log

= −

(4)

where [Ti] and [N] are concentration of Ti and N in steel, respectively, in weight per-

cents,

is temperature, K,

and

are constants. Their values depend on the temperature

and state of steel (liquid or solid) /21,22/.

TiN particles decorate the austenite grain boundaries and mechanically block their

movement. Only in the case of very high heat input (either high temperature or long

time), TiN particles will start to coarsen. In the cast condition TiN particles are present in

diameter 1 to 3 micrometer, which will hardly be dissolved during reheating. During the

coarsening, small particles become dissolved, and large particles become even larger,

coarser. During this process, some segments of grain boundaries can be TiN-free, and can

start growth particularly in fusion zone /14, 19/.

4.2. Influence of particles on phase transformation

Influence of particles on phase transformations can be observed as: (i) influence on

recrystallization and (ii) influence on austenite decomposition.