BASIC METALLURGY OF FUSION WELDING

BASIC METALLURGY OF FUSION WELDING

A typical fusion welded joint varies in metallurgical structure from fusion zone to the base material with consequential variation in mechanical properties.

This is because of the fact that fusion welding processes result in melting and solidification with very high temperature gradient within a small zone with the peak temperature at the centre of the fusion zone.

In general, a weld can be divided in four different zone as shown schematically in figure

The fusion zone ( FZ) can  be characterized as a mixture of completely molten base metal and filler melt . if the consumable electrodes are in used with high degree of homogeneity where the mixing in primarily motivated by convection in the molten weld pool.

The main driving forces for the convection transport of heat and resulting mixing of molten metal in the weld pool are  1) Buoyancy force  2) Surface tension gradient force

3) Thermomagnetic force  4) Friction force

Similar to the casting process, the microstructure in the weld fusion zone is expected to change significantly due to re-melting & solidification of metal at the temperature beyond the effective liquidise temperature.

Temperature limit in this zone is from 1600 degree Celsius to 1535 degree Celsius , which is the highest temperature of the welding process. It is highest liquidise section in the welding process. It is above the austenitic structure.

Weld Interface  which is also refer to as mushy zone is a narrow zone consist of partially melt base material which has not got opportunity for mixing . This zone separates the fusion zone & heat affected zone .Temperature limit consider in this zone is 1538 to 1495 °C.formation of Austenitic structure course or dence depends on the carbon percentage.

The Heat affected Zone( HAZ) is the region that experiences a peak temperature that is weld below the solidus temperature while high enough that can change the microstructure of the material.

The amount of change in the microstructure in HAZ depends on the amount of heat input ,peak temp reached ,time at the elevated temperature and the rate of cooling . as a result marked changes in the micro structure , the mechanical properties are also change in HAZ and usually this zone remains as the weakest section in a weldment.

This zone can be bifurcated into four part according to variation in the temperature

a)    Coarse grain HAZ temperature 1495 to 1148° C . It is purely liquid austenitic zone

b)    Fine grain HAZ : 1148°C  to 912°C temperature variation. This region of fine grain HAZ.upto 0.2 % carbon. it is purely Austenite structure. it is austenite  & cementite structure

c)    Intercritical HAZ  : Temperature region is 912 to 770 °C . Martensite formation is there .

d)    Tempered HAZ : 723° C  to 600°C  temperature variation . it is Ferrite & Perlite upto 0.18% of carbon percentage & Perlite & Cementite above 0.18% Carbon.

e)      Unaffected Base Metal : From 600°C down there is unaffected base Metal .It is hardly Perlite & Cementite Structure , which is going to stable.

When we weld the material in the fraction of second the material is going to all this stags,Welding joint is the weakest section in the material , hence welding should done very careful following all the rule of AWS & ASME.

PRAKASH PATIL

ASNT LEVEL 2 CERTIFIED

SPECIALY IN RT INSPECTION