SURFACE MODIFICATION OF MARTENSITIC STAINLESS STEEL

    It is a common practice in various engineering applications to modify the surface properties of material requiring specific surface characteristics of any component over that of its core material to suit various service conditions. It is most commonly achieved by modification of physical, chemical, mechanical and metallurgical characteristics of the surface of a metal. There are many sophisticated treatment like sputtering and treating by plasma, laser, ion and electron beam but surface modification is commonly carried out by mechanical treatment, heat treatment and fusion of the substrate as well as extra deposition on it. From the experiments done it is derived that surface fusion by gas tungsten arcing can be precisely applied for appreciable surface modification up to significant depth of a metal substrate. One effective method to achieve desired micro structural transformation is gas tungsten pulse arcing (GTPA). The precisely controlled thermal distribution in the arcing system makes it suitable for this process.

   
     GTPA due to its high peak current compared to GTA can be operated at higher energy input. But due to lower mean current, net heating of the base material remains lower. Thus by using GTPA better heat control along with wider fusion zone and larger depth of penetration is achieved. The GTPA has the ability to control energy input as well as its distribution in the entire process of surface modification of the substrate by fusion. It is achieved by manipulation of the solidification behavior and nature of phase transformation in the matrix through a control over the depth of fusion.

    The surface modification process on annealed 5mm thick stainless steel gave some results. It was carried out with the help of a Fronius Magic wave 1700 pulsed TIG welding machine. The characteristics of ripple in fusion zone were examined for different f and ᶲ values. It was observed that more number of ripples are formed per unit length with the increase of either f or ᶲ. This has resulted in relatively finer surface ripples at higher f and ᶲ. The cooling rate and solidification behavior of fusion zone have been analyzed. It has been observed that there is significant increase in cooling rate as well as primary solid growth rate for both cases that is increasing ᶲ and maintaining f constant and increasing f and maintaining ᶲ constant. Comparatively larger time for heat dissipation in between pulsed arcing at low frequency is reason for the relatively low cooling rate at lower f. it reduced the temperature gradient of fusion zone to its relatively large area of hot surrounding heat sink. A relative lowering of fusion zone width with decrease of ᶲ from 0.3 to 0.25 may have happened due to some predominant compromising effect of ᶲ in reduction of heat buildup in fusion over the effect of f on enhancement of the same up to certain extent. The controlled fusion by gas tungsten pulsed arcing and its consequent thermal behavior has significant effect on the microstructure of fusion zone and heat affected zone. The change in cooling characteristics of the matrix during solidification of the fusion zone is the reason for hardness increase. Welding inspector courses in Ernakulam