MICROWAVE WELDING

In the field of joining of metals research are taking place for innovative ideas to come. One such new idea is microwave joining. It is one of the newly developing and advanced technique. But the thing is most people are unfamiliar with this type of process. It is a kind of fusion welding using frequency of 2.54 GHz. Microwave technique uses microwave energy which interacts with the material at the joint interface. It can be transmitted, reflected or absorbed when interact with materials. For processing materials like glazing of sprayed ceramic composite surfaces microwave energy can be used.

Microwave energy is widely used in composite and ceramics because of its nature. The nature of composite and ceramics is that they absorb microwave. As metals tend to reflect microwave radiations less work is done in the field of metals. This method was experimented on mild steel and stainless steel as substrates. For joining butt joint is preferred as it is easy to handle. Substrates are surrounded by refractory brick after applying the interfacing material between the faces. When microwave comes in contact with charcoal, which is used to provide extra heating, it starts burning thus increasing the temperature at the joint. The faces of substrates get totally wet. The molten region coverts into a shape of joint after cooling at atmospheric situation.

Taguchi method was developed by Dr. Genichi Taguchi to improve the quality of manufacturing goods. It is also applied in engineering field. The quality engineering set by Dr. Taguchi in 20^th century was regarded as greatest achievement in engineering. In his concept he mainly focuses on engineering strategies which includes upstream and shop-floor quality engineering. In upstream method small-scale experiments and robust designs for large scale production is used while cost based real time system is used in shop floor technique. Three concepts where there in Taguchi’s philosophy. One was that quality should be planned into the manufactured goods. Best worth is achieved by reducing the deviation on or after the target. This was his second. The expenditure of quality should be calculated as a function of deviation from the standard and the losses should be measured system-wide was his third.

After doing the Taguchi’s method using L9 orthogonal array some results were obtained. The value of hardness is lower at 600 sec as the material doesn’t fuse properly than 700s and 800s. There is decrease in hardness due to excessive heat at 800s. The percentage of interfacing powder will result in proper fusion and less porosity will be observed. Since the hardness of the stainless steel base metal is higher than mild steel base metal the SS-SS joint has higher hardness. Among the three parameters, substrates has the major influence on the hardness followed by percentage Ni based powder used and time. Like that new technologies and ideas are coming in the field of welding and inspection. welding inspector course in kochi

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

WELDING OF ULTRAHIGH STRENGTH STEEL

Structural steels with very high strength levels capable of developing minimum yield strength of nearly 1400 MPa is called ultrahigh strength steels. In recent times, the ultrahigh strength steels are in a great demand for critical structural aircraft and aerospace application. In aircrafts, automobiles, power plants, chemical industries etc. for technical as well as economic reasons the dissimilar combination of steels are necessary. Maraging steel and medium alloy medium carbon steel are used in these situations. Maraging steels are a class of precipitation hard enable martensitic steels. It develop strength due to the precipitation of inter metallic compounds.

Fabrication process which is widely employed for ultrahigh strength steels is fusion welding. Gas tungsten arc welding and electron beam welding are the two fusion welding employed. Three types of filler materials are used which include maraging steels filler, austenitic stainless steel filler and medium alloy medium carbon steel filler. The austenite get transformed to martensite during cooling in the fusion zone in maraging steels welds. In medium alloy steel weld the columnar grain growth is prevalent at the center. After post weld aging no variation is observed in microstructure as compared to as welded condition in respect of austenite stainless steel and medium alloy medium carbon steel filler welds.

The hardness survey was done on different filler welds. Austenite stainless steel weld metal shows the lowest hardness. The region of weld of medium alloy medium carbon steel filler shows the highest hardness. The hardness of maraging steel filler weld is as likely as parent metal. The residual stress at the center of the weld metal zone is compressive in case of maraging steel and medium alloy medium carbon steel weld metals. When it comes to the case of austenite weld the residual stress at the center of the weld metal zone is tensile. welding inspection courses in Kochi.

The similar metal welds will show symmetrical fusion zone and heat affected zones, whereas the dissimilar metal welds shows un symmetrical fusion zone and heat affected zones. The difference in the thermal conductivity of materials is the reason for the unsymmetrical nature. In medium alloy medium carbon steel the width of heat affected zone is more when compared to maraging steel. The residual stress in similar metal weld of maraging steel is compressive and in similar metal weld of medium alloy medium carbon steel it is tensile. In case of dissimilar metal weld it is compressive in maraging steel and tensile in medium alloy medium carbon steel.