Summary of Root Bead Welding
A perfect root bead should be free from undercuts, porosity, incomplete fusion, insufficient penetration, and excessive penetration (see Fig. 5-14). All of these defects can be avoided by learning and practicing the correct welding procedures.
It should be kept in mind that these defects are the responsibility of the welder. Porosity cannot be blamed on the equipment, but rather on not cleaning the weld sufficiently when grease, oil, and rust are present prior to welding. Other causes of porosity are defective electrode coatings such as chipped coatings, flaking of the coating, and coatings containing an excessive amount of moisture. All of these electrode defects can be detected by the welder before he strikes the arc.
Proper root opening and edge penetration help to attain complete penetration. The composition of the weld metal is also affected by the edge preparation. When the spacing and the edge preparation are correct, the welder will be able to manipulate the electrode comfortably so that there will be a better intermixing of the base metal and the filler metal.
Restraint cracking occurs when small welds are made on thick metal sections, such as heavy-wail pipe. This subject is discussed at length in Chapter 13. Suffice to say here that the size of the root bead must be large enough to withstand the shrinkage stresses without cracking. This calls for careful attention to the condition of the joint before welding.
The presence of hydrogen in the weld or in the base metal heat - affected zone becomes dangerous if the microstructure in both of these areas becomes martensite, with hardness exceeding 30 Rockwell. The danger increases as the carbon content increases. The base metal heat - affected zone needs particularly close attention to prevent cold cracking. Hydrogen alone cannot be blamed for under bead cracking; other contributing variables in the immediate vicinity of the weld area must be considered as well. The following welding procedures can also con-
Manifold - Welds been x-ray-(Perfect) and hydrostaticiy tested OK for receuvubg station, (crude oil)
tribute to cold cracking: 1) reaction stress from restraint of the base member; 2) residual stress from the unequal stress caused by contraction of the base metai and weld metal; 3) the development of short range stress due to the transformation; and 4) structure stress resulting from the austenite-to-martensite transformation. This transformation is influenced by the alloying elements, especially the carbon content in respect to higher stress levels; it is likely to increase further from the melting of the base metal and inducement of filler metal into the weld. The use of premium electrodes carrying the prefix LC (Low Carbon) is strongly recommended,
Multiaxial stress develops in those alloys with limited toughness and ductility. There are limited options for accommodating this high stress. If hydrogen is present in such a weld, then very little energy will be needed to promote cracking. Hydrogen in a weld of hardenable steel is a powerful promoter of cracking.
Carbon steel generally contains less than 0.30 percent carbon. When joined alloy steel containing 0.10 percent carbon will usually call for countermeasures against the influence of hydrogen. Even the addition of manganese to a level of about 1.25 percent in a carbon steel that contains 0.30 percent carton might need precautions against hydrogen when welding heavy sections.
It is not difficult to decide when countermeasures are needed against hydrogen if consideration is given to the principle of anticipated microstructure, as discussed earlier. The reasoning for such apprehension is clear. Such steel in heavy sections with tend to form martensite in the heat-effective zone, unless welding was conducted with controlled heat input to secure a relatively slow cooling rate.
Welding procedure are not just for information, but must be implemented, The specifics cannot be ignored during welding of an alloy material, nor can the welder fail to follow these written instruction. For example, if preheating and the interpass temperature are ignored, the metal experiences a cooling rate that would likely lead to a martensite structure with poor ductility and toughness. Steel has a tendency to develop a higher level of hardness with an increasing cooling rate. Therefore, it can be difficult to determine if a service failure is due to defective welding or simply weld induced brittleness.
it is important to control cracking in medium or higher carbon steel, and in steel which incorporates any alloying element to achieve qualities such as strength and hardness, corrosion resistance, impact resistance and toughness, and toughness at sub-zero temperature. Sometimes adding a single alloying element will work; in other cases multiple elements are more effective. For instance, it is uncommon to find iron without carbon as a strengthening additive. Carbon is more powerful and effective than any other element when added in small quantities. Its effectiveness increases as it is increased for tensile strength and hardness.
Carbon, as an alloying element, is soluble in high-temperature iron to an appreciable extent, but is soluble in the low-temperature form only to a limited extent. With the addition of alloy to a point where its high temperature crystalline form exists and the alloying element enters the lattices to form a solid solution. Upon cooling the alloy to the point where the crystalline transformation occurs, the alloying element suddenly experiences the full effects of limited solution (solid), and precipitation takes place. This results in embrittlement and reduced ductility in the HAZ.
The cooling rate must be regulated to produce a fine dispersion of precipitation throughout the metal so that it has high ductility and a fine grain structure. Otherwise, the structural condition will result in a marked decrease in ductility. Transformation hardening is the principal mechanism used to increase the hardness and tensile strength of carbon and alloy steel. Tensile strength moves to higher levels when the carbon and manganese content increase. In the alloy steels, moderate addition of carbon also increase these mechanical properties, but reduces ductility and toughness.
Depending upon the welding process employed, the conditions under which steel hardens to its martensite structure varies. Also, residual stress, reaction stress, and hydrogen can cause delayed cracking during extended service, particularly in large heavy weldments. Alloying steel along with steel making practices in recent years has led to improvements in terms of toughness. This is especially true when welding high strength, low alloy material and medium-high alloys. A minute fissure or void in such a weld can or will be accepted, depending on the, the joint design, and the severity and variation of load carrying capacity.
In recent years, toughness has received much more attention within the microstructure and the heat affected zone, along with the fusion line. Molten alloy exhibits much more of the stronger carbide forming elements; it has a slower dissolving rate from its higher temperatures, and higher preheating, requirements, approximately to 450° F. Caution may be necessary to control grain growth by carefully preheating and the use of temp sticks to indicate when the appropriate preheat has been reached. The success in welding carbon and alloy steel is determined by the familiarity with the microstructural characteristics of each particular type of steel, and avoiding the development of an unsuitable structure (low toughness) in or adjacent to the weld joint. In merging the chemical composition of carbon steel with low and high alloy, the addition of an alloying element from a filler metal can improve a particular quality or may unexpectedly appear as a hindrance to improving toughness. Other important factors include the microstructural changes involving the transformation of austenite, ferrite, and martensite. Grain size, cooling rate, residual elements, and multiple alloy may also bring about synergistic effects, though little is yet known about such synergy. Preheating and postheating can also help attain the required level of toughness while avoiding cracking and other metallurgical difficulties.
Emphasis must be given to the type of the base metal (chemical composition) to be welded, steel making practices, wall thickness, joint design, and, in most cases, the work function of such a welded component. In addition, the welding engineer and metallurgist need to determine the effects of microstructures of a weld based on a hydrogen free weld, or how to maintain a hydrogen-free condition. Preheating, interpass temperature, and post weid heat treatment are very important. Even considering which electrode to use for a weld is important to the welding engineer in terms of dilution, pick-up, and recovery. The welding engineer must consider all these facts and write an operation sheet which adequately reflects the above concerns, A qualified welder can prevent defects such as porosity, slag inclusion, incomplete fusion, excessive penetration, incomplete penetration, undercutting, root bead cracking during welding, and distortion caused by shrinkage. Of all of these, cracking seem to be the most unwanted defect. Defects in the heat affected zone can be caused by the lack of adherence to proper procedure in terms of preheating and interpass temperatures.