The Weld Defects #2
Another type of weld defect is crack. Note that weld crack is different from undercut or incomplete fusion described in previous post, though they may visually look alike. Undercut and incomplete fusion are created during welding process. On the other hand, crack developes after the welding process. Usually, undercut and incomplete fusion initiate cracks when structures exposed to load. There are many types of cracks in and around the weld as shown in Figure 1 below. The question is: Why can this happen? I am not an expert in crack but I am trying to explain this based on what I think, in simplest way possible.
1. Toe Crack and Root Crack
Going back to undercut, it is clearly seen that toe crack is initiated by undercut when the metal exposed to tension. In the same fashion, root crack is also initiated by incomplete fusion.
2. Weld Interface Crack
Weld interface crack is also initiated by incomplete fusion. The cause of this crack is poor bonding strength between weld pool material and base metal. It can be due to undercurrent which causes base metal not melted and fused properly with the weld pool. It can be also due to breach in welding procedure. From my fabrication experience in Indonesia, welder seldom does pre-heat and post-heat treatment because of the friendly tropical weather. In other countries where welding sometimes done in winter, heat treatment before and after welding is a must.
Arc welding is performed by striking a highly-concentrated and high-temperature electric arc to a very small area of base metal. In seconds, metals melt, fuse and solidify. That means the weld joint expands and contracts in high speed because the surrounding base metals act as cooler in the absence of pre-heat and post-heat treatment. Any impurities or weak bonding on the weld interface will become weld interface crack when weld pool cool down and contract. Joining 2 different metals with different coefficient of expansion can also produce this kind of crack.
Fig. 1 Types of cracks in weld.
3. Other Weld Cracks
Inadequate heat treatment can also be the cause of various weld crack such as: Heat-Affected Zone (HAZ) crack, root surface crack, underbead crack, and weld metal crack. Sudden change in temperature causes local brittleness which is susceptible to crack when metal deforms.
4. Design Consideration
When it comes to design, crack defect is more dangerous than undercut or incomplete fusion. First, crack is smaller in size and requires equipment of higher sensitivity to detect. Second, it propagates under relatively low load and causes sudden failure which is usually catasthropic. The best way to overcome this problem is to perform detailed stress analysis to find the hot spot. Hot spot here means spot of high stress and high-cycle load since crack propagation also depends on number of load cycle.
Optimize structural strength of hot spot region by means of reinforcement. Prepare detailed welding procedure and inspection. If cost saving is among consideration, use heat treatment on critical spot only. Perform nondestructive testing of appropriate accuracy efficiently on the selected hot spot based on hot spot map supplied by designer and structural analyst.
See also: Design Considerations in Heavy Construction.
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