We recently helped out a customer in determining why some of their welds were cracking. It was determined that cracking on their parts was due to rapid cooling and improperly sized welds. The discussions we had before and after determining the cause were quite interesting. We went over the typical causes for cracking with our customer. Here is a brief summary of our meetings.

COLD CRACKING OF WELDS 4. HYDROGEN BEHAVIOUR The figure 3 below illustrates the drastic decrease of hydrogen solubility in steel as the temperature / 2 CRACKING +); + Figure 7 Susceptibility to cold cracking of steels cracking. TEMPERATURE in the weld metal.

Please note that these are not all the causes for cracks. Generally speaking we can separate cracks in two groups, which have to do with the timing of the cracks: hot cracking and cold cracking. Essentially hot cracking occurs as you weld or shortly thereafter while cold cracking can occur hours or even days after welding is completed.

Hot Cracking Hot cracking is mainly due to high amounts of elements with low melting temperatures in the base material. Slightly elevated amounts of these elements is usually not enough to cause cracking, but rather it couples with other issues such as improper width to depth ratio of the weld bead. Some elements used in steel making like sulfur and phosphorus have low melting points. Because of these low melting points these elements are pushed to the center of the weld bead as it is solidifying. These elements can then get trapped in the middle of the weld when the puddle does not stay in a molten state long enough. When this occurs we get a centerline crack as the one pictured below.

Power matte crack. The same way in which water in a glass freezes the weld puddle solidifies from the outside edges towards the middle. Tramp elements are sometimes caught in the middle causing cracks. To prevent hot cracking we want to do the following: • Have proper width to depth ratio on weld beads (1:1 to 1.4:1 is ideal). Sometime joint redesign is needed to accomplish this.

• Select base materials carefully. • Select the right process. Processes like SAW which provide the best penetration can sometime be an issue depending on the joint details. Cold Cracking Cold cracking is much worse than hot cracking simply because it may be hours or days before the crack appears. This means the welded assembly may already be out the door and in service. Cold cracking is characterized by a crack starting on the base material and not on the weld as is the case with hot cracking. It is sometimes referred to heat affected zone (HAZ) cracking or hydrogen assisted cracking.

Cold cracking occurs when three conditions are present: enough sensitive material involved, sufficient level of hydrogen, and a high level of residual stress. We typically have little control over the level of residual stress so the goal is to control the other two variables. High cooling rates and materials with high carbon content create conditions susceptible to cold cracking. The faster the cooling rate the more likely we are to get cold cracks.