Preheating (in accordance with AWS D10.10M)
Preheating is defined as the application of heat to the base metal immediately before welding, brazing, soldering, thermal spraying or cutting.
Concurrently, "Interpass" heating is applicable to the weld area (weld metal, Heat Affected Zone (HAZ), and adjacent base metal) prior to start of each pass in a multi-pass weld.
One reason for Preheating is to prevent hydrogen cracking, also identified to as hydrogen embrittlement, in the weld metal and/or HAZ. Our objective in Preheat/Interpass heating is to remove all the contributing items to hydrogen embrittlement. As you can see from Figure1, the factors that will produce a crack at ambient temperature are: Stress, Hydrogen, and Microstructure. This objective is accomplished by the interaction of several effects including: driving off moisture prior to the start of welding, reducing the cooling rate, and increasing the rate of hydrogen diffusion. A second reason for Preheat/Interpass heating is the redistribution of solidification stresses that result from the greater time for this to occur afforded by the slower cooling rate. A third reason for Preheat/Interpass heating is to reduce the cooling rate in materials that form hard or brittle micro structural constituents when cooled to rapidly from welding temperatures. By doing this we remove the factors that lead to hydrogen embrittlement. If this is not done, it could lead to a failure as seen in Figure 2.
BAKEOUT (in accordance with AWS D10.10M)
Although a standard term for this process is not recognized by AWS, such heating is performed to remove hydrogen from material prior to manufacture fabrication or repair activity. At the temperatures commonly used for such heating, removal of atomic hydrogen (H), as opposed to molecular hydrogen (H2), is generally the objective. Molecular hydrogen (H2), which is trapped at voids such as inclusions, weld defects, blisters, etc. as seen in Figure 3, will not be removed unless the temperature is raised sufficiently to dissociate it to atomic hydrogen. The temperature required to accomplish such dissociation is typically near that used for PWHT. When molecular hydrogen is present, care must be exercised such that temperature and hydrogen partial pressure do not result in conditions under which high temperature hydrogen attack can occur. As a result temperature limitations may be imposed.
One common source of hydrogen is the service environment, such as found in wet Hydrogen Sulfide (H2S) service, which is an acid and causes the same effect as seen in Figure 4. Therefore, this heating is frequently applied to service exposed material prior to a repair activity. The purpose for removing the hydrogen is to prevent hydrogen-induced (delayed) cracking in the weld metal and/or HAZ. Since the objective is to facilitate diffusion to free surfaces, time-temperature parameters are selected such that sufficient hydrogen mobility is provided to accomplish the desired degree of removal during the allotted hold time.
In simplified terms, in environments where Hydrogen is present in contact with the metal, the hydrogen works its way into the metal over time and temperature fluctuations. This is undesirable when doing a weld repair on this metal due to the pockets of hydrogen build up will cause the repair to crack during the cooling process. Therefore, the answer to this is a Bakeout which simply is holding the metal at the repair site to a certain elevated temperature for a specified amount of time. This expands the metal structure enough and gives enough time for the hydrogen to escape to the atmosphere. This dramatically reduces the chance of the weld repair cracking after cool down.