Postheating temperature

For hardenable steels the carbon content is used to select preheat, interpass and postheat temperatures. 

Diagram for selecting minimum preheat, interpass and postheat temperatures for alloy and lean alloy steels giving fully hardened HAZs. Note relaxation of preheat temperatures may be possible with low hydrogen processes and thin sections, particularly at low C contents. Confirmation should be obtained by joint simulation tests. For class of steel refer to Table 4.3... 

Diagram for selecting minimum preheat, interpass, and postheat temperatures (carbon and C-Mn steels with fully hardened HAZs). 

For values of this parameter, F, up to 115, a steel is graded carbon steels . For values from 116-145 the steel is graded carbon-manganese steels and examples can be found in Table 4.3. For these two grades the relationship between carbon content and expected HAZ hardness is given in Fig. 4.7, and its subsequent use to establish minimum preheat, interpass, and postheat temperatures is exactly as described previously. 

Colloidal synthesis The aqueous synthesis routes could not yield rare earth oxides directly without postheat treatment, while the dry routes usually lead to products with relatively wide size distribution and the nanocrystals could not be dispersed as colloidal solutions. Therefore, it is highly desirable to synthesize R2O3 nanocrystals in suitable nonaqueous solutions. However, the decomposition of rare earth precursor and crystallization of rare earth oxide nanocrystals would require an elevated temperature. Therefore, the solvents are usually with a high boiling point, which are called "high-boiling solvents."... 

The structure of the flame itself consists of two major zones, the primary reaction zone and a secondary reaction or postheating zone. The primary reaction zone, the area of the flame just above the burner surface, is the region where combustion, atomization, and excitation occurs. Some typical flame combustion products formed in this region are CO, C02, H2, N2, and H20 molecules, as well as O, H, OH-, and C- radical species. The secondary reaction zone is a much cooler region where the flame gases mix with atmospheric components that may include impurities and emission interferants. Between these two zones lies a smaller, but important, intermediate region, where little reaction occurs. In this region of the flame, the fraction of the atoms in the ith excited state, a is controlled only by the prevalent temperature and can be represented by the Boltzmann distribution ... 

The decision to maintain weld temperature as a postheat, together with an estimate of the time for which it must be held, must be based on a consideration of hydrogen concentration at the end of welding and some critical concentration below which cracking will not occur when the weld cools to ambient temperature. At present there is little information on these critical concentrations although it is possible to make cautious estimates. These estimates can then be used to calculate postheating times for different joint thicknesses and temperatures. Diagrams of the type shown schematically in Fig. 

Combined thicknesses greater than the values indicated by the broken lines on Fig. 4.2 decrease the time to cool to temperatures lower than 300 °C, and hence increase the amount of hydrogen retained at room temperature. As a result, some increase in preheat, or the use of postheating, may be required for very large thicknesses beyond the broken lines. 

As the expected HAZ hardness increases, higher temperatures are predicted and it becomes necessary to select appropriate times for which postheat should be held to assist hydrogen removal. These times may be obtained from the hydrogen removal curves which are described and listed in Chapter 5. If the times involved appear unacceptably long, the possibility of tempering before the weld cools out can be considered. If this is not possible careful use of a... 

Lamination Temperature, °C Postheat treatment Bond strength, g/cm Faiiure interface... 

---