Welds preheating

Mild steel is the first, and generally the most weldahle class of steel. It has very low hardenability and any suitable electrode may be used. For thin section welds preheat is not necessary, and with CE values less than 0.30 no control of bead size is needed for any combined thickness. Overhead and vertical-down welding can be performed successfully at 1.3 kj/mm on any thickness up to a CE of... 

Weak Acid. Stainless steels (SS) have exceUent corrosion resistance to weak nitric acid and are the primary materials of constmction for a weak acid process. Low carbon stainless steels are preferred because of their resistance to corrosion at weld points. However, higher grade materials of constmction are required for certain sections of the weak acid process. These are limited to high temperature areas around the gau2e (ca 900°G) and to places in which contact with hot Hquid nitric acid is likely to be experienced (the cooler condenser and tail gas preheater). 

Elimination. Since slag is less dense than the weld metal, it will float to the surface if unhindered by rapid solidification. Therefore, preheating the components to be welded or high weld heat input may prevent slag entrapment. 

Residual stresses caused by large temperature differences between the weld bead and adjacent metal can be reduced by preheating the metals to be welded, especially if the sections are thick, or by stress-relief-annealing following the welding procedure. Proper welding techniques, especially the use of appropriate weld filler metals to minimize weld-metal shrinkage, can minimize residual stresses. 

Poor Weldability a. Underbead cracking, high hardness in heat-affected zone. b. Sensitization of nonstabilized austenitic stainless steels. a. Any welded structure. b. Same a. Steel with high carbon equivalents (3), sufficiently high alloy contents. b. Nonstabilized austenitic steels are subject to sensitization. a. High carbon equivalents (3), alloy contents, segregations of carbon and alloys. b. Precipitation of chromium carbides in grain boundaries and depletion of Cr in adjacent areas. a. Use steels with acceptable carbon equivalents (3) preheat and postheat when necessary stress relieve the unit b. Use stabilized austenitic or ELC stainless steels. 

In making field welds, the temperature of structural members should preferably be above 0°F. In the weld areas, steel should be preheated before welding or cutting operations. 

P. Hydrodesulfurization unit hydrogen preheat exchanger shell blisters, intergranular Assuring, and decarburization in weld metal postweld heat treated at 1150°F. 

Fig. 5.17 U se of heat sinking or preheating reed switches for soldering or welding can prevent heat stress damage.

The requirements for preheating shall be per para. GR-3.5 and Table GR-3.5, and as shown in the engineering design specification, and shall apply when welding the test sample(s). 

Preheat temperature shall be checked by use of temperature-indicating crayons, thermocouple pyrometers, or other suitable means to ensure that the temperature specified in the WPS is obtained prior to and maintained during welding. 

Dissimilar Materials. When materials having different preheat requirements are welded together, it is recommended that the higher temperature shown in Table GR-3.5 be used. 

Interrupted Welding. If welding is interrupted, the rate of cooling shall be controlled or other means shall be used to prevent detrimental effects in the piping. The preheat specified in the WPS shall be applied before welding is resumed. 

For preheat and PWHT, the term nominal thickness is the thickness of the weld as defined below. 

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