Wednesday, October 12, 2011

Preheat and Interpass Temperature

Preheat and Interpass Temperature, that's the topics that Piping & Fabrication will discuss today. Ferritic materials undergo metallurgical phase changes when cooling from welding to ambient temperature. Mild steels which contain no more than 0.20 percent carbon and 1 percent manganese can be welded without preheat when the thickness is 1 in (25 mm) or less. However, as the chemical composition changes by increases of carbon, manganese, and silicon or the addition of chromium and certain other alloying elements, preheating becomes increasingly important since the higher carbon and chrome molybdenum steels can develop more crack-sensitive martensitic, matensitic-bainitic, and other mixed phase structures when cooled rapidly from welding temperatures.

There is also a potential for hydrogen from SMAW electrode coatings or from moisture on the base metal surface to be dissolved in the weld. Also as the weld cools, stresses caused by shrinkage are imposed on the parts and distortion can result; and as thickness increases, thermal shock from the heat of welding can induce cracking more readily.

Preheating prior to welding is a solution to most of these problems. Preheating slows the cooling rate of the weld joint and results in a more ductile metallurigical structure in the weld metal and HAZ. It permits dissolved hydrogen to diffuse more readily and helps to reduce shrinkage, distortion, and possible cracking caused by the resultant residual stresses. It raises the temperature of the material sufficiently high to be above the brittle fracture transition zone for most materials.

The codes vary regarding preheat requirements. Some have mandatory require ments while others give suggested levels. For example, for carbon steel welding, the B31.1 Code requires preheating to a temperature of 175_F (80°C) when the carbon content exceeds 0.30 percent and the thickness of the joint exceeds 1 in. B31.3 recommends preheating to 175°F (80°C) when the base metal specified strength exceeds 71 ksi or the wall thickness is equal to or greater than 1 in (25 mm). ASME III Section suggests a preheat of 200_F (95_C) when the maximum carbon content is 0.30 percent or less and the wall thickness exceeds 11/2  in for P No. 1 Gr. No. 1, or 1 in (25 mm) for P No. 1 Gr. No. 2. It also suggests a 250°F (120°C) preheat for materials with carbon in excess of 0.30 percent and wall thicknesses exceeding 1 in (25 mm). The ASME B31.4 and B31.8 Codes require preheat based on carbon equivalents. When the carbon content (by ladle analysis) exceeds 0.32 percent, or the carbon equivalent (C + 1/4 Mn) exceeds 0.65 percent, preheating is required. The reader is advised to consult the specific codes for preheating requirements. See Table A6.4 for some typical preheat requirements. It should be noted that for the 9Cr-1Mo-V (P No. 5B Gr. 2) material some manufacturers suggest a preheat of 350_F (177_C) for GTAW and 400 to 450°F (204 to 232°C) for other types of welding regardless of thickness.

While it is preferred that preheat be maintained during welding and into the postweld heat treatment cycle without cooling, this may not always be practical. The B31.1 Code permits slow cooling of the weld to room temperature provided the completed weld deposit is a minimum of 3/s in (9.5 mm) or 25 percent of the final thickness, whichever is less. For P No. 5B and P No. 6 materials some type of intermediate stress relief is required. For the 9Cr-1Mo-V material it is recommended that the finished weld be heated to 500°F (260°C), held at that temperature for 2 hours, and allowed to cool slowly in still air by wrapping it with insulating material.

Too much heat during welding can also be a problem. Where notch toughness is a requirement, prolonged exposure to temperatures exceeding 600°F (316°C) can temper the base metal. Controlling the interpass temperature is required to minimize this problem. Interpass temperature control means allowing the temperature of the joint to cool below some specified level before the next pass is deposited.

Because of its martensitic structure, a maximum interpass temperature of 600°F (316°C) should be observed when welding 9Cr-1Mo-V material. In welding of austenitic stainless steels, sensitization of the base metal HAZ will result from the heat and welding. Here the solution is to weld with as low a heat input as possible at the highest possible speed to minimize the precipitation of carbides (sensitization). A maximum interpass temperature of 300 to 350°F (149 to 177°C) is usually employed.

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