Pre Heat for P11, P22, P91 Materials

Unless there is a specific prohibition against using flame (oxy-acetylene / oxy-propane / oxy-mapp etc), then use it. Simply be aware of the dangers of spot heating and keep the flame neutral; you don't want to add carbon via excessive spot heat and a rich flame, nor erode the material for excessive spot heating and a reducing flame.

Generally, you want to preheat an area at least 3 thicknesses to each side and the ends of the proposed weld to remove surface moisture, and minimize thermal shock (and the resulting stresses) to the parent material.

Using flame may be acceptable per the Boiler Code as it gives a choice of methods, including preheating with flame. However, one should restrict this to small bore piping, but should be carefully considered when welding pipe to valves as these have different heat dissipation characteristics than other fittings - elbows for example. For large bore pipe, the application of flame for preheating is cumbersome and expensive as the manpower required overcomes any cost savings one may expect from reducing the cost of contracting a company to provide the electrical resistance type. Be careful with the size of the "weed-burner" as it is necessary to make sure the heat is not causing localized hot spots. Also, a sweeping motion in the application of the flame is necessary. The temperature must be checked at three inches from the weld and on at least three points around the circumference of the pipe before welding starts. If a welder's technique is to weld a part of the weld and move on to another part, the preheat must be checked to assure that the minimum has been maintained at the portion where the new welding is to start.

I agree with J-rod, this practice of using torches shall be limited only to small bore as for the large bore piping uniformity of heating would not be there with open flame. Secondly, for pre-heat purpose only oxy-propane flame is to be used as oxy-acetylene flame contains hydrogen which is detrimental to the materials you mentioned.

Regards,

Ashfaq Anwer

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