Furnace Heat Treatment. To assure that heat treatments attain the results intended (i.e., correct heating and cooling rates, desired holding temperature in all parts, etc.), it is very important that all controlling and recording instruments be calibrated on a regular basis. The furnace should be inspected and a temperature survey made to assure that all locations within it are capable of attaining and maintaining specific temperatures within some reasonable tolerance. This is particularly important if the zone temperatures are used as the basis for acceptance of the heat treatment. If there is any concern, it might be advisable to attach thermocouples directly to the parts being heat-treated. When piping subassemblies are placed in the furnace, they should be supported to permit exposure of the underside to the radiant and convection heating surface. Supports should be located so as to avoid sagging. Care should be taken to avoid any flame impingement directly on surfaces being heat-treated. The ends of assemblies being heat-treated should be closed but not sealed to minimize oxidation of the inside surfaces. Occasions may arise where special surface finishes on the pipe inside surface or on flow meter sections could be adversely affected by oxidation caused by heat treatment. In such cases the inside of the assembly can be purged with an inert gas to minimize oxidation.
Assemblies should be so placed as to assure the uniform application of heat. Heating and cooling rates must be selected to assure heating through the full thickness and to minimize distortion caused by uneven heating. The faster the rate of heating or cooling, the more probability of distortion. Assemblies with massive flanges, fittings, or other unusual configurations should be treated more carefully than those with butt welds only. Many of the codes have specified heating and cooling rates which are considered reasonable. Local Heat Treatment. When an assembly is too large for a furnace to accommodate, it may be fabricated in sections which are individually furnace heat-treated and later joined by welding. The final butt welds may then be locally heat-treated
in the same fashion as field welds. The most common practice is the use of induction or resistance heating. When preheating is an essential part of the welding operation, the induction or resistance equipment can be used for preheating, maintaining preheat during welding, and, finally, stress relieving.
A proper stress-relieving operation will assure that the weld and HAZ through the full thickness will attain the required temperature for the required time. The B31.1 Code requires that the heated band be at least 3 times the thickness of the thickest part being joined. With induction or resistance heating the heating elements themselves often have greater coverage. Depending on the massiveness of the joint being heated, one or more pieces of heating equipment may be needed. Controlling and recording thermocouples are located on or adjacent to the weld. Usually locally heat-treated shop welds are in the 5G position (pipe horizontal, weld vertical). For small pipe sizes, a single thermocouple located at the 12 o’clock position may suffice, but for larger diameters and heavier walls at least two and preferably four, located at 90° intervals, should be employed to assure uniformity of heating. Judicious use of insulating material should be employed to minimize heat loss. When joining parts of differing masses, concentrate more heating effort on the more massive part.
If it is necessary to locally stress-relieve a branch connection, not only the branch weld itself but the entire circumference of the header for a distance of at least 2 times the header thickness on either side of the branch should be heated. Heating of the weld alone, while resulting in a satisfactory stress relief, could distort the header significantly. Heating and cooling during local stress relief of pipe to pipe joints can be more rapid than for furnace applications since there is less chance of distortion unless, of course, the heating is not applied uniformly. Ends of the assembly should be closed but not sealed to reduce heat loss on the inside surface due to air flow. The main concern is assurance that the inside surface of the weld attains the required temperature for the required time. Local stress relieving with torches or gas ring burners can be effectively employed but must be limited to situations where controlled heating and cooling rates are not a factor.
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