Code Requirements, something that Piping & Fabrication will explain to all of you, after I'm back from my duty on South Pacific Viscose, and now this is the detail.s
Postbending and Postforming Requirements. The designer of the piping system should specify the type of heat treatment required to assure appropriate physical, metallurgical, or corrosion-resistant properties. As an example, a normalize or normalize and temper may be required to assure certain notch toughness properties for nuclear or low-temperature applications, or a carbide solution heat treatment for cold worked austenitic stainless steel may be required to preclude IGSCC. This should be agreed upon well before any fabrication starts.
The codes have certain mandatory heat treatment requirements which must be observed as a minimum, normally a stress-relieving treatment. Such heat treatment is usually in accordance with the postweld heat treatment tables given in the applicable code. Differing requirements apply depending on whether the bending or forming was performed hot or cold. According to B31.3, cold bending is performed at a temperature below the transformation range (below the lower critical), and hot bending is performed at a temperature above the transformation range (above the upper critical). B31.1 and ASME Section III make the break between hot and cold bending at a temperature 100°F (38°C) below the lower critical.
B31.3 requires heat treatment after cold bending when (1) specified in the engineering design, (2) the calculated elongation will exceed 5 percent for materials requiring notch toughness properties, and (3) the calculated elongation will exceed 50 percent of the specified minimum elongation indicated in the material specification for P-No.1 through P-No.6 materials. For hot bending and forming, heat treatment
is required for all thicknesses of P-Nos.3, 4, 5, 6, and 10A materials. B31.1 and ASME Section III on the other hand require heat treatment after bending or forming in accordance with the postweld heat treatment table of the applicable code for P-No.1 materials with a nominal wall thickness exceeding 3/4 in unless the bending or forming was completed above 1650°F (900°C). All ferritic alloy materials of NPS 4 (DN 100) or larger or with a nominal wall thickness of 1/2 in or greater which are hot bent or formed must receive an annealing, normalizing and tempering, or a tempering heat treatment to be specified by the designer, or if cold bent or formed, the heat treatment at the required time and temperature cycle specified in the postweld heat treatment table for the material involved.
The codes have no requirements for postbending or forming heat treatments of austenitic stainless steels or nonferrous materials. Postwelding Heat Treatment Requirements. Before applying any post-welding
heat treatment (PWHT), it should be noted that for work under ASME Section IX, postwelding heat treatment is an essential variable for welding procedure qualification. For ferritic materials there are five possible conditions of heat treatment, each requiring separate qualifications. These are:
1. No PWHT
2. PWHT below the lower critical temperature (stress relief)
3. PWHT above the upper critical temperature (normalize or anneal)
4. PWHT above the upper critical temperature, followed by heat treatment below the lower critical temperature (normalize and temper)
5. PWHT between the upper and lower critical temperatures.
For other materials, two conditions apply: no PWHT or PWHT within a specified temperature range.
Accordingly, for shop work, it may be necessary to qualify welding procedures for several possible heat-treatment situations. For field work only the no heat treatment or stress-relieving situations will normally apply.
When required by the codes, heat treatment consists of a stress-relieving operation. Other heat treatments such as annealing, normalizing, or solution heat treatment may be applied but are not mandatory. However, the welding procedure must have been qualified for the heat treatment applied.
Each code has its own definition regarding governing thicknesses, its own exemptions, differing temperature and holding requirements, heating and cooling rates, etc., reflecting the differing concerns and needs of individual industries. The codes are also constantly evolving as the committees obtain and review new data. Accordingly, the reader should refer to the applicable edition of the code of interest
for requirements. At the time of this writing, the following is a comparison of the heat treatment
requirements for carbon steel materials.
B31.1 requires heat treatment of P-No. 1 Gr.Nos. 1, 2, and 3 in the temperature range of 1100 to 1200°F (600 to 650°C) for 1 h/in (1 h/25 mm) of thickness for the first 2 in (50 mm) plus 15 min for each additional inch over 2 in (50 mm), with a 15-min minimum. Exempted are welds with a nominal thickness of 3/4 in (19 mm) or less, and a 200°F (95°C) preheat must be applied when either of the base metals exceed 1 in (25 mm). The nominal thickness is defined as the lesser of the thickness of the weld or the thicker of the base metals being joined at the weld. The thickness of the weld is further defined as the thicker of the abutting edges in a groove weld, the throat of a fillet weld, the depth of a partial penetration weld, and the depth of the cavity for repair welds. Thickness as it relates to branch welds is a function
of the header thickness, the branch thickness, and reinforcing pad thickness.
B31.1 also requires controlled heating and cooling at temperatures above 600°F (316°C). The rate shall not exceed 600°F/h (335°C/h) or 600°F/h (335°C/h) divided by one-half the maximum thickness at the weld in inches, whichever is less. Section III requires heat treatment of P-No. 1 materials in the temperature range of 1100 to 1250°F (600 to 675°C) for 30 min when the thickness is 1/2 in (12.7 mm)
or less, for 1 h/in (1 h/25 mm) of thickness for thickness over 1/2 to 2 in (12.7 mm to 50 mm), and 2 h plus 15 min for each additional inch of thickness over 2 in (50 mm). In this case the thickness is defined as the lesser of (1) the thickness of the weld, (2) the thinner of the pressure retaining parts being joined, or (3) for structural attachment welds, the thickness of the pressure retaining material. ASME Section III exempts P-No. 1 materials in piping systems from mandatory heat treatment based on thickness and carbon content. When the materials being joined are 11/2 in (38 mm) or less, the following exemptions apply: (1) a carbon content of 0.30 percent or less with a nominal thickness of 11/4 in (32 mm) or less,
(2) a carbon content of 0.30 percent or less with a nominal wall thickness of 11/2 in (38 mm) when a preheat of 200°F (95°C) is applied, (3) a carbon content over 0.30 percent with a nominal wall thickness of 3⁄4 in (19 mm) or less, and (4) a carbon content over 0.30 percent and a nominal wall of 11/2 in (38 mm) or less when a preheat of 200°F (95°C) is applied. ASME Section III also requires controlled heating and cooling. Above 800°F (430°C) the rate shall not exceed 400°F/h (225°C/h) divided by the maximum thickness in inches but not to exceed 400°F/h (205°C/h). The rate need not be less than 100°F/h (55°C/h).
Time and temperature recordings must be made available to the Authorized Nuclear Inspector. B31.5 requires heat treatment of P-No. 1 material greater than 3/4 in (19 mm) in the temperature range of 1100 to 1200°F (600 to 650°C) for 1 h/in (1 h/25 mm) of wall thickness with a 1 h minimum. The governing thickness is the thicker of the abutting edges for butt welds and the throat thickness for fillet socket and seal welds. Controlled heating and cooling rates are specified. B31.3 has similar requirements except that differing thickness definitions are applied to branch, fillet, and socket welds, and there are no specified heating or cooling rates. B31.4 and B31.11 both require stress relieving when the wall thickness exceeds
11⁄4 in (32 mm), or 11⁄2 in (38 mm) if a 200°F (95°C) preheat is applied. No specific temperature is specified. B31.8 on the other hand requires stress relief if the carbon content exceeds 0.32 percent, the carbon equivalent (C + 1⁄4 Mn) exceeds 0.65 percent, or the wall thickness exceeds 11⁄4 in (32 mm). Carbon steels are to be heat treated at 1100°F (600°C) or higher as stated in the qualified welding procedure. Requirements for postweld heat treatment of many different ferrous alloy steels are given in the various codes. As in the case of the carbon steels, there are variations in requirements from code to code. In the case of welding dissimilar metals, the codes most often specify that the heat treatment which invokes the higher temperature requirement be applied to the weld joint. In applying this criteria many factors should be considered. See the section ‘‘Dissimilar Metals’’ for some options. Another possibility is to take advantage of longer-time and lower-temperature heat treatments permitted by some codes.
In the end, the best source of information for specific requirements regarding heat treatment is the particular code mandated by law or contract. Where none is invoked, the various codes can be used as guides.
Anyway, thanks for all the support and start from today, I will try to continue post this blog, just to share the knowledge from Piping and Fabrication.
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