Welding of Ferrous Piping Materials is the topics that Piping & Fabrication will explore and please enjoy it.
Carbon Steels. Carbon steels are classed as P-No.1 by ASME Section IX. The vast majority of carbon steel pipe is used for services below 775°F (413°C). Joints are most often V bevels with commercial backing rings or open butt roots and are welded out with SMAW, SAW, GMAW, and FCAW. For services which require high quality, GTAW root welds with SMAW, SAW, and FCAW weld-outs are most prevalent. Most carbon steel filler metal is produced to weld 60,000- and 70,000-psi material. More often than not fabricators use the 70,000-psi filler for all carbon steel welding. For SMAW the most popular electrode is E-7018, although for open-butt root pass welding using SMAW, E-6010 is still the choice.
FCAW welding is rapidly replacing SMAW because it can deposit at a much higher rate. Preheating and postweld heat treating are required depending on the carbon content and wall thickness. For typical preheat and postweld heat treatment requirements. When working to a specific code, be sure to use the requirements found in that code.
Carbon Molybdenum Steels. Carbon molybdenum steels are classed as P-No 3. Currently this material has very little use because of unfavorable experience with graphitization at temperatures over 800°F (427°C).
Chromium Molybdenum Steels. The chromium molybdenum steels are primarily used for service temperatures from 800 to 1050°F (427 to 565°C). They range from 1⁄₂ Cr-1⁄₂ Mo to 9 CR-1 Mo-V and are classed by ASME Section IX as P-No. 3, P-No. 4, and P-No. 5 A and 5 B. The preponderance of usage is in the 11⁄4 Cr-1⁄2 Mo-Si and 21⁄ Cr-1 Mo grades. Welding usually consists of GTAW root welds with filler metal added or preplaced inserts. The balance of the weld is made by SAW for welds which can be performed in the 1G position and SMAW for fixed position welds. FCAW is rapidly overtaking SMAW for these materials also.
The 9Cr-1Mo-V material is a relatively recent addition to the list of chromium molybdenum steels for use in high-temperature service. Its great advantage over other chrome moly steels is its high-temperature strength. It has allowable stresses comparable with those of austenitic stainless steels. This results in a lesser wall thickness and consequently less weight to support and considerably less volume of filler material. A tighter line configuration can be anticipated because the lesser section modulous will result in smaller reactions at the terminals due to expansion loadings. This material also has an advantage over austenitic stainless steels in that its coefficient of thermal expansion is less than that of the stainlesses, again resulting in lower end reactions for the same configuration. On the down side, 9Cr-1Mo-V is typically amartensitic structure at room temperature and requires great care in bending, welding, and postbending and welding heat treatment.
For hot bending, a temperature of 1740 to 1920°F (950 to 1050°C) is preferred. Bending in the temperature range of 1560 to 1740°F (850 to 950°C) should be avoided. After hot bending, a normalize at 1900 to 1990°F (1040 to 1090°C) is required to put carbides back into solution. The normalize is followed by a tempering heat treatment between 1350 and 1440_F (730 and 780_C). Both are followed by cooling in still air.
Welding is extremely critical. The latest ASME Section II Part C, lists 9Cr-1Mo- V filler materials. SFA 5.5 lists E9018-B9 for SMAW electrodes, and SFA 5.28 lists ER90S-B9 for rods and electrodes for gas-shielded welding. Storage and handling of electrodes is very critical (see Filler Metals). Preheat and interpass temperatures and postwelding cooling should be scrupulously observed (see Preheat and Interpass Temperature). Postwelding stress relief is a necessity. The current ASME B31.1 Code requires a range of 1300 to 1400°F (700 to 760°C), but some literature indicates that a range of 1360 to 1440°F (740 to 780°C) may be more desirable for reasonable hardness and good ductility. The time at temperature should be 1 h per in of thickness, and heating and cooling rates above 800°F (427°C) should be limited to 100°F (55°C) per h.
Martensitic and Ferritic Stainless Steels. The martensitic and ferritic grades of stainless steels are not often encountered in piping systems. They are a group of steels with chromium contents ranging from 11.5 to 30 percent. Martensitic stainless steels are those which are capable of transformation to martensite under most cooling conditions and therefore can be hardened. Ferritic stainless steels on the
other hand contain sufficient chromium and other ferrite formers such as aluminium, niobium, molybdenum, and titanium so that they cannot be hardened by heat treatment. ASME Section IX classes martensitic stainless steels as P-No.6 and ferritic stainless steels as P-No.7. The user should consult the Welding Handbook16 for suggested welding processes and the applicable code for specific preheating and postweld heat-treatment requirements.
Austenitic Stainless Steels. Austenitic stainless steels are classed as P No. 8. Piping systems of austenitic stainless steels represent a fairly significant proportion of a fabricator’s and/or installer’s work, since they appear in nuclear power plants, chemical plants, paper mills, food processing facilities, and other applications where cleanliness and corrosion resistance are mandatory and even in fossil power plants where their high-temperature properties are needed. Most root welding is done by the GTAW process, and the inside of the root is protected by purging with argon, helium, or nitrogen to prevent formation of hard chromic oxides. GTAW is used for weld-out in lighter walls, and combinations of GTAW, SMAW, and SAW are used for heavier sections. Filler metal must contain some ferrite to preclude microfissuring as described in the section ‘‘Filler Metals.’’ To minimize the precipitation of carbides (sensitization) during welding, interpass temperatures are usually limited to 300 to 350°F (150 to 175°C). Heat treatment after welding is not mandatory. For corrosion services, heating during fabrication could be detrimental since it would serve to enhance sensitization. The effects of sensitization can be mitigated by a carbide solution heat treatment as described in the section ‘‘Heat Treatment.’’ Low-carbon grades of stainless steels welded with L grade electrodes are also used in services where sensitization can be a problem.
Low-Temperature Steels. The term low-temperature steel is applied to a variety of steels which exhibit good notch toughness properties at temperatures down to cryogenic levels. The B31.1 and B31.3 Codes permit the use of most steel down to -20°F (-29°C). Below this, certain grades of carbon and nickel steel with good toughness and austenitic stainless steels are needed. Welding procedures and welding filler metals must be tested to assure suitability for the intended service. B31.3 gives details of such requirements. Root pass welding using GTAW, with SMAW and SAW weldout, is commonly used. Some FCAW is used in the carbon steels and low-nickel steels. A preheat of 200°F (95°C) is suggested by B31.3 for low-nickel steels followed by a postweld heat treatment consisting of a stress relieve at 1100 to 1175°F (600 to 630°C) when the wall exceeds 3⁄₄ in (19 mm). For 9 percent nickel steel a preheat of 50°F (10°C) and a stress relieve at 1025 to 1085°F (552 to 585°C) followed by cooling at a rate greater than 300°F/h (167°C/h) down to 600°F (316°C) is required. Certain nonferrous materials are also suitable for low-temperature service.
Carbon Steels. Carbon steels are classed as P-No.1 by ASME Section IX. The vast majority of carbon steel pipe is used for services below 775°F (413°C). Joints are most often V bevels with commercial backing rings or open butt roots and are welded out with SMAW, SAW, GMAW, and FCAW. For services which require high quality, GTAW root welds with SMAW, SAW, and FCAW weld-outs are most prevalent. Most carbon steel filler metal is produced to weld 60,000- and 70,000-psi material. More often than not fabricators use the 70,000-psi filler for all carbon steel welding. For SMAW the most popular electrode is E-7018, although for open-butt root pass welding using SMAW, E-6010 is still the choice.
FCAW welding is rapidly replacing SMAW because it can deposit at a much higher rate. Preheating and postweld heat treating are required depending on the carbon content and wall thickness. For typical preheat and postweld heat treatment requirements. When working to a specific code, be sure to use the requirements found in that code.
Carbon Molybdenum Steels. Carbon molybdenum steels are classed as P-No 3. Currently this material has very little use because of unfavorable experience with graphitization at temperatures over 800°F (427°C).
Chromium Molybdenum Steels. The chromium molybdenum steels are primarily used for service temperatures from 800 to 1050°F (427 to 565°C). They range from 1⁄₂ Cr-1⁄₂ Mo to 9 CR-1 Mo-V and are classed by ASME Section IX as P-No. 3, P-No. 4, and P-No. 5 A and 5 B. The preponderance of usage is in the 11⁄4 Cr-1⁄2 Mo-Si and 21⁄ Cr-1 Mo grades. Welding usually consists of GTAW root welds with filler metal added or preplaced inserts. The balance of the weld is made by SAW for welds which can be performed in the 1G position and SMAW for fixed position welds. FCAW is rapidly overtaking SMAW for these materials also.
The 9Cr-1Mo-V material is a relatively recent addition to the list of chromium molybdenum steels for use in high-temperature service. Its great advantage over other chrome moly steels is its high-temperature strength. It has allowable stresses comparable with those of austenitic stainless steels. This results in a lesser wall thickness and consequently less weight to support and considerably less volume of filler material. A tighter line configuration can be anticipated because the lesser section modulous will result in smaller reactions at the terminals due to expansion loadings. This material also has an advantage over austenitic stainless steels in that its coefficient of thermal expansion is less than that of the stainlesses, again resulting in lower end reactions for the same configuration. On the down side, 9Cr-1Mo-V is typically amartensitic structure at room temperature and requires great care in bending, welding, and postbending and welding heat treatment.
For hot bending, a temperature of 1740 to 1920°F (950 to 1050°C) is preferred. Bending in the temperature range of 1560 to 1740°F (850 to 950°C) should be avoided. After hot bending, a normalize at 1900 to 1990°F (1040 to 1090°C) is required to put carbides back into solution. The normalize is followed by a tempering heat treatment between 1350 and 1440_F (730 and 780_C). Both are followed by cooling in still air.
Welding is extremely critical. The latest ASME Section II Part C, lists 9Cr-1Mo- V filler materials. SFA 5.5 lists E9018-B9 for SMAW electrodes, and SFA 5.28 lists ER90S-B9 for rods and electrodes for gas-shielded welding. Storage and handling of electrodes is very critical (see Filler Metals). Preheat and interpass temperatures and postwelding cooling should be scrupulously observed (see Preheat and Interpass Temperature). Postwelding stress relief is a necessity. The current ASME B31.1 Code requires a range of 1300 to 1400°F (700 to 760°C), but some literature indicates that a range of 1360 to 1440°F (740 to 780°C) may be more desirable for reasonable hardness and good ductility. The time at temperature should be 1 h per in of thickness, and heating and cooling rates above 800°F (427°C) should be limited to 100°F (55°C) per h.
Martensitic and Ferritic Stainless Steels. The martensitic and ferritic grades of stainless steels are not often encountered in piping systems. They are a group of steels with chromium contents ranging from 11.5 to 30 percent. Martensitic stainless steels are those which are capable of transformation to martensite under most cooling conditions and therefore can be hardened. Ferritic stainless steels on the
other hand contain sufficient chromium and other ferrite formers such as aluminium, niobium, molybdenum, and titanium so that they cannot be hardened by heat treatment. ASME Section IX classes martensitic stainless steels as P-No.6 and ferritic stainless steels as P-No.7. The user should consult the Welding Handbook16 for suggested welding processes and the applicable code for specific preheating and postweld heat-treatment requirements.
Austenitic Stainless Steels. Austenitic stainless steels are classed as P No. 8. Piping systems of austenitic stainless steels represent a fairly significant proportion of a fabricator’s and/or installer’s work, since they appear in nuclear power plants, chemical plants, paper mills, food processing facilities, and other applications where cleanliness and corrosion resistance are mandatory and even in fossil power plants where their high-temperature properties are needed. Most root welding is done by the GTAW process, and the inside of the root is protected by purging with argon, helium, or nitrogen to prevent formation of hard chromic oxides. GTAW is used for weld-out in lighter walls, and combinations of GTAW, SMAW, and SAW are used for heavier sections. Filler metal must contain some ferrite to preclude microfissuring as described in the section ‘‘Filler Metals.’’ To minimize the precipitation of carbides (sensitization) during welding, interpass temperatures are usually limited to 300 to 350°F (150 to 175°C). Heat treatment after welding is not mandatory. For corrosion services, heating during fabrication could be detrimental since it would serve to enhance sensitization. The effects of sensitization can be mitigated by a carbide solution heat treatment as described in the section ‘‘Heat Treatment.’’ Low-carbon grades of stainless steels welded with L grade electrodes are also used in services where sensitization can be a problem.
Low-Temperature Steels. The term low-temperature steel is applied to a variety of steels which exhibit good notch toughness properties at temperatures down to cryogenic levels. The B31.1 and B31.3 Codes permit the use of most steel down to -20°F (-29°C). Below this, certain grades of carbon and nickel steel with good toughness and austenitic stainless steels are needed. Welding procedures and welding filler metals must be tested to assure suitability for the intended service. B31.3 gives details of such requirements. Root pass welding using GTAW, with SMAW and SAW weldout, is commonly used. Some FCAW is used in the carbon steels and low-nickel steels. A preheat of 200°F (95°C) is suggested by B31.3 for low-nickel steels followed by a postweld heat treatment consisting of a stress relieve at 1100 to 1175°F (600 to 630°C) when the wall exceeds 3⁄₄ in (19 mm). For 9 percent nickel steel a preheat of 50°F (10°C) and a stress relieve at 1025 to 1085°F (552 to 585°C) followed by cooling at a rate greater than 300°F/h (167°C/h) down to 600°F (316°C) is required. Certain nonferrous materials are also suitable for low-temperature service.
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