Copper Nickel 90/10, 70/30 Buttweld Pipe Fittings Manufacturers, Suppliers – ASME B16.9 Elbows, Tees, Caps, Olets, Reducers, Stub Ends, Crosses, Laterals, Return Bends

  • We have around 30 years experience in manufacturing of Copper Nickel buttweld pipe fittings in size ranging from 1/2” to 60”, schedule from Sch5s to SchXXS. Metallica specializes in manufacturing elbows, tee, reducer, caps, crosses, stub ends, etc conforming to ASME, ANSI, and DIN standard. We also produce custom size pipe fittings made according to drawings from our clients. Our products are widely used in oil & gas, petrochemical, refineries, power plant, shipbuilding, metallurgy, paper making and other industries.
  • A Copper Nickel buttweld pipe fitting is a type of pipe fitting, which is designed to be welded on site to connect pipes together. Welding of an Copper Nickel buttweld pipe fitting to pipes or other fittings is generally done to allow change in direction of the piping system. Copper Nickel Buttweld fittings are generally identified or inquired in nominal pipe sizes with specified pipe schedule. Copper Nickel Buttweld fitting’s dimensions and tolerances are defined as per ASME B16.9 standard. This helps create full penetration weld without any extra preparation needed for the Copper Nickel buttweld fitting.
  • Manufacturing Copper Nickel buttweld fittings – A Copper Nickel buttweld pipe fitting is made by the process of hot forming that includes bending and forming to shape. The starting material of butt weld fitting is a pipe that is cut to length, heated and molded into specific shapes by means of dies. Heat treatment is also done to remove residual stresses and obtain desired mechanical properties.
  • Being one of the largest volume Copper Nickel buttweld pipe fittings manufacturer and supplier in Mumbai, we can guarantee you the lowest prices. At Metallica Metals, we believe in selling in volume and increasing our customer base across the globe, enabling us to work on low margins. We have an extensive trader network across India, USA, Europe and the Middle East, who buy from us on a regular basis.
  • In addition to traders, we have many EPC contractors, companies and end users in various industries as our customers. We are leading Copper Nickel buttweld fittings suppliers for various applications ranging from low/medium/high pressure or temperatures.
  • ASME 16.9 covers overall dimensions, tolerances, ratings, testing, and markings for factory-made wrought Copper Nickel butt-welding fittings in sizes NPS 1⁄2 through NPS 48 (DN 15 through DN 1200). The term “wrought” denotes fittings made of seamless or welded pipes, tubing, plates, or forgings. These fittings include long radius elbows, reducing elbows, long radius returns, short radius elbows, short radius 180° returns, 3D elbows, straight tees, straight crosses, reducing tees, reducing crosses, lap joint stub ends, caps, concentric reducers, and eccentric reducers.Butt Weld Pipe Fittings Manufacturers and Suppliers

Copper Nickel Buttweld Pipe Fittings Manufacturers and Suppliers

Standard Types of Copper Nickel Buttweld Fittings

Copper Nickel 180 Degree Short Radius ElbowsCopper Nickel 180 Degree Long Radius Elbows
Copper Nickel 90 Degree Short Radius ElbowsCopper Nickel 90 Degree Long Radius Elbows
Copper Nickel Reducer ConcentricCopper Nickel Reducer Eccentric
Copper Nickel Lap Joint Stub EndCopper Nickel End Cap
Copper Nickel Tee StraightCopper Nickel Tee Reducing
Copper Nickel 45 Degree ElbowsCopper Nickel 30 Degree Elbows
  1. Copper Nickel Pipe Elbows – Steel elbow is used to be installed between two lengths of pipe or tube allowing a change of direction, usually these carbon steel elbows distinguished by connection ends.
  2. Copper Nickel Pipe Bend – Pipe bend can be made of variety materials, like carbon steel, alloy steel, stainless steel, low temperature steel or high-performance steel, etc.
  3. Copper Nickel Pipe Tees – We can provide seamless tube tees and welding tube tees, which can also be divided into straight tees and reducing tees. Usually, the pipe tees have the same inlet and outlet sizes.
  4. Copper Nickel Pipe Cross – A pipe fitting, having four openings in the same place, at right angles to each other. A pipe cross is a kind of pipe fitting. It is used in the place where four pipes meet together. The pipe cross may have one inlet and three outlets, or three inlets and one outlet.
  5. Copper Nickel End Caps – End Cap, Carbon Steel cap, Stainless Steel Caps, ANSI, ISO, JIS and DIN Standards, Measures 1/2 to 56 Inches. Based on different materials, pipe caps include carbon steel cap, stainless steel cap, and alloy steel cap etc.
  6. Copper Nickel Pipe Reducer – Pipe reducer are tube fittings that are widely used in a number of industries in order to providing greatest connection flexibility in connecting fractional tubes in various installations.
  7. Copper Nickel Stub End – Stub Ends are fittings used in place of welded flanges where rotating back up flanges are desired. They are also called Lap Joints and Vanstone Flared Laps. A rotating back up flange seats itself against the back surface of the Stub End.
  8. Copper Nickel Coupling Fittings – Coupling fittings are used for fluid conduits in which each end portion of two tubes to be connected is provided with a swaged-on adapter carrying the necessary means to connect or disconnect the tubes without disturbing the swaged-on adapter.

Production & Supply Range of Copper Nickel Alloy 90/10 (C70600, 2.0872) Buttweld Fittings

GradesCU-NI-90/10 (UNS No C70600)
SpecificationASTM B366 / ASME SB366
StandardASME/ANSI B16.9, ASME B16.28, MSS-SP-43
Thickness5s, 10s, 40s, 80s, 10, 20, 40, STD, 60, 80, XS, 100, 120, 140, 160, XXS available with NACE MR 01-75
Werkstoff Nr2.0872
EquivalentsCuNi10Fe1Mn
Size1/2″ to 24″ NB to 24″NB in
TypeSeamless / Welded / Fabricated
ProductionHot-Rolled (HR) / Cold-Rolled (CR)
Surface FinishPickled, Turned or Ground
ApplicationChemicals & Fertilizers Industries, Sugar Mills, Cement Industries, Ship Building, Paper Industries, Oil & Gas, Petrochemicals, etc.

Production & Supply Range of Copper Nickel Alloy 70/30 (C71500, 2.0882) Buttweld Fittings

SpecificationASTM B366 / ASME SB366
GradesCU-NI-70/30 (UNS No C71500)
StandardASME/ANSI B16.9, ASME B16.28, MSS-SP-43
Thickness5s, 10s, 40s, 80s, 10, 20, 40, STD, 60, 80, XS, 100, 120, 140, 160, XXS available with NACE MR 01-75
Werkstoff Nr2.0882
EquivalentsCuNi30Cr2, CuNi30Mn
ProductionHot-Rolled (HR) / Cold-Rolled (CR)
Size1/2″ to 24″ NB to 24″NB in
TypeSeamless / Welded / Fabricated
Surface FinishPickled, Turned or Ground
ApplicationChemicals & Fertilizers Industries , Sugar Mills , Cement Industries , Ship Building , Paper Industries , Oil & Gas , Petrochemicals, etc.
  • Trademark Notice: Some names are trade names and/or trademarks of specific manufacturers. We are not affiliated with any manufacturer(s). Orders will be filled to meet specifications from any available source(s). Names are listed solely for reference to help identify products consistent with listed specifications.

ASTM Product Specification of Copper Nickel Alloy

B469Seamless Copper Alloy Tubes for Pressure Applications
B466Seamless Copper Nickel Pipe and Tube
B552Seamless & Welded Copper Nickel Tubes for Water Desalting
B543Welded Copper & Copper Alloy Heat Exchanger Tube
B608Welded Copper Alloy Pipe
B467Welded Copper Nickel Pipe
B151Copper Nickel Zinc Alloy (Nickel Silver) & Copper Nickel Rod & Bar
B111Copper & Copper Alloy Seamless Condenser Tubes & Ferrule Stock
B359Copper & Copper Alloy Seamless Condenser & Heat Exchanger Tubes with Integral Fins
B171Copper Alloy Plate & Sheet for Pressure Vessels, Condensers & Heat Exchangers
B122Copper Nickel Tin Alloy, Copper Nickel Zinc Alloy (Nickel Silver), & Copper Nickel Plate, Sheet, Strip & Rolled Bar

Chemical Properties of Copper Nickel 90/10 Buttweld Fittings

Element(in %)
Cu88.7
Fe1.3
Ni1.0

Mechanical Properties of Copper Nickel 90/10 Buttweld Fittings

PropertiesMetricImperial
Tensile strength, ultimate303-414 MPa43900-60000 psi
Tensile strength, yield (depending on temper)110-393 MPa16000-57000 psi
Elongation at break (in 254 mm)42.00%42.00%
Machinability (UNS C36000 (free-cutting brass) = 100%)20%20%
Shear modulus52.0 GPa7540 ksi
Poisson’s ratio0.340.34
Elastic modulus140 GPa20300 ksi

Physical Properties of Copper Nickel 90/10 Buttweld Fittings

PropertiesMetricImperial
Density8.94 g/cm30.323 lb/in3

Chemical Properties of Copper Nickel 70/30 Buttweld Fittings

Element(in %)
Cu65 min
Mn1.0 max
Pb0.05 max
Ni29 – 33 max
Fe0.4 – 1.0 max
Zn1 max

Mechanical Properties of Copper Nickel 70/30 Buttweld Fittings

PropertiesMetricImperial
Tensile strength, ultimate372-517 MPa54000-75000 psi
Tensile strength, yield (depending on temper)88.0-483 MPa12800-70100 psi
Elongation at break (in 381 mm)45.00%45.00%
Charpy impact107 J78.9 ft-lb
Machinability (UNS C36000 (free-cutting brass) = 100%)20%20%
Shear modulus57.0 GPa8270 ksi
Poisson’s ratio0.340.34
Elastic modulus150 GPa21800 ksi

Physical Properties of Copper Nickel 70/30 Forged Fittings

PropertiesMetricImperial
Density8.94 g/cm30.323 lb/in3
Melting point2140°F1171°C

What is Copper Nickel Alloy?

Cupronickel or copper-nickel (CuNi) is an alloy of copper that contains nickel and strengthening elements, such as iron and manganese. The copper content typically varies from 60 to 90 percent. (Monel metal is a nickel-copper alloy that contains a minimum of 52 percent nickel.) Despite its high copper content, cupronickel is silver in color. Cupronickel is highly resistant to corrosion by salt water, and is therefore used for piping, heat exchangers and condensers in seawater systems, as well as for marine hardware. It is sometimes used for the propellers, propeller shafts, and hulls of high-quality boats. Other uses include military equipment and chemical, petrochemical, and electrical industries.

Properties of Copper Nickel 90/10 and 70/30 Buttweld Fittings

  • The addition of nickel to copper improves strength and corrosion resistance without reducing the good ductility.
  • Copper-nickel alloys have excellent resistance to marine corrosion, strength and durability.
  • Aside from being very corrosion resistant to sea water, copper nickel alloys are very thermally stable.
  • Ductile
  • Can be hardened only by cold working
  • Low temperature co-efficient of electrical resistance.
  • The nickel content in these copper nickel alloys also enables them to retain their strength at elevated temperatures compared to copper alloys without nickel.

Difference Between Copper Nickel 90/10 and Copper Nickel 70/30

Copper-nickel alloy properties vary with composition. The two main copper nickel alloys are 90/10 (90% copper, 10% nickel) and 70/30 (70% copper, 30% nickel). The 70/30 is the stronger and has greater resistance to seawater flow, but the 90/10 will provide good service for most applications and, being less expensive, tends to be more widely used. Both alloys contain small but important additions of iron and manganese which have been chosen to provide the best combination of resistance to flowing seawater and to overall corrosion. Copper 90/10 and Copper 70/30 both show outstanding resistance to corrosion, particularly sea water; however, Copper 70/30 is stronger and has superior resistance to impingement corrosion. On the other hand, a slightly superior resistance to biofouling is exhibited by the 90/10 alloy.

Applications of Copper Nickel 90/10 and Copper Nickel 70/30 Buttweld Fittings

  • Seawater pipework
  • Offshore fire water systems
  • Heat exchangers and condensers
  • Sheathing of legs and risers on offshore platforms and boat hulls
  • Hydraulic lines
  • Fish cages for aquaculture
  • Desalination units

Types of Copper Nickel 90/10 and Carbon Nickel 70/30

Cupro Nickel 70/30 Bright BarCupro Nickel 90/10 Bright Bar
70/30 Cu-Ni Plates90/10 Cu-Ni Plate
Copper Nickel 70/30 TubesCopper Nickel 90/10 Tubes
UNS C71500 Cu-Ni FittingsUNS C70600 Cu-Ni Fittings
WERKSTOFF NR. 2.0882 FlangesWERKSTOFF NR. 2.0872 Flanges
70/30 Round Bar90/10 Round Bar
Cupro Nickel 70/30 ForgingsCupro Nickel 90/10 Forgings
Cu-Ni 70/30 PipesCu-Ni 90/10 Pipes
70/30 Copper Nickel Blocks90/10 Copper Nickel Blocks
70/30 Cupro Nickel Plate Cuttings90/10 Cupro Nickel Plate Cuttings
UNS C71500 Cu-Ni Forged FittingsUNS C70600 Cu-Ni Forged Fittings
70/30 Cupro Nickel Threaded Fittings90/10 Cupro Nickel Threaded Fittings

Testing & Inspection of CuNi 90/10 (C70600, 2.0872), CuNi 70/30 (C71500, 2.0882) Buttweld Fittings

Testing (Destructive, Non Destructive)

Positive Material Identification – PMI TestingHydrostatic Test
Chemical Analysis – Spectro AnalysisHydrogen-Induced Cracking (HIC) Test
Mechanical Testing Such as Tensile, Elongation, Reduction of AreaSulfide Stress Corrosion Cracking (SSC), NACE TM 0177
Micro TestRadiography Test
Macro TestDye Penetrant Test (DP Test)
Hardness TestUltra Sonic Test (UT)
Pitting Resistance TestEddy Current Testing
Intergranular Corrosion (IGC) TestImpact Test
Flaring TestBend Test
Flattening Test

Inspection

  • Our internal inspection reports would be provided for all the goods before shipment.
  • Buyers or their associates are most welcome to visit us for personal inspect
  • We regularly have inspections ongoing at our premises from internationally acclaimed Inspection Agencies such as TUV, BVIS, SGS, Llyods, DNV etc.

Prices for CuNi 90/10 (C70600, 2.0872), CuNi 70/30 (C71500, 2.0882) Buttweld Fittings

Prices for CuNi 90/10 (C70600, 2.0872), CuNi 70/30 (C71500, 2.0882) Buttweld Fittings are directly co related to prices of the raw materials such as iron ore, metal scrap, chromium, nickel and various other alloying elements. The production cost of heat resistant stainless steel and nickel alloy plates is high as they contain high amount of nickel and chromium. Meanwhile, mild steel material is among the lowest cost steel available, followed by carbon steel, 400 series stainless steels, aluminium, alloy steel, and 300 series stainless steel. Special alloys such as titanium, Inconel, Monel and Hastelloy cost very high as the nickel, chromium and moly content is very high.

For special and discounted prices for various types of CuNi 90/10 (C70600, 2.0872), CuNi 70/30 (C71500, 2.0882) Buttweld Fittings, please feel free to contact us through email, phone or whatsapp. We are also regularly publishing our price list for various steel products on our blog.

Documents Provided – Domestic Sales or Export of CuNi 90/10 (C70600, 2.0872), CuNi 70/30 (C71500, 2.0882) Buttweld Fittings

Certificate of OriginRaw Material Test Reports
Commercial InvoiceHeat Treatment Charts
Packing ListQuality Assurance Plan (QAP)
Fumigation CertificatesNABL approved Laboratory Test Reports
Letter of GuaranteeMaterial Test Certificates
ROHS CertificateCertificate of Compliance/Conformity

Price Terms for CuNi 90/10 (C70600, 2.0872), CuNi 70/30 (C71500, 2.0882) Buttweld Fittings

We can offer any of the following price basis depending on your requirement:

  • Ex-works
  • FOR Site in India
  • FOB Nhava Sheva
  • CFR, CIF, CPT your destination port
  • Door to Door Delivery Duty Paid as well as Duty Unpaid

Delivery Time of CuNi 90/10 (C70600, 2.0872), CuNi 70/30 (C71500, 2.0882) Buttweld Fittings

  • If the items are available in stock, we take 2 days from the date of receipt of advance payment to clear the shipment and send it to port.
  • And in-case the items are not available in stock, the time taken for manufacturing depends on the size, quantity and grade.
  • However we can claim the fact that we can compete with anyone in India on Delivery time and most of the time our production time is the least.
  • We have tied up with strong logistic partners in India which can undertake any type of shipping activities and make sure that there is no delay in shipment.

Marking & Packing of CuNi 90/10 (C70600, 2.0872), CuNi 70/30 (C71500, 2.0882) Buttweld Fittings

  • All our products will have complete marking details which are co-related to the test certificates we supply.
  • In case of orders for cut piece of Pipes, Sheets or Round Bars, the marking is transferred from the product from which it is cut. In such cases we provide Letter confirming the marking transfer and picture of the product from which it is cut.
  • All goods are packed in wooden cases or pallets.

Suppliers of CuNi 90/10 (C70600, 2.0872), CuNi 70/30 (C71500, 2.0882) Buttweld Fittings in India

Mumbai, Delhi, Bangalore, Hyderabad, Ahmedabad, Chennai, Kolkata, Surat, Pune, Jaipur, Visakhapatnam, Kanpur, Nagpur, Lucknow, Thane, Bhopal, Indore, Pimpri Chinchwad, Patna, Vadodara, Ghaziabad, Ludhiana, Agra, Nashik, Faridabad, Meerut, Rajkot, Kalyan-Dombivli, Vasai-Virar, Varanasi, Srinagar, Aurangabad, Dhanbad, Amritsar, Navi Mumbai, Allahabad, Howrah, Ranchi, Gwalior, Jabalpur, Coimbatore, Vijayawada, Jodhpur, Madurai, Raipur, Chandigarh, Guntur, Guwahati, Solapur, Hubli–Dharwad, Mysore, Tiruchirappalli, Bareilly, Moradabad, Tiruppur, Gurgaon, Aligarh, Jalandhar, Bhubaneswar, Salem, Mira-Bhayandar, Warangal, Jalgaon, Kota, Bhiwandi, Saharanpur, Gorakhpur, Bikaner, Amravati, Noida, Jamshedpur, Bhilai, Cuttack, Firozabad, Kochi, Nellore, Bhavnagar, Dehradun, Durgapur, Asansol, Rourkela, Nanded, Kolhapur, Ajmer, Akola, Gulbarga, Jamnagar, Ujjain, Loni, Siliguri, Jhansi, Ulhasnagar, Jammu, Sangli-Miraj & Kupwad, Mangalore, Erode, Belgaum, Ambattur, Tirunelveli, Malegaon, Gaya, Thiruvananthapuram, Udaipur, Kakinada, Davanagere, Kozhikode, Maheshtala, Rajpur, Sonarpur, Rajahmundry, Bokaro, South Dumdum, Bellary, Patiala, Gopalpur, Agartala, Bhagalpur, Muzaffarnagar, Bhatpara, Panihati, Latur, Dhule, Tirupati, Rohtak, Sagar, Korba, Bhilwara, Berhampur, Muzaffarpur, Ahmednagar, Mathura, Kollam, Avadi, Kadapa, Kamarhati, Sambalpur, Bilaspur, Shahjahanpur, Satara, Bijapur, Kurnool, Rampur, Shimoga, Chandrapur, Junagadh, Thrissur, Alwar, Bardhaman, Kulti, Nizamabad, Parbhani, Tumkur, Khammam, Ozhukarai, Bihar Sharif, Panipat, Darbhanga, Bally, Aizawl, Dewas, Ichalkaranji, Karnal, Bathinda, Jalna, Eluru, Barasat, Kirari Suleman Nagar, Purnia, Satna, Mau, Sonipat, Farrukhabad, Durg, Imphal, Ratlam, Hapur, Arrah, Anantapur, Karimnagar, Etawah, Ambarnath, North Dumdum, Bharatpur, Begusarai, New Delhi, Gandhidham, Baranagar, Tiruvottiyur, Pondicherry, Sikar, Thoothukudi, Rewa, Mirzapur, Raichur, Pali, Ramagundam, Haridwar, Vijayanagaram, Tenali, Nagercoil, Sri Ganganagar, Karawal Nagar, Mango, Thanjavur, Bulandshahr, Uluberia, Katni, Sambhal, Singrauli, Nadiad, Secunderabad, Naihati, Yamunanagar, Bidhannagar, Pallavaram, Bidar, Munger, Panchkula, Burhanpur, Raurkela Industrial Township, Kharagpur, Dindigul, Gandhinagar, Hospet, Nangloi Jat, Malda, Ongole, Deoghar, Chapra, Haldia, Khandwa, Nandyal, Morena, Amroha, Anand, Bhind, Bhalswa Jahangir Pur, Madhyamgram, Bhiwani, Berhampore, Ambala, Morbi, Fatehpur, Raebareli, Mahaboobnagar, Chittoor, Bhusawal, Orai, Bahraich, Vellore, Mehsana, Raiganj, Sirsa, Danapur, Serampore, Sultan Pur Majra, Guna, Jaunpur, Panvel, Shivpuri, Surendranagar Dudhrej, Unnao, Chinsurah, Alappuzha, Kottayam, Machilipatnam, Shimla. Adoni, Udupi, Katihar, Proddatur, Saharsa, Hindupur, Sasaram, Hajipur, Bhimavaram, Kumbakonam, Dehri, Madanapalle, Siwan, Bettiah, Guntakal, Srikakulam, Motihari, Dharmavaram, Gudivada, Phagwara, Narasaraopet, Suryapet, Miryalaguda, Tadipatri, Karaikudi, Kishanganj, Jamalpur, Ballia, Kavali, Tadepalligudem, Amaravati, Buxar, Jehanabad, Aurangabad, Gangtok

Exporters of CuNi 90/10 (C70600, 2.0872), CuNi 70/30 (C71500, 2.0882) Buttweld Fittings

  • America: Antigua and Barbuda, Argentina, Aruba, Bahamas, Barbados, Belize, Bermuda, Bolivia, Brazil, Canada, Cayman Islands, Chile, Colombia, Costa Rica, Cuba, Dominica, Dominican Republic, Ecuador, El Salvador, Grenada, Guatemala, Guyana, Haiti, Honduras, Jamaica, Mexico, Nicaragua, Panama, Paraguay, Peru, Puerto Rico, Suriname, Trinidad and Tobago, United States, Uruguay, Venezuela
  • Europe – Albania, Andorra, Austria, Belarus, Belgium, Bosnia and Herzegovina, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia, Euro area, Faroe Islands, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Isle of Man, Italy, Kosovo, Latvia, Liechtenstein, Lithuania, Luxembourg, Macedonia, Malta, Moldova, Monaco, Montenegro, Netherlands, Norway, Poland, Portugal, Romania, Russia, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey, Ukraine, United Kingdom
  • Africa – Algeria, Angola, Benin, Botswana, Burkina Faso, Burundi, Cameroon, Cape Verde, Central African Republic, Chad, Comoros, Congo, Djibouti, Egypt, Equatorial Guinea, Eritrea, Ethiopia, Gabon, Gambia, Ghana, Guinea, Guinea Bissau, Ivory Coast, Kenya, Lesotho, Liberia, Libya, Madagascar, Malawi, Mali, Mauritania, Mauritius, Morocco, Mozambique, Namibia, Niger, Nigeria, Republic of the Congo, Rwanda, Sao Tome and Principe, Senegal, Seychelles, Sierra Leone, Somalia, South Africa, South Sudan, Sudan, Swaziland, Tanzania, Togo, Tunisia, Uganda, Zambia, Zimbabwe
  • Asia – Afghanistan, Armenia, Azerbaijan, Bahrain, Bangladesh, Bhutan, Brunei, Cambodia, China, East Timor, Georgia, Hong Kong, Indonesia, Iran, Iraq, Israel, Japan, Jordan, Kazakhstan, Kuwait, Kyrgyzstan, Laos, Lebanon, Macao, Malaysia, Maldives, Mongolia, Myanmar, Nepal, Oman, Palestine, Philippines, Qatar, Saudi Arabia, Singapore, South Korea, Sri Lanka, Syria, Taiwan, Tajikistan, Thailand, Turkmenistan, United Arab Emirates, Uzbekistan, Vietnam, Yemen
  • Australia – Australia, Fiji, Kiribati, New Caledonia, New Zealand, Papua New Guinea, Samoa, Solomon Islands, Tonga, Vanuatu