GB/T 21832.1-2018Welded austenitic-ferritic duplex stainless steel tubes and pipes - Part 1: Tubes for heat exchangers (English PDF)

4.1 Outer diameter and wall thickness

4.1.1 The outer diameter D of steel tubes shall not be greater than 203 mm; the wall thickness S shall not be greater than 8.0 mm; and their dimensions shall be as specified in GB/T 21835. Steel tubes of other outer diameters and wall thicknesses can be supplied in accordance with the requirements of the purchaser, which shall be as agreed on by the supplier and the purchaser. D - the outer diameter, in mm.

5.6.2 Underwater air-tightness

5.6.2.1 For steel tubes of outer diameter not greater than 50.8 mm, carry out underwater air-tightness test in place of hydrostatic test one by one. Test pressure for underwater air-tightness test shall not be less than 1.0 MPa; and test medium is compressed air. Under the test pressure, steel tubes shall be fully immersed in water; the standup pressure time shall not be less than 10 s; and steel tubes shall be free from leakage.

5.6.2.2 Other test pressures can be used for air-tightness test in accordance with the requirements of the purchaser, which shall be as agreed on by the supplier and the purchaser and stated in contracts.

5.6.3 Eddy current testing

The supplier can use eddy current testing to replace hydrostatic test. The artificial defects of contrast sample tubes for eddy current testing shall comply with the specifications for acceptance level E4H of GB/T 7735-2016.

5.7 Metallographic structure

5.7.1 The metallographic structure of steel tubes shall be austenitic-ferritic; the austenitic content of the base metal zones shall be 40% ~ 60%; and the austenitic content of the weld zones (the heat affected zones) shall be 35% ~ 65%.

5.7.2 Austenitic contents of other ranges can be specified, which shall be as agreed on by the supplier and the purchaser and stated in contracts.

5.8 Surface quality

5.8.1 The inner-outer surfaces of steel tubes shall be smooth, without cracks, folds, undercuts, lack of penetration or weld sags. These defects shall be removed completely; the removing depth shall not exceed the lower deviation of nominal wall thickness; the actual wall thickness at the removing spots shall not be less than the minimum value allowed by wall thickness.

5.8.2 The surfaces of steel tubes allow the existence of local scratches, indentations and pits, but their depth shall not exceed 50% of the lower deviation of wall thickness.

Those exceeding can be polished, however, the actual wall thickness at the polishing spots shall not be less than the minimum value allowed by wall thickness. Other local defects are allowed, which will not affect the minimum value allowed by wall thickness.

5.8.3 Reinforcements of inner-out welds shall comply with the following specifications.

a) not greater than 0.1 mm for steel tubes of S <= 1.0 mm;

7.1 Inspection and acceptance

The inspection and acceptance of steel tubes shall be carried out by the quality and technology supervision department of the supplier.

7.2 Group-batch rules

Except that chemical composition can be inspected and accepted by furnace, the other test items of steel tubes shall be inspected and accepted by batch. Each batch consist of steel tubes of the same designation, heat number, specification, welding process and heat treatment system (heat). The quantity of each batch of steel tubes shall not exceed those specified as follows.

a) 400 steel tubes for D <= 40 mm;

b) 200 steel tubes for 40 mm < D <= 40 mm;

c) 100 steel tubes for D > 140 mm.

7.3 Sampling quantity

The sampling quantity for all tests of each batch of steel tubes shall be as specified in Table 4.