Materials of construction.

(a) Each portable tank must be constructed of carbon or alloy steels. Materials included in part UHT in Section VIII of the ASME Code (IBR, see § 171.7 of this subchapter) or equivalent materials are not authorized. Any materials used in the tank shell must conform to a recognized national standard and must be suitable for the external environments in which the tank will be carried. The minimum elongation for any material must be 20 percent or greater.

(b) The maximum stress allowed for a material shall be determined using one of the following methods:

(1) 1.5 times the specified values for the material at 93 °C (200 °F) in Section VIII of the ASME Code;

(2) Derived by test for the actual yield and tensile strengths at 93 °C (200 °F) for the actual group of plates used to fabricate the tank using the methods described in §178.270-3(d); or

(3) Derived from the minimum yield and tensile strengths at 93 °C (200 °F) specified by the national standard to which the material is manufactured using the methods described in § 178.270-3(d).

(c) Maximum allowable stress values, derived for an actual group of plates, that are based on actual tensile and yield strengths of the material at 93 °C (200 °F) shall not be greater than 120 percent of the specified minimum yield and tensile strength specified in the national standard to which the material is manufactured.

(d) The maximum allowable stress values must be derived from the following criteria:

(1) For austenitic steels;

(i) When the yield strength is determined using the 0.2 percent offset, 93.75 percent of the yield strength.

(ii) When the yield strength is determined using the 1.0 percent offset, 75 percent of the yield strength.

(2) For carbon and low alloy steels, the yield strength is determined using the 0.2 percent offset. The maximum allowable stress value is the lower of 93.75 percent of the yield strength or 37.5 percent of the tensile strength.

(e) For purposes of these specifications, tensile strength, yield strength and elongation must be determined using a specimen having a gauge length:

L0 = 5.65(S0)1/2

where: L0=the gauge length of the specimen—millimeters (inches); and S0=the cross sectional area of the specimen—square millimeters (square inches).

Tensile tests and analysis of results must be in accordance with ISO 82, “Steels-Tensile Testing” (IBR, see § 171.7 of this subchapter). The yield strength in tension shall be the stress corresponding to a permanent strain of 0.2 percent of the gauge length, except that for high alloy austenitic steels the yield strength shall be the stress corresponding to a permanent strain of 0.2 or 1.0 percent of the gauge length as appropriate. The elongation must be at least 20 percent.

(f) If maximum allowable stress values or minimum tank wall thicknesses are based on the actual yield strength, the actual tensile strength, or the actual elongation for the material used to fabricate the tank, the test records or certification of test results by the material producer or tank manufacturer must be approved by the approval agency, retained by the tank manufacturer for a period not less than 15 years, and made available to any duly identified representative of the Department or the owner of the tank.

[Amdt. 178-65, 46 FR 9895, Jan. 29, 1981, as amended at 64 FR 10782, Mar. 5, 1999; 68 FR 75750, Dec. 31, 2003]