49 CFR § 178.50 - Specification 4B welded or brazed steel cylinders.
(a) Type, size, pressure, and application. A DOT 4B is a welded or brazed steel cylinder with longitudinal seams that are forged lap-welded or brazed and with water capacity (nominal) not over 1,000 pounds and a service pressure of at least 150 but not over 500 psig. Cylinders closed in by spinning process are not authorized.
(b) Steel. Open-hearth, electric or basic oxygen process steel of uniform quality must be used. Content percent may not exceed the following: Carbon, 0.25; phosphorus, 0.045; sulphur, 0.050. The cylinder manufacturer must maintain a record of intentionally added alloying elements.
(c) Identification of material. Pressure-retaining materials must be identified by any suitable method that does not compromise the integrity of the cylinder. Plates and billets for hotdrawn cylinders must be marked with the heat number.
(d) Manufacture. Cylinders must be manufactured using equipment and processes adequate to ensure that each cylinder produced conforms to the requirements of this subpart. No defect is permitted that is likely to weaken the finished cylinder appreciably. A reasonably smooth and uniform surface finish is required. Exposed bottom welds on cylinders over 18 inches long must be protected by footrings. Welding procedures and operators must be qualified in conformance with CGA C-3 (IBR, see § 171.7 of this subchapter). Seams must be made as follows:
(1) Brazing materials. Brazing materials must be by copper brazing, by copper alloy brazing, or by silver alloy brazing. Copper alloy composition must be: Copper, 95 percent minimum; Silicon, 1.5 percent to 3.85 percent; Manganese, 0.25 percent to 1.10 percent.
(2) Brazed circumferential seams. Heads attached by brazing must have a driving fit with the shell, unless the shell is crimped, swedged, or curled over the skirt or flange of the head, and be thoroughly brazed until complete penetration by the brazing material of the brazed joint is secured. Depth of brazing of the joint must be at least four (4) times the minimum thickness of shell metal.
(3) Welded circumferential seams. Circumferential seams are permitted by the welding process.
(4) Longitudinal seams in shells. Longitudinal seams must be a forged lap joint design. When brazed, the plate edge must be lapped at least eight (8) times the thickness of the plate, laps being held in position, substantially metal to metal, by riveting or electric spot-welding; brazing must be done by using a suitable flux and by placing brazing material on one side of seam and applying heat until this material shows uniformly along the seam of the other side.
(e) Welding or brazing. Only the attachment of neckrings, footrings, handles, bosses, pads, and valve protection rings to the tops and bottoms of cylinders by welding or brazing is authorized. Attachments and the portion of the cylinder to which they are attached must be made of weldable steel, the carbon content of which may not exceed 0.25 percent except in the case of 4130X steel, which may be used with proper welding procedure.
(1) For cylinders with outside diameters over 6 inches, the minimum wall thickness must be 0.090 inch. In any case, the minimum wall thickness must be such that calculated wall stress at minimum test pressure (paragraph (i)(4) of this section) may not exceed the following values:
(2) Calculation must be made by the formula:
(g) Heat treatment. Cylinder heads, bodies or the completed cylinder, formed by drawing or pressing, must be uniformly and properly heat treated by an applicable method shown in table 1 of appendix A of this part before tests.
(h) Opening in cylinders. Openings in cylinders must comply with the following:
(1) Any opening must be placed on other than a cylindrical surface.
(3) Each opening in a cylindrical type cylinder, except those for pressure relief devices, must be provided with a fitting, boss, or pad, securely attached to container by brazing or by welding.
(4) If threads are used, they must comply with the following:
(i) Threads must be clean cut, even without checks, and tapped to gauge.
(ii) Taper threads must be of a length not less than as specified for American Standard taper pipe threads.
(iii) Straight threads, must have at least four (4) engaged threads, must have tight fit and a calculated shear strength at least ten (10) times the test pressure of the cylinder; gaskets are required for straight threads and must be of sufficient quality to prevent leakage.
(5) The closure of a fitting, boss, or pad must be adequate to prevent leakage.
(i) Pressure testing. Each cylinder must successfully withstand a pressure test as follows: (1) Lot testing.
(i) At least one (1) cylinder randomly selected out of each lot of 200 or fewer must be tested by the water jacket or direct expansion method as prescribed in CGA C-1 (IBR; see § 171.7 of this subchapter). The testing equipment must be calibrated as prescribed in CGA C-1. All testing equipment and pressure indicating devices must be accurate within the parameters defined in CGA C-1.
(iii) The minimum test pressure must be maintained at least 30 seconds and sufficiently longer to ensure complete expansion. Any internal pressure applied after heat-treatment and prior to the official test may not exceed 90 percent of the test pressure. If, due to failure of the test apparatus or operator error, the test pressure cannot be maintained, the test may be repeated in accordance with CGA C-1, section 5.7.2.
(iv) Permanent volumetric expansion may not exceed 10 percent of the total volumetric expansion at test pressure.
(2) Pressure testing.
(i) The remaining cylinders in the lot must be tested by the proof pressure, water-jacket, or direct expansion test method as prescribed in CGA C-1. The minimum test pressure must be maintained for the specific timeframe and the testing equipment must be calibrated as prescribed in CGA C-1. Further, all testing equipment and pressure indicating devices must be accurate within the parameters defined in CGA C-1. If, due to failure of the test apparatus or operator error, the test pressure cannot be maintained, the test may be repeated in accordance with CGA C-1, sections 5.7.2 or 7.1.2, as appropriate. Determination of expansion properties is not required.
(j) Mechanical test. A mechanical test must be conducted to determine yield strength, tensile strength, elongation as a percentage, and reduction of area of material as a percentage as follows:
(1) Testing is required on two (2) specimens removed from one (1) cylinder, or part thereof, heat-treated as required, as illustrated in appendix A to this subpart. For lots of 30 or fewer, mechanical tests are authorized to be made on a ring at least 8 inches long removed from each cylinder and subjected to the same heat treatment as the finished cylinder.
(2) Specimens must comply with the following:
(i) When a cylinder wall is 3/16 inch thick or less, one the following gauge lengths is authorized: A gauge length of 8 inches with a width not over 1 1/2 inches, a gauge length of 2 inches with a width not over 1 1/2 inches, or a gauge length at least twenty-four (24) times the thickness with a width not over six (6) times the thickness.
(ii) The specimen, exclusive of grip ends, may not be flattened. Grip ends may be flattened to within one inch of each end of the reduced section.
(iii) When the size of a cylinder does not permit securing straight specimens, the specimens may be taken in any location or direction and may be straightened or flattened cold, by pressure only, not by blows. When specimens are taken and prepared using this method, the inspector's report must show detailed information regarding such specimens in connection with the record of mechanical tests.
(iv) Heating of a specimen for any purpose is not authorized.
(3) The yield strength in tension must be the stress corresponding to a permanent strain of 0.2 percent of the gauge length. The following conditions apply:
(i) The yield strength must be determined by either the “offset” method or the “extension under load” method as prescribed in ASTM E 8 (IBR, see § 171.7 of this subchapter).
(ii) In using the “extension under load” method, the total strain (or “extension under load”) corresponding to the stress at which the 0.2 percent permanent strain occurs may be determined with sufficient accuracy by calculating the elastic extension of the gauge length under appropriate load and adding thereto 0.2 percent of the gauge length. Elastic extension calculations must be based on an elastic modulus of 30,000,000. In the event of controversy, the entire stress-strain diagram must be plotted and the yield strength determined from the 0.2 percent offset.
(iii) For strain measurement, the initial strain reference must be set while the specimen is under a stress of 12,000 psig, and strain indicator reading must be set at the calculated corresponding strain.
(iv) Cross-head speed of the testing machine may not exceed 1/8 inch per minute during yield strength determination.
(v) The yield strength must not exceed 73 percent of the tensile strength.
(k) Elongation. Mechanical test specimens must show at least a 40 percent elongation for a 2-inch gauge length or at least 20 percent in other cases. However, elongation percentages may be reduced numerically by 2 percent for 2-inch specimens, and by 1 percent in other cases, for each 7,500 psig increase of tensile strength above 50,000 psig. The tensile strength may be incrementally increased by four increments of 7,500 psig for a maximum total of 30,000 psig.
(l) Flattening test -
(1) Cylinders. After pressure testing, a flattening test must be performed on one cylinder taken at random out of each lot of 200 or fewer by placing the cylinder between wedge-shaped knife edges having a 60 degree included angle, rounded to a half-inch radius. The longitudinal axis of the cylinder must be at a 90-degree angle to knife edges during the test. For lots of 30 or fewer, flattening tests are authorized to be performed on a ring of at least 8 inches long removed from each cylinder and subjected to the same heat treatment as the finished cylinder.
(2) Pipes. When cylinders are constructed of lap welded pipe, an additional flattening test is required, without evidence of cracking, up to six (6) times the wall thickness. In such case, the rings (crop ends) removed from each end of the pipe, must be tested with the weld 45 °F or less from the point of greatest stress.
(m) Acceptable results for flattening tests. There must be no evidence of cracking of the sample when it is flattened between flat plates to no more than six (6) times the wall thickness. If this test fails, one additional sample from the same lot may be taken. If this second sample fails, the entire lot must be rejected.
(n) Rejected cylinders. Reheat treatment is authorized for a rejected cylinder in accordance with this paragraph (n). After reheat treatment, a cylinder must pass all prescribed tests in this section to be considered acceptable. Repair of brazed seams by brazing and welded seams by welding is authorized. For cylinders with an outside diameter of less than or equal to six (6) inches, welded seam repairs greater than one (1) inch in length shall require reheat treatment of the cylinder. For cylinders greater than an outside diameter of 6 inches, welded seam repairs greater than three (3) inches in length shall require reheat treatment.
(i) On shoulders and top heads whose wall thickness is not less than 0.087-inch thick;
(ii) On side wall adjacent to top head for side walls which are not less than 0.090 inch thick;
(iv) On a metal plate attached to the top of the cylinder or permanent part thereof; sufficient space must be left on the plate to provide for stamping at least six retest dates; the plate must be at least 1/16-inch thick and must be attached by welding, or by brazing. The brazing rod must melt at a temperature of 1100 °F. Welding or brazing must be along all the edges of the plate;
(v) On the neck, neckring, valve boss, valve protection sleeve, or similar part permanently attached to the top of the cylinder; or
(2) Embossing the cylinder head or sidewall is not permitted.