GB/T 3639-2021Seamless cold-drawn or cold-rolled steel tubes for precision applications (English PDF)

6.1 Dimensions

6.1.1 Steel tubes are usually delivered, based on nominal outer diameter (D) and nominal wall thickness (S). According to the requirements of the buyer AND after negotiation between the supplier and the buyer, the steel tube can also be delivered, based on the nominal outer diameter and the nominal inner diameter (d) OR the nominal inner diameter and the nominal wall thickness.

6.1.2 The dimensions of the steel tube shall meet the requirements of Table 1.

6.1.3 For steel tubes, which are delivered in cold working (+C, +LC) state, the allowable deviation of the nominal outer diameter and the nominal inner diameter shall meet the requirements of Table 1.For steel tubes, which are delivered in the heat treatment (+SR, +A, +N) state, the allowable deviation of the nominal outer diameter and the nominal inner diameter shall meet the requirements of Table 2.

6.1.4 The allowable deviation of steel tube's wall thickness is ±10%S or ±0.10 mm (whichever is larger).

6.1.5 According to the requirements of the buyer, after negotiation between the supplier and the buyer, as indicated in the contract, the steel tubes, which have dimensions and/or allowable deviations other than those specified in Table 1 and Table 2, are available.

6.2 Length

6.2.1 Ordinary length

The ordinary length of steel tube is 3000 mm ~ 12000 mm. After negotiation between the supplier and the buyer AND indicated in the contract, it may deliver the steel tubes, which have a length shorter than 3000 mm but not shorter than 2000 mm; the quantity of steel tubes shall not exceed 5% of the total quantity of the batch.

6.2.2 Fixed length and multiple length

6.2.2.1 According to the request of the buyer, after negotiation between the supplier and the buyer, as indicated in the contract, the steel tube can be delivered according to the fixed length or the multiple length. The total length of the fixed-length and multiple-length of the steel tube shall be within the ordinary length range. The allowable deviation of the fixed length shall meet the following requirements.

a) When the length is not more than 5000 mm, it is;

b) When the length is greater than 5000 mm, it is.

6.2.2.2 For steel tubes, which are delivered in multiple length, the margin of each cut shall meet the following requirements.

a) When the outer diameter is not more than 159 mm. it is 5 mm ~ 10 mm;

b) When the outer diameter is more than 159 mm. it is 10 mm ~ 15 mm.

6.3 Curvature

6.3.1 The curvature per meter of the steel tube shall not be greater than 1.5 mm/m.

7.1 Steel designation and chemical composition

7.1.1 The designation and chemical composition (melting analysis) of the steel shall meet the requirements of 10, 20, 35, 45, 25Mn in GB/T 699; the Q355B, Q420B in GB/T 1591; the 25CrMo and 42CrMo in GB/T 3077, where P, S content shall not be greater than 0.025%.

7.1.2 When the purchaser requires a finished product analysis, it shall be specified in the contract. The allowable deviation of the chemical composition of the finished steel tube shall meet the requirements of GB/T 222.

7.1.3 According to the requirements of the buyer AND after negotiation between the supplier and the buyer, other steel tubes can be supplied.

7.2 Manufacturing method

7.2.1 Steel smelting method

Steel shall be smelted, by electric arc furnace plus out-of-furnace refining OR oxygen converter plus out-of-furnace refining methods.

7.2.2 Manufacturing method of steel tube

The steel tube shall be manufactured by cold drawn or cold rolled seamless method. When the purchaser specifies a certain method to manufacture steel tubes, it shall be specified in the contract.

7.3 Delivery state

Steel tubes shall be delivered in one of the states, which are listed in Table 3.

7.4 Mechanical properties

The longitudinal mechanical properties of the steel tube, at room temperature, in the delivery state, shall meet the requirements of Table 4.

7.5 Process performance

7.5.1 Flattening

According to the requirements of the buyer, after negotiation between the supplier and the buyer, as indicated in the contract, the steel tubes, which have a ratio of wall thickness to outer diameter less than 15%, in the designations of 10, 20, Q355B, in the +A and + N states, can be subject to the flattening test.

After the specimen is flattened, the distance H between flat plates, after flattening, is calculated according to formula (1). After the test, there shall be no cracks or fractures on the specimen.

7.5.2 Flaring

According to the requirements of the buyer, after negotiation between the supplier and the buyer, as indicated in the contract, the steel tubes, which have an outer diameter of not more than 150 mm AND a wall thickness of not more than 10 mm, in the designations 10, 20, Q355B, in the state of +A and +N, can be subject to flaring test. The top core taper of the flaring test is 60°. The flaring rate of the outer diameter of the specimen shall meet the requirements of Table 5.After the test, the specimen shall not show cracks.

7.7 Surface roughness

7.7.1 For steel tubes, which have an outer diameter not greater than 260 mm, the inner and outer surface roughness Ralpha shall meet the following requirements.

a) When delivered in +SR, +A, +N states, the outer surface is not more than 4 µm;

b) When delivered in +C and +LC states, the inner and outer surfaces of the steel tube are not greater than 4 µm; wherein the requirements for the inner surface are applicable to steel tubes, which have a nominal inner diameter or a calculated inner diameter of not less than 15 mm.

7.7.2 For steel tubes, which have an outer diameter greater than 260 mm, the inner and outer surface roughness Ralpha shall not be greater than 6 µm.

7.7.3 The surface roughness is measured, in the axial direction of the steel tube, according to the arithmetic mean deviation Ralpha of the profile, which is specified in GB/T 1031, AND the recommended sampling length.

Designation Outer diameter flaring rate/% 7.7.4 According to the requirements of the buyer, after negotiation between the supplier and the buyer, as indicated in the contract, it may specify other surface roughness requirements.

7.8 Surface quality

7.8.1 The inner and outer surfaces of steel tubes shall be smooth. The depth of local pits, scratches, small scratches shall not exceed 0.08 mm. The actual wall thickness of the steel tube, at these defects, shall not be less than the minimum allowable wall thickness deviation.

7.8.2 Defects on the inner and outer surfaces of the steel tube can be removed, by appropriate methods. The actual wall thickness, at the removal location, shall not be less than the minimum allowable wall thickness deviation.

7.8.3 The inner and outer surfaces of cold-worked steel tubes may have phosphate and lubricant adhesion layers, which are formed in the manufacturing process. The inner and outer surfaces of the delivered steel tube, in the heat-treated condition, may have an oxide film layer, that does not affect the surface inspection; however, there shall be no loose oxide scale.

9.1 Inspection and acceptance

The inspection and acceptance of steel tubes are carried out, by the supplier's quality and technical supervision department.

9.2 Group-batching rules

Steel tubes are inspected and accepted in batches. Each batch shall be composed of steel tubes of the same designation, the same heat number, the same specification, the same heat treatment system (heat) or the same delivery state. The number of steel tubes in each batch shall not exceed 500.

9.3 Sampling quantity

The sampling quantity of each batch of steel tubes shall meet the requirements of Table 6.