GB/T 20409 Multi - Rifled Seamless Steel Tubes For High Pressure Boiler

GB/T 20409 Multi-rifled Seamless Steel Tubes for High-pressure Boiler
 
Application:High-quality carbon structural steel, alloy structural steel cold-drawing or cold-rolled internal thread tube for boilers suitable for producing high pressure and above pressure
 
Size:Length--8-12m
           OD--28.6-66.7mm
           WT--6.38-18.33mm
           Curvature--≤1.0mm/m
 
Surface Quality:
1.The internal and external surfaces of female threaded tubes are not allowed to have cracks, folds, knots, folds, and delaminations. These defects should be completely removed. The depth of defect removal should not exceed 10% of the wall thickness, and the actual wall thickness at the filling removal site should not be less than the wall thickness. The minimum allowable thickness deviation, the clearance should be rounded off
2.The shape of the thread should be kept intact. There must be no drawing, curling or collapse.
3.The inner and outer straight surfaces of internally threaded tubes (including non-sharpened core rods) shall be allowed to have a depth of not more than 4% of the minimum wall thickness and a maximum of 0.2 mm.
4.Other local imperfections that do not exceed the wall thickness to allow negative deviations are allowed to exist.
5.Oxide scales on the inner and outer surfaces of the internally threaded tube should be removed so that the thin oxide layer does not interfere with the inspection.
 
 

The grade and chemical composition of steel

Number	Brand	chemical composition(mass fraction)/ %
		C	Mn	Si	Cr	Mo	S	P
1	20G	0.17~0.23	0.35~0.65	0.17~0.37	--	--	≤0.020	≤0.025
2	20MnG	0.17~0.23	0.70~1.00	0.17~0.37	--	--	≤0.020	≤0.025
3	25MnG	0.22~0.29	0.7~1.00	0.17~0.37	--	--	≤0.020	≤0.025
4	12CrMoG	0.08~0.15	0.40~0.70	0.17~0.37	0.40~0.70	0.40~0.55	≤0.020	≤0.025
5	15CrMoG	0.12~0.18	0.40~0.70	0.17~0.37	0.80~1.10	0.40~0.55	≤0.020	≤0.025
Note1:Residual element content of 20G,20MnG,25MnG should satisfy: Cu≤0.20%,Cr≤0.25%,Ni≤0.25%,V≤0.08%,Mo≤0.15%；The remaining elements of other brands should satisfy: Cu≤0.20%,Ni≤0.30%. Note2:The acid soluble aluminum in 20G steel should not be more than 0.010%. Note3:The nitrogen content of the steel produced by the oxygen converter plus the refining of the furnace should not be more than 0.008%.

 

The size of the internal screw tubes and the allowable deviation of the thread parameters

Code	Project	“A” teeth types allowable deviation	“B” teeth types allowable deviation
A	O.D≤38.1mm 38.1mm＜O.D＜50.8mm 50.8mm≤O.D≤63.5mm O.D≥63.5mm	±0.15mm ±0.20mm ±0.25mm ±0.30mm	--
B	O.D≤38.1mm 38.1mm＜O.D＜50.8mm 50.8mm≤O.D≤63.5mm O.D≥63.5mm	--	±0.15mm ±0.20mm ±0.25mm ±0.30mm
C	I.D of the root of the screw Thread: O.D-(1.11×Minimum W.T×2) O.D≤38.1mm 38.1mm＜O.D＜50.8mm 50.8mm≤O.D≤63.5mm O.D≥63.5mm	±0.15mm ±0.20mm ±0.25mm ±0.30mm	±0.15mm ±0.20mm ±0.25mm ±0.30mm
E	Minimum I.D	Negative deviation to zero	Negative deviation to zero
F	Minimum W.T	+22% 0	+22% 0
G	Thread tips	--	--
I	Thread top width	--	--
	Height of thread: O.D≤38.1mm 38.1mm＜O.D＜50.8mm 50.8mm≤O.D≤63.5mm O.D≥63.5mm	±0.3mm	±0.18mm ±0.20mm ±0.23mm ±0.28mm
K	Side Angle of thread.	±15°	±15°
O	radius of rounded corner at the root of thread	--	--
M	Thread pitch	±3.2mm	±3.2mm
N	Thread lead	±3.2mm×thread tips	±3.2mm×thread tips
	Helix angle	Rating	Rating
T	Thread top width (axial)	±1.3mm	±1.3mm
U	Thread top (axial) groove width	--	--
For the B type, the maximum deviation of any thread height can exceed the allowable deviation specified in the table, but it should not exceed the allowable deviation of 0.08mm, and the average height of all threads should be in line with the given deviation range. The rating is based on the axis of the pipe.