Chapter II
Manufacturing Process and
Quality Control System
58
62 2.1 Manufacturing Process………
2.1.1 Preparation of molten iron 2.1.2 Pipe Casting
2.1.3 Finishing / coatings
2.2 Quality Control System………
2.2.1 Production procedure of ductile iron pipe 2.2.2 Quality control and inspection frequency 2.2.3 Joint performance test
2.1.1 Preparation of molten iron
2.1.1.1 Molten iron smelting
The Fe in the iron ore is reduced in the blast furnace, which will also keep suitable composition of Si, Mn, S, P, and other trace elements in molten iron.
With the storage function, the metal mixer can be utilized between the blast furnace and electric furnace for the buffering and thermal insulation, so as to obtain the suitable composition of molten iron.
The composition and temperature of molten iron can be adjusted in the electric furnace to achieve suitable values for spheroidisation.
2.1.1.2 Spheroidization and Inoculation
After the molten iron’s spheroidization through the addition of magnesium, the precipitated graphite will change its form from flake to spheroid during solidification, thus avoiding incision of flake graphite to the continuity of iron matrix, so that enabling the ductile iron with excellent ductility, flexibility and impact resistance.
Inoculation is an important procedure during production of ductile iron, through which graphite in the ductile iron can become more rounded and refined, so that the spheroidizing level can be elevated, and the pipe wall can become denser in structure; in addition, the product prasticity and toughness can be improved too.
The metal-stream inoculation is performed along with the centrifugal casting simultaneously.
Zinc coating Annealing Weighing
Spray of external
finishing layer Packing storage Composition adjustment
in the electric furnace Spheroidizing Centrifugal casting
2.1.2 Pipe Casting
2.1.2.1 Preparation of Sand Cores
The pipes with different joints are produced with pipe molds with different joint-sand-cores. The core sand mixed with resin is added into the sand core mold, tamped and solidified for the centrifugal casting.
2.1.2.2 Centrifugal casting
The high-quality ductile iron pipe is produced by the centrifugal casting process: After spheroidization, the qualified high-temperature molten iron flows through a launder into the metal mold rotating at high speed, and then is distributed on the internal surface of metal mold under the centrifugal force, finally it is solidified into a pipe. The impurities and gas in molten iron is sufficiently excluded due to the centrifugal force. The gradual inward solidification makes the pipe wall extremely dense, so as to save materials to the most and improve the mechanical properties of pipe.
2.1.2.3 Annealing
Annealing treatment is a necessary and important procedure during production of ductile iron pipe.
Generally, the elongation of ductile iron pipe by centrifugal casting is 2~5%, inferior to the requirement specified by ISO 2531. In order to enhance elongation, the cast pipe shall be annealed, that is, the pipe is heated to certain temperature in the annealing furnace, and then cooled to below certain temperature along with the proper annealing curve, and continuously cooled in ambient temperature. After the annealing treatment, the mechanical properties of ductile iron pipe can reach the indexes in the Table 1 and comply with ISO 2531:
Sand cores
Figure 1 Manufacturing Process for Ductile Iron Pipe
Annealing Mechanical properties test
Zinc coating Finished Products Warehouse
2.1.3 Finishing / coatings
2.1.3.1 Zinc spray on external wall
After the annealing treatment, the external surface of the pipe is sprayed with zinc coating complying with ISO 2531 by means of the electric arc spraying process: utilize low voltage and high current power to melt zinc wire; atomize the molten zinc with high-speed compressed air stream, and then spray it on the external wall of pipe in the form of micro droplet.
2.1.3.2 Hydrostatic Test
Each ductile iron pipe and fitting manufactured by Xinxing shall be subject to hydrostatic test in accordance with ISO 2531, qualified result of which is the prerequisite for the product’s entering into the next procedure .
2.1.3.3 Cement mortar and other lining
Taking into consideration the sanitation requirements of urban water supply to prevent water pollution against rust, it is necessary to conduct anti-corrosion protection for pipe from inside through application of internal lining. Without special requirement, all ductile iron pipes produced by Xinxing shall be internally lined with cement mortar complying with the portable water requirements.
According to the requirements of ISO 4179, Xinxing adopts the centrifugal lining process, through which the cement mortar lining surface features smoothness, uniformity and flatness, as well as high bonding strength with pipe wall, and is not easy to fall off.
Pipes with cement mortar lining centrifugally applied shall be placed in the environment with high temperature and humidity, so as to accelerate the maturation of cement mortar lining.
2.1.3.4 Finish coating, package and storage
After the spraying of finishing coating and marking, the cured pipe (DN ≤ 300) will be packed and put in storage, and then shipped to the designated location as scheduled.
Type of casting
Minimum tensile strength
Rm/MPa Minimum elongation A /%
DN80~2000 DN80~1000 DN1100~2000
Centrifugal ductile iron pipe 420 10 7
Fittings, non-centrifugal ductile
iron pipe 420 5 5
Note: 1. The yield strength (RP 0.2) can be measured by agreement between the supplier and purchaser. Where:
RP 0.2 ≥ 270MPa, when A ≥ 12% for DN80 ~ 1000; or RP 0.2 ≥ 270MPa, whenA ≥ 10% for DN ＞ 1000 and above.
RP 0.2 ≥ 300MPa under other conditions.
2. For centrifugal ductile iron pipes between DN 80 ~ 1000,when the wall thickness class is above K12, minimum elongation is 7%.
Table 1 Mechanical Properties of Ductile Iron Pipes
Inoculant Molten iron Ferrosilicon,
2.2 Quality Control System
2.2.1 Production procedure of ductile iron pipe
No. QC Item Testing Frequency Inspection Equipment Dealing with Nonconforming Products
1 Composition of molten iron
from the blast furnace Each furnace Spectrometer Return
2 Composition of molten iron
from FM furnace Each furnace Spectrometer Readjust
3 Temperature of molten iron
from FM furnace Each furnace Thermometer Readjust
4 Composition of nodulized
molten iron Each pack Spectrometer Return
5 Core appearance Full testing Visual inspection Repair or destroy
6 Core dimensions ≥ 5 pcs/shift/specification Caliper Destroy
7 Pipe mold dimensions Each piece Micrometer, dial indicator Rework 8 Casting temperature Sampling test every 3~10
pieces Thermometer Adjustment or return
9 Appearance Each piece Visual inspection Adjustment
10 Pipe weight Each piece Platform scale Adjustment
11 Furnace temperature Test along production, record
every half-hour Thermal instrumentation Adjustment
12 Chain speed Test along production, record
every half-hour Inverter Adjustment
13 Appearance Each piece Visual inspection Reject
14 Mechanical property 1 piece/batch Universal Testing Machine Reject
15 Zinc coating weight 1 time/day Electronic balance Rework
16 Pipe weight Full testing Electronic scale Reject
17 Dimensions Full testing Standard round and card
board Reject
18 Pressure Test Full testing Pressure test machine Reject
19 Ratio of cement mortar Each bucket Electronic scales, water meter Adjustment
20 Lining appearance Each piece Visual inspection Reject or rework
21 Lining thickness ≥ 1 time/hour Steel needle, steel ruler Reject or rework
22 Curing time Each furnace Watch Adjust
23 Curing temperature Each furnace Thermometer
24 Appearance Full testing Visual inspection Reject or rework
25 Film thickness ≥ 3 pcs/shift/specification Coating thickness tester Rework 26 Lining thickness ≥ 3 pcs/shift/specification Coating thickness tester Reject or rework
Tested item Test requirements Test conditions Test method
1. Positive internal hydrostatic pressure
Test pressure:(1.5PFA +5) bar Test duration: 2h No visible leakage
Joint of maximum annulus,aligned and withdrawn, with maximum shear load
Refer to ISO 2531/
Maximum pressure variation during the test: 0.09 Bar
Joint of maximum annulus, aligned and withdrawn, with maximum shear load Joint of maximum annulus, deflected
3. Positive external aligned, with maximum shear load
4. Cyclic internal hydrolic pressure
24,000 cycles
Test pressure: Between PMA and (PMA-5) bars
No visible leakage
Joint of maximum annulus, aligned and withdrawn, with maximum
shear load
a. 0.9 bar under the atmospheric pressure
W/2 W/2
2.2.2 Quality control and inspection frequency 2.2.3 Joint performance test
Each type of joint newly designed by Xinxing shall be subject to performance test strictly in accordance with ISO10804-1 and EN 545. It shall pass third party certification before put into market.
——Purpose and inclusion of performance test
It is to verify the mechanical strength and leak tightness of the joint under the most unfavorable applicable conditions of casting tolerance and joint deflection. The specific contents and requirements are listed in Table 3 for performance test of the flexible joint.
Table 3 Inclusion and Requirement of Performance Test for Pipe Joint Table 2 Quality Control and Inspection Frequency
——Performance test apparatus
Figure 2 apparatus for positive/negative internal hydrostatic pressure and cyclic internal hydraulic pressure
Figure 3 apparatus for positive external hydrostatic pressure