Martensitic stainless steel seamless pipe KL-HP12CR for pipelines has excellent weldability, mechanical properties and corrosion resistance. The weldability is improved by reducing the carbon and nitrogen content. Reducing carbon content also significantly improves carbon dioxide corrosion resistance, and the corrosion rate is lower than 0.127 mm/a in carbon dioxide environments at temperatures up to 160°C and 2.0 MPa. Due to the addition of molybdenum, the sulfide corrosion resistance (SSC) is improved. This new type of steel pipe can be used in hydrogen sulfide environments with pH values of 4.0 and 0.001 MPa. This type of steel pipe has a strength of X80 grade and has sufficient low-temperature toughness when it is actually used in pipelines. Post-weld heat treatment for several minutes, lower carbon content, and addition of titanium can effectively prevent intergranular stress corrosion cracking (IGSCC) in the heat affected zone. This kind of steel tube is expected to be further used to transport liquids containing corrosive gases. For example, carbon dioxide is a low-cost, economical material with a high life cycle.
People are increasingly concerned about the reduction of oil resources. The temperatures and pressures of the oil wells and gas wells currently being mined have reached unprecedented heights. The liquids produced have carbon dioxide, which causes more corrosion. Therefore, when the liquid is to be transported before the corrosive substance and water are removed, it becomes extremely important to prevent the flow line and the collecting line pipe from being corroded by carbon dioxide. In addition, these liquids usually contain trace amounts of hydrogen sulfide and therefore also need to prevent chloride stress cracking. In such a corrosive environment, for the use of carbon steel as a pipeline material, the conventional anti-corrosion method is to inject a rust inhibitor into the liquid and use a rust inhibitor to prevent corrosion. However, this increases production costs, especially in offshore pipelines, so rust inhibitors are less used, especially considering life-cycle costs. Another reason not to use rust inhibitors is to worry about pollution caused by leakage accidents. Therefore, there is a need for an economical material that does not require the use of rust inhibitors. Existing corrosion resistant alloys for pipelines, including duplex stainless steels, have the disadvantage of high material costs. In contrast, martensitic stainless steels are generally less weldable and require preheating and longer post-weld heat treatment. Therefore, considering the pipe laying efficiency, martensitic stainless steel is rarely used in pipelines. However, martensitic stainless steels have appropriate carbon dioxide corrosion resistance and are less expensive than duplex stainless steels.
To this end, a Japanese steel company adopts a large number of steelmaking technology measures, such as reducing the content of carbon and nitrogen, controlling and adding alloying elements to improve the weldability of martensitic stainless steels, and developing good weldability and corrosion resistance. Martensitic stainless steel seamless pipe for pipeline.
1 The process of development
1.1 target characteristics
The goal of development is as follows:
(1) Weldability: Welding does not require preheating;
(2) Maximum heat-affected zone hardness: HV350 or lower;
(3) Carbon dioxide corrosion resistance: resistance to 5% NaCl, partial pressure of carbon dioxide of 3.0 MPa, corrosion environment at 150 °C;
(4) Sulfur sulfide stress corrosion resistance (SSC): resistant to 5% NaCl, 0.001 MPaH2S, pH 4.0;
(5) Strength: X80 grade (550MPa or higher yield strength);
(6) Low temperature toughness: 100 J or higher Charpy impact toughness absorption energy at -40°C.
1.2 chemical composition design
The design of the chemical composition of the steel pipe shall take into account the influence of the alloying elements on the weldability, corrosion resistance, hot workability and other characteristics of the martensitic stainless steel. In particular, the study of weldability was based on the chemical composition of KO-13Cr (0.20C-13Cr-0.03N) used for petroleum pipes in a carbon dioxide environment while maintaining the same corrosion resistance in the base material. According to the results of Table 1 on the effect of chemical composition on hot workability and other properties, the chemical composition of this steel was finally determined as 12Cr-5Ni-2Mo-0.01N, 0.015C or less.
1.2.1 Weldability
Because martensitic stainless steels tend to have weld cracks during welding, preheating is required in practice to prevent cracking. Welding cracks are caused by hydrogen dissolved in the weld metal and the weld heat affected zone, and hardening and residual stress induced by martensitic transformation in the heat affected zone. Therefore, an effective means for preventing welding cracks in terms of materials is to reduce the content of carbon and nitrogen to suppress hardening induced by martensitic transformation. Table 1 shows the results of a Y-shaped groove weld crack resistance test for low C+N martensitic stainless steels. The steel used in the crack resistance test contains 0.03% carbon or nitrogen. At the same time, both the carbon and nitrogen in the steel will be as low as 0.01%, and the steel will not be subjected to a cracking test and will be preheated at 30°C. The results show that if the content of carbon and nitrogen is reduced to 0.01%, welding without preheating is possible. Existing steelmaking techniques can reduce the carbon and nitrogen content to such low levels.
Table 1 Y-groove welding crack resistance test results of low C+N martensitic stainless steel
material
pre-heat temperature
30°C
70°C
100°C
0.03C-0.01N
11Cr-1.0Ni-0.5Cu
Cracked
Cracked
Cracked
0.01C-0.03N
11Cr-1.0Ni-0.5Cu
Cracked
Cracked
Cracked
0.01C-0.01N
12Cr-1.0Ni-0.5Cu
No crack
No crack
No crack
12Cr-1.0Ni-1.0Cu
No crack
No crack
No crack
12Cr-2.0Ni-0.5Cu
No crack
No crack
No crack
Board thickness: 15mm
Welding material: 410HSMAW, 4 Ф (diffusible hydrogen; 4.28cm3/100g)
Welding conditions: Current: 160A
Voltage: 24 to 26V
Speed: 150mm/min
Test conditions: room temperature: 30°C,
Humidity: 60% RH
1.2.2 Carbon dioxide corrosion resistance
Lowering the carbon content also improves the carbon dioxide corrosion resistance of the steel. Tests have shown that different chemical compositions of martensitic stainless steels have different carbon dioxide corrosion resistance. The relationship between corrosion rate and carbon dioxide index is determined by Cr-10C+2Ni. Increasing the chromium and nickel content and reducing the carbon content improve the carbon dioxide corrosion resistance of the steel. This is presumably because lowering the carbon content reduces the content of chromium carbide, thereby increasing the amount of chromium dissolved, thereby effectively preventing corrosion.
1.2.3 Sulfide Resistance Stress Corrosion Resistance
Since the sulfide stress corrosion of martensitic stainless steels originates from pitting corrosion and improves the pitting resistance, the sulfide corrosion cracking resistance can be improved. It is known that the alloying element molybdenum improves the pitting resistance of the steel. Tests have shown that there is no difference in the test results of increasing nickel content from 4% to 5%, while increasing molybdenum content from 1% to 2%, the occurrence of sulfide stress cracking tends to low pH, high partial pressure of hydrogen sulfide , Or more harsh environment. This phenomenon shows that adding 1% of molybdenum can fully ensure the sulfide corrosion resistance under the environment of 5% NaCl, 0.001 MPaH2S, and pH 4.0, which is the goal of developing such steel. However, since the pitting resistance of the heat-affected zone may be lower than that of the base metal, adding 2% of molybdenum ensures stable pitting resistance.
2 characteristics of the new type of steel pipe
The characteristics of the newly-developed steel pipe were tested. The sample was a seamless pipe with an outer diameter of 273 mm and a wall thickness of 12.7 mm. The chemical composition was listed in Table 2 and quenched and tempered to obtain the X80 grade product. . This product and the use of 25Cr duplex stainless steel as welding material, the first pass with gas tungsten arc welding (GTAW), the second pass gas metal arc welding (GMAW) for the annular weld. The chemical composition of the welding material is shown in Table 2, and the welding conditions are shown in Table 3. No preheating or post-weld heat treatment was performed.
Table 2 Chemical composition of base metal and welding wire for girth welds wt%
material
C
Cr
Ni
Mo
N
Base metal
<0.015
12
5.1
2
0.01
GTAW welding wire
0.01
25.3
9.5
4
0.27
GMAW welding wire
0.02
25.1
9.6
4
0.27
Table 3 Annular weld welding conditions
Road times
Welding method
Welding materials
Position of welding
Protective gas
Inter-layer temperature
Electric current
Voltage
speed
Heat input
A
V
Mm/min
kJ/mm
1
GTAW
Ф2.0mm
5G
100% Ar
<25°C
148
13.5
44
2.7
2
GMAW
Ф1.2mm
5G
100% Ar
25°C
145
15
75
1.7
2.1 Mechanical properties
Table 4 shows the tensile test results. The strength is set to X80 grade, and the welded joint is fractured in the matrix metal, indicating high performance. The profile of the welded joint shows that the maximum hardness of the heat affected zone is about HV330, which meets the design goal of HV350 or lower. The Charpy impact test results on the welded joints show that the absorption energy at -80°C and -40°C is about 200J, which proves that the newly developed steel has excellent low temperature toughness.
Table 4 Tensile test results of welded joints and base metal
material
Yield strength, MPa
Tensile strength, MPa
Elongation rate, %
Broken position
Welded joint
-
856
30
Base metal
Base metal
634
827
34
-
2.2 Carbon dioxide corrosion resistance
The immersion test was conducted in an environment of high temperature and high partial pressure of carbon dioxide, and the carbon dioxide corrosion resistance of the steel was evaluated by measuring the weight loss. Assuming an acceptable corrosion rate of 0.127mm/a, newly developed materials are suitable for 160°C, 2.0MPa CO2 partial pressure.
2.3 Sulfide stress corrosion resistance
A tensile-loaded tensile sulfide stress corrosion test was used to evaluate the sulfide corrosion resistance of welded joints. The aqueous solution was mixed with 5% or 10% NaCl, 0.5% CH3COOH was added, and when CH3COONa was used, the pH was adjusted from 3.5 to 5.0. The partial pressure of hydrogen sulfide mixed into the test gas was 0.001 to 0.007 MPa. The applied stress is 567 MPa, and its yield strength is equivalent to 90% of the base metal. The test results show that despite the sulfide stress corrosion cracking in the heat affected zone at pH 3.5, sulfide stress corrosion cracking did not occur at a pH of 4.0 and a partial pressure of hydrogen sulfide of 0.001 MPa.
(3) Intergranular stress corrosion cracking in annular welds
According to reports, the laboratory study found that the intergranular stress corrosion cracks produced on the annular welded joints in the high-temperature carbon dioxide environment have chemical compositions similar to those of the newly developed steel. In addition to this, it has been reported that a molybdenum-free material with a chemical composition similar to that of a newly-developed steel used in actual pipelines has gas leakage due to intergranular stress corrosion cracking.
3.1 The mechanism of intergranular stress corrosion cracking
In order to verify the effect of welding conditions on the sensitization behavior, the specimens used in the stress corrosion cracking test were subjected to two passes of simulated welding heat cycles. In order to carry out the tests under harsh conditions, the corrosion environment is 牶 pH value of 2.0, U-shaped bend test method, apply more strain. The test results show that some specimens are cracked after the second pass of the thermal cycle. Only the first pass of the sample did not crack.
These results indicate that the causes of intergranular stress corrosion cracking are as follows: During the high-temperature heating cycle, carbon is dissolved, and carbide is precipitated in the grain boundary of the prior austenite in the subsequent thermal cycle, and carbides at the grain boundary are precipitated. Chromium-depleted zones are formed nearby, which in turn sensitizes the material.
3.2 Method of preventing intergranular stress corrosion cracking
Since intergranular stress corrosion cracking is presumably caused by the chromium depletion zone, possible methods of preventing intergranular stress corrosion cracking include post-weld heat treatment to restore chromium diffusion, reducing the carbon content to a very low level, and adding titanium to suppress Precipitation of chromium carbides.
In order to determine the effect of post-weld heat treatment, materials containing 100 ppm of carbon were sensitized with two passes of the heating cycle followed by a third pass of heating cycle under different conditions. A U-shaped flexural stress corrosion cracking test similar to that described above was used to evaluate the prepared specimen. The test results showed that the samples sensitized had no cracks when heated within the range of 550-700°C for several minutes. This effect may be due to heat treatment increasing the diffusion of chromium, which reduces the chromium depletion zone. Using short-time post-weld heat treatment (minutes), intergranular stress corrosion cracking can be prevented, which does not interfere with the actual laying efficiency of the pipe.
In order to determine the effect of reducing the carbon content and the addition of titanium, materials with different carbon contents and titanium contents were evaluated. The sample was subjected to a heating cycle of 450° C. and 1000 s. This condition easily caused sensitization and a similar U-shaped bending stress corrosion cracking test was performed. As the test conditions changed, a gap was created in the U-shaped bend of the sample. The test results show that reducing the carbon content and adding titanium can inhibit cracking. This is presumably because the dissolution of carbon is inhibited during the welding and it is converted into titanium carbide to suppress the precipitation of chromium carbide which causes chromium depletion. Therefore, reducing the carbon content and adding titanium are effective methods for improving the resistance to intergranular stress corrosion cracking of the material.
4 Conclusion
The new martensitic stainless steel seamless pipes have improved weldability by reducing the carbon and nitrogen content and have excellent corrosion resistance and mechanical properties by optimizing other alloying elements.
The main characteristics of this new type of steel pipe are as follows:
(1) The new type of steel pipe has excellent weldability without welding cracks.
(2) The strength of the new steel pipe is X80 grade, and the low-temperature toughness Charpy impact test at -40°C has an energy absorption of about 200J or more.
(3) The steel has excellent carbon dioxide corrosion resistance and the corrosion rate is 0.127 mm/a or less at 160°C and 2.0 MPaCO2.
(4) The steel has excellent sulfide stress corrosion resistance under the conditions of pH 4.0 and 0.001 MPa partial pressure of hydrogen sulfide.
(5) Intergranular stress corrosion cracking can be prevented after a short-time (a few minutes) post-weld heat treatment. Lowering the carbon content and adding titanium can effectively improve the resistance to intergranular stress corrosion cracking of the material.
Since this new type of steel pipe has excellent weldability, mechanical properties, and corrosion resistance, it can be used to transport liquid lines containing corrosive gases, such as carbon dioxide, and therefore it will become low.
