15CrMo steel plate is a heat-resistant steel with pearlite structure, which has high thermal strength (δ b ≥ 440MPa) and oxidation resistance at high temperatures, and has a certain ability to resist hydrogen corrosion. Due to the high content of Cr, C, and other alloying elements in steel, the hardening tendency of the steel is more obvious and the weldability is poor.

15CrMo weldability

Welding materials

Regarding the welding characteristics of 15CrMo steel,

Plan I: Welding preheating, using ER80S-B2L welding wire, T1G welding base, E8018-B2 welding rod, electrode arc welding cover, and local heat treatment after welding.

Post Weld Heat Treatment

The specimens welded using Scheme I should undergo local high-temperature tempering treatment after welding. The heat treatment process is as follows: the heating rate is 200 ℃/h, the temperature is raised to 715 ℃ and held for 1 hour and 15 minutes, the cooling rate is 100 ℃/h, and the temperature is lowered to 300 ℃ before air cooling. Specifically, JL-4 tracked electric heater (1146 × 310) is used to wrap around the weld seam, and aluminum silicate cotton is used for insulation with a thickness of 50mm. The temperature control adopts DJK-A electric heater automatic temperature controller.

Welding Procedure Qualification Test Results

Test plan: Tensile test, Bending test, Impact toughness test, aky (J/cm2)

Tensile strength δ b/Mpa, fracture location, bending angle surface, bending back, welding seam, fusion line, heat affected zone (HAZ)

Plan I 550/530 base metal 50. Qualified 84.8 162 135.6

Scheme II 525/520 base metal 50. Qualified 79.4 109.2 96.7

15CrMo welding process

2.1 Welding Materials

Based on the weldability of 15CrMo steel and the working characteristics of high-pressure pipelines on site, and referring to the welding process card provided by foreign countries, we have selected two schemes for welding tests based on past experience.

Plan I: Welding preheating, using ER80S-B2L welding wire, T1G welding base, E8018-B2 welding rod, electrode arc welding cover, and local heat treatment after welding.

Option II: Use ER80S-B2L welding wire, T1G welding base, E309Mo-16 welding rod, welding rod filling for arc welding cover, without heat treatment after welding. The chemical composition and mechanical properties of welding wire and electrode are shown in Table 1.

Table 1 Chemical Composition and Mechanical Properties of Welding Materials

Model C Mn Si Cr Ni Mo S P δ b/Mpa δ,%

ER80S-B2L≤0.05 0.70.41.2 <0.20.5 ≤0.025 ≤0.025 ≤500 25

E8018-B2 0.070.7 0.3 1.1 0.5 ≤0.04 ≤0.03 550 19

E309Mo-16≤0.12 0.5～2.5 0.9 22.0～25.0 12.0～14.0 2.0～3.0≤0.025≤0.035 550 25

2.2 Preparation before welding

The specimen is made of 15CrMo steel pipe with a specification of φ 325 × 25, and the groove type and size are shown in Figure 1.

Before welding, use an angle grinder to polish the inside and outside of the groove and within 50mm of the groove edge until a metallic luster is exposed, and then clean it with acetone.

The test piece is in a horizontal fixed position with a gap of 4mm between the joints. Six points are uniformly spot welded along the circumference using manual tungsten inert gas arc welding, and the length of each point should not be less than 20mm. The welding rod is baked according to the specifications in Table 2.

Table 2 Welding Rod Baking Specification

Welding rod model, baking temperature, insulation time

E8018-B2 300 ℃ 2h

E309Mo-16 150 ℃ 1.5h

Process parameters

According to Plan I, preheating is required before welding. According to the formula proposed by Tto Bessyo et al. for calculating preheating temperature:

In the formula To=350 √ [C] -0.25 (℃), To - preheating temperature, ℃.

[C]=[C]x [C]p [C]p=0.005S[C]x

[C] In the formula x=C (Mn Cr)/9 Ni/18 7Mo/90,

[C] X - component carbon equivalent;

[C] P - carbon equivalent size; S - specimen thickness (S=25mm in this article);

[C]x=C （Mn Cr）/9 7/90Mo=0.361

[C] If p=0.045, then To=138 ℃

Therefore, the preheating temperature is selected as 150 ℃. Use an oxygen acetylene flame to heat the specimen. First, use a temperature measuring pen to roughly determine the surface temperature of the specimen (estimated by the speed of color change in the handwriting). Finally, use a semiconductor thermometer to measure. At least three measurement points should be selected to ensure that the specimen as a whole reaches the required preheating temperature.

During welding, manual tungsten inert gas arc welding is used as the base layer for the first layer. To avoid dents on the back of the weld seam at the upward welding position, the wire feeding method uses the inner filling method, where the welding wire is fed into the pipe through the gap between the two ends. The remaining layers are welded using electrode arc welding, with a total of 6 layers and one weld bead per layer. The welding process parameters for Scheme I and Scheme II are shown in Tables 3 and 4. Weld according to Plan I

Table 3 Welding Process Parameters for Scheme I

Weld bead name Welding method Welding material Welding material Specification/mm Welding current/A Arc voltage/V Preheating and interlayer temperature Heat treatment specification

Bottom layer tungsten plate argon arc welding ER80S-B2L φ 2.4 110 12

Filler layer electrode arc welding E8018-B2 with a diameter of 3.2 and a temperature range of 85-90, 23-25150 ℃, and 715 ℃. ×75min

Cover layer electrode arc welding E8018-B2 φ 3.2 5 85-90 23-25

Table 4 Welding Process Parameters for Scheme II

Weld bead name Welding method Welding material Welding material Specification/mm Welding current/A Arc voltage/V Preheating and interlayer temperature Heat treatment specification

Bottom layer tungsten plate argon arc welding ER80S-B2L φ 2.4 110 12

Filler layer electrode arc welding E309Mo-16 with a diameter of 3.2, ranging from 90 to 95 and 22 to 24//

Cover layer electrode arc welding E309Mo-16 φ 3.2 90~95 22~24

When welding, the interlayer temperature should not be lower than 150 ℃. To prevent the cooling of the specimen caused by interrupted welding, two welders should alternate during welding, and insulation and slow cooling measures should be taken immediately after welding.

2.4 Post weld heat treatment

3 Welding Procedure Qualification Test

After welding, the test piece shall undergo 100% ultrasonic testing according to JB4730-94 "Non destructive Testing of Pressure Vessels" standard, and the weld seam shall be qualified as Grade I. Conduct welding procedure qualification tests in accordance with the JB4708 "Welding Procedure Qualification for Steel Pressure Vessels" standard. The evaluation results are shown in Table 5.

Table 5 Welding Procedure Qualification Test Results

Test plan: Tensile test, Bending test, Impact toughness test, aky (J/cm2)

Tensile strength δ b/Mpa, fracture location, bending angle surface, bending back, welding seam, fusion line, heat affected zone (HAZ)

Plan I 550/530 base metal 50. Qualified 84.8 162 135.6

Scheme II 525/520 base metal 50. Qualified 79.4 109.2 96.7

From the results of the tensile test, it can be seen that all the tensile specimens of the two schemes broke at the base metal, indicating that the tensile strength of the weld is higher than that of the base metal; All the bending tests are qualified, indicating that the plasticity of the weld seam is good. According to the impact toughness test results in Table 5, it can be seen that the impact toughness of Scheme I is significantly higher than that of Scheme II, which proves that the post weld heat treatment specification of Scheme I is relatively ideal. High temperature tempering not only improves the joint structure and performance, but also makes the toughness and strength match appropriately. According to the mechanical performance results at room temperature, both recommended welding process schemes can be used for on-site construction. Scheme I uses welding rods with similar composition to the base metal, and the weld performance matches the base metal. The weld should have high thermal strength and be not easily damaged after long-term use at high temperatures. The difficulty lies in the strict standards for post weld heat treatment, and improper control of tempering temperature, insulation time, heating and cooling rates can actually lead to a decrease in weld performance. Scheme II uses austenitic stainless steel welding rods for welding, which can eliminate post weld heat treatment. However, due to the different expansion coefficients between the weld and the base material, carbon diffusion and migration can occur during long-term high-temperature operation, which can easily lead to damage of the weld in the fusion zone. Therefore, from the perspective of reliability, it is more reliable to use Scheme I for welding on site.

4 Conclusion

Both welding schemes are feasible for welding thick walled high-pressure pipes made of 15CrMo steel. In order to ensure that the weld performance matches the base material and has high thermal strength, adopting scheme I yields better results. The key is to strictly control the post weld heat treatment process.

Although Scheme II can eliminate post weld heat treatment, the possibility of carbon migration and diffusion in the weld at high temperatures leading to weld damage cannot be ignored. Therefore, it should only be used with caution when post weld heat treatment is not possible.

Formula for calculating the weight of 15crmo steel plate: length x width x thickness x 0.00785=kg/m

The outer ring turns yellow

15CrMo alloy steel plate is rolled with secondary rolling material for one pass. Does the outer ring start to turn yellow or rust? What is the reason for this?

To remove the oxide scale on the surface of 15CrMo alloy steel plate, the immersion continuous acid washing method is currently commonly used. After acid washing, the surface of 15CrMo alloy steel plate often contains acid solution, so it needs to be cleaned with cold or warm water. However, yellow rust often appears on the surface of 15CrMo alloy steel plate after washing. Seriously affecting the surface quality of the finished product. Japan has studied the yellowing mechanism to eliminate this defect. Taking hydrochloric acid as an example, there are the following reactions:

FeCl2+2H2O=Fe (0H) 2+2HCl (1) acid washing process

Drying process of 2Fe (OH) 2+O2=2FeO · OH+H2O (2)

Equation (1) represents the equilibrium state on the wet plate surface in aqueous solution, where Fe (OH) 2 and HCl do not appear yellow.

Equation (2) represents the steel plate that begins to dry. Due to the action of oxygen in the air, Fe (OH) 2 oxidizes and becomes insoluble in water. FeO · OH turns into yellow rust on the surface of 15CrMo alloy steel plate.

Chemical composition

Chemical composition

Chemical composition of brand (mass fraction) (%)

C Mn Si Cr Mo Ni Nb+Ta S P

15CrMo 0.12~0.18 0.40~0.70 0.17~0.37 0.80~1.10 0.40~0.55 ≤0.30 _ ≤0.035 ≤0.035

Mechanical properties

Brand tensile strength MPa yield point MPa elongation (%)

15CrMo 440~640 235 21

Application examples

Seamless pipes for specialized purposes such as petroleum, petrochemical, and high-pressure boilers include seamless pipes for boilers, seamless steel pipes for geology, and seamless pipes for petroleum

Common specifications

Material specifications: Thickness * Width * Length (mm) can be customized for rolling steel mills across the country. Weight (ton) Name:

15crmo 8 * 1500-4200 * 6000-18800M 198.65T alloy structural steel plate

15crmo 12 * 1500-4200 * 6000-18800M 186.618T alloy structural steel plate

15crmo 25 * 1500-4200 * 6000-18800M 258.366T alloy structural steel plate

15crmo 30 * 1500-4200 * 6000-18800M 241.624T alloy structural steel plate

15crmo 45 * 1500-4200 * 6000-18800M 263.254T alloy structural steel plate

15crmo 55 * 1500-4200 * 6000-18800M 283.318T alloy structural steel plate

15crmo 60 * 1500-4200 * 6000-18800M 169.563T alloy structural steel plate

15crmo 70 * 1500-4200 * 6000-18800M 569.356T alloy structural steel plate

15crmo 80 * 1500-4200 * 6000-18800M 231.315T alloy structural steel plate

15crmo 90 * 1500-4200 * 6000-18800M 341.318T alloy structural steel plate

15crmo 100 * 1500-4200 * 6000-18800M 461.318T alloy structural steel plate

15crmo 110 * 1500-4200 * 6000-18800M 598.359T alloy structural steel plate

15crmo 120 * 1500-4200 * 6000-18800M 431.621T alloy structural steel plate

15crmo 130 * 1500-4200 * 6000-18800M 388.654T alloy structural steel plate

15crmo 140 * 1500-4200 * 6000-18800M 348.351T alloy structural steel plate

15crmo 150 * 1500-4200 * 6000-18800M 645.982T alloy structural steel plate