Quality Control of A182 F91 Forged Pipe Forging Process

ASTM A182 F91 forged pipe is an important raw material for boiler internal pipe fittings, which has a significant impact on the quality of pipe fittings. Therefore, controlling its quality is of great significance. To control the quality of raw materials, steel ingots, forging process, heat treatment, and conduct non-destructive testing and physicochemical analysis. Through quality verification, it has been proven that this process is feasible and can effectively control quality.

0. Introduction

ASTM A182 F91 forging pipe belongs to sleeve forgings, which is an important raw material for boiler inner pipe fittings. It has a great influence on the quality of pipe fittings, so it is of great significance to control its quality. 91 series steel is actually a kind of metamorphic steel formed by adding strengthening elements V, Nb, N and so on on the basis of the original 9Cr-1Mo steel. It is usually called martensitic heat-resistant steel, but in the American standard, the amount of Cr less than 10 % is classified as ferrite series, so the name in the standard is still ferrite type ( in fact, the microstructure type can be obtained by different heat treatment processes. Different phases, so do not have to stick to the name ). In China GB 5310-2008, the same kind of steel 10Cr9Mo1VNb ( seamless steel tube for high pressure boiler ) is less N than another brand 1Cr9Mo1VNb in China. The real same as the United States 91 steel is 1Cr9Mo1VNb ( N ) heat-resistant steel. Now taking the finished product of Φ400 x Φ280 x 980 as an example, the quality control of this material forging is introduced.

1. Design forging drawings

Design the forging diagram, and see Figure 1 for the forging blank diagram. After calculation, the weight of the selected steel ingot is 937 kg. Choosing the weight and specification of the steel ingot is very important. It is necessary to consider the taper of the core rod and the additional materials generated by the uneven polygonal shape of the inner and outer surfaces, as well as the machining allowance and burning loss.

Figure.1 Forging Stock Diagram

2. Control of steel ingots

Northeast Special Steel produces the steel ingots, and the smelting method is electric furnace smelting + outside furnace refining and vacuum degassing. We require that the coarse and fine inclusions of the four types of non-metallic inclusions be ≤ 1.5 and the total amount be ≤ 4.5, which is superior to the provisions of GB 5310-95; H. The content of O should be controlled within [H] ≤ 3PPM, [O] ≤ 40PPM; The content of the five harmful elements Pb, Sn, As, Sb, and Bi is controlled below 200PPM, and the chemical elements meet the standard requirements, as shown in Table 1 below.
Table.1 Chemical Composition of 91 Steel (wt/%)

Note: What is not within the standard range refers to the maximum content.

2.1 Hazards of non-metallic inclusions

• Sulfides reduce the corrosion resistance, plasticity, toughness, and fatigue resistance of steel;
• Oxides and silicates disrupt the continuity of the steel matrix and lead to stress concentration, thereby reducing the plasticity, toughness, and fatigue resistance of the steel;
• Nitrides significantly increase the brittleness of steel and are prone to crack formation.

The impact of hydrogen and oxygen content control on the quality of steel ingots is as follows:

• The main harm of hydrogen to steel is hydrogen induced cracking defects;
• Oxygen can reduce the mechanical properties of steel, and the formation of oxide inclusions can also reduce the plasticity, toughness, and fatigue resistance of steel.

2.3 Hazards of Five Harmful Elements

• Their melting point is relatively lower than that of steel. When steel is in a solid state, they are still in a liquid state. Hence, they are called low melting point elements;
• When their content in steel exceeds a certain limit, they will significantly reduce the high-temperature mechanical properties, increase the high-temperature brittleness of the steel, reduce the strength and toughness of the steel, and make the steel brittle;
• They often coexist in one body, causing severe segregation and rarely exist alone, thus having a greater destructive effect on steel.

3. Forging process control

Heating the gas heating furnace to the forging temperature, the initial forging temperature is less than 1200 ℃, and the actual control is 1180 ℃. The final forging temperature is greater than 800 ℃, but the actual control is 850 ℃. The heating time is greater than 12.5 hours to achieve complete austenitization. The initial forging temperature, the heating temperature of the billet, is generally the initial forging temperature, known as the initial forging temperature. Generally, it is 150-200 degrees below the liquidus, and the heating temperature should be as high as possible without overheating. The higher the initial forging temperature, the better the plasticity of the material and the smaller the deformation resistance; The higher the initial forging temperature, the wider the forging temperature range, the longer the forging time, and the fewer forging heats (heating times for forging), which can improve productivity and reduce energy consumption. Final forging temperature: During the forging deformation process of the billet, the heat gradually dissipates, the temperature decreases, the tendency for work hardening increases, the plasticity decreases, and it is difficult to continue deformation and may crack. Therefore, forging deformation must stop at an appropriate temperature, which is called the final forging temperature. The final forging temperature should generally not be lower than the recrystallization temperature of the metal to avoid deformation difficulties caused by work hardening and even workpiece fracture and scrapping. However, it should not be too much higher than the recrystallization temperature. Otherwise, the recrystallization grains will become coarse after stopping forging, resulting in a narrower range of forging temperature. Heating insulation is to ensure uniform temperature, microstructure, and malleability of the billet. However, prolonged insulation at high temperatures can lead to overheating. Therefore, the insulation time for heating is limited to the hot penetration of the billet, which depends on the power of the heating furnace, furnace temperature, and the thermal conductivity of the metal material.

Forge and press on the 3150 fast forging machines, with a forging ratio requirement greater than 6, after the process of pier roughening, elongation, pier roughening, punching, elongation, expansion, furnace cooling after forging, and then annealing heat treatment. The heating curve is shown in Figure 2.

Figure.2 Heating Curve

4. Heat treatment control

Treat the inner and outer surfaces of the annealed forging to remove the oxide skin and expose the metallic luster. Conduct ultrasonic and magnetic particle testing, and after passing the preliminary inspection, perform normalizing and tempering heat treatment. According to the differences in composition, the temperature of AC₁ and AC₃ in F91 is between 800 ℃ and 830 ℃, while the temperature of AC₃ is between 890 ℃ and 940 ℃. The Ms temperature (the starting temperature of martensitic transformation) is quite high, around 400 ℃ (750 ° F). The Mf temperature (martensitic transformation end temperature) is above 100 ℃ (210 ° F); F91 material undergoes a wide range of cooling rates, where the austenite structure is completely transformed into martensitic structure. This rate range is usually satisfied by normalizing, so the heat treatment process for F91 material is generally normalizing + tempering. The heat treatment curve is shown in Figure 3.

5. Hardness and metallographic inspection results

The hardness test results are shown in Table 2.
The results of metallographic examination are shown in Fig.4. After heat treatment, the 20 × 20 × 10mm sample was taken and the metallographic structure was tested in the laboratory. The metallographic structure was lath tempered martensite.
Table.2 Hardness Inspection Results

Table.3 Chemical Composition Analysis Results

Table.4 Tensile Test Results

Table.5 Impact Test Results

Table.6 Bending Test Results

Figure.3 Heat Treatment Curve

Figure.4 A182 F91 Metallographic Structure 400x

6. Chemical composition inspection

The chemical composition inspection is a sampling analysis, as shown in Table 3.

7. Mechanical performance test

The sampling direction is circular sampling, and the results of the tensile test, impact test, and bending test are shown in Table 4, Table 5, and Table 6, respectively.

8. Precision turning

After finishing the vehicle, ultrasonic and magnetic particle testing will be carried out, and after passing the inspection, identification will be made and delivered for use.

9. Conclusion

Quality control was carried out on the forged pipe through chemical composition analysis, non-metallic inclusion detection, metallographic structure, hardness inspection, room temperature mechanical performance testing, non-destructive testing, and other processes. All indicators met the requirements of ASTM A182 F91 standard. This quality control method for raw materials and the forging process can effectively ensure forging quality and can be applied to the forging of other materials.
Author: Hou Zhiyong, Wu Huacheng