Low Temperature Impact Toughness of 16MnD Steel Forgings

The 16MnD steel forgings for low temperature pressure equipment are introduced. After the quenching and tempering treatment, the numerical values ​​of the mechanical properties of the steel show abnormal fluctuations in the low temperature impact energy of the steel. The reason for this fluctuation is due to the presence of non-metallic inclusions in the steel forgings; or in the performance heat treatment, the quenching cooling is insufficient; or the tempering temperature is too high or too low. According to the approximate proportional relationship between tensile strength (R _m ) and Brinell hardness of steel forgings, the relationship between Rm and HBW of 16MnD steel is calculated by using the actual test data of mechanical properties in production: R _m /HBW≈3.3335. Using this formula, the minimum and maximum values ​​of the range of R _m values ​​required by the technical standards are directly substituted into the formula to determine the hardness range within the range of tensile strength requirements, which serves as the basis for the tempering temperature of the selected quenching and tempering treatment; And after the quenching and tempering treatment, the hardness value of the surface of the measured workpiece is brought into the formula to predict the mechanical properties that the forging may reach. At the same time, the technical measures to improve the low temperature impact toughness of 16MnD steel forgings are introduced.

Because 16MnD steel has low carbon content and low alloying elements, it has good weldability and low price, and is widely used in the manufacture of forgings for low temperature pressure equipment. In actual production, after the forgings are subjected to quenching and tempering treatment, sometimes the low temperature impact test at -40 °C, the impact sample appears brittle fracture, the notched impact absorption energy is very low, the test results are far from the specifications specified in the forging technical standards. The production cycle was delayed, the manufacturing cost of forgings was increased, and the timely delivery of products was affected.

In the forging manufacturing industry, low-alloy steel forgings for low-temperature pressure-bearing equipment often exhibit low-temperature impact toughness. The test results of mechanical properties of forgings show that the strength and plasticity test values ​​of forgings are normal and stable, while the low temperature impact toughness is more volatile. In view of this, to explore the factors affecting the low temperature impact toughness of 16MnD steel forgings, it is necessary to study the method to improve the low temperature impact toughness of 16MnD steel forgings.

1. Forging technical requirements

The chemical composition of steel for 16MnD steel forgings for low temperature pressure equipment shall comply with Table 1. After the forgings have been quenched and tempered, the samples shall be taken according to the NB/T47009-2010 standard. The mechanical properties test results shall comply with Table 2.

2.16MnD steel forgings quenching and tempering process

　　 16MnD steel forgings are generally simple in shape, with low carbon content and low alloying elements. If the forgings have no defects, no quenching cracks will occur during quenching and quenching. 16MnD steel forgings quenching and quenching temperature is 930 °C ± 10 °C, quenching cooling medium is selected from tap water, low cost, pollution-free and environmentally friendly, in order to ensure the low temperature impact toughness of forgings, the workpiece is quenched and deep-cooled during quenching and cooling until the workpiece It is cold in the water. When the workpiece is quenched and heated, in the range of the critical temperature of the steel (Ac1 ~ Ac3), it should be heated rapidly to increase the superheat of the steel, increase the austenite nucleation rate, refine the austenite grains, and quench the cooling. After that, a fine uniform quenched structure is obtained. The process curve of the tempering treatment of 16MnD steel forgings is shown in the attached drawing. The mechanical properties of 16MnD steel forgings after heat treatment are listed in Table 3. It can be seen from the data in Table 3 that under the normal quenching and tempering condition of the forging, the comprehensive mechanical properties are relatively stable, and there is no large fluctuation, especially the low-temperature impact absorption energy of the forgings is also high.

3. Factors affecting the low temperature impact toughness of 16MnD steel forgings

(1) Influence of non-metallic inclusions After the quenching and tempering treatment of 16MnD steel forgings, the mechanical properties of the samples are taken at the locations specified in the technical standards. Sometimes the impact energy of the two impact specimens in the three impact specimens is low-temperature impact absorption. High, and one of the impact specimens has a particularly low absorption energy at low temperature impact (see Table 4). This is often because there are brittle non-metallic inclusions on the cross section below the V-notch of the impact specimen. Because non-metallic inclusions are present in forgings, especially brittle inclusions, the bonding force between the metals is severely damaged, resulting in a significant reduction in the low-temperature impact absorption energy of the forgings. If this happens during the mechanical test of the forging, according to the technical requirements of the forging, three impact samples can be taken near the original sampling part of the forging, and the low temperature impact test is repeated. The test result is generally ideal, and it is qualified. The determination of whether or not to proceed is in accordance with the technical standards for forgings.

(2) Influence of tempering temperature In the performance heat treatment of 16MnD steel forgings, because the tempering temperature is too low or the tempering holding time is too short, the mechanical properties of the forgings have high strength, low plasticity and toughness, resulting in low temperature impact absorption of the forgings. The energy is unqualified and needs to be reworked. In this case, a method of increasing the tempering temperature or extending the tempering holding time may be employed to appropriately reduce the strength of the forging and improve the ductility and toughness of the forging.

In the mechanical properties test of forgings, there was a phenomenon that the tensile strength of forgings was high, the yield strength was low, the yield ratio was low, and the low-temperature impact 16MnD steel forgings had low absorption energy (see Table 5). This is because the tempering temperature of the forging is too high during the quenching and tempering process, which exceeds the critical point Ac1 of the steel. Part of the austenitization occurs in the steel, and the austenite structure appears in the forging structure during tempering heating. When the air is cooled from the tempering temperature, this part of the austenite will undergo a normalizing structure transformation (if the tempering is released and the cold is released, this part of the austenite will undergo a quenching structure transformation), this normalizing structure and tempering Compared with the treated tissue morphology, the low-temperature impact absorption energy of the forging is greatly reduced.

(3) There is a case of the ductile-brittle transition temperature of steel. After the quenching and tempering treatment of 16MnD steel forgings, the mechanical properties test results are that the strength (Rm, ReL), hardness (HBW), and plasticity (A, Z) are everywhere. In the normal range, but the low temperature impact energy (AKV) value is very low, the test results of the three impact samples are unqualified (see Table 6).

The problem in Table 6 arises because the test temperature at the low temperature impact test is lower than the ductile-brittle transition temperature (FATT50) of the 16MnD steel. When the forging steel is below the FATT50 temperature, the material is in a brittle state and its AKV value. It is also very low; if the test temperature is higher than the FATT50 temperature, the material is in a tough state and its AKV value will be at a normal level. Any factor that increases the ductile-brittle transition temperature of steel will reduce the low temperature impact toughness of steel forgings.

(4) Effect of metal grain size of forgings If there are overheated structures such as coarse grains and Wei's structure in the forging metal, the bonding between the metal grains will be weakened, and the brittle phase will be precipitated on the grain boundaries. The embrittlement elements are segregated at the grain boundaries, resulting in a significant reduction in the impact toughness of the steel forgings. Refining the forging die to make the metal grain size smaller, can increase the impact toughness of the steel, reduce the ductile-brittle transition temperature of the steel, and improve the low-temperature impact absorption energy of the forging. Because the grain of the forging is refined, the total grain boundary area which hinders the crack propagation is increased, the concentration concentration of the impurity elements on the grain boundary is reduced, thereby avoiding the metal-brittle brittle fracture and improving the impact toughness of the steel.

(5) Other factors affecting harmful elements (S, P, Sb, Sn, As, etc.) in steel for forgings; looseness, segregation, microcracking and other defects in forgings; size of forgings, sampling parts of impact samples, etc. Will affect the low temperature impact toughness of forgings. In the test of the mechanical properties of forgings, the volatility of the energy absorbed by the low temperature impact needs to be analyzed.

4. Forging performance prediction method

(1) We put 16MnD steel forgings

The mechanical performance test data in production practice was analyzed, analyzed and calculated. In the range of hardness 141 to 177HBW, the data of R _m , R _eL and HBW are integrated, and the arithmetic mean is taken. The calculated tensile strength (R _m ) and yield strength (R _eL ) are as follows. Forecast calculation formula:

R _m /HBW=3.3335(1)

R _m =3.3335HBW(2)

R _eL /Rm=0.7475(3)

R _eL =2.4918HBW(4)

It is well known that there is an approximate proportional relationship between the tensile strength and the Brinell hardness of steel. The prediction formula of tensile strength is based on this and can be applied in actual production inspection.

(2) NB/T47009-2010 "low alloy steel forgings for low pressure bearing equipment" technical standards, only the heat treatment state of the forgings and the required mechanical properties, but does not specify the surface hardness of the forgings after heat treatment. We can substitute the lowest value of the Rm requirement range of 480 MPa and the highest value of 630 MPa into the formula (1) or (2), and calculate the corresponding hardness values ​​of 144 HBW and 189 HBW. When preparing the forging and quenching process, the tempering temperature can be determined by referring to the hardness range of 144 to 189HB.

(3) After the 16MnD steel forgings have been quenched and tempered, it is necessary to check the Brinell hardness value of the surface of the workpiece, and substitute the detected surface hardness values ​​into equations (2) and (4), respectively, and calculate the Rm and ReL values. It can be predicted whether the Rm and ReL values ​​of the forgings are within the acceptable range specified by the technical standards. If the predicted value deviates from the upper limit of the acceptable range, it indicates that the forging has high strength and low plastic toughness. It is necessary to increase the tempering temperature and retemper, to reduce the hardness and strength of the forging, and to improve the plasticity and toughness; if the predicted value deviates from the qualified range The lower limit value indicates that the strength of the forging is lower than the technical standard, and the forging must be re-tempered. When reworking and quenching, the tempering temperature should be appropriately reduced.

However, please pay special attention to the inspection of the hardness of the forging after quenching and tempering, which is carried out on the surface of the workpiece. According to the specification of the sampling part of the mechanical properties of the forging, there is a certain distance from the surface of the forging. For 16MnD low hardenability steel, the greater the distance from the surface of the forging, the lower the hardness. The hardness of the sampling part of the mechanical property sample shall be corrected according to the forging technical standard and with reference to the hardenability curve of the steel, and the surface hardness value of the test shall be appropriately corrected. The corrected hardness value is then substituted into equations (2) and (4) to calculate, which is closer to the actual situation of the forging, and the predicted value is more accurate.

5. Method for improving low temperature impact toughness of 16MnD steel forgings

(1) For forging steel, when smelting, select suitable deoxidizer, reduce oxide in steel, improve the purity of steel, reduce non-metallic inclusions in steel; strictly control S, P, Sb, Sn, As, Pb, etc. Harmful element content; adding an appropriate amount of refined grain elements to obtain a calming essence of fine grain steel.

(2) Steel must have sufficient forging ratio when forging, break the dendrites in the steel, fully compact the internal metal, reduce the internal defects of the forgings, increase the internal metal density; avoid no forging ratio heating, prevent forgings Severe overheated tissue, coarse crystals and mixed crystals.

(3) After forging, it is necessary to carry out suitable post-forging heat treatment, the purpose is to refine the metal grains of the forging, eliminate the defects such as coarse crystal, mixed crystal and superheated structure in the forging structure, improve the microstructure of the forging, and do a good job for performance heat treatment. Organizational preparation.

(4) When the 16MnD steel forgings are subjected to performance heat treatment, when the temperature is raised to (650±20) °C, according to the cross-section of the workpiece, the temperature is properly maintained, and then the furnace power is rapidly heated to pass the temperature of Ac1 to Ac3 to increase the phase change of the steel. The superheat degree is used to increase the nucleation rate of austenite and achieve the purpose of refining austenite grains. After the quenching is released, the workpiece should be quickly quenched into the cooling water (the water should be circulated and stirred), and the intense, sufficient and deep cooling should be carried out to improve the quenching and subcooling, so that the forging structure is fully transformed, and the quenching transformation product is more uniform and fine.

(5) For the 16MnD steel forgings, the amount of furnace should be properly adjusted during the heat treatment, and the workpiece should be kept at a certain distance to make it heat and cool evenly. After quenching and tempering in the fashion furnace, the quenching stress is eliminated, and excellent comprehensive mechanical properties, especially low temperature impact toughness, are obtained.

6. Conclusion

(1) The quality of steel for forgings is an intrinsic factor to ensure excellent comprehensive mechanical properties, especially low temperature impact toughness, of 16MnD steel forgings.

(2) The reasonable high quality of heating, forging and post-forging heat treatment in the forging production process is the necessary condition for obtaining the excellent comprehensive mechanical properties, especially the low temperature impact toughness, for the performance heat treatment of 16MnD steel forgings.

(3) The high-quality performance heat treatment of 16MnD steel forgings is the key link to obtain excellent comprehensive mechanical properties, especially low temperature impact toughness.

About the author: Wang Songlin, Ren Xiufeng, Qin Yingchao, Guo Shengqi, Zhang Dandan, Gaoning, Shandong Elite Heavy Industry Co., Ltd.