Two 4.6t steel ingots, made of 1Cr17Ni2, have serious transverse cracks during the forging process, and one of them has serious longitudinal cracks, which has a great impact on the batch products.
In order to avoid similar problems in subsequent production, a series of analysis was made on the causes of ingot cracking.
Ingot heating process: keep the ingot at 500 ℃ for 2h, raise the temperature to 850 ℃ at the rate of 100 ℃ / h, keep the ingot for 2h, raise the power to 1180 ℃, keep the ingot for 6h, and then take out the furnace for forging.
Severe cracking occurred during the first fire compaction.
2. On site observation
There are many transverse cracks on the surface of the ingot, which are obviously in the shape of wide openings.
The longitudinal crack is a direction that basically runs through the length of the ingot and is located in the center of the billet.
The crack opening width is small, and the head and tail end faces of the ingot are broken, as shown in Fig. 1 to Fig. 4.
The original fracture has been oxidized and is grayish black, which belongs to the morphology caused by the high temperature of the typical fracture.
3. Test analysis
3.1 Low magnification test analysis
The hot acid leaching test is carried out on the cross-section test piece at the crack of the ingot. The results are shown in Table 1.
Table 1 hot acid leaching test of cross-section test piece
|General porosity / grade||Center porosity / grade||Ingot segregation / grade||Defect morphology|
|1.5||2.5||3.5||There are many cracks, the longest is about 6cm|
The steel ingot cross-sectional test piece is basically square, and there is an open crack in the middle of one side, with a depth of about 6mm, which is the vertical depth of the macro steel ingot longitudinal crack.
Columnar crystal pattern can be seen on the edge, and there are several small cracks on the test piece, with a maximum length of about 10mm, as shown in Fig. 5-fig. 7.
The test results show that the ingot shape segregation is serious after the ingot forging (only pressing Square), and it is unqualified.
The small cracks observed are related to the height of columnar crystals in the ingot casting.
3.2 fracture test analysis
The artificial fracture is a typical shell fracture, as shown in Fig. 8.
The test results show that the shell fracture is an abnormal fracture, and the cause of its formation needs to be further analyzed.
3.3 Chemical composition analysis
Take samples on the surface of steel ingot and R / 2 for chemical composition analysis.
See Table 2 for the results.
The chemical composition meets the technical requirements of 1Cr17Ni2 steel.
Table 2 chemical composition of 1Cr17Ni2 steel (mass fraction) (%)
|R / 2||0.15||0.53||0.012||0.013||0.49||16.8||1.77||0.018|
3.4 Non metallic inclusion detection
Take high magnification sample on the test piece for non-metallic inclusion detection, and evaluate it according to GB / T10561-2005 standard rating chart microscopic inspection method for determination of non-metallic inclusion content in steel.
See Table 3 for the results.
Table 3 test results of non-metallic inclusions (grade)
|Position||Class A||Class B||Class C||Type D||Class Ds|
|R / 2||1.0||1.5||0.5||0.5||0.5|
The purity of the ingot is qualified, but there are many inclusions of class B alumina.
3.5 Metallographic analysis
Metallographic structure and grain size of samples at different positions are tested, and the results are shown in Table 4.
Table 4 metallographic structure and grain size test
|Position||Grain size / grade||Metallographic structure|
|Edge||5.0||Low carbon ferrite + ferrite + intergranular carbide + lamellar structure|
|R / 2||3.5||Low carbon ferrite + ferrite + intergranular carbide + lamellar structure|
|Core||3.5||Low carbon ferrite + ferrite + intergranular carbide + lamellar structure|
|Columnar crystal region||3.5||Low carbon ferrite + ferrite + intergranular carbide + lamellar structure (structure distribution retains columnar crystal morphology)|
The test results show that the microstructure is low-carbon martensite + ferrite + intergranular carbide + lamellar structure, and the carbides are continuously distributed along the grain boundary and precipitated along the original columnar crystal, which will produce greater brittleness and reduce the mechanical properties of the steel.
The metallographic structure of each part is shown in Fig. 9 to Fig. 14.
3.6 Crack analysis
The micro morphology of low magnification small crack is different in width, intermittent, fuzzy in boundary and discontinuous in tip.
There are very fine cracks beside the small cracks, which are distributed in intermittent linear and island shapes.
After being etched by hydrochloric acid aqueous solution of high iron chloride, the microstructure of small cracks and microcracks is mainly distributed along the ferrite with columnar crystal distribution, and there is no obvious change in the microstructure near the crack, as shown in Fig. 15-fig. 18.
The results show that the small cracks and micro cracks of forgings are closely related to the carbides distributed along the as cast columnar crystals.
3.7 Micro fracture analysis
The macroscopic shell like fracture is characterized by cleavage feathers and tear ridge lines formed between parallel cleavage under scanning electron microscope, and the cast free crystal surface and second phase particles and inclusions can be seen locally, as shown in Fig. 19-fig. 22.
The micro cleavage crack source is located on the free crystal surface of the grain boundary.
The energy spectrum determines that it mainly contains C, Al, Si, Cr, Ni and other elements, among which al, Si, Cr and other elements.
The composition of the micro region is higher than the average level, and the composition of the Ni element is lower than the average level.
The composition of the cleavage micro region is close to the macro chemical composition.
The results show that the shell like fracture is caused by the micro segregation of aluminum in the steel.
The chemical composition test results show that the ingot material meets the technical requirements of 1Cr17Ni2 steel.
However, the structure uniformity of the ingot is poor, and the ingot segregation is grade 3.5, which is unqualified.
Ingot segregation is caused by component segregation and impurity accumulation at the junction of columnar crystal region and central equiaxed crystal region of ingot.
There are many small cracks in the low magnification columnar crystal region of the ingot, and the micro crack morphology is similar to the carbide morphology of as cast columnar crystal.
The structure of the ingot after forging is low-carbon martensite + ferrite + carbide + lamellar structure, and the grain size is 3.5-5.0.
The structure distribution in the columnar crystal region still retains the columnar crystal shape. There are a large number of continuously distributed carbides on the grain boundary, which can increase the brittleness of the structure.
The shell like fracture in the columnar crystal region of the ingot is an abnormal fracture.
The micro fracture shows cleavage and tear ridge, which indicates that the ingot is brittle.
The micro cleavage crack source is located on the free crystal surface of the grain boundary, and the energy spectrum of C, Cr, Al, Si and other elements are on the high side.
The analysis shows that it is caused by the presence of Cr containing carbides and the second phase containing Al.
When the Al is close to 0.09%, the shell like fracture is easy to appear in the columnar crystal region.
When aluminum deoxidation is adopted, once the aluminum content is not strictly controlled, it is easy to cause a large amount of aluminum residue.
Although the original aluminum content in the molten steel does not exceed the standard, at the end of solidification of the ingot, due to the low melting point of aluminum, the aluminum concentration in the residual molten steel is significantly increased, which is enough to make the second phase containing aluminum precipitate from the molten steel in dendrite form, which is a kind of micro segregation phenomenon.
When the crystallization is slow, the dendrite aluminum containing second phase precipitated from the residual molten steel is pushed to the grain boundary of the primary crystallization. When the matrix crystallization speed exceeds a certain critical speed, it is trapped in the growing crystal, and finally increases the intergranular fracture sensitivity.
The heating process of steel ingot is 500 ℃ for 2h, heating to 850 ℃ at the rate of 100 ℃ / h for 2h, heating to 1180 ℃ at the power for 6h, and then out of the furnace for forging.
1Cr17Ni2 is a martensitic ferritic duplex stainless steel with brittleness at 475 ℃.
Avoid heating for a long time in the temperature range of 400-525 ℃.
When 1Cr17Ni2 steel is heated above 900 ℃, the grain growth tendency increases obviously, and the brittleness increases, which worsens the forging conditions.
The chemical composition of the ingot meets the technical requirements of 1Cr17Ni2 steel, but the microstructure uniformity is poor and the ingot segregation is serious.
The structure of steel ingot after forging is poor.
The main reason for ingot cracking is the improper heating process design of the ingot, which easily leads to 475 ℃ brittleness.
Moreover, the ingot casting cools slowly, and the aluminum containing phase precipitates in the columnar crystal region, resulting in an increase in the sensitivity of intergranular fracture.
Under the joint action of the two, the forging cracking is finally caused.