Material 14Cr17Ni2 belongs to martensitic ferritic stainless steel.
The alloy has good corrosion resistance and high mechanical properties.
It has good corrosion resistance to oxidizing acids and aqueous solutions of organic acids.
It is a common material for manufacturing key structural parts and fastening parts with corrosion resistance.
As such components are used in the environment of withstanding tension, shear and impact, the materials shall not only meet the conventional mechanical properties, but also have good impact toughness.
However, the hot processing process is complicated. In the production process over the years, the mechanical properties are often unqualified.
According to statistics, it is mainly reflected in the impact energy, which has seriously affected the production cost and delivery date, and caused many unnecessary losses.
In this post, it is proved that 14Cr17Ni2 has obvious temper brittleness at medium temperature through experimental research.
The phenomenon of temper brittleness was discovered very early.
During the first World War, the phenomenon of embrittlement of gun steel was discovered and called “Krupp disease”.
The small fluctuation of chemical composition significantly affects the structure and properties of steel, especially the change of carbon and chromium content has a great impact on the impact energy.
Therefore, in actual use, the composition should be controlled and adjusted reasonably according to the application conditions and the requirements of all aspects.
In this post, 11 furnaces of 14Cr17Ni2 alloy forged bars with different chemical compositions were treated by the same heat treatment system, and then the Charpy impact tests of U-notch specimens were carried out.
The effects of different chemical compositions on impact energy were compared and analyzed;
The Charpy impact test of U-notch specimen was carried out after one furnace of forged bar was treated with different tempering temperatures, and the effects of different tempering temperatures on impact energy were compared.
Test materials and methods
11 furnaces of 14Cr17Ni2 alloy forged rods with a diameter of 90mm are used.
See Table 1 for the chemical composition.
Table 1 chemical composition of forged rod (wt%)
In order to study the effect of tempering temperature of 14Cr17Ni2 on impact energy, firstly, No. 10 forged rod was subjected to 990 ℃ / 1.5h / oil cooling, and after oil cooling quenching, it was tempered by holding at 300 ℃, 380 ℃, 400 ℃, 450 ℃, 520 ℃, 550 ℃, 600 ℃ and 680 ℃ for 4h / water cooling;
In order to study the effect of 14Cr17Ni2 chemical composition on impact energy, 11 furnace forged bars were quenched at 990 ℃ / 1.5h/oil cooling, and then tempered at 300 ℃ and 520 ℃ respectively.
According to GB / T 229-2020 metallic materials Charpy pendulum impact test method, the sample size is 55mm × 10mm × 10mm, the notch shape is U-shaped notch, and the longitudinal room temperature impact test is completed.
The metallographic structure was observed by Olympus Gx71 metallographic microscope.
The corrosion agent was CuCl2 (5g) + HCl (100ml) + ethanol (100ml).
Analysis of test results
Effect of tempering temperature on impact energy of 14Cr17Ni2
Fig. 1 shows the impact energy of 14Cr17Ni2 under different tempering temperatures.
By comparing the relationship between different tempering temperatures and impact energy, it is found that within the tempering range of 300 ℃ ~ 450 ℃, the impact energy of this material decreases from 100J to 19J, showing a significant decrease;
However, within the tempering range of 300 ℃ ~ 680 ℃, the impact energy generally decreases first and then increases.
When tempering at 680 ℃, the impact energy rises to 78J, and the lowest point of impact energy is 19J around 450 ℃.
It shows that the material has obvious temper brittleness.
The temperature range of temper brittleness is 350 ℃ ~ 550 ℃, and the temper temperature has great influence on the impact energy of the material.
Fig. 1 impact energy of 14Cr17Ni2 at different tempering temperatures
By comparing the macro morphology of the fracture under different tempering temperatures (Fig. 2) and that under different tempering temperatures (Fig. 3), the macro morphology of the fracture tempered at 520 ℃ is typical intergranular brittle fracture.
There are many bright surfaces on the fracture surface, and each bright surface is a grain boundary.
The macroscopic morphology of the fracture surface tempered at 600 ℃ is ductile fracture, which belongs to transgranular fracture and river pattern.
The fracture surface after tempering at 600 ℃ has obvious dimples and shear lips.
Fig. 2 macro morphology of fracture under different tempering temperatures
Fig. 3 fracture morphology under different tempering temperatures
The quenched structure of 14Cr17Ni2 is lath martensite + ferrite.
After tempering, martensite decomposes and carbide precipitates, while ferrite does not change.
When tempering at 200 ℃ ~ 300 ℃, the carbide precipitation in and between laths in the matrix structure gradually increases, but it still presents fine dispersion distribution, so the impact energy is high.
When tempering at 350 ℃, a small amount of carbides precipitate at the grain boundary.
When tempering at 400 ℃ ~ 550 ℃, the carbides precipitated between the laths and on the grain boundary increase obviously, and are distributed along the grain boundary and the lath with high dispersion.
The carbides precipitated at the grain boundary greatly reduce the impact energy of the steel, and cause the steel to appear obvious brittleness tendency and intergranular fracture at this time.
When the tempering temperature is higher than 600 ℃, the carbides are partially dissolved and the brittleness disappears.
Effect of chemical elements on impact energy of 14Cr17Ni2
14Cr17Ni2 steel belongs to martensite ferritic stainless steel, and the structure in quenched state is martensite + δ-ferrite + retained austenite.
During tempering, M23C6 type carbides precipitated from martensite and δ-ferrite and accumulated on the grain boundary, and martensite decomposed into tempered sorbite.
Fig. 4 is the metallographic microstructure corresponding to different impact energies.
It can be seen from Fig. 4 that the metallographic microstructure with impact energy of 52J and 35J has grain size of grade 5.
The biggest difference between the two is that the latter precipitates a large number of carbide particles on the grain boundary.
The carbide precipitation on the grain boundary greatly reduces the impact energy of the steel, and leads to the obvious brittle tendency and intergranular fracture of the steel at this time.
Fig. 4 metallographic microstructure corresponding to different impact energies
The main chemical components of 14Cr17Ni2 include C, Cr, Ni, Si, Mn, P and S.
The elements Mn and Si do not belong to the main alloy elements in 14Cr17Ni2.
Within the range of chemical composition, they have little influence on the structure of steel.
Therefore, the structure and properties of steel are mainly affected by C, Cr and Ni.
Ni itself has no obvious effect on temper brittleness.
Since chromium carbide affects the impact energy of forgings, the content of C and Cr in raw materials will directly affect the impact energy of forgings.
Fig. 5 shows the impact energy of 14Cr17Ni2 with different C and Cr contents.
The impact energy at 300 ℃ and 520 ℃ have almost the same trend, which verifies the conclusion that the impact energy of raw materials is the main reason.
Fig. 5 impact energy of 14Cr17Ni2 with different C and Cr contents
It can be seen from Fig. 5 that the impact energy of 14Cr17Ni2 generally decreases with the increase of Cr content, while when the Cr content is not different, the impact energy decreases with the increase of C content.
(1) Within the tempering range of 300 ℃ ~ 680 ℃, the impact energy of 14Cr17Ni2 tends to decrease first and then increase.
The lowest point of impact energy appears around 450 ℃, and the impact value is generally low within the range of 350 ℃ ~ 550 ℃, lower than 39J (GJB 2294A-2014).
It shows that the material has obvious temper brittleness, and the temper temperature has a great influence on the impact energy of the material.
350 ℃ ~ 550 ℃ is the temper brittleness interval temperature of the material.
(2) The fluctuation of the chemical composition of raw materials has a great influence on the impact energy of 14Cr17Ni2 material.
With the increase of the content of element C and Cr, the impact energy of the material shows a general downward trend.
(3) When 14Cr17Ni2 is tempered at 350-540 ℃ and the impact energy of the material is required, the content of elements C and Cr in the raw material shall be strictly controlled.