Material 14Cr17Ni2 belongs to the martensitic ferritic stainless steel category and has good corrosion resistance and high mechanical properties. It is resistant to oxidizing acids and organic acids in aqueous solutions.
This alloy is commonly used for manufacturing structural parts and fastening parts that require corrosion resistance. The components are subject to tension, shear, and impact, and must not only have conventional mechanical properties but also good impact toughness.
However, the hot processing of 14Cr17Ni2 is complicated and often results in unqualified mechanical properties. According to statistics, the impact energy is mainly affected, which increases production costs, delays delivery dates, and causes unnecessary losses.
This post presents the results of experimental research that proves 14Cr17Ni2 has obvious temper brittleness at medium temperatures. The phenomenon of temper brittleness was first discovered during World War I and was referred to as “Krupp disease.”
The small fluctuations in chemical composition can significantly impact the structure and properties of steel, and changes in carbon and chromium content have a significant effect on impact energy.
Therefore, in practical use, the composition should be controlled and adjusted according to the application conditions and requirements. Eleven furnaces of 14Cr17Ni2 alloy forged bars with different chemical compositions were treated with the same heat treatment system, and Charpy impact tests of U-notch specimens were performed.
The impact of different chemical compositions on impact energy was compared and analyzed. In addition, the Charpy impact test of U-notch specimens was carried out after one furnace of forged bars was treated with different tempering temperatures, and the impact of different tempering temperatures on impact energy was 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%)
| Serial number: | C | Cr | Ni | Mn | Si | S | P |
| 0.11~0.17 | 16~18 | 1.5~2.5 | ≤0.8 | ≤0.8 | ≤0.015 | ≤0.03 | |
| 1 | 0.15 | 16.1 | 1.77 | 0.56 | 0.28 | 0.0018 | 0.022 |
| 2 | 0.14 | 16.2 | 1.77 | 0.56 | 0.28 | 0.0018 | 0.022 |
| 3 | 0.15 | 16.21 | 2.27 | 0.5 | 0.58 | 0.0018 | 0.02 |
| 4 | 0.16 | 16.3 | 2.23 | 0.37 | 0.054 | 0.0026 | 0.013 |
| 5 | 0.14 | 16.45 | 2.26 | 0.6 | 0.12 | 0.0004 | 0.022 |
| 6 | 0.14 | 16.46 | 2.27 | 0.62 | 0.15 | 0.0003 | 0.023 |
| 7 | 0.15 | 14.46 | 2.25 | 0.46 | 0.1 | 0.0024 | 0.012 |
| 8 | 0.14 | 16.47 | 2.3 | 0.34 | 2.3 | 0.0015 | 0.02 |
| 9 | 0.16 | 16.49 | 2.26 | 0.52 | 0.31 | 0.0018 | 0.013 |
| 10 | 0.15 | 16.5 | 2.23 | 0.24 | 0.51 | 0.0012 | 0.011 |
| 11 | 0.12 | 16.51 | 2.34 | 0.47 | 0.34 | 0.006 | 0.017 |
To examine the impact energy of 14Cr17Ni2 as a result of tempering temperature, a No. 10 forged rod was first subjected to a 990 ℃ heat treatment for 1.5 hours using oil cooling. After oil quenching, the rod was tempered by holding it at temperatures of 300 ℃, 380 ℃, 400 ℃, 450 ℃, 520 ℃, 550 ℃, 600 ℃ and 680 ℃ for 4 hours and then cooled with water.
To investigate the effect of 14Cr17Ni2’s chemical composition on impact energy, 11 furnace forged bars were subjected to a 990 ℃ heat treatment for 1.5 hours using oil cooling, followed by tempering at 300 ℃ and 520 ℃ respectively.
The sample size for the Charpy pendulum impact test, as per GB/T 229-2020 metallic materials method, was 55mm × 10mm × 10mm with a U-shaped notch and was performed at room temperature in the longitudinal direction.
The metallographic structure was observed using an Olympus Gx71 metallographic microscope.
The corrosion agent used was a mixture of CuCl2 (5g), HCl (100ml), and 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 was discovered that within the tempering range of 300°C to 450°C, the impact energy of the material significantly decreases from 100J to 19J.
However, within the tempering range of 300°C to 680°C, the impact energy decreases initially and then increases. When tempering at 680°C, the impact energy increases to 78J, with the lowest point of impact energy around 450°C at 19J.
This indicates that the material exhibits obvious temper brittleness. The temperature range for this temper brittleness is 350°C to 550°C, and the tempering temperature has a significant impact on the material’s impact energy.
Fig. 1 impact energy of 14Cr17Ni2 at different tempering temperatures
By comparing the macroscopic morphology of the fracture under different tempering temperatures (Fig. 2 and Fig. 3), it can be seen that the fracture tempered at 520 ℃ exhibits a typical intergranular brittle fracture. This is characterized by numerous bright surfaces, each of which corresponds to a grain boundary.
On the other hand, the macroscopic morphology of the fracture surface tempered at 600 ℃ displays ductile fracture, characterized by transgranular fracture and a river pattern. This is evident from the presence of clear dimples and shear lips on the fracture surface.
Fig. 2 macro morphology of fracture under different tempering temperatures
Fig. 3 fracture morphology under different tempering temperatures
The microstructure of 14Cr17Ni2 after quenching is composed of lath martensite and ferrite.
Upon tempering, the martensite undergoes decomposition and the formation of carbides, while the ferrite remains unchanged.
When tempered at temperatures between 200℃ and 300℃, the precipitation of carbides in the matrix structure increases gradually and remains finely dispersed, resulting in high impact energy.
At 350℃, a limited amount of carbides precipitate at the grain boundaries.
Between 400℃ and 550℃, the amount of carbides precipitated between the laths and at the grain boundaries increases significantly, leading to a dispersed distribution along the laths and grain boundaries.
This increased precipitation of carbides at the grain boundaries leads to a significant decrease in the impact energy of the steel and a noticeable tendency towards brittleness and intergranular fracture.
When the tempering temperature exceeds 600℃, the carbides begin to dissolve, and the steel’s brittleness is reduced.
Effect of chemical elements on impact energy of 14Cr17Ni2
14Cr17Ni2 steel is a type of martensitic ferritic stainless steel. Its quenched state structure is composed of martensite, delta-ferrite, and retained austenite.
During tempering, M23C6 carbides precipitate from the martensite and delta-ferrite and accumulate on the grain boundaries. This results in the decomposition of martensite into tempered sorbite.
Figure 4 shows the metallographic microstructure at various impact energies. It can be observed that the metallographic microstructure with impact energies of 52J and 35J have a grain size of grade 5. However, the latter has a higher number of carbide particles precipitated on the grain boundaries.
The presence of these carbides on the grain boundaries significantly decreases the impact energy of the steel, making it more brittle and prone to intergranular fractures.
Fig. 4 metallographic microstructure corresponding to different impact energies
The primary alloy elements in 14Cr17Ni2 are C, Cr, and Ni, while Si and Mn have a minimal impact on the steel’s structure and properties. The presence of Ni has no significant effect on temper brittleness. The impact energy of 14Cr17Ni2 forgings is primarily determined by the content of C and Cr, as chromium carbide influences the impact energy.
This is demonstrated in Fig. 5, which displays the impact energy of 14Cr17Ni2 with varying levels of C and Cr. The impact energy at both 300°C and 520°C shows a similar trend, reinforcing the conclusion that the impact energy of the raw materials is the primary factor.
Fig. 5 impact energy of 14Cr17Ni2 with different C and Cr contents
It is evident from Figure 5 that the impact energy of 14Cr17Ni2 generally decreases as the Cr content increases. However, when the Cr content remains constant, the impact energy decreases as the C content increases.
Conclusion
(1) The impact energy of 14Cr17Ni2 within the tempering range of 300 ℃ to 680 ℃ tends to decrease first and then increase. The lowest point of impact energy occurs around 450 ℃, and the impact value is generally low within the range of 350 ℃ to 550 ℃, lower than 39J (GJB 2294A-2014).
This indicates that the material has significant temper brittleness and the tempering temperature has a significant impact on its impact energy. The interval temperature of 350 ℃ to 550 ℃ is considered the temper brittleness interval temperature for the material.
(2) The fluctuation of the chemical composition of raw materials greatly affects the impact energy of 14Cr17Ni2. As the content of elements C and Cr increases, the impact energy of the material shows a general downward trend.
(3) To ensure the impact energy of 14Cr17Ni2 when tempered at 350-540 ℃, the content of elements C and Cr in the raw material must be strictly controlled.
