The proportion of molds in aerospace, automotive, automation and daily necessities manufacturing is increasing.
Cold work die steel is used for stamping, drawing, bending, cold heading, wire rolling and bending of metal or non-metallic materials.
In order to meet the impact, wear, bending, shearing and other conditions of the tool and die, the material should have excellent strength and toughness.
Cr12MoV steel is a high chromium micro deformation die steel with high wear resistance, hardenability and thermal stability.
It is often used to manufacture cold working dies and tools under high wear resistance, micro deformation and high load service conditions.
Although steel has high strength, hardness and good wear resistance, it has poor toughness.
A drawing die is made of Cr12MoV steel. Its technological process is: forging → blanking → machining → quenching → tempering → grinding.
A Cr12MoV drawing die of a certain unit was quenched at 1030 ℃ and tempered at 200 ℃, and cracks were found after tempering.
The failed drawing die is shown in Fig. 1a.
Taking the test block from the failure drawing die, it can be seen that the fracture fringe is radially distributed along the longitudinal direction, which belongs to brittle cleavage fracture, as shown in Fig. 1b.
Fig. 1 Macro morphology of failed drawing die
1. Test method
OBLF direct reading spectrometer was used to detect the chemical composition of the failed drawing die;
Cut the sample with a wire cutting machine, grind and polish the sample, and then etch it with 4% nitric alcohol;
Use DM2000X inverted metallographic microscope to inspect the metallography of the sample;
Test the hardness with a digital Rockwell hardness tester.
2. Test results and analysis
1. Chemical composition detection
Use direct reading spectrometer to detect the chemical composition of the failed drawing die and the raw materials of the same batch of plates. See Table 1 for the results.
Test 1 shows the chemical composition of the failed drawing die, and test 2 shows the chemical composition of the raw materials of the same batch of plates as the drawing die.
The composition meets the requirements for the chemical composition of Cr12MoV tool and die steel in GB/T 1299-2014, as shown in Table 1.
Table 1 Chemical Composition of Cr12MoV (Mass Fraction) (%)
| Element | C | Si | Mn | Cr | V | Mo | S | P |
| criterion | 1.45~1.70 | ≤0.4 | ≤0.4 | 11.0~12.5 | 0.15~0.30 | 0.40~0.60 | <0.02 | <0.03 |
| Detection 1 | 1.46 | 0.31 | 0.29 | 11.22 | 0.21 | 0.41 | 0.013 | 0.022 |
| Detection 2 | 1.49 | 0.30 | 0.29 | 11.50 | 0.21 | 0.41 | 0.011 | 0.022 |
2. Hardness test
Brinell hardness test was carried out for raw materials of the same batch, and the hardness was 229HBW and 241HBW, meeting the requirements of GB/T 1299-2014 Tool and Mold Steel Annealing State 207~255HBW.
3. Metallographic examination
According to the requirements of GB/T 1299-2014 Tool Steel, the Cr12MoV steel delivered in annealed state shall be inspected for the unevenness of eutectic carbide, and the qualification level shall be as specified in Table 2.
The thickness of this sheet is 60mm, and the qualification level of eutectic carbide non-uniformity is ≤ 4.
Table 2 Qualification Level of Eutectic Carbide Heterogeneity of Cold Work Die Steel
| Diameter or side length of steel/mm | Eutectic carbide nonuniformity qualification level/grade | |
| 1 groups | 2 groups | |
| ≤ | ||
| ≤50 | 3 | 4 |
| 50~70 | 4 | 5 |
| 70~120 | 5 | 6 |
| 120~400 | 6 | agreement |
| ＞400 | agreement | agreement |
(1) Metallographic examination of invalid drawing die
Take a test block from the drawing die with cracks for metallographic inspection.
Fig. 2 shows the metallographic structure of 100 times that of the drawing die.
The matrix is tempered martensite, a small amount of retained austenite, granular eutectic and secondary carbide.
It can be seen from Fig. 2a that the carbide aggregates into piles, and the massive and strip carbide particles are relatively large, which are banded and reticulated.
The eutectic carbide unevenness is rated as Grade 6 according to the fourth level diagram of GB/T 14979-1994 standard, which does not meet the requirements of GB/T 1299-2014 for tool and die steel.
The network carbide in the raw material is not improved during forging, which promotes the increase of brittleness at the grain boundary, cracks appear under the effect of quenching stress, and the cracks extend along the direction of carbide distribution, as shown in Fig. 2b.
Fig. 2 Quenching and tempering structure (100 ×）
Fig. 3 shows the metallographic structure of drawing die 500 times, and the maximum size of massive carbide is 0.165mm.
According to JB/T 7713-2007, the rating of massive carbide is greater than Grade 5, as shown in Table 3.
Generally, the carbide grade of Cr12MoV steel die shall not be greater than Grade 3.
These agglomerated massive carbides are not easy to dissolve into the matrix under normal quenching heating temperature, which causes the matrix with poor carbon and alloy elements to be in an overheated state, and the grains grow up.
After quenching, coarse quenched martensite structure is obtained.
Coarse carbide and martensite are both microstructures with great brittleness, which are easy to crack when quenched or under heavy load, and most cracks extend along the direction of carbide distribution.
Fig. 3 Quenching and tempering structure (500 ×）
Table 3 Grade of Massive Carbide
| Level/grade | 1 | 2 | 3 | 4 | 5 |
| Maximum size of massive carbide/mm | 0.009 | 0.013 | 0.017 | 0.021 | 0.025 |
(2) Metallographic inspection of raw materials of the same batch of plates
Take test blocks from raw materials of the same batch of plates for metallographic inspection.
Fig. 4 shows the 100 times metallographic structure of the raw material, and the matrix is sorbite, white particles, strip carbides and eutectic carbides.
The carbide particles in the strip shape are relatively coarse and distributed in a strip and network form.
The unevenness of eutectic carbide is rated as Grade 6 according to the fourth level chart in GB/T 14979-1994, which does not meet the requirements for tool and die steel in GB/T 1299-2014.
Fig. 4 Raw material organization (100 ×）
Fig. 5 shows the metallographic structure of 500 times of the raw material, and the maximum size of the massive carbide is 0.093mm.
According to JB/T 7713-2007, the rating of the massive carbide is greater than Grade 5, as shown in Table 3.
The edge of raw material carbides is sharp, and the existence of sharp corners can easily cause stress concentration and increase the risk of quenching cracking and grinding cracking.
Fig. 5 Raw material organization (500 ×）
3. Process analysis and improvement
By re forging the raw materials, the eutectic carbide is crushed and refined, and the coarse dendritic eutectic carbide is broken to improve the uniformity of carbide distribution.
Spheroidizing annealing pretreatment is added before heat treatment and quenching.
The annealing process is 550 ℃+860 ℃ (180min)+750 ℃ (300min), which is cooled to room temperature in the furnace.
The quenching process is 550 ℃ (preheating 1)+850 ℃ (preheating 2)+1030 ℃ quenching, reducing the heating rate from 15 ℃/min to 10 ℃/min.
The tempering temperature shall be adjusted to 230 ℃ and the tempering shall be conducted twice.
The drawing die is heat treated with a new process. The metallographic structure is shown in Fig. 6.
The matrix is tempered martensite, a small amount of retained austenite, white block and granular carbide.
The non-uniformity of carbide is level 2. The hardness of the drawing die tempered by this process is 60.3HRC and 61.5HRC, meeting the technical requirements of 58~62HRC. No quenching cracks are found.
Fig. 6 Tempering metallographic picture after process improvement (100 ×）
4. Conclusion
1) The main reason for the failure of the drawing die is that the carbides in the raw materials gather into piles, the blocky and strip carbides are relatively coarse, and they are banded and reticulated, and the degree of unevenness of eutectic carbides is 6.
The size of massive carbide is 0.069mm, and its grade is greater than Grade 5.
Coarse carbide particles and severe carbide segregation cause stress concentration and crack under quenching stress.
2) By re forging the raw materials, the eutectic carbide is crushed and refined, and the coarse dendritic eutectic carbide is broken to improve the uniformity of carbide distribution.
Before heat treatment and quenching, spheroidizing annealing pretreatment is added to reduce the heating rate and quenching stress during quenching, so that products meeting technical requirements can be produced.
