Die casting mold is an important process equipment used in production process of die-casting, which has an important impact on quality of casting. Quality of die casting mold affects strength and surface quality of casting. Since liquid alloy enters cavity at a high temperature and high speed, it causes impact on mold parts, and die casting mold has higher requirements on wear resistance, corrosion resistance, strength and so on. Excellent molds can not only reduce frequency of replacement, reduce production costs, improve production efficiency, but also help to form excellent castings. Therefore, how to improve quality of die casting mold and extend their service life is an important issue to be solved in actual production and research. HHD steel is a new type of hot work die steel with high hardenability. It has good hardenability and can obtain a toughened structure of 10 ~ 20nm carbides with a layered structure between low-carbon lath martensite crystals and a dense oxide film resistant to high temperature oxidation. Soft nitriding process is widely used for improving wear resistance and corrosion resistance of die casting mold parts, and is used for surface treatment of mechanical structural parts or mold parts. Soft nitridation is process of infiltrating carbon and nitrogen atoms on the surface of materials through various chemical or physical means. Hardness and fatigue strength of surface nitrided layer structure after soft nitridation treatment are excellent, but research on soft nitridation system of die casting mold parts is still less. Effect of soft nitriding treatment temperature on properties of die-casting HHD steel is mainly studied. 

 

Different nitriding, nitriding plus oxidation treatment processes and parameters are used to perform nitriding treatment on the surface of HHD mold steel, as shown in Table 1.

 

A. Sample 1 (530 ℃)

 

B. Sample 2 (550 ℃)

 

C. Sample 3 (570 ℃)

 

D. Sample 4 (590 ℃)

Figure 1 Thickness of infiltration layer at different soft nitriding temperatures

From Figure 1, HHD steel substrate and transition layer between soft nitrided layer and substrate can be distinguished. In same processing time, salt bath temperatures of samples are grouped. Bath temperatures are separated by 20℃. From screenshots of four groups, it can be seen that thickness of nitriding layer of four groups of samples gradually increases with increase of processing temperature. When nitriding temperature reached 590℃ (sample 4), a bright white transition layer appeared in nitriding layer and substrate (see Figure 1 (d)). At this time, thickness of specimen and transition zone was the widest.

 
Figure 2 Hardness distribution at different soft nitriding temperatures

Hardness is one of important indicators of soft nitriding treatment of mold steel. Hardness of test infiltration layer is changed from surface to substrate. Test results are shown in Figure 2. It can be seen from Fig. 2 that surface hardness of sample under temperature treatment process at 590℃ is the highest, which can reach 1605 HV, while surface hardness of sample under treatment temperature of 530℃ is the lowest, which is 1145 HV. Surface hardness of sample was 1362HV at 570 ℃. Through testing hardness, surface morphology of transition zone and matrix of four groups of samples, it was found that hardness of transition zone of sample was the highest, hardness of matrix was the lowest, and thickness of nitrided layer was the largest under temperature treatment process of 590℃. When treated at 530℃, hardness of transition zone is the lowest, thickness of nitrided layer is the thinnest, and matrix hardness is the largest. According to test results shown in Figure 2, hardness of four treatment processes decreased with increase of test depth. Hardness at 550℃ at a distance of 85 to 115 μm from surface was higher than that of the other three processes. Hardness of the other three processes is 530, 570, and 590℃ in order. Nitriding temperature relative to substrate at this time is equivalent to tempering substrate, which causes hardness of HHD steel to decrease.

 
Fig. 3.Effect of 120min soft nitriding temperature on impact toughness of infiltration layer

Effect of soft nitriding temperature on impact toughness of infiltrated layer and substrate under infiltrated layer was also tested in the test. Test results are shown in Figures 3 and 4 (Matrix refers to substrate without soft nitriding). Results shown in Figure 3 indicate that soft nitriding treatment is at 530 ~ 590℃. Increase of treatment temperature can improve impact toughness of infiltration layer to a small extent, but impact toughness of soft nitrided sample is lower than that of Nitrided HHD steel.

 

Fig. 4.Effect of 120-minute soft nitriding temperature on impact toughness of substrate core

Figure 4 shows effect of soft nitriding temperature on impact toughness of core of HHD steel. According to curve in Figure 4, it can be seen that impact toughness of core of substrate is improved after soft nitriding treatment of HHD steel at 530 ~ 590℃ for 120min; a peak occurs at 570℃, and then impact toughness decreases at 590℃, but it is higher than 530℃ and 550℃. This is also because soft nitriding treatment is equivalent to reducing impact toughness caused by tempering substrate. 

 

A. Sample 1

 

B. Sample 2

 

C. Sample 3

 
D. Sample 4
Figure 5 Cracks after 500 thermal fatigue cycle tests

Figure 5 shows effect of soft nitriding process on initiation and propagation of thermal fatigue cracks on the surface of HHD steel after 500 thermal fatigue cycle tests. It can be seen from Figure 5 that there are 13 microcracks on the surface of sample 1, crack width is 2 ~ 5μm, and the longest main crack length is about 120μm; there are 8 microcracks on the surface of test 2, but crack width is larger, which is 4 ~ 7.2 μm, the longest main crack length is about 216 μm; there are 7 micro-cracks on the surface of sample 3, crack width is small, in the range of 2 ~ 3 μm, the longest main crack length is about 127 μm; there are 6 micro-cracks on the surface of sample 4, crack width is about 3 μm, the longest main crack length is the longest, about 290 μm.

 

A. Sample 1

 

B. Sample 2

 

C. Sample 3

 
D.  Sample 4
Figure 6 Cracks after 1000 thermal fatigue cycle tests

Figure 6 shows effect of soft nitriding process on the initiation and propagation of thermal fatigue cracks on the surface of HHD steel after 1000 thermal fatigue cycle tests for four kinds of samples. Compared with 500 fatigue cycle morphologies, surface crack of preformed notch has larger extension and expansion after 1000 thermal fatigue cycle tests. There are 8 cracks on the surface of sample 1 with a crack width of about 10 μm and the longest main crack length of about 260 μm; 7 cracks on the surface of sample 2 with a crack width of about 10 μm and the longest main crack length of about 870 μm; 7 cracks on the surface of sample 3 with a crack width of about 14 μm and the longest main crack length is about 1005 μm. Surface cracks of sample 4 are distributed around preformed notch in a network shape, about 8 cracks, but crack width is small, about 9 μm. The longest main crack length is about 830 μm.

 

A. Sample 1

 

B. Sample 2

 

C. Sample 3

 
D. Sample 4
Figure 7 Cracks after 2000 thermal fatigue cycle tests.

Figure 7 shows effect of soft nitriding process on the initiation and propagation of thermal fatigue cracks on the surface of HHD steel after 2000 thermal fatigue cycle tests for four samples. It can be seen from Figure 7 that crack size of sample 1 continued to increase, and there were 8 main cracks on the surface, but length was relatively short. Except for the longest main crack of about 425 μm, remaining cracks were less than 300 μm and crack width was about 15 μm. There are 10 cracks on the surface of sample 2, which are relatively evenly distributed around preformed notches, width is relatively close to about 16 μm. The longest main crack length is slightly increased compared to 1000 thermal fatigue cycle tests, crack length is about 950 μm; There are 8 cracks on the surface of sample 3 with a width of about 11 μm and the longest crack length of about 750 μm. A serious oxide scale exists on the surface of Sample 4, most of cracks are covered by it. Crack width is about 10 μm and the longest main crack length is about 685 μm.

 

In summary, after thermal fatigue test testing, order of thermal fatigue resistance of HHD steel after different soft nitriding treatment temperatures and different numbers of thermal fatigue cycles is obtained and is shown in Table 2.

 
Fig. 8 Effect of different nitriding treatment temperature on wear at 120 min (+ oxidation treatment)

High-temperature wear tests were performed on HHD steel samples at different nitriding temperatures. As shown in Figure 8, as nitriding temperature increased, ability of HHD steel to resist high-temperature wear gradually increases.

 

A. Morphology of Sample 1

 

B. Morphology of peeling layer of sample 1

 

C. Morphology of sample 2

 

D. Morphology of peeling layer of sample 2

 

E. Morphology of sample 3

 

F. Morphology of peeling layer of sample 3

 

G. Sample 4 furrow morphology

 
(H) Morphology of peeling layer of sample 4
Fig. 9 Effect of soft nitriding treatment temperature on the surface morphology of HHD steel at high temperatures 

Fig. 9 shows effect of soft nitriding treatment temperature on micro-morphology of high-temperature wear surface of HHD steel. It can be seen from Fig. 9 that after high-temperature abrasion test, layered spalling occurred on the surface of sample; at 530 ~ 590℃, with increase of soft nitriding temperature, area of spalled layer, depth and number of furrows can be reduced. Increasing temperature of soft nitriding treatment can improve high temperature wear resistance of HHD steel, and with gradual increase of temperature, high temperature wear resistance of HHD steel is gradually improved.