White corners of thin-walled gray iron castings are common defects in castings [1-4]. Generally, small castings have thin walls and are cast in green sand. Although the chemical composition of the molten iron is qualified, due to the influence of the casting wall thickness and the thermal conductivity of the casting, the thick and thin parts of the same casting. Both the inside and the outside may get a different organization. Especially the corners of the castings are prone to white mouths, which cause difficulties in machining, resulting in the so-called "hard material". Most of the parts of gray cast iron "material hard" occur in the part of the rough part. Such as: edges and corners, grooves, convex surfaces, surfaces, etc. Material hardness has a lot to do with the tendency of white mouth. Aiming at the difficult machining problems in the actual production of castings of a certain company, this paper conducts a systematic study, analyzes the causes of "hard materials", and proposes corresponding solutions.

1 Experimental materials and methods

Casting pig iron 22#, 26# and a machine casting number 0# were sampled on site. Sampling by wire cutting was performed respectively, and the observation of optical tissue and scanning tissue was performed. Chemical on cast iron and castings
Composition test to exclude the influence of trace elements on the processing performance of castings. The castings were sampled for metallographic observation in ZEISS optical and scanning microscopes, HBS-3000 digital Brinell hardness tester and HTM-1000TM micro hardness tester were used for hardness testing. The chemical composition of pig iron and castings is shown in Table 1.

2.1 Chemical composition analysis

China's cast iron pig iron standard (GB/T 718-2005) [5], in the standard, the Si content of 22# pig iron is 2.00% ~ 2.40%, and the Si content of 26# pig iron is 2.40% ~ 2.80%. According to Table 2, a company’s pig iron 22# and 26# test showed that the Si content of 22# pig iron was 1.86, which did not meet the lower limit of the standard.
It meets the standard, and the Mn content is also low. 26# pig iron P and S content is too high, P content reaches level 5, S content exceeds the standard, and contains a certain amount of Cr. The test composition of casting 0# shows that only Cr content of whitening elements has reached the whitening tendency, and the content of other trace elements has not reached the minimum content of causing whitening, so the impact is negligible. Compared with the selection of the five elements in the "Casting Handbook" [6], it can be seen that the carbon content of the castings in this study is relatively high, the Si content is relatively low, and the Mn content is relatively low.

2.2 Hardness test

In the HBS-3000 digital display Brinell hardness tester, the test is 1875 N, the indenter diameter is 2.5 mm, and the hardness of the 5 tests is shown in Table 2. On the digital microhardness tester, the white area in the optical photo was marked with microhardness. The results are shown in Table 3. Therefore, although the average macroscopic hardness of the matrix is very low, only the Brinell hardness is about 145 HB, the hardness of its local area is very high, reaching the Vickers hardness of about 1 000 HV. The smaller the pit, the higher the hardness. According to the literature, the hardness of phosphorus eutectic is 500~700 HV, ledeburite ≤ 800 HV, and carbide> 900 HV.

Therefore, the hardness analysis results show that the white area is hard and brittle cementite carbide, which basically excludes phosphorus eutectic, which is the main reason for the hard material. In order to accurately determine the composition of this carbide, energy spectrum analysis is required.

2.3 Energy spectrum analysis

The partial enlargement of the optical white area is shown in Fig. 2 and Fig. 3. It is characterized by the distribution of recessed holes in the matrix and the characteristic of eutectic. Therefore, the energy analysis of this area shows that the elements contained in the recessed part of the area are Fe, P and C element, so it is judged as Fe3 (C, P), P element is stored
Segregation. The P element in the recessed part is higher, not a eutectic product, but a hole formed by the final solidification and shrinkage. Figure 4 Energy spectrum analysis results show that in addition to Fe, P and C elements, the white area contains Cr and V, forming alloy carbides, which are harder and harder.
Take cutting.

2.4 Organizational analysis

The optical photo shows the metallographic structure of the casting made by etching with 4% nitric acid alcohol, as shown in Figure 5. Among them, a, b, c, and d are the core structure of the casting, and e, f, g, and h are the edge structure of the casting. a, b, c, d and e, f, g, h correspond to 50, 100, 200, and 1,000 times tissue photos. The scanned tissue photo is shown in Figure 6, and the arrow points to the white area in the corresponding optical tissue photo, which is carbide. The white block areas are carbides, the flakes are graphite, and the gray areas are pearlite. It can be seen that the metallographic structure is ferrite + pearlite + graphite + carbide, pitted structure. The whiteness of the edges is obviously more serious than that of the heart. Comparing with GB/T7216-2009, it can be seen that [7], the heart tissue is the initial
The raw star-shaped graphite F type has a length of about 150 μm and a width of about 5 μm. This is formed by high-carbon molten iron under relatively large subcooling conditions. The edge layer structure is fine curly graphite gathered in a chrysanthemum-like distribution of type B graphite. The length is about 100 μm and the width is 3 μm. Determine the number of carbides
The amount of carbide in the heart tissue is about 5%, reaching level 3. The amount of carbides in the edge tissue is about 10%, reaching level 4. When the carbon is in the form of graphite, the graphite can be used for lubrication during machining, and the cutting is easy. When carbon exists in the form of carbide (Fe3C), because Fe3C cementite is hard and brittle, machining is difficult, especially when it contains other alloying elements (such as Cr), alloy cementite ((Fe, M) 3C) This compound is harder and more difficult to cut, and the so-called "hard material" problem occurs during machining [8]. Therefore, in the casting process of gray iron parts, it is necessary to reduce the amount of carbon to avoid the appearance of carbides, and take some measures to promote carbon graphitization if necessary.

3 Analysis and discussion

The main factors affecting the machining performance of castings are the chemical composition of the cast iron and the solidification cooling rate. The carbon content and silicon content in the chemical composition of cast iron are the two most important controlling factors. The cooling rate of the casting mainly depends on the wall thickness of the casting. When the content of carbon and silicon in cast iron is constant, the thinner the casting wall, the greater the tendency of cast iron to whiten. When the wall thickness of the casting is constant, the greater the total content of carbon and silicon in the cast iron, the more thorough the degree of graphitization of the cast iron.

The carbon equivalent of the casting in this study is 4.36%, which is a high-carbon equivalent casting; the Si/C ratio is 0.46, which is low. Increasing the carbon equivalent makes the graphite flakes thicker, the number increases, and the strength and hardness decrease. Increasing Si/C can reduce the tendency of white mouth.

In the production of gray cast iron, the influence of overheating and the effect of gestation also need to be considered. Increasing the temperature of the molten iron within a certain range can make the graphite refinement, the matrix structure finer, the tensile strength increased, and the hardness decreased. It is necessary to comprehensively consider the composition of the charge, the smelting equipment, and the energy factors of the chemical composition. The inoculation treatment is to add the inoculant to the molten iron to change the metallurgical state of the molten iron before the molten iron enters the casting cavity, and to increase the non-spontaneous nucleus is graphite refinement. Thereby improving the microstructure and performance of cast iron. Common inoculants include ferrosilicon, calcium silicon and graphite. Combining our products and production costs, it is recommended to use ferrosilicon (75% silicon, the addition amount is about 0.4% of the weight of the molten iron). Second, barium ferrosilicon and strontium ferrosilicon. Ferrosilicon inoculates quick-acting effect, reaching the peak within 1.5 min, and declining to non-pregnant state after 8~10 min, which can reduce the degree of supercooling and white mouth tendency, increase the number of eutectic clusters, form A-type graphite, improve the uniformity of the section, and increase the resistance. Tensile strength is 10-20MPa. Disadvantages: poor resistance to decay. If the late inoculation process is not used, it is not ideal for large wall thickness differences and long pouring time.

Barium ferrosilicon has a stronger ability to increase the number of eutectic clusters and improve section uniformity than ferrosilicon. The ability to resist decline is strong, and the inoculation effect can be maintained for 20 minutes. Suitable for various grades of gray cast iron parts, especially suitable for large-scale thick-walled parts and production conditions with long pouring time.

Strontium ferrosilicon has 30% to 50% higher whiteness reduction ability than ferrosilicon, and has better section uniformity and anti-decay ability than ferrosilicon. At the same time, it does not increase the number of eutectic clusters, is easy to dissolve, and has less slag. Thin-walled parts, especially parts requiring shrinkage and leakage with high eutectic clusters are not desired.

The Mn content of the castings in this study is low. Manganese itself is an element that hinders graphitization, but manganese can offset the strong whitening effect of sulfur. Therefore, within the limit of offsetting the effect of sulfur, manganese actually plays a role in promoting graphitization. Practice has proved that the increase in manganese content can not only increase and refine pearlite, but it is not harmful to appropriately relax the control of sulfur. Therefore, it is recommended to appropriately increase the Mn content.

4 Conclusion

The main reason for the machining difficulty of castings in this study is the appearance of cementite carbides, especially the cementite carbides of alloys containing Cr, V and other elements are the main reason for machining difficulties. To improve this problem, the first idea is to reduce or eliminate carbides in the organization. Changing the composition of castings and adjusting the production process are effective ways. Combined with the specific production situation of the castings in this study, the following production suggestions are put forward:

• (1) To increase the silicon content, the first choice is to add an inoculant before pouring. For ferrosilicon (75% silicon), barium ferrosilicon and strontium ferrosilicon can also be used according to the pouring time and on-site effects. It is recommended to use compound inoculants (Si-Ba and RE-Si).
• (2) Increase the manganese content in the casting to offset the strong white mouth effect of sulfur.
• (3) Improve the quality of pig iron. 26#Pig iron P and S content is too high.
• (4) Reduce Cr content in castings. The high content of Cr (>0.1) in the castings can already produce the effect of whitening. Cr can significantly increase the hardness and damage the machining performance.