TPT-January-2015

Article

Shandong SiFang Technical Development Co Ltd

the suitable ratio of Cr/C can result in excellent comprehensive performance of hardness and toughness. Element Mo distribution in the phases of high chromium alloy is about 50 per cent in Mo 2 C, about 25 per cent in M 7 C 3 carbide effective to improve hardness and wearing resistance, in addition about 23 per cent dissolved in the matrix, which significantly improves alloy hardenability. The combination of V and C can generate primary carbide and afterwards secondary carbide, so that the solid solution amount of carbon in the matrix will decline and the point Ms will increase. Certain amounts of V can refine microstructure during solidification, having a favourable effect on elimination of bulky columnar grain. The binding force between W and C atoms is large, improving martensite stability in high temperature, thereby increasing red hardness. In hot quenching, partial W carbide dissolves into the matrix to improve hardenability. Ni can strengthen the matrix to improve the material comprehensive performance, and improve hardenability. Ti can form stable TiC nuclei, and refine eutectic microstructure. Adding Nb can make the precipitation of high hard Nb (CN) particles (2,000HV) in the matrix, and these particles have a high melting point, regular shape, small size and uniform distribution in the matrix. Therefore, adding Nb can control the solidification segregation effectively, improving its comprehensive performance and saving Ni or Cr. The chemical composition of new high chromium die steel has been designed based on service condition of crimping dies and effects of various alloying elements listed in Table 2. 3.2 Research and development of cast form processing Cast form processing is another key factor to ensure the die manufacturing quality. The new casting processing and equipment have been designed according to the crimping die shape and casting character of new high chromium die steel. Electrical furnace smelting and refining ensure purity of molten steel and advisable casting system ensures quick uniform filling, reducing segregation effectively.

spares, so Baosteel is urgent to purchase new dies. Due to the intellectual property protection of the German manufacturer of the original dies, Baosteel has difficulty getting manufacturing details. Because of the tiny field of crimping die manufacturing and manufacturing difficulty, official publication and peer communication have demonstrated that crimping dies in large diameter pipe plants have not been manufactured of a similar material in China. So far the research on crimping processes has concentrated on crimping width setting under different pipes and different dies while, as for the die itself, there is less research on the die manufacturing, including the material selection and the manufacturing process [3-6] . After the service condition analysis and research of the crimping die, neither the chemical composition nor the forging processing of the original German manufacturer has been imitated simply, and the cast processing solution of new high chromium alloy has been developed eventually. The detailed manufacturing drawings of the dies have been determined with the original dies through geometrical measurement of three-coordinates measuring machine and checking of the die assembly matching of the equipment body. 3.1 Research and development of new high chromium alloy High chromium alloy is a material of chemical composition that offers a wide range of uses; different chemical composition and heat treatment processing result in distinct microstructure, hardness and toughness. The development of new material must been in accordance with the service condition of the crimping dies. In high chromium alloys the basic and important alloying elements are C and Cr, whose content has a decisive effect on the amount of carbide, the quantity ratio of M 7 C 3 carbide and total carbide, hardness, toughness and hardenability. The microhardness of M 7 C 3 carbide is about 1,800HV, while the microhardness of M 3 C carbide is only about 800HV, therefore 3 Self-development of crimping die

The developed special device breaks up initially formed dendrites, and the small dendrites distribute throughout the alloy liquid, forming effective crystal nucleus, which result in fine uniform equiaxed grains in the cast dies; meantime, casting defects, such as shrinkage porosity, are avoided generating in order to obtain high quality casting blanks.

Figure 3: Ultrasonic testing results of cast dies (distance-amplitude curve)

C

Si

Mn

P

S

Cr

Ni

Mo

Ti

V

Cu

RE

1.80~ 2.80

≤ 1.0

≤ 1.0

≤ 0.05

≤ 0.05

12.0~ 22.0

0.8~ 3.0

≤ 0.30

≤ 1.0

≤ 1.0

0.01~ 0.5

0.5~3.0

Table 2: Chemical composition of new high chromium die steel (mass per cent, %) [7]

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J anuary 2015