Prevention of casting defects of pressure bearing base of 00bob cylinder

00bob cylinder is the key pressure bearing high temperature steel casting of supercritical steam turbine. Its function is to separate the flow passage part of steam turbine from the atmosphere and form a closed steam chamber to ensure the energy conversion process of steam in the steam turbine. The product is made of zg25crmov-1 + 7 cast steel. When the design of the pouring system is unreasonable and the pouring operation is improper, it is easy to produce sand holes, air holes, cracks, etc.

1) Two 00bob cylinders are completed in molding and core making, and the box is closed and poured two days later. During this period, the workshop environment temperature is 10-15 ℃, and the relative humidity is 9% – 11%; the main material of molding and core making is furan resin sand with the strength of 8.5mp. After the box is closed, the negative pressure treatment is carried out without floating sand and other sundries.

2) The purity of molten steel meets the requirements of JB / t10087-2001 technical conditions for pressure bearing steel parts of steam turbine. During the pouring process, riser boiling occurs in both parts.

3) After machining, ut flaw detection was carried out on the split surface, and it was found that the two products were all near the riser, and there were defects such as sand inclusion, porosity and cracking in the area with a depth of 60-100mm.

Defect prevention plan

1) Improvement measures for sand inclusion prevention: change the layout of the riser, and arrange the riser in the middle of the matrix away from the end of the core; arrange the slag collecting tank between the risers of the split face to ensure the floating of steel slag and impurities; ensure enough water for pouring steel to avoid the generation of shrinkage cavity [3]; change the support mode of the sand core, fix the end of the core, and support the fixed sand core.

2) Improvement measures for prevention of air hole: control the screening of reclaimed sand to ensure that the remaining volume of 100 mesh screening is ≤ 10% and the micro powder is ≤ 2%; the sand core and sand mold are arranged in the forward direction and bypass the air outlet, and the air permeable rope is added in the sand mold to avoid turbulence and gas entrapment.

3) crack prevention measures: sand core filling with dry sand and foam to ensure the sand core yield; Ricoh surface treatment is smooth and dry, avoiding chilling cracks.

All the 6 sets of improved products are qualified. It has been proved by practice that the improvement of core stability and yield, the control of recycled sand screening and micro powder and the increase of air permeability are effective in solving the defects of sand inclusion, air hole and crack of 00bob cylinder, and the internal quality and surface quality of this kind of products are improved.

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