Improvement of strength and hardening process of t

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Improvement of strength and hardening process of investment casting shell

Abstract: through the analysis of the requirements for the strength performance of shell and different hardeners, under the condition that the binder and refractory materials remain unchanged, the mixed hardening process of ammonium chloride and crystalline aluminum chloride is applied, and good economic results are achieved

key words: investment casting shell strength hardener

manufacturing shell is a key process in investment casting process. It not only determines the dimensional accuracy and surface roughness of castings, but also directly affects the manufacturing cost and production efficiency of castings. Years of practice has proved that due to the high residual strength of the shell, it brings difficulties to the sand cleaning and alkali boiling process of castings. Our factory consumes 4688.6 tons of steam (cost up to 257900 yuan), 26.8 tons of caustic soda (cost up to 92800 yuan) and crystalline aluminum chloride 1 in the shell making process every year, which further reduces the demand for raw materials by 6214 tons (cost up to 421600 yuan), occupying a large amount of production funds. Therefore, the hardening process of crystalline aluminum chloride, which affects the strength of shell, was improved. The mixed hardening process of ammonium chloride and crystalline aluminum chloride was applied, and good economic results were achieved

1 Analysis of the relationship between shell strength and hardener

from shell making, pouring to cleaning, there are three different strength indexes of shell, namely normal temperature strength, high temperature strength and residual strength. There is a certain relationship between the three intensities, but the formation mechanism and influencing factors are not completely the same. For example, on May 12 this year, if the strength at room temperature is insufficient, parts are easy to fall off in the shell making process, and are easy to deform or crack in the dewaxing process; If the high temperature strength is insufficient, shell deformation and fire escape (breakout) will occur during roasting and pouring; If the residual strength is too high, it will directly affect the shelling of the mold shell and the difficulty of sand removal of the casting

how to adjust the relationship between the three strengths of the shell so that it has high room temperature strength, sufficient high temperature strength and as low residual strength as possible is what we hope. According to the current situation of shell making process, it is necessary to analyze and improve the common hardeners under the condition that the binder and refractory materials remain unchanged

1.1 analysis of the characteristics of ammonium chloride hardener

as a hardener of sodium silicate shell, the hardening reaction formula of ammonium chloride is as follows:

2nh4cl+2o →

msio2 The SiO2 colloid generated from the reaction result of (n-1) h2o+2nacl+2nh3 ↑ +2h2o

will firmly bond the quartz powder and sand particles in the mold shell, so that the mold shell can obtain strength

ammonium chloride is the earliest sodium silicate shell hardener. Its main advantages are fast diffusion hardening, short shell making cycle, low shell residual strength and good shelling. Compared with the crystalline aluminum chloride hardener, the shell has poor high-temperature strength, is easy to fuzz during storage, releases ammonia during the hardening reaction, pollutes the air, has poor working conditions, and the equipment corrosion is relatively serious

1.2 analysis of the characteristics of crystalline aluminum chloride hardener

crystalline aluminum chloride is used as the hardener of water glass shell. In the hardening process, aluminum chloride and water glass are mutually neutralized and mutually promoted hydrolysis process; In this process, the pH value and stability of water glass decrease, resulting in the precipitation of silicon gel. At the same time, due to the role of water glass, the pH value of aluminum chloride increases, and Al (OH) 3 gel is precipitated through a series of hydrolysis and polymerization reactions to form a silica gel and aluminum gel coagulum, so the strength of crystalline aluminum chloride shell is high. The reaction formula is as follows

3 (2O) +2alcl3 →

3msio2 (n-1) H2O + 2Al (OH) 3 + 6nacl

crystalline aluminum chloride is used as hardener for sodium silicate shell. Its main advantage is that the shell has high temperature strength and thermal deformation resistance, and its thermal shock stability is better than that of ammonium chloride shell; After fully aging storage, the shell has high strength at room temperature; No harmful gas is released during hardening, which does not pollute the air; The hardening process is easy to control and the performance is stable. Its main disadvantage is that the hardening speed is slow, and it needs to be stored for a long time after hardening; The residual strength of shell is high and the shelling performance is poor

based on the above analysis results of ammonium chloride hardener and crystalline aluminum chloride hardener, their effects on shell strength are different. If the two hardeners are mixed in a certain proportion to form a mixed hardener, the advantages of the two can be brought into play and the disadvantages can be overcome

2 development and application of mixed hardening

ammonium chloride is mixed with crystalline aluminum chloride as a hardener to form an NH4Cl AlCl3 complex. The ammonia produced in the hardening process reacts with aluminum chloride to produce ammonium chloride, which is redissolved into the hardener, which not only eliminates the ammonia taste, but also maintains the hardener composition. Under the condition of ensuring a certain amount of cl- concentration required by the hardener except for low speed, the highest mass fraction of two single hardeners (ammonium chloride 20% and crystalline aluminum chloride 30%) is mixed in reverse with the concentration difference decreasing by 2% and 3%. Based on the experience of no ammonia precipitation and the use of a single hardener, the mass fraction of ammonium chloride is determined to be 8% - 10%, and the mass fraction of crystalline aluminum chloride is 20% - 24%, and the process test plan is formulated

2.1 shell making process of mixed hardener

(1) paint preparation and paint viscosity shall be in accordance with cicba/b02 The requirements of "paint preparation" shall be implemented, and the paint viscosity shall be taken as the upper limit of the process

(2) the specification and operation method of shell making and sanding shall be in accordance with cicba/b02 The regulations and requirements of shell making shall be implemented

(3) the hardening and drying of shell making shall be carried out according to the requirements specified in Table 1. Table 1 hardening and drying process parameters

hierarchical shell making line shell making manual shell making hardening temperature/℃ hardening time/min drying temperature/℃ drying time/min hardening time/min air drying time/min surface layer room temperature 8 ~ 1020 ~ 2512 ~ 149 ~ 1025 ~ 30 reinforcement layer room temperature 8 ~ 1025 ~ 3512 ~ 149 ~ 1025 ~ 30

2.2 test and application

according to the above shell making process, four kinds of precision castings were selected and four rounds of manual shell making process tests were carried out, 742 sets of shells are made, and 81.2 tons of molten steel are poured

the first round (March, 1998): 162 groups of 4 castings were tested, and 19 groups of castings were poured and the fire escape rate was 11.73%

(1) process parameters: w (alcl3.6h2o) =18.9% ~ 21.1%, w (NH4Cl) =9.65% ~ 10.23%, density 1.13 ~ 1.15 g/cm3, pH 2.0

the hardening time is 9 min, and the air drying time is 25 ~ 30 min

(2) analysis: the main reason for shell fire is the low mass fraction of alcl3.6h2o in the mixed hardener and the insufficient high-temperature strength of shell

(3) results: the residual strength of the shell is low, and the sand cleaning effect is good. The alkali boiling process can be cancelled for the two castings. The alkali boiling time of two castings is shortened to 12 ~ 18 h

the second round (March April 1998): 132 groups of 4 kinds of castings were tested, and there was no fire escape during pouring

(1) process parameters: w (alcl3.6h2o) =23.65% ~ 23.80%, w (NH4Cl) =9.00% ~ 9.24%, density 1.17 ~ 1.18 g/cm3, pH 2.0

other process parameters are the same as those in the first round

(2) analysis: the mixed hardener is adjusted, the mass fraction of alcl3.6h2o meets the requirements of the test plan, the high temperature strength of the shell is higher than that of the first round, and the quality of the shell is stable

(3) results: the residual strength of the shell is higher than that of the first round, and the sand removal effect is not obvious. The process parameters should be adjusted to reduce the residual strength

the third round (April, 1998): 219 groups of three castings were tested, and 8 groups of pouring ran away, with a run away rate of 3.65%

(1) process parameters: w (alcl3.6h2o) =23.6% ~ 23 engineer Yang is the manager of the technical center responsible for the research and development of PA products 8%, w (NH4Cl) =9.0% ~ 9.2%, the density is 1.16 ~ 1.17 g/cm3, and the pH value is 2.0

the second layer of coating is changed to the surface (quartz) coating, and other parameters are the same as those of the first round of process

(2) analysis: in order to reduce the residual strength of the shell, under the condition that other processes remain unchanged, the second layer (reinforcement layer coating) is changed to quartz coating, and the high-temperature strength of the shell decreases, resulting in fire escape

(3) results: the residual strength of the shell is lower than that of the second round, and the sand cleaning effect is improved, but there is a casting hole that needs to be boiled with alkali, and the residual strength needs to be further reduced

the fourth round (April, 1998): 229 groups of 2 kinds of castings (with holes) were tested, and 10 groups of pouring flashed, with a flashover rate of 4.36%

(1) process parameters: w (alcl3.6h2o) =23.7% ~ 23.8%, w (NH4Cl) =9.05% ~ 9.10%, density 1.16 ~ 1.17 g/cm3, pH 2.0

the second and third layers of coating are changed to quartz coating, and the other are the same as the first round of process parameters

(2) analysis: in order to eliminate the alkali boiling of some castings with holes and shorten the alkali boiling time, the first three layers of coating and sanding are changed into quartz refractories, and the residual strength of the shell is reduced through the phase transformation of quartz at 573 ℃. The last three layers are high alumina refractories to ensure that the shell has sufficient high temperature strength. For the fire escape rate of 4.36%, it has been completely solved after adjusting the coating layer

(3) results: the adjusted process parameters meet the relationship between the three strengths of the shell. Some castings cancel the alkali boiling process, and some castings shorten the alkali boiling time from the original 36 hours to 12 hours

based on the above test results, the manual shell making was transferred to mass production on May 3, 1998, and the shell making line was partially transferred to mass production from August 8, 1998. The shell quality is stable, the sand cleaning effect is good, the composition of hardener is stable, and its content decreases slowly, which has good economic benefits

3 economic analysis and conclusion

(1) shorten the hardening (drying) time and improve the production efficiency by 1.72 times

(2) some castings (9 kinds, 484.87 t in total) cancel alkali boiling and some castings (10 kinds, 417.29 t in total) shorten alkali boiling time and reduce production cost

(3) reduce the consumption of steam and raw materials, and save 383000 yuan of production funds. (end)

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