DEPHOSPHORIZATION AND DESULFURIZATION OF MOLTEN DUCTILE IRON AND ITS EFFECT ON DUCTILITY

Transcription

1 1 DEPHOSPHORIZATION AND DESULFURIZATION OF MOLTEN DUCTILE IRON AND ITS EFFECT ON DUCTILITY By Rub Nawaz Ansari (Deputy Manager, Bolan Castings Ltd. Pakistan) PROBLEM: The importance of phosphorus in the chemistry of the Ductile is vital. It is kept at a level of % maximum. The only way to keep it at this level is to use the raw material which should be low in the phosphorus. The consistency in the raw material of the foundry is not much appreciable in Pakistan. The chemistry fluctuates from supplier to supplier and source to source. Therefore it becomes sometimes very hard to keep the phosphorus below the desired level. The surprises result in sub standard production of the ductile iron which has not the up to the mark mechanical properties and premature failure is observed at the end. If there is some fluctuation in sulfur level that has been treated with the soda ash. The sulfur issue is obvious as higher values of sulfur in the melt simply do effect the Mg treatment and consequently nodules are not produced. In other words if the sulfur level is high it do not let the ductile iron to be produced and the problem is highlighted at the earlier stages and addressed accordingly. But it is not in the case of phosphorus. Phosphorus let the ductile iron to be produced but with some low ductility. After mechanical testing or even in service of the part foundry man comes to know that the mechanical properties were not up to the mark. Ductile Iron foundries do not have any method to reduce the phosphorus level in the melt if is introduced through the raw material. In this research it is tried to keep the phosphorus level at minimum level through dephosphorization of the liquid metal. The principles used in dephosphorization in Pig Iron and

2 2 steel with additives that promote P2O5 in the slag. The most suitable way found is ladle treatment as we could not take risk to add additives in the furnace which may harm its lining. Previous Workings: We found from the literature that oxygen lancing with basic slag forming additives has very good results for the removal of the phosphorus from the steel. We also learned from the literature that Na2CO3 has been used as desulfurizing and dephosphorizing agent for the liquid pig iron. According to Takaharu Mori Ya and Masanobu Fujii, It has been known for some time that dephosphorization, simultaneously with desulfurization, can be attained by oxidation refining using a Na2C03 or Na20 system slag, but it can hardly be said that the basic investigations are satisfactory. ( Dephosphorization and Desulfurization Pig Iron by Na2CO3 by Takaharu Mori Ya and Masanobu Fujii) Na2CO3 has also been used in the ductile iron desulfurization. But the practice tells it does not have any considerable impact on the dephosphorization in the case of ductile iron. According to Hitoshi Ono, Tamenori Mas Ui and Hisashi Mori, The slagging rate of CaO has a great effect on the slag-metal reaction in the steel making process. For this reason, many fundamental and practical studies have so far been made on the mechanism of CaO dissolution into slag and the dissolution rate. In hot metal dephosphorization by lime injection, however, it is not always clear how the injected lime forms slag in hot metal and reacts with phosphorus. It might be thought that the lime injected with oxygen into hot metal forms slag rapidly and participate in the dephosphorization reaction.

3 3 ( Dephosphorization Kinetics and Reaction Region in during Lime Injection with Oxygen By Hitoshi Ono, Tamenori Mas Ui and Hisashi Mori) Experiments Basis: It is not advisable to lance oxygen for this purpose as it will oxidize the necessary elements like C and Si as well and the cost of re-addition of these will be in creased. The author tried to combine the additives of pig iron and steel together for the simultaneous desulfurization and dephosphorization of the ductile iron without any oxygen lancing which was successful to certain limit. Objectives The project focused on: i. The study of process for reduction of phosphorus percentage. ii. The investigation of the elongation and strength of ductile iron. SCOPES i. The Ladle treatment was used to remove phosphorus from the melt. ii. The process was designed for simultaneous removal of sulfur and phosphorus. iii. Fused Lime with Soda Ash ware used as dephosphorising agent. iv. Standard UTS wedge was cast to study the mechanical properties. Materials: The compositions of the available pig iron in the market are as below:

4 4 CHEMICAL COMPOSITION OF PIG IRON C Si Mn S P Imported Imported Imported Table 1: chemical composition of available pig iron Typical Compositions of the Ductile Iron GGG450 Produced without Dephosphorization The following composition is taken for a ferritic S.G. Iron. The cast part is supposed to bear shock loads as well as the nominal stress. Carbon Silicon Mn S P UTS Kg/mm2 Elongation % Table 2: Current compositions of ductile iron without dephosphorization

5 5 Nodule Micrograph 100 X Methodology: Additives: The additives chosen for this purpose were: 1. Soda Ash (Na2CO3) 2. Fused Lime Powder. (CaO) Both of the additives were sourced from the local sources in powder form. Furnace: Treatment Ladle: Size of the batch: Induction coreless medium frequency furnace was used to melt. Simple open mouth ladle with one ton capacity was used as treatment ladle. 500 kg liquid metal Temperature: 1430 C Stirring Time: 10 min

6 6 Experimental Work: In a series of experiments the concentration of the additives and the method of addition were altered one by one. We started from using lime powder 1% of the liquid bath size and end up to 3%. However the Soda Ash was kept 2% of the bath size in all the experiments. The best results we obtained when Soda Ash (10kg i.e., 2% of the bath size) and Lime Powder (5 kg i.e., 1% of the bath size) were premixed in the mixer to make a homogeneous mixture before putting them into the ladle bottom. The hot metal was tapped over it. Rest of 5 kg (i.e., 1% of the bath size) of the pure lime was added with the stream of the tapped liquid metal. Results The results of the experiments were as follows: We do not see any significant effect on C, Si and Mn by the treatment. We also observe no significant change in nodule size and count of the ductile iron. However the amount of S and P has been decreased as shown the following table. Experiment No Carbon Silicon Mn S P UTS Kg/mm2 Elongation % Table 3 : Ductile Iron compositions after dephosphorization Discussion The phosphorus is reduced from 0.043% to 0.022% with this process that means 48.86% reduction in Phosphorus level is observed with this process as shown in the table below.

7 7 Initial Average Final P Phosphorus Experiment Lime addition Reduction Phosphorus level Reduction No. %age in P %age (%) %ge % % % % % % Table 4: Phosphorus reduction values Process Efficiency 0,045 0,04 Ph os ph or us % 0,035 0,03 0,025 0,02 Final P level 0,015 0,01 0, Experiment No.

8 8 Similarly we see the increase in elongation with reduced phosphorus level. We observe a maximum of 34% increase in the elongation as shown in the table. Experiment No. Lime addition %age Previous Ave Elongation % Current Elongation % Increase in Elongation % Increase %age Elongation % % % % % % Table 7.3 Elongation values If we compare the data of phosphorus level and elongation, we find that there is increase in elongation with decrease in phosphorus level keeping all other factors effecting elongation constant as shown in the table. Experiment No. Final P level After treatment Elongation % Table 7.4 Phosphorus contents and Elongation values

9 , , , ,5 11 After treatment Elongation % 0,041 0,034 0,028 0,025 0,022 0,023 After treatment Elongation % Conclusions We concluded from the results of the experiments as following: 1. The results of the experiments that the process is efficient up to a certain limit. 2. The phosphorus and sulfur can be eliminated simultaneously from the ductile iron melt by use of this process. 3. By keeping all other factors affecting constant the ductility of the ductile iron increases with the reduction of the phosphorus level. 4. The fluidity of the metal did not create any problem so we did not need to increase the carbon equivalent value through carbon or silicon increase. 5. The process has a low cost as both additives are readily available in the local market at lower prices.

10 10 6. Based upon the data regarding the ductility and quality assurance together with its relatively low cost, the potential use of low phosphorus s-g iron castings would be expected for diverse machine parts with high quality and reliability. REFERENCES: 1. {Takaharu Mori Ya and Masanobu Fujii} Dephosphorization and Desulfurization Pig Iron by Na2CO3 Molten%20Pig%20Iron%20by%20Na2CO3.pdf (SUT student web portal, Thailand) 2. {Hitoshi Ono, Tamenori Mas Ui and Hisashi Mori} Dephosphorization Kinetics and Reaction Region in during Lime Injection with Oxygen (J-STAGE, Japan Sceince and Technology Information Aggregator, Electronic,) 3. {Haruyoshi Tanabe and Masayuki Nakada} Steelmaking Technologies Contributing to Steel Industries, (JFE Steel Corporation)