0　Introduction

The microstructure of the weldment depends on the composition of a flux for given welding parameters. It has been verified by various researchers[1-3]. The various factors that affect the weld microstructure are alloying elements, weld oxygen content, prior austenite grain size and cooling rate[4]. The variations of oxygen and the change in the shape and size of inclusions have a definite effect on the microstructure of the weld[5]. It is found that acidic fluxes deposit higher oxygen proportion to the welds in comparison to basic fluxes. Using high basic fluxes, low oxygen welds and high acicular ferrite welds are observed while acidic fluxes generate high oxygen content in the welds and bainitic microstructures are found. The various oxides like titanium, boron and vanadium promotes the formation of oxides. These elements like boron and titanium may form oxide inclusions and these are trapped in the weld and facilitate the formation of acicular ferrite[6]. The microstructure is mainly decided by the chemical composition of the flux, austenite grain size and cooling rate[7]. The weld oxygen content has a significant effect on weld metal microstructure. Coarse grain bainite structure is formed with 0.01% oxygen while the structure is changed as this oxygen content is increased. Acicular ferrite is formed with 0.025% oxygen in the weld[8].

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The weld oxygen content also affects the AF formation to a certain extent in the weld[9]. The high weld oxygen promotes proeutectoid and side plate ferrite and Mn content promotes bainite and acicular ferrite[10].

一般说来，城市发展进程中的利益结构有三种形式：一是主导者与职能单位的关系，即一方(主导者)雇佣另一方(职能单位)或以承包方式使其承担某种项目；二是，组织之间的谈判协商关系，指多个组织进行谈判协商，利用各自的资源进行合作以求能更好地实现各自单位的利益；三是系统的协作，指各个组织相互了解，结合为一，有着共同的想法，通力合作，从而建立起一种自我管理的网络(格里•斯托克，1999)。在城市更新中前两种关系普遍存在，但是系统合作的关系还在探索与发展之中。依据西方城市更新的经验，城市政府与开发商、房地产商的之间密切合作是城市更新顺利进行的重要保障，也是国内城市更新中值得重点关注的领域。

The alloying elements like C, Mn, Ni and Mo also affect the weld metal microstructure and mechanical properties. The carbon content reduces the width of the coarse grained allotriomorphic ferrite and other ferrite structures. The carbon diffuses in to the remaining austenite during the transformation of austenite into pearlite and ferrite. High carbon content reduces the formation of widmanstatten ferrite and allotriomorphic ferrite and increase the acicular ferrite[11]. The alloying element Mn also reduces the formation of acicular ferrite. The weld Mn also promotes the formation of micro phases such as retained austenite, martensite and degenerated pearlite.

The ANOVA Table 6 shows that the factors CaF2 and FeMn both are having significant effect on weld pearlite content. Beside these two factors, interaction effects of AB and AC are also significant. The second order quadratic model developed for weld pearlite content is given in equation (1). The pearlite content is given in percentage.

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The design matrix in coded form is given in the Table 1 and the three factors are shown in Table 2. The wire, plate composition and welding parameters are shown in Table 3 and Table 4 respectively. Measured responses showed in Table 5. The measured responses and the various elements transfer to the welds are given in Table 6 and Table 7 respectively.

Table 1　Design matrix in coded form (wt%)

Table 2　The three factors and their levels

Table 3　The wire and plate compositions (wt%)

Table 4　The welding parameters

Table 5　Measured responses

1　Experimental procedure

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(1) Twenty fluxes were designed as per RSM by using central composite design. The designed matrix in the coded form is given in Table 1.

(2)The base fluxes CaO, SiO2 and Al2O3 were selected and mixed in the ratio of 7:10:2 as per binary and ternary phase diagrams. The additives CaF2, FeMn and NiO were added in the range of 2%-8%.

(3)Twenty fluxes were prepared by agglomeration technique.

The following steps were followed:

(4)Beads on plates were made on 18 mm thick low carbon steel plates of the given composition. The three levels, three factors and composition of the plate and wire are given in the Table 2 and Table 3.

Table 7 shows the statistical data for model given in Table 6. The curve in the Fig.1 shows that the pearlite percentage in the weld increases with increasing CaF2 content in the flux. This can be attributed to the change in weld oxygen contents and the elements transfer to the weld with increasing CaF2 additive in the flux. The weld oxygen content in the weld is reduced with increase of CaF2 content in the flux[14]. The weld pearlite content increases with increasing oxygen content in the welds. The weld pearlite content decreases with decrease in oxygen content to a certain extent but after a certain limit it increases as weld oxygen is further reduced. This has been shown in the Fig.2. The weld oxygen has been associated with the negative ΔMn quantity[15]. So, the negative Mn quantity represents the amount of oxygen associated with the weld. Large negative value of Mn indicated large oxygen in the weld. The positive value of Mn shows that Mn content has been transferred from the flux to the weld and it is associated with a small amount of oxygen content. The pearlite content is high when the oxygen content is either too high or too low. This has been depicted in Fig.2. The weld metal oxygen has a very significant effect on weld metal ferrite formation kinetics and ferrite morphology[16 ].

(6)Beads were laid one over the other in order to minimize the dilution effect of base plate.

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(7)10%dilution effect of the base plate has been assumed.

(8)Mathematical models were developed using Design Expert 8.0.7.1 software.

(9)Chemical analysis of the bead (powder was extracted from the top bead with the help of a drill) was done in Spectro Analytical Lab, Okhla, New Delhi.

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(10)The transfer of manganese was calculated by a ΔDelta quantity=analyzed composition expected composition.

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The ANOVA Table 6 shows that the interaction effects of CaF2 and FeMn and CaF2 and NiO both are significant for pearlite formation. These interaction effects are shown in the Fig.5 and Fig.6 respectively. Fig.5 shows that the weld pearlite content increases with increasing both CaF2 and FeMn additives. The maximum value is obtained at 8% of CaF2 and 2% of FeMn contents and minimum value of pearlite content is obtained at middle levels of the both the factors. Fig.6 shows that the pearlite content also increases with increasing CaF2 band NiO. The maximum value of pearlite content is observed at 8% of CaF2 and 2% of NiO, while it is found minimum at 8% of NiO additive and 2% of CaF2. Fig.7 shows combined effect of CaF2 and NiO on pearlite content.

Weld oxygen content decides the microstructure and controls the elements transfer to the weld. The ANOVA Table 8 shows that CaF2 and NiO both are significant factors for weld oxygen transfer. Besides the individual factor, interaction of factors, AB and ABC is also significant for the model. The cubic model developed for oxygen content is given in equation (2).

(11)Measured responses weld oxygen content and pearlite contents are given in Table 5.

2　Results and discussions

The inclusions type, composition, shape, volume and size also show a significant effect on microstructure and mechanical properties. These inclusions mainly depend upon the oxygen and other elements like Al, Ca, Ti present in the weld. These inclusions may be generated due to deoxidation process[12]. The oxygen is not soluble in solid steel, only a fraction of this oxygen content is soluble in steel. This oxygen reacts with the available cationic species and as the temperature falls it may produce nonmetallic inclusions. These inclusions also affect the microstructure. The specific type inclusions help in formation of a special microstructure called acicular ferrite due to a lattice match between the inclusion and acicular ferrite. The basicity of the flux also affects the weld oxygen content and consequently the formation of various microstructures. The high weld oxygen content shows high inclusion content but the grain size is refined with high oxygen contents[13].

Content of pearlite= 7.286 + 2.34 CaF2-3.40FeMn+0.669 NiO-0.166CaF2× FeMn-0.181CaF2×NiO + 0.366 FeMn2

(1)

Table 6　ANOVA for pearlite content

(5)Welding parameters such as voltage, current and travel speed were kept constant. These parameters are given in Table 4.

Table 7　Statistical data for model given in Table 6

The variation of pearlite content with the FeMn additive is shown in the Fig.3.This shows that initially the weld pearlite content is reduced to a minimum and after that it increases. This might be attributed to the change in weld oxygen and Mn contents with the addition of FeMn in the flux. The FeMn content works as a deoxidizer and the Mn content to the weld might be increased with the addition of FeMn to the flux. The Mn content initially might be less because of low FeMn content but it may increase with further addition of FeMn to the flux. The weld Mn content and oxygen both may decide the pearlite content in the weld. The pearlite content is reduced with increasing carbon equivalent of the welds as shown in Fig.4. This may be attributed to the combined effect of various elements transferred to the welds on carbon equivalent. The pearlite content increases with increasing basicity index (BI) of the flux. This can be inferred from the Fig.5.This can be because of reduction in weld oxygen content with increasing BI and the Fig.4 also confirms this that the pearlite content is high in both the conditions when the weld oxygen is either very low or very high.

The ANOVA F-value of 13.48 shows that the model is significant. The “P value” is less than 0.001, which shows that there are only 0.015 chances that this model may be insignificant. The lack of fit is also insignificant relative to the pure error. The R-squared of 0.861 6 shows that 86.16% of the variance in the model can be explained uniquely or combined by the independent variables. The Pred. R-squared and Adeq. R-squared are in close agreement.

Fig.1　Effect of CaF2 on pearlite content

Fig.2　Effect of weld oxygen on pearlite content

Fig.3　Effect of FeMn on pearlite content

Fig.4　Correlation of pearlite content with CE

Fig.5　Effect of BI on pearlite content

3　Combined effects of flux constituents on pearlite content

The expected composition was calculated from the below given relation:

Fig.6　Combined effect of CaF2 and FeMn on pearlite content

Fig.7　Combined effect of CaF2 and NiO on pearlite content

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Oxygen content in the weld = 0.294 + 3.015×10-3CaF2 + 0.265FeMn-0.241NiO-0.0704 CaF2×FeMn + 0.0464CaF2×NiO + 0.251FeMn×NiO +

(2)

Table 8　ANOVA for response surface reduced cubic model

The ANOVA F-value of 9.94 shows that the model is significant. There are only 0.86% chances that this model may be insignificant. The “P value” is 0.0086. The lack of fit is also insignificant relative to the pure error. The R-squared of 0.9598 shows that 95.98% of the variance in the model can be explained uniquely or combined by the independent variables. The Adj. R-squared value is 0.8633.

The variation of weld oxygen content with CaF2 is shown in Fig.8. It shows that the weld oxygen is reduced with increasing CaF2 content in the flux[14,19]. The weld oxygen content is also reduced by adding NiO additive in the flux. This has been shown in Fig.9.This may be explained in correlation with the decomposition of NiO. Being a basic oxide it may produce less oxygen on decomposition. The Fig.10 shows that the weld oxygen is reduced with increasing FeMn additive in the flux. This may be attributed to the fact that Mn acts as a deoxidizer and it reduces the weld oxygen content[17-18]. The weld Mn content increases with addition of FeMn in the flux. The weld oxygen is reduced with increasing BI of the flux and it has been shown in Fig.11.This is in agreement with the past research.

Fig.8　Variation of weld oxygen with CaF2 content

Fig.9　Variation of weld oxygen with NiO additive

Fig.10　Effect of FeMn on weld oxygen content

Fig.11　Effect of BI on weld oxygen content

4　Combined effect on weld oxygen

The combined effect of CaF2 and FeMn shows that the weld Oxygen increases upto 5% addition of both the additives but after that it becomes constant. This has been depicted in Fig.12.The results have been given on the basis of the contour diagrams. This contour diagram has been shown in Fig.13.

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Fig.12　Combined effect of CaF2 and FeMn on weld oxygen

Fig.13　Contour diagram for combined effect on weld oxygen

5　Validation of the model

The validation of the model was done by selecting two fluxes within the given range.The results are tabulated in Table 9 given below and the error lies within the permissible limit. It shows that the model can be used for predicting the values of pearlite content and weld oxygen content for the given flux composition.

Table 9　Experimental and predicted values (wt%)

6　Conclusions

(1) The pearlite content increases with increasing weld oxygen and CaF2 content in the flux.It is high in both the conditions when the oxygen is either very high or very low.The pearlite content also increases with increasing both CaF2 and FeMn additives.

(2) CaF2 and NiO both as individual factors show synergistic effect on weld pearlite content while as a mixture these show anti synergistic effect. CaF2 and NiO both ad individual components are having anti synergistic effect on weld oxygen while as a mixture both are showing synergistic effect.

(3) The weld oxygen after an initial increase, becomes constant for further addition of CaF2 and FeMn is reduced with increasing both FeMn and NiO while after an initial decrease, it increases with further addition of CaF2.

(4) The weld oxygen is reduced with increasing CaF2 FeMn and NiO contents.

(5) This experiment may help in deciding the microstructure of the welds by selecting the suitable flux for welding as per our requirement.

References

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[2]　Kent william kettel. Correlation of flux composition and inclusion characterstics in submerged arc welding of HY-100 Steel. California: Noval post graduate school, 1993.

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