THE RELATIONSHIP BETWEEN CONTINUOUS COOLING RATE AND MICROSTRUCTURE IN THE HEAT AFFECTED ZONE ( HAZ ) OF THE DISSIMILAR WELD BETWEEN CARBON STEEL AND AUSTENITIC STAINLESS STEEL

This paper investigates the changed in the microstructure in HAZ of the dissimilar weld between carbon steel and 304 austenitic stainless steel. Continuous cooling transformation diagrams (CCT), peak temperature profiles and cooling rates can be used to predict the change in the microstructure of the HAZ during the welding process. Optical microscopy, X – ray, SEM and TEM were used to determine the phases which formed in HAZ of carbon steel and austenitic stainless steel. The results of this study indicate that grain size in HAZ depended on the temperature at that point could be reached during the welding. Fully Martensitic layer observed at the interface in carbon side due to the combination of the rapid cooling subsequent to weld and local chemical composition. Cooling rate played the rule on forming Widmanstatten ferrite, Bainite and Pearlite. On the other hand, microstructures and grain size in HAZ of austenitic stainless steel were not affected by temperature and cooling rate. Carbides precipitation (M23C6, M7C3), however, were found in the boundary of grains.


Introduction
Mechanical properties of steels are strongly connected to their microstructure obtained after the welding process.It was found in previous studies that the microstructures of weldments were affected by composition, peak temperature and cooling rate [1][2][3][4][5][6][7][8][9][10].Especially, cooling rate was an important variable which played a key role in phase transformation, microstructure as well as mechanical properties.In welding processes, as the cooling rate in the weld zone and heat affected zone (HAZ) varied with cooling distance from the fusion line of the weld, the phases formed from the transformation was observed to also vary with cooling distance [7].For example with low carbon steel, increasing the cooling rate resulted in the gross microstructure changing from coarse, polygonal Ferrite to acicular Ferrite to upper Bainite.A blocky or elongated Martensitic -Austenite microconstituent was also observed [1,2].Cooling rate also significantly affected the formation of intermetallic phases and corrosion resistance and hot cracking in stainless steel [11][12][13][14].Differences in the cooling rate appeared to provide dramatic effects on the microhardness of steels, depending on the carbon content of steels [5,10,15,16].The samples cooled with salty water had higher hardness values possibly due to the formation of Martensitic after quenching, while air-cooled samples were mostly Ferrite [7].The microhardness increased with the increasing cooling rate and carbon content due to solid solution hardening and formation of the Martensitic phase.Thus heat treatment (heating and cooling) was used to obtain desired properties of steels such as improving the toughness, ductility or removing the residual stresses [4].
In practice, the cooling rate depends on a number of variables, for example, heat input, process efficiency, preheat temperature, material properties and wire feeding rate.Rosenthal developed an equation for predicting cooling rates and peak temperatures in HAZ of weldment for thin plates.On the other hand, cooling rate and peak temperatures could be measured by thermocouple [17][18][19][20][21].
Because weld metals are subjected to continuous cooling upon solidification, the resultant microstructures should be predictable from continuous cooling transformation (CCT) diagrams [24][25][26][27].Therefore, before presenting the results of the metallographic analysis, a brief summary of the type of weld metal microstructures can determine from CCT diagrams base on material compositions and cooling rates.

Experimental procedure
As received plates of carbon steel and 304 stainless steel were machined to dimensions of 275 x 85 x 3 mm, then edge joint preparation and cleaning were required before welding.The chemical composition of the base metal and filler are presented in Table .1.The welding joints were made by shielded metal arc welding (SMAW).The welding parameters in the processes shown in Table .2. Thermocouples (Ktype) were used to measure the transient temperature distribution during welding.The position of thermocouples is shown in Table .3.
Table 1 The  The weld specimens were cut from the aswelded plates with 20 x 5 x 3 mm.Metallography samples were polished and etched in 5g FeCl3 + 15cm 3 HCl + 50÷100cm 3 H2O for stainless steel and weld metal and 3% HNO3 for carbon steel side.The microstructures and hardness test were measured by OM LEICA MDS4000M and hardness tester ARK600.Xray (D500), FE-SEM (JSM7600F), and TEM (JEM1400 Plus) were used to determine the phases, carbides which formed in HAZ of carbon steel and austenitic stainless steel.In order to study the effect of cooling rate on microstructures in HAZ of the dissimilar metal weld between carbon steel and stainless steel, phases transformation can be predicted by using CCT diagrams.Fig. 3 illustrates CCT diagrams of carbon steel and stainless steel that were measured by thermocalc software.
Fig. 3 CCT diagram of carbon steel and stainless steel by JmatPro

Microstructure in HAZ of carbon steel
The HAZ microstructures and grain sizes of carbon steel changed significantly from the fusion line to base metal.The average grain size near the fusion boundary was counted by 4.5 grade level, then reduced to 10.9 grade level in the base metal.Depending on cooling and peak temperature, the existed phases are varied in each zone.Fig. 4 shows the microstructures in each separate region in HAZ of carbon steel.The high cooling rate at the fusion boundary of up to 110 0 C/s caused forming the fully Martensitic layer along the fusion line.Lower cooling rates which present in Table .3 were measured in the experimental lead to mixed phases in others HAZ.In regions where the peak temperature is higher than Ac3 870 0 C, and cooling rate is higher 2 0 C/s, Austenitic transformed to Martensite or Bainite.Widmanstatten ferrite can be formed

Microstructure and carbide precipitation in HAZ of stainless steel
Austenite was observed in HAZ of stainless steel (Fig. 6a).There was not phase transformation in HAZ of stainless steel during the welding process, however, carbide precipitation can be observed in grain boundary (Fig. 6b).The X-ray's results show that the Cr23C6 located in the grain boundary while the Cr7C3 was in the HAZ zone of stainless steel (Fig. 7).The morphology of carbides Cr23C6 and Cr7C3 can be observed by TEM (Fig. 8).The CCT of stainless steel indicates that carbide M23C6 formed at the high temperature that ranges from 900 -950 0 C; whereas carbide M7C3 was precipitated at the lower temperature from 780 -900 0 C. Carbide M2(C,N) can be found when the temperature ranges from 600 -880 0 C with low cooling rate and the retention time of the material in the sensitization temperature range is long enough for precipitation to take place.Carbide precipitation did not occur immediately next to the fusion line, it usually occurred at a short distance away from it.This phenomenon can be explained by using thermal cycle during the welding process.The material next to the fusion boundary experienced the highest peak temperature and cooling rate.Consequently, the cooling rate through the precipitation range was too high to allow carbide precipitation to occur.Consider the microstructure in HAZ zone where the peak temperature was lower 600 0 C, there were no signs of grain boundary precipitation.

Conclusion
The relationship between cooling rate and microstructures in the HAZ of the dissimilar weld between carbon steel and austenitic stainless steel can be summarised:  The changing of peak temperature and cooling rate from the fusion line to base metal caused the forming distinct phases in HAZ of carbon steel.In the cooling rate range, from 110 0 C/sec at the fusion line to 1.1 0 C/sec in the base metal, the microstructure changed from the fully Martensitic layer to Bainite to Pearlite.Widmanstatten Ferrite was found near the fusion boundary. There was not phase transformation in HAZ of stainless steel during the welding process, however, carbide precipitation can be observed in the grain boundary.Carbide M23C6 or M7C3 was found at the Austenitic grain boundary.

Fig. 5
Fig. 5 Transmission electron micrograph (TEM) showing a Martensite lath and Bainitic structure in HAZ of carbon steel

Fig. 6
The microstructure in HAZ of stainless steel, (a) HAZ stainless steel, (b) Carbide precipitation at grain boundary by SEM (a) (b) Fig. 7 The X-ray's results in HAZ of stainless steel, (a) near the fusion boundary (b) HAZ (a) (b) Fig. 8 The TEM images in grain-boundary (a) and HAZ zone (b) of stainless steel DOI 10.12776/ams.v23i4.1002p-ISSN 1335-1532 e-ISSN 1338-1156

Table 4
Cooling rate in HAZ of carbon steel