Different points were marked on the horizontal line through the region of base metal, HAZ and weld metal for four welded samples with different welding speed/heat input to determine the hardness of the respective points which is shown in figure 19.
Figure 21. Hardness measurement for four welded samples with variable welding speeds.
a)40 cm/min b) 60 cm/min c) 80cm/min d) 98cm/min.
Hardness results obtained for 4 different heat input is shown in the figure 22. It is evident from the figure that when the heat input is 2,7 KJ/mm, the highest one during experiment, the minimum hardness of 294 HV is obtained. Alternatively, in case of lowest heat input (1,1 KJ/mm), the maximum hardness of 363 HV is acquired which is quite high when compared to those obtained with highest heat input. From these results, its more than clear that the hardness values are increased through the HAZ and the weld metal with decreasing heat input. It can be explained by the fact that the low heat input is followed by fast cooling resulting in higher hardness and strength in the weld.
Figure 22. Hardness results for different heat inputs. a) Q- 2,7 KJ/mm; b) Q- 1,8 KJ/mm; c) Q - 1,35 KJ/mm; d) Q - 1,1 KJ/mm.
The maximum hardness obtained in the weld for different heat inputs of 40 cm/min, 60cm/min, 80cm/min and 98cm/min are 294 HV, 325 HV, 331 HV, and 363 V respectively which is shown in the table 13.
Table 13 Influence of heat input on the maximum hardness with variable welding speeds. a - 40cm/min, b- 60cm/min, c- 80cm/min, d- 98cm/min.
Weld Welding speed
cm/min
Heat input KJ/mm
Maximum hardness HV
a) 40 2,7 294
b) 60 1,8 329
c) 80 1,35 331
d) 98 1,1 363
From this table, the relation between heat input and the maximum hardness when using different welding speeds can be generated as shown in figure 23. The result shows that the hardness and thus strength in the HAZ and weld metal increase with increasing welding speeds.
Figure 23. Influence of heat input on the maximum hardness with variable welding speeds.
a - 40cm/min, b- 60cm/min, c- 80cm/min, d- 98cm/min.
The result also revealed that the hardness in weld metal is higher than that of base metal in all the welds. Interestingly, the higher values of hardness are achieved in HAZ in all cases.
In general, HAZ softening is common phenomena in high strength steel due to tempering softening and transformation softening. However, the hardness results do not support that general literature findings as the HAZ was found to be harder than base metal and weld metal in all cases which is quite shocking.
5 CONCLUSION
Hot cracking is less likely to occur during bead on plate SAW welding of 24 mm thick S500ML although different welding speed are set. So, no hot cracking can be observed in SAW welding within certain heat input range i.e. if the heat input is less than 2.7 KJ/mm.
The D/W ratio of weld bead doesn’t exceed 1 with the applied welding conditions and parameters during SAW welding which also explains the absence of hot cracking in the welds.
Heat input decreases with increase in welding speed. The depth of penetration increases with increasing welding speed until it reaches the maximum penetration. After that the penetration decreases with increasing welding speed. This effect of increasing welding speed on the depth of penetration confirms to a general pattern. The bead on plate experiment on 24mm thick S500 ML showed that the best penetration is achieved at speeds between 60 -70 cm/min and the corresponding heat input for that speed is about 1,8 KJ/mm when the constant current and voltage of 600 A and 30 V with 4 mm electrode wire are used. With increasing welding speed, weld bead width and the height decreases.
The hardness distribution through the weld increases with decreasing heat input. The maximum hardness is obtained when the heat input is the minimum. Hardness values in weld metal are higher than that of base metal which can be explained by the use of over- matching filler wire. But the most surprising conclusion was that the hardness distribution in the HAZ was found to be higher than that of base metal and weld metal. This is quite contrasting to the previous research which says thermomechanically controlled high strength steel tend to soften in HAZ.
For hot cracking to occur in the weld or HAZ, the main elements that must be present in the weld are strain on the solidifying metal, presence of impurities like Sulphur and the shape of the weld bead. The cracking occurs at the final stage of the solidification due to solidification shrinkage and thermal contraction when the tensile stress across adjacent grains exceed the strength of solidifying metal. Solidification cracking normally occurs at the weld metal at the grain boundaries where impurity elements precipitate into. Coarse columnar grains are
more cracking susceptible than fine equiaxed grains because they are relatively more ductile and can deform to resist the shrinkage strain easily. Hot cracking is likely to occur in deep and narrow welds. The two main conditions for occurrence of hot cracking are:
a) When depth to width ration of the weld bead is greater than 1.
b) When units of cracking susceptibility (UCS) value of the weld exceed 25 for fillet welds and 20 for butt welds.
Concerning the weldability, challenges and problems in welding of high strength steels, following conclusions can be drawn.
Wide range of types of high strength steel are available in the market and their evolution seems unstoppable that will definitely raise the requirements for welding process and procedures. It’s a big challenge for welding machine manufacturers to update with the progress and fulfil the requirements of newly evolved HSS. Its more challenge for them to present the equipment with reasonable price which also meets the demand and requirement of industrial application. For instance, consumable selection is normally wide with low strength steels but for S700, the options can be narrow.
The main governing factors for weldability of HSS are heat input, cooling time, preheat/inter-pass temperature, carbon equivalent and selection of consumable. Cooling time from 800 ˚C to 500 ˚C is one of the principle factors that dictates the nature of microstructures formed especially in the HAZ.
It’s necessary to maintain low heat input to maintain similar weld joint as base metal in terms of microstructures and mechanical properties but most of the conventional arc welding methods are known for low thermal energy density and high heat input. The consequences of high heat input are larger and softer HAZ, larger grains, low toughness and hence susceptible to several weld defects. HAZ softening is the common characteristics of TMCP and QT steels. Heat input and cooling time influence the amount of softening in HAZ. High heat input and slow cooling rate leads to wider HAZ.
Welding conditions applicable for HSS produced through one production route (e.g. TMCP) may not be suitable for those manufactured from other production routes (e.g. Quenched and tempering).
The welding conditions and the chemical composition of the steel has a huge influence on microstructures and the mechanical properties. Higher the carbon content and alloying elements in the steel, more sensitive the steel will be towards thermal cycle during welding.
This leads to greater changes in the microstructures and properties of weld when compared to the base metal.
Recommendation for future research
HAZ is the common phenomena in HSS especially manufactured through thermomechanically controlled rolling and quenched and tempering. But HAZ was found to be higher than base metal and weld metal when bead on plate submerged arc welding was performed on thermomechanical rolled S500ML with 24mm thickness. It would also be interesting to investigate the possibility of hot cracking in thermomechanically rolled HSS with yield strength over 700 Mpa and in quenched and tempered HSS.
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