3.1. Microstructural Evaluation
Fig 2 exposes the SEM image of all five cast TRIP 780 sheets of steel consisting of multiphase of bainite (B), ferrite (F), retained austenite (RA) and martensite (M)[24] . Fig 2 (a) shows the region of weld pulse includes the initial weld. This microstructure has martensite and some bainite in some areas. Fig 2 (b) shows the tempered structure along with the remained ferrite in this area. The grains are homogeneous in steel 3 (S3). It was difficult to isolate the annealed region because grains can be detected stretching and more refined with tempered martensite packs with a different orientation as observed in fig 2 (c).
In steel 4 (Fig. 2d), wide coarser polygonal aniline structures were formed in the weld nugget. Martensite was formed adjacent to each other with different orientations after the annealed grains toward the weld center. Figure 2 (d) and 2 (e) exposed that the martensite is getting fine at the adjacent welding. A substantial amount of bainite, martensite, retained austenite, and mostly ferrite is present in less than 1% of Mn as microstructures. A lower % of Mn is often relevant to the austenite variability and increases the cooling period’s ferritic reactions.
3.2 Phase Evolution of cast TRIP steels
Fig 3 shows the XRD patterns for heat-affected zone, base metal, and fusion zone of welded samples. The XRD patterns indicate that the presence of delta ferrite, austenite, martensite, and retained austenite phases due to the reaction of SiC reinforcement in TRIP steel. Peak intensity was found to vary with Si composition. It is observed that the peak shift was observed with increasing Si wt%.
Fig 3 (a) shows the high peaks denoted ferrite and austenite in the base metal; these are the steel's essential elements. Base metal peaks reveal that the amount of retained austenite increases with increasing the Si reinforcement in each steel. Heat affected zone XRD peaks (fig 3 b) reported retained austenite existence in each steels weld joints [25]. Fig 3 (c) shows the amount of retained austenite decreased in the fusion zones. It is evident from the XRD pattern that the diffraction intensity and broadening of the peaks vary with the volume fraction of Si reinforcement.
3.3 Hardness
The hardness profiles of weld regions are shown in Fig. 4. The fusion zone exhibited high hardness values then the base meal. All five Cast TRIP steels showed fusion zone hardness above 450 HV; In this case, steel 4 and steel 3 showed less hardness 460,485. While steel 1, steel 2, and steel 5 exhibited the maximum hardness of 545 HV, 520 HV, and 615 HV, respectively. Steel 1, Steel 2, Steel 5 have lower carbon substances; however, Cr % is higher with the comparison of steel 4 and steel 3. Enhancement of Cr and interaction of Ti with Mn might cause an increase in the hardness of martensite.
The chemical composition of steel was influenced more in the fusion zone hardness during the resistance spot welding process and found retained austenite at a minimum level. Silica and carbon particulates was well-bonded in TRIP steels, it has constraint to increases the dislocation density and plastic deformation. The presence of high chemical components such as carbon, Cr and Mn content obtained increase hardness in the fusion zone. Steel 4 has the lowest hardness due to the interface among micro alloying elements and the formation of nonmetallic presences that affect the TRIP steel hardness.
Steel 5 has less carbon content than other considered steel. It has resisted the optimum energy absorption and deflection without affect the strength because the retained austenite percentage is low in the heat affected zone. Figure 5 shows the load curve in terms of the height of TRIP steels penetration depth. As shown in Fig. 5 the rate of indentation in constant load was respectively higher for Steel 4 and Steel 5 than other samples, which are due to transformed martensite to cementite and softening of ferrite tempered.
3.4 Shear stretching and cross stretching
The tensile shear strength (TSS), cross-tension strength (CTS), and elastic modulus results are shown in Table 3. Cross-tension strength test results were showed not many differences. Steel 1, Steel 2, Steel 3 exhibits the CTS value of 2.14 ± 0.25 KN, 2.21 ± 0.36 KN and 2.26 ± 0.29 KN, respectively. The highest cross-tension strength value of 3.05 ± 0.33 was observed from sample 5. Elastic modulus of steel 1 (209 ± 2 GPa), steel 2 (205 ± 3 GPa), steel 3 (216 ± 4 GPa), steel 4 (200 ± 2 GPa) and steel 5 (225 ± 3 GPa) were observed. Sample 5 exhibits the highest elastic modulus than all other steels in this study. A high elastic modulus demands a higher force to achieve a certain deformation. The ratio of cross-tension strength (CTS) and tensile shear strength (TSS) is called ductility ratio. Steel 5 and steel 3 have the highest ductility ratio as 0.165, 0.149 respectively.
Table 3
Results of ductility ratio and Elastic Modulus.
Samples
|
Cross-tension strength Max. load (KN)
|
Tensile shear strength Max. load (KN)
|
Ductility ratio (CTS/ TSS)
|
Elastic Modulus, GPa
|
Steel 1
|
2.14 ± 0.25
|
15.2 ± 0.25
|
0.141
|
209 ± 2
|
Steel 2
|
2.21 ± 0.36
|
15.9 ± 0.36
|
0.139
|
205 ± 3
|
Steel 3
|
2.26 ± 0.29
|
17.6 ± 0.29
|
0.149
|
216 ± 4
|
Steel 4
|
2.23 ± 0.25
|
17.3 ± 0.25
|
0.135
|
200 ± 2
|
Steel 5
|
3.05 ± 0.33
|
18.5 ± 0.33
|
0.165
|
225 ± 3
|
Maximum force and ductility ratio are shown in Fig. 6. Steel 5 exposed maximum tensile shear strength load peak with an average of 18.5 ± 0.33 KN and ductility ratio (CTS/TSS) of 0.165 ± 0.004. Steel 1 exhibits the lowest value of tensile shear strength, cross-tension strength, values of 15.2 ± 0.25 KN, 2.14 ± 0.25 KN respectively. Low ductility ratios were observed in steel 4 due to higher CTS results than all other weld steels.
Figure 7 shows the displacement in cross-tension weld samples. Maximum displacement and energy absorbed were high in steel 4 (6.86) due to the excellent relationship among these properties. Low displacement percentage observed in steel 1 (3.34), steel 2 (3.98), steel 3 (4.45), and steel 5 (4.42). The amount of both displacement and energy absorption has improved by 51%. This improvement along with the observed microstructure for steel 4 (Fig. 2d) showed the best situation for the fracture type. The existence of less tightness, along with an almost equiaxed and finer structure, makes brittle martensite flexible and makes the state of the interface closer to the peripheral.
Figure 8 shows the weld nugget failure energy for tensile shear and cross-tension tensile test. Results exposed that steel 1, steel 2, steel 3, and steel 5 were low energy absorbed due to hardness high in the heat-affected zone and fusion zone of all spot weld joints, which illustrates breakability. Displacement and energy absorbed by steel 4 were certainly high compared to the rest of weld steels. Carbon content was less in steel 4 and required sustaining high displacement and low load.
3.5 Fracture surface analysis
Figure 9 shows the fracture surfaces SEM image of weld steels. Figure 9 (a) shows the failure of steel 1 sample with a combination of interface and peripheral fracture with about 40% of the interface fracture in the weld nugget. The weld nugget in Fig. 9(b) fractured in pull-out and almost peripheral mode. Separation in the weld occurred from its edge at the weld boundary line. Some intergranular fracture and cleavage facets were observed in steel 2. In this type of tension, the crack traverse moves from the joint groove in the weld nugget environment. Fractures resulting from cross-tension of pulsed samples showed Pull out fracture type, which is due to the joining of grain boundaries in the HAZ region, adjacent to the weld nugget. It is determined that the hardness of the weld nugget and the lower HAZ region, increase the desire to pull out failure in cross-fracture. Figure 9 (c) shows the weld nuggets adjacent detachment in the annealed region, which includes equiaxed cavities, dimples that are fractured and ductility. In Fig. 9 (d), fracture adjacent to the weld includes dendritic solidification of grains, growth orientation and microvoids combination is clear observed.
Steel 4 observed 70% yield load and crack propagation does not begin from the joint. According to Fig. 9 (d), as soon as it reaches to the weld region, changes its path with regard to re-austenitic grains which converted from columnar mode to half equiaxed mode. Also due to temper in the welding region, its hardness decreases and the failure toughness improves. The failure in sheet thickness at the annealed region occurs in the form of intergranular with coarse grains, and at the weld center, pull-out detachment is observed. Failure modes for steel 4 with a lower hardness were almost a pull-out fracture. A thickness and pull out separation were obtained, which was accrued by repeating the test in HAZ fracture zone and ruptured area. Steel 5 observed 20 % yield load and mostly transgranular fracture areas with predominantly cleavage surfaces in the exterior layers of the weld and microvoids combination in the center of the weld detected in Fig. 9 (e). Dimples and dendrites were observed in the fracture surfaces.