# Clinching of High-Strength Steel Sheets with Local Preheating
## 3 Results and Discussion
### Local Preheating of the Clinched Joint Zone
#### 3.4.1 Zones with Gradient Microstructure Prior to Clinching, Steel No. 1.0941  
Local heating (RSW) was applied to two overlapping galvanized steel sheets of grade 1.0941 in order to transform the microstructure from the original ferritic-martensitic microstructure to a gradient fine-grained ferritic-sorbitic microstructure. Different states of microstructure were observed on the upper sheet US-1.0941 (upper sheet = US) and lower sheet LS-1.0941 (lower sheet = LS) sheets at the Cu-Cr electrode contact sites and in their surroundings (Figure 14a).  
In zone \(P1\), there was a ferritic-martensitic microstructure (Figure 15a), which continuously transitioned to the original ferritic-martensitic microstructure of sheet grade 1.0941 and to a fine martensitic microstructure in the local heating zone (Figure 14b). The microstructure in Figure 14b was formed after rapid cooling. This area was heated to a temperature in the interval Ac1-Ac3, and a two-phase ferritic-martensitic structure was formed by rapid cooling.  
Figure 13: Schematic of microhardness measurement HV0.3 of clinched joint: (**a**) marking of measured areas of CL with steel No. 1.0941, (**b**) measured values HV0.3 of CL with steel No. 1.0941 in P1, P2, and center, (**c**) marking of measured areas of CL with steel No. 1.0935, (**d**) measured values HV0.3 of CL with steel No. 1.0935 in P1, P2, and center.  
The microstructure of zone \(P2\) was transformed from a fine-grained ferritic-martensitic microstructure to austenite without an increase in the austenitic grain size as a result of local rapid intense heating to temperatures above Ac3 of material 1.0941. By the same intense heat dissipation to the surrounding of the heating zone, the austenite was transformed into a fine-martensitic microstructure (Figure 16). A martensitic microstructure was formed in the central region in zone \(C\), characterized as a lens during the spot welding (Figure 17). There was intense heat dissipation into the electrode body at the contact points of the cooled Cu-Cr electrodes and the overlapping sheets. In zone \(E\), there was then a region to a depth of about 0.3 mm in which no microstructure transformation occurred. This region was not heated above the temperature of Ac1 (Figure 18).  
Figure 14: Microstructure of steel No. 1.0941 in the heating zone: (**a**) defining the zones, (**b**) zones from P1 to P2.  
Figure 15: Microstructure in the local heating zone of steel No. 1.0941—zone \(P1\).  
The microhardness of each zone with a gradient microstructure was measured in the region of the original base material (BM), zone _P1_, with a ferritic-martensitic microstructure, zone _P2_, with a fine martensitic microstructure, and the central region, which included zone \(C\), with a martensitic microstructure, and zone \(E\), in which no microstructural transformation occurred. The microhardness was measured on the upper sheet (Figure 19a) and the lower sheet (Figure 19b).  
Figure 16: Microstructure in zone _P2_ of steel No. 1.0941.  
Figure 17: Microstructure in zone \(C\) of steel No. 1.0941.  
The microhardness values in the _P1_ zone were slightly increased with respect to the microhardness values of the original ferritic-martensitic microstructure. The microhardness values of the fine-grained sorbitic microstructure of the _P2_ zone were twice the microhardness values of the original ferritic-martensitic microstructure. In the central zone, the martensitic microstructure had almost twice the microhardness values of the original ferritic-martensitic microstructure. The microstructure gradient formed by the narrow ferritic-martensitic region at the point of contact of the electrodes with the sheets (zone _E_) caused by the intense heat dissipation into the water-cooled electrodes was manifested by the microhardness values at the level of the original matrix (Figure 20a,b).