# Influence of Different Base Materials on the Microstructure and Properties of Clad Rebar
## 3 Results and Discussions
### Metallographic Analysis of the Composite Interface  
Figure 7 shows the metallographic images of the carbon-steel side of the various rebars. It can be observed from the figure that the composite interface has a proper bonding state and no visible cavities, cracks, or oxides. This can be attributed to the cleaning treatment performed on the surface of the billets before they were assembled.  
Figure 7: Metallographic images of the carbon-steel side of various rebars: (**a**) 20 MnSiV/316L rebar; (**b**) 35\(\#\)/316L rebar; (**c**) 45\(\#\)/316L rebar; (**d**) 55\(\#\)/316L rebar.  
The carbon-steel side mainly consists of pearlite and ferrite. Furthermore, a pure ferrite region can be observed at the composite interface and is called the decarbonization zone. The above phenomenon can be primarily attributed to the amounts of carbon contained in the four base materials, which are 0.22%, 0.35%, 0.46%, and 0.58%, respectively. The carbon content in clad 316L stainless steel is 0.021%. There is a relatively large difference between the concentration of carbon in the base and clad layers. Therefore, in the forming process of the rebars, carbon elements will diffuse from the base layer to the cladding layer, leading to decarbonization on the carbon-steel side. As the carbon content in the base layer increases, the width of the decarbonization zone expands from 30 to ~50 \(\upmu\)m.  
Figure 8 shows the metallographic images of the stainless-steel side of various rebars. The stainless-steel side has a single austenitic structure, and some grains exhibit equiaxed crystal distribution. Figure 8c shows the metallographic image of the stainless-steel side of the 45#/316L rebar. A certain number of granular precipitates can be observed at the stainless-steel grain boundary. Figure 8d shows the metallographic image of the stainless-steel side of the 55#/316 rebar. Compared with the 45#/316L rebar, the 55#/316L rebar had a significantly increased number of precipitates at the grain boundary of the stainless-steel side and a large number of web-like precipitates with continuous distribution inside the grains.  
Point scans were performed on the precipitates by energy-dispersive X-ray spectroscopy (EDS). Table 4 shows the point scan analysis of the precipitates. It is evident that the main components of the precipitates at the stainless-steel side were carbon, chromium, and iron, suggesting that the precipitates are chromium-rich carbides. The forming mechanism of chromium-rich carbides is based on the fact that carbon is interstitially diffused, while silicon and Mn are substitutionally diffused in the iron lattice, and the activation energy of interstitial diffusion is far less than that of substitutional diffusion [20]; thus, carbon will first cross the composite surface to the stainless-steel side. Compared with nickel, chromium and carbon present stronger affinity, and thus carbon preferentially reacts with chromium on the stainless-steel side to generate stable chromium-rich carbides.  
Figure 8: Metallographic images of the stainless-steel side of various rebars: (**a**) 20 MnSiV/316L rebar; (**b**) 35#/316L rebar; (**c**) 45#/316L rebar; (**d**) 55#/316L rebar.  
A closer observation of Figure 8c also reveals that chromium-rich carbides continued to precipitate at the grain boundary of stainless steel because of a relatively large number of defects at the grain boundary of stainless steel. The carbon atoms experienced smaller resistance when diffusing along the grain boundary. Therefore, when carbon crosses the composite interface, it would manly diffuse along the grain boundary of stainless steel and react with chromium at the grain boundary to form chromium-rich carbides.