[9]
pointed out the average nanohardness of martensite in ultra- fine-grained steel is 5.74GPa , while the hardness of fine grain in size of 1-2μm of ferrite was 2.92GPa above the 2.01GPa value of coarse ferrite. Fig.6 shows SEM image of nanoindentation in CGHAZ and FGHAZ. From Fig.6 it is obtained that nanoindentation size is smaller than the coarse grain, while it gets through a couple of fine grains. ICHAZ exhibits temper softening behavior and has the minimum hardness. Compared with the microhardness, the distribution behavior of nanohardness values is similar to that of microhardness values, as shown in Fig.5. However, the variation in nanohardness values of all zones is much larger than microhardness measuresments. The reason is that microhardness indents average over lots of grains and grain boundaries. Nanohardness values are also larger than microhardness values on the whole, which coincides well with the present study[[ ] T. Ohmura , T. Hara, K. Tsuzaki. Scripta Materialia, Vol.49 (2003) ,1157-1162 ]. Maybe because the shape of indenter and indentation diameter are different. In addition, Microhardness indents average over grains and grain boundaries, which reflects the mechanism of grain boundaries strengthening. Nanohardness reveals the contribution of the matrix strength. The elastic modulus, also known as Young's modulus, is essentially a measurement of the stiffness of a material. Thus it is commonly used in design and engineering applications. On the whole, the elastic modulus of metallic material is not much sensitive to the microstructure. Fig.5 Micro mechanical properties of weld joints Fig.5 SEM image of nanoindentation (a) CGHAZ (b)FGHAZ The relationship of micro mechanical properties and microstructure Nanoindentation hardness variation has a strong relationship with microstructure. The Microstructure in CGHAZ and FGHAZ are composed of lath martensite with very small cementite precipitates which are evenly distributed, as indicated by arrows in Fig. 7a and 7b. The detail characterisation of cementite precipitates of HAZ is revealed by using TEM technique. Cementite precipitates are regularly distributed with the length of 50-100nm as indicated by arrows in Fig. 8. Cementite precipitates and high dislocation density in CGHAZ and FGHAZ are two key factors to strengthen the microstructure with the principle of precipitation hardening and dislocations strengthening, which increased the value of nanohardness. The grain in FGHAZ is fine with size of 1-4μm, so nanoindentation get through a couple of grains. Grain-boundary hardening plays a key role in increasing the hardness by the refinement of the microstructure. Choi Y[9] believed that the presence of cementite particles and refinement of the grain size in the fine-grained strain-induced dynamic-transformation ferrite (SIDTF) are expected to be the strengthening mechanisms. Joonoh Moona[10] evaluated the strength of lath martensite by Vickers hardness and nanoindentation test. He analyzed that deformed CGHAZ had a higher strength than undeformed CGHAZ, which is due to the contribution of grain boundary strengthening and precipitation strengthening. The weld metal with the microstructure of lath bainite, whose start temperature (Bs) is relatively high, released a lot of residual stress and transformation stress. That is the why the nanoindentation of weld metal is lower. Additionally, there is little precipitates from weld metal by SEM observation, as shown in Fig. 7c. Fig. 7 SEM image of small precipitates of weld joints (a) CGHAZ (b)FGHAZ(c)WM Fig.8 TEM image of nanoscale precipitates at the region of HAZ Conclusions 1. The microstructure of the weld joints is inhomogenous. The base metal is composed of tempered bainite and the weld metal is mainly of lath Bainite with a little of martensite. The weld metal can equally match the base metal with good toughness. 2. CGHAZ and FGHAZ are composed of lath martensite with a lot of very small cementite precipitates and high dislocation density. This is the main reason why its nanohardness is higher than that of weld metals and base metal. Besides, the refinement of the structure plays key role in increasing the hardness. 3. The variation in microhardness values is not as large as that for nanohardness measuresments since microhardness indents average over lots of grains and grain boundaries. Nanohardness values are larger than microhardness values since the shape of indenter and indentation diameter are different and Microhardness indents average over grains and grain boundaries, which reflects the mechanism of grain boundaries strengthening, while nanohardness reveals the contribution of the matrix strength. References
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