Investigation On Indentation Test Method For Mechanical Properties Of Micro-zones Of Chromium Molybdenum Vanadium Steel Welded Joints

The inhomogeneity of microstructure and mechanical properties of the welded joints are usually regarded as"metallurgical defects",which is the root cause of variable failure modes,complex failure mechanisms and premature failure of joints.Therefore,it is necessary to characterize the distribution of heterogeneous mechanical properties of the joint in detail,analyze the damage and fracture behavior of the joint from the perspective of multiphase materials（base metal,heat-affected zone,weld metal）,and provide scientific reference for joint structure design,condition monitoring,safety assessment,etc.However,the size of the micro-zone of the joint,especially the heat-affected zone,is small,which brings many inconveniences and limitations to the test and characterization of the non-homogeneous mechanical properties of the joint.The continuous ball indentation method has many advantages,such as small volume of test materials,high differentiation of test results,and unique advantages in characterization of local mechanical properties.But,the existing ball indentation method is mainly aimed at the single homogeneous materials and lacks consideration of the characteristics of the"multiphase composition material"of the joint.According to the above problems,this paper detailed the microstructure characterization for the micro-zones sizes and microstructure features of joint.The ball indentation method is used to test mechanical properties of local micro-zones of the joint.Combined with finite element method,considering the indenter diameter,indentation depth,indentation position deviation,the influence of the heterogeneous mechanical properties of the joint on indentation test results is mainly discussed.And the indentation test method for the features of"multiphase composition material"of the joint is cleared.The main work and conclusions are as follows:（1）According to the characteristics of the welding process,two cross-joint paths of the upper submerged arc welding and the bottom argon arc welding were selected respectively,and the heterogeneous microstructure and micro-hardness distribution of the CrMoV joints were characterized by optical microscope,scanning electron microscope,and micro-hardness test.The results show that the microstructure inhomogeneity of the joint is mainly reflected in two aspects:on the one hand,it is across the joint direction.According to the microstructure morphology and grain size,the joint can be divided into base metal,fine grain zone,coarse grain zone,and weld metal.The widths of the heat-affected zone of the upper submerged arc welding and the bottom argon arc welding are about 1.9 mm and 2.8 mm,respectively.The distribution of the micro-hardness across the joint is inhomogeneous,which is consistent with the change of the microstructure.The coarse grain zone has largest grain size and the highest hardness.The hardness of the weld zone is slightly higher than that of the base metal due to the presence of a large amount of carbon-rich M/A structure.On the other hand,the microstructure of the weld and coarse grain zone are obviously different in the direction of weld thickness.Because of the large submerged arc weld bead,high heat input,slow cooling rate and long residence time at high temperature,the weld metal has obvious columnar and equiaxed grains,and the microstructure of coarse grain zone is lath bainite with high dislocation density.However,the bottom argon arc welding bead is slender,the heat input is low,and the cooling rate is fast,which leads to fine grains in the weld zone,and granular bainite in the coarse grain zone,so the micro-hardness of the upper submerged arc welding seam and coarse grain zone is higher.（2）The continuous ball indentation method was used to test the mechanical properties of the local micro-zones of the joint.The distributions of tensile properties across the joint under the two test paths are consistent,and the mechanical properties between micro-zones are significantly different,which are consistent with the microstructure evolution across the joint,indicating the applicability of the indentation method to test the mechanical properties of micro-zones of the joint.By observing the morphology and damage of residual indentation,it is found that obvious Pile-up occurs around the indentation,and the Pile-up height of each micro-zone is different.During the indentation process,voids may be formed at the bottom of the indentation and within the range of 45°-60°with the loading axis.The formation of voids is related to the grain size and precipitates of the material under the indenter.When the grains are small and there are many precipitates,it is easy to form voids.The testing result of the indentation located at the interface of adjacent micro-zones due to the deviation of the indentation position deviates from the center value of the measured micro-zone.And the Pile-up and damage on both sides of the indentation are asymmetrical,which show that it is different from the testing for homogeneous materials,because the micro-zones of the joint are small and the mechanical properties between them are different.So in order to test the true mechanical properties of micro-zones of the joint,the possible influence of the adjacent micro-zones and the deviation of the indentation position must be considered.（3）The influence of heterogeneous mechanical properties of joints on the test results of indentation method was investigated by FEM.It is found that adjacent micro-zones have no influence on the test results when the HAZ center is tested by the current common indentation diameter（D=0.5 mm,1 mm,1.5875 mm）and the indentation depth(h_max=100μm,200μm,300μm).Combined with the effects of the those two on the Pile-up height,it is more appropriate to select the larger indenter diameter D=1.5875 mm and the smaller indentation depth h_max=100μm in order to obtain the accurate mechanical properties of the micro-zone.And it is found that the plastic zone radius beneath the indenter is the same as the Pile-up range around the indentation,so the plastic zone radius can be obtained by measuring the Pile-up range.When the indentation position deviates from the HAZ center,but the edge of the indenter does not cross the boundary of the adjacent micro-zone,the material under the indenter is uniform,and the test results are consistent with that in the center of the micro-zone.However,when the edge of the indenter crosses the interface between micro-zones,the asymmetric indentation is formed because of the heterogeneous material beneath the indenter.And the calculation result deviates from the center value of the micro-zone with Pile-up heights of two sides respectively.According above work,it is illustrated that ensuring the homogeneity of the materials under the indenter the accurate mechanical properties of the micro-zones can be obtained.And the applicability and reliability of the indentation method to test the mechanical properties of micro-zones are confirmed.