Research On The Evaluation Of Stainless Steel Properties By Automated Indentation Testing Method

In industrial production, the mechanical properties of materials affect the use of equipment directly. During the service of the equipment, the mechanical properties of the material will change with the changing of operation conditions, thereby affecting the performance of equipment. For example, strain aging embrittlement and other material performance degradation phenomenon will even have a significant impact on equipment safety. Although the testing instruments of material mechanical properties have made a significant progress, but how to make mechanical properties be tested non-destructively in the equipment without damage has become a research focus.The author introduced the principle of material mechanical properties tested by automated ball indentation. The automated ball indentation device obtains the load-indentation depth curve according to the load-unload process of the indenter. The true stress-true strain data points are calculated by the σ,-εp relationship of the hardness/micro-hardness test. Then the true stress-true strain curve of the material will be fitted according to the power-law strengthen equation σ=Ks" . Based on this, the material tensile strength, yield strength, indentation deformation energy and other mechanical properties will be calculated according to empirical formulas. Its characteristic of rapid and semi-non-destructive detection meets online testing on site. The author used common pressure vessels steel S30408, S31603, S32168 and S30403, etc. as the study material. Conventional tensile test and automated ball indentation test were carried out on material in different states respectively. Correlate and contrast the mechanical properties data of materials obtained by two types of tests. The tensile strength of stainless steel with different states obtained by the tests was compared. The result shows that the value of the tensile strength tested by conventional tensile test has a good correlation with that of automated ball indentation test and the automated ball indentation method can be applied to the engineering. The yield strength of stainless steel with different states tested by two types experiment has big offset. The author corrected empirical formula of the automated ball indentation calculating yield strength by re-associate the data so that the formula can be applied to measure the yield strength of stainless steel.As the automated ball indentation test will leave pits with different sizes on the surface of material and a certain degree of residual stress, resulting in stress concentration, this may affect mechanical properties of surface material. The author simulated the automated ball indentation process combining with true stress-true strain data acquired from conventional mechanical tests by finite element software ABAQUS. At the same time, external loads were applied to study the stress distribution influenced by indentation residual stress and applied loads at the pits. This will provide theoretical support for eliminating the effects of pits.