Effect Of Heat-treatment On Cavitation-erosion Resistance Of Matensitic Stainless Steel

Cavitation erosion is caused by the growth and collapse of vaper cavities or bubbles due to local pressure fluctuation in liquid. The bubbles collapse takes place in very short period and causes very high stress and temperature. Erosion corrosion also occurs in turbine blades because of the large amount of sand in rivers. Cavitation erosion and erosion corrosion can be called CE for short. CE is a common problem in engineering parts in contact with a liquid, and particularly, it is one of the main forms of failure in turbine blades. How to control and reduce it has become a big technical problem in urgent need of solution. Researching and developing CE-resistant materials is one of the most effective ways to solve this problem.Presently,0Cr13Ni5Mo is the conventional material for turbine blades. But the application of ultra-low-carbon matensitic stainless steel in this field is not very wide.In this study, the tested materials were two types of ultra-low-carbon matensitic stainless steel namely00Cr13Ni5Mo and00Cr16Ni5Mo. The samples from the two types of steel are in six different heat-treatment states. And the micro structural analysis, cavitation tests and erosion corrosion tests are performed using Scanning Electron Microscope (SEM), a550W ultrasonic vibratory processor and a rotating disk apparatus, respectively. The aim of this study is to investigate the influence of chemical composition, tempering temperature on the micro structure, mechanical properties and CE resistance of the tested materials.Analyzing and comparing the metallographs, scanning electron micrographs and mass-loss curves of cavitation and erosion corrosion we can draw several conclusions as follows:The normalized microstructure of00Cr13Ni5Mo consists mainly of lath matensite yet that of00Cr16Ni5Mo lath matensite and δ ferrite due to its higher Cr content. The tempered microstructure of00Cr13Ni5Mo consists mainly of lath matensite and reversed austenite, that of00Cr16Ni5Mo lath matensite,δ ferrite and reversed austenite. For the two types of stainless steel, the sample tempered at600℃properly has the maximum amount of reversed austenite.Good cavitation resistance requires an excellent combination of hardness and toughness. The experimental results show that tempering at a high temperature(>550℃) is no good to, and even has a deleterious effect on the improvement of cavitation resistance of00Cr13Ni5Mo due to the significant reduce of hardness induced by the presence of reversed austenite. However,00Cr16Ni5Mo can possess a good combination of hardness and toughness after tempered at650℃owing to its different microstructure. Therefore, tempering at a proper temperature may be a effective way to improve the cavitation resistance of00Cr16Ni5Mo.The reversed austenite generated during tempering process plays a deleterious effect on the cavitation corrosion resistance in3%NaC1solution. This is because that the electrode potential of austenite is different from that of martensite, which accelerates the formation of micro-cells resulting in corrosion damage.Hardness plays a significant role in materials’erosive-resistance. The erosion corrosion behaviors of00Cr13Ni5Mo and00Cr16Ni5Mo agree with this rule.00Cr13Ni5Mo or00Cr16Ni5Mo tempered at500℃may be a good candidate when hydropower stations are choosing turbine materials to be used in rivers containing sand.