Strength–hardness statistical correlation in API X65 steel

Accurate determination of pipe yield strength (as an indication of material resistance to plastic collapse) is of great importance to pipeline engineers. In this research, Vickers hardness data from 100 tested pipes (API X65) was used to derive strength–hardness statistical correlation. First, hardness data were measured in different sub-zones of weldment (i.e. weld metal, HAZ, and base metal). Next, tensile properties of base and weld metal were measured on flat tensile strips. The minimum, maximum, mean and standard deviation of test data were calculated then for statistical variation and difference of mean value in each test zone. All test data were described by probability density function (PDF), and zones with the largest variance were determined. It was shown that hardness data could be used for yield strength prediction in API X65 steel with reasonable accuracy. Discussion on weldment microstructure correlation with strength and hardness data concluded the paper.