engleski

Investigation of the Influence of Improvement on the Effect of Strain hardening of 34CrMo4 in the Production of Seamless Steel Pressure Vessels from Pipes

This paper analyzes the process of making a seamless pressure vessel O229 mm on a spinning machine. The pressure vessels are exposed to high operating pressure when in use (up to 200 bar). Therefore, the material of the pressure vessels must meet specific requirements to prevent cases of disaster and damage. It is required to pass a burst test on one out of the 200 pressure vessels. If the burst test meets the requirements and demands, a pressure vessel is randomly selected from the same batch, and is used to make the test samples for testing the mechanical properties. Only if these results meet the standard prescribed limits, the pressure vessels can be delivered to the user. The purpose these tests, by controlling the mechanical properties of samples made from the finished pressure vessels before and after heat treatment by improving 34CrMo4 steel, and comparing them with the required standard values, is to assess the technological parameters and possible approaches in future research that could contribute to the reliability of pressure vessels in use. Before heat treatment and improvement, the analysis of test results shows that the tendency to plastic deformation is relatively small, but there is the tendency to local deformations. Local deformations indicate a possible breaking of uncontrolled pieces of pressure vessels (“fragments”) in terms of burst. In the samples taken after improvements of the pressure vessel, the yield point is 46 % higher than before heat treatment and reaches a value of approximately 950 MPa. Tensile strength reaches a value of about 1050 MPa, which is about 17 % higher than before heat treatment. The analysis of the curve recorded during tensile testing showed that the effect of deformation strengthening is less pronounced and that it has almost linear character. Plasticity is not lower, but deformation at destruction has increased from 12.8 to 16.7 %. For the deformed neck of the improved pressure vessel, strength characteristics are almost unchanged, compared to the samples of the improved body of pressure vessel. Minimal deformations at destruction increased from 12.8 to 15.1 %, and reinforcements in this case has almost linear form. Based on this analysis, it was concluded that the results of recording the tensile curve can be used as the indicators of the presence of anomalies that could contribute to unsatisfactory results on the burst tests. It helped as the basis for a recommendation of possible direction for the research: the selection of heat treatment parameters/properties at static tensile testing.

seamless pressure vessel

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