Failure Studies on Flow Formed High Strength Pressure Vessel: A Case Study

Abstract

High strength thin walled flow formed tubes are manufactured from AISI 4130 medium carbon low alloy steel. Starting with an ultimate tensile strength of 650 MPa, the material has recorded a tensile strength of 1250–1300 MPa corresponding to a percentage thickness reduction of 88. It has been observed that material with higher impurity levels and inclusion ratings are more vulnerable to development of micro cracks at higher percentage thickness reduction. Deformed inclusions like MnS act as stress raisers leading to initiation of micro cracks. Hard to deform inclusions like silicates create high stress concentration at inclusion-matrix interface, leading to de-cohesion and finally cracking. The presence of dissolved gas contents, particularly hydrogen, are harmful in flow forming. Hydrogen embrittlement is a serious problem which is likely to lead to cracking of the flow formed tube. It could be concluded from this study that clean steel (electro slag refined) processed through hardening and tempering route with a maximum percentage reduction in thickness of 88 or less can give consistently very high strength of the order of 1250–1300 MPa for AISI 4130 steel.