​​​​Callum O'Dwyer

This project is limited in scope to examining the behaviour of simple plain carbon steels under heat treatment conditions designed to model what could be caused by a fire. The 1.25-5% Chromium steels commonly used for pressure vessels will not be covered.

 

The steel commonly used for low pressure vessels in the refinery industry is ASTM A517 grade 70. This is a low carbon steel. The British Standard equivalent is EN 10028 P355GH. Both these steels have ultimate strengths in the range 490-620 MPa. The older American standard permits higher carbon levels than the British Stand presumably to achieve the strength requirements. Levels of carbon of 0.31% are allowed for thicker plate materials. The British Standard only permits carbon levels of 0.21%.

 

However due to advances in modern steel making, there are practises which produce small grain sized steels which allow the strength requirements to be achieved at lower carbon contents. This is important as far as welding is concerned. Higher carbon contents can lead to problems in the weld heat affected zones (HAZs). Since pressure vessels are of all welded construction this is a major major concern.

Thus for this project a plain carbon steel with a 0.22% carbon content was chosen to model the pressure vessel steel.

 

There was some complication in acquiring the desired standards of steel with the technical department, which is explained in more depth in Project Difficulties. However, the grades of steel which were eventually used were found to be a low carbon steel, estimated 0.1%C and a higher composition of at least 0.5%C. 

 

More information about the theoretical basis behind the Heat Treatment and Cooling is found in the Phase Diagrams, TTT plots and CCT diagrams section. The theoretical basis for the impact testing can be found in Brittle-Ductile Temperature Graph, the Mechanical Properties of various Iron-Carbon Alloys here, and the predictions here.